April 5, 2001 GSA Release No. 01-08

Geologists Will Explore New Earthquake Findings April 9–11

I. Introduction II. Presentation Highlights III. Complimentary Media Registration and Other Information

(II) Presentation Highlights

MONDAY, APRIL 9

EARTHQUAKES: FOCUS ON SAN ANDREAS

In a session titled "Active Tectonics and Paleoseismology of the San Andreas Fault System," scientists will examine new findings on large earthquake ground motion, a major overdue San Andreas earthquake in the San Bernardino and Palm Springs areas, a complex region where the San Andreas and San Jacinto faults merge into a system of active faults that enter the Gulf of California, and new findings on California's second largest earthquake in the last 200 years. (Monday, April 9, 8:00-11:30 a.m., Sheraton Universal, Studio II)

In combination, evidence in the following two papers suggest that a large San Andreas earthquake is overdue for California's San Bernardino and Palm Springs regions.

The Paleoseismic Record at Burro Flats: Evidence for a 300-year Average Recurrence for Large Earthquakes on the San Andreas Fault in San Gorgonio Pass, Southern California
Doug Yule, Department of Geological Sciences, California State Univ., Northridge, CA, 818-677-6238, and Kerry Seih, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA.

Recent studies of the structurally complex San Andreas fault at the San Gorgonio Pass and the Burro Flats site in the San Bernardino strand of the San Andreas fault reveal a complete five-event record relating roughly to 1500-1850, 1400-1550, 1300-1450, 700-1100, and 450-800 A.D.. The range of recurrence intervals for these events is from 100 to 471 years, or an average of 300 years. These events correlate with those found at the Indio site and with events at sites along the Mojave segment, including Pitman Canyon, Wrightwood, and Pallett Creek. Evidence from the San Andreas fault system in San Gorgonio Pass supports the idea that relatively infrequent large earthquakes may rupture in the Coachella Valley, San Bernardino, and Mojave segments of the fault. Since the last event was between 1500 and 1850 A.D., another event should have occurred by now.

Paleoseismic Studies of the San Andreas Fault at the Plunge Creek Site, Near San Bernardino, California
Sally F. McGill, Department of Geological Sciences, California State Univ - San Bernardino, CA, 909-880-5347, and Safaa A. Dergham, Department of Geological Sciences, California State Univ. Long Beach, Long Beach, CA.

Fifty-two samples of faulted sediments from trenches across the south branch of the San Andreas fault at the Plunge Creek site, near San Bernardino, California, have been dated and studied. They indicate than an earthquake ruptured the south branch of the San Andreas fault at the Plunge Creek site sometime between about AD 1440 and AD 1650.

Relation of the Southern San Jacinto Fault Zone to the Imperial and Cerro Prieto Faults
Harold Magistrale, Department of Geological Sciences, San Diego State Univ., San Diego, CA, 619-594 6741.

The San Jacinto fault zone is part of the San Andreas fault system, and carries a large portion of the plate motion. The connections of the south end of the San Jacinto fault zone to other faults is covered by the sediments that fill the Imperial Valley, so it has been difficult to tell how the plate motion is distributed between different faults heading south to the Gulf of California. This work uses very accurate earthquake locations (hypocenters) found using a new earth model to define the faults buried under the sediments. One fault defined in this way connects the San Jacinto fault zone to the Imperial fault just north of the international border. Another fault, the Cerro Prieto, has been known to exist south of the border. This fault can now be traced to north of the border where is connects to another branch of the San Jacinto fault zone. An offset between those two faults provides the perfect environment for a geothermal field.

As a result of this work, we know how many, and where, faults cross the border; this will allow better seismic hazard evaluations and tectonic models to be developed.

Paleoseismology along the Owens Valley Fault: Accounting for the San Andreas Discrepancy
Jeffrey Lee, Geological Sciences, Central Washington Univ, Ellensburg, WA 98926; 509-963-2801, et al.

