As a means of testing the effect of secondary arrivals on regional locations, we selected events associated with and surrounding the 1997 November 8 Tibet Mw=7.5 mainshock. This event produced a significant surface rupture (top image) and has a strike-slip mechanism [Velasco et al., 2000]. The lower image shows all historical events near the mainshock as listed in the EDR catalog. We analyzed arrivals from regional stations (Introduction) for all of the events in order to determine if any existing S phases were not listed in the EDR catalog. We were able to confidently pick 98 new S and Sn phases to add to the EDR data set.
To test the possible accuracy of relocations (assuming the P and S velocity models are known), we performed a series of synthetic tests (see Synthetic Tests, parts 1 and 2). With synthetic tests, we can determine the optimal parameters for locating regional events using secondary phases and apply them to actual data (Relocations). For all locations, we used the EvLoc location algorithm [Bratt and Bache, 1988; Nagy, 1996].
|InSAR View of Surface Rupture from Main Shock -
Can also see slight rupture to the south, denoted as secondary rupture.
Interferometric map showing the coseismic surface displacement e ld.One full-color cycle (blue-red-yellow-blue) represents 50 cm of ground shift away from satellite along the radar line of sight. Uncolored areas are zones of low phase coherence that have been masked before phase unwrapping. Small color discontinuities observed at frame boundaries are due to differences of incidence angles between adjacent tracks in overlapping regions.
Figure from Peltzer, et. al.
|Catalog and Synthetic Locations
Historical events (EDR catalog) surrounding the mainshock and surface rupture. From these events we selected 14 (Mb4.5) along with their station/arrival pairs to create synthetic arrival data (Synthetic Tests). The surface rupture acts as a ground-truth for the events.
Catalog Locations with Secondary Phases |
Ground Truth Constraints on Relocations Near a Large-Magnitude Event
Synthetic Tests Pt. 1 | Synthetic Tests Pt. 2 | Relocations of Actual Data | Relocations Using S Phases for the Asia Study Area | Summary