The final activity of the SCEC Summer Undergraduate Internship Program is to attend the SCEC Annual meeting and present a poster describing project results. This year's interns were also asked to prepare very brief presentations summarizing their projects which were given during the first session of the meeting. Both the presentations and posters were outstanding.
Three-dimensional excavation of the San Andreas Fault at the Burro Flats Paleoseismic site near Banning, California. Clay Stevens and Doug Yule, California State University, Northridge Evidence from trenches opened in 1999 at Burro Flats suggested that the San Andreas fault at San Gorgonio Pass has remained dormant since about A.D. 1450. However, the 1999 trenches exposed a limited area of a complex, 150-m-wide stepover basin and may have missed evidence of more recent event(s). This summer an expanded trench network was excavated to search for evidence of more recent events. The new trenches show clear evidence of post-A.D. 1450 faulting and folding. The focus of my SCEC internship has been to construct contour maps of marker horizons and isopach maps of the intervening strata deposited in the last 550 years. Three marker horizons have been mapped over an area of about 1000 square meters. Marker horizon 84, near the base of the 550 year-old section, is faulted and folded into a syncline-anticline-syncline triad with a maximum structural relief of about 1.5 m that diminishes gradually to <0.25 m from south to north. Horizon 85, in the middle of the section, is not faulted and shows a maximum structural relief of about 0.6 m that also diminishes to the north. Horizon 88, near the top of the section, shows structural relief of 0.3 m or less. The modern ground surface is undeformed, dipping uniformly toward the southwest perpendicular to the fault. Sediment above horizon 85 is interpreted to have buried the folds and minor faulting that formed during the most recent event. Horizon 84 drapes a paleo-surface created by the penultimate event. The greater structural relief of horizon 84 is therefore attributed to deformation from both the most recent and penultimate events. Age data constrain the most recent event to between A.D. 1500 and 1800 and the penultimate event to between A.D. 1400 and 1550.
Hector Mine: Postseismic Deformation from ERS InSAR Allison Jacobs, David Sandwell
and Lydie Sichoix The Hector Mine earthquake occurred on October 16, 1999, in the Mojave Desert of California. Four days later, the European Space Agency satellite, ERS-2, flew over the region, bouncing radar waves off the desert surface to produce the first co-seismic synthetic aperture radar (SAR) image of the area. The collection of this data was also highly important for studying postseismic deformation since the most significant earthquake deformation usually occurs within the first 40 days after the main shock. We were fortunate to match this key image with data received on June 21, 2000, at the Scripps Institution of Oceanography satellite ground station, and create a postseismic interferogram covering this critical time period. Using the interferogram we contoured the line-of-sight (LOS) displacement in the fault region. The three main deformation features seen in the line of sight displacement map are a region of subsidence (40 mm line of sight increase) on the northwestern side of the fault, a region of uplift (65 mm line of sight decrease) located to the northeast of the primary fault bend, and a linear trough running along the main rupture of the fault. These features coincide well with a characteristic left-bending, right-lateral strike-slip fault that ideally exhibits contraction on the restraining side of the fault bend and extension along the opposite side. In this initial postseismic interferogram, the areas of subsidence and uplift combine to display a maximum amount of displacement of nearly 105 mm. A second interferogram made from images taken 39 days and 139 days after the earthquake shows a significant decrease in the amount of displacement (maximum displacement of 50 mm). This smaller deformation indicates that most of the relaxation occurs within about 40 days of the main shock. The co-seismic and postseismic interferograms created for this study are on display at http://topex.ucsd.edu.
Seismic Hazard Assessment of the San Joaquin Hills Using GIS Daniel E. Raymond and Lisa Grant The San Joaquin Hills region
of Orange County is undergoing rapid commercial and residential
development. The anticlinal structure of the area is likely due
to a combination of tectonic uplift and seismic activity on local
fault systems. The goal of this project was to determine the
hazards of the fault zones in the area while there may still
be time to translate these findings into sound planning practices
for the area.
Seismic Sleuths 2001: Revision
of a Tool of Scientific Literacy Kathryn van Roosendaal and Robert
de Groot Seismic Sleuths, an earthquake education curriculum for grades 7-12, was developed jointly by AGU and FEMA in 1995. In 1999, NSF funded a revision of the curriculum under the direction of the Southern California Earthquake Center (SCEC). Revisions will reflect advances in science and technology especially the use of geodesy in earth science. This effort will provide an innovative opportunity to improve earth science education in several domains. Instruction in the classroom generally centers on teaching the conclusions of scientific investigation. Seismic Sleuths 2001 will be a resource for a basic integrated approach to the study of seismic phenomena while being a doorway into current research and controversies in earth science. The curriculum will harness the power of the World Wide Web forming partnerships across academe, government, and industry in order to tap the multi-disciplinary nature of earth science. The primacy of relevance, so well demonstrated in the original curriculum, will be enhanced and expanded to include issues related to all hazards that face citizens in the United States. Seismic Sleuths 2001 will allow individuals to make sense of the world around them, take action to reduce risk, and use technology that is an integral part of their lives without eclipsing the science that comprises the backbone of the curriculum.
Inconclusive Evidence for Fault Zone Trapped Waves on the Bullion Fault Marie C. Ammerman and Ralph J.
