EARTHQUAKES Chapter 13 STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time Earthquakes are the result of stress that builds up over time. STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time Earthquakes are the result of stress that builds up over time. Stress gradually builds as tectonic forces deform rocks. STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time Earthquakes are the result of stress that builds up over time. Stress gradually builds as tectonic forces deform rocks. When the stress exceeds the strength of the rocks STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time Earthquakes are the result of stress that builds up over time. Stress gradually builds as tectonic forces deform rocks. When the stress exceeds the strength of the rocks the fault slips, causing an earthquake. STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time Earthquakes are the result of stress that builds up over time. Stress gradually builds as tectonic forces deform rocks. When the stress exceeds the strength of the rocks the fault slips, causing an earthquake. The process repeats again and again. Rocks deform as strain develops Strike-slip fault ROCKS DEFORM ELASTICALLY, THEN REBOUND DURING AN EARTHQUAKE RUPTURE TIME 1 A farmer builds a stone wall across a strike-slip fault. TIME 2 The relative motion between blocks on either side of the locked fault causes the ground and the stone wall to deform. Focus Epicenter TIME 3 A new fence is built across the already- deformed land. TIME 4 The rupture displaces the fault, lowering the stress. The elastic rebound straightens the rock wall, but the fence exhibits a reverse curve. Stress Earthquakes Time Local rock strength Stress may build more quickly or less quickly and the strength of the fault may vary, causing earthquakes to occur at varying times and with varying amounts of slip. Focus 0 Seconds Rupture expands circularly on fault plane, sending out seismic waves in all directions. 5 Seconds Rupture continues to expand as a crack along the fault plane. Rocks at the surface begin to rebound from their deformed state. 10 Seconds The rupture front progresses down the fault plane, reducing the stress. 20 Seconds Rupture has progressed along the entire length of the fault. The earthquake stops. Fault cracks at surface Fault crack extends Spring Mass Spring Mass The mass is loosely coupled to Earth. Recording pen Earth moves up Recording pen Earth moves up Upward movement of the Earth causes downward relative movement of the mass, and vice versa. Earth moves down Earth moves down The pen traces the differences in motion. Earth moves left Earth moves right Earth moves side to side Mass Hinge Focus Mantle Seismograph Core SP Seismic waves travel through Earth and over its surface. Body waves P waves S waves Surface waves Love waves (left-right, or back and forth) Rayleigh waves (rolling) Minutes Surface waves The waves travel at different speeds and arrive at the seismograph at different times. 0 PS Compression wave P waves are compressional waves that travel quickly through rock. P waves push and pull particles in the direction of their path of travel. A section of rock expands and then contracts Shear-wave crest Shear-wave crest S waves S waves push material at right angles to their path of travel. A section of rock shears from a square to a parallelogram. S waves travel at about half the speed of P waves. Wave direction Surface waves ripple across Earths surface. The ground surface moves in a rolling, elliptical motion. Rayleigh wave Wave direction The ground shakes sideways, with no vertical motion. Love wave Seismograph Focus Seismograph Epicenter Seismic waves arrive at distant seismographic stations at different times. Distance traveled from earthquake epicenter (km) Time elapsed after start of earthquake (min) 3-minute interval at 1500 km , Seismogram A 11-minute interval at 8600 km 8-minute interval at 5600 km Seismogram B Seismogram C S wave P wave Because P waves travel faster than S waves, the interval between their travel-time curves increases with distance. By matching the observed interval to the curves, a geologist can determine the distance from the station to the epicenter. 1500 km A A B B Epicenter 5600 km 8600 km C C If the geologist then draws a circle around each seismographic station, the point at which the circles intersect is the earthquakes epicenter. Amplitude =23 mm Richter magnitude Amplitude (mm) Interval between S and P waves (s) Distance (km) P S S-wave interval = 24 secondsP-wave A geologist measures the amplitude of the largest seismic wave and the time interval between the P- and S-waves to determine the distance from the epicenter. Amplitude =23 mm Richter magnitude Amplitude (mm) Interval between S and P waves (s) Distance (km) P S S-wave interval = 24 secondsP-wave A geologist measures the amplitude of the largest seismic wave and the time interval between the P- and S-waves to determine the distance from the epicenter. By connecting the points, the geologist determines the Richter magnitude. Seismographic stations Fault First motion (pull toward epicenter) First motion (pull toward epicenter) First motion (pull toward epicenter) First motion (pull toward epicenter) First motion (push away from epicenter) First motion (push away from epicenter) Determining a left-right or right- left movement World seismicity from 1976 to 2002 Transform fault (lateral shearing) Rift valley (divergence) Mid-ocean ridge (divergence) Lithosphere Asthenosphere Transform fault (lateral shearing) Rift valley (divergence) Normal faulting Mid-ocean ridge (divergence) Lithosphere Asthenosphere Lithosphere Asthenosphere Transform fault (lateral shearing) Rift valley (divergence) Normal faulting Mid-ocean ridge (divergence) Shallow earthquakes coincide with normal faulting at divergent boundaries and with strike-slip faulting at transform boundaries. Lithosphere Asthenosphere Deep-ocean trench (convergence) Large shallow earthquakes occur mainly on thrust faults. Intermediate- and deep- focus earthquakes occur in the descending slab. Southern California fault traces San Andreas faultSan Gabriel Mountains North American Plate North American Plate Pacific Plate Pacific Plate Los Angeles Motion of Pacific Plate relative to motion of North American Plate The Big Bend causes the Pacific Plate to compress against the North American Plate, causing thrust faulting. Here, the San Andreas fault is parallel to plate motion, and the faulting is right-lateral strike slip. Southern California earthquakes (July 1970-June 1995) Northridge 1994 Magnitude 6.9 San Fernando 1971 Magnitude 6.7 Landers 1992 Magnitude 7.3 July 1970June 1995 Key: 5+