SCEC Award Number 15030 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Joint seismotectonic and source spectra analysis of the Ventura Basin and San Gorgonio Pass SCEC Special Fault Study Areas, southern California
Investigator(s)
Name Organization
Egill Hauksson California Institute of Technology John Shaw Harvard University
Other Participants Thomas Goebel (PostDoc) or TBN PostDoc, Andreas Plesch - Research Associate
SCEC Priorities 1a, 2d, 4d SCEC Groups USR, Seismology, EFP
Report Due Date 03/15/2016 Date Report Submitted 03/10/2016
Project Abstract
 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.
Intellectual Merit This project relates to many key SCEC objectives and will improve our understanding of earthquake activity across southern California. In particular, our high-resolution studies of seismicity provide better delineation of fault structures and make possible more advanced seismicity studies by us and other SCEC researchers. Our analyses provide fundamental insights into micro-earthquake activity, the crustal strain field, major faults, and crustal geophysics.
Broader Impacts The outreach activities consisted of publishing the results of the research in peer-reviewed journals. Also, the focal mechanism catalog is being distributed to researchers via the Southern California Earthquake Data Center (SCEDC). We have also presented results at SCEC workshops.
Exemplary Figure A perspective view looking west of the Ventura basin and Fillmore swarm. The different color surfaces are major late Quaternary faults represented in the SCEC Community Fault Model (CFM (Plesch et al., 2007). The coast line is shown as a white jagged line. Hypocenters for the 2015 Fillmore swarm are highlighted by magnitude, and occur along the down-dip extent of the Simi-Santa Rosa fault zone Nicholson et al., (2014).