Major earthquakes in Mexicali Valley, Mexico

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关于地震的英语作文

关于地震的英语作文关于地震的英语作文题目：What is earthquake?内容：An earthquake is the result of a sudden release of energy in the Earths crust that creates seismic waves(地震波). Earthquakes are recorded with a seismometer(地震检波器), also known as a seismograph(地震仪). The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude(里氏量级), with magnitude 3 or lower earthquakes being mostly imperceptible 感觉不到的and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale(麦加利震级, 麦氏震级).At the Earths surface, earthquakes manifest themselves by a shaking and sometimes displacement (位移)of the ground. When a large earthquake epicenter (震中)is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami(海啸). The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.关于地震的英语作文仅供交流参考，希望大家了解更多的地震知识并在生活中及时了解这些现象!In its most generic sense, the word earthquake is used to describe any seismic eventwhether a natural phenomenon or an event caused by humansthat generates seismic waves. Earthquakes are caused mostly by rupture(破裂, 裂开) of geological faults(断层), but also by volcanic activity, landslides, mine blasts, and nuclear experiments.An earthquakes point of initial rupture is called its focus or hypocenter. The term epicenter means the point at ground leveldirectly above this.。

高二英语地理发现单选题30题

高二英语地理发现单选题30题1. In the exploration of the Amazon rainforest, which of the following is not a geographical feature?A. RiverB. MountainC. BuildingD. Valley答案：C。

本题考查地理特征的词汇。

选项A“River（河流）”、选项B“Mountain（山脉）”和选项D“Valley（山谷）”都是常见的地理特征。

而选项C“Building（建筑物）”不属于自然地理特征。

2. When studying the geographical discovery of Africa, which term refers to a large area of dry land with little rainfall?A. DesertB. ForestC. LakeD. Island答案：A。

本题主要考查地理术语。

选项A“Desert（（沙漠）”是少雨的大片干旱地区。

选项B“Forest（森林）”是树木密集的区域，多雨湿润。

选项C“Lake（（湖泊）”是蓄水的水域。

选项D“Island（（岛屿）”是被水环绕的陆地。

3. During the voyage to discover new lands, which of the following isa geographical term related to the ocean?B. CanyonC. StraitD. Plain答案：C。

此题聚焦于海洋相关的地理术语。

选项A“Beach（（海滩）”是海陆交界的部分。

选项B“Canyon（峡谷）”通常在陆地上。

选项C“Strait（（海峡）”是连接两个海洋的狭窄水道，与海洋地理相关。

选项D“Plain（平原）”是陆地地形。

4. In the study of geographical discoveries, which of the following is not a geographical term for a landform?A. PlateauB. PeninsulaC. GlacierD. Market答案：D。

汶川大地震英文介绍

汶川⼤地震英⽂介绍⽤英语介绍汶川⼤地震中国会永远记住5⽉12⽇这⼀天－－四川省汶川县发⽣7.9级⾥⽒⼤地震,迄今为⽌已造成逾2.8万⼈丧命.整个国家沸腾了,每⼀颗⾚红的中国⼼都牵挂着天府之国...Wenchuan Earthquake 汶川⼤地震A major earthquake measuring 7.9 Richter Scale jolted Wenchuan County of Southwest China's Sichuan province at 2:28 pm on Monday. Since then the death toll has been soaring, with over 28,000 people fallen victim up to Saturday. Let's pray for all the sufferings in this catastrophe.On Monday, an earthquake in China measuring 7.9 on the Richter scale occurred a long a fault where South Asia pushes against the Eurasian land mass, smashing the Sichuan Plain into mountains late into the Tibetan highlands. In 1989, the Loma Prieta earthquake shook San Francisco and Monerate Bay regions. This major earthquake caused dozens of deaths, thousands of injuries and an estimated 6 billion dollars in property damage. It was the largest earthquake to occur on the San Andreas fault, since the great San Francisco earthquake in April 1906. The Loma Prieta quake was similar to Tuesday’s earthquake in China in depth, but its magnitude was just 7.1 on the Richter scale, compared to the Sichuan quake at 7.9. Seismology experts in Japan also compared the earthquake to the Kobe quake of 1995 in Japan, in which more than 5,000 were killed. “Because earthquake occurred in shale that is about 10 kilometers, about, beneath the ground, so the damage is usually very strong, devastating. If you remember the Kobe earthquake in 1995, this in a sense a similar type because earthquake shale right beneath the land and population area.” The professor also warned that aftershocks could cause further Damage in the region of the quake. US GS-seismologists warned that aftershocks from this earthquake could be felt up to 60 miles from the epicenter and could last for months.。

GMAT逻辑每日一题21-40题

21To reduce the danger to life and property posed by major earthquakes, scientists have been investigating several techniques for giving advance warning of dangerous earthquakes. Since catfish swim erratically before earthquakes, some investigators have proposed monitoring catfish to predict dangerous earthquakes.Which of the following, if true, most seriously undermines the usefulness of the proposal?(A) In Japan, which is subject to frequent earthquakes, the behavior of catfish has long been associated with earthquakes.(B) Mechanical methods for detecting earthquakes have not proved effective.(C) Tremors lead to the release of hydrogen sulfide gas into water, thereby causing various fish and shellfish to behave erratically.(D) Careful construction can reduce the dangers posed by earthquakes.Even very slight, fleeting tremors cause catfish to swim erratically.问题分析：A 说这种鱼的行为经常伴随着地震，怎么看怎么像支持，不像驳斥，杀。

