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Offshore Pacific-North America lithospheric structure and Tohoku tsunami observations from a southern California ocean bottom seismometer experiment

Monica D. Kohler

Published September 20, 2016, SCEC Contribution #6399, 2016 SCEC Annual Meeting Talk on 9/13 11:00 (PDF)

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The motivation for the offshore ALBACORE seismic experiment was to identify the physical properties and deformation styles of the western half of the Pacific-North America plate boundary in southern California. An array of 34 ocean bottom seismometers (OBSs) was deployed in 2010-2011, extending from the eastern California Borderland into Pacific oceanic plate 300 km west of the Patton Escarpment. The ALBACORE OBSs, together with 65 stations of the onshore Southern California Seismic Network, were used to measure ambient noise correlation functions and Rayleigh-wave dispersion curves which were inverted for 3D shear-wave velocities. The resulting shear-wave velocity model illustrates plate boundary deformation including both thickening and thinning of the crustal and mantle lithosphere within the California Borderland and at the westernmost edge of the North American continent. The velocity model defines the transition from continental lithosphere to oceanic tectonic environment, and indicates the persistence of uppermost mantle volcanic processes associated with East Pacific Rise spreading adjacent to the Patton Escarpment. One of the most prominent of these seismic structures is a low-velocity anomaly underlying San Juan Seamount, suggesting ponding of magma at the base of the crust, resulting in thickening and ongoing adjustment of the lithosphere due to the localized loading. Complementary to this, mapping of two active transpressional fault zones in the California Borderland - the Santa Cruz-Catalina Ridge fault and the Ferrelo fault - was carried out to characterize their geometries using over 4500 line-km of new multibeam bathymetry data recorded during the OBS deployment cruise, combined with existing data. The geometry of the fault systems shows evidence of multiple segments that could experience through-going rupture over distances exceeding 100 km. Transpression on west- and northwest-trending structures persists as far as 270 km south of the Transverse Ranges, extension persists in the southern Borderland, and these faults show potential for dip-slip rupture.

The 2011 Tohoku tsunami was serendipitously recorded by the ALBACORE seafloor co-located pressure gauges, demonstrating how dense array data can illustrate and validate predictions of linear tsunami wave propagation characteristics. Phase and group travel times were measured for the first arrival in the pressure gauge tsunami waveforms filtered in narrow period bands between 200 and 3000 s. For each period, phase velocities were estimated across the pressure gauge array based on the phase travel-time gradient using eikonal tomography. Clear correlation is observed between the phase velocity and long-wavelength bathymetry variations. In the deep open ocean, phase velocity dispersion is observed for both short and long periods. The pressure gauge tsunami records across the entire array show multiple, large-amplitude, coherent phases arriving one hour to more than 36 hours after the initial tsunami phase. Beamforming and back-projection analysis of the tsunami waveforms reveals locations of the bathymetric features in the Pacific Ocean that contributed to the scattered, secondary tsunami arrivals in southern California.

Key Words
Borderland, shear-wave velocity model, tsunami

Kohler, M. D. (2016, 09). Offshore Pacific-North America lithospheric structure and Tohoku tsunami observations from a southern California ocean bottom seismometer experiment. Oral Presentation at 2016 SCEC Annual Meeting.

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