Displacements on the Imperial, Superstition Hills, and San Andreas Faults Triggered by the Borrego Mountain Earthquake

The Borrego Mountain earthquake of April 9, 1968, triggered \nsmall but consistent surface displacements on three \nfaults far outside the source area and zone of aftershock \nactivity. Right-lateral displacement of 1-2 1/2 cm occurred \nalong 22, 23, and 30 km of the Imperial, Superstition Hills, \nand San Andreas (Banning-Mission Creek) faults, respectively, \nat distances of 70, 45, and 50 km from the epicenter. \nAlthough these displacements were not noticed until 4 days \nafter the earthquake, their association with the earthquake \nis suggested by the freshness of the resultant en echelon \ncracks at that time and by the absence of creep along most \nof these faults during the year before or the year after the \nevent. Dynamic strain associated with the shaking is a \nmore likely cause of the distant displacements than is the \nstatic strain associated with the faulting at Borrego Mountain \nbecause (1) the dynamic strain was much larger and \n(2) the static strain at the San Andreas fault was in the \nwrong sense for the observed displacement. The principal \nsurface displacements on the Imperial fault took place within \n4 days of the earthquake and may have occurred simultaneously \nwith the passage of the seismic waves, but the \npossibility of delayed propagation to the surface is indicated \nby a 1971 event on the Imperial fault in which the surface \ndisplacement followed the triggering earthquake by 3-6 days. \nAll three of the distant faults are "active" in that they show \nevidence of repeated Quaternary movement, and surface displacements \noccurred only along those segments where the \nfault trace is well delineated in surface exposures, at least \nin uncultivated areas. This is the first documented example \nof fault displacement triggered by seismic shaking far from \nthe source area, although such displacement has probably \ngone undetected many previous times here and in similar \ntectonic environments. This phenomenon forces us to be \nmuch more conservative in estimating the probabilities of \ndamage from surface displacements along active faults in \nseismic regions.