SGES 1302 INTRODUCTION TO EARTH SYSTEM Lecture 19: Earthquakes 2

Earthquake Hazards An earthquake of magnitude-5 can be catastrophic in one region, but relatively harmless in another. There are many factors that determine the severity of damage produced by an earthquake. Identifying earthquake hazards in an area can sometimes help to prevent some of the damage and loss of life. For example, the design of certain buildings can affect earthquake damage. The most severe damage occurs to unreinforced buildings made of brittle building materials such as concrete. Wooden structures, on the other hand, are more resilient and generally sustain less damage. The type and characteristics of the earth material under the building can also affect the earthquake damage. For example, loose sandy or silty may liquify during an earthquake and seismic waves can be amplified in some hard rock materials.

The Effects of Earthquake Ground shaking Ground shaking (and tsunamis) is the most widespread seismic hazards. Shaking generally causes the most damage and loss of life. Unlike ground rupture, shaking is very difficult to avoid. Damages to structures/buildings caused by shaking can be mitigated to a large extent by engineering design. Key factors controlling response of near-surface materials: Thickness of Quaternary deposits Degree of consolidation (void ratio) Degree of cementation Depth to water table

The Effects of Earthquake Types of shaking-induced ground failures: Settlement of cohesionless (sandy) alluvial materials – vertical contraction resulting from loss of void space Liquefaction of cohesionless (sandy) alluvial materials. Ground oscillation: Occurrence of large surface waves, as a surficial layer that overlies a liquefied layer fractures and blocks bob up and down. Lateral spreading (slopes <3°): Horizontal translation of surficial blocks in response to liquefaction of underlying subhorizontal layers Flow failure (slopes >3°): Translation of surficial blocks in response to liquefaction of underlying inclined layers (same mechanism as lateral spreading) Failure of quick clays: Collapse of open clay structure and loss of shear strength.

The Effects of Earthquake Ground Displacement: Earthquakes can cause large scale displacement of the landscape. The 1906 San Francisco earthquake caused a 430 km rupture along the San Andreas fault and a 7 m right-lateral displacement near the epicenter. During the 2004 Sumatra earthquake, a 5 m upward thrust of the seafloor generated the deadly 2004 Indian Ocean tsunami. Ground displacement in urban areas can be disastrous, causing collapse of buildings, destroying roads and bridges, severing gas and water pipelines and electricity lines.

The Effects of Earthquake Landslides & rockfalls: A large earthquake’s violent shaking can dislodge large masses of unstable soils and rocks from hillsides, causing them to move downslope. Earthquake generated landslides can occur on land and on the seafloor. The 2008 Sichuan earthquake triggered thousands of landslides in the rugged Sichuan province.

The Effects of Earthquake Liquefaction: It is the conversion of unconsolidated sediment with some initial cohesiveness and strength into a mass of water-saturated sediment that flow like water (liquefy). It causes the loss of shear strength as liquids have essentially no shear strength. Liquefaction causes a once firm ground to become a slurry of mud and can cause widespread collapse of buildings (eg. the 1995 Kobe earthquake). Sands and silty sands most susceptible to liquefaction are: well-sorted, very young (<100 years), have a high water table (< 3 m).

The Effects of Earthquake Seiches: Seismic waves cause the water in an enclosed (eg. lake) or partially enclosed (eg. bay) body of water to move back and forth across the basin, with water level rising and falling. The back and forth motion of water can strip the soils around a lake down to bare bedrock and can cause a dam to overflow. Seiches can develop at great distances from a powerful earthquake (eg. the great Alaskan earthquake set off seiches across the US as far as Texas, 6000 km away, where water rose and fell 2 m in bays along the Texas Gulf coast). Seiches can continue over long after the earthquake (eg. the 1959 Madison Canyon earthquake set off seiches in Hebgen Lake every 17 min for 11 hours).

The Effects of Earthquake Tsunami It is a large ocean wave generated by vertical motions of the seafloor during an earthquake. Though less common, it can also be generated by a submarine landslide. These motions displace the entire column of water overlying the fault, creating bulges and depressions in the water. The disturbance then spreads out from the epicenter in the form of extremely long waves. While these waves are in the open ocean, their height is generally less than 1 m. When the waves enter shallow water, however, they can form huge breakers with heights occasionally exceeding 30 m. These enormous wave heights, together with open-ocean speeds between 500 and 800 km/h, make tsunamis dangerous threats to coastal areas both near to and far from a earthquake’s epicenter. Generally, tsunami consists of several waves that arrive at irregular intervals over several hours.

Principal Earthquake Zones Concentration of earthquakes worldwide near plate boundaries, with a small but significant number of intra-plate quakes. Almost 80 percent of all earthquakes occur on the Circum-Pacific Belt and about 15 percent on the Mediterranean-Asian Belt across southern Europe and Asia.

Principal Earthquake Zones

Earthquake Forecasting There is currently no completely reliable way to forecast the exact time and location of the next earthquake. Instead, earthquake forecasting is based on calculating the probability of an earthquake. The probability of an earthquake’s occurrence is based on two factors: the history of earthquakes in an area and the rate at which strain builds up in the rocks. Most earthquakes occur in long, narrow bands called seismic belts. The probability of future earthquakes is much greater in these belts than elsewhere on Earth. The pattern of earthquakes in the past is usually a reliable indicator of future earthquakes in a given area. Seismometers and sedimentary rocks can be used to determine the frequency of large earthquakes. The history of an area’s seismic activity by can be used to generate seismic-risk maps.

Earthquake Forecasting Recurrence rates Earthquake-recurrence rates along a fault can indicate whether the fault ruptures at regular intervals to generate similar earthquakes. The earthquake-recurrence rate along a section of the San Andreas fault at Parkfield, California, for example, shows that a sequence of earthquakes of approximately magnitude-6 shook the area about every 22 years from 1857 until 1966. In 1987 seismologists forecasted a 90-percent probability that a major earthquake would rock the area within the next few decades. In September, 2004, a magnitude-6 earthquake struck. Extensive data was collected before and after the 2004 earthquake. The information obtained will be invaluable for predicting and preparing for future recurrent earthquakes around the world.

Earthquake Forecasting Seismic gaps Probability forecasts are also based on the location of seismic gaps. Seismic gaps are sections located along faults that are known to be active, but which have not experienced significant earthquakes for a long period of time.

Earthquake Forecasting Stress accumulation The rate at which stress builds up in rocks is another factor seismologists use to determine the earthquake probability along a section of a fault. Eventually this stress is released, generating an earthquake. The stress accumulated in a particular part of a fault, together with the amount of stress released during the last earthquake in a particular part of the fault, can be used to develop stress accumulation map.