Thrust fault slip rates deduced from coupled geomorphic
and tectonic models of active faults and folds in the San Francisco Bay
Area: Collaborative Research with Arizona State University, and University
of California, Davis
Dr. J Ramón Arrowsmith
Arizona State University
Department of Geology, ASU
Tempe, AZ 85287-1404
Office: (602) 965-3541
Fax: (602) 965-8102
University of California, Davis
Department of Geology, University of California, Davis
Davis, CA 95616
Office: (530) 752-6808
Fax: (530) 752-0951
Element II: Evaluate Urban Hazard and Risk. Determine the
geometry, location, and rate of deformation on fold and thrust-fault structures
in SF Bay Area.
Key words: Neotectonics, Quaternary Fault Behavior, Tectonic Geomorphology,
Surface Deformation, Fault Segmentation
Table of Contents:
III. Data Availability
IV. Non-Technical Summary
Resulting From NEHRP Research
The Southern San Francisco Bay Area shows partitioning
of deformation into strike-slip faulting and thrust faulting. The
strike-slip component of deformation is accommodated along the San Andreas,
Hayward, and Calavaras faults while thrust faults accommodate the component
of plate-normal convergence. Many of these thrust faults are poorly
studied or identified; however, we see evidence of their activity in the
active uplift of the Santa Cruz Mountains in the southern Bay Area.
These faults may pose significant seismic hazard to the southern Santa
Clara Valley; therefore, the identification of the location of these faults,
their recent activity, and their slip rate is necessary to make an accurate
earthquake evaluation of the Southern Santa Clara Valley.
This study hypothesizes that the topography in the
Santa Clara Valley is influenced by movement along these thrust faults
in the area. An analysis of topography may provide information about
the location of these faults and an idea of the relative magnitude of the
slip rates along these faults. We performed such morphometric analyses
to identify areas of high uplift and movement along identified faults.
Moreover, we developed a process-based numerical model that simulates the
response of the landscape to active faulting. We will use these models
in combination with our morphometric analyses to constrain fault slip rates.
In addition, the error associated with applying the models to study areas
will be evaluated.
We performed a morphometric analysis of the Southern
Santa Clara Valley Thrust Zone (SSCVTZ) in the area surrounding Loma Prieta,
California. This area was northeast of the epicenter of the 1989
Mw = 7.1 Loma Prieta Earthquake which caused extensive
damage to the San Francisco Bay Area. We calculated several morphometric
parameters for the area. One parameter, the topographic residual, is calculated
by subtracting a surface interpolated from the stream bottoms in a landscape
from a surface interpolated from the ridge lines in a landscape.
This parameter measures the relief within a drainage basin; therefore,
high values of the topographic residual often correlate with high uplift
rates. Our results show that there is a strong correlation between
active uplift (manifest by high exhumation rates deterined by related research
on apatite fission track dating) and the topographic residual parameter
in the Loma Prieta area (Figure 1).
Figure 1: The topographic residual
surface is the difference between the surface interpolated from the tops
of the ridges in a landscape and the surface interpolated from the bottom
of the stream channels in a landscape. The topographic residual surface
is a geomorphic parameter which measures how steeply channels are incised
into the landform. In areas undergoing rapid uplift, the channels
will incise steeply into the surrounding soil introduced by the uplift,
and ridge lines may not drop as quickly. Therefore, high values of
the residual should correlate with rapidly uplifting areas. A morphometric
analysis of the landforms surrounding Mt. Loma Prieta (above) shows that
high values (orange and red)of the topographic residual correlates with
high uplift rates.
In addition to performing morphometric analyses of the
Loma Prieta area, we developed standardized methods for calculating morphometric
parameters. One of the major problems with extracting quantitative
information from morphometric parameters is the non-reproducibility of
results due to the amount of human interpretation made in the calculation
of these parameters. The methods we developed are performed automatically
on digital topography (such as 7.5 minute USGS Digital Elevation Models),
and are fully reproducible and uniquely defined by three input parameters
(critical flow accumulation, inverted flow accumulation, nearest neighbor
search radius). We intend to provide these tools to the scientific
community in order to standardize the calculation of morphometric parameters.
Morphometric analyses used in isolation cannot yield
quantitative information about fault slip rates because of the role of
geomorphic processes in determining the morphology of landscapes.
In order to extract quantitative information from this morphology, we must
consider how morphometric parameters are effected by differing geomorphic
and tectonic rates. By constraining geomorphic rates using field
investigation and remotely sensed data, we can calibrate our morphometric
analyses and extract fault slip rates from the shape of the landforms.
We developed a process-based two-dimensional numerical model which simulates
landscape development in areas undergoing active crustal deformation (a
schematic flowchart of the model is presented in Figure
2). We have reproduced the development of fault scarp-scale
features (10s of meters; Figure 3) as well
as the initiation and development of channel networks (Figure
4). We will use this model to extract slip rates along the
SSCVTZ based on our morphometric investigations.
Figure 2: Schematic representation
of our process-based numerical hillslope development model. Both
geomorphic and tectonic processes are considered when simulating landscape
development. These models will be used to gain intuition about how
tectonic rates affect the morphology of landscapes. Finally, we intend
to use these models to calibrate our morphometric investigations of the
SSCVTZ to estimate slip rates from the shape of the landscape in that area.