The second largest earthquake to hit California in the last 200 years occurred along the Owens Valley fault in 1872. Despite the earthquake's estimated magnitude of ~7.5, there are no specific age estimates for pre-1872 earthquakes which one would expect for such a large earthquake. However, our new paleoseismic and geochronologic data allows us to estimate that a penultimate earthquake happened between 3,300 ± 500 and 4000 ± 400 thousand years ago. This type of study is crucial for understanding earthquake history and for estimating when future earthquakes might occur. If we assume uniform return time between earthquakes, our data shows an earthquake recurrence interval of between 4,400 and 2,800 years. Therefore, it is unlikely than an earthquake will occur along the Owens Valley fault for a very long time.

Precarious Rock Constraints on Ground Motion for Historic and Pre-historic Earthquakes
James N. Brune, Seismological Laboratory, University of Nevada - Reno, Reno, NV, 775-784-4975.

Some recent estimates have indicated that there are often relatively large ground motion for large earthquakes (compared to those for moderate-sized earthquakes). But according to Brune's research, this is not necessarily so. Precariously balanced rocks in southern California provided constraints on ground motion for historic earthquakes such as the 1899 and 1918 San Jacinto earthquakes (magnitudes about 7), the 1952 Kern County earthquake (magnitude about 7.5), and the 1812 and 1857 San Andreas fault earthquakes (magnitudes near 8). They also provide evidence of ground motion of prehistoric (but relatively recent) events. Precarious rock constraints on ground motion for these earthquakes are consistent with recordings from the recent Turkey and Taiwan earthquakes, and support the idea that large earthquakes do not always cause large ground motions. On the other hand, lack of precarious rocks in some places may indicate the presence of unmapped faults which have shaken the ground in these areas.

ASIAN TECTONICS

Since Asia is the youngest of the continents, sessions such as "Tectonics of Eastern Asia with Emphasis on Tibet and Adjacent Regions " will provide insight into Earth's evolutionary history. (Monday, April 9, 1:20-4:30 p.m., Sheraton Universal, Studio I)

Did Tarim (+North China) Collide with Qiangtang (+South China) in Both the Devonian and the Triassic?
Eric Cowgill and Paul Kapp, Department of Earth and Space Sciences, UCLA, Los Angeles, CA, 310-206-1761.

Asia is the only continent to have experienced relatively recent and widespread growth and it provides a great opportunity for us to understand how continents are both constructed and deformed. For example, the southern Indo-Asian collision zone is a mosaic of three components. The Kunlun mountain belt lies along the northwestern edge of the Tibetan Plateau, the Qiangtang block to the south makes up much of the central part of the Tibetan plateau, and the Tarim block to the north is presently a huge intracontinental basin within the interior of China. Cowgill and Kapp have compared the timing of deformation and metamorphism within the Qiangtang block to that of the Kunlun mountains. They suggest that the Kunlun mountains together with the Tarim basin and North China collided with the Qiangtang twice, once in the middle to late Paleozoic, and then again ~50 million years later in the early Mesozoic. If this hypothesis is correct, it seems that a supercontinent of a considerable size briefly formed in the middle to Late Paleozoic. This continent was eventually amalgamated with continental fragments derived from the Gandwana supercontinent in the south between the middle Mesozoic and early Tertiary as represented by the accretion of the Lhasa block and India continent onto the southern margin of Asia.

TUESDAY, APRIL 10

EARTHQUAKES: THE SAN FERNANDO VALLEY AND SANTA MONICA

In January 2001 Gary Fuis of the US Geological Survey, et al., made national news by showing that a basin of soft sediments beneath the San Gabriel Valley makes the area more vulnerable to earthquake damage than was previously thought. The study, published in GSA's journal GEOLOGY, used data from a series of test explosions conducted in 1994, known collectively as the Los Angeles Region Seismic Experiment (LARSE). The LARSE data continues to help scientists locate hidden earthquake hazards and determine where the strongest shaking is likely to occur.

