2.1 Introduction to Earthquake Concept

The phenomenon of earthquakes not only suggests destruction that strikes without warning but poses problems of construction for the engineer or designer who must build lasting structures in the earthquake zones of the world. What actually causes earthquakes? Can the resulting damage be avoided?

2.1.1 Basic Definitions and Seisoic Activities in the World

The earth is made of layers, divided into the crust, mantle, and core. The crust is the Earth's surface, a thin, hard layer of rock, broken into many pieces. Each of these pieces is known as a crustal plate. Some form continents, others the ocean floor, but they are always moving. According to the theory of plate tectonics, the earth's crust is made of six major plates and nine smaller ones that lie on the mantle--a thicker, denser layer of hot, soft, molten rock. These plates float around on the mantle, in a hot, soft zone.

The core is made up of even hotter rocks below the mantle, and currents of burning rock rise up through the mantle. These currents spread out once they hit the bottom surface of the crust. This behavior tends to tear the crust, pulling it apart, grinding and colliding plates against others. Continental drift (when major plates are slowly but steadily moved apart) also contribute, carrying plates until they collide. It is through these collisions that mountain ranges also are formed. This movement of our dynamic planet produces earthquakes and volcanoes.

Earthquakes occur most frequently (about 95% of the time) at the point where two plates scrape against one another. When these two plates move against each other, the crack is known as a fault. When plates jam against one another, stress builds up in between.When the pressure becomes too great, it bends and snaps free with a jerky motion. This sudden motion is an earthquake.

The parts of a fault are (1) the fault plane, (2) the fault trace, (3) the hanging wall and (4) the footwall. The fault plane is where the action is. It is a flat surface that may be vertical or sloping. The line it makes on the Earth's surface is the fault trace. Where the fault plane is sloping, the upper side is the hanging wall and the lower side is the footwall. When the fault plane is vertical, there is no hanging wall or footwall.

Any fault plane can be completely described with two measurements: its strike and dip. The strike is the direction of the fault trace on the Earth's surface. The dip is the measurement of how steeply the fault plane slopes-if you dropped a marble on the fault plane, it would roll exactly down the direction of dip.

It's important to know a fault's type: normal, reverse or strike-slip. The type reflects the kind of forces that are acting on the fault.

• Normal faults form when the hanging wall drops down. The forces that create normal faults are pulling the sides apart, or extensional.
• Reverse faults form when the hanging wall moves up. The forces creating reverse faults are compressional, pushing the sides together.
• Together, normal and reverse faults are called dip-slip faults, because the movement on them occurs along the dip direction-either down or up, respectively.
• Strike-slip faults have walls that move sideways, not up or down. That is, the slip occurs along the strike, not up or down the dip. In these faults the fault plane is usually vertical, so there is no hanging wall or footwall. The forces creating these faults are lateral or horizontal, carrying the sides past each other. Strike-slip faults are either right-lateral or left-lateral. That means someone standing near the fault trace and looking across it would see the far side move to the right or to the left, respectively.

In reality, many faults show a combination of dip-slip and strike-slip motion. Geologists use more sophisticated measurements to analyze these fault movements.

At its most simple level, a powerful earthquake can be felt by people in the area, and the damage it causes can be seen. The strength of an earthquake is usually measured on the Richter Scale.

The Richter Scale is designed to measure earthquake magnitudes, regardless of the location. For measuring the speed, or acceleration of the ground, a scientific scale measurement instrument, seismograph, is used. The Richter scale for earthquake measurements is logarithmic. This means that each whole number step represents a ten-fold increase in measured amplitude. Thus, a magnitude 7 earthquake is 10 times larger than a 6, 100 times larger than a magnitude 5 and 1000 times as large as a 4 magnitude. This is an open ended scale since it is based on measurements not descriptions. An earthquake detected only by very sensitive people registers as 3.5 on this scale, whilst the worst earthquake ever recorded reached 8.9.

