Ground Shaking

Introduction

According to the Natural History Museum, London, the ground upon which we build is anything but solid. The Earth Gallery illustrates how rocks flow, melt, shatter, are squeezed and folded. But more than that, the continents that support the earth's civilizations are in constant motion. Hundreds of millions of years ago the continents were joined, but now they are dispersing ever so slowly. Once, the east coast of South America nestled neatly against the west coast of Africa. Now, separated by the Atlantic Ocean, they lie 9600 km apart. The idea that buildings are founded upon stationary ground is an illusion. From the perspective of geological time, the earth's crust is in a state of dynamic flux.

The scientific understanding of this dynamic process known as continental drift or tectonic plate movement - the basic cause of most earthquakes - dates back only 100 years. Prior mythology and speculation that sought to explain earthquake occurrence and its prevention is deeply embedded in many cultures. For example, some peoples attributed earthquakes to subterranean beings holding up the world. Whether in the form of fish, animals or people, when they changed position to relieve their unrelenting burden, the earth shook. Many cultures possessed or still possess their own god or gods of earthquakes. Peoples like the Central Asian Turks valued jade as a talisman credited with the power to protect them from, among other dangers, earthquakes. Aristotle's influential belief was closer to the mark. It dismissed the activities of gods or other creatures in favour of natural phenomena. Namely, 'that mild earthquakes were caused by wind escaping from caves within the bowels of the earth and severe shocks by gales that found their way into great subterranean caverns.' 1

It is not surprising that people sought to explain the occurrence of earthquakes, which happened without warning and so quickly devastated their communities. Although it appears that some animals, fish and insects sense and react to earthquakes before they are felt by humans, earthquakes strike suddenly. Often a rumbling is heard several seconds before shaking begins, and within a few seconds the initial tremors have grown into violent shaking. At other times a quake strikes like an instantaneous pulse. A reporter covering the October 2005 Pakistan earthquake recounts the experience of a Balakot boy searching through the rubble of his school where 400 of 500 of his fellow students had been buried alive. The boy recounted that the collapse occurred so suddenly, prompting the reporter to explain: ' How quick is hard to comprehend. At another school a teacher told a colleague of mine from the Daily Telegraph how he had just arrived at the door of his classroom. The children stood up. As they began their morning greeting of ' Good morning, Sir' the earthquake hit. The teacher stepped back in surprise, the roof collapsed. They all died, all 50 of them, just like that. No wobbling walls and dashes for the door. No warning. One second you have a classroom full of children in front of you, and the next, they are dead'.2

If the potential source of an earthquake attack is both known with reasonable confidence and is also some distance from a major city, an early warning system can be implemented. For instance, earthquakes most likely to damage Mexico City originate along the Guerrero coast some 280 km to the west. The 72 seconds that the earthquake waves take to travel to the city afford sufficient time for people to flee low-rise constructions or move to a safer location within their building. Commercial radio stations, the internet and audio alerting systems such as local sirens alert people to impending danger.3 Several other cities, including Tokyo, have also installed early warning systems, but these allow far less time for preventative actions.4 Unfortunately, for the vast majority of us living in seismic zones, any warning remains a dream.

Upon sensing initial ground or building movement, sufficient time usually elapses for the occupants to experience uncertainty and then fear. After realizing that the movement is not caused by a passing heavy vehicle but by an earthquake, one questions whether the vibrations are a precursor to more severe ground motion. While low-intensity earthquake shaking may be experienced as a gentle shock or small vibrations, during intense shaking people cannot walk steadily. They may be thrown over, or if sleeping, hurled out of bed. The perception of earthquake shaking is also usually heightened by what is happening in the immediate vicinity of the person experiencing a quake. Objects sliding, toppling or falling - be they building contents or elements of buildings such as suspended ceiling tiles, or dust from cracking plaster and concrete - all increase the psychological and physical trauma of a quake.

Apart from the poorest of communities for whom even partial earthquake protection is unaffordable, most of the disastrous effects of earthquakes are avoidable. Earthquake-resistant construction greatly reduces the loss of life from a damaging quake, as well as lessening economic losses and disruption to societal activities. Architects and structural engineers achieve earthquake-resistant buildings by following the principles and techniques outlined in this book. These are incorporated into new buildings with minor additional cost. The exact per centage increase in construction cost depends on many factors including the type and weight of building materials, the seismicity of the region and local code requirements. However, it is certainly far less expensive than improving the seismic performance of existing buildings.