Fuis will co-chair a session titled "Heart of the Transverse Ranges: Geology and Tectonics of the LARSE II Region (San Fernando Valley, East Ventura Basin, and San Gabriel Fault)," in which scientists will share new information on area faulting and earthquake dynamics based on analysis of the LARSE data. (Tuesday, April 10, 8:00 a.m.-11:45 a.m. and 1:30 p.m.-4:30 p.m., Sheraton Universal, Studio IV) Highlights include:

Preliminary Seismic Images from the Los Angeles Region Seismic Experiment, Phase II (LARSE II)
Gary S. Fuis, US Geological Survey, Menlo Park, CA, 650-329-4758; et al.

Using LARSE data, the authors have developed preliminary images of the San Fernando, San Andreas, and Northridge faults - the faults responsible for the San Fernando earthquake in 1971 and the Northridge earthquake in 1994. They have also imaged the Santa Monica fault. This new view of fault configurations and their interconnections will contribute greatly to our understanding of the "machinery" of southern California earthquakes.

Aspects of the Quaternary Geology of the San Fernando Valley, California
John C. Tinsley III, US Geological Survey, Menlo Park, CA, 650-329-4928

The San Fernando Valley experienced near-record levels of strong ground motion during the Northridge earthquake. The result was widespread damage from strong shaking and ground failure. Tinsley will describe subsurface conditions of the area that account for observed patterns of damage, along with implications for future earthquake potential.

Structure of the San Fernando Basin, California, Based on Analysis of Gravity and Magnetic Data
V. E. Langenheim, US Geological Survey, Menlo Park, CA, 650-329-5313; et al.

Study of gravity data in and around the San Fernando Valley has confirmed the presence of a deep basin underneath the valley. This basin is deeper than previously thought, with a floor perhaps as deep as 8 kilometers. The basin's configuration, and interaction of the 1994 Northridge earthquake with the Verdugo fault that runs along its eastern margin, will be discussed.

Structures Possibly Related to the 1971 San Fernando and 1987 Whittier Narrows Earthquakes Based on the Analysis of Magnetic and Gravity Data
Thomas G. Hildenbrand, US Geological Survey, Menlo Park, CA, 650-329-5303; et al.

Magnetic and density differences between rocks on opposite sides of two different faults in southern California allow scientists to determine the depth, dip, and lateral connection of these faults. The San Fernando fault, where it penetrates bedrock, dips ~60 degrees northeast to a depth of ~10 kilometers and connects with the Sierra Madre fault zone. The Whittier fault dips steeply northeast and extends northwest, where it is buried under sedimentary rocks, to connect or nearly connect with a major fault system consisting of the Santa Monica, Hollywood, and Raymond faults. This system crosses the central part of the Los Angeles region.

Variability of Site Response in the San Fernando Valley from the 1994 Northridge Earthquakes Using Aftershock Data and Seismic Reflection Modeling
William J. Stephenson and Stephen J. Hartzell, US Geological Survey, Denver, CO, 303-273-8573

This study shows how the shallow underlying geologic structure (i.e., upper few hundred meters) influenced the shaking experienced at the surface from aftershocks of the Northridge earthquake.

LARSE II: Towards an Understanding of the Subsurface Structure in the Santa Monica Area
Shirley Baher and Paul Davis, Dept. of Earth and Space Science, UCLA, Los Angeles, CA, 310-825-3021

This study shows how deeper geological structure (i.e., ~3 kilometers), acting much like a lens, focused seismic energy from the Northridge earthquake into Santa Monica and caused more damage than in adjacent areas.

PLANETARY GEOSCIENCE: NEW PERSPECTIVES ON MARS AND MERCURY

In a session titled "Geology Beyond Earth: Recent Results from the Planets and Their Moons," scientists will update our understanding of Mercury, Mars, and Jupiter. (Tuesday, April 10, 8:00-11:30 a.m. and 1:50-4:00 p.m., Sheraton Universal, Studio II)

Topography of the Polar Ice Caps on Mars: Recent Results from the Mars Orbiter Laser Altimeter
Anton B. Ivanov, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 818-354-9478.

