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Seismicity in a weak crust: the transtensional tectonics of the Brawley Seismic Zone section of the Pacific-North America Plate Boundary in Southern California, USA

Hauksson, Egill and Stock, Joann M. and Husker, Allen L. (2022) Seismicity in a weak crust: the transtensional tectonics of the Brawley Seismic Zone section of the Pacific-North America Plate Boundary in Southern California, USA. Geophysical Journal International, 231 (1). pp. 717-735. ISSN 0956-540X. doi:10.1093/gji/ggac205.

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The Brawley Seismic Zone (BSZ) is a ∼58-km-long section of the Pacific–North America Plate boundary that connects the southernmost San Andreas Fault (SAF) and the Imperial Fault in southern California. We analyse the BSZ as two segments: a north segment, dominated by SAF tectonics, and a south segment that accommodates a higher level of seismicity. The south segment includes a ∼6 km wide right lateral step-over that includes the Salton Sea geothermal field (SSGF) and Holocene subaerial lava domes at the south end of the Salton Sea, called Salton Buttes. In general, the 40 yr (1981–2021) of BSZ seismicity is characterized by transitory sequences that often are accommodated simultaneously or in quick succession on intricate ladder-like faults. These sequences rarely reactivate faults associated with previous seismicity alignments but rather activate adjacent faults, sometimes located within less than 1 km. They can include several events of similar size as the mainshock, followed by bursts of aftershocks sometimes located away from the mainshock rupture. The seismicity rate and the spatial geometrical complexity varies between the BSZ-north and BSZ-south segments. The 24-km-long BSZ-north accommodates a ∼12-km-long linear trend of seismicity that extends from the SAF terminus into the Salton Sea, where moderate-sized northeast striking sequences form the rungs in a ladder-structure in a weak crust. The seismicity in this area is most likely influenced by the stress state of the SAF. In contrast, the 34 km long BSZ-south segment, which also has a weak crust, has accommodated larger sequences that illuminate irregular ladder-type faulting, with aftershocks defining linear distributions striking either north or northeast. The focal mechanisms exhibit a mostly strike-slip style of faulting with minor dip-slip faulting in the south Salton Sea area as well as the Mesquite basin to the south. The state of stress, as determined from focal mechanisms, consists of almost horizontal σ₁, and σ₃ with vertical σ₂. The activation angle between the trend of σ₁ and the preferred nodal plane of the largest nearby event decreases systematically from north to south along the long-axis of the BSZ. In the step-over zone, the Holocene volcanism and the frequent seismicity sequences suggest crustal extension as well as associated reduced crustal strength as compared to other parts of the BSZ. The presence of weak, thin, hot crust and distributed ladder-like or conjugate predominantly strike-slip faulting suggests that the whole BSZ acts as a several kilometre wide fault zone where no prominent principal slip surfaces are required to accommodate moderate-sized (M6+) earthquakes.

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URLURL TypeDescription
Hauksson, Egill0000-0002-6834-5051
Stock, Joann M.0000-0003-4816-7865
Husker, Allen L.0000-0003-1139-0502
Additional Information:Partial funding for this research was provided by NSF grant EAR-1728690 and USGS Cooperative Agreement G20AP00037. We used GMT from Wessel et al. (2013) to make the figures. We appreciate comprehensive reviews by D. Trugman and L.M. Jones and the support provided by more than 20 SCSN and SCEDC staff members who maintain stations and communications systems, as well as data flow and archiving. The authors have no conflicts of interest with this work to declare.
Group:Seismological Laboratory
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Issue or Number:1
Record Number:CaltechAUTHORS:20220916-666042000
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:117029
Deposited By: Olivia Warschaw
Deposited On:25 Oct 2022 20:54
Last Modified:25 Oct 2022 20:54

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