NEAR-SURFACE SEISMIC IMAGING ACROSS THE PITAYCACHI FAULT, NORTHEASTERN SONORA, MEXICO

The Pitaycachi normal fault of northeastern Sonora, Mexico is the source of the 3 May 1887 Bavispe earthquake (M_w ~7.4), which represents the largest historical earthquake of the southern Basin and Range Province and testifies to the continuing deformation occurring in this region. This fault bounds the eastern margin of the San Bernardino Valley, extending roughly north-south for over 100 km with fault-scarp exposures from the 1887 earthquake of up to 2 meters. In the fall of 2001, a near-surface seismic survey across this fault was undertaken to attempt to image shallow structures associated with the fault in order to better constrain recent fault activity. Seismic data were acquired with a 60-channel recorder, with 500 ms records, and with a three-pound sledgehammer as the source. Ray-trace modeling and tomographic inversion of first arrivals show that unconsolidated sediments in the footwall are thin (~1 m) and overlie more consolidated course alluvial-fan sediments. Unconsolidated sediments near the surface in the hanging wall are greater than 6 m thick and show significant lateral and vertical velocity variations. Extremely dry conditions result in extraordinarily low compressional-wave velocities (as low as 175 m/s) in relatively loose sediments exposed at the surface. On the basis of correlations of the Sierra Madre Occidental basalt flows, total throw on the fault was estimated previously at over 4000 meters since fault initiation at about 23 Ma, giving a total slip rate of 0.17 mm/yr. Although previous estimates of Quaternary slip rates on this fault are believed to be significantly less, fault activity remains vigorous. The nature of the faulting in the San Bernardino Valley is similar to Quaternary fault scarps flanking numerous basins in southern Arizona, and provides evidence of continued extensional deformation in the southern Basin and Range Province.