Longbase Strainmeter Recordings

Longbase Strainmeter Data: Sierra El Mayor/Cucapah (Baja California) Earthquake M 7.2 April 4, 2010

Laser Strainmeters – Plate Boundary Observatory instruments in green:

Site	Component	Lat (N)	Long (W)	Azimuth	Distance
SCS	NS	33.247	115.944	-0.3	73
SCS	EW	33.247	115.944	89.7	73
DHL	NS	33.392	115.788	-4.3	87
DHL	EW	33.392	115.788	85.6	87
PFO	NS	33.609	116.455	0.3	129
PFO	EW	33.609	116.455	90.4	129
PFO	NW	33.609	116.455	135.3	129
GVS	NS	34.176	118.221	14.7	288
CHL	NS	35.692	120.216	-0.4	535
CHL	EW	35.692	120.216	90.3	535

Distance is in km, measured from approximate North end of rupture.

The First Day

At most of the sites, we observed rapid strain changes that began at the same time as the arrival of the seismic waves. The dynamic strains from these waves were large enough that for 10-20 seconds the laser beam was moved too far to form an interference pattern. We thus do not have a record of the coseismic offset, and have set this to zero for each of these sites: SCS, DHL, and PFO. (This was not necessary for GVS and CHL recordings.)

SCS	DHL	PFO

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The First Week

Over the first week, the strains at both of the closer sites (SCS and DHL) show a gradually-decaying postseismic strain; at DHL there are several small and relatively abrupt offsets whose relative amplitude on the two independent components argues that they were caused by creep events on the San Andreas fault. The two most distant sites (CHL and GVS) show little response in comparison, apart from day-long transient behavior. The records at Piñon Flat Observatory (PFO) show a change in the rate of strain, with signs that consistent with slip on the San Jacinto fault, which is similar to what was seen postseismically in response to the Anza, Caifornia, earthquake of 2005.