A LOW-STRESS-DROP, LOW-MAGNITUDE EARTHQUAKE WITH SURFACE FAULTING: THE IMPERIAL, CALIFORNIA, EARTHQUAKE OF MARCH 4, 1966 By JAMES N. BRUNE AND CLARENCE

Right-lateral surface displacement reaching 11⁄2 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 kin, a net moment of 2 X 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. FIELD AND SEISMIC OBSERVATIONS Surface-faulting. Right-lateral surface displacement of about 11⁄2 cm occurred along the Imperial fault 8 km east of E] Centro, California, in association with a magnitude 3.6 earthquake that was centered in this area on March 4, 1966. Although the principal area of faulting was slightly closer to E1 Centro than to Imperial (Figure 1), the earthquake is herein referred to as the Imperial earthquake to avoid confusion with the much larger (M = 7.1) "El Centro earthquake" (or "Imperial Valley earthquake") of 1940. Both the 1940 and 1966 events were caused by displacement on the Imperial fault, which is one of the many members of the San Andreas fau]t system in the northern Gulf of California structural province. The Imperial fault was first recognized and named at the time of the 1940 earthquake (Buwalda and Richter, 1941), although the existence of a major structural break along this line is also indicated by geophysical evidence (Biehler et al, 1964) as well as by continuous low Quaternary fault scarps along parts of the fault trace. On March 6, 1966 Brune noticed a 11⁄2 cm horizontal offset in the white line of Highway 80 (Figure 2), 281 m west of the James road intersection, at the very point where the 1940 offset on the Imperial fault is known to have taken place (J. P. Buwalda, unpublished field notes). Observations in June 1966 along adjacent parts of the Imperial fault showed that en-eehelon cracks indicative of small right-lateral