HR: 0800h
AN: S11B-1015    [Abstracts]
TI: Relative Earthquake Location Using Surface Waves
AU: * Ammon, C J
EM: cammon@geosc.psu.edu
AF: Penn State, 440 Deike Building, University Park, PA 16870 United States
AB: Earthquake locations are fundamental parameters necessary for the in situ study of earthquake physics and faulting and are needed to map and to quantify Earth's deformation. Within dense continental seismic networks, earthquakes can be accurately and precisely located. However, for many important tectonic environments, existing catalog data are neither accurate nor precise. In particular, for isolated continental and off-shore areas, earthquake locations are estimated primarily using distant observations, which limits our resolution of hypocentral parameters. Surface waves are generally viewed as a complicated component of the seismic wave field. One can think of a number of reasons why using surface-wave phase shifts in event location might be complicated - most notable are waveform depth dependence and azimuthal variations in phase from faulting geometry changes. However, these problems are smaller than intuition may suggest, and phase shifts can be measured with better precision than one might think. Further, the large and relatively slowly varying horizontal slowness of the Rayleigh waves can produce accurate relative event epicentroid (epicenter of the event centroid) locations for events with similar faulting parameters. Using longer period observations (25-75 seconds) allows a stable linkage of events 10's of km apart. I illustrate these ideas with several examples, and show the value of using surface-waves to estimate relative earthquake epicentroid locations in remote areas. Specifically, I use double-difference phase-shift measurements from intermediate-period Rayleigh waveforms to constrain the relative locations of earthquake epeicentroids. I apply the method to about 40 strike-slip earthquakes in the vicinity of the Panama Fracture Zone. The resulting relocations more closely mimic the plate-boundary geometry than the initial NEIC locations and the mean phase shift residual is just above one second. Distances between the initial and final locations in preliminary inversions ranged from about 10 to 50 km. The median distance shift was 23 km. Centroid time shifts ranged from 0 to 4 seconds. The method provides an important tool for investigating spatial seismicity patterns associated with large events and their aftershocks, slow-slip events and background seismicity, and other important seismic processes operating in complex tectonic environments.
DE: 7230 Seismicity and seismotectonics
DE: 7255 Surface waves and free oscillations
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting