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 1434-HQ-97-GR-03113 Dr. J Ramón Arrowsmith George Hilley Arizona State University Department of Geology, ASU Tempe, AZ 85287-1404 Email: ramon@pangea.stanford.edu Office: (602) 965-3541 Fax: (602) 965-8102 http://www.public.asu.edu/~arrows Roland Bürgmann University of California, Davis Department of Geology, University of California, Davis Davis, CA 95616 Email: burgmann@geology.ucdavis.edu Office: (530) 752-6808 Fax: (530) 752-0951 http://www-geology.ucdavis.edu/faculty.html#burgmann 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:

I. Introduction
II. Results
III. Data Availability and Acquisition
IV. Non-Technical Summary
V. Publications Resulting From NEHRP Research

Introduction
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.

Results
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 M_w= 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 previously done.

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 6).

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.

Data Availability and Acquisition
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/.

Non-Technical Summary
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 hazard assessment.

Publications 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.