We use precise hypocenter patterns and focal mechanisms to investigate the presence or absence of a continuous strike‐slip fault at depth connecting the San Bernardino strand of the San Andreas fault with the Coachella Valley segment of the Banning fault. We inverted 560,000 arrival times from 23,000 earthquakes (1981–1993) for high‐quality hypocenters and three‐dimensional P wave velocity structure in a 1° by 2° area centered on the San Gorgonio Pass. Cross‐sectional plots of relocated earthquakes reveal an abrupt 5 to 7 km high step in the maximum depth of hypocenters. The step riser defines a near‐vertical, locally curved surface that extends westerly more than 60 km from the Coachella Valley segment of the San Andreas fault to the San Jacinto fault. A hypothetical continuous vertical San Andreas fault through San Gorgonio Pass would cross the step at an oblique angle. We suggest that the step is the expression of the contact between different basement rock types juxtaposed by large‐scale right‐slip motion on the ancestral San Andreas fault. South of the step in Peninsular Ranges type basement (intrusives), brittle failure occurs down to about 20‐km depth, while north of the step in San Bernardino type basement (Pelona schist), brittle failure occurs to only about 13‐km depth. The step provides a piercing plane that should be offset about 3 km right laterally by an active, continuous, vertical San Andreas fault. Within the resolution of our mapping the step is not offset in this manner, implying either that there has not been a throughgoing vertical fault at depth, that a throughgoing fault has not experienced enough slip to offset the step, or that a throughgoing fault is not vertical and dips north over the top of the step. Hypocentral patterns and focal mechanisms indicate distributed deformation (thrust, normal, and strike‐slip faulting) over a large volume in the San Gorgonio Pass region; there is no evidence of hypocenter or slip vector alignments that would indicate a throughgoing, continuous, near‐vertical San Andreas fault. In summary, we find no evidence indicating a continuous fault at seismogenic depth connecting the San Bernardino strand and Coachella Valley segment of the San Andreas fault zone. We speculate that this is because the 3 km of right slip on the San Bernardino stand of the San Andreas fault and Coachella Valley segment of the Banning fault has not been sufficient to form a single new structure through the 15‐ to 20‐km gap between the two previously unconnected segments. This implies that large earthquake rupture on the San Andreas fault may be inhibited from propagating through San Gorgonio Pass, thus limiting the maximum magnitudes on the southern San Andreas fault.

We invert earthquake P-wave arrival times to image the 3D distribution of P wave velocities in the Mesozoic Peninsular Ranges batholith and nearby areas in southern California. There is a 3% velocity conwast between the eastern and western Peninsular Ranges at 4 and 20 km depth (west side faster) and a 1 to 1.5% velocity conwast across the San Andreas fault zone (south side faster) in the San Bernardino region at 4 to 14 km depth. The San Andreas velocity contrast is due to the juxtaposition of different rock types by slip along the fault zone. The Peninsular Ranges batholith velocity conwast is due to a difference in rock composition across the batholith. The maximum gradient in the crustal velocities is coincident with a compositional boundary within the batholith that reflects emplacement of the batholith across juxtaposed oceanic and continental crust Quaternary fault development has been in part concentrated at this boundary.

Abstract. Accurate forecasting of large earthquakes in the San Jacinto fault zone depends on the correct determination of fault segmentation. We have searched for discontinuities in the fault zone using as data geological maps of the fault traces, relocated earthquake hypocenters, focal mechanisms, slip rates, and historic large earthquake rapture zones. We identify nine principal discontinuities in the northern San Jacinto fault zone. These are characterized as structural discontinuities (bends, steps, branches, intersections), geodetic discontinuities (slip rate), and lithologic discontinuities. Most of the discontinuities have more than one defining characteristic. We also analyzed the local strain field orientation along the fault zone. Significant lateral discontinuities in the strain-field orientation are coincident with many of the structural and geodetic discontinuities we identify. Segment lengths defined from the data vary from about 7 to 35 km. This range of segment lengths is consistent with the lengths of large historical earthquake ruptures. However, evidence from small earthquake hypocenter patterns suggests the possibility that in some areas large earthquakes may rapture multiple adjacent segments. In these cases, maximum magnitudes may be larger than those observed historically. Because of the short historic record and the sparse paleoseismic record, probabilistic estimates of earthquake recurrence for this fault zone have depended heavily on assumed characteristic lengths of individual rupture segments. Ours and previous delineations of segment lengths have varied considerably, indicating that assumed segment length is a major source of uncertainty in the recurrence calculations. This source of uncertainty should be taken into account in calculations of the reliability of the probabilistic hazard estimates.

Two lines of evidence suggest that large earthquakes that occur on either the San Jacinto fault zone (SJFZ) or the San Andreas fault zone (SAFZ) may be triggered by large earthquakes that occur on the other. First, the great 1857 Fort Tejon earthquake in the SAFZ seems to have triggered a progressive sequence of earthquakes in the SJFZ. These earthquakes occurred at times and locations that are consistent with triggering by a strain pulse that propagated southeastward at a rate of 1.7 kilometers per year along the SJFZ after the 1857 earthquake. Second, the similarity in average recurrence intervals in the SJFZ (about 150 years) and in the Mojave segment of the SAFZ (132 years) suggests that large earthquakes in the northern SJFZ may stimulate the relatively frequent major earthquakes on the Mojave segment. Analysis of historic earthquake occurrence in the SJFZ suggests little likelihood of extended quiescence between earthquake sequences.

P and S wave amplitude data from digital, three‐component seismograms of local earthquakes recorded on a temporary seismic array have been used to tomographically invert for the three‐dimensional attenuation structure in the upper crust of Long Valley caldera, California. Differential attenuation (∂Q−1) values are obtained through the use of a spectral ratio technique that minimizes the site effect on the spectral ratios. The spectral ratios are inverted for ∂Q−1 using the LSQR algorithm. Zones of high, positive P wave ∂Q−1 (greater P wave attenuation) are observed at a depth of 4–5 km beneath the east flank of Mammoth Mountain and at 6–8 km beneath the southwestern edge of the resurgent dome. A region of positive S wave ∂Q−1 is seen at 7–8 km beneath the southern edge of the resurgent dome, whereas at 6–7 km, the S wave results are near average. The region of positive S wave ∂Q−1 at 7–8 km beneath the resurgent dome may indicate the roof of a magmatic system. The positive P wave ∂Q−1 at 4–5 km beneath Mammoth Mountain and 6–8 km beneath the resurgent dome may be due to the presence of a compressible fluid such as supercritical water. These zones may represent some of the source regions for the Long Valley geothermal system.