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


Synthetic Tests and Relocation Procedure

  • Perform synthetic tests to assess whether we have adequate azimuthal station coverage to obtain precise locations on the fault.
  • Select several events of various magnitudes and place them on the main and secondary ruptures to generate synthetic arrival data. Add gaussian noise (dependent on estimated pick and model error) to the picks, and perform the relocations.
  • Perform this set of inversions ten times, with each run generating gaussian noise for the picks (Figure 2). Tests how well events could be constrained on the ruptures given normal arrival pick errors, with the velocity model known.
  • Fix depth to assess the lateral resolution. Calculate the average mislocation to be ~5 km.
  • Discovered that error ellipse estimates appear to be slightly low, as two of the ten events lie outside of the 90% error ellipse.
  • Attempted to include S phases as a means of improving the relocations for all the events shown in Figure 1 that had S-wave picks (Figure 3).
  • PPCs were used for P phases in the locations (no PPCs for S phases calculated).
  • When including the S phases, many events could not be relocated due to residual restrictions (Figure 3 left).
  • Mainshock location displays the southern bias observed in the catalog location.
  • When S phases are excluded (Figure 3 right), the mainshock is located on the main surface rupture.
  • Decided not to use S phases for relocating any of the candidate secondary rupture events.
  • Figure 2 - Relocations and 90% error ellipses for synthetic data. Data generated for select events around and including the main Manyi event on 08NOV1997. Selected events were placed on the main and secondary ruptures (black dots) to generate synthetic arrival data with added gaussian noise. This was done to test how well events could be constrained on the ruptures given normal arrival pick errors, with the velocity model known. Depth fixed at 33 km. Inset: Average distance from actual synthetic origin for ten synthetic location runs.
    Figure 3 - Relocations and error ellipses for EDR catalog locations around the 08NOV1997 Tibet mainshock only for those events which had S phases. 2-D travel-time corrections were used. Catalog locations are shown as black dots. a) S phases included in the relocation. Using S phases in the relocations reintroduces the southern bias in the mainshock (yellow) previously observed using P phases only without 2-D travel-time corrections. Several events were not relocated due to residual restrictions. b) Relocations without including S phases. The mainshock relocates to the trace of the main rupture.

    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