A low-stress-drop, low-magnitude earthquake with surface faulting: The Imperial, California, earthquake of March 4, 1966

Abstract

Right-lateral surface displacement reaching 1 1/2 centimeters occurred over a ten-kilometer section of the Imperial fault in association with a magnitude 3.6 earthquake on March 4, 1966, the smallest known earthquake yet associated with surface displacement. The displacement is documented by field observations of en-echelon cracking in pavement and the offset of the white center line of Highway 80. The association of the observed displacement with the March 4 earthquake is supported by the shallow depth of the earthquake source, the high excitation of waves in the top layer of sediments, the high excitation of Love waves of period 8-15 seconds, the distribution of aftershocks, and the agreement between the source moment as calculated from the observed faulting and from the amplitudes of Love waves. Calculations based on faulting theory indicate a fault depth of 1.1 km, a net moment of 2 × 10²² dyne-cm, a stress drop of 1.1 bar and an energy release of 10¹⁷ ergs. The remarkable internal consistency of the various calculations provides strong support for the faulting mechanism. It is suggested that low stress drops and relatively large fault lengths may be associated with many other small earthquakes and that allowance must be made for a wide range in the stress drops and fault lengths for any given magnitude range.