ML3.7 and ML3.8 Doublet Earthquakes in Pomona
A Pair of ML3.7 and ML3.8 Earthquakes Occurred at 4:43AM and 5:06AM PDT, Respectively, on September 19th, 2013.
CISN/SCSC Executive Summary (E. Hauksson, K. Hutton, A. Guarino, V. Thomas, D. Given, N. Scheckel, Z. Newman)
Version 1 - 11:20AM 9/19/2013
Pomona - Two earthquakes occurred early this morning, two miles northwest of Pomona, CA. The first event, a ML3.7, occurred at 4:43AM PDT (local time), and was followed by a slightly larger ML3.8 at 5:06AM PDT. We call this a "doublet," as there are two events with similar location and magnitude that occur within a short time period. These events were both felt throughout the San Gabriel Valley, and as far away as southern Orange County, northern Los Angeles County, and the Inland Empire.
These are but two additional events in a longer series of events, culminating in ten days of activity near this area, with a sequence that produced a cluster of 25 small earthquakes producing a northeast trend, extending for a distance of 1 to 2 miles. The strike of this trend aligns with the strike of the nodal planes for the focal mechanisms (as both are very similar), but it is difficult to identify the causative fault with a series of events so small. Much larger events sometimes produce surface rupture or robust aftershock sequences, both of which help identify the causative fault more conclusively, when used in conjunction with the moment tensor solution and/or focal mechanism.
The focal mechanism for both events is likely left-lateral strike-slip, provided the fault plane at 220/228 degrees is the same as the strike for the northeast trend of events, which is likely the case.
The CIIM Map above (Community Internet Intensity Map, aka "Did You Feel It?") shows felt reports with intensity of at least MMI II from southern Orange County east to the Inland Empire, and to northwest Los Angeles County. Since there are two maps, the number of reponses are split between the maps. The total number of people who have reponded as of 11:00AM on 9/19/2013 was 1059.
Sequence over last ten days in orange, M3.7 and M3.8 doublet in red.
The waveforms above were captured from the M3.8 event, which occurred at 5:08AM. P waves and S waves can be seen marked on flags in red above. The blue stations are analog stations, whereas the black are digital stations. An aftershock is visible at the end of the wavetrain, seen on the two blue stations. The preliminary magnitude for the small aftershock was calculated at M1.7 from our real-time system.