AT&G
- Spotila's active tectonics and geomorphology research
Fault interaction along the North Frontal thrust
system, southern California
Scope: The segmentation of thrust faults has
important implications for earthquake rupture dynamics and the seismogenic
potential of active faults. In Los Angeles, for example, thrust faults of
the Transverse Ranges
are broken into many segments by intersecting strike-slip faults. If they
rupture in small pieces, the resulting earthquakes are moderate, like the 1994
M6.7 Northridge earthquake. If the strike-slip faults do not represent
segment boundaries, these thrust faults may experience larger ruptures
associated with greater earthquake size, which has not yet been observed but
which would be devastating to Los Angeles. Unfortunately, L.A. is not an
ideal place to study this problem of seismic hazards and fault interaction
dynamics, because its young (Holocene) tectonic geomorphology has been largely
modified by human activity. To address this problem, we are studying the
recent rupture history of an analogous structural setting to the east, where
the reverse faults of the Transverse Ranges are intersected by the Eastern
California shear zone. This setting provides an interesting structural
problem, because the two fault systems actually intersect. Although no
cross-cutting relationship can be determined, it seems likely that one fault
system has been rendered inactive by the other. Some of these strike-slip
faults have ruptured in the Holocene, suggesting perhaps the thrust fault has
gone inactive. Out work has shown, however, that at least some segments
of the North Frontal thrust system are active and have ruptured in the past
10,000 yrs. We have proposed an alternative model, in which westward
propagation of the Eastern California shear zone has rendered the eastern half
of the thrust system inactive, while leaving the western half active (Spotila
and Anderson, 2004). During our
paleoseismic investigations, we also refined techniques for characterizing
trenching sites on reverse faults in arid environments using ground-penetrating
radar (Anderson et al., 2003).
Personnel: Kevin Anderson
(M.S. 2002), John Hole
(Virginia Tech)
Funding: National Earthquake Hazards Reduction
Program (NEHRP, USGS), Award
#00HQGR0028, 3/00-3/02.
Links: There are numerous on-going neotectonic
projects that relate to this study, including work by Missy Eppes (UNC Charlotte) and Les McFadden (U.
New Mexico) on the tectonics and soil development on structures along the North
Frontal thrust system, John Matti, Fred Miller, Bob Powell, and the USGS
Southern California Mapping Project (SCAMP)
on the general structure and geology of the rangefront, Tom
Rockwell (SDSU) and Charlie Rubin
(CWU) on paleoseismology of the Eastern California shear zone, and the general
efforts of the Southern California Earthquake
Center and Caltech Seismo Lab
to constrain earthquake hazards in southern California. Here's a link to PRIMELAB (Purdue), where we had
cosmogenic ages measured on some samples from faulted fanglomerates along the
rangefront.
Photo of the North Frontal thrust fault exposed in a trench
through young alluvium. The 1.7 m dip-slip offset of a carbonate paleosol
horizon is readily visible. This offset must postdate the age of detrital
charcoal retreived from the hanging-wall one meter left of the tape measure,
which we have dated with radiocarbon as 9.7 Ka. This demonstrates
Holocene activity along the North Frontal thrust system.
Here is an airphoto of the North Frontal thrust system. The trench site was in the area shown
by the box.
Below are two ground-penetrating radar profiles across the
thrust fault. Figure A shows the
location trenched in the photograph above (trench log is below). The second figure shows a nearby
profile that was not trenched. This
site runs along a recent drainage channel incised into an 8-meter high scarp in
older alluvium. Potential
trenching sites are indicated by the south-dipping reflectors. The north-dipping reflector is assumed
to be a buttress-unconformity along a bend in the channel.
Below is a summary figure from our trench log (same trench
shown in above photograph):
Click
here for more photos of
the North Frontal thrust system.
Comments to: spotila@vt.edu