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Geological Structure and Tectonic Evolution

Geological Sturcture and Tectonic Evolution of the California Continental Borderland

Dr. Mark Legg is recognized as one of the leading experts on faulting and seismotectonics of the southern California offshore region based on more than 30 years experience. He has:
  • Mapped the major active and ancient fault systems of the area offshore southern California and northern Baja California.
  • Compiled, analyzed, and mapped the offshore seismicity and earthquake character.
  • More than 35 publications in scientific journals, books, and maps resulting from offshore geological/geophysical research.
  • Research Grants from the National Science Foundation, U.S. Geological Survey, Federal Emergency Management Agency, University of California, Office of Naval Research, Southern California Earthquake Center, National Academy of Sciences.

Tectonic Processes

Tectonic Processes - Active Deformation Associated with Transpression and Transtension

Based on three decades of mapping active faults and seafloor deformation offshore southern California, combined with earthquake focal mechanism studies, Dr. Legg has developed substantial expertise in the character (geometry, style, kinematics, and dynamics) of active oblique faulting. The tectonic evolution of southern California involved major episodes of oblique rifting (extensional strike-slip or transtension) and subsequent oblique contraction (compressional strike-slip or transpression). Understanding the 3D geometry of oblique-slip fault systems and their evolution through time is important for evaluating future earthquake hazards, and tsunami hazards in offshore areas. Furthermore, most (if not all) of the major hydrocarbon resources being produced in southern California exist in ancient transtensional structures that were subsequently inverted (basin inversion) to form excellent hydrocarbon reservoirs. Exploitation of these abundant hydrocarbon resources, both onshore and offshore, requires accurate mapping of the subsurface structure that forms the reservoirs that trap the oil and natural gas. Other mineral resource accumulations including ground water, hydrothermal systems, and precious metals are also closely related to ancient and active oblique-slip fault systems.

Tsunami Generation and Hazards

Tsunami Generation and Hazards to Coastal Populations and Infrastructure

2002 EERI/FEMA Professional Fellowship

USC Tsunami Research Center Associate

Legg Geophysical provided tectonic faulting and landslide source models for local tsunami generation offshore California. These models were used to prepare the California Tsunami Inundation Maps for emergency response planning. Site-specific tsunami hazard evaluations have been prepared for commercial and industrial clients.

Direct observation and measurements of recent major trans-Pacific tsunamis were made at coastal sites in southern California. We have documented tsunami wave heights and currents in coastal waterways that will provide important calibration of tsunami hazard models used to assist emergency planners in reducing tsunami hazards.

Collaboration with Caltech scientists investigating offshore earthquakes and recent tsunamis.

Recent tsunami publications including:
  • Barberopoulou, A., Legg, M.R., Gica, E., and Legg, G., 2013, Multiple wave arrivals contribute to damage and tsunami duration on the US West Coast: Springer, (in press)

  • Wilson, R., Admire, A., Borrero, J., Dengler, L., Legg, M., Lynett, P., Miller, K., Ritchie, A., Sterling, K., and Whitmore, P., 2012, Observations and impacts from the 2010 Chilean and 2011 Japanese tsunamis in California (USA): Pageoph, Topical Volume Tsunamis 2011

  • Barberopoulou, A., J.C. Borrero, B., Uslu, M.R. Legg, and C.E. Synolakis, 2011, A second generation of tsunami inundation maps for the State of California: Pure and Applied Geophysics, 168 (2011), p. 2133-2146.

  • Barberopoulou, A., M.R. Legg, B. Uslu, and C.E. Synolakis, 2010, Reassessing the tsunami risk to major ports and harbors of California I: San Diego: Natural Hazards, Springer, published online, 18 p. DOI 10.1007/s11069-010-9681-8

  • Wilson, R.I., L.A. Dengler, M.R. Legg, K. Long, and K.M. Miller, 2010, The 2010 Chilean Tsunami on the California Coastline: Seismological Research Letters, Volume 81, Number 3, p. 545-546.

  • Borrero, J. C., M.R. Legg, and C.E. Synolakis, 2004, Tsunami sources in the southern California bight: Geophysical Research Letters, v. 31, p. L13211.

  • Legg, M.R., J.C. Borrero, and C.E. Synolakis, 2004, Tsunami hazards associated with the Catalina fault in southern California: Earthquake Spectra, vol. 20, p. 917-950.

Earthquake Hazards and Risk Assessment

1980 USGS-sponsored research: National Earthquake Loss Assessment: Sensitivity to Alternative Risk Mapping Procedures - with Ronald Eguchi and John H. Wiggins.

This research demonstrated that seismic hazard maps for making decisions regarding earthquake mitigation funding, building code design parameters, and so forth must be carefully prepared because expected losses derived from damage algorithms (vulnerability) applied to the national building stock (exposure) can vary substantially for minor changes in expected hazard levels due to the exponential character of both vulnerability and hazard relationships. Inaccurate hazard mapping due to inadequate background data, such as short historical record, incomplete geological knowledge, or arbitrary adjustment of map values, may provide unreliable estimates of long term costs (annualized losses) from earthquake. Cost-effective mitigation strategy requires accurate hazard maps, damage algorithms, and exposure data. In the year 2000, FEMA produced a new assessment of annualized losses due to earthquake in the United States based on the USGS National Seismic Hazard Maps and the HAZUS loss assessment program.


Academic History

PhD (Geological Sciences) University of California, Santa Barbara (1985)

Dissertation: Geologic Structure and Tectonics of the Inner Continental Borderland Offshore Northern Baja California, Mexico

MS (Oceanography) Scripps Institution of Oceanography (UC San Diego 1980)

Thesis: Seismicity and Tectonics of the Inner Continental Borderland of Southern California and Northern Baja California, Mexico

BS (Space Sciences & Mechanical Engineering) Florida Institute of Technology (1973)

Senior Research Project: Build and test a thermistor “hot-wire” anemometer for use in the laboratory wind tunnel.

Major Subjects

Marine Geology and Geophysics

Advanced Structural Geology, Sedimentary Petrology, Geophysics, Earthquake Seismology, Exploration Seismology, Plate Tectonics, Gravity and Geomagnetism, Geophysical Inverse Theory, Submarine Geology


Physical Oceanography, Dynamical Oceanography, Wave Dynamics, Marine Biology/Biological Oceanography

Mechanical Engineering

Statics, Elasticity, Continuum Dynamics, Fluid Dynamics, Aerodynamics, Propulsion, Solid Mechanics, Materials Science, Thermodynamics, Heat Transfer, Linear Systems, Electrical Circuits, Nuclear Reactor Engineering, Vibrations

Space Sciences

Atmospheric Physics, Planetary Astronomy, Tropical Meteorology, Electromagnetic Fields and Waves,, Statistical Thermodynamics, Synpotic and Dynamic Meteorology

Mathematics and Applied Physics, Geometrical Optics, Physical Chemistry

Case Studies

Examples of past projects will be found on the Case Studies page. These cases will contain example graphics and results demonstrating the quality of past projects.

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