1. UseIT Tutorial # 3 Earthquakes in the Southern California Fault System Tom Jordan June 16, 2011

3. Three Types of Plate Boundary Transform Fault lateral motion Spreading Center divergent motion Subduction Zone convergent motion

4. Present-Day Mosaic of Plates

5. Active Faulting in California Direction of Pacific Plate motion

6. San Andreas Fault System

7. San Andreas System

8. Pacific - North America Plate Boundary

11. Significant Earthquakes in Southern California during the 20 th Century

12. Los Angeles Region

13. San Andreas fault Los Angeles Region

14. San Andreas fault Sierra Madre fault Whittier fault Newport-Inglewood fault Palos Verdes fault Hollywood-Santa Monica-Malibu Coast fault Santa Ynez fault Raymond fault Los Angeles Region

16. Puente Hills “Blind” Thrust Fault

18. What causes earthquakes? Sudden slip on a fault that has reached its breaking strength (“tectonic” earthquake)

19. 1906 M 7.9 1857 1680 M 7.7 On average, large earthquakes recur on the San Andreas fault about every 100-150 years Earthquakes on the San Andreas Fault Pacific plate motion relative to the North American plate 50 mm/yr

20. San Andreas Fault Offset by 130 m in 3700 years 130 m / 3700 yr = 35 m/kyr 5 m of slip per eqk implies ~ 7 eqk/kyr or, on average, ~ 1 eqk every 140 yr

21. San Andreas Fault Fence built across San Andreas faults near Bolinas, California, was offset by 3 m 1906 San Francisco Earthquake

22. Strike-slip fault TIME 1 A farmer builds a stone wall across a strike-slip fault. ROCKS DEFORM ELASTICALLY, THEN REBOUND DURING AN EARTHQUAKE RUPTURE

23. 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.

24. Rocks deform as strain develops Strike-slip fault ROCKS DEFORM ELASTICALLY, THEN REBOUND DURING AN EARTHQUAKE RUPTURE Focus Epicenter 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. TIME 3 A new fence is built across the already- deformed land.

25. 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.

26. fault displacement time Yield stress time stress Reid’s (1910) Elastic Rebound Theory Base stress fault trace  Map view Recurrence Interval

27. Reid’s (1910) Elastic Rebound Theory What happens during the earthquake? ~ 150 years

28. 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 Rupture expansion during a large (M7) earthquake

32. Foreshocks and Aftershocks

33. Aftershocks of 27 Feb 2010 Chile Earthquake (M8.8) 500 km

34. Two Ways to Measure Earthquake Size MagnitudeMagnitude –Measures the size of the rupture on a fault (e.g., on the San Andreas fault) IntensityIntensity –Measures the size of the ground shaking at a particular site (e.g., here in this classroom)

35. Earthquake Magnitude For each increase of 1 unit in magnitude: –Energy increases by a factor of 33 –Fault area increases by a factor of 10 –Fault slip increases by a factor of 3.3

36. 30 km x 20 km = 600 km 2

37. Aftershocks of 27 Feb 2010 Chile Earthquake (M8.8) 500 km 600 km x 100 km = 60,000 km 2

38. Earthquake Magnitude For each increase of 1 unit in magnitude: –Energy increases by a factor of 33 –Fault area increases by a factor of 10 –Fault slip increases by a factor of 3.3

39. Frequency-Magnitude Statistics An increase of one magnitude unit corresponds to an order of magnitude decrease in the number of earthquakes.

40. Shaking Intensity

41. Isoseismic (“equal shaking”) map for the Northridge earthquake of January 17, 1994 (M 6.7)

42. Shaking Intensity Isoseismic (“equal shaking”) map for the great San Francisco earthquake of April 18, 1906 (M7.8)

43. Shaking Intensity 500 km M 7.6M 7.9

44. End Tutorial #3