DEFORMATIONS TRIGGERED BY THE M=7.2, APRIL 04, 2010, EL MAYOR EARTHQUAKE ON FAULTS IN THE MEXICALI VALLEY

Since 1996, the Center for Scientific Research and Higher Education of Ensenada, CICESE, has been operating a network of geotechnical instruments (REDECVAM) for continuous recording of deformation phenomena in the Mexicali Valley. Currently, the network includes three crackmeters, four surface tiltmeters, two borehole tiltmeters, and five piezometers installed in the shallow aquifer; all instruments have sampling intervals in the 1 to 20 minutes range. The goal of the network is to monitor deformation related to tectonic (seismic and interseismic) phenomena, as well as antropogenic deformation caused by the deep fluid extraction in the Cerro Prieto Geothermal Field. The instruments are installed along the faults which limit the Cerro Prieto pull apart basin.

Almost all of the installed instruments worked during the April 4 event and recorded coseismic deformation.

The observed cosesimic slip for the April 4 earthquake on the Saltillo fault is about 2„Ÿ4 cm vertical and 1„Ÿ1.5 cm right lateral, while tilt attained 3000 µradians. The main feature observed since 1996 along the Saltillo fault is the presence of slow slip events, 2„Ÿ3 times per year, with magnitudes of 1„Ÿ3 cm recorded in the crackmeter, 100„Ÿ300 µradians tilt, and durations of the order of days. Comparison between coseismic slip and slow slip suggest that the area of observed coseismic deformation is much wider than during a typical slip event.

The Cerro Prieto fault had about 6 cm of recorded vertical slip (10 cm measured on the surface) and tilt of about 700 µradians, suggesting a narrow zone of deformation, also observed in the field.

There was no instrument working on the Morelia fault during the earthquake, but surface measurements suggest up to 30 cm of vertical slip, and a fresh rupture was observed along this fault between the Cerro Prieto Geothermal Field and the Imperial fault. There is no discernible fissure along the Imperial fault around Tamaulipas.

Coseismic step-like groundwater level changes in the range .2 - 4.7 meters were recorded at 4 wells. Using the volume strain efficiency calculated by Sarychikhina et al. (2009) for wells in the Mexicali Valley, the cosismic volumetric strain field in the depth of the piezometers was calculated to be of the order of 10^-6 - 10^-5.