Annual Meeting - SCEC.org

Poster Abstracts
• The SCEDC has revamped its website. The changes make it easier for users to search the archive, discover
updates and new content.
• Post processing on El Mayor Cucapah 7.2 sequence continues. To date there have been 11847 events reviewed.
Updates are available in the earthquake catalog immediately.
• A double difference catalog (Hauksson et. al 2011) spanning 1981 to 6/30/11 will be available for download at
www.data.scec.org and available via STP.
• A focal mechanism catalog determined Yang et al. 2011 is available for distribution at www.data.scec.org.
• Waveforms from Southern California NetQuake stations are now being stored in the SCEDC archive and
available via STP as event associated waveforms.
• As part of a NASA/AIST project in collaboration with JPL and SIO, the SCEDC will receive real time 1 sps
streams of GPS displacement solutions from the California Real Time Network
(http://sopac.ucsd.edu/projects/realtime; Genrich and Bock, 2006, J. Geophys. Res.). These channels will be archived
at the SCEDC as miniSEED waveforms, which then can be distributed to the user community via applications
such as STP.
• STP sac output now includes picks from the SCSN.
• The SCEDC is exploring the feasibility of archiving and distributing waveform data using cloud computing such
as Google Apps. A month of continuous data from the SCEDC archive will be stored in Google Apps and a client
developed to access it in a manner similar to STP. The data is stored in miniseed format with gzip compression.
Time gaps between time series were padded with null values, which substantially increases search efficiency by
make the records uniform in length.
• In the past year, the SCEDC has collaborated with with NCEDC and USGS Golden, and IRIS to review the
StationXML schema, make suggested changes, and and determine what software would be needed to develop
StationXML writers and readers. Information on StationXML can be found at http://www.data.scec.org/xml/station/
PALEOSEISMOLOGY AND SLIP RATE OF THE SAN GORGONIO PASS FAULT ZONE AT MILLARD
CANYON: TESTING THE LIKELIHOOD OF THROUGH-GOING SAN ANDREAS RUPTURES (A-142)
J.D. Yule and P.L. McBurnett
Understanding the San Andreas fault (SAF) behavior in San Gorgonio Pass is essential to assess earthquake and hazard risks
in southern California. Within the San Gorgonio Pass region the SAF system becomes complex and diffuse. Here, strike-slip
segments to the north and south intersect with the thrust-dominated San Gorgonio Pass fault zone (SGPFZ). Two end-member
hypotheses explain this 20 km wide, left-stepping zone in terms large earthquake behavior. The ‘enabler’ model suggests a
through going rupture of M7.8 that ruptures from the Salton Trough to the Mojave Desert whereas the ‘barrier’ model forecast
< M7.5 ruptures to the north and south that are arrested by the complexity of SGPFZ. Trenches excavated at the northern
Millard Canyon site test these competing hypotheses by constraining the timing of large ruptures on the SGPFZ and
comparing them chronologies on the SAF to the north and south of the Pass.
Two ~55 m-long and 2-5 m-deep trenches cross a 2.5 m-high scarp that cuts an abandoned terrace riser. Paleoearthquakes are
recognized by growth strata relations and secondary reverse faults. On lapping sequences buttress ground surface rupture
events with intervals of unconformities between each on lapping unit. Ample detrital charcoal was present allowing dating of
on lapping sequences constraining rupture timing and uplift rates. Displacement in Trench 1 shows growth folding of fluvial
material with minor faulting. Preliminary total displacement measurements suggest ~9 m of total offset within the last ~2700
yrs giving an average slip rate of 3 mm/yr. Four individual faulting events can be seen implying recurrence intervals of ~600-
700 yrs.
Our preferred model is the ‘enabler’ which allows SAF ruptures to propagate through the SGPFZ. Recurrence intervals change
from ~100-200 yrs, as is common throughout the SAF, to intervals of ~600-700 yrs coinciding with lower slip rates. The SGPFZ
complex structure may discourage through going ruptures, as with the 1812 rupture ending in Burro Flats, with through
going ruptures occurring every 600-700 yrs on average. We also cannot rule out local ruptures contained within the SGPFZ as
a possible source of faulting without surface rupture continuing north or south of the zone. Since 1906 most segments of the
258 | Southern California Earthquake Center