Overdue?

Abstract:

