Vertical crustal deformation associated with the 1979 M = 6.6 Imperial Valley, California Earthquake: Implications for fault behavior

Abstract

Elevation changes in the Imperial Valley, California, derived from repeated leveling surveys for the time period including the 1979 M = 6.6 earthquake, provide some constraints on fault geometry and slip distribution associated with this event. Many of the first‐order features of the observed vertical movements are well matched by simple models consisting of variable slip on planar faults in an elastic half‐space using fault offsets inferred from strong ground motion observations (Archuleta, 1984) and measured afterslip. The geodetic and seismic observations suggest that significant slip is confined to depths above 13 km with maximum right‐lateral offset reaching about 1.5 m on the Imperial fault. Dip slip occurs predominantly in the sediments on the upper 5 km of the Imperial fault and on the Brawley fault. Right‐lateral afterslip is confined to the upper 5 km of the Imperial fault and reaches about 30 cm for the period 1979–1981. In contrast, observed elevation changes near the 1979 hypocenter (on southern end of Imperial fault) and in the Brawley Seismic Zone show significant deviations from those predicted by models of fault slip inferred from strong ground motion measurements. Specifically, the geodetic data suggest that slip on the Imperial fault is significantly lower in the vicinity of the earthquake hypocenter than along the central and northern sections of the fault. In addition, there is marginal evidence that the dip of the Imperial fault changes along strike from approximately vertical just north of the U.S.‐Mexico border to between 70° and 80° near the northernmost extent of the 1979 surface break. This change in dip may be related to a change in local strike along the fault. Large elevation changes (>15 cm) also occur within the Brawley Seismic Zone well north of the primary surface faulting. While these movements are consistent with a number of possible fault models, our prefered interpretation based on geodetic and seismic observations (aftershock locations and focal mechanisms) involves right‐lateral, aseismic slip on a northwest striking fault along the east side of the Brawley Seismic Zone and conjugate left‐lateral faulting on a northeast striking fault (possibly associated with an M = 5.8 aftershock). Buried creep on this same right‐lateral fault in the Brawley Seismic Zone can also account for vertical deformation during the postseismic period of the 1940, M = 7.1 Imperial Valley earthquake as well as deformation during the interseismic period between the 1940 and 1979 events. Substantial right‐lateral slip in the Brawley Seismic Zone suggests that a significant part of the shear strain released during and following the 1940 and 1979 earthquakes on the Imperial fault is transferred through the Brawley Seismic Zone to the southern end of the San Andreas fault.