Speculation about life on Mars is often based on the idea that if water can be found, then there's a chance of finding evidence of life. The polar ice caps and polar layered deposits on Mars are its major known reservoirs of water. Measurements made by the Mars Orbiter Laser Altimeter (MOLA) instrument on the Mars Global Surveyor Spacecraft (MGS), has enabled an accurate reconstruction of these ice caps. The measurements are performed with precision of near 1 m, which even allows the tracking of changes of seasonal CO2 snow cover. It's important to understand the current state of the polar layered deposits since they may provide a record of climate changes on Mars and that would provide insight into the history of water cycle on the red planet. Ivanov will present this report on behalf of the MOLA Science Team.

Mercury Radar-bright Polar Features
Martin A. Slade, III, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 818-354-2765, et al.

While the Mariner 10 flew by Mercury three times between 1974-75, it was only able to map about 50 percent of each pole and only 45 percent of Mercury's surface as a whole. Images have been made of Mercury's north pole using the recently upgraded Arecibo Observatory in Puerto Rico and the Goldstone Solar System Radar in California. Radar observations from both facilities have been mapping, with ever-increasing resolution, the areas that the Mariner 10 missed. Starting in 1991, much work has focused on the polar areas, which show very radar-bright areas inside of craters near the poles. The only viable explanation for these radar-bright areas is water ice trapped in areas which are permanently shadowed from the sun, and exhibiting "coherent backscatter." The north polar region has been mapped in great detail over the last few years, and images from Arecibo have been recently published. The south pole of Mercury is not accessible to Arecibo until 2005, so Slade and his colleagues took advantage of a brief window in February 2001 to map this region at a higher resolution than ever achieved before and compared observations at two frequencies. The experimenters now hope to better understand the properties and the nature of the radar-bright areas near Mercury's poles.

WEDNESDAY, APRIL 11

EARTHQUAKES: DAM SAFETY

A poster session in the Sheraton Universal Grand Ballroom provides for further investigation of "Active Tectonics of the Los Angeles Basin." Authors are available to discuss their work from 9:00 a.m.-11:00 a.m. Highlights of this session include the following paper:

Results of a Probabilistic Seismic Hazard Study for the Santa Barbara Area, Southern California
Larry W. Anderson and Roland LaForge, US Bureau of Reclamation, Denver, CO, 303-445-3170

The US Bureau of Reclamation recently completed a seismic hazard evaluation for four small dams in the Santa Barbara area: Glen Anne, Lauro, Ortega, and Carpinteria. The study included 18 known faults as well as randomly occurring earthquakes. Reclamation engineers will use the results to assess the safety of the dams under earthquake conditions.

ENVIRONMENTAL GEOSCIENCE: FUTURE OF THE SALTON SEA

Session: "Environmental Geology, Engineering, and Hydrogeology," sponsored by the AAPG Division of Environmental Geosciences (Wednesday, April 11, 8:45 a.m.-11:00 a.m., Sheraton Universal Terrace B/C)

The Salton Sea: A Cost Benefit Analysis of its Future
Ivan Colburn, Dept. of Geological Sciences, California State University, Los Angeles, CA, 323-343-2413

The Imperial Valley's Salton Sea formed as a freshwater lake in 1905. Brackish agricultural waste water flowing into the lake over nine and a half decades from Imperial and Coachella valley agricultural fields gradually turned it into a "hypersaline" lake with a surface area larger than any other lake in California. Current plans to sell Imperial Valley water to the San Diego County Water Authority will reduce the sea's volume, further increasing its salinity and altering its shoreline. By 2010 the sea's volume will have shrunk 78 percent and its salinity will have increased 400 percent or more over current levels. Colburn will discuss whether there are any cost-effective methods of countering this situation with implications for the sea's future.

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