HR: 0830h
AN: S21D-0337    [PDF]
TI: Three-Dimensional Source-Scanning Algorithm: Imaging Seismic Sources of Earthquake or Non-earthquake Origins
AU: Shan, S
EM: sjshan@shaw.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Road, Sidney, BC V8L4B2 Canada
AU: * Kao, H
EM: hkao@nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Road, Sidney, BC V8L4B2 Canada
AB: A scanning algorithm is developed to image the seismic source of earthquake or non-earthquake origin in a 3-D heterogeneous media. For a given time segment, the scanning process first calculates the theoretical arrival times from one grid point inside the velocity model to all seismic stations. This is efficiently done by retrieving the pre-calculated travel time tables stored online. The corresponding amplitudes at all stations are stacked to give the "brightness" of that point, and this calculation is systematically repeated for all grid points to image the snapshot of seismic sources in the region at that particular time. The scanning then proceeds to the next time segment. In reality, we conduct a preliminary scan at a time interval of 5 s with a spatial resolution of 1 km to identify coherent seismic events. The scanning is subsequently done at shorter time intervals (down to 0.1 s) to locate the brightest spot. The two biggest advantages of this algorithm are (1) it is completely free from human picking errors and thus is perfect for automatic analysis, and (2) it maps out the geometric configuration of the finite source rather than just outputs a single point in the model space. Control tests using synthetic waveform data indicate that this algorithm is robust even under a high level of background noise. We apply this algorithm to a local earthquake (Mw 4.3) in the northern Cascadia subduction zone. The scanned image shows a vertical rupture plane striking E-W, consistent with one of the nodal planes determined from moment-tensor inversion of regional broadband waveforms. The point with the maximum brightness is remarkably close to the hypocenters determined by conventional earthquake location methods. Application of this algorithm to the tremor activity of the latest Episodic Tremor and Slip (ETS) event beneath the southern Vancouver Island indicates that the ETS tremors have east-dipping sources located in the vicinity of the inferred plate interface.
DE: 7215 Earthquake parameters
DE: 7230 Seismicity and seismotectonics
DE: 7294 Instruments and techniques
SC: Seismology [S]
MN: 2003 Fall Meeting