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PSUsing
Petroleum Industry 3D Surveys to Improve Understanding of Active Faults: The
Palos Verdes Fault
in San Pedro Bay, California*
By
Andrew W. Rigor1, Mark R. Legg2, Robert J. Mellors1, and Robert D. Francis3
Search and Discovery Article #40088 (2003)
*Adapted for online presentation from award-winning poster at Student Expo, Norman, OK, March, 2003.
1San Diego State University ([email protected])
2Legg Geophysical
1California State University Long Beach
Abstract
Large earthquakes
on the Palos Verdes fault
may be destructive to the nearby urban areas of the
densely populated Los Angeles Basin. We interpret an industry 3D seismic survey
(244 km2, CDP spacing 24.6 m inline, 50.3 m cross-line), and process
and interpret three intersecting shallow 2D profiles (16 channel, 0.95 s depth,
3.125 m CDP spacing), to map the shallow geometry (<3.0 s two-way travel time)
of the
fault
zone in San Pedro Bay. Several reflectors, tied to known
stratigraphic boundaries using well logs, provide timing of interpreted features
that are used to improve understanding of the
fault
activity and associated
seismic hazard.
We observe five
distinct fault
segments in the area of the 3D survey. Each segment consists of
one primary strand that is near vertical to at least 3.0 s two-way travel time,
and one to five secondary strands forming a zone that varies from 700-2400 m
width. Several of these
fault
strands break latest
Quaternary sediment and are associated with bathymetrically observed deformation
at the surface. Deformation character at
fault
bends is consistent with a
right-slip dominated
fault
zone. Seismic reflections of growth and no-growth
sequences above anticlines to the west of the
fault
and in the Quaternary
deposits of the Wilmington graben to the east of the
fault
are imaged in the
2D surveys. These alternating styles in
vertical deformation may be associated with lateral movement of sedimentary
sequences along bends in the main
fault
strand.
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(Complete captions accompany full-size images and follow text.)
Click here to view sequence of Figure4(a), (b), and (c).
Click here to view sequence of Figure 6(a), (b), (c), (d), and (e).
Why Study the
3D Seismic Reflection Data (Figure 2)
2D Seismic Reflection Data (Figure 3)
The Palos
Verdes Time slices
reveal a releasing bend in the southern part of the study area and a
restraining bend in the center of the study area (Figures 4 and
5). The
Anticlines
within the study area are the San Pedro Bay anticlinorium and the “Beta
ear” anticline west of the
The narrow,
tilted wedge of Quaternary strata southwest of the
3D data provided by Chevron-Texaco. Research funded by Southern California Earthquake Center.
Figure 1.
Index map. The box on the inset map shows the location of the Palos Verdes
Figure 2.
Chair-cut visualization of Beta oil field 3D survey, cut at line 120, CDP 650, and time 1800 ms. Positive amplitudes are in red; negative
amplitudes, in blue. The general northwest trend of the
Figure 3. A
fence diagram of the locations of three 2D lines on top of the 1436-1492
ms 3D time-slice. Green vertical lines show well locations with log
data. Palos Verdes
Figure
4(a-c). Beta 3D seismic time slices. (a) 188 ms, (b) 368 ms, and (c)
1468 ms. Green lines show intersection of high resolution 2D seismic
lines with 3D time slice. The main trace of the Palos Verdes
Figure 5.
The location of the segments and features of the Palos Verdes
Figure
6(a-e). Individual cross-sections through segments A through E are
labeled A-A’ through E-E’ and shown with interpreted faults and
horizons. Anticlines within this survey are the San Pedro Bay anticlinorium (SPBA) and the “Beta ear” anticline to the west of the
Figure 7. A
broad anticline, perpendicular to the
Figure
8(a-b). Close-ups of 300 ms of 2D data from (a) line 80a, above, and (b)
81, below, have been annotated to show approximate location of the
Quaternary wedge of sediments overlying the Palos Verdes Hills and San
Pedro Bay anticlinoria (unconformity in green), the Palos Verdes Figure 9. 2-D, E-W seismic line
81 that extends across and beyond the study area. The Palos Verdes
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