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Group 2 – Earthquake Geology | Poster Abstracts
Newberry Springs, California ~30 km east of Barstow. The trenches exposed fluvial, lacustrine, and
playa sediments, consisting of pebble gravel and coarse-grained sands, silts, and clays. Although
motion on two parallel fault strands that cross the site appears to be predominantly strike-slip,
secondary components of vertical movement have created a down-dropped block between them.
Twelve optically stimulated luminescence dates bracket surface rupture ages. The three most
recent surface ruptures corroborate the pattern of earthquake clustering originally proposed in the
Eastern California Shear Zone (ECSZ) by Rockwell et al. (2000). Specifically, the most recent event
occurred during latest Holocene time between 0.6 to 1.1 ka and 2.0 ka. This age suggests that the
Calico fault has ruptured as part of the ongoing latest Holocene cluster. The penultimate and antepenultimate
surface ruptures both occurred during a brief interval between 5-6 ka (event 2 [5.0-5.6
ka] and event 3 [5.6 to 6.1 ka or possibly 7.3ka]). Both of these ruptures fall within the penultimate
seismic cluster on the ECSZ. One possible explanation for two earthquakes within the same cluster
could be that the Calico fault slips ~2x faster than nearby dextral faults (Oskin et al., 2008). This
higher slip rate may be accommodated via multiple ruptures during clustering time periods.
Alternatively, the study site may be an overlap zone between northern and southern Calico
ruptures. The 1.5m vertical separation associated with event 3 is much larger than separation in
event 2. This suggests that event 3 exhibited larger displacement at the trench site. Evidence for a
possible fourth event between 6.1 to 7.3ka and 8.4ka is also present in our southern trench.
However, the age is not well defined. The absence of evidence for earthquakes between 2-5 ka falls
within the seismic lull on the ECSZ found by Rockwell et al. (2000) – a time period when
paleoseismic evidence suggests a cluster of activity in the network of faults beneath the Los
Angeles region and central Transverse Ranges (Dolan et al., 2007).
2-041
RECENT OFFSET MEASUREMENTS, COACHELLA VALLEY SEGMENT, SAN
ANDREAS FAULT Williams PL, Hudnut KW, and Seitz GG
The Coachella Valley Segment (CVS) of the southern San Andreas fault (SSAF) has not ruptured in
a large or great earthquake during the past ~330 years (Sieh and Williams, 1990). Given that
recurrence intervals of ~200 years have been found for the CVS (Fumal et al., 2002; Williams and
Seitz, 2007; Philibosian et al., 2007), the long quiescence points to a particularly high seismic hazard
(e.g. WGCEP, 2008). This paper presents detailed geomorphic slip-per-event evidence for the
southern 50 km of the CVS. A sufficient sample of the fault's past behavior should indicate what to
expect from future earthquakes. Detailed descriptions of channel offsets are presented for 65
independent sites, some of these preserve evidence of multiple offset events. Compilation of the
survey indicates that the latest five earthquakes produced moderate, 3±1 meter offsets, with
cumulative displacement of 15-17 meters. Interpreted in the context of the paleoseismic data
referenced above, these data suggest a CVS slip rate of 16±1mm/yr. The data reported here were
initially developed by use of aerial reconnaissance and field mapping, and were subsequently
checked, refined and extended utilizing Lidar data obtained from the NCALM B4 project. Several
independent features in the range of 6-25 meters were discovered during examination of the Lidar
dataset, and in many cases the Lidar substantially improved assessment of fault location and
confirmed the sizes of larger offsets.
2-042
AN UPDATE ON ACCESS TO GEOEARTHSCOPE LIDAR TOPOGRAPHY DATA
Crosby CJ, Nandigam V, Arrowsmith JR, Balakrishnan S, Baru C, and Phillips DA
The recently completed GeoEarthScope airborne LiDAR (Light Detection And Ranging)
topography acquisition will provide unprecedented data adjacent to active faults throughout the
plate boundary region of western North America. Totaling more than 5000 square kilometers, these
2008 SCEC Annual Meeting | 159