Utilizing Results from InSAR to Develop Seismic Location Benchmarks and Implications for Seismic Source Studies

Search for Secondary Rupture Event

As the first step in determining the candidate events for the secondary rupture, we determined which of the events from the EDR catalog were within the time period for the InSAR satellite pass and after the mainshock (08 NOV1997 - 02 DEC 1997) (Peltzer et al., 1999). 42 were possible candidates for the secondary rupture (Figure 1).
  • Placed the 42 candidate events on the secondary rupture, fixed the location and depth (3 km), and relocated using the IASP91 velocity model. We chose a value of 3.0 s for SDOBS as a first cut for the 42 candidate events, leaving 11 events for further analysis.
  • Any events with less than five regional defining stations before relocation and less than 50% of phases with absolute residuals below 4.0 s were discarded.
  • Limited the remaining events to those with a relatively large percentage of manually repicked phases, leaving six candidate events. Discarded events that were obviously at too great a distance to be associated with the secondary rupture, which left three events (Figure 1).
  • Estimated the size of the event that caused the secondary rupture to be between Mw = 4.8-5.1, using the equation for moment magnitude (Lay and Wallace, 1995).
  • The secondary rupture is ~5 km in length and has a displacement measured at least 0.1 m. The magnitude range was determined using a range of rupture depths from 1-5 km. After analyzing the waveforms of the three candidate events, we were able to discard one event based on the low SNR of the first arrivals at relatively near stations (MAKZ and LSA).
    In order to investigate the two remaining candidate events (Figure 1), we analyzed waveforms for the first P arrivals and the Rayleigh waves for station MAKZ (Figure 4). Separation between events suggested by Rayleigh waves indicates that neither event could be associated with the secondary rupture.
  • Also used several different iterations of the JHD approach to attempt to identify secondary rupture event. Results were inconclusive.

    Introduction | Surface Rupture | Data and Velocity Models | Synthetic Tests and Relocation Procedure | Search for Secondary Rupture Event | Relocation Results | Waveforms for Candidate Events | Relocation of Candidate Events | Conclusions and Recommendations | References