HR: 1340h
AN: S53A-0186    [Abstracts]
TI: Evidence of Two Source Mechanisms for Cascadia Intraslab Seismicity From an Anomalous Secondary P Arrival
AU: * Medema, G F
EM: guy@ess.washington.edu
AF: University of Washington Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310 United States
AU: Crosson, R S
EM: bob@ess.washington.edu
AF: University of Washington Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310 United States
AU: Creager, K C
EM: kcc@ess.washington.edu
AF: University of Washington Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310 United States
AB: A prominent secondary P arrival is observed at stations on the Olympic Peninsula from 13 intraslab earthquakes in the subducting Juan de Fuca (JDF) slab beneath western Washington. The events occur in two regions at a depth range of 45-55 km. Ten of the events locate beneath south central Puget Sound, and 3 are off the SE tip of Vancouver Island. These two regions are coincident with the flanks of the JDF arch structure. Intraslab events outside the 45-55 km depth range do not show the secondary phase, nor do events within this depth range located between these two regions. First arriving P at Olympic stations for all intraslab events have apparent velocities consistent with upper mantle paths, while the secondary P arrivals have apparent velocities of approximately 6.0 km/s, consistent with crustal paths. Careful comparison of slab arch models to hypocenters suggests that earthquakes with a secondary P originate above the slab Moho in the subducted oceanic crust, and those without the secondary phase are in the upper mantle of the JDF slab. Two independent source mechanisms are proposed for Cascadia intraslab seismicity with most of the earthquakes occurring in the slab upper mantle and relatively few in the subducted crust. The pattern of larger (M $>$ 2.5) events without secondary P displays a linear, NW trend. Magnetic anomaly patterns [Wilson, 2002] reveal an offset in the isochrons coincident with this linear distribution, suggesting a link between these slab upper mantle events and structure. The largest historic Cascadia intraslab events including the 1949 M7.1, 1965 M6.5 and 2001 M6.8 earthquakes occur in the south Puget Sound region where most of the events with the secondary phase are located. Trace data are not available at Olympic Peninsula stations for the two earlier events and is clipped for the 2001 main shock, so the existence of a secondary phase for these events is indeterminate. However, the two largest aftershocks from the 2001 event and all other well-recorded events in this region show the secondary phase and are therefore interpreted to be in the subducted oceanic crust. This suggests that the largest events may originate in the subducted oceanic crust.
DE: 7218 Lithosphere and upper mantle
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting