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Meeting Date: Monday, May 4th
    - Social Hour Begins at 6:00PM

Location (Map Below): DoubleTree Club

Speaker: Mark Legg

Topic: High­-resolution mapping of two large­-scale transpressional fault zones in the California Continental Borderland: Santa Cruz­-Catalina Ridge and Ferrelo


New mapping of two active transpressional fault zones in the California Continental Borderland, the Santa Cruz­Catalina Ridge fault and the Ferrelo fault, was carried out to characterize their geometries, using over 4500 line­km of new multibeam bathymetry data collected in 2010 combined with existing data. Faults identified from seafloor morphology were verified in the subsurface using existing Borderland seismic reflection data including single­channel and multi- channel seismic profiles compiled over the past three decades. The two fault systems are parallel and are capable of large lateral offsets and reverse­slip during earthquake faulting. The geometry of the fault systems shows evidence of multiple segments that could experience through­going rupture over distances exceeding 100 km. Published earthquake hypocenters from regional seismicity studies further define the lateral and depth extent of the historic fault ruptures. Historical and recent focal mechanisms obtained from first­motion and moment tensor studies confirm regional strain partitioning dominated by right­slip on major through­going faults with reverse- oblique mechanisms on adjacent structures. The two systems resemble onshore active fault zones and provide additional information for understanding the full Pacific­North America plate boundary evolution, as well as transpressional fault systems along the plate boundary. Because of their potential for dip­slip rupture, they may also be capable of generating local tsunamis that would impact southern California coastlines, including populated regions in the Channel Islands.

Speaker Information:

Dr. Legg received a B.S. in Space Sciences & Mechanical Engineering from the Florida Institute of Technology in 1973. At F.I.T, he also studied Physical Oceanography for the M.S. program. Dr. Legg continued his graduate studies in Oceanography at the Scripps Institution of Oceanography, with a National Science Foundation Graduate Fellowship, where he received his M.S. in 1980. In 1985, Dr. Legg completed his Ph.D., Geological Sciences, at the University of California, Santa Barbara. His post­graduate research focused upon the geology and seismotectonics of the California Continental Borderland, with detailed studies of the inner borderland west of San Diego and northern Baja California. While a graduate student, Dr. Legg conducted earthquake hazard and risk analysis research for the J.H. Wiggins Company, and helped the California Division of Mines & Geology map faults and earthquakes in the borderland. After the Ph.D., Dr. Legg expanded his marine seismic exploration skills at the Amoco Tulsa Research Center. Since 1988, Dr. Legg returned to Southern California to continue his offshore faulting and earthquake hazards studies. Recent offshore faulting and earthquake research efforts have included submersible dives (Alvin, Turtle, and Delta) to study borderland faults directly, multibeam swath bathymetry mapping of seafloor structure, and acquisition, processing, and interpretation of MCS and single­channel seismic profiles throughout the borderland. He continues to educate the larger community regarding the seismic hazards of the borderland including the potential for locally­generated tsunamis through seminars and publication. As President of Legg Geophysical, a small consulting firm that specializes in Earth Sciences and Risk Analysis, Dr. Legg is involved in many diverse projects relating to both natural and man­made hazards funded by both government and commercial agencies. He is also a participating scientist with the Southern California Earthquake Center, and adjunct professor at San Diego State University, and a Visiting Assistant Research Geophysicist at UC Santa Barbara. His collaboration with scientists at Oregon State University and UC Santa Barbara to produce more accurate maps of the Borderland bathymetry for fault and tectonic studies led to the discovery of the large crater structures to be discussed. Most recently, Dr. Legg's work focuses on obtaining high­resolution multichannel seismic reflection images of active tectonic structures in the California Continental Borderland, so that more accurate and precise understanding of the regional tectonic evolution may be achieved.

Address: 7 Hutton Dr. Santa Ana, CA

Meal: Regular or Vegetarian meals Complimentary tea and coffee will also be available

Cost: Members $30; Non-members $35; Students and Professors $15, $5 late fee for reservations made after 5pm the Friday prior to the meeting.

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