Enhanced imaging of fault zones in southern California from seismic
reflection studies
University of Nevada, Reno, USA, (1997)Abstract
This dissertation focuses on processing and migration techniques to
enhance the seismic images of two types of faults that are diffcult
to map in the subsurface: normal and blind thrust. I examine normal
faulting in Death Valley, California, with deep crustal reflection
data; and blind thrust faulting in the Los Angeles basin, California,
with earthquake data. The main findings of my research can be summarized
in two parts: 1. Enhanced imaging of a normal fault with seismic
reflection data: Motivated by the need to image faults to test Cenozoic
extension models for the Death Valley region of the western Basin
and Range province, an area of strong lateral velocity variations,
I examine the geometry of normal faulting in southern Death Valley
by seismic depth imaging. I analyze COCORP (Consortium for Continental
Reflection Profiling) Death Valley Line 9 to attain an enhanced image
of shallow fault structure to 2.5 km depth. Previous workers used
standard seismic processing to infer normal faults from bed truncations,
displacement of horizontal reflectors, and diffractions. I use a
detailed velocity model obtained by nonlinear optimization of first-arrival
times picked from shot gathers, examine the unprocessed data for
fault reflections, and use a Kirchhoff prestack depth imaging procedure
to properly handle lateral velocity variations and arbitrary dips.
Fault-plane reflections reveal the listric true-depth geometry of
the normal fault at the Black Mountains range front in southern Death
Valley. This is consistent with the concept of low-angle extension
in this region and strengthens its association with crustal-scale
magmatic plumbing. 2. Enhanced imaging of crustal faults with earthquake
data: An inexpensive means to understand further the geometry of
active faults in southern California arises from the use of aftershock
recordings to image crustal structures. The advent of regional seismic
networks that record digital seismograms from hundreds of stations
makes this crustal reflectivity profiling possible even in the absence
of conventional active-source seismic data. I show it is feasible
to image fault structure using three-dimensional, wide-angle prestack
Kirchhoff migration. I achieve this with the use of aftershock traces
recorded on the short-period vertical stations of the Southern California
Seismic Network. This work complements seismicity and focal mechanism
work by imaging reflectivity volumes and cross sections rather than
having to associate events with certain faults. Further, it can image
below the seismogenic zone to resolve current geologic controversies
on how pro- posed faults extend below focal depths. I demonstrate
the validity of these images as showing reflective structures, and
the ability to use clipped high-gain seismograms as sign-bit data
to yield valid geometric imaging. Work with data from the 1991 Sierra
Madre earthquake sequence images the prominent lower crustal reflective
zone observed beneath most of the San Gabriel Mountains by the Los
Angeles Region Seismic Experiment Line 1. Aftershocks of the 1994
Northridge earthquake allow me to image a north-dipping structure
that may represent the fault plane of a crustal-penetrating blind
thrust. The images serve as a test for the existence and geometry
of thrust ramps and detachments proposed from balanced-section reconstructions
of shallow-crustal profiles and borehole data. My results are more
consistent with a thick-skinned tectonic regime in the vicinity of
the Northridge earthquake, rather than a thin-skinned model. In addition,
my work leads to the following key points that summarize the overall
importance of fault zone imaging from seismic reflection studies
in the region: it is possible to image active fault zones, but I
achieve geometric imaging only, without property descriptions. I
constrained tectonic style in two provinces: extensional (Death Valley)
and contractional (Los Angeles basin). Each case required three-dimensional
treatment of sources and receivers, asymptotic assumptions, and some
knowledge of velocities. Normal faulting in Death Valley may be defined
as an intra-plate feature because it does not provide evidence that
faulting cuts the Moho; whereas the Elysian Park Thrust may be an
inter-plate feature because it appears to cut the Moho.