In-situ Seafloor Sediment Characterization From Background Noise and Earthquakes Recorded in the Gulf of California

We present an in-situ evaluation of the response of the Gulf of California (GoC) seafloor sediments to passive dynamic loads. Horizontal-to-vertical (H/V) spectral ratios are used to characterize the local seafloor sediment response in terms of the distribution of ground motions with their respective resonant frequencies. Ambient noise, local and regional earthquakes are used as passive dynamic loads. An analysis and description of the GoC seafloor background noise is also presented. The data was recorded by fourteen stations with three-component broadband sensors and a differential pressure gauge (Ocean Bottom Seismograph, OBS) of the Sea of Cortez Ocean-Bottom Array (SCoOBA) seismic experiment. Earthquakes and other natural seismic signals were recorded for nearly 12 months between October 2005 and October 2006. High frequency local events with impulsive onset arrivals as well as emergent onset arrivals are frequent on all stations. Earthquakes from regional and teleseismic distances are abundant and well defined on several stations simultaneously. The background noise clearly shows changes of the noise levels at intervals of approximately 6 hours in long period signals. Two sub-arrays were centered within the Alarcon and Guaymas basins, ~20 km spacing, and four additional instruments were deployed at ~100 km spacing. The array successfully recorded micro- seismic activity, dozens of local events, M > 3.5, and two large (M ~ 6) events with numerous foreshocks and aftershocks. The H/V spectral ratio technique offers a fast and inexpensive means to obtain information of the preferential vibration modes of soft sediment systems. This information is useful for the design of marine structures because makes use of background noise and earthquake signals. The main advantage is that there is no need for active sources to conduct the study. The method is also well suited for modeling shallow sediments, which usually cover most of the seafloor. H/V spectral ratios were computed for all signals and compared between each other to characterize the effect of the seafloor marine sediments on the seismic records.