Electrical resistivity imaging of a seismic region in northern Baja California, Mexico

Abstract

Magnetotelluric (MT) observations collected at 17 sites along a 45–km profile were used to investigate the electrical conductivity structure in the Ojos Negros seismic zone, in northern Baja California, Mexico. In this region, seismic activity registered by a local network consists mainly of micro–earthquakes that occur at depths between 13 to 15 km. Epicenters mapped at the surface show that seismic activity is not aligned with known faults but is scattered over the Ojos Negros valley. We interpret the MT data using 2–D inversion codes applied to series and parallel invariant responses, and also to traditional Groom–Bailey's TE and TM responses. The resistivity models obtained for both approaches are similar. The models reveal a high conductivity anomaly closely correlated with the distribution of hypocenters at depth. Most of the seismic events cluster in a zone of maximum resistivity gradient located at the rim of the high–conductive anomaly. The shape of the conductivity anomaly and the spatial attitude of the seismic events suggest that Ojos Negros fault is a major listric structure accommodating normal and dextral strike–slip movement. The large resistivity contrast across the fault–plane is probably related to a compositional contrast between metamorphic rocks in the footwall and plutonic rocks in the hanging wall of the fault. The fault–plane at depth seems to separate ductile, highly conductive metamorphic rocks from more brittle, mid–resistivity shattered granitic rocks. The MT resistivity model and the earthquake data together yield an explanation that either technique could not provide alone, throwing light on the physical conditions of the rocks in the middle crust.