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The bridge from earthquake geology to earthquake seismology

David D. Jackson

Published September 20, 2016, SCEC Contribution #6413, 2016 SCEC Annual Meeting Talk on 9/12 11:00 (PDF)

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Recurrence intervals of large earthquakes exceed the instrumental earthquake record, so we rely on paleo-seismic and surface deformation evidence to infer long-term earthquake rates and recurrence statistics. Paleo-seismic studies generally imply quasi-periodic recurrence and greater rates for large earthquakes than those deduced from instrumental data coupled with Gutenberg-Richter magnitude relations.

Under SCEC leadership a team of paleo-seismic experts compiled the most reliable data available for use in the third Uniform California Earthquake Rupture Forecast (UCERF3). They reported dates of observed displacements at 32 sites on 13 named faults in California. Corrected for multiple-site ruptures, the total reported paleo-event rate at those sites exceeds about 4 per century, yet none has occurred since 1916. Such a long hiatus is extremely unlikely for a Poisson process and even less probable for an ensemble of quasi-period processes.

Hypotheses for the hiatus of paleo-events include (1) extreme luck, (2) unexplained regional fault interaction, or (3) mistaken identification of non-seismic displacements as evidence of large earthquakes, or counting multiple branches of single ruptures as separate ruptures. The first can be rejected with 99% confidence. There is no evidence for the second in the pre-1916 paleo-seismic history or in any theoretical models yet reported. Temporal clustering of large quakes (“supercycles”?) could in principle explain the hiatus, but individual site records and rate-state frictional models suggest quasi-periodic recurrence instead. The third hypothesis could explain the observed quiescence because mistaken identity would be prevented by instrumental seismic data during the last century. In any case the paleo-event hiatus poses a serious challenge to earthquake forecast models. It also begs the larger question of how 2-dimensional fault-based observations and models can be reconciled with 3-D instrumental earthquake observations.

Key Words
paleoseismology, recurrence, supercycle,

Jackson, D. D. (2016, 09). The bridge from earthquake geology to earthquake seismology. Oral Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability (EFP)