GMEG - Geology and Geophysics
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What is this project about?The project combines traditional surficial geologic mapping with subsurface geophysical modeling to create a three dimensional (3D) map of the Earth's crust in the vicinity of the San Andreas Fault Zone in central California. The goal is not only to make a 3D map the can be manipulated for viewing, but in integrate an earth history dimension (the 4th dimension - 4D) with the goal for advanced modeling and interpretation of the past, present, and future evolution of the crust and landscape along the San Andreas Fault system. A 3D/4D mapping project products will have widespread application for studies of earthquake and fault motion behavior, as well as applications relevant to impacts on infrastructure and civil planning and development, and education.
See Geologic Maps of the San Fancisco Bay Region.
Background: Recent success in modeling the geology of fault faces (Hayward, Parkfield) and building the northern California 3D geologic map for use in strong motion simulations leads us to propose systematic modeling of the San Andreas Fault in northern and central California. The San Andreas Fault (SAF) is a world-renowned icon of both earthquake science and plate tectonics. The great San Francisco earthquake of 1906 signaled the birth of earthquake science in the US. Interest in the SAF was heightened when it was recognized to be an exceptional example of a transform fault, one of the fundamental plate boundary types in plate tectonic theory. Because of its prominent role in these two important disciplines and because it poses a threat to people throughout the state, it has received intense scrutiny for the past 100 years.
|Click on images for a larger view, or see these and more on the Science Tour (a series of linked illustrated web pages).
Left: Map of the San Andreas Fault System
Below right: 3D geologic block diagrams of the Hayward Fault Zone
WHAT scientific questions hope to be addressed?
Despite intense study, we are only marginally closer to understanding some of the most fundamental aspects of the behavior of the San Andreas Fault:
• Why are some sections capable of producing great earthquakes whereas others produce only small ones?
• Why do earthquakes start and stop where they do?
• Why do parts of the fault creep continuously whereas other parts seem to fail only during earthquakes?
• What controls fault segmentation (reaches of the fault that will rupture during an earthquake) and do earthquakes repeatedly rupture the same segments?
• What are asperities and what controls their location and behavior?
• What controls rupture velocity?
• Perhaps most importantly, what are the likely consequences of future large earthquakes on the San Andreas Fault in terms of injury, loss of life, and economic dislocation?
Addressing the last question first, computer simulations of ground shaking from the 1906 San Francisco and the 1989 Loma Prieta earthquakes (http://earthquake.usgs.gov/regional/nca/1906/simulations/) indicate that quantitative predictions of shaking sufficiently precise to be used as inputs to test building designs and on which to base disaster planning are within our grasp. 3D geologic maps are at the very foundation of our ability to make such predictions. Such maps do not exist for most of the SAF, and where they do exist, they need refinement.
With regard to the other questions, we argue that perhaps the most important reason that answers about the behavior of the SAF remain elusive is that the geology at seismogenic depths is unknown. Where we know something about the deep geology, intriguing relationships are emerging between fault-face geology and seismicity (Hayward Fault) or segments that have ruptured (Loma Prieta, Parkfield, Coyote Lake).
Even knowing the geology at seismogenic depths will likely not be sufficient to completely understand fault behavior because the behavior is actually a three-dimensional process through time (4-D). Previous efforts at palinspastic reconstruction of the continental margin have largely been two dimensional (x-t or strike-slip offset through time), although some small-scale three-dimensional (x-y-t, or surface offsets in map view through time) models have been constructed (for example, Atwater and Stock, 1998, Int. Geol. Rev., v. 40). With 3-D geologic maps, we have an opportunity for the first time to do detailed reconstruction of the full suite of deformation processes active in the fault zone through time to create 4-D (x-y-z-t, or volumetric deformation through time) geologic reconstructions. A reconstruction such as this will improve our understanding of the development of the San Andreas Fault system, including how the 3-D geology has affected that development. Moreover, a 4-D reconstruction should give better constraints to our estimates of long-term slip amounts and rates, uplift and erosion rates, and basin formation rates. Because this kind of reconstruction has not yet been accomplished, the full scope of the scientific potential is at present unknown.
This project rests on past experience gained during the initial Venture Capital Project (3-Dimensional Geologic Maps, Howell and Jachens, 1997) and the now ending 3D Geologic Maps and Visualization NCGMP Project. What we learned is that the key ingredient to successfully constructing 3D geologic maps is to maintain a close, integrated, and continuous working interaction between geologists, geophysicists, and other geoscientists dealing with their own individual data sets and expertise.
|WHAT goals and objectives will the project hope to accomplish?
The goals of this project are:
Goal 1: to collect, interpret, and publish regional potential field geophysical data and Earth surface geological data,
Goal 2: to construct a digital three-dimensional geologic map along the San Andreas Fault between the Transverse Ranges and Point Arena,
Goal 3: to build on this 3D map to construct a four-dimensional (4D) ‘map’ of the evolution of the fault and surrounding regions through ‘San Andreas’ time, and 4) to use these products in collaboration with our colleagues in the Earthquake Hazards Program and academia to address questions of fault behavior, particularly those laid out in the ‘Statement of Problem’ section. The map will nominally include geology out to 15 km on either side of the fault, with flexibility to extend the coverage farther where geologic problems dictate. The map will extend 15-20 km
below sea level.
