Earthquakes
Violent Ground Motion - loud noise, knocked down. Collapse of buildings, fire, landslides, and dam failures, Most earthquakes occur at plate boundaries, Prevention - Government mandates as to location and design of structures. Shoddy construction kills tens of thousands.
What is an Earthquake? - A shaking or vibration of the ground caused by deformed rocks breaking along a fault
Elastic Rebound Theory - two blocks are pushed in opposite directions, energy stored in rocks overcomes friction and the blocks slip suddenly along the fault
Slip - distance of the displacement
focus - point at which slop initiates
epicenter - geographic point on the earth's surface directly above the focus.
Intense vibrations or seismic waves travel outward from the focus.
Studying Earthquakes - Seismograph - an instrument to measure ground motion. Uses a spring (vertical motion) or hinge (horizontal motion) to isolate a mass from the earth's surface.
Seismic Waves
P-waves - wave motion (compression/expansion) is parallel to direction of wave propagation. P-waves can travel through solid, liquid or gas. Fast
S-waves - wave motion is perpendicular to direction of wave propagation. S-waves only travel through solids. Slower than P-waves,
Surface waves - move along the surface of the earth, slower than S-waves, but amplitude of ground displacement is high.
Locating the Epicenter - the difference in time between the arrival of P and S waves is related to distance traveled. If the distance to 3 seismographs is known, the earthquake epicenter can be determined.
Measuring the Size of an Earthquake
Richter Magnitude - related to the size (or amplitude) of the ground movement. Scale is logarithmic (magnitude 8 earthquake is 100 times larger than a magnitude 6).
Large earthquakes are less frequent than small earthquakes. 800,000 earthquakes each year between 2 and 3.4. A magnitude 8 earthquake only happens about every 5 years.
Modified Mercalli Intensity Scale - measure of the destructiveness of an earthquake, which depends on building codes, nature of the soil, distance from epicenter.
Determining Fault Mechanisms from Earthquake Data - The geographic distribution of first motions (push/pull) on ground surface from P-wave arrivals can determine the fault orientation and the type of fault motion (normal, reverse or strike-slip) or nuclear explosions.
Earthquakes occur primarily at Plate Boundaries
Shallow-Focus Earthquakes at Divergent Boundaries
Narrow belts of tensional (normal fault) earthquakes coincide with mid-ocean ridge crests
Strike-slip (shear) earthquake occur along transform faults that offset ridge segments
Deep-focus Earthquakes at Convergent Boundaries
Almost all deep-focus earthquakes occur along the inclined plane of a subducted plate. These earthquakes are produced by thrust faulting (compression).
Shallow-Focus Earthquakes within Plates
New Madrid, Missouri (1812) - rift valley
Charleston, NC (1886) - probable rift valley
Earthquake Destructiveness - About 100 earthquakes with Richter magnitudes between 6 and 7 occur each year.
Loma Prieta (south of San Francisco) in 1989 (magnitude 7.1) and Northridge (near LA) in 1994 (magnitude 6.9) caused billions of dollars in damage even though they had less than 1/100 the amount of energy as truly great earthquakes (e.g, San Francisco (1906, 8.3), Tokyo (1923, 8.2), Chile (1960, 8.6) and Alaska (1964, 8.6)
How Earthquakes Cause Their Damage
Ground vibrations shake structures and cause them to collapse (Armenia in 1988)
Fires ignited from ruptured gas lines or downed power lines (San Francisco, 1906)
Avalanches (liquefaction (Alaska, 1964; Loma Prieta, 1989)
Tsunamis - 20 m wall of water traveling at 800 km/hr
Mitigating the Destructiveness of Earthquakes
Seismic-risk map of the United States - in high risk areas, building codes required to resist earthquakes.
Early-Warning Indicators - Many small earthquakes, Tilting of ground surface, Changes in Electrical Properties, Changes in water level in wells.
Seismic Gap Method - most likely place for an earthquake to occur is at a locked portion of a fault where the time since the last earthquake exceeds the average interval between earthquakes. Southern California: recurrence time for great earthquakes is 100 to 150 years. Last great earthquake was in 1857.