Sudden Surges of Seismicity within Natural Slow Growing and Long Duration Seismicity Swarms near Cahuilla Valley in the Central Peninsular Ranges, Southern California

Egill Hauksson, Zachary E. Ross, & Elizabeth S. Cochran

Published August 15, 2018, SCEC Contribution #8765, 2018 SCEC Annual Meeting Poster #069

The most recent Cahuilla swarm that started back in mid 2016 has grown steadily in number of events (~10,000 of M>0.3) and presently extends over an almost north-south linear trend of ~7 km. Since late on 11 August 2018 the seismicity accelerated with a ~120 event foreshock sequence, which culminated with a mainshock of Mw4.4 on 15 August 2018. The mainshock was followed by more than 200 aftershocks of M>0.3 over a period of 12 hours. This new activity extends the spatial distribution of the sequence ~1 km to the southwest. The b-value decreased from ~0.94 to ~0.74 during this most recent activity, suggesting that a new region, possibly of higher state of stress, was being activated.

Three major Cahuilla Valley swarms occurred in 1980-1981, 1983-1984, and 2016/6-2018, with the latest swarm still ongoing. The first two swarms had no clear mainshock, with the largest events of M3.6 and M3.7. They had similar moment release with about 100 events of M≥1.5, but the ongoing swarm now has a Mw4.4 mainshock and has generated more than 180 events of M≥1.5. These swarms are prolific in generating small earthquakes down to M0.3 or less suggesting abundant availability of trigger ignitions along the joints or foliations of the plutonic Mesozoic rock. Successive seismicity surges, which appear to originate at the swarm initiation focus, lengthen the spatial extent of the swarm lineament, thus providing a mechanism for how natural swarms increase in spatial extent. The style of faulting that is predominantly strike-slip with a small normal component is consistent with possible counter-clockwise block rotation of smaller crustal blocks. Such rotation could be driven by geometrically complex plate-boundary stress loading along the two nearby major late Quaternary strike-slip faults, Elsinore and San Jacinto faults.

Key Words
Seismiciy swarms, batholith

Citation
Hauksson, E., Ross, Z. E., & Cochran, E. S. (2018, 08). Sudden Surges of Seismicity within Natural Slow Growing and Long Duration Seismicity Swarms near Cahuilla Valley in the Central Peninsular Ranges, Southern California. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology