Source Parameters of the Shallow 2012 Brawley Earthquake, Imperial Valley

Abstract

Resolving earthquake parameters, especially depth, is difficult for events occurring within basins because of issues involved with separating source properties from propagational path effects. Here, we demonstrate some advantages of using a combination of teleseismic and regional waveform data to improve resolution following a bootstrapping approach. Local SS-S differential arrivals from a foreshock are used to determine a local layered model which can then be used to model teleseismic depth phases: pP, sP, and sS. Using the cut-and-paste (CAP) method for which all strike (θ), dip (δ), rake (λ), and depth variations are sampled for several crustal models. We find that regional data prove the most reliable at fixing the strike, whereas the depth is better constrained by teleseismic data. Weighted solutions indicate a nearly pure strike-slip mechanism (θ 59° 1°) with a centroid depth of about 4.0 km and an M w of 5.4 for the mainshock of the 2012 Brawley earthquake. Online Material:Figures showing examples of waveform fits, comparison of synthetic teleseismic waveforms using different grid sizes in the FD calculation, effect of soft sediment on teleseismic Green's functions, and sensitivity of regional and teleseismic data on the focal mechanisms.

Report on the August 2012 Brawley Earthquake Swarm in Imperial Valley, Southern California

“…The teleseismic centroids tend to be deeper, in part, because the depth resolution is less. Chu and Helmberger (2013) also determined a regional double-couple solution using the "cut-and-paste" method (Zhu and Helmberger, 1996) as well as a local velocity model appropriate for the Imperial Valley basin. They found that using a local velocity model, they needed a depth of ∼4 km for the centroid.…”

“…Late Quaternary fault traces (magenta) from Jennings and Bryant (2010) are also shown. 6.9 ‡ 227°82°SE 6°Focal mechanism from first motions and S= P ratios 5.4 5.5 238°87°NW −7°Regional cut-and-paste method (Chu and Helmberger, 2013) 5.4 4.0 239°90°1°Teleseismic cut-and-paste method (Chu and Helmberger, 2013) *NW, dips to the northwest; SE, dips to the southeast. † SCSN, Southern California Seismic Network; USGS, United States Geological Survey; CMT, Centroid Moment Tensor.…”

Lithospheric waveguide beneath the Midwestern United States; massive low-velocity zone in the lower crust

“…As in previous studies [e.g., Chu et al ., , ], we find the mechanisms reported by SLU catalog [ Herrmann et al ., ] to be quite accurate, but the depths can be refined by adding depth phases at teleseismic distances (e.g., pP and sS) and extended P nl at regional distances. But the relative delay of depth phases does depend on the crustal model especially for events situated in basins with thick sediments [ Chu and Helmberger , ]. We present a summary of the source modeling efforts for the Oklahoma event in Figure , assuming the Herrmann crustal model in Figure .…”

“…The depth estimates vary with models, where different crustal structures change the differential times between direct arrivals and their depth phases (pP, sP, and sS). For example, soft sediment structures can also cause uncertainties about earthquake depth as for the Brawley earthquake in the Imperial Valley, California [ Chu and Helmberger , ]. We have included results from our preferred model CRS in Figure d which increases the source depth slightly to 5.5 km.…”

A numerical study of strike‐slip bend formation with application to the Salton Sea pull‐apart basin

“…Here the relative Pacific‐North American plate motion is accommodated by the San Andreas Fault (SAF), the transform plate boundary, and numerous subparallel faults, including the Imperial Fault (IMF) and the San Jacinto Fault (SJF). Located at the stepover between the southern SAF and the IMF, the Salton Sea pull‐apart basin developed in the past 0.5~0.1 Ma as the stepover is connected by the Brawley Fault [ Brothers et al ., ; Fuis and Mooney , ], shown as an active seismic belt with recent earthquake swarms [ Hauksson et al ., ; Lohman and McGuire , ] and moderate earthquakes [ Chu and Helmberger , ; Hauksson et al ., ]. The Salton Sea region features high heat flow, low seismic velocity in the upper mantle [ Tape et al ., ], active crustal extension [ Crowell et al ., ], significant basin sedimentation [ Dorsey , ], and active volcanism [ Schmitt and Vazquez , ; Schmitt et al ., ], all typical to pull‐apart basins [ Cunningham and Mann , ].…”