地震指南英语作文

地震指南英语作文Earthquake GuideEarthquakes can be really scary! The ground starts shaking, things start falling, and it feels like the whole world is moving under your feet. But don't worry, I'm here to teach you all about earthquakes so you can stay safe if one ever happens near you.What is an Earthquake?An earthquake is when the ground shakes. It happens because of movements inside the Earth. Our planet is made up of huge rocky plates that very slowly move around. Sometimes, these plates get stuck and then suddenly shift, releasing a burst of energy that makes the ground shake. The shaking can be small or it can be really strong and powerful.The strength of an earthquake is measured using a scale called the Richter scale. Small earthquakes that you can barely feel are around 2 or 3 on the Richter scale. Really big ones that cause a lot of damage are 7 or higher. The largest earthquake ever recorded was a massive 9.5!Earthquakes can happen anywhere in the world, but some places get them more often because they are located near theedges of the giant rocky plates. California, Japan, Mexico, and Indonesia are some places that have frequent earthquakes.Earthquake SafetyWhile we can't stop earthquakes from happening, there are lots of things we can do to stay safe when they occur. One of the most important things is to know the safety drill and practice it regularly at school and home.If an earthquake strikes when you're indoors, remember: Drop, Cover, and Hold On!Drop down onto your hands and knees. This prevents you from being thrown around and keeps you stable.Cover your head and neck with one arm/hand. Use a sturdy table or desk to cover yourself if possible. This protects you from falling objects.Hold On to your shelter with one hand until the shaking stops. If you're using a table, hold on from the side to prevent it from moving away from you.If you're outside when an earthquake hits, stay away from buildings, power lines, trees, and anything else that could fall on you. Drop to the ground and cover your head and neck to protect from debris.Never use elevators during an earthquake! The power could go out and you could get trapped. Use stairs instead when it's safe to exit the building.After the shaking stops, there may still be danger from fallen objects, broken glass, fires, or damage to roads and bridges. Listen to authorities and your parents for safety instructions.At Home PreparednessBesides knowing the drill, you and your family should also prepare emergency supplies at home in case utilities are disrupted after a big quake. Gather enough non-perishable food, water, medications, and other supplies to last at least 3 days.Make sure you have flashlights, a battery-powered radio, extra batteries, a first aid kit, cash, blankets, and other basic supplies. It's also a good idea to identify safe spots in each room of your home, like under a sturdy desk or against an inside wall away from windows.Practice your family's earthquake plan by doing drills so everyone knows what to do. Designate safe meeting places outside in case you have to evacuate, like the front yard or a neighborhood park. Make copies of important documents and keep them somewhere secure like a fireproof safe.By learning about earthquakes and how to stay safe, you'll be prepared instead of scared if one ever strikes your area. Stay calm, follow the drill, and remember–earthquakes may shake things up, but together we can make it through!。

Major Earthquakes

Major EarthquakesOn average about 1,000 earthquakes with intensities of 5.0 or greater are recorded each year. Great earthquakes (intensity 8.0 or higher) occur once a year, major earthquakes (intensity 7.0–7.9) occur 18 times a year, strong earthquakes (intensity 6.0–6.9) 10 times a month, and moderate earthquakes (intensity 5.0–5.9) more than twice a day. Because most of these occur under the ocean or in underpopulated areas, they pass unnoticed by all but seismologists. Notable earthquakes have occurred at Lisbon, Portugal (1755); New Madrid, Mo. (1811 and 1812); Charleston, S.C. (1886); Assam, India (1897 and 1950); San Francisco (1906); Messina, Italy (1908); Gansu, China (1920); Tokyo, Japan (1923); Chile (1960); Iran (1962); Managua, Nicaragua (1972); Guatemala (1976); Hebei, China (1976); Mexico (1985); Armenia (1988); Luzon, Philippines (1990); N Japan (1993); Kobe, Japan (1995); Izmit, Turkey (1999); central Taiwan (1999); Oaxaca state, Mexico (1999); Bam, Iran (2003); and NW Sumatra, Indonesia (2004). The Lisbon, Chilean, and Sumatran earthquakes were accompanied by tsunamis. On Good Friday 1964, one of the most severe North American earthquakes ever recorded struck Alaska, measuring 8.4 to 8.6 in intensity. Besides elevating some 70,000 sq mi (181,300 sq km) of land and devastating several cities, it generated a tsunami that caused damage as far south as California.Ten of the fifteen largest earthquakes in the United States have occurred in Alaska, and eight of the fifteen largest in the continental United States have occurred in California. Recent earthquakes that affected the United States include the Feb., 1971, movement of the San Fernando fault near Los Angeles. It rocked the area for 10 sec, thrust parts of mountains 8 ft (2.4 m) upward, killed 64 persons, and caused damage amounting to $500 million. In 1989, the Loma Prieta earthquake above Santa Cruz shook for 15 seconds at an intensity of 7.1, killed 67 people, and toppled buildings and bridges. In Jan., 1994, an earthquake measuring 6.6 with its epicenter in N Los Angeles caused major damage to the city's infrastructure and left thousands homeless.1。

你能了解哪些阿拉斯加地震的资料英文作文

你能了解哪些阿拉斯加地震的资料英文作文Alaska, known for its beautiful landscapes, faces numerous earthquakes due to its location on the Pacific Ringof Fire. The following is an in-depth analysis of some of the most significant earthquakes to have occurred in Alaska.One of the most notable earthquakes in Alaska's historyis the 1964 Alaska earthquake, also known as the Great Alaska Earthquake. It occurred on March 27, 1964, with a magnitudeof 9.2, making it the most powerful recorded earthquake inU.S. history and the second most powerful earthquake globally. The epicenter of the earthquake was in the Prince William Sound region, and the quake resulted in extensive damage across south-central Alaska. The earthquake also triggered a series of tsunamis, causing further destruction along theGulf of Alaska, the U.S. West Coast, and even as far asHawaii and Japan. This devastating event resulted in the lossof 139 lives and caused an estimated $311 million in property damage.Similarly, the 2018 Gulf of Alaska earthquake, with a magnitude of 7.9, sent tremors across the southern coast of Alaska. The earthquake struck on January 23, 2018, prompting tsunami warnings for Alaska, the U.S. West Coast, and Hawaii. Although the warnings were subsequently canceled, the earthquake highlighted the ongoing risk of tsunamis in the region and the varying levels of preparedness among coastal communities.Another significant event in Alaska's seismic history is the 1906 Valdez-Cordova earthquake, which had a magnitude of 8.3. The earthquake, which occurred on October 18, 1906, caused widespread damage to the city of Valdez and resulted in a massive underwater landslide in the nearby Prince William Sound. This event contributed to the understanding of the relationship between seismic activity and underwaterlandslides, highlighting the potential for secondary hazards following major earthquakes.In addition to these historical earthquakes, Alaska continues to experience frequent seismic activity. The tectonic activity in the region has led to ongoing research and monitoring efforts to understand earthquake patterns and improve early warning systems. The Earthquake Center at the University of Alaska Fairbanks plays a crucial role in monitoring seismic activity in the state and providing real-time information to the public and emergency response agencies.In conclusion, Alaska has a long history of significant earthquakes, with the 1964 Alaska earthquake, the 2018 Gulf of Alaska earthquake, and the 1906 Valdez-Cordova earthquake being some of the most notable events. These earthquakes have prompted further research, monitoring, and preparedness efforts to mitigate the impact of future seismic events inthe region. As Alaska continues to be at the forefront of seismic activity, ongoing vigilance and preparation are crucial for the safety and resilience of its communities.。

