1. Earthquake Hazards in Western Washington The mega-thrust at the coast Shallow earthquakes in the crust of Puget Sound Deep earthquakes in the subducted Juan de Fuca Plate Which should you worry about?

2. Three kinds of earthquakes: Plate Interface (megathrust, Cascadia, 1700) Intraslab (deep, Wadati-Benioff, Nisqually 2001) North American Plate (Shallow, crustal earthquakes, Seattle, 900)

4. Deep Earthquakes Wadati-Benioff zone earthquakes Intraslab earthquakes Occur inside the cold subducting lithosphere Occur to depths of nearly 700 km in many subduction zones where the oceanic plate is old and cold In Cascadia, the subducting Juan de Fuca plate is young and warm; deepest earthquakes are only 100 km

5. Earthquake Hazards in Western Washington The mega-thrust at the coast Shallow thrusts in the crust of Puget Sound Normal faulting in the subducted Juan de Fuca Plate There have been 3 events in the last 60 years: 1949, 1965 and 2001. There WILL be more in YOUR lifetime. The last one caused $1,000,000,000 in losses that were largely preventable.

6. The Nisqually Earthquake February 28, 2001 10:54:33 AM PST Magnitude 6.8

16. Intraslab Seismicity Largest intraslab earthquakes are in south sound region at base of microseismicity, including 1949 M7.1; 1965 M 6.5 and 2001 M6.8 Deepest earthquakes are downstream from large events Intraslab seismicity is virtually absent north and south of arch Intraslab focal mechanisms are widely scattered but generally are in-plane tension Should we prepare for M7 or M8 intraslab earthquakes?

18. SHOW WEB ANIMATION!

24. Discussion

25. The largest intraslab earthquakes tend to occur at the base of the seismic zone.

26. 3-D Tomographic Inversion Active-Source Data ~92,000 first arrivals from active source experiments (e.g. SHIPS) ~92,000 first arrivals from active source experiments (e.g. SHIPS) 1200 wide-angle reflection times from SHIPS 1200 wide-angle reflection times from SHIPS

27. Previous Tomography (Preston et al. 2003)

29. Complication #2: Gouge zones

30. Fault zones and Faults The ideal fault: San Gabriel Fault Photo Matt D’Allessio

31. You are here Press, 20-12 Pacific and North American plates are main players

32. USGS Prof. Paper, 1-2 North America map Show Movie #01

33. Subduction under Alaska & Aleutian Is. 8 cm/yr of plate convergence –almost strike-slip in Aleutian Is. Site of large underthrusting earthquakes –1964 Alaska Earthquake M W =9.2 Volcanic arc & eruptions

34. Not subduction! Embarrassing. (UT Austin) Strike-slip Alaska - Pacific boundary

35. Queen Charlotte Fault System

36. Alaska Most dangerous faults in US –8 quakes over M=8 in last 100 years Sparsely populated Main fault is subduction thrust –Only surfaces on ocean floor –Many secondary faults Also has volcanoes, tsunamis

37. 1899-1979 Alaska quakes Stars - M > 7.5 Triangles - 6.5 to 7.5 Yanev, p. 175 6.7 & 8 quakes in 2002

38. Tectonic Setting of N. America Old stable interior East coast “passive margin” West coast “active margin” –Pacific - N. Am. plate interactions Transform motion at SAF Subduction under Alaska & Aleutian Is. –Juan de Fuca - N. Am. interactions Subduction under Northern California, Oregon, Washington, and British Columbia

39. US Geology

40. US Topography Topographic action mark tectonic activity

41. Press, 20-6c Continental collision is cause of Appalachian Mts

42. Now Passive margin US Europe Rifting

43. USA seismicity Notice that all the action is in the West –most dramatic topography in the West Yosemite, Cascade Mts., Big Sur coast, etc. –few earthquakes in the East Some faults as far east as Yellowstone No action to west in Pacific plate either –Hawaii is special case - hot spot volcano

44. US Seismicity

45. Geology of western US - Fabric related to deformation

46. Western boundary of North American Plate Type of boundary depends on orientation Plate boundaries move and change in time Past 30 My saw major changes & development of San Andreas Fault From 80 to 30 My Farallon plate subducted under west coast. –Juan de Fuca & Cocos plates are remnants of Farallon plate

47. Cartoon of Western N. America tectonics Vidale

48. Details: US and Mexico coast Three little plates subducting offshore Oregon, Washington, and B. Columbia –Juan de Fuca Plate –Gorda Plate –Explorer Plate Spreading ridge splitting Gulf of California –Separating Baja from N. America –Oblique because ridges are combined with transform faults Cocos Plate subducting to the south

49. Exotic terrain placement Accretion of fragments to continent

50. Exotic terrains Western N. Am (Cordillera) assembled from small pieces over past 200 My

51. Changes in west coast A mid-ocean ridge subducted –Before that, just subduction on coast –First hit near LA 25 Mya –San Andreas fault system started then As ridge is subducting, two triple junctions are moving apart on coast –Mendocino Triple Junction moving north NA-Pacific-Juan de Fuca junction –NA-Pacific-Rivera junction off Baja California

53. Note present San Andreas fault Show movie #02

54. Oregon-Washington M 9 every 1000 years, last in 1700 Recent quakes –M 7.0 in 2000, 6.5 in 1965 in Seattle –M 7.1 in 1949 in Olympia Main fault is subduction zone Also volcanoes (like Mt. St. Helens) Not adequately prepared We’ve looked at these faults before –But biggest surface offshore

55. Cascadia subduction zone 4 cm/yr convergence rate

56. Pacific - Juan de Fuca spreading Subduction of J. de F. under N. Am. Iacopi, 27 Note: Plate motions Triple Junctions

57. Cartoon view Show movie #09

58. Also smaller faults in Seattle

59. Oregon- Wash. Close-up Yeats, 5-2 Note: Volcanoes San Andreas Triple Junctions Mt. Hood From Portland

60. Mt Rainier and Seattle

61. Subduction under Pacific Northwest 4 cm/yr of plate convergence –like small convection cell - up at ridge down at subduction zone Site of great underthrusting earthquakes –None in historic record –But evidence for magnitude 9 quake in 1700 Tsunami sands in buried marsh Tsunami in Japan in 1700 Volcanic arc & eruptions –Mt. St. Helens, Mt. Rainier, Mt. Shasta

62. Oregon-Washington X-sect Cascadia cross-section Keller, 7-5 Ridge Olympic Mts Cascade Mts

63. Tsunami sand associated with the 1700 earthquake covering a Native fire pit on the banks of the Salmon River in Oregon. Fire-cracked rocks, charcoal and ashes delineate the fire pit that was dug into the sandy beach.

64. Last 30 Mya of west coast tectonics Cape Mend.

67. ETS Every 15 months Creager & Wech * BS Array

68. Tremor Migration for four ETS events Wech, Creager and Melbourne, JGR, submitted

70. Tremor and Slip summed over 4 ETS events Wech, Creager and Melbourne, JGR, submitted

71. Episodic Tremor and Slip (McCausland et al., GRL 2005)

72. Cascadia