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Description

Most were killed when a double decked freeway in Oakland collapsed. About 10 months earlier, an earthquake with moment magnitude 6. The death toll in the latter earthquake was about 25,! Similarly the Moment Magnitude 7. Most buildings were made of poorly reinforced concrete. While architecture and building codes can reduce risk, it should be noted that not all kinds of behavior can be predicted. Although codes are refined each year, not all possible effects can be anticipated. For example different earthquakes show different frequencies of ground shaking, different durations of ground shaking, and different vertical and horizontal ground accelerations.

Old buildings cannot cost-effectively be brought up to code, especially with yearly refinements to code.

Key Objectives

Even with construction to earthquake code, buildings fail for other reasons, like poor quality materials, poor workmanship, etc. Ground Motion - Shaking of the ground caused by the passage of seismic waves, especially surface waves, near the epicenter of the earthquake are responsible for the most damage during an earthquake and is thus a primary effect of an earthquake.

The intensity of ground shaking depends on: Local geologic conditions in the area.

In general, loose unconsolidated sediment is subject to more intense shaking than solid bedrock. Size of the Earthquake.


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In general, the larger the earthquake, the more intense is the shaking and the duration of the shaking. Distance from the Epicenter. Shaking is most severe near the epicenter and drops off away from the epicenter. The distance factor depends on the type of material underlying the area. There are, however, strange exceptions.

For example, the Mexico City Earthquake magnitude 8. Damage to structures from shaking depends on the type of construction. Concrete and masonry structures are brittle and thus more susceptible to damage wood and steel structures are more flexible and thus less susceptible to damage.

Faulting and Ground Rupture - Ground rupture generally occurs only along the fault zone that moves during the earthquake, and are thus a primary effect. Thus structures that are built across fault zones may collapse, whereas structures built adjacent to, but not crossing the fault may survive. Aftershocks - These are smaller earthquakes that occur after a main earthquake, and in most cases there are many of these were measured after the Alaskan Earthquake. Aftershocks occur because the main earthquake changes the stress pattern in areas around the epicenter, and the crust must adjust to these changes.

Aftershocks are very dangerous because they cause further collapse of structures damaged by the main shock. Aftershocks are a secondary effect of earthquakes. Fire - Fire is a secondary effect of earthquakes. Because power lines may be knocked down and because natural gas lines may rupture due to an earthquake, fires are often started closely following an earthquake. The problem is compounded if water lines are also broken during the earthquake since there will not be a supply of water to extinguish the fires once they have started.

Landslides - In mountainous regions subjected to earthquakes ground shaking may trigger landslides, rock and debris falls, rock and debris slides, slumps, and debris avalanches. These are secondary effects.

Assessing Seismic Hazard and Risk Globally for an Earthquake Resilient World

Liquefaction - Liquefaction is a processes that occurs in water-saturated unconsolidated sediment due to shaking. In areas underlain by such material, the ground shaking causes the grains to lose grain to grain contact, and thus the material tends to flow. Liquefaction, because it is a direct result of ground shaking, is a primary effect.


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You can demonstrate this process to yourself next time your go the beach. Stand on the sand just after an incoming wave has passed. The sand will easily support your weight and you will not sink very deeply into the sand if you stand still. But, if you start to shake your body while standing on this wet sand, you will notice that the sand begins to flow as a result of liquefaction, and your feet will sink deeper into the sand. Changes in Ground Level - A secondary or tertiary effect that is caused by faulting.

Earthquakes may cause both uplift and subsidence of the land surface. During the Alaskan Earthquake, some areas were uplifted up to Tsunami - Tsunami a secondary effect that are giant ocean waves that can rapidly travel across oceans, as will be discussed in more detail later. Earthquakes that occur beneath sea level and along coastal areas can generate tsunami, which can cause damage thousands of kilometers away on the other side of the ocean. Flooding - Flooding is a secondary effect that may occur due to rupture of human made dams and levees, due to tsunami, and as a result of ground subsidence after an earthquake.

World Distribution of Earthquakes The distribution of earthquakes is called seismicity. This makes sense, since plate boundaries are zones along which lithospheric plates move relative to one another. Earthquakes along these zones can be divided into shallow focus earthquakes that have focal depths less than about km and deep focus earthquakes that have focal depths between and km. Earthquakes at Diverging Plate Boundaries.

Diverging plate boundaries are zones where two plates move away from each other, such as at oceanic ridges. In such areas the lithosphere is in a state of tensional stress and thus normal faults and rift valleys occur. Earthquakes that occur along such boundaries show normal fault motion, have low Richter magnitudes, and tend to be shallow focus earthquakes with focal depths less than about 20 km. Such shallow focal depths indicate that the brittle lithosphere must be relatively thin along these diverging plate boundaries.