Archuleta Aftershocks recorded by stations in the Bullion Wash Array lacked the arrival of seismic waves that characterize the fault zone trapped waves observed by Li et al., 1994. The arrivals following the S wave were more prominent on fault parallel and vertical components of the seismometers than on the fault perpendicular components-inconsistent with a Love wave type mode trapped in a fault zone. The waves following the S wave did not show consistent amplitudes on stations within the fault zone with a decrease in amplitude for stations farther away from the fault trace. The apparent lack of fault zone trapped waves may be due to the juvenile nature of the Bullion fault zone, i.e., it is not well enough developed to generate trapped waves. The observed post-S waves may be basin edge generated waves caused by a wedge of sediments that underlies the array (Dibblee, 1967). The presence of edge generated surface waves would explain why the waves are more prominent on fault parallel and vertical components.
Pressure Solution Slip-faulting Associated with the Quatal Thrust Fault Kevin Mass and Jan Vermilye This study is based on the analysis
of pressure solution strain structures to infer stress orientations.
Sandstone and conglomerate outcrops located in Quatal Canyon
(northern Ventura County, CA) have many types of pressure solution
deformation structures. Small pressure solution slip-faults (cm
to m in length) are a type of deformation structure that can
be analyzed to infer the local and regional principal stresses.
Field measurements of these faults are recorded at six locations
in Quatal Canyon. These six data locations are then combined
in order to determine the regional inferred stress axes.
Constraints on the SCEC 3D velocity Model from Gravity Data: Two Dimensional Gravity Modeling of the Central and Eastern Transverse Ranges in the Los Angeles Region Nancy Natek and Mousumi Roy The goals of this project are
to test the consistency of crustal seismic velocity structure
and gravity data in the Transverse Ranges. Following earlier
work by Roy and Clayton, we use 2D gravity models based on density
structures inferred from crustal tomography. We use version 2
of the Southern California Earthquake Center (SCEC) velocity
model and an empirically derived scaling relation between seismic
velocities and density. The density structures thus inferred
are used to forward model gravity along 2D profiles.
Refinement of Near-Surface P and S Wave Velocities in the SCEC 3-D Velocity Model Using 3-D Waveform Modeling Tracy Pattelena, University of
California, Santa Cruz An important part of ground motion studies opts to predict full theoretical seismograms for an earthquake at a given site. Our study focuses on the ground motion in a portion of the San Fernando Valley (SFV) where control on the near-surface S wave velocity, a critical parameter for accurate prediction of strong ground motion, is mostly indirect and in many areas not well constrained. We use 1-Hz 3D finite-difference simulations to propagate waves for a M=5.1 Northridge aftershock through two 10 km by 10 km by 5 km models of the SFV subsurface: (1) a subset of the SCEC southern California reference 3D seismic velocity model (version 2.0), and (2) model 1 modified with slower velocities in the near-surface material taken from a tomographic model. Model 2 contains near-surface P and S velocities up to 70% and 50% less than those of model 1, respectively, in the upper 500 m. Compared to the response of model 1, the tomographically refined model (2) generates peak ground velocities (PGV) and cumulative kinetic energies (CKE) up to a factor of two in localized areas of the model. While the synthetics for model 2 improve the fit to data at the five stations only slightly, our simulations suggest that version 2.0 of the SCEC velocity model is not complex enough to reproduce the 1-Hz duration and peaks in the SFV basin.
Afterslip Models of Postseismic Deformation from the 1999 M7.1 Hector Mine Earthquake Teresa Baker, Massachusetts Institute
of Technology, Following the October 16, 1999
M=7.1 Hector Mine earthquake there has been a focused effort
by SCEC researchers to collect postseismic surface deformation
data in the region surrounding the fault rupture. Using campaign
and continuous GPS data collected by SCIGN, the USGS, USC, UCLA,
UCSD and MIT, we calculated postseismic velocities at more than
50 sites. By measuring the postseismic deformation we can quantify
the crustal response to the earthquake and model the distribution
of afterslip on the fault at depth. This is important for understanding
the mechanics of the lower crust and the postseismic period of
the earthquake cycle.
Setting the Stage for a HAZUS User Group in Southern California Alexandra Jordan, Mark Benthien,
and Jill Andrews This summer I worked on a project with Mark Benthien and Jill Andrews to help promote the computer application, HAZUS. HAZUS, or "Hazards US", is a PC-based Geographic Information Systems software used for earthquake loss mitigation. The application is used to collect an over report of the amount of damage that may occur to one area affected by an earthquake. Our main goal, along with working with HAZUS and learning more about the application, was to help form a Southern California User Group. At the beginning of the summer we traveled to the San Francisco Bay Area and joined a workshop for users of the already existing Bay Area User Group. By observing and talking with many of the participants we were able to get an idea of the future goals and accomplishments geared toward the formation of a Southern California User Group. The Bay Area User Group has acted as a model for the kind of user group we are hoping to establish. To gain more knowledge on what people in the southern California region already know and think about HAZUS, I designed a survey of 10 questions related to the application. I called between 25 and 30 people and asked them about their interests and knowledge of HAZUS. Most people seemed enthusiastic to participate in future functions related to HAZUS and hoped to hear more information about our progress. These results will provide the SCEC Board of Directors with the information needed to structure a Southern California User Group.
Nancy Natek and Kevin mass at dinner.
Teresa Baker, Allison Jacobs, and Marie Ammerman at dinner. |
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