Paleoseismology of the North Panamá Deformed Belt from Uplifted Coral Platforms at Moín and Limón, Costa Rica

Eldon Gath, Tania Gonzalez, and Christopher Madugo, Earth Consultants International & Walter Montero, University of Costa Rica

In 1991, the Caribbean coastal area between Moín and Limón was coseismically uplifted up to 1.8 m by a M_s7.6 earthquake on the western segment of the North Panamá Deformed Belt (NPDB), resulting in the emergence of a broad coral reef platform. Designers of a proposed new harbor asked the highly relevant question "when is the next one?" as part of the Probabilistic Seismic Hazard Analysis (PSHA) process. Review of documents showed that multiple large earthquakes have been felt in the region during its ~270 years of historical record, with an event in 1822 described somewhat similarly to the 1991 event. Geologic reconnaissance revealed multiple older platforms that provided an opportunity to quantify the timing and rate of 1991-like uplift earthquakes on the Limón section of the NPDB. Mapping and hand surveying identified 6 pre-1991 coral platforms and wave cut notches preserved on the coastal-facing slope beneath a set of higher marine terraces that cap the coast between Moín and Limón. 48 coral samples were obtained but only 12 were suitable for U-series dating due to diagenetic alteration of the original aragonite to calcite. Assuming that these coral die offs were caused by coseismic uplift events, and using additional dated corals from published sources and one radiocarbon-dated shell, we have interpreted and temporally constrained as many as 12 events in the past ~7,000 years. Our preliminary work, indicates that the penultimate earthquake did occur in 1822, but was smaller than 1991. We calculate an average slip rate of 3.8±0.3 mm/yr on the NPDB using a geophysically-determined fault dip of 27°, and an average earthquake recurrence of ~600 years, but return periods are irregular and clustered, suggesting that the region experiences two types of events: large events with ~1400-year return periods that could reflect multi-segment ruptures of the NPDB, and smaller events with recurrence intervals in the 100s of years. These results, along with other regional fault sources, helped to populate the PSHA logic tree and develop realistic ground motions used for the facility design.

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Mitigation of Surface-Fault Rupture: Updating California's Alquist-Priolo Earthquake Fault Zoning Act for the New Millennium

Eldon Gath, Earth Consultants International & Roy J. Shlemon, Shlemon & Associates

Following the 1971 Sylmar earthquake, enactment of the 1972 California Alquist-Priolo (A-P) "Earthquake Fault Zoning Act" has been an important program to mitigate surface-fault rupture. While the legislative intent to prohibit new structures from being built across future fault rupture hazards was progressive for its time, the A-P has become a 42-year old law in need of revision and modernization to reflect the four decades of improved knowledge of geologic fault hazards and earthquake engineering of soils and structures. When the A-P was first enacted, Kerry Sieh's pioneering paleoseismic studies of the San Andreas fault were still 5 years in the future; and few geologists understood or could realistically quantify the fault rupture process, much less its temporal and spatial predictability. Today, many geologists specialize in this field and have the skills to quantify discrete coseismic fault displacements. Engineers can now provide very practical structural mitigation alternatives, in contrast to the current fault avoidance, regardless of fault displacement, recurrence or general kinematics. Accordingly, the public, and our profession, is better served by upgrading the A-P for consistency with the risks associated with other natural hazards. We suggest replacing the A-P defined "active fault" term [any amount of Holocene-age surface rupture] with "hazardous fault," defined as one with movement within 500 years of its average recurrence. This definition is consistent with the 10% probability of exceedance in 50 years depicted on current CGS/USGS seismic hazard maps and used in CA building codes for residential construction. Recognizing that fault offsets, regardless of age and to some extent magnitude, can be mitigated by performance-based structural design, we also propose that the A-P be amended to permit any mitigation alternative that results in a life-safety solution per the CA building codes, recognizing that in many cases, avoidance might still be the only alternative.

Eldon, a consulting engineering geologist, has more than 30 years of experience in the identification, investigation, and remediation of geologic hazards, involving land use planning, environmental assessments, field exploration programs, and presentation of findings. He has particular experience with the evaluation of active faults for construction site planning, the development of seismic safety programs and policies, and is currently engaged in efforts to modernize California's 40-year old active fault zoning act (Alquist-Priolo Earthquake Fault Zoning Act).

Eldon is the President of Earth Consultants International, a geological consulting firm [helping our clients solve complex earth-science problems around the world] that he co-founded in 1997, following 12 years with Leighton Consulting in southern California. He has considerable international experience including field projects in Turkey, Panama, Mexico, Costa Rica and Papua New Guinea, as well as project involvement in many others.

Eldon is a graduate of the University of Minnesota, Institute of Technology, with a BS degree in Geology in 1978. He has been in graduate school ever since; MS program at Cal State LA (1982-1990), PhD program at UC Riverside (1993-1996), PhD program at UC Irvine (1998-2008), but despite getting very close, he has never managed to complete the degree due to his busy consulting responsibilities, professional organization involvement, and travel schedules; or conflicted priorities, if you ask his advisors.

Eldon has received several research grants from the U.S. Geological Survey's National Earthquake Hazard Research Program, the Southern California Earthquake Center, and the National Science Foundation for earthquake geology research in California, including paleoseismology of the Whittier fault, tectonic development of the San Joaquin Hills, tectonic geomorphology of the Eastern Los Angeles Basin, and the seismic hazards of the Santa Ana Mountains. He served as the geosciences member on a National Research Council panel to develop the research agenda for the NEES program, he served on the LA County Land Development Technical Advisory Committee for a decade, served on the California Board for Engineering, Land Surveyors and Geologists' Technical Advisory Committee for two years, and has participated multiple times as an Occupational Expert for the US Department of Labor.

Eldon is a frequently invited speaker to local southern California colleges. Since his first professional presentation on the Whittier fault at AGU in 1987, he has given over one hundred presentations before professional, academic, and public groups, and has published dozens of papers on a wide range of geological and professional practice topics, several of which have received awards for outstanding presentations and papers.

In 1995 he was awarded the Aki Award for Outstanding Paper Presentation at the California Academy of Sciences Annual Meeting for Active tectonic structures in the eastern Los Angeles basin, then in 2007 received the Outstanding Presentation Award at the AAPG Annual Meeting for Quaternary geomorphic development and seismic hazards of Orange County, California. Along with coauthors, he has received the 2010 GSA E.B. Burwell Outstanding Paper Award for The Geology of Los Angeles, and the 2012 AEG Claire P. Holdredge Outstanding Paper Award for Paleoseismology of the Pedro Miguel fault, Panama Canal. He served as South Coast Geological Society (SCGS) President in 1987, AEG Southern California Section Chair from 1990-1992, AEG Treasurer, Vice President, and President from 1993-1997, received the AEG Floyd T. Johnston Service Award in 2008, was elected a Fellow by GSA in 2011, and was made an Honorary Member of the SCGS in 2012. He is a member of AEG, GSA, AIPG, AAPG, EERI, IAEG, IAPG, AAAS, AGU, SSA, PDAC, and all local geological societies.