Project Abstract
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The 2015 Fillmore swarm occurred about 6 km west of the City of Fillmore in Ventura, California, and was located beneath the eastern part of the actively subsiding Ventura basin at depths from 11.8 km to 13.8 km, similar to two previous swarms in the area. Template-matching event detection showed that it started on the 5th of July 2015 at 2:21 UTC with a ~M1.0 earthquake. The swarm exhibited unusual episodic spatial and temporal migrations, and diversity in nodal planes of focal mechanisms as compared to the simple hypocenter defined plane. It was also noteworthy because it consisted of >1,400 events of M≥0.0, with M2.8 being the largest event. We suggest that fluids released by metamorphic dehydration processes, migration of fluids along a detachment zone, and cascading asperity failures caused this prolific earthquake swarm. Other mechanisms such as simple mainshock-aftershock stress triggering or a regional aseismic creep event are less likely. Dilatant strengthening may be a mechanism that causes the temporal decay of the swarm as pore pressure drop increased the effective normal stress, and counteracted the instability driving the swarm.
Our analysis showed differences in stress drop characteristics between the San Gorgonio Pass (SGP) and Ventura (VB) Special Fault Study Areas. The SGP has significant internal variation in stress drop magnitudes, and also exhibits systematically higher stress drops than does VB. We demonstrate that the higher scatter in SGP is not a generic artifact of our method but an expression of underlying differences in source processes. Our results suggest that higher ambient stresses, which can be deduced from larger focal depth and more thrust faulting, may only be of secondary importance for stress drop variations. Instead, the general degree of stress field heterogeneity and strain localization may influence stress drops more strongly, so that more localized faulting and homogeneous stress fields favor lower stress drops. In addition, higher regional loading rates, for example, across the VB potentially result in stress drop reduction whereas localized slow loading rates on structures within the SGP result in anomalously high stress drop estimates. |
