Rupture Directivity of Moderate Earthquakes in Northern California

Abstract

We invert peak ground velocity and acceleration (PGV and PGA) to estimate rupture direction and rupture velocity for 47 moderate earthquakes (3:5 ≥ M ≥ 5:4) in northern California. We correct sets of PGAs and PGVs recorded at stations less than 55-125 km, depending on source depth, for site amplification and sourcereceiver distance, then fit the residual peak motions to the unilateral directivity function of Ben-Menahem (1961). We independently invert PGA and PGV. The rupture direction can be determined using as few as seven peak motions if the station distribution is sufficient. The rupture velocity is unstable, however, if there are no takeoff angles within 30°of the rupture direction. Rupture velocities are generally subsonic (0:5β-0:9β); for stability, we limit the rupture velocity at v 0:92β, the Rayleigh wave speed. For 73 of 94 inversions, the rupture direction clearly identifies one of the nodal planes as the fault plane. The 35 strike-slip earthquakes have rupture directions that range from nearly horizontal (6 events) to directly updip (5 events); the other 24 rupture partly along strike and partly updip. Two strike-slip earthquakes rupture updip in one inversion and downdip in the other. All but 1 of the 11 thrust earthquakes rupture predominantly updip. We compare the rupture directions for 10 M ≥ 4:0 earthquakes to the relative location of the mainshock and the first two weeks of aftershocks. Spatial distributions of 8 of 10 aftershock sequences agree well with the rupture directivity calculated for the mainshock.

FinDer v.2: Improved real-time ground-motion predictions for M2–M9 with seismic finite-source characterization

“…We expect that this is true for the majority of events. This observation would be also consistent with earlier studies (Boatwright 2007;Seekins & Boatwright 2012) that find evidence for rupture directivity in small-to moderate-sized earthquakes (3.5 ≥ M ≥ 5.4). We expect that resolving fault-plane ambiguities of small to large earthquakes within a few seconds could be extremely useful to constrain the faults along which rupture is occurring.…”

Rupture process for micro-earthquakes inferred from borehole seismic recordings

“…Nine and eight MRFs were used to determine the slip distributions of the Berkeley and El Cerrito earthquakes, respectively. Boatwright (2007) and Seekins and Boatwright (2010) also reported on rupture directivity using the peak ground velocity/acceleration measurements. The rupture directivities for the Berkeley and El Cerrito earthquakes inferred from the finite-source modeling are consistent with those reported by U.S. Geological Survey from the peak ground velocity observations (e.g., http://earthquake.usgs.gov/regional/nca/rupture/?id=20111020-1441 and http://earthquake.usgs.gov/regional/nca/rupture/?id= 20120305-0533).…”

Pore pressure sensitivities to dynamic strains: Observations in active tectonic regions

“…Because of data gaps, inadequate sampling rates, and other factors, the number of events with high-frequency pore pressure records at particular boreholes ranges from 12 to 36, and in one case (station B001), only a single event was recorded. I also included two events that do not follow the magnitude-distance relationship in equation (1): (1) the 2011 M W 4.5 San Juan Bautista (SJB) earthquake in Northern California, to examine the potential for directivity effects at local distances [Seekins and Boatwright, 2010], and (2) the teleseismic 2011 M W 9 Tohoku-oki earthquake, offshore of Japan [Simons et al, 2011], because the associated Rayleigh waves are approximately harmonic at these epicentral distances and have strongly polarized strain components that can be used in cross-spectrum analyses (described later). (1)), which I require to be valid for at least one station.…”