Utilizing Results from InSAR to Develop Seismic Location Benchmarks and Implications for Seismic Source Studies
Michael L. Begnaud, Aaron A. Velasco, Lee K. Steck
Los Alamos Seismic Research Center, Los Alamos National Laboratory
- The verifiability of ground-based nuclear explosion monitoring (GNEM) depends on the ability to effectively monitor small magnitude (< 4) events, with an implicit goal of locating seismic events within a contiguous area smaller than 1000 km2.
- Most relocation procedures suffer from inadequate testing against accurate information about a seismic event. Nuclear tests, mining explosions, or specific calibration explosions have all been used to assess event locations. Deployments of seismic instruments to record and locate aftershock sequences can also give a level of accuracy much greater than sparse global networks of stations (Myers and Schultz, 2000).
- A large (Mw = 7.5) earthquake occurred in a remote northern region of the Tibetan plateau. The event was a vertical strike slip event and appeared anomalous in nature due to the lack of large aftershocks (Velasco et al., 2000).
- The event had an associated surface rupture that was identified and modeled using Interferometric Synthetic Aperture Radar (InSAR) (Peltzer et al., 1999).
- InSAR can provide independent and accurate information (ground truth) regarding ground surface deformation and/or rupture. The Tibetan mainshock created a surface rupture of over 180 km in length (Figure 1), and occurred on a previously mapped fault that was determined using SPOT imagery (Tapponnier and Molnar, 1977).
- The east-west orientation of the fault rupture provides excellent ground truth for latitude, but is of limited use for longitude. However, a secondary rupture, which occurred 50 km south of the mainshock rupture trace (Peltzer et al., 1999), can provide ground truth with accuracy within 5 km.
- Relating surface deformation to seismic events is trivial when events are large and create significant surface rupture, such as the mainshock. However, the smaller secondary rupture presents a challenge.
- This 5-km-long rupture is believed to have a thrust mechanism (Peltzer, G., personal communication), which implies that the earthquake/ aftershock that generated it may be near but not necessarily on the surface trace of the fault.
- Identifying the secondary event within the aftershock sequence would provide a small ground-truth event, a valuable calibration point in this region.
- Can also use the event to validate the regional location methods, velocity models, and empirical calibration approaches being used for GNEM R&E. InSAR is providing unique insight for anelasticity in the crust, and co-locating an event may reveal unique insight into the rupture.
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 |
Please direct comments and questions to the principal author.