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Geol 104: Earthquakes Earthquakes • What are they? – An earthquake is a trembling or shaking of the ground. • What causes them? – Earthquakes are caused by the sudden release of stored [elastic] strain energy by brittle fracture. Terminology of Earthquakes • Focus or Hypocenter - the point within the Earth where the rocks rupture. • Epicenter - the point on the Earth’s surface directly above the focus. • Focal Depth - distance of focus from Earth’s surface. Elastic Rebound Theory • Tectonic forces slowly build up with time, initially the rocks deform elastically. • Eventually the build up of stored energy exceeds the strength of the rock and it fractures. • The stored elastic strain energy is rapidly released and propagates outward as vibration waves called seismic waves. 1 Geol 104: Earthquakes Seismic Waves • Body waves – Seismic waves that travel through the interior of the Earth. As the waves propagate, the rocks are elastically deformed by change in either volume or shape. Types of Body Waves – Compression (P) waves - deform rocks through change in volume. • P-waves have greatest velocity (4-7 km/s in crust and ~8 km/s in mantle). As such, they are the first waves to arrive at a distant point. – Shear (S) waves - deform rocks through change in shape. • S-waves travel ~2 km/s slower than P-waves, and their velocity depends on the density and resistance to shearing of the material. Fluids do not have shear strength and thus cannot transmit S-waves. Seismic Waves • Surface waves – Seismic waves that travel along the surface of the Earth. Slowest moving waves collectively referred to as L or Long waves. • Love waves - transverse side-to-side wave motion in a horizontal plane parallel to Earth’s surface. • Rayleigh waves - backward rotating, circular motion similar to water molecule in ocean waves. 2 Geol 104: Earthquakes Measuring Seismic Waves • Seismographs - detect and record vibrations/motion of the Earth caused by an earthquake. – Inertial seismographs – Strain seismographs Locating an Earthquake • Travel time curves – Use the time lag between P and S seismic waves to determine distance from the epicenter to the seismograph (but not direction). • Three-point construction – Intersection point of time lag distance circles from three different seismographs yields location of epicenter. Earthquake Energy • Intensity – Mercalli Intensity Scale • Qualitative measure of intensity based upon amount of vibration people feel and extent of damage. 3 Geol 104: Earthquakes Earthquake Energy • Magnitude – Richter Magnitude Scale • Quantitative scheme based upon maximum amplitude of strongest body wave. • The Richter scale is logarithmic; each increase in magnitude corresponds to a 10-fold increase in the amplitude of the wave. • More importantly, each increment of magnitude corresponds to a 30-fold increase in energy released. Earthquake Energy • Seismic Moment Scale – Based upon the shear strength of the rock, rupture area of the fault and the average displacement (slip) on the fault. – The seismic moment scale provides a better measure of energy released by large (>6 on Richter scale) earthquakes. Earthquake Magnitudes, Characteristic Effects and Frequencies Richter Mercalli Magnitude Intensity <3.4 I 3.5-4.2 4.3-4.8 4.9-5.4 5.5-6.1 6.2-6.9 II-III IV V VI-VII VII-IX 7.0-7.3 X 7.4-7.9 >8.0 XI XII Characteristic Effects Number Annually Usually not felt by people. 800,000 Felt indoors by some people. Felt by many people, windows rattle. 30,000 4,800 Felt by all; dishes break, doors swing. Slight building damage, plaster cracks. Much building damage, chimneys fall, houses move on foundations. 1,400 500 100 Serious damage, bridges twisted, walls fractured, many masonry buildings collapse. Great damage, most buildings collapse. Total damage, waves seen on ground surface, objects thrown into the air. 15 4 one every 5-10 yrs 4 Geol 104: Earthquakes So how much elastic energy can rocks accumulate before failure? • ~100 joules of energy can be accumulated in 1 m3 of elastically deformed rock. • 100 joules is equivalent to ~25 calories of heat energy (about two peanut M&Ms). • But, when billions to trillions of m3 of rock are strained the results are impressive... • The 1989 Loma Prieta quake released ~1015 joules and the 1906 San Francisco quake released ~1017 joules (about the same amount of energy as a hydrogen bomb blast). Distribution of Earthquakes • ~80% originate in the circum-Pacific seismic belt that corresponds to the volcanic belt known as the Ring of Fire. • ~15% originate in the MediterraneanHimalayan belt that extends from Gibraltar to Southeast Asia. • Lesser seismic belts follow mid-ocean ridge systems. Tectonic Settings of Earthquakes • Divergent Boundaries – Tensional stress – Low magnitude and shallow foci (often <20 km). – Rocks are weaker in tension and temperature is higher near the surface (brittle-ductile transition nearer surface). 5 Geol 104: Earthquakes Tectonic Settings of Earthquakes • Transform Boundaries – Shear stress – High magnitude and shallow foci (<100 km). – Shear stress can build at “locked” segments (Stick-slip behavior). – Temperature follows normal geothermal gradient. – Water may act to lower strength. Tectonic Settings of Earthquakes • Convergent Boundaries – Subduction zones • Bending within down-going slab can result in tensional stress and small magnitude earthquakes at very shallow depth. • Sliding of down-going slab in the rigid lithosphere yields large magnitude earthquakes at depths to ~100 km. • Sinking through ductile asthenosphere, earthquakes are within the down-going slab down to 670 km. Tectonic Settings of Earthquakes • Convergent Boundaries • Bennioff zone - zone of earthquake foci dipping into the mantle away from a trench due to subduction. – Collision zones • • • • Where two continental lithosphere plates converge. Very strong compressive forces. Rocks are much stronger in compression High magnitude earthquakes at depths to 300 km.. 6 Geol 104: Earthquakes Tectonic Settings of Earthquakes • Intraplate (Bowling Green) – Most often associated with failed rift zones and reactivation of ancient faults. – Shallow foci but can have high magnitude due to high strength of continental basement rocks. Review - Earthquakes – Elastic rebound theory – Types/characteristics of seismic waves – Locating earthquakes – Intensity and magnitude of earthquakes – Energy released by earthquakes. – Geographic distribution of earthquakes – Tectonic settings and depth/magnitude of associated earthquakes. 7