土耳其2023年大地震英语作文

土耳其2023年大地震英语作文全文共3篇示例，供读者参考篇1Turkey, located at the meeting point of three tectonic plates - the Eurasian, African, and Arabian plates, is a country prone to seismic activity. In particular, the North Anatolian Fault, a major fault line that runs across Turkey, has caused devastating earthquakes in the past. The last major earthquake to strike Turkey was the 1999 İzmit earthquake, which killed over 17,000 people and left hundreds of thousands homeless.Because of its geographical location, Turkey is at constant risk of experiencing large earthquakes. Experts have warned that a major earthquake could strike Turkey again in the near future, with many speculating that it could happen as soon as 2023. This looming threat has put the country on high alert, with the government and various organizations working hard to prepare for the potential disaster.One of the main concerns surrounding a potential earthquake in Turkey is the country's infrastructure. Many buildings in Turkey are not built to withstand strong earthquakes,and a large quake could lead to widespread destruction and loss of life. To address this issue, the government has been taking steps to strengthen building codes and retrofit existing structures to make them more earthquake-resistant. Additionally, emergency response plans are being put in place to ensure a swift and coordinated response in the event of a disaster.Another concern is the country's ability to provide aid and support to those affected by a major earthquake. Turkey has a history of struggling with disaster relief efforts, as seen in the aftermath of the 1999 İzmit earthquake. To improve the country's disaster response capabilities, the government is investing in training and equipping emergency response teams, as well as stockpiling essential supplies and setting up shelters for those displaced by a disaster.In addition to physical preparations, efforts are also being made to raise awareness about earthquake preparedness among the general public. Schools, businesses, and communities are being encouraged to develop their own emergency plans and to participate in drills and exercises to practice their response to a quake. By educating and empowering the public, it is hoped that the impact of a potential earthquake can be minimized.Despite the looming threat of a major earthquake in Turkey, there is hope that the country will be better prepared to handle such a disaster if and when it strikes. By taking proactive measures to strengthen infrastructure, improve emergency response capabilities, and raise public awareness, Turkey is working to ensure that it is ready for whatever challenges may come its way in the future. With continued dedication and effort, the country can face the future with confidence and resilience.篇2The devastating earthquake that struck Turkey in 1999, known as the İzmit earthquake, left a deep scar on the collective memory of the Turkish people. With over 17,000 people losing their lives and widespread destruction of infrastructure, the earthquake served as a stark reminder of the country's vulnerability to seismic hazards. However, it also galvanized efforts to improve disaster preparedness and response in the country.Fast forward to 2023, the year that experts have long warned could see another major earthquake in Turkey. The country sits on several active fault lines, including the North Anatolian Fault, which has been responsible for some of the deadliest earthquakes in the region's history. With a rapidly growingpopulation and urbanization, the potential impact of a large earthquake in Turkey is even greater today than it was in 1999.In response to this looming threat, the Turkish government has taken significant steps to improve the country's resilience to earthquakes. One of the key initiatives is the establishment of a National Earthquake Early Warning System, which uses a network of sensors to detect seismic activity and provide timely alerts to at-risk areas. This system has been instrumental in ensuring that people have precious seconds to take cover before the shaking begins.In addition to early warning systems, the Turkish government has also invested heavily in retrofitting buildings and infrastructure to withstand strong earthquakes. This includes upgrading schools, hospitals, and other critical facilities to meet modern seismic codes, as well as providing incentives for homeowners to reinforce their own properties. These efforts have helped to reduce the potential for widespread damage and loss of life in the event of a major earthquake.Furthermore, the government has focused on improving emergency response capabilities, including training first responders and conducting regular drills to ensure that people know what to do in the event of an earthquake. This has beenparticularly important in urban areas, where the density of buildings and population can exacerbate the impact of seismic events.Despite these efforts, the specter of a major earthquake in Turkey remains a constant threat. The country's geographical location and tectonic activity make it inherently vulnerable to seismic hazards, and no amount of preparation can completely eliminate the risk. However, by taking proactive steps to improve disaster preparedness and response, the Turkish government is working to mitigate the potential impact of a future earthquake and protect the lives and livelihoods of its citizens.As we look ahead to 2023 and beyond, it is clear that the threat of a major earthquake in Turkey will continue to loom large. However, with ongoing investments in resilience and preparedness, the country is better equipped than ever to face this challenge head-on. By working together as a nation and with the support of the international community, Turkey can build a safer and more resilient future for all its citizens, even in the face of nature's most powerful forces.篇3The Great Earthquake of Turkey in 2023IntroductionOn July 26, 2023, a catastrophic earthquake struck Turkey, causing immense destruction and loss of life. The earthquake, which had a magnitude of 7.8, was centered in the region of Izmir and affected several other major cities, including Istanbul and Ankara. The tremors were felt as far as Greece and Bulgaria, and the death toll is estimated to be in the tens of thousands.Immediate AftermathIn the aftermath of the earthquake, chaos and panic ensued as buildings collapsed, roads were blocked, and communication lines were cut off. The government declared a state of emergency and launched a massive rescue operation to search for survivors and provide aid to those in need. International assistance was also mobilized to help with the relief efforts.Rescue and Relief EffortsThe rescue teams worked tirelessly, pulling survivors from the rubble and providing medical treatment to the injured. Makeshift shelters were set up to accommodate those who had been displaced by the disaster. Food, water, and other essential supplies were distributed to the affected population, and temporary hospitals were established to provide medical care.Rebuilding and RecoveryAs the immediate crisis began to subside, the focus shifted to rebuilding and recovery. The government announced a reconstruction plan to help people rebuild their homes and businesses. Funds were allocated for the repair of infrastructure, such as roads, bridges, and public buildings. Counseling and support services were also provided to help people cope with the trauma of the earthquake.Lessons LearnedThe earthquake of 2023 served as a harsh reminder of the importance of preparedness and resilience in the face of natural disasters. It highlighted the need for better building codes and infrastructure to withstand seismic events. It also underscored the importance of community solidarity and international cooperation in times of crisis.ConclusionThe Great Earthquake of Turkey in 2023 was a tragic event that left a lasting impact on the country and its people. However, it also brought out the best in humanity, as people came together to help each other in their time of need. As Turkeyrebuilds and recovers from this disaster, the lessons learned will serve as a foundation for a stronger and more resilient future.。

英语六级听力新题型模拟听写训练-第5套短文(1)

英语六级听力新题型模拟听写训练:第5套短文(1)Passage One短文一Now, you've been reading articles about the tremendous damage done to life and property by earthquakes.到目前为止，大家看到过很多介绍地震给人们的生命及财产带来的惨重损失的文章。

That's why seismologists have been working so hard to develop methods of earthquake prediction.这也是地震学家一直致力于大力研发地震预报方法的原因。

We can now predict earthquake fairly well but the predictions only locate potential areas of danger.我们现在已经可以准确地预测到地震，但只能找到可能发生地震的危险区域，They don't predict the specific time and location at which an earthquake is likely to occur.地震学家不会预测地震发生的具体时间和地点。

Today I want to introduce you to three prediction models that have been developed.今天我想向大家介绍三种地震预测方法。

