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Development of sensors and techniques to assess earthquake hazards and submarine slope stability

Abstract

Reducing vulnerability from geohazards such as submarine landslides and earthquakes requires identifying susceptible regions and modeling the consequences. We introduce innovative instruments and techniques that have the potential to advance preparedness and mitigation efforts. We develop optical fiber strainmeters to monitor deformation along unstable slopes and in a vertical borehole at the SAFOD observatory. With this latter strainmeter, we record coseismic strain-steps from local microearthquakes in addition to teleseismic events, which we compare to accelerations from a nearby seismometer to derive local phase velocities. We also study a seafloor crack within the Santa Barbara Basin that might be the beginning of an imminent submarine landslide. We deployed a new seafloor acoustic ranging system which detected no motion across the crack above a 99% confidence level of ± 7 mm/yr over two years of monitoring. Combined with sub- bottom CHIRP profiles with < 1 m accuracy exhibiting no evidence of internal deformation, we conclude that the elongated scarp-like crack is most likely a relict feature from a previous failure. Probabilistic seismic hazard analysis suggests that an ̃M̃ <̲ 7 earthquake is required within the basin to explain a previous landslide, arguing against a recent relocation of the 21 Dec. 1812 earthquake (M7.1) to the San Andreas Fault. Finally, we study the anomalous Ms 5.3 earthquake of 10 Feb. 2006 in the Gulf of Mexico. Surface wave full-waveform inversion suggested a source of either a shallow landslide translating on a near sub-horizontal surface, or sub-horizontal or vertical faulting within shallow, low velocity sediments. We integrated the results of two industrial seismic exploration surveys to relocate the epicenter. The geology around the relocation is consistent with the sliding source model, and geotechnical modeling suggests that the sharp relief is capable of producing a large landslide. To test the landslide hypothesis, we surveyed the region acquiring multibeam, sidescan, and sub-bottom seismic profiles. The results of that survey indicate no evidence of a large debris flow. This suggests a source mechanism within the shallow sedimentary section on either a near sub-horizontal or near vertical plane

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