Seismologists use global networks of seismographic stations to accurately map earthquakes around the world. After studying the worldwide earthquake distribution,the pattern of earthquake types, and the movement of Earth's rocky crust, scientists proposed that plate tectonics was the main underlying cause of earthquakes. The theory of plate tectonics arose from several previous geologic theories and discoveries. Scientists now use the plate tectonics theory to describe the movement of the Earth's plates and how this movement causes earthquakes. They also use the knowledge of plate tectonics to explain the locations of earthquakes, mountain formation, and deep ocean trenches, and to predict which areas will be damaged the most by earthquakes. It is clear that major earthquakes occur most frequently in areas with features that are found at plate boundaries: high mountain ranges and deep ocean trenches. Earthquakes within plates, or intraplate tremors, are rare compared with the thousands of earthquakes that occur at plate boundaries each year, but they can be very large and damaging.

Earthquakes that occur in the area surrounding the Pacific Ocean, at the edges of the Pacific plate are responsible for an average of 80%of the energy released in earthquakes worldwide. Japan is shaken by more than 1,000 tremors greater than 3.5 in magnitude each year. The western coasts of North and South America also are very active earthquake zones, with several thousand small to moderate earthquakes each year.

Another series of faults traces through southern Europe and Central Asia. Some of the world's worst earthquakes, and some of the most recent ones, have occurred over faults that crisscross through Turkey and Iran.

2.1.2 Structural Damages Caused by Earthquakes

The extent of earthquake damage depends on what areas are hit. If an unpopulated region is struck, there will be low loss of life or property. If it hits a large city, there may be many injuries and destruction. Many of the areas at risk near the Pacific Ocean are largely populated. Major earthquakes hitting those areas today could produce catastrophic damage.

Most global cities have at least tripled their populations in the last hundred years, so a major earthquake could collapse skyscrapers, factories, and power plants. Millions of people could feel the shock waves, not only of the initial tremor, but through aftershocks, tsunamis, landslides, floods, and social effects.

Earthquakes have the power to uproot trees and send them crashing into buildings. They can trigger landslides and avalanches, and cause flooding and tsunamis. Human structures are also at risk. It is interesting to note that tall buildings will sustain the least damage if they are located directly at the epicenter. This is because they can withstand the up-and-down motion of P-waves and S-waves. Far away from the epicenter the motions shake buildings from side to side. These buildings are often knocked off their foundations.

Buildings with thick, heavy walls do not resist shock waves well. Violent earthquakes often cause structures to collapse, burying people underneath. Brick buildings are the most vulnerable. Chimneys and heavy roof tiles are often shaken off during tremors and can crash into bystanders or passerbys.

Construction on soft or filled-in soil suffer the most because they feel shock waves most directly. Buildings on bedrock suffer less damage because the ground is firm.

The shaking of the earth is sometimes not the greatest disaster. It is in the ensuing fires and floods that the greatest damage often occurs. In the 1906 San Francisco earthquak, it was the subsequent fires that did the majority of the damage. An earthquake can also destroy dams high above a city or valley, causing floods to sweep away everything in their path.

An important aspect of post-earthquake study is the realization of the important role that the quality of construction plays. Earthquakes do not respect theories, calculations or divisions of responsibility. Many instances of poor quality construction are invariably exposed in earthquake damaged buildings. Badly placed reinforcement, poorly compacted concrete, incomplete grouting of masonry and loose or missing bolts in structural steelwork are some of the common examples of poor quality. Other examples of faulty construction can put lives at risk: falling masonry or cladding, ceiling tiles dislodged, window frames separating from the walls and toppling inwards or outwards, and escape paths blocked by jammed doors and fallen masonry. Usually these types of failure could have been avoided with very little expense.

An important category in earthquake building failure is when the building is so badly damaged that it has to be demolished, although it has not collapsed. For the owner and the insurance company the costs are similar whether the building collapses or is demolished. For the occupants it is the difference between life and death.

Where two buildings are close together, or where there is a movement joint in a building, the two sides are very likely to pound against each other during an earthquake. Major structural damage can result, particularly where the floor levels differ. The cause lies in the closeness of the two structures and in the flexibility of the buildings, factors which are within the control of the designer.

Modern buildings are often assembled from many separate components. Older ones commonly have timber floors with joists poorly tied to the supporting walls. Any lack of tying together in a building is quickly exposed by seismic (earthquake) action. The nature of seismic ground motion inevitably leads to differential movement between separate components, and in the absence of structural continuity, differential movement will occur.