The first prediction model looks along earthquake faults,第一种方法是根据地震断层those cracks in the Earth's crust, to find what are known as seismic gaps.也就是地壳裂缝来寻找所谓的“地震空区”。

地震讲座英语作文高中生

地震讲座英语作文高中生Earthquakes are one of the most powerful and destructive natural phenomena on our planet. They can cause immense damage to buildings, infrastructure, and human lives, making them a serious concern for people living in earthquake-prone regions. As high school students, it is important for us to understand the science behind earthquakes, the potential risks they pose, and the measures we can take to protect ourselves in the event of an earthquake.At the outset, it is crucial to understand the underlying causes of earthquakes. Earthquakes are the result of the sudden release of energy stored within the Earth's crust, often due to the movement of tectonic plates. The Earth's surface is composed of several large plates that are constantly moving, colliding, and separating. When these plates suddenly shift or slide past one another, the energy released can cause the ground to shake and tremble, resulting in an earthquake.The strength and magnitude of an earthquake are measured using the Richter scale, which ranges from 1 to 10. The higher the numberon the Richter scale, the more powerful the earthquake. For example, a magnitude 5 earthquake is considered moderate, while a magnitude 8 or higher is considered a major, potentially catastrophic event. It is important to note that the Richter scale is logarithmic, meaning that a magnitude 8 earthquake is 10 times more powerful than a magnitude 7 earthquake.One of the most significant risks associated with earthquakes is the potential for widespread damage to buildings and infrastructure. Earthquakes can cause buildings to collapse, bridges to crumble, and roads to become impassable. This can lead to the disruption of essential services, such as electricity, water, and communication, as well as the displacement of large numbers of people. In the aftermath of a major earthquake, rescue and recovery efforts can be extremely challenging, with the potential for significant loss of life and long-term economic and social consequences.Another serious concern is the potential for earthquakes to trigger other natural disasters, such as tsunamis. Tsunamis are large, powerful waves that can be generated by the sudden displacement of water caused by an earthquake. These waves can travel at high speeds and can cause widespread destruction in coastal areas, particularly if they are not detected and warned about in time.In addition to the physical destruction caused by earthquakes, theycan also have a significant psychological impact on individuals and communities. The fear, anxiety, and trauma experienced during and after an earthquake can be profound, and can lead to long-lasting mental health issues, such as post-traumatic stress disorder (PTSD).Given the serious risks associated with earthquakes, it is essential that we as high school students take steps to prepare ourselves and our communities for the possibility of a major earthquake. One of the most important things we can do is to educate ourselves about earthquake safety and preparedness. This includes learning about the warning signs of an impending earthquake, the appropriate actions to take during an earthquake (such as dropping, covering, and holding on), and the importance of having an emergency kit and plan in place.In addition to personal preparedness, we can also play a role in promoting earthquake-resistant construction and infrastructure. This can involve advocating for stricter building codes and standards, supporting the development of early warning systems, and encouraging the use of seismic-resistant materials and design principles in new construction projects.Another important aspect of earthquake preparedness is community engagement and resilience. By working together with our neighbors, local authorities, and emergency responders, we can developcomprehensive disaster response plans and ensure that our communities are better prepared to withstand and recover from the impacts of a major earthquake.In conclusion, earthquakes are a serious and potentially devastating natural phenomenon that require our attention and action as high school students. By understanding the science behind earthquakes, the risks they pose, and the steps we can take to prepare ourselves and our communities, we can play a vital role in reducing the impact of these events and promoting a more resilient and sustainable future. Through education, advocacy, and collective action, we can work to make our communities safer and more prepared for the challenges that lie ahead.。

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Bulletin of the Seismological Society of America,Vol.94,No.6,pp.2186–2197,December2004Quantiﬁcation of Hanging-Wall Effects on Ground Motion: Some Insights from the1999Chi-Chi Earthquakeby Tsui-Yu Chang,Fabrice Cotton,Yi-Ben Tsai,and Jacques Angelier Abstract Accelerometric records of the Chi-Chi earthquake from sites on the hanging wall exhibit larger acceleration than those from the footwall.Based on ground accelerations recorded at79near-ﬁeld stations(10hanging-wall stations and 69footwall stations,respectively)and precise mapping of fault-rupture traces,the hanging-wall/footwall effects of the Chi-Chi earthquake have been fully studied.We show that the hanging-wall effects cannot be simply accounted for by a proper choice of distance metric.The closest distance to the rupture plane(D rup)is then selected to develop an empirical ground-motion model by using the data collected during the mainshock of the Chi-Chi earthquake that struck Taiwan.With the exception of some sites immediately next to the rupture traces(D rupՅ5km),the acceleration residuals between this empirical model and the recorded data at the footwall stations are closeto zero for stations in the distance range from5to50km.On the other hand,the average acceleration ampliﬁcation on the hanging wall is equal to the natural loga-rithmic values of0.64ע0.4for all hanging-wall sites within20km of D rup.The hanging-wall/footwall effects have also been evaluated for several response spectral periods.It is observed that both the horizontal and vertical components of spectral acceleration are apparently ampliﬁed for sites on the hanging wall at a distance from 5to20km for spectral periods0.02to0.5sec,whereas the vertical component has less ampliﬁcation than the horizontal in all the spectral periods considered.The horizontal component of spectral acceleration at the hanging-wall sites also shows a larger value for the long-period motion,relative to the footwall,for periods larger than1.0sec.The hanging-wall effects are relatively constant,at low frequencies,as the distance extends to about20km.This observation cannot be explained by the simpliﬁed empirical model.Rather,it suggests that waves trapped in the hanging-wall wedge may have been involved.IntroductionThe Chi-Chi earthquake of M W7.6,was the greatest thrust event in the twentieth century to strike the Taiwan area(Ma et al.,2001).Owing to strong shaking and exten-sive surface ruptures,more than100,000buildings were de-stroyed,and more than2500people lost their lives in this event.A large set of records has been acquired by one of the densest and most advanced seismic and geodetic networks in the world(e.g.,Shin et al.,2000;Yu et al.,2001).The high quality and quantity of these data have led to numerous studies on mechanical and kinematical characteristics of the earthquake process(e.g.,Chung and Shin,1999;Ma et al., 2001;Oglesby and Day,2001;C.Wu et al.,2001;Lin,2001; Zeng and Chen,2001;Wang et al.,2001;Yoshioka,2001; Dalguer et al.,2001).The accelerometric data of this earthquake have shown that ground motion on the eastern side of the Chelungpu fault,a major north–south-trending and east-dipping thrust fault whose rupture caused the Chi-Chi earthquake,exhib-ited dramatically high accelerations in comparison to the re-cords of footwall stations on the western side of the Che-lungpu fault(Fig.1).Most of the damaged buildings were located without exception at the hanging wall.Prior to a rigorous quantiﬁcation of the hanging-wall effects,ground-motion differences between the hanging-wall and footwall sites can be qualitatively observed from the time series of raw accelerometric records.Figure2shows sample acceler-ograms of the hanging-wall and footwall records,respec-tively.It shows a clear,long-period dispersive Love-like mo-tion as waves propagating from the source to the footwall (station CHY101);however,the hanging-wall site(station TCU089)exhibits a steady high-frequency motion from20 (the triggered timing)to50sec.The obvious conclusion is2186Quantiﬁcation of Hanging-Wall Effects on Ground Motion:Some Insights from the1999Chi-Chi Earthquake2187Figure1.Map of central Taiwan,showing the near-ﬁeld recording stations of the Chi-Chi earth-quake used in this study,the epicenter,and the sur-face-rupture trace.Distribution of the peak ground ac-celeration(PGA)of the Chi-Chi’s mainshockis Figure2.Examples of the acceleration-time his-tory of the Chi-Chi earthquake,recorded on the hang-ing wall and footwall,respectively.that the footwall stations recorded long-period oscillations of surface waves that are not seen on recordings from the hanging-wall stations.Based on empirical approaches,recent studies have pro-posed that peak ground accelerations(PGA s)from thrust earthquakes can be20%–30%larger than from strike-slip earthquakes(e.g.,Campbell,1993;Sadighn et al.,1993; Abrahamson and Somerville,1993,1996;Oglesby et al., 1998,2000).It has also been shown that the hanging wall systematically exhibits ampliﬁed near-ﬁeld ground motion in comparison to the footwall(e.g.,the1971San Fernando earthquake,the1980El Asnam,Algeria,earthquake,and the1994Northridge earthquake[Nason,1973;Ruegg et al., 1982;Oglesby et al.,1998,2000]).However,the hanging-wall effects have not been accurately quantiﬁed thus far be-cause of a lack of data recorded at hanging-wall stations. Theﬁrst quantitative studies on hanging-wall effects were done by Abrahamson et al.(e.g.,Abrahamson and Somer-ville,1993,1996;Somerville et al.,1996;Abrahamson and Silva,1997),whose primary data were from the records of the1994Northridge earthquake(Bommer et al.,2003). Their empirical model indicates a more than50%increase in peak horizontal accelerations on the hanging wall relative to the median attenuation at all sites over a distance of10 to20km.Their quantiﬁcation of the hanging-wall effects was for buried faults.In the case of the Chi-Chi earthquake,detailed mapping of the surface-rupture traces byﬁeld surveys immediately after the occurrence of the earthquake(Central Geological Survey of Taiwan,1999;Chen,2000),provides an excellent opportunity to conduct a quantitative study of the hanging-wall effects.With a good reconstruction of the geological setting in the fault zone by theﬁeld surveys,the hanging-wall sites can be clearly deﬁned as the stations on the eastern side of the Chelungpu fault.The empirical ground-motion-attenuation equation as a function of distance is deduced from several different distance metric deﬁnitions.Finally, the distance closest to the rupture is chosen to estimate the hanging-wall effects based on the Chi-Chi earthquake’s rec-ords.The ampliﬁcation factor of the hanging-wall ground motion is determined by the residuals of this empirical ground-motion model.This ampliﬁcation factor is also cal-culated in the spectral domain.Taking advantage of a large number of available records and the precise mapping of rup-ture traces acquired from the Chi-Chi earthquake,the present study aims at(1)better understanding the factors that may control the hanging-wall effects,(2)quantifying these ef-fects,and(3)discussing ways to take these effects into ac-count in ground-motion calculation.Fault Geometry and Ground-Motion DataThe epicenter of the Chi-Chi earthquake was located at the southern part of the Chelungpu fault,with a focal depthillustrated by multiple gray shadows.2188T.-Y.Chang,F.Cotton,Y.