||April 5, 2001
GSA Release No. 01-08
Geologists Will Explore New Earthquake Findings April 911
(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
- 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,
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,
- 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
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
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
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.
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
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,
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
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.