Individuals, businesses and communities respond differently to the potential hazards posed by quakes. Although most earthquake-prone countries possess codes of practice that stipulate minimum standards of design and construction, particularly in developing countries, the majority of people are at considerable risk. Due to their economic situation or lack of appreciation of their seismic vulnerability, their homes and workplaces possess little if any seismic resistance. Every community in a seismically active zone should have numerous strategies to cope with a damaging quake. Some communities, due to their preoccupation with day-to-day survival, take a fatalistic approach that excludes any preventative or preparatory actions. Others implement civil defence and disaster management planning. Although not reducing the risk of injury or loss of life nor damage to buildings and infrastructure significantly, these initiatives reduce the trauma following a quake and assist post-earthquake restoration.

Quakes strike at the heart of a community. When they damage and destroy buildings, people and animals are injured and killed. Quakes destroy the basic necessities of life, demolishing shelter, ruining food and water supplies and disrupting people's livelihoods. Conversely, buildings that perform well during an earthquake limit its impact on people and their basic needs. The aim of this book is to reduce earthquake-induced devastation by providing architects and engineers with the knowledge to design both new and rehabilitated buildings that possess adequate seismic resistance.

Understanding earthquakes

This section explains why architects might need to design earthquake-resistant buildings. It introduces the basic geological mechanisms causing earthquakes, explaining where and when earthquakes occur and the characteristics of ground shaking relevant to buildings. The focus here is upon those aspects of earthquakes over which we as designers have no control. Having outlined in this chapter what might be termed the earthquake problem, the remaining chapters deal with the solutions. For more detailed yet not too highly technical information on the basics of earthquake occurrence, the reader can refer to one of several general introductory texts.5

Why earthquakes occur

Compared to the 6400 km radius of the earth, the thickness of the earth's crust is perilously thin. The depth of the continental crust averages 35 km, and that of the oceanic crust only 7 km. While an analogy of the earth's crust as the cracked shell of a hen's egg exaggerates the thickness and solidity of the crust, it does convey the reality of a very thin and relatively brittle outer layer underlain by fluid - molten rock. Convection currents within the earth's viscous mantle, powered by vast amounts of thermal energy radiating from the earth's core, generate forces sufficiently large to move the continents. The earth's tectonic plates are like fragments of a cracked egg shell floating on fluid egg white and yolk. They move relative to each other approximately 50 mm per year; apparently about as fast as our fingernails grow (Fig. 1.1).

In some places, tectonic plates slip past each other horizontally. In others, such as where an oceanic plate pushes against a continental plate, the thinner oceanic plate bends and slides under the continental plate while raising it in a process known as subduction (Fig. 1.2). Due to the roughness of the surfaces and edges of tectonic plates, combined with the huge pressures involved, potential sliding and slipping movements generate friction forces large enough to lock-up surfaces in contact. Rather than sliding past each other, rock in a plate boundary area (say along a fault line) absorbs greater and greater compression and shear strains until it suddenly ruptures (Fig. 1.3). During rupture, all of the accumulated energy within the strained rock mass releases in a sudden violent movement - an earthquake.

The mechanical processes preceding an earthquake can be likened to the way we snap our fingers. We press finger against thumb to generate friction (Fig. 1.4(a)), then also using our finger muscles we apply

▲ 1.1 Tectonic plates and their annual movement (mm). The dots indicate positions of past earthquakes

(Reproduced with permission from IRIS Consortium).

▲ 1.1 Tectonic plates and their annual movement (mm). The dots indicate positions of past earthquakes

(Reproduced with permission from IRIS Consortium).

Subducting oceanic plate

Earthquake foci

▲ 1.2 Subduction of an oceanic plate under a continental plate.

Subducting oceanic plate

Earthquake foci

Original Strain position of builds up blocks of land deforming separated by the rock a fault

After rupture the land rebounds

Fault movement

▲ 1.2 Subduction of an oceanic plate under a continental plate.

▲ 1.3 Increase of strain adjacent to a fault plane and the subsequent energy release and fault displacement.

a sideways force at the interface between the surfaces (Fig. 1.4(b)). If the initial pressure is low, they slide past each other without snapping. Increasing the pressure and the sideways force distorts the flesh. When the sliding force exceeds the friction between thumb and finger, the finger suddenly snaps past the thumb and strikes the wrist as the pent-up strain converts to kinetic energy (Fig. 1.4(c)).