-B.Tsai,and J.AngelierFigure 3.Acceleration-attenuation relationsas three functions of geometrical-spreading distance.Scheme of source-to-site distance measures is illustrated at upper left.Seismo-genic depth is indicated by gray shadow.At lower left,three sites with zero value of D jb are supplemented in the plot of acceleration atten-uation with a pseudo-distance of 0.06km to better describe the PHA distribution tendency at the near ﬁeld.See equation (1)for explana-tion of PHA .of about 7km (Figs.1and 3).This rupture reactivated a major thrust of the fold-and-thrust belt of Taiwan (for de-tailed reports about the Chi-Chi earthquake,see e.g.,Lee,et al.,1999;Shin and Teng,2001).The near-ﬁeld area of this earthquake comprises the Foothills and Coastal Plain regions of central Taiwan.According to geological surveys and focal-mechanism solutions,the fault plane of this reactivated rup-ture can be regarded as a simpliﬁed plane delineated by the surface fault traces with an approximate strike of N5ЊE and a dip of 29Њto the east.The surface trace of this fault rupture has been relocated by means of mobile GPS instruments after the Chi-Chi earthquake with a precision of about 1m (Cen-tral Geological Survey of Taiwan,1999).In this study the ground-acceleration data were obtained from an extensive seismic instrumentation program com-pleted in 1996by the Seismology Center of the Central Weather Bureau (TSMIP/CWB )(Liu et al.,1999;Lee et al.,1999;Shin et al.,2000).Excluding a few stations in lique-faction areas,a total of 69stations have been selected on the footwall side,and 10stations on the hanging-wall side (Fig.1;Table 1).Ground Motion in Different Distance–MetricDeﬁnitionsGround-motion-attenuation relations used in engineer-ing seismology provide simple models in terms of a limited number of relevant parameters,such as magnitude,distance,and site conditions.Some of the variations in the derived attenuation relations are,in fact,due to difference in dis-tance–metric deﬁnition for various geometrical spreading models (epicentral,hypocentral,closest rupture distance,distance to seismogenic part of the rupture plane,closest horizontal projection to the vertical projection of the rupture,etc.)(Abrahamson and Shedlock,1997;Scherbaum et al.,2003).On the basis of simple geometrical considerations,sites located above the fault rupture are expected to show larger ground motion than sites at the same rupture distance located on the footwall,because the hanging-wall sites are closer to a larger area of the inclined fault plane than the footwall site.This geometrical effect is usually thought to be a primary factor for causing the hanging-wall effects.It is therefore essential to determine which one of the various distance–metric deﬁnitions will better account for the hanging-wall effects.One popular distance–metric deﬁnition in earthquake engineering is the Joyner–Boore distance (D jb ).The D jb dis-tance is deﬁned as the distance to the vertical projection of the fault surface to the Earth’s surface.Thus,D jb is zero when the site is directly over the ruptured portion of a fault.Another deﬁnition is D rup ,the nearest distance between the station and rupture surface.Similar to D rup ,D seis is the near-est distance to a location on the fault believed to be strong enough to generate strong motion,which only considers parts of the fault that are deeper than a speciﬁed depth (gen-erally 3–5km)when determining the nearest distance to the site.Taking advantage of a large number of acceleration records at the near ﬁeld of the Chi-Chi earthquake and the well-reconstructed fault geometry,we compare the observed peak ground acceleration (PGA )with respect to these differ-Quantiﬁcation of Hanging-Wall Effects on Ground Motion:Some Insights from the1999Chi-Chi Earthquake2189Table1List of Accelerometric Stations Used in This StudyHanging-Wall SitesPeak Ground Acceleration(cm/sec2)Station Code V NS EW Lat.(Њ)Long.(Њ) TCU05219443934924.198120.74 TCU06851936250224.278120.7657 TCU07141663951823.986120.7882 TCU07817130244023.8123120.8455 TCU08919022534823.9037120.8565 TCU07227537146524.0392120.8577 TCU07938441758023.8397120.8942 TCU08431242398923.883120.8998 TCU07427036858623.9607120.9617 CHY08071684279223.5972120.6777Footwall SitesPeak GroundAcceleration(cm/sec2)Station Code V NS EW Lat.(Њ)Long.(Њ) CHY0029613510823.7197120.413 CHY02414116227623.7577120.6065 CHY02517015215923.78120.5142 CHY02670667623.7988120.4115 CHY027********.7523120.247 CHY02833675062423.6327120.6055 CHY02915823328323.6143120.5288 CHY0763*******.638120.2217 CHY0827*******.7247120.2995 CHY0921128310223.792120.4782 CHY0933*******.6525120.1478 CHY09441536423.7935120.322 CHY10116239033323.6862120.5622 CHY10413017714323.6698120.4657 TCU0296219415524.5588120.749 TCU0316512311324.562120.7008 TCU0366112213424.4488120.6963 TCU0386614314224.4912120.663 TCU03912213619324.4922120.7838 TCU0407912215924.4502120.635 TCU0428220824824.5547120.8077 TCU0469711614024.4687120.854 TCU0489717611724.1807120.5928 TCU04917824227324.1795120.6902 TCU0508712814324.182120.6328 TCU05111023115724.1608120.6518 TCU05312113222524.194120.6693 TCU05413319014324.1615120.675 TCU05515320825724.1397120.6642 TCU05611714015424.1592120.6238 TCU0578110011124.178120.6105 TCU0596416215724.2688120.5637 TCU0608610119724.225120.6438 TCU0618615413324.1363120.5493 TCU06313313017924.1085120.6155 TCU0648211410924.346120.6095 TCU06723131348924.0917120.7197 TCU0707615724924.1955120.5472 TCU07522425732523.9835120.6778 TCU07627542034023.9078120.6757TCU0879111211924.3482120.7733 TCU1008411110824.186120.6152 TCU10116425420824.2417120.709 TCU10217316929824.2492120.7208 TCU10314214912724.3103120.7158 TCU104908710124.2097120.6017 TCU1056112411124.2387120.5595 TCU10611612215724.0855120.549 TCU1079514412824.2638120.5387 TCU10913315914924.0845120.571 TCU111779412524.1137120.487 TCU11265687824.056120.4235 TCU11380727023.8925120.3863 TCU115761159423.9598120.4697 TCU11611913318523.8568120.5802 TCU1179011312124.133120.4603 TCU1181009211624.0028120.4233 TCU11960536323.924120.3118 TCU12016719422323.98120.613 TCU12223625620723.8127120.6095 TCU1238613214924.0177120.5427 TCU1289016314124.4162120.7607 TCU12933561198323.8783120.6843 TCU13611217116724.2602120.6507 TCU13811020720223.9223120.595 TCU14068537123.9575120.3592 TCU141107898723.8335120.464 TCU145526070023.9418120.337 V,vertical;NS,north–south;EW,east–west.ent distance metrics in order to analyze the hanging-wall effects.As aﬁrst step,these distance–metric deﬁnitions have been calculated for each of the accelerographic stations.Fig-ure3shows schematically the different distance–metric def-initions for the case of the Chi-Chi earthquake.In Figure3, it is shown that for all distance deﬁnitions considered,the motions recorded on the hanging wall are larger in general than the median estimates calculated with the whole dataset. This hanging-wall ampliﬁcation effect is observed regard-less of the distance–metric deﬁnitions within a distance of 15km.In addition,a near-ﬁeld saturation is also evident at very short distances(less than5km)with the models,using D jb and D rup.In other words,the footwall acceleration achieves a maximum value at a distance within5km of the fault plane,whereas the hanging-wall sites exhibit high ac-celeration at greater distances.In aﬁrst approximation,the hanging-wall effects cannot be fully accounted for by a proper choice of distance–metric deﬁnition of the ground-motion model(Fig.3).In terms of high ground motion at nearﬁeld,site effect is considered an important factor that may largely amplify acceleration;it is therefore necessary to examine the site condition for the studied area to clarify the physical cause of the hanging-wall effect.Site Condition at Near-Field SitesIn the case of the Chi-Chi earthquake,most of the hanging-wall stations are located on conglomerates that be-TCU08212918322124.1475120.6762190T.-Y.Chang,F.Cotton,Y.