The potential for earthquakes along reverse faults in the Los Angeles region has been dramatically illustrated by the damaging San Fernando, Whittier Narrows, and Northridge earthquakes in 1971, 1987, and 1994 [Kamb et al., 1971; Hauksson and Jones, 1989; Hauksson et al., 1994]. These earthquakes have sparked fundamental questions regarding the potential size of earthquakes generated along reverse faults in the greater Los Angeles region.
Although there is debate on the maximum size of earthquakes in the greater Los Angeles region (e.g., Dolan et al., 1995 and Rubin, 1997), our data suggest that prehistoric earthquakes at the Loma Alta site are significantly larger than other historical earthquakes along reverse faults in the Los Angeles region. Large slip displacements suggest that the Sierra Madre fault breaks across multiple, relatively short segments (as defined by Crook et al., 1987.), unlike the 1971 San Fernando earthquake that ruptured a single fault segment.
During 1997, I (together with Scott Lindvall and Tom Rockwell) compiled and synthesized geologic and detrital radiocarbon data from our paleoseismic and geomorphic studies along the Sierra Madre fault system. Evidence for two prehistoric earthquakes across the Sierra Madre fault, along the southern flank of the central Transverse Ranges in the Los Angeles Region, indicate large slip earthquakes are significantly larger than previously thought. Restoration slip along the fault indicates a minimum of ~4.0 m of slip from the most recent earthquake and a total cumulative slip of ~10.5 m for the past two prehistoric earthquakes. This implies that the average slip at this site is as much as 5 m in each of the past two earthquakes. Upward termination of faults, colluvial wedges, and offset stratigraphic units provide evidence for large earthquakes that occurred over the past 15 ka. Large ground motion displacement exceeding several meters have the potential for disrupting lifeline systems and must be considered for future seismic assessment of the greater Los Angeles region.
In 1997, I modified our existing Total Station survey software and implemented a beta version of "Survey". The software now has a Graphical User Interface that filters raw data downloaded from the Total Station. The software currently works on Microsoft Windows 95 and NT operating systems. We plan to port the code to the Macintosh OS. The software: 1) converts raw data (vertical and horizontal angles and slope distance) into Cartesian coordinates, 2) the filtered data (Easting, Northing, and Elevation) is easily opened, edited and manipulated by standard spreadsheet programs such as Microsoft Excel, 3) with multiple occupation sites, the data can easily be transformed into a single reference framework, and 3) data can be easily rotated onto a vertical plane. Here, radial side shots of scarp profiles or trench wall exposures can be easily projected to a plane perpendicular to the trench wall or to the fault scarp.
Papers published or in review from the last few years of SCEC support:
McGill, S. F., and C. M. Rubin, in review, Surficial slip distribution on the central Emerson Fault during the 28 June 1992 Landers earthquake, J. Geophys. Res.
Rubin, C. M., S. Lindvall, T. Rockwell, final preparation, Paleoseismic evidence of large slip earthquakes along the Sierra Madre fault in Altadena, California, Science.
Rubin, C. M., and K. Sieh,, Long Dormancy, Low Slip Rate and Similar Slip-per-event for the Emerson Fault, Eastern California Shear Zones, J. Geophys. Res., 102, 15,319-15,333, 1997.
Rubin, C. M., Systematic underestimation of earthquake magnitudes from large intracontinental reverse faults: Historical ruptures break across segment boundaries, Geology 24, 989-992, 1996.