Seismologists issue special report
on SP, Fillmore earthquake swarm
July 17, 2015
By Peggy Kelly
Santa Paula News
There’s been an earthquake swarm in the Santa Clara River Valley now numbering — or by the time you read this topping — 200 small shakers that started more than week a week ago.
Known officially as the Fillmore Swarm, seismologists have noted that the bulk of the shakers have been four to five miles west of Fillmore, which puts the activity four to five miles east of Santa Paula.
And according to a special report on the activity, the cluster has been occurring very near to or directly under the Toland Road Landfill, just where opponents to the landfill expansion said in the mid-1990s they feared an active fault was located.
The earthquakes, according to the Southern California Seismic Network (SCSN) started July 5 at 6 a.m. and as of 12 p.m. July 13 there had been a total of 181.
But in all there were 37 earthquakes July 13 mapped in the area between Santa Paula and Fillmore, although the number that occurred after noon has not been broken out.
The swarm was addressed in a special report by the SCSN “consists of a number of clusters” the most intense activity peaking on July 9 with 10 to 13 seismic events per hour. And the activity was about halfway between the San Cayetano Fault to the north and the Oak Ridge Fault to the south.
Preliminary data points to fault movement being thrust faults five to nine miles beneath the surface, “in an area of known earthquake faults,” according to the report.
The size of the swarm area is approximately 2km by 1km, or 1.2 miles by .06 miles.
The SCSN has not recorded a similar swarm in Ventura County since 1981 and considers this ongoing seismic event “very unusual” in light of the lack of larger magnitude 3 or above events that could be expected during such a swarm. The quakes have ranged in magnitude from less than 1.0 to 2.8 magnitude.
The SCSN reports the swarm “is slowly and inconsistently migrating to the northwest, under the Toland Road Landfill.”
At this point the SCSN reports that 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.”
The seismic activity slowed July 14 with only four events recorded with magnitudes ranging from 1.3 to 2.6.
In the mid-1990s the Venture Regional Sanitation District, which owns the landfill, overcame the objections of the Santa Paula and Fillmore communities and lawsuits filed by growers and others to expand the Toland Road Landfill from 150 tons a day to 1,500 tons a day.
The main thrust of the opponent’s argument was that the Culbertson fault line ran under the unlined, existing landfill that is close proximity to the San Cayetano fault. The VRSD claimed that their soil testing showed the Culbertson fault had not been active for more than 11,000 years.
The Board of Supervisors on a split vote allowed the expansion to go forward.
The Southern California Seismic Network is the clearinghouse of all things earthquakes and includes Caltech and USGS among other agencies.
To read the SCSN report visit:
www.scsndev.gps.caltech.edu/index.php/earthquakes/speqrep/20150709-m2-8-fillmore-swarm/