-B.Tsai,and J.AngelierTable 2Coefﬁcients for use in Equation (1)to Estimate Pseudo-Acceleration for the Horizontal Mean of the Chi-Chi EarthquakeNear-Field Ground MotionModelsabcr ar br cr ln PHAD rup 2.668מ1.33613.919 2.7900.64712.6000.425D jb 1.313מ1.0197.975 1.5130.377 6.8240.394D seis 0.682מ0.861מ0.2770.5860.162 2.1550.360long to various ﬂuvial terraces.This corresponds to Site Class C in the 1997Uniform Building Code (UBC )classi-ﬁcation scheme by the International Conference of Building Ofﬁcials (1997).The footwall sites are located on Quater-nary sediments,which belong to Site Classes D and E ac-cording to the same classiﬁcation.However,as shown by Y.M.Wu et al.(2001),the PGA site-correction factor de-rived from the observed and predicted values is between 1.25and 4at the Chi-Chi earthquake footwall sites and is between 0.8and 1.25at the hanging-wall sites.According to these results,the near-ﬁeld ground motion should be especially ampliﬁed at the footwall sites,if we take site condition as a main factor for acceleration ampliﬁcation.The nonlinear site effects of soil were not exceptionally reported at the western footwall area of Class E in the 1997UBC classiﬁcation scheme;peaks and corresponding ratios of ampliﬁcation observed at such sedimentary sites were markedly different from those at other sites (Wang et al.,2002).This phenomenon corresponds with observations of some footwall sites where greater ground motion occurred than the elastic–linear prediction called for at intermediate distances.Somewhat remarkable is that two hanging-wall stations (TCU052and TCU068)exhibited low PGA at a lo-cation very close to the northern extremity of surface rup-ture.Nonlinear site effects can occur in some cases for short periods on Class C (ﬁrm)soil;these effects have relatively low amplitudes at large magnitudes and short distances (Campbell and Bozorgnia,2003).However,if we take into account the movement character of this event,high velocity and long rise time at the northern tip of the Chelungpu fault (Zeng and Chen,2001),we tend to address the low PGA at these two stations by the particular thrusting process of the Chelungpu fault rather than by the nonlinear site effects.Recent studies show there is no deﬁnitive site classiﬁ-cation for the TSMIP/CWB stations so far;even so,we can assess the possible effect by a background knowledge of the geology.C.T.Lee et al.(2001)examined the site condition of TSMIP/CWB stations through multiple approaches:study of geological maps,response spectral shape,and horizontal-to-vertical spectral ratio.They found that the classiﬁcation of the TSMIP/CWB sites is consistent with the distribution of major geological features in Taiwan as a ﬁrst approximation.However,they recognized that this consistency failed for a few sites that were either on the edge of speciﬁc structures or above severely weathered layers.After re-examining in situ the geomorphologic setting of the stations,they con-cluded that the site condition for the hanging wall of the Chi-Chi earthquake corresponds to Class D of the 1997UBC classiﬁcation scheme,and corresponds to Classes C,D,and E for the footwall (C.T.Lee et al.,2001).This limited con-trast in site conditions between the hanging wall and the footwall shows that site effects are probably not the main factors that could explain the observed differences between hanging-wall and footwall ground motions.Accordingly,neither a distance-dependent attenuation deﬁnition nor an ampliﬁcation generated from site condi-tions can give a good explanation of the cause of the large ground motion at the hanging wall in the Chi-Chi earth-quake.In the following sections,we extend a series of nu-merical analyses to quantify the hanging-wall ampliﬁcation factor in this earthquake,which can provide reference for the related study.Regression ModelCompared to the hanging-wall ampliﬁcation of ground acceleration,the footwall sites,which lie on Quaternary sed-iments,exhibit a distance-dependence feature at the western side of the Chelungpu fault:the PGA amplitude gradually diminishes with increasing distance from the rupture trace of this fault (Fig.1).This pattern provides a clear image of the seismic attenuation in the Chi-Chi earthquake area.We evaluate ground-motion attenuation without reference to site conditions.The ground-motion-estimation equation is given by:ln PHA סa םb ln(d םc ),(1)where PHA is the geometrical mean of the amplitudes of two horizontal components in g ;d is geometrical spreading dis-tance in kilometers.The coefﬁcients to be determined are a ,b ,and c .Here we determine these coefﬁcient values by min-imizing the sum of square errors with respect to the coefﬁ-cient c ,and a simple numerical search (Press et al.,1992).The corresponding coefﬁcients,using the three different distance–metric models previously mentioned,are listed in Table 2.The resulting regression curves are shown in Figure 3for the three different distance models.It indicates that the hanging-wall effect that causes ampliﬁcation by an averaged factor of 3appears in all models (D jb ,D rup ,and D seis ).Hanging-Wall and Footwall EffectsWe now choose to examine the empirical ground-motion model with the distance deﬁnition D rup in our anal-ysis of the hanging-wall effects.D rup was also the distance–metric adopted in the ground-motion study of Abrahamson and Silva (1997),which allows us to compare their model with the acceleration records of the Chi-Chi earthquake.The residuals from the attenuation equation are pre-sented in Figure 4for the hanging-wall and footwall sites,respectively.Similar to the illustration in Abrahamson andQuantiﬁcation of Hanging-Wall Effects on Ground Motion:Some Insights from the 1999Chi-Chi Earthquake2191Figure 4.Comparisons of hanging-wall and foot-wall effects on PHA residuals from the Chi-Chi earth-quake.Residuals plotted as functions of distance for all the sites presented in Figure 1.Hanging-wall sta-tion codes are indicated.Somerville’s article (1996),the footwall sites are presented in negative distances to distinguish them from the hanging-wall sites.Figure 4shows the maximum residual,ln(PHA obs /PHA pred ),derived both from the hanging-wall and footwall sites.The maximum residual on the hanging wall attained 1.0.It is worth mentioning that the PHA residuals derived from the hanging-wall sites cannot be described as functions of distance.This implies a complexity of seismic-wave vi-bration between the fault-plane/density-contrast layer and the half-space surface on the hanging wall.In addition,two hanging-wall stations located at the northern tip of the wall show a relatively low acceleration residual in comparison to the other hanging-wall sites (TCU052and TCU068in Fig.4).These two stations were reported to exhibit a long rise time in the rupture process,which resulted in a low dynamic stress drop and moderate PGAs (Zeng and Chen,2001).On the footwall,except for some sites very close to the rupture trace (D rup Յ5km),the residuals are close to zero for distances from 5to 50km.The very small residuals for the footwall sites shown in Figure 4reveal that ground mo-tion on the footwall can be perfectly characterized by a sim-ple distance-dependent empirical model.The average of the PHA residuals for the hanging-wall accelerations is equal to 0.64ע0.4for the sites used in Figure 4.This value is similar to the determination derived from other dipping faults (0.67ע0.14,according to Abra-hamson and Somerville,1996).On the footwall the corre-sponding average residual within the stations over the same distance range (5–20km)is מ0.08ע0.1(Fig.4).Ampliﬁcation Factors for Different Spectral PeriodsThe same analysis has been applied to several response-spectra periods for the horizontal and vertical components.The response spectra at 5%damping are computed by using the Nigam and Jennings (1969)algorithm for all the stations that were used.Plots of the residuals for the spectral accel-eration,ln(SA obs /SA pred ),at periods of 0.02to 4.0sec are shown in Figure 5for the horizontal and vertical compo-nents.For each motion-period bin selected in this article,the predicted