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

摘要

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 M-L 3.6, M-L 3.7, and M-w 4.4. Each successive swarm had larger cumulative seismic moment release with about 314 and 411 events of M1.5, while the third swarm has produced about 451 events of M1.5 (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 similar to 40 degrees 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 600days each, exhibit clear bilateral spatial migration for distances of up to similar to 7-8km and reach their full length in about 20months. The slow spatial-temporal development of the swarms corresponds to a fluid diffusivity of 0.006 to 0.01m(2)/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.