2015/07/09 M2.8 Fillmore Swarm

A swarm of earthquakes started at 6 am on 5th of July 2015 in Ventura, southern California.

As of 12 pm on 13 July a total of 181 earthquakes of M1.0 to M2.8 have been recorded by the CISN, 38 of which have been M2.0 or greater. The swarm consists of a number of clusters, the most intense activity occurring on July 9th. Between July 5th and July 8th, 1 – 5 events occurred per day, magnitudes up to M2.3. The rate increased during July 9th, peaking with 10-13 events per hour. Further pulses of activity occurred on July 12th and July 13th with a peak of 9 events between 1 and 2 am on July 13th.

Temporal evolution of the swarm showing the hiatus and spurts of activity. The top plot focuses on the days of highest activity rate, while the lower plot shows the time period to 11 Aug 2015.

The swarm is located about 3 miles west of the City of Fillmore, in farming areas in the middle of the Ventura basin.

The Fillmore swarm as of 1pm on 13 July 2015. The size of the swarm area is approx. 2km by 1 km. The red dot in Fillmore is the closest seismic station.

The Fillmore swarm at noon on 9 July 2015. The figure shows the San Cayetano fault at the north edge, and the Oak Ridge fault at the south edge. Fillmore swarm epicenters are in the middle (magenta) and the city of Fillmore to the right.

These events are mostly not being felt because they are small and located away from an urban area.

The swarm is located about halfway in between the San Cayetano Fault and the Oak Ridge Fault. It does not appear to be associated with faults mapped at the surface in the region. The preliminary data indicate that the fault movement is mostly thrust faulting on northeast (or southwest) striking planes. This is also consistent with the trend of the epicenters.

These events are located about 8 to 14 km (5-9 miles) beneath the Earth’s surface and are occurring in an area of known active earthquake faults, both of which suggest that they are of tectonic origin. However, at this time, a direct connection to anthropogenic activity cannot be ruled out with complete certainty because more detailed research is needed.

Map and cross sections showing the event depth distribution. The large oilfields are also shown outlined in blue and labelled. The dots are scaled with magnitudes which are all in the M1 to M2 range.

Since 1981 the SCSN has not recorded a similar earthquake swarm in the Ventura region. The large number of small earthquakes and lack of larger magnitude earthquakes of mag 3 or larger is very unusual. The swarm is slowly and inconsistently migrating to the northwest, under the Toland Road Landfill.

This earthquake swarm may be a reflection of fluid migration at depth or a slow slip event at depth, which could be caused by the compressional tectonics and high natural fluid pressures in the basin.

A kmz file showing the earthquakes and California faults can be found Here.

Caltech and USGS seismologists continue to monitor the situation real-time and will provide up-to-date information if there are significant new developments.

Dr. Jen Andrews (jrand@gps.caltech.edu) and Dr. Egill Hauksson, Caltech (hauksson@caltech.edu)