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The San Fernando Valley and adjacent mountains are part of the Transverse Ranges physiographic province that is composed of parallel, east-west trending mountain ranges and sediment-filled valleys. With regard to seismicity and crustal mobility, the province is one of the most active in the United States. The distinctive geological structure of the Transverse Ranges is dominated by the effects of north-south compressive deformation resulting in thrust faulting, strike-slip faulting, and bedrock folding. These are attributable to convergence between the “Big Bend” of the San Andreas fault and northwestern motion of the Pacific Plate, and expressed, for example, in thrust faulting like that exhibited by the Northridge earthquake, the 1971 San Fernando earthquake, and the 1987 Whittier Narrows earthquake.
The Transverse Ranges of southern California comprise several east-west trending mountain blocks bounded by major faults and interspersed with broad valleys. The aftershock areas of the 1994 Northridge, 1987 Whittier Narrows, and 1971 San Fernando earthquakes and principal faults are shown for their relations to the physiography.
The western Transverse Ranges are an outstanding example of a geological model of folds and uplifts produced by slip on buried faults. This model is used to help explain the occurrence of earthquakes like the Northridge and Whittier Narrows earthquakes that occurred beneath lowlands many kilometers away from the surface traces of faults that bound nearby mountain ranges. The model has helped identify several major blind thrusts throughout southern California and is being used to postulate the locations of many other buried faults. Investigators then use modern geological and seismological methods to verify the locations and dimensions of these faults.
USGS scientists are attempting to clearly map the crustal structure of the Earth beneath the Los Angeles region, including the subdivision of the crust into blocks, the properties of those blocks, and the nature of the faults that bound them. Scientists are also determining how stress is being applied to this collage of blocks because the faults accumulate strain that most likely will be relieved in the form of earthquakes. Knowing the geometry and interrelations of the blocks, and the stresses being applied, is fundamental to forecasting earthquake scenarios for planning and engineering design.
To learn more about crustal structure, the USGS and the Southern California Earthquake Center (SCEC) began a program of seismic imaging known as the Los Angeles Region Seismic Experiment (LARSE). The program began in 1993 and was expanded in 1994 to cover the region of the Northridge earthquake. This experiment was designed to produce reflection and refraction images of the crust along three lines crossing the region, including the offshore environment. Each line represents a massive data-collection system using a few hundred sources and thousands of receivers to produce images of faults and other features to depths of more than 15 kilometers. For example, 640 seismographs assembled from many institutions in North America were used for the experiment along the onshore segment of Line 1. Through the northern Los Angeles basin and San Gabriel Mountains, explosions were spaced 1,000 meters apart and the seismographs 100 meters apart to produce both a reflection and a refraction image of the crust. The first data set and preliminary interpretations available from the LARSE are from Line 1. Data sets from Lines 2 and 3 will be analyzed during 1996 and 1997. The Line 1 data show excellent results with great promise for describing the crustal structure of the Los Angeles region.
The offshore segment of Line 1 targeted the Catalina, San Pedro basin, and Palos Verdes Hills faults. Onshore, principal targets along Line 1 were the top of geologic basement rocks beneath the Los Angeles basin that had never before been imaged. Line 1 was also situated for imaging the blind thrust fault system that caused the 1987 Whittier Narrows earthquake, the Sierra Madre fault system that caused the 1991 Sierra Madre earthquake, and the San Andreas fault.
The lines imaged by the LARSE (green) were selected to cross known faults (shown in purple onshore and red offshore) and suspected features, such as buried thrust faults. The lines target such onshore features as the San Andreas fault and the buried thrust faults that produced the 1987 Whittier Narrows and 1994 Northridge earthquakes.
Data from Line 1 show the Catalina and San Pedro basin faults, and evidence of the top of basement rocks beneath the sedimentary and volcanic rocks of the Catalina and San Pedro basins. A possible blind thrust fault offshore shows up as a reflection within basement rocks. An excellent record from near the crest of the San Gabriel Mountains (p. 18; not shown in the accompanying section) shows a prominent reflective zone as shallow as about 22 kilometers. This zone, interpreted to lie chiefly or entirely in the lower crust, almost certainly represents an important change of physical properties, or a “block” boundary within the crust.
The cross section of the Earth’s crust offshore from Los Angeles shows features to a depth of about 6kilometers. Note the contrast of relatively flat-lying features above basement in the Catalina basin and the heavily distorted features in the San Pedro basin. Reflections within the basement in the eastern part of the section may be evidence of a blind thrust fault. “FZ” indicates known fault zones (see map on p. 19). This image was produced using a sonogram-like technique (see p. 21).
Preliminary images from the LARSE have illuminated many of the structural features originally targeted including offshore faults, basement rocks beneath the Los Angeles basin, and deep crustal structure beneath the San Gabriel Mountains. Given the generally high quality of the data, more refined analysis will likely resolve structures in the upper crust and additional deeper structures. The LARSE data from Line 1 are a promising beginning in defining the various crustal blocksand their bounding faults that make up the tectonic framework that causes the earthquakes in the Los Angeles region.
Using the CAT Scan-like technique, scientists analyze sound and other waves transmitted (or “refracted”) through the Earth. Waves traveling more slowly in some directions than in others allow one to outline the shape of regions of slower wave-speed transmission. (In a CAT Scan of the brain, tissue and cavities attenuate X-ray energy differently, and an image of the brain can be constructed using the different patterns of energy transmission.) In the case of a fault in the Earth, waves passing through it are commonly slowed down, and their patterns can be used to delineate the zone surrounding the fault. The zones of low wave speed surrounding faults are believed to be caused in part by tiny open cracks in the rock that are induced by high strain near the faults. Low wave speed can also be caused by chemical effects of circulating ground water that alters the composition of rock in a fault zone.