▲ 1.5 A surface fault with considerable vertical displacement. The 1999 Chi Chi, Taiwan earthquake.

(Reproduced with permission from Chris Graham).

▲ 1.5 A surface fault with considerable vertical displacement. The 1999 Chi Chi, Taiwan earthquake.

(Reproduced with permission from Chris Graham).

Epicentre

Epicentre

Epicentral distance

Fault plane

▲ 1.6 Illustration of basic earthquake terminology.

Seismic waves

Fault plane

▲ 1.6 Illustration of basic earthquake terminology.

▲ 1.4 Experience the build-up of tectonic strain and energy release by snapping your fingers. Apply pressure normal to your finger and thumb (a), next apply sideways force (b), and then feel the sudden snapping when that force exceeds the friction between thumb and finger (c).

The surface along which the crust of the earth fractures is an earthquake fault. In many earthquakes the fault is visible on the ground surface. Some combination of horizontal and vertical displacement is measurable, often in metres (Fig. 1.5). Chapter 15 discusses the wisdom of building over or close to active surface faults. The length of a fault is related to the earthquake magnitude (defined in a later section). For example, the fault length from a magnitude 6 quake is between 10-15 km, and 100-200 km long for a magnitude 8 event. The vertical dimension of a fault surface that contributes to the total area ruptured is also in the order of kilometers deep. The point on the fault surface area considered the centre of energy release is termed the focus, and its projection up to the earth's surface, a distance known as the focal depth, defines the epicentre (Fig. 1.6).

The length of the focal depth indicates the damage potential of an earthquake. Focal depths of damaging quakes can be several hundred kilometers deep. While perhaps not producing severe ground shaking, these deep-seated earthquakes affect a wide area. In contrast, shallower earthquakes concentrate their energy in epicentral regions. They are generally more devastating than deeper quakes where occurring near built-up areas. The focal depth of the devastating 2003 Bam, Iran earthquake that killed over 40,000 people out of a population of approximately 100,000, was only 7 km, while that of the similar magnitude 1994 Northridge, California quake was 18 km.The relatively low loss of life (57 fatalities) during the Northridge earthquake was attributable to both a greater focal depth, and more significantly, far less vulnerable building construction.

Epicentral distance

Seismic waves

Where and when earthquakes strike

Relative movement between tectonic plates accounts for most continental or land-affecting earthquakes. Seventy per cent of these quakes occur around the perimeter of the Pacific plate, and 20 per cent along the southern edge of the Eurasian plate that passes through the Mediterranean to the Himalayas. The remaining 10 per cent, inexplicable in terms of simple tectonic plate theory, are dotted over the globe (Fig. 1.7). Some of these intraplate quakes, located well away from plate boundaries are very destructive.

A reasonably consistent pattern of annual world-wide occurrence of earthquakes has emerged over the years. Seismologists record many small but few large magnitude quakes. Each year about 200 magnitude 6, 20 magnitude 7 and one magnitude 8 earthquakes are expected. Their location, apart from the fact that the majority will occur around the Pacific plate, and their timing is unpredictable.

Although earthquake prediction continues to exercise many minds around the world, scientists have yet to develop methods to predict

▲ 1.7 Geographic distribution of earthquakes. Each dot on the map marks the location of a magnitude 4 or greater earthquake recorded over a period of five years. (Reproduced with permission from IRIS Consortium).

precisely the location, time and magnitude of the next quake in a given geographic region. However, based upon a wide range of data including historical seismicity, measurements of ground uplift and other movement, and possible earthquake precursors such as foreshocks, scientists' predictions are more specific and refined than those of global annual seismic-ity discussed previously. The accuracy of such predictions will improve as seismological understanding continues to develop. Here are several examples of state-of-the-art predictions from peer reviewed research:

• 'There is a 62 per cent probability that at least one earthquake of magnitude 6.7 or greater will occur on a known or unknown San Francisco Bay region fault before 2032' ,6

• The probability of the central section of the New Zealand Alpine Fault rupturing in the next 20 years lies between 10 and 21 per cent,7 and

• The probability of Istanbul being damaged by an earthquake greater or equal to magnitude 7 during the next thirty years is 41 ± 14 per cent.8

Several other valid generic predictions regarding quakes can be made; a large quake will be followed by aftershocks, a quake above a given magnitude event is implausible within a given geographic region, and certain size quakes have certain recurrence intervals.

