The 4 December 1948 earthquake (Mw 6.4): Evidence of reverse faulting Beneath the Tres Marías escarpment and its implications for the Rivera-North American relative plate motion

the largest historical earthquake along the northern segment of the rivera-north american plate boundary occurred on 4 december 1948. this event caused casualties and important damage in the penal colony on the tres marías islands. seismograms were collected from regional and teleseismic stations in order to determine the focal mechanism solution. the resulting mechanism shows high-angle reverse faulting with p axes oriented ne-sw. the mechanism obtained agrees with the solutions of two smaller-magnitude earthquakes located also on the tres marías escarpment. the estimated magnitude for the 1948 event is mw6.4, based on the amplitude of the surface waves recorded at the de bilt (netherlands) seismic station. the slip direction of the resulting mechanism is interpreted as reflecting the relative motion between the rivera and the north american plates. thus the rivera plate appears to be underthrusting beneath the tres marías escarpment in a ne-sw direction. the source mechanism obtained here helps to constrain the relative motion between these two plates where data is sparse.