Using the sonogram-like technique, scientists piece together faint echoes (the waves reflected from buried rock layers and faults) into a picture of the objects that produced the echoes. (In a sonogram of a pregnant woman, the unborn baby actually reflects sound-wave energy back to the surface of the abdomen, and thus can be imaged.) In some cases, one can see the fault itself. In most cases, however, one can only see the fault indirectly, such as in the offset of one or more layers in the rock cut by the fault. A sonogram-type image is shown on page 20.
|Imaging Faults with Seismic Methods
A fault is a break in rock along which movement occurs or has occurred. In the upper part of the Earth’s crust where rocks are brittle (generally 0-16 kilometers deep), movement occurs on faults in jerks we call earthquakes. Because earthquake shaking is commonly strongest at points on the Earth’s surface closest to the movement on a fault, it is important to get a depiction or image of faults at depth in order to determine danger zones on the surface. Scientists create images of faults using sound waves generated by explosions or other means at the surface, or by waves from earthquakes. They form images by either of two techniques that can be likened to two medical imaging techniques—“sonograms” and “CAT Scans.”
Digital Maps and Databases -- Living Electronic Documents on the Geology of Southern California
The Northridge earthquake occurred on a concealed, previously unidentified fault directly beneath an urban area. The event thus issued another reminder that, like buried faults, there remain many geological elements that need to be identified to extend our knowledge of the earthquake hazards in southern California. Particular elements within our reach of understanding include the locations of other blind thrust faults. Other such elements are slip rates and earthquake recurrence intervals on major faults that threaten the Los Angeles region, such as thrust faults of the Sierra Madre-Cucamonga system and strike-slip faults of the Newport-Inglewood system.
To help increase geological knowledge and identify deficiencies in geological information, the USGS is compiling special digital information for the region. The information includes a digital map and a database pertaining to the locations and rates of activity of faults and folds in the region. The purposes of this compilation are to (1) evaluate regional seismic hazards; (2)create derivative hazards maps; (3)evaluate the distribution and quality of existing data; and (4) focus on future research. The concept and content of the map and database were developed and refined during meetings with the California Division of Mines and Geology (CDMG) and SCEC in 1994-95. These meetings, culminating in a SCEC-sponsored workshop in March 1995, were convened specifically to confirm the needs of those who would likely be using the map and database in the future. All principals in academia, industry, and government were invited to conceptualize the product and balance its design with the uses they anticipated.
|SCEC Knowledge Transfer, with funding from the National Science Foundation and FEMA, sponsored a 2-day workshop November 9-10, 1995, entitled “Addressing Seismic Hazards in Southern California: Establishing Dialog Among Academia, the Insurance Industry, and Risk Assessment Professionals.” For information on the workshop, subsequent meetings, and other SCEC activities contact: SCEC Knowledge Transfer University of Southern California Mail Code 0742 Los Angeles, CA 90089-0472 Phone: 213/740-1560 E-mail: ScecInfo@usc.edu|
The USGS is taking careful steps to assign systematic quality ratings to information throughout the database. This is especially important in that much information needed for seismic-hazards evaluations typically varies within a range of uncertainty. A specific value used for the database might represent, for example, the value agreed upon by consensus among many investigators, or a value determined from scientific literature. Thus, a database user will need a systematic and explicit reliability rating for the data. The USGS approach will provide database users with enough information to make their own judgments about the quality of evaluations of parameters such as slip rates and other hazards indicators.
|The WWW home page for the digital geologic map and database is currently (1996) under construction. Check the Northridge Earthquake Home Page for linkages: http://geohazards.cr.usgs.gov|
The digital fault and fold map for southern California highlights blind thrust systems and other principal faults. Faults exposed at the surface are color-coded according to slip rates and earthquake recurrence intervals, with the red and orange features having higher rates of activity than the green and blue. Specific data on all faults are included in the geologic database.
For 1995-96, the USGS gave priority to synthesizing and evaluating all fault-related information based on the boundaries of 1:100,000-scale maps for Los Angeles and Long Beach. This information will be available as printed copy and electronically from the World Wide Web (WWW). Future phases of the project will enlarge geographic coverage, and incorporate additional information on folds and the age and deformation of deposits related to concealed faults. The digital map and database are designed to be continually updated through use of the WWW for reviews by users, and corrections and additions by the USGS. Thus, these products will become more comprehensive with time and remain state-of-the-art as long as they are maintained on the WWW for periodic upgrading.
Meaningful earthquake-hazards evaluations need to be based on complete and reliable fault databases. Information users need a good sense of the reliability of the data and maps they will apply to decision making. Having reliability ratings for seismic-hazards information helps increase levels of confidence among decision makers, engineers and others, and helps ensure the most appropriate uses of the information. The USGS and other NEHRP agencies have primary responsibilities for expressing the quality of the information they provide in systematic and explicit terms.
Maps and databases on seismic hazards are more likely to be successfully used if the customers for these products are involved with conceptualizing their design. Additionally, successful use depends upon ready accessibility of maps and information. Using World Wide Web technology allows significant interaction between producers and users of the digital maps and databases, and makes information readily and increasingly accessible as Internet access grows for all.
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