In the hours and even months following a moderate to large earthquake, aftershocks or small earthquakes continue to shake the affected region. Although their intensities diminish with time, they cause additional damage to buildings weakened by the main shock, like the magnitude 5.5 aftershock that occurred a week after the 1994 Northridge earthquake. Post-earthquake reconnaissance and rescue activities in and around damaged buildings must acknowledge and mitigate the risks aftershocks pose.

Some predictions, such as a region's maximum credible earthquake, are incorporated into documents like seismic design codes. Based mainly upon geological evidence, scientists are confident enough to predict the maximum sized quake capable of occurring in a given region. For example, the largest earthquake capable of being generated by California's tectonic setting is considered to be magnitude 8.5. Its return period, or the average time period between recurrences of such huge earthquakes is assessed as greater than 2500 years.

Structural engineers regularly use predicted values of ground accelerations of earthquakes with certain return periods for design purposes. The trend is increasing for seismic design codes to describe the designlevel earthquake for buildings in terms of an earthquake with a certain average return period. This earthquake, for which even partial building collapse is unacceptable, is typically defined as having a 10 per cent probability of being exceeded within the life of a building, say 50 years. The return period of this design earthquake is therefore approximately 500 years.

The probability p of an earthquake with a given return period T occurring within the life of a building L can be calculated using Poisson's equation, p = I — e—L/T. For example, if L = 50 years, and T = 500 years, the probability of this event being exceeded during the lifetime of the building is approximately 0.I or I0 per cent.

Special buildings that require enhanced seismic performance, like hospitals and fire stations, are designed for larger quakes. In such cases design earthquake return periods are increased, say to 1000 or more years. Designers of these important buildings therefore adopt higher design acceleration values; the longer the return period, the larger the earthquake and the greater its ground accelerations. Figure I.8 shows a portion of a typical seismic map.9 Most countries publish similar maps.

(Adapted from a 1996 US Geological Survey map).

Earthquake magnitude and intensity

Seismologists determine the position of a quake's epicentre and its magnitude, which relates to the amount of energy released, from seismograph records. The magnitude of a quake as determined by the Richter Scale relates logarithmically to the amount of energy released. An increase of one step in magnitude corresponds to an approximate 30-fold increase in energy, and two steps, nine hundred times more energy. The 1976 Tangshan earthquake in China, the twentieth century's most lethal earthquake that caused approximately 650,000 fatalities, was magnitude 7.7.I0 The largest ever recorded quake was the magnitude 9.5 in the I960 Great Chilean earthquake which, even with its devastating tsunami, had a significantly lower death toll. So the value of magnitude itself does not indicate the impact of a quake. Large earthquakes in regions distant from built-up areas may pass almost unnoticed. Another form of measurement describes the degree of seismic damage a locality suffers or is likely to suffer.

While each earthquake is assigned a single magnitude value, the intensity of earthquake shaking varies according to where it is felt. A number of factors that include the earthquake magnitude, the distance of the site from the epicentre, or epicentral distance (see Fig. I.6) and the local soil

▲ 1.8 A map of an area of the U.S.A. showing horizontal acceleration contours expressed as a percentage of the acceleration due to gravity. The values, applicable to low-rise buildings founded on rock, have a 10% probability of exceedence in 50 years.
▼ 1.1 Partial summary of the Modified Mercalli Intensity (MMI) Scale

Intensity

Description

I to III

Not felt, except under special circumstances.

IV

Generally felt, but not causing damage.

V

Felt by nearly everyone. Some crockery broken or items overturned. Some

cracked plaster.

VI

Felt by all. Some heavy furniture moved. Some fallen plaster or damaged

chimneys.

VII

Negligible damage in well designed and constructed buildings through

to considerable damage in construction of poor quality. Some chimneys

broken.

VIII

Depending on the quality of design and construction, damage ranges

from slight through to partial collapse. Chimneys, monuments and

walls fall.

IX

well designed structures damaged and permanently racked. Partial

collapses and buildings shifted off their foundations.

X

Some well-built wooden structures destroyed along with most masonry and

frame structures.

XI

Few, if any masonry structures remain standing.

