Application of array-based early warning system to tsunami offshore Ventura, California

Yuqing Xie, & Lingsen Meng

Submitted August 8, 2017, SCEC Contribution #7445, 2017 SCEC Annual Meeting Poster #019

Extreme scenarios of M 7.5+ earthquakes on the Red Mountain and Pitas Point faults can potentially generate significant local tsunamis in southern California. The maximum water elevation could be as large as 10 m in the nearshore region of Oxnard and Santa Barbara. Recent development in seismic array processing enables rapid tsunami prediction and early warning based on the back-projection approach (BP). The idea is to estimate the rupture size by back-tracing the seismic body waves recorded by stations at local and regional distances. A simplified source model of uniform slip is constructed and used as an input for tsunami simulations that predict the tsunami wave height and arrival time. We demonstrate the feasibility of this approach in southern California by implementing it in a simulated real-time environment and applying to a hypothetical M 7.7 Dip-slip earthquake scenario on the Pitas Point fault. Synthetic seismograms are produced using the SCEC broadband platform based on the 3D SoCal community velocity model. We use S-wave instead of P-wave to avoid S-minus-P travel times shorter than rupture duration. Two clusters of strong-motion stations near Bakersfield and Palmdale are selected to determine the back-azimuth of the strongest high-frequency radiations (0.5-1.5 Hz). The back-azimuths of the two clusters are then intersected to locate the source positions. The rupture area is then approximated by enclosing these BP radiators with an ellipse or a polygon. Our preliminary results show that the extent of 1294 square kilometers rupture area and magnitude of 7.6 obtained by this approach is reasonably close to the 1849 square kilometers and 7.7 of the input model. The average slip of 7.3 meters is then estimated according to the scaling relation between slip and rupture area, which is close to the actual average dislocation amount, 8.3 meters. Finally, a tsunami simulation is conducted to assess the wave height and arrival time. The errors of -3 to +9 s in arrival time and 0.4 m in wave amplitude are reasonably small for early warning purpose. The blind zone for early warning is the region north of the outcrop of Pitas Point faults and has a scale close to the length of the fault, 43 km. The warning time is above 15 min in the nearshore region west of Cojo Bay Beach and south of Oxnard.

Key Words
tsunami, early warning, backprojection

Citation
Xie, Y., & Meng, L. (2017, 08). Application of array-based early warning system to tsunami offshore Ventura, California. Poster Presentation at 2017 SCEC Annual Meeting.


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