The researchers will use the locations of secondary ocean microseisms noise to improve the temporal resolution of changes in crustal velocity, and to map the locations of coastal reflections of primary ocean waves. The proposed microseisms source location technique is based on an algorithm developed for real-time earthquake locations and is suitable for areas like Southern California where multiple simultaneous sources are located within a few hundred km offshore and where there is a well distributed seismic array on land. The use of source locations overcomes a problem with standard correlation methods that require a random distribution of sources to achieve precise measurements, which translates to long correlation times and hence a reduced temporal resolution. Using a set of source locations, a shorter synthetic seismogram can be used in the correlation. The coastal reflection points will be determined by forming an adjoint problem with the hypothesized reflection source and an incident primary wavefield determined from high resolution Wave Action Models (WAM's).
The source locations procedure is demonstrated with both a synthetic and real data examples. The results show that there are often many source points active at any given time. They provide two independent approaches: (1) Synthetic Aperture Radar (SAR) measurements of the intensity of wave-wave interactions at the source regions ? a necessary condition for microseisms generation. A new airborne system that will be flown in June 2008 over the offshore S. California to look for opposing ocean wavefields which are indicative of the source points. (2) A comparison between the amplitudes of the observed micoseisms and estimated amplitudes calculated using the source locations and the theory of ocean microseisms.
The nature of the coastal reflections is currently poorly understood. This research will provide the opportunity to develop a model for this process and to ultimately to generate a description of the complete wave-field that can be incorporated into the next generation of Wave Action Models. Microseisms provide a means to continuously monitor the Rayleigh wave speed across the Los Angeles basins as a potential proxy for stress changes. The use of sources in this procedure increases the temporal resolutions and removes the problem of systematic bias due to the assumption of random source locations.
This research bridges three communities: radar interferometry, oceanography and seismology and hence has the potential to spur research in all these areas. Determining the mechanisms for coastal reflections will significantly enhance the community wave action models. The application to southern California can be extended to several other areas including northern California, Cascadia, and Japan. The procedure established by this research could become part of the standard seismic monitoring of Southern California