Figure 3: Simulation of the
development of landforms resulting from incision into a fault scarp.
In this model, two channels are specified in order to transport material
across the scarp. The surrounding hillslopes respond to the incision
by processes such as creep and rainsplash (diffusive). This model
includes the processes of channelization, rainsplash and creep, and the
interaction between these processes. In previous models, one set
of processes had to be specified and results were one dimensional.
Zero sediment flux boundary conditions were specified along all boundaries
of the model, leading to the formation of fans in front of the scarp.
Values for the diffusivity of hillslope materials were set to 10 m3/ka,
the horizontal unit scale was set to 10 meters and the vertical unit scale
was set to 10 meters. Modeling geomorphic redistribution over a fault
scarp in more than one dimension using process-based models has not been
Figure 4: Simulated development
of a drainage basin over 2,000 years. In this model, precipitation,
infiltration, runoff, hillslope and fluvial sediment transport, hillslope
and fluvial sediment deposition, channel formation and headward growth,
and landsliding are all considered. By 1,000 years, a realistic drainage
network (dashed black lines) has been established and continues to be modified
over the next 1,000 years. No tectonic input is considered in this
model. The boundary conditions for this model include constant elevation
boundary conditions along the south, east and west boundaries and constant
flux (Qwater = 0.0, Qsediment = 0.0) along the northern boundary.
Chanelization is initiated when the logarithm of the product of the local
slope and contributing area exceed some critical value. The model
unit dimensions are 10 meters in the horizontal dimensions and 10 meters
in the vertical dimension.
In order to bridge the gap between the simulations
and extracting tectonic rates from the landscape, we need to constrain
the rates of geomorphic processes. We made high-resolution Real-Time
Kinematic Global Positioning System (RTKGPS) measurements of landforms
cutting across two fault zones in the SSCVTZ. In the vicinity of
Rancho San Antonio Open Space Preserve, the Berrocal and Monte Vista Fault
Zones cut fluvial terraces and produce steep topography (Figure
5). The measurements will be used to evaluate recent activity
along the Berrocal and Monte Vista Fault Zones as well as estimate geomorphic
rates in the area. In addition, the study shows that collecting data
using the RTKGPS technology provides a means of accurately measuring small
landforms (accuracy < 2 m) for a morphometric analysis and for comparing
field observations with our numerical models (Figure
Figure 5: We surveyed landforms
crossing the Monte Vista and Berrocal Fault Zones using high resolution,
Real-Time Kinematic GPS Total Station surveying equipment. Terraces
and the surrounding hillslopes in Rancho San Antonio Open Space Preserve
were surveyed. The dashed gray box shows the location of the surveying
data shown to the right. We plan to use this surveying data with
our numerical faulting and hillslope development models in order to estimate
the slip rate on the Berrocal and Monte Vista Faults.
Figure 6: This figure is a
three-dimensional view of the data collected within the surveying area.
This view is looking east across the landform. Note the significant
vertical exaggeration in this figure. The Berrocal Fault is directly
behind the crest of the hill in the northeast corner of this figure.
We acquired four types of digital data in our study.
Digital Orthophoto Quadrangles (DOQ) were used to identify suspect geomorphic
features. Digital Elevation Models (DEM) were used to perform the
small-scale morphometric analysis of the Loma Prieta area. The hydrology
of the area was determined from Digital Line Graphs (DLG) of the surface
hydrological features and from flow routing and accumulation algorithms
performed on the DEMs. We downloaded these data from the Bay Area
Regional Database (BARD). In addition, we used the General Distribution
Of Geologic Materials In The Southern San Francisco Bay Region, California:
A Digital Map Database (OFR-93-693) to create regional geologic maps and
identify suspect faults in the area. These data were obtained from
the USGS web server at ftp://wrgis.wr.usgs.gov/pub/geologic/ca/of93-693/ssfb_m1.tar.Z.
This technical summary and other information related to this project are
available at http://www-glg.la.asu.edu/~at/.
Slip along thrust faults in the Southern Santa Clara Valley
area form the Santa Cruz Mountains and may pose significant seismic hazard
to this area. We use the shape of the landscape in order to identify
the location of active faults. We develop a process-based numerical
model in order to understand how fault slip rates along these faults affect
the topography of the area. These models will be used in combination
with topographic data to constrain slip rates along the thrust faults.
We collected high-resolution topographic data to constrain geomorphic rates
in the area. These rates will be used as input to our numerical models.
The end result of our research will be 1) the development of techniques
for identifying the location of faults in areas of active crustal deformation
and the determination of their slip rates based on the shape of the surrounding
landforms, and 2) the determination of these rates along faults in the
Southern Santa Clara Valley in order to provide input to a regional seismic
Resulting From NEHRP Research
Hilley, G.E., Arrowsmith, J R., Bürgmann, R., Investigation of
Active Deformation Using a Landscape Development Model and Field Examination
of Landforms and Geology Along the Northeastern Margin of the Southern
Santa Cruz Mountains, Geological Society of America Abstracts with Programs,
1997 Annual Meeting, 1997.