Geology 111G Lecture 21 16 April 2005

Earthquakes 2

Interpreting Earthquakes (cont'd)

Elastic Rebound Hypothesis

Effects of Earthquakes

Earth Structure from Seismic Waves

Moho

Low Velocity Zone

I. Causes of Earthquakes. Elastic rebound hypothesis predicts that an earthquake is the sudden release of strain built up in rock; upon failure, rock on opposite sides of a fault moves in opposite directions. Permits prediction that failure takes place after a certain amount of strain buildup, which is indicated by progressive tilting or warping of earth's surface in the vicinity of faults.

A. Recurrence interval. Expected time interval between quakes.

1. San Francisco Earthquake of April 18, 1906 resulted in 7 meters of offset. A 450 km segment of the fault failed. This is a plate-margin fault with estimated displacement of 6-7 cm/year. Dividing the offset by the displacement gives an estimated recurrence interval of 100-116 years.

2. Intraplate faults, such as the Wasatch fault zone in Utah also pose serious hazards. These are more difficult to determine as to recurrence interval since we have no observable displacement. The technique for determining recurrence is to date ancient events using radiocarbon techniques. Expectation here is for magnitude 6-7 quakes every 400 years or so.

II. Effects of Earthquakes

A. Fire: broken gas and water lines.

B. Structural damage.

1. Damage by shaking. Masonry structures (brick, stone, adobe, cinder block) are more prone to collapse than wood and steel frame construction, which have inherently greater elasticity. Masonry requires significant reinforcement. Shaking influenced by substrate.

2. Damage by settling. Structures on unconsolidated or saturated sediment.

a. Liquefaction. Saturated sediment loses cohesion during shaking, causes uneven settling.

C. Tsunami: Seismic sea waves triggered by subaqueous fault displacement or major landslides underwater. Very long wavelength (4/1/46 Unimak Island quake had wavelength of 160 km, traveled 800 km/hour, with amplitude of only 1m in deep water but 12-18 m in Hawaii). Tsunamis are accentuated on the shelf.

III. Structure of the Earth, revisited. Our knowledge of earth's internal structure comes from seismic wave behavior.

A. Crust. Waves are refracted due to increasing density , and hence increased velocity downward. They thus follow curved pathways. P-wave velocities increase downward from 6 to 7 km/sec.

1. Mohorovicic discontinuity. Base of crust marked by abrupt increase in wave velocity.

B. Mantle. P-wave velocities range from in excess of 8 km/sec to about 14 km/sec near the core. This is because the rocks are dense, composed mainly of Fe-rich minerals olivine and pyroxene.

1. Low velocity zone: Seismic discontinuity at about 100 km beneath continents, 50-80 km beneath oceans. This is a zone of marked decrease in seismic velocity which marks the base of the lithosphere. There is no evidence for a compositional (density) change here, so it is interpreted as an abrupt increase in temperature, possibly accompanied by partial melting or rock. This boundary is where the lithosphere is decoupled from the asthenosphere.

C. Core. Lies below 2900 km.

1. Seismic waves traveling 11,000 km or less move along a predictable path.

2. Compressional earthquake waves traveling more than 11,000 km arrive late, while shear waves don't arrive at all.

a. P-waves are refracted at core-mantle boundary and travel longer paths, which delays them. Those going through inner core accelerate at its outer boundary (5100 km)

b. S-waves are attenuated at core-mantle boundary, indicating that outer core is liquid.