Slow‐Growing and Extended‐Duration Seismicity Swarms: Reactivating Joints or Foliations in the Cahuilla Valley Pluton, Central Peninsular Ranges, Southern California.

Three prolific earthquake swarms and numerous smaller ones have occurred since 1980 in the Mesozoic igneous plutonic rocks of the Perris block of the Peninsular Ranges, Southern California. The major swarms occurred in 1980–1981, 1983–1984, and 2016–2018, with the latest swarm still ongoing. These swarms have no clear mainshock, with the largest events of ML 3.6, ML 3.7, and Mw 4.4. Each successive swarm had larger cumulative seismic moment release with about 314 and 411 events of M ≥ 1.5, while the third swarm has produced about 451 events of M ≥ 1.5 (as of September 2018). The concurrent strike‐slip faulting occurred on north to northwest striking planes but with no orthogonal northeast trending seismicity alignments. These shallow swarms are probably driven by intrablock Pacific‐North America plate boundary stress loading of the two bounding major late Quaternary strike‐slip faults, the Elsinore and San Jacinto faults. The state of stress within the Cahuilla Valley pluton has a ~40° angle between the maximum principal stress and the average trend of the swarms, suggesting that migrating pore fluid pressures aid in the formation and growth of zones of weakness. These swarms, which last more than 600 days each, exhibit clear bilateral spatial migration for distances of up to ~7–8 km and reach their full length in about 20 months. The slow spatial‐temporal development of the swarms corresponds to a fluid diffusivity of 0.006 to 0.01 m2/s, consistent with very low permeability rocks as expected for this block. There is no geodetic or other evidence for a slow slip event driving the swarms. Plain Language Summary: There have been three unusual seismicity swarms in the Cahuilla Valley, Peninsular Ranges, over the last 40 years: 1980–1981, 1983–1984, and 2016–2018, with the latest still ongoing. They occurred within granite rocks that underlie the Cahuilla Valley pluton. The first two lasted for about 2 years, and the current swarm is still ongoing more than 2.5 years after it began. These swarms differ from mainshock‐aftershock sequences because the largest events occur months after the swarm initiation event. These swarms are probably driven by plate boundary stress loading of the two bounding major late Quaternary strike‐slip faults, the Elsinore and San Jacinto faults, and aided by changes in pore fluid pressures. Key Points: Since 1980, three prolific natural earthquake swarms occurred between the Elsinore and San Jacinto faults, in the central Peninsular RangesThese swarms are driven by interseismic plate boundary strain rates and modulated by pore fluid pressure in low‐permeability plutonic rockShallow depths, absence of mapped faults, and nonlinear seismic moment release suggest concurrent slip on foliations in the plutonic rock [ABSTRACT FROM AUTHOR]