SEMINAR - Paul Bedrosian

Electromagnetic Imaging of Active Fault Zones

Paul A Bedrosian
US Geological Survey
Crustal Imaging and Characterization Team

Suspected impact crater is center of teen's universe

His discovery launches 3-year research project

By Sharon A. Heilbrunn

EL CAJON – After years of research, an East County teen has perhaps accomplished a first – identifying an ancient meteorite crater in California.
Sam Spevack, a Grossmont Middle College High School senior, found what scientists believe is a possible crater near Stockton that was created by a meteorite hitting the Earth millions of years ago.
Sam, 17, has been working on the project for three years. “It's kind of like a hobby,” he said.
He found the crater when his father, geophysicist Bennett Spevack, was finishing projects near the area.
Sam noticed the feature while looking at seismic data his father was using and decided to investigate further. “I was able to recognize what I thought looked like an impact crater,” Sam said.
The formation – dubbed the Victoria Island Structure and measuring about five kilometers in diameter – is buried about one mile beneath the Earth's surface. Its circular shape is similar to known impact craters.
If Sam's theory is proved – scientists are working on that now – it will be the only known impact crater in California.
In November, Sam was honored as a regional winner in the Siemens Competition in Math, Science and Technology, a highly respected event for high school students doing graduate-level work.
To enter, he submitted a 20-page report on the project. At the competition, he delivered a PowerPoint presentation to a panel of judges, his peers and members of the public. Although he didn't win first place, he was one of 30 finalists out of more than 1,000 entries.
“I was slightly nervous,” Sam said. “The competition was really amazing. All the projects were really interesting.”
Jared Morrow, assistant professor of geology at San Diego State University, has mentored Sam as he looks for evidence of the impact.
“Sam is extremely diligent,” Morrow said. “Very hardworking. He's very bright, very well-rounded.”
Sam's interests spill into the musical world. He plays violin, clarinet and piano. He recently sent applications to several universities, including Cornell, University of California San Diego, San Diego State University and UC Santa Barbara's College of Creative Studies.
“I really like musical composition,” he said, noting that he might pursue a career in that field.
Sam has spent nearly 300 hours on the impact crater project, taking little steps each day that involve mapping, lab work, research, writing and reading.
Even though the work can be tedious, it's gratifying for him to remember that if he hadn't made this finding, “the impact would still be unnoticed under the Earth's surface,” Sam said.
He will continue to research physical and geochemical data with Morrow. Eventually, they will present their findings in a peer-reviewed scientific journal, with the hope that it will be accepted by the larger scientific community.
The project is a long-term one, Morrow said, but Sam is being patient – and optimistic.
“Right now, it's a suspected impact,” Sam said. “That's a short step away from being proven.”

Source San Diego Union Tribune
Sharon A. Heilbrunn is a community news assistant in the East County office.
Sharon Heilbrunn: (619) 593-4957; sharon.heilbrunn@uniontrib.com

Seismogenic, Electrically Conductive, and Fluid Zones at Continental

George R. Jiracek, Victor M. Gonzalez, T. Grant Caldwell,
Philip E. Wannamaker, and Debi Kilb

We explore the idea that fluid occurrence below the seismogenic zone plays an active role in the rupture process by examining how fluids spatially relate to seismicity at three continental plate boundaries: South Island of New Zealand, the Himalaya, and San Andreas fault, USA. With this objective, we project earthquake hypocenters onto magnetotelluric (MT) electrical resistivity cross-sections. MT detection of conductive zones in the crust containing low fractions of fluids (less than 1%) requires an interconnected network of fluid-filled porosity facilitated by shearing, fracturing, and/or grain-edge wetting. Mechanisms promoting fluid reservoirs in the ductile crust include: 1) stalling of upward propagating porosity waves, 2) tectonically induced neutral buoyancy, and 3) development of ductile shear zones. Distinct conductive horizons are detected at depth in the ductile crust in New Zealand and the Himalaya where the tectonic convergence is high. In the Parkfield segment of the San Andreas fault, where convergence is low, there is high conductivity in the ductile crust but it forms a sub-vertical corridor to the surface with no distinct top. The tops of sub-horizontal conductive zones are ~20 km depth in New Zealand and ~25–40 km in the Himalaya where the seismogenic crust extends only to 12 and 25 km depth, respectively. The deep conductive layer in New Zealand may have originated as a “water sill” facilitating water-weakening, localized deformation, and eventually becoming a water-rich, anisotropic, mylonized, ductile shear zone. Fluid exchange through the active Alpine fault may initiate or be initiated by fault rupture. Localized, unstable flow in deep fluidized zones detected by MT may trigger earthquakes above.

Jiracek, G. R., Gonzalez, V. M., Caldwell, T. G., Wannamaker, P. E., and Kilb, D., 2007, Seismogenic, electrically conductive, and fluid zones at continental plate boundaries in New Zealand, Himalaya, and California, USA: in D. Okaya, T. Stern, and F. Davey, eds., A continental plate boundary: Tectonics at South Island, New Zealand, Geophys. Mono. Ser. 175, AGU., Washington, DC, 347-369.


A Continental Plate Boundary: Tectonics at South Island, New Zealand

David Okaya, Tim Stern, Fred Davey, Editors

A Continental Plate Boundary offers in one place the most comprehensive, up-to-date knowledge for researchers and students to learn about the tectonics and plate dynamics of the Pacific-Australian continental plate boundary in South Island and about the application of modern geological and geophysical methods. It examines what happens when convergence and translation occur at a plate boundary by

• describing the geological and geophysical signature of a continental transform fault;

• identifying the diverse vertical and lateral patterns of deformation at the plate boundary;

• assessing an apparent seismicity gap on the plate boundary fault and fast-moving plate motions;

• comparing this plate boundary to other global convergent continental strike-slip plate boundaries;

• documenting the utility of the double-sided onshore–offshore seismic method for exploration of a narrow continental island; and

• Providing additional papers presenting previously unpublished results.

This volume will prove invaluable for seismologists, tectonophysicists, geodesists and potential-field geophysicists, geologists, geodynamicists, and students of the deformation of tectonic plates.

Geophysical Monograph Series, Volume 175, 350 pages, hardbound, 2007, ISBN: 978-0-87590-440-5, AGU Code GM1754405