Earthquake damage

• Ground motion due to seismic waves• Fault rupture of the ground surface• Fire• Liquefaction• Tsunami

Earthquake Risk

• The risk that an earthquake will occur close to where you live depends on whether or not tectonic activity that causes deformation is occurring within the crust of that area.

• For the U.S., the risk is greatest in the most tectonically active area, near the plate margin in the Western U.S.

• The San Andreas Fault which forms the margin between the Pacific Plate and the North American Plate, is responsible for about 1 magnitude 8 or greater earthquake per century.

• Historically, large earthquakes have also occurred in the area of New Madrid, Missouri; Charleston, South Carolina; and an area extending from New Jersey to Massachusetts.

Earthquake Damage

• Earthquakes don't kill people, buildings do. This is because most deaths from earthquakes are caused by buildings or other human construction falling down during an earthquake.

• Earthquakes located in isolated areas far from human population rarely cause any deaths.

• Damage from earthquakes can be classified as follows: 1-Ground Shaking - Shaking of the ground caused by the passage of seismic waves near the epicenter of the earthquake is responsible for the collapse of most structures. The intensity of ground shaking depends on distance from the epicenter and on the type of bedrock underlying the area.   – In general, loose unconsolidated sediment is subject to

more intense shaking than solid bedrock.– Damage to structures from shaking depends on the type

of construction. Concrete and masonry structures, because they are brittle are more susceptible to damage than wood and steel structures, which are more flexible.

Ground shaking is amplified in unconsolidated materials

• 2-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. 

• 3-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. In the 1906 earthquake in San Francisco more than 90% of the damage to buildings was caused by fire. 

• 4-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 loose grain to grain contact, and thus the material tends to flow.

5-Tsunamis - Tsunamis are giant ocean waves that can rapidly travel across oceans. Earthquakes that occur along coastal areas can generate tsunamis, which can cause damage thousands of kilometers away on the other side of the ocean.

• Tsunamis, or seismic sea waves • Earthquakes under the ocean• Destructive waves called “tidal waves”• Result from “push” of fault block • In open ocean height is > 1 meter • In shallow coast water wave can be > 30

meters • Very destructive

Tsunamis

Formation of a tsunami by displacement of the ocean floor

The speed of a wave moving across the ocean correlateswith the ocean depth.

Tsunamis

Alaska, 1964Indonesia, 1992

Niigata, 1964

Tsunami travel times (in hrs) to Hawaii from various points in the Pacific Basin

6-Rapid Mass-Wasting Processes - In mountainous regions subjected to earthquakes ground shaking may trigger rapid mass-wasting events like rock and debris falls, rock and debris slides, slumps, and debris avalanches.

Eartquake’s Forecasting

• Historic and prehistoric records of past major earthquakes should reflect that periodicity, allowing one to forecast the time window for the next earthquake.

Major seismic gaps (Stretches with little or no seismic activity) in the Western Hemisphere

Studies of the dates of large historic and prehistoric earthquakes along majorFault zones have suggested that they may be broadly periodic, occurring at more or less regular interval.

On average, major earthquakes occur every 20 yrs, and the last one in 1966

The Earthquake Cycle

• Quiescence, building up of energy. • Foreshocks minutes to days before main shock.• Main shock, largest event in the cycle.• Aftershocks occur in the following days to weeks.• Adjustment activity may last years to centuries, and cover the

area surrounding the main shock.• Quiescence again, lasting much longer than the other phases

(hundreds of years in active area to millions of years).• The cycle is not periodic (earthquakes do not occur after equal

time periods) because stress build-up at any given site is affected by the earthquake activity in the surrounding area.

Fluid Injection to minimize major Earthquakes

• The idea is that, because increased pore pressure decreases friction and shear strength, facilitating slip along fault zones, faults “lubricated” by fluid injection could release built-up stress by slipping. However, in locked fault zones along which considerable stress had already built up, the result might be a major earthquake induced by human activity. There is also no simple correlation between pore pressure and resultant earthquake intensity. The method might best be applied, at least initially, to sections of faults not long quiescent, so that the stress could be released continually before it builds to dangerous levels.

Sand boils are caused by soil losing its internal cohesion (friction between soil particles) during the strong shaking of a large earthquake, a process called liquefaction

How to Avoid Earthquake Hazards

• Primary earthquake-related hazards include: landslides (avoid areas historically prone to landslides; undertake slope-stability studies); ground motion (stay away from fault zones, and especially filled land near fault zones; also, bedrock is safer material to build on than is unconsolidated sediment; alternatively, try earthquake-resistant structural designs); tsunamis (listen for warnings; avoid coastal areas); liquefaction (avoid areas known to be prone to it); coastal flooding from tectonic movements (not much can be done in this case). A secondary hazard is fire (build valving systems into pipelines and water lines to minimize the risks).