Earthquakes at Transform Fault Boundaries. Transform fault boundaries are plate boundaries where lithospheric plates slide past one another in a horizontal fashion. Another way of looking a seismic risk that is more useful to construction designers and engineers, and therefore to the development of building codes is based on expected horizontal ground acceleration.

The Earthquake Hazards Program

Ground accelerations of 0. Figure 3. Hazards Associated with Earthquakes Possible hazards from earthquakes can be classified as follows: Ground Motion - Shaking of the ground caused by the passage of seismic waves, especially surface waves near the epicenter of the earthquake are responsible for the most damage during and earthquake. The intensity of ground shaking depends on: Local geologic conditions in the area. In general, loose unconsolidated sediment is subject to more intense shaking than solid bedrock. Size of the Earthquake. In general, the larger the earthquake, the more intense is the shaking and the duration of the shaking.

Distance from the Epicenter. Shaking is most severe near the epicenter and drops off away from the epicenter. The distance factor depends on the type of material underlying the area. There are, however, strange exceptions.

Earthquake Hazard Analysis Methods: A Review

For example, the Mexico City Earthquake magnitude 8. Damage to structures from shaking depends on the type of construction. Concrete and masonry structures are brittle and thus more susceptible to damage wood and steel structures are more flexible and thus less susceptible to damage. Faulting and Ground Rupture - Ground rupture only occurs along the fault zone that moves during the earthquake.

Thus structures that are built across fault zones may collapse, whereas structures built adjacent to, but not crossing the fault may survive. Aftershocks - These are usually smaller earthquakes that occur after a main earthquake, and in most cases there are many of these were measured after the Alaskan Earthquake. Aftershocks occur because the main earthquake changes the stress pattern in areas around the epicenter, and the crust must adjust to these changes. Aftershocks are very dangerous because they cause further collapse of structures damaged by the main shock.

Fire - Fire is a secondary effect of earthquakes.

Earthquake Hazard and Risk in Canada

Because power lines may be knocked down and because natural gas lines may rupture due to an earthquake, fires are often started closely following an earthquake. The problem is compounded if water lines are also broken during the earthquake since there will not be a supply of water to extinguish the fires once they have started.

Landslides - In mountainous regions subjected to earthquakes ground shaking may trigger landslides, rock and debris falls, rock and debris slides, slumps, and debris avalanches. Liquefaction - Liquefaction is a processes that occurs in water-saturated unconsolidated sediment due to shaking. In areas underlain by such material, the ground shaking causes the grains to lose grain to grain contact, and thus the material tends to flow. You can demonstrate this process to yourself next time your go the beach.

Stand on the sand just after an incoming wave has passed. The sand will easily support your weight and you will not sink very deeply into the sand if you stand still. But, if you start to shake your body while standing on this wet sand, you will notice that the sand begins to flow as a result of liquefaction, and your feet will sink deeper into the sand. Changes in Ground Level - A secondary or tertiary effect that is caused by faulting.

Earthquakes may cause both uplift and subsidence of the land surface. During the Alaskan Earthquake, some areas were uplifted up to Tsunamis - Tsunamis are giant ocean waves that can rapidly travel across oceans, as will be discussed in more detail later. Earthquakes that occur beneath sea level and along coastal areas can generate tsunamis, which can cause damage thousands of kilometers away on the other side of the ocean.

Flooding - Flooding is a secondary effect that may occur due to rupture of human made dams, due to tsunamis, and as a result of ground subsidence after an earthquake. This makes sense, since plate boundaries are zones along which lithospheric plates move relative to one another. Earthquakes along these zones can be divided into shallow focus earthquakes that have focal depths less than about km and deep focus earthquakes that have focal depths between and km.

Earthquakes at Diverging Plate Boundaries. Diverging plate boundaries are zones where two plates move away from each other, such as at oceanic ridges. In such areas the lithosphere is in a state of tensional stress and thus normal faults and rift valleys occur. Earthquakes that occur along such boundaries show normal fault motion, have low Richter magnitudes, and tend to be shallow focus earthquakes with focal depths less than about 20 km. Such shallow focal depths indicate that the brittle lithosphere must be relatively thin along these diverging plate boundaries. Examples - all oceanic ridges, Mid-Atlantic Ridge, East Pacific rise, and continental rift valleys such as the basin and range province of the western U.

Earthquakes at Transform Fault Boundaries. Transform fault boundaries are plate boundaries where lithospheric plates slide past one another in a horizontal fashion. The San Andreas Fault of California is one of the longer transform fault boundaries known. Earthquakes along these boundaries show strike-slip motion on the faults and tend to be shallow focus earthquakes with depths usually less than about km.

Earthquake Risk

Richter magnitudes can be large. Earthquakes at Converging Plate Boundaries. Convergent plate boundaries are boundaries where two plates run into each other.