Ambient Near-Coastal Seismic Noise and Temporal Monitoring

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The researchers will use the locations of secondary ocean microseisms noise to improve thetemporal resolution of changes in crustal velocity, and to map the locations of coastalreflections of primary ocean waves. The proposed microseisms source location techniqueis based on an algorithm developed for real-time earthquake locations and is suitable forareas like Southern California where multiple simultaneous sources are located within afew hundred km offshore and where there is a well distributed seismic array on land. Theuse of source locations overcomes a problem with standard correlation methods thatrequire a random distribution of sources to achieve precise measurements, whichtranslates to long correlation times and hence a reduced temporal resolution. Using a setof source locations, a shorter synthetic seismogram can be used in the correlation. Thecoastal reflection points will be determined by forming an adjoint problem with thehypothesized reflection source and an incident primary wavefield determined from high resolutionWave Action Models (WAM's).The source locations procedure is demonstrated with both a synthetic and real dataexamples. The results show that there are often many source points active at any giventime. They provide two independent approaches: (1)Synthetic Aperture Radar (SAR) measurements of the intensity of wave-waveinteractions at the source regions ? a necessary condition for microseisms generation. Anew airborne system that will be flown in June 2008 over the offshore S. California tolook for opposing ocean wavefields which are indicative of the source points. (2) Acomparison between the amplitudes of the observed micoseisms and estimatedamplitudes calculated using the source locations and the theory of ocean microseisms.The nature of thecoastal reflections is currently poorly understood. This research will provide theopportunity to develop a model for this process and to ultimately to generate a descriptionof the complete wave-field that can be incorporated into the next generation of WaveAction Models. Microseisms provide a means to continuously monitor the Rayleighwave speed across the Los Angeles basins as a potential proxy for stress changes. Theuse of sources in this procedure increases the temporal resolutions and removes theproblem 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 theseareas. Determining the mechanisms for coastal reflections will significantly enhance thecommunity wave action models. The application to southern California can be extendedto several other areas including northern California, Cascadia, and Japan. The procedureestablished by this research could become part of the standard seismic monitoring ofSouthern California