3D Images of Faults Within the Crafton Hills Fault Zone From the June 16th, 2005, Yucaipa, California, Earthquake: Implications for the Geometry of the San Andreas Fault.

A magnitude 4.9 earthquake occurred near Yucaipa on June 16th, 2005, followed by over 100 aftershocks, one of which reached magnitude 3.9. Most of the aftershocks are confined to depths below 10 km. We relocated the aftershocks using the double-difference hypocenter location method. Two dense, separate earthquake clusters can be observed, one related to the main shock, the other to the largest aftershock. These clusters outline two ENE trending, nearly vertical, sub-parallel faults about 600 m apart. The up dip projection of these faults reaches the Earth's surface in the vicinity of several fault traces attributed to the Crafton Hills fault zone. Several other faults related to this fault zone can be imaged from the relocated events of Richards-Dinger and Shearer (2000). A third, less dense alignment of events within the aftershock cloud delineates at depth a fault with a 60 degrees dip to the SW whose up dip projection emerges very close to the trace of the San Bernardino segment of the San Andreas fault. The few focal mechanisms (Hauksson 2000) available for events in the vicinity of this fault all show a left-lateral component of slip on a SW dipping plane. The dip-slip component is normal for some mechanisms and reverse for some others, but always smaller than the strike-slip component. These observations provide improved constraints on the deep geometry of significant faults in the greater San Bernardino area, including the San Andreas fault. Could the lack of seismicity on the San Bernardino segment of the San Andreas mean that at depth the fault is not vertical, but dips to the SW, possibly following an old normal fault?