Finite fault modeling of oceanic strike-slip earthquakes

Abstract:

The depth extent of seismic rupture in oceanic lithosphere is thought to be limited by the 600º to 800ºC isotherm with the thermal structure generally characterized by a half-space cooling model. However, previous studies constraining this limit represent a limited number of individual faults and earthquakes and use a wide range of different methods. Observations of significantly deep slip at the 800ºC isotherm, supershear rupture velocities, and along-strike differences in seismic slip have been made for oceanic strike slip earthquakes (McGuire and Beroza, 2012; Yue et al., 2013; McGuire et al., 2012). To examine how seismic rupture is controlled, we look at a variety of earthquakes in different settings using the same method of finite fault modeling. We choose the largest and best recorded oceanic strike-slip earthquakes from tectonic settings of interplate transform, intraplate fracture zones, and strike-slip plate boundaries. These earthquakes are located in the Indian Ocean, near the South Sandwich Islands, on the edge of the Scotia Plate, off the coast of Alaska, and west of Australia, rupturing lithosphere with ages from 0 to 70 My. We first determine first motion and point source mechanisms from the first arriving P waves and later arriving pP, sP, and SH waves. Using the nodal planes of these mechanisms, we perform finite fault modeling at a range of constant rupture velocities and hypocenter depths. We determine which slip asperities are well-constrained by limiting the extent of the preferred model until the fit to the data is affected significantly. The rupture directivity, rupture speed, depth extent of slip, and along-strike distribution of slip is then compared between events to identify relationships to the tectonic setting, thermal structure inferred from lithospheric age, or other possible mechanisms for controlling slip. These strike-slip earthquakes also provide examples of rupture along a bimaterial fault plane, which have been shown to have a relationship between stress loading direction, rupture directivity, and rupture speed by previous studies. The results of this study will determine seismic slip distribution in the relatively simple structure of oceanic lithosphere, and provide a comparison for the more complex structure of continental strike-slip faults.