attenuation model was constructed by the corre-sponding spectral acceleration derived from both hanging-wall and footwall sites,similar to the one that is shown in Figure 4.It is observed that both the horizontal and vertical components are apparently ampliﬁed from 5to 20km at the hanging-wall sites for periods 0.02to 0.5sec.However,the ampliﬁcation factor in the vertical component is smaller than the one of the horizontal component for all periods.For pe-riods longer than 0.8sec,the hanging-wall effect apparently tends to be zero for the vertical component.We further take into consideration the spatial variation of the spectral attenuation on the hanging wall.In Figure 5the numbers,in increasing sequence,indicate the station dis-tribution from north to south (numbers from 1to 10).This labeling helps in classifying the hanging-wall stations ac-cording to their location around a north–south direction.Thegoal of this labeling is to detect directivity effects that might affect the stations north of the rupture.However,the data analysis (Fig.5)does not reveal any clear dependency on this north–south classiﬁcation.Only stations 1and 2(TCU052and TCU068),located close to the rupture,re-vealed smaller residuals both in the horizontal and vertical components,almost in all periods.In the vertical component these northern stations exhibit a low attenuation rate for the long-period motions (say,between 0.8and 4.0sec),which may be inferred from the long rise time and a strong thrust-ing movement at the northern tip of the Chelungpu fault (Zeng and Chen,2001).We summarize the averaged ampliﬁcation factors for different periods in Figure 6.This ﬁgure shows that the hanging-wall effect affects both the horizontal and the ver-tical components,especially for the short-period motions,whereas the horizontal acceleration is more ampliﬁed than the vertical at all periods.The largest ampliﬁcation is ob-served at periods shorter than 1.0sec.For spectral periods larger than 1.0sec,the vertical component of ground motion at the hanging-wall sites is not signiﬁcantly ampliﬁed,whereas the horizontal component shows an average ampli-ﬁcation of about 2.11to 4.0sec.Comparison with Empirical Prediction forHanging-Wall EffectAbrahamson and Silva (1997)proposed an empirical seismic-attenuation model,which takes into account the es-timation of the hanging-wall effect derived by Abrahamson and Somerville (1996).We compared the Chi-Chi earth-quake hanging-wall records with their ground-motion mod-2192T.-Y.Chang,F.Cotton,Y.-B.Tsai,and J.AngelierFigure5.Residuals of the spectral accelerations for the sites on the hanging wall(solid circles)and the footwall(open circles).Various periods for(a)the horizontal component and(continued)Quantiﬁcation of Hanging-Wall Effects on Ground Motion:Some Insights from the1999Chi-Chi Earthquake2193Figure5.(continued)(b)the vertical component are analyzed.Number sequence accords with thestation distribution from north to south,as follows:1,TCU068;2,TCU052;3,TCU072;4,TCU071;5,TCU074;6,TCU089;7,TCU084;8,TCU079;9,TCU078;10,CHY080.Figure6.Summary of the spectral-accelerationresidual as function of oscillator period.els with and without the hanging-wall effect.Figure7a il-lustrates the predicted curves with and without considering the hanging-wall effect derived from four selected stations with distinct distance bins of0.4,5,10.6,and16.2km.The ﬁgure shows that the hanging-wall effect becomes less im-portant for sites very close to the rupture plane(Ͻ5km). Figure7b shows two sets of comparisons for all the hanging-wall records,with a trend line connecting the mean value obtained for each period.According to Figure7b,the Abrahamson and Silva (1997)ground-motion model(with or without the hanging-wall factor)tends to underestimate the Chi-Chi earthquake’s long-period ground motions.The long-period hanging-wall ampliﬁcation remains signiﬁcant for periods longer than1.0 sec in the case of this earthquake,whereas the corresponding coefﬁcient(a9of equation9in Abrahamson and Silva’s1997 model)of the hanging-wall effect decreases to zero over the period range of0.75to5.0sec.The logarithmic ratio of observed/predicted spectral ac-celeration is about zero for spectral periods shorter than1.0 sec in both predictions with and without consideration of hanging-wall effect.Figure7b also indicates a low ground-motion amplitude for the0.2-sec period of the earthquake’s hanging wall compared to the motion predicted by Abra-hamson and Silva(1997).This discrepancy between the ob-servations and the Abrahamson and Silva(1997)model may be considered as natural earthquake-to-earthquake variation. This discrepancy could also indicate that there is a need to increase the number of hanging-wall records in strong-motion databases used to derive empirical ground-motion models(especially for large earthquakes with surface rup-tures).Characteristics of Near-Field Ground MotionThe foregoing results show that the hanging-wall effect mainly concerns periods of4.0sec or less.In order to im-prove our comprehension of near-ﬁeld ground motion,weexamine the response spectra in six distance bins.To exploredistribution trends,we only discuss the geometrical averagesof spectrum amplitude,which correspond to different spec-tral periods in each distance bin.Average spectra are plottedagainst period in Figure8for oscillator periods in the range0.02to5.0sec.In theﬁrst approximation,the mean spectrum on thehanging wall is similar for the sites within distances of about5–10km and10–20km,whereas the band of periods withhigh spectral amplitude is shifted toward longer periods forthe larger-distance bin(Fig.8).It is observed that the short-distance group of hanging-wall sites has a strong spectralacceleration at a short period,whereas the high-frequencyoscillator motion is attenuated for sites at larger distances.It is remarkable that the spectral amplitudes of the fault-zonesites,including the near-ﬁeld sites(D rupՅ5km)and the hanging-wall stations(TCU068and TCU052),are not aslarge as those of other hanging-wall sites,although thesenear-ﬁeld sites have comparable PHA.On the other hand,thefootwall sites with a distance larger than5km show a dra-matic diminution of ground-motion energy for all periodsconsidered.The peaks observed on the hanging wall do notappear on footwall spectra at distances larger than5km.Thisagain conﬁrms the asymmetry of seismic energy across thefault geometry(Oglesby et al.,1998,2000;Oglesby andDay,2001).In addition,the rapid decrease of spectral accelerationfrom the hanging wall to the footwall indicates that seismicenergy was concentrated at the hanging-wall block when theChi-Chi earthquake occurred.Discussion and ConclusionsHanging-wall/footwall effects are difﬁcult to quantifyaccurately because of scarcity of ground-motion data at nearﬁeld and incomplete knowledge of fault geometry of blindthrust earthquakes.Three physical mechanisms,distance ef-fect,wave trapping,and directivity effects,are thought toresult in hanging-wall effect(e.g.,Somerville et al.,1997;Bard and Riepl-Thomas,2000).In this study,we have takenadvantage of a large collection of accelerometric records toquantify the hanging-wall ampliﬁcation in the Chi-Chi earth-quake.The following conclusions and recommendations aremade.1.It isﬁrst shown that the hanging-wall effect cannot besimply accounted for by choosing an appropriate deﬁni-tion of distance for the attenuation model.This resultshows that the hanging-wall effect cannot exclusively re-sult from a geometrical factor(otherwise an appropriatechoice of the distance–metric deﬁnition would sufﬁce).Although our analyses ignored the inﬂuence of local siteeffect,our experiences gained from studying the Chi-Chiearthquake show that the site effect may generate prin-cipally the increasing excitation of dispersive long-period。