Aftershocks, generally of much smaller magnitude than the main seismic shock which they follow, play no explicit part in the design process. Nevertheless they play a significant part in the immediate post-earthquake rescue and survival operation. The further damage caused by aftershocks to already damaged buildings is greater than their magnitude would otherwise suggest. Following major earthquakes many structures brought to the brink of collapse by the main shock are destroyed by subsequent lesser shocks.

Concentrations of force occur where there are abrupt changes in structural stiffness or mass distribution. For this reason building form should be regular and symmetrical, as far as the functional requirements permit.

2.1.3 Ground Behavior and Foundations

Geo-technical factors have been responsible for considerable damage to structures and constructed facilities in past earthquakes. Most geo-technical earthquake engineering problems can be broadly divided into two categories: response problems and failure problems. Response problems involve the effects of geo-materials on the amplitude, frequency content, and duration of earthquake ground motions; little or no permanent soil deformation is involved.

The tendency of soil to amplify ground motions has long been known, and nonlinear amplification effects are now well recognized by seismologists and geotechnical engineers. Failure problems involve permanent deformations of soil masses, generally through transient or long-term exceedance of the soil shearing resistance. Both types of problems must be addressed to advance the development of performance-based geotechnical earthquake engineering.

Both response and failure problems frequently involve the interaction of geo-materials with structures such as buildings, bridges, and piers. Because the interaction generally takes place through the foundation of the structure, these problems are frequently referred to as "soil-foundation-structure interaction" problems.

Other important problems, however, involve the response and/or failure of soils in the absence of nearby structures (e.g., stability of slopes adjacent to highways) or in situations in which the structure has little effect on the seismic performance of the soil (e.g., stability of soft soil beneath a bridge approach embankment).

The violent shaking of the ground temporarility increases lateral and vertical forces, disturbing the inter-granular stability of non-cohesive soils, and imposing strains directly on surface material where the fault plane reaches the surface. A transient increase in lateral and vertical forces places at risk any soil structures capable of movement. The resulting types of damage are landslips and avalanches. Results of the 1970 earthquake in Peru and the 1964 earthquake in Anchorage, Alaska, show that this damage can be on a massive scale. One village, Yungay, Peru, was destroyed almost entirely with the loss of 18,000 lives by a debris flow involving tens of millions of tonnes of rock and ice.

The disturbance of the granular structure of soils by shaking leads to consolidation of both dry and saturated material, due to the closer packing of grains. For saturated sands, shaking may increase pore pressure to the point where it exceeds the confining soil pressure, resulting in temporary liquefaction. This is an important affect. It can lead to massive failure in bearing and piled foundations, the collapse of slopes, embankments and dams. It can cause the phenomenon of "boiling" where liquefied sand flows upwards in surface pockets. It is also possible for some unstable soils to heave.

Shear movements in the ground may be at the surface or entirely below it. Where the earthquake fault reaches the surface, permanent movements of considerable magnitude may occur, in meters rather than centimeters. Surface shear movements may also take place as a result of other soil displacement (e.g., landslips or consolidation). Sub-surface shear movements can occur in weaker strata, leading to damage of embedded or buried structures. Sub-surface shear movements also reduce the transmission of ground motion to the surface, effectively putting an upper bound on the surface motion.

In considering the more spectacular permanent ground displacements that can result from ground shaking, it should not be forgotten that elastic displacements also occur. They are critical in the design of piles, underground pipelines and culvert-type structures. Failures in underground piping and ductwork are common in earthquakes and have important implications for the post-earthquake emergency services.

Failures of building foundations in earthquakes are not uncommon but are nearly always caused by failure of the supporting soil. Overturning failures due to uplift occur rarely, far less often than calculations would suggest. This rarity is probably due to the effective reduction in stiffness that accompanies uplift, which correspondingly reduces the force exerted by the ground acceleration. There can be no doubt that substantial tension from overturning forces can develop at foundation level. Examination of some lower failed columns in Caracas, following the 1967 earthquake, showed that they had failed in tension due to a combination of overturning forces and vertical ground acceleration.

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