At the surface, the map detail will be at resolution appropriate for visualization at a scale of 1:100,000, but by necessity the 3-D map will have lower resolution at depth, the level of detail at the base of the 3-D map being controlled by the geophysical signatures of the various deep crustal units. The map will be constructed of 3D surfaces in a rules-based architecture.
|Plate tectonic map of northern California.|
WHAT tasks are part of this project?The project will be organized into three tasks:
TASK 1) 3D/4D geologic map construction and assembly
TASK 2) surface geologic studies
TASK 3) geophysical/subsurface geologic mapping
New geologic mapping is needed in three areas, and this new mapping will be supported by concurrent gravity and magnetic interpretations in cases of ambiguity or where critical relationships are concealed. New subsurface geologic mapping is needed along most of the fault, and will follow the philosophy of starting with the surface geologic map and progressively carrying critical contacts into the subsurface. This work will depend heavily on modeling of potential field anomalies, detailed analysis of seismic tomography models, analysis of micro-seismicity, and integration of drill hole data. Construction of the 4D geologic map will require integration of the results of tasks 1) and 2) within a framework that explicitly incorporates time-histories of along-fault, cross-fault and vertical deformation. This task will be sequenced geographically, with the Bay Region section (between the San Andreas/Calaveras junction and the San Andreas/San Gregorio junction) being the focus of the initial effort.
HOW …is the project being conducted ?The 3D geologic map will be assembled in EarthVision (TM), a rules-based geologic structure building software system which has served us well in constructing past models. The system is designed for 3D geologic maps so the explicit incorporation of temporal evolution will require innovative applications of the EarthVision system or application of new computing/visualization software. Specifically, we will need software that can handle 3-D bodies moving and changing shape through time. It seems likely that 3-D animation software will be more effective in handling this problem. Answering the question of what tools are required will part of the work of this Project.
|A serpentinite diaper in a fault zone in the Northern San Francisco Bay region.|
WHY is this research important?Background: Geologic mapping developed during previous Northern California Projects is known to have been used in land use and(or) emergency planning by the cities of Mill Valley, Oakland, Berkeley, San Jose, Benicia, Pittsburg, and Albany, as well as Contra Costa, Solano, and Napa County, and California Dept. of Transportation, Dept. of Energy, and Public Utilities Commission, plus Lawrence Berkeley National Lab (U.S. Dept. of Energy) and National Park Service. These maps have also been used in the preparation of Seismic Hazard Zone Maps by CGS.
In FY08, modeled shaking based on the 1989 Loma Prieta earthquake has yielded bands of amplified shaking related to the 3-D shape of the Cupertino basin and corresponding to areas of concentrated damage observed after the real earthquake. 3-D mapping of the Hayward Calaveras Fault has revealed a direct connection at large earthquake depth, raising the possibility that the two faults could rupture together to produce earthquakes 2-4 times larger than that estimated for the Southern Hayward Fault alone. This observation has led to inclusion a combined rupture in the ongoing modeling of the Hayward Fault for the 140-year commemoration of the 1868 earthquake as well as follow-up studies.
In FY09 we developed a new tectonic model of San Andreas-normal deformation in the Coalinga region that suggests seismic hazard there is independent of SAF strike-slip. We began collaboration with PG&E to study the 3D geology and seismology of the Central Coast Ranges adjacent to the SAF. We developed a new technique to map the 3D extent of folded magnetic layers.
|Relevance and Impact: The recent successes of computer simulations in of the 1906 San Francisco and 1989 Loma Prieta earthquakes demonstrated that accurate estimates of ground shaking are possible provided they are based on realistic 3D geologic maps with physical properties assigned according to geologic unit and 3D position. A graphic example of this success was the strong ground shaking at Santa Rosa from the 1906 earthquake simulation, which is in accord with the severe damage suffered by Santa Rosa in 1906 that has always been a puzzle, given that Santa Rosa was far from the epicenter. The output from such simulations can be used to test building designs and by disaster planners as input to computer models that predict the numbers and distribution of deaths, injuries, and property losses. The project will deliver
a 3D geologic map of the crust surrounding the entire onshore reach of the San Andreas Fault north of the Transverse Ranges, the foundation on which simulations can be built. Without a suitable 3D geologic map, the other steps are not possible.
This work can also have an important impact in preparing for future earthquakes. Assessing the possible effects of future earthquakes is a more difficult task than simulating past ones because there are many more unknowns—location, magnitude, and rupture dynamics must all be specified. Recent work on the Hayward, San Andreas, Calaveras Faults suggests that analysis of the 3D geology along the fault may be the key to defining or constraining these parameters without waiting for an earthquake to do so. The impact of our work will be significant for planning for future earthquakes on the San Andreas Fault. Moreover, the San Andreas Fault serves as a model for strike-slip faults worldwide, so the results of this project will have significance well beyond coastal California.
|Geologic map and cross section of the Wilbur Hot Springs area, northern California.|
|The impact of the 4-D reconstruction is in advancement of scientific technique and understanding. No such reconstruction has yet been attempted, but the required building-blocks of a 3-D geologic map and significant age controls are now possible. Although the work is immediately relevant to the understanding of the San Andreas Fault and its ongoing processes, because this is a previously untested approach the complete spectrum of its impact and relevance is unknown.|