XII

Most construction severely damaged or destroyed.

conditions influence the intensity of shaking at a particular site. An earthquake generally causes the most severe ground shaking at the epicentre. As the epicentral distance increases the energy of seismic waves arriving at that distant site as indicated by the intensity of shaking, diminishes. Soft soils that increase the duration of shaking as compared to rock also increase the intensity. One earthquake produces many values of intensity.

Another difference between the magnitude of an earthquake and its intensities is that, whereas the magnitude is calculated from seismograph recordings, intensity is somewhat subjective. Intensity values reflect how people experienced the shaking as well as the degree of damage caused. Although several different intensity scales have been customized to the conditions of particular countries they are similar to the internationally recognized Modified Mercalli Intensity Scale, summarized in Table 1.1. Based on interviews with earthquake survivors and observations of damage, contours of intensity or an isoseismal map of an affected region, can be drawn (Fig. I.9).11 This information is useful for future earthquake studies. It illustrates the extent, if any, of an earthquake's directivity, how the degree of damage

Ground Acceleration Map Loma Prieta
▲ 1.9 A map showing the distribution of Modified Mercalli Intensity for the 1989 Loma Prieta, California earthquake. Roman numerals represent the intensity level between isoseismal lines, while numbers indicate observed intensity values. (Adapted from Shephard et a., 1990).

varies over a region with increasing epicentral distance, and how areas of soft soil cause increased damage.

The nature of earthquake shaking

At the instant of fault rupture, seismic waves radiate in all directions from the focus. Like the waves emanating from a stone dropped into a pond, seismic waves disperse through the surrounding rock, although at far greater velocities. But unlike the ever increasing circles of pond waves, the spread of seismic waves can take more elliptical forms. In these situations where the earthquake energy partially focuses along one certain direction, the earthquake exhibits directivity. The extent of directivity, which causes more intense damage over the narrower band in the line of fire as it were, is unpredictable. Directivity depends on several geological factors including the speed at which the fault rupture propagates along its length.

P-wave

P-wave

Compressed rock --"'

Expanded rock

S-wave

▲ 1.10 Dynamic ground movements caused by the propagation of P- and horizontal S-waves.

S-wave

▲ 1.10 Dynamic ground movements caused by the propagation of P- and horizontal S-waves.

Of the three types of waves generated by fault rupture, two travel underground through rock and soil while the third is confined to the ground surface. P-waves, or Primary waves, travel the fastest. They move through rock in the same way as sound waves move through air, or as a shock wave travels along a metal rod when it is struck at one end. They push and pull the soil through which they pass. S-waves or Shear waves, of most concern to buildings, move soil particles side to side, vertically and horizontally (Fig. 1.10). They propagate from the focus at a speed of about 3 km/sec. Surface waves are the third type of waves. Named after the scientists who discovered them, Love waves vibrate only in the horizontal plane on the earth's surface while Rayleigh waves also have a significant vertical component. Their up-and-down motion is similar to ocean waves. The author vividly recalls the peaks and troughs of Rayleigh waves travelling along the road when once, as a boy, he was riding to school.

Horizontal S-waves, Love and Rayleigh waves, all of which move the ground to-and-fro sideways, cause the most damage to buildings. Buildings are far more susceptible to horizontal rather than vertical accelerations. The snake-like action of these waves induces into the foundations of buildings horizontal accelerations that the superstructures then amplify. The waves also transmit horizontal torsion rotations into building foundations. The primary focus of seismic resistant design is to withstand the potentially destructive effects of these waves.

▲ 1.11 A building tipped onto its side and cantilevered from its base experiences 1.0 g acceleration acting vertically.

P-waves S-waves

P-waves S-waves

▲ 1.12 North-south components of acceleration, velocity and displacement histories from Sylmar, California, during the 1994 Northridge earthquake. (Adapted from Norton etal., 1994).

Time (seconds)

▲ 1.12 North-south components of acceleration, velocity and displacement histories from Sylmar, California, during the 1994 Northridge earthquake. (Adapted from Norton etal., 1994).

Characteristics of ground shaking

From the perspective of designing seismic resistant buildings, the three most important characteristics of ground shaking are the value of peak ground acceleration, the duration of strong shaking and the frequency content of the shaking. Recorded peak ground accelerations of damaging earthquakes range from 0.2 g to over I.0 g where g is the acceleration due to gravity. A I.0 g horizontal acceleration at the base of a rigid building induces the same force as if the building were tipped onto its side to cantilever horizontally from its base (Fig. I.II). Very few buildings can survive such a large force. The higher the level of ground acceleration, the greater the horizontal earthquake forces induced within the building. As explained in Chapter 2, the horizontal flexibility of the superstructure of a building amplifies the ground shaking commonly by a factor of up to two to three times.

Earthquake acceleration records are obtained from seismographs which record the rapidly changing accelerations or velocities throughout the duration of a quake. Mathematical manipulation of these records produces corresponding graphs of velocity and displacement against time (Fig. I.I2).I2 Ground motions are easiest to visualize from the graph of displacement against time. Figure I .I 2 shows a movement of 0.2 m in one direction and just over 0.3 m in the other in a period of approximately I.5 seconds. An appreciation of the maximum inertia forces generated within buildings during this quake is gained from noting the far higher frequency accelerations from which the peak ground acceleration can be determined. The accelerations last for such small periods of time their displacements are smoothed out in the displacement-against-time graph.

The duration of strong shaking also affects the degree of earthquake damage a building sustains. Just as a losing boxer, reeling from blow after blow to the body desperately awaits the end of the round, so a building is concerned about the duration of a quake. The longer shaking feeds dynamic energy into a building, causing more and more energy to be absorbed by the structure, while the extent of damage and its severity grows. In conventional reinforced concrete construction, once beams and columns crack, further load cycles cause the concrete on either side

▲ 1.13 A scratch plate accelerometer record of a small earthquake. It shows directionally-random horizontal accelerations. The numbered rings indicate acceleration values expressed as a decimal of the acceleration due to gravity. (Reproduced with permission from GNS Science).
Recordings

Position of recording

r

Soft sediment

Bedrock

Stiff sediment

▲ 1.14 A cross-section through a geological setting near Wellington, showing acceleration records at five sites during a small earthquake. Note how the accelerations increase and frequencies reduce above deeper and soft sediments.

(Reproduced with permission from J. Taber).

▲ 1.14 A cross-section through a geological setting near Wellington, showing acceleration records at five sites during a small earthquake. Note how the accelerations increase and frequencies reduce above deeper and soft sediments.

(Reproduced with permission from J. Taber).

of cracks to be ground away, both weakening the structure and making it more flexible.

The duration of strong shaking correlates with earthquake magnitude and soil type.13 Duration increases with magnitude. For a magnitude 6 earthquake, expect approximately 12 seconds of strong shaking, but the duration of a magnitude 8 quake increases to over 30 seconds. If a site is underlain by soil rather than rock, the duration of strong shaking doubles.

The frequency content of earthquake shaking at a given site is also significantly affected by the ground conditions. On a rock site, most of the earthquake energy is contained within frequencies of between 1 and 6 cycles per second. In contrast, soil sites reduce the frequency of high energy vibrations. As discussed in Chapter 2, the degree to which a building superstructure amplifies ground motions - and consequently requires enhanced seismic resistance - depends on how close the frequencies of energy-filled vibrations match the natural frequency of the building.

Another important characteristic of ground shaking is its random directivity. Even though the predominant shaking of a quake may be stronger in one particular direction, for design purposes ground shaking should always be considered totally random in three dimensions. Figure 1.13 shows an example typical of the chaotic and irregular movements caused by earthquakes. Random directional shaking has major consequences for earthquake resistant buildings. As discussed in the Chapter 2, buildings must be designed for earthquake forces acting in any direction.

Importance of ground conditions

The influence of soil in reducing the frequency of ground shaking measured in cycles per second while increasing its duration and severity has been mentioned. Local soil conditions, particularly deep layers of soft soil as may be found in river valleys or near estuaries, significantly amplify shaking. They also modify the frequency content of seismic waves by filtering out higher frequency excitations (Fig. 1.14). Although this effect is observed in many quakes it was particularly evident in a local area of Mexico City during the 1985 Mexico earthquake. A small area of the city built over a former lake bed is underlain by deep soft clay. During the earthquake this soft soil deposit behaved

Position of recording like a bowl of soft jelly shaken by hand. The soil amplified the vibrations of the seismic waves in the bedrock at the base of the soft soil by factors greater than five times and shook to-and-fro with a natural frequency of 0.5 cycles per second. This shaking, considerably slower than that measured on bedrock nearby, caused modern high-rise buildings with similar natural frequencies to resonate. Some collapsed, and many were badly damaged.

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