Earthquaaaaakes… - a sudden vibration or trembling in the Earth

MagnitudeEarthquake Effects Estimated Number Each Year Examples 2.5 or less Usually not felt, but can be recorded by seismograph. 900, : Smallest earthquake detectable by people. 2.5 to 5.4 Often felt, but only causes minor damage. 30, : Energy released by the Hiroshima atomic bomb. 5.5 to 6.0 Slight damage to buildings and other structures to 6.9 May cause a lot of damage in very populated areas : About 120 shallow earthquakes of this magnitude occur each year on the Earth. 6.7: Northridge, California earthquake January 17, 1994.

Northridge California Earthquake, 1994

MagnitudeEarthquake Effects Estimated Number Each Year Examples 7.0 to 7.9 Major earthquake. Serious damage : Major earthquake threshold. 7.4: Turkey earthquake August 17, More than 12,000 people killed. 7.6: Deadliest earthquake in the last 100 years. Tangshan, China, July 28, Approximately 255,000 people perished. 8.0 or greater Great earthquake. Can totally destroy communities near the epicenter. One every 5 to 10 years 8.3: San Francisco earthquake of April 18, : December 26, 2004 Sumatra earthquake. 9.5: Most powerful earthquake recorded in the last 100 years. Southern Chile on May 22, Claimed 3,000 lives.

Tangshan, China Earthquake, 1976

Sumatra Earthquake (and Tsunami), 2004

Formation of Earthquakes Quick release of stored potential energy into the kinetic energy of motion Most are produced along faults, tectonic plate boundary zones, or along mid-oceanic ridges Large masses of rock moving past each other can become locked due to friction Friction is overcome when the accumulating stress has enough force to cause a sudden slippage of the rock masses Magnitude of shock wave released into surrounding rocks controlled by: –quantity of stress built up from friction –distance the rock moved when the slippage occurred –ability of the rock to transmit the energy contained in the seismic waves Sometime after the main shock wave, aftershocks can occur because of the continued release of frictional stress

Distribution of earthquake epicenters from 1975 to About 90% of all earthquakes occur at a depth between 0 and 100 kilometers. (Source: U.S. Geologic Survey, National Earthquake Information Center).

Earthquake Waves Earthquakes are a form of wave energy transferred through bedrock Motion is transmitted from the point of sudden energy release, the earthquake focus, as spherical seismic waves that travel in all directions outward Point on the Earth's surface directly above the focus is termed the epicenter (draw diagram!)

Measuring Earthquakes Using a seismograph –Converts the wave energy into a standard unit of measurement like the Richter scale –In the Richter scale, units of measurement are referred to as magnitudes. –An increase of one unit on the Richter scale represents an increase of about 40 times in the amount of seismic energy released

Earthquake Damage Earthquakes with magnitudes of 8 and greater rare, but can lead to almost total devastation over a large area Smaller earthquakes more frequent, but combined energy release is small compared with that from one great earthquake –About 3 million earthquakes of magnitude 4 to release as much seismic energy as a single magnitude 8 earthquake Highest Richter magnitudes are not necessarily the most devastating, nor do they necessarily cause the greatest loss of life Depends on: –Depth (shallow earthquakes are more destructive than deeper ones) –Population density –Rock types (some waves can travel through more easily) –Soil conditions (wet coastal area?) –Local building standards (developed vs. developing country)

What can we do to protect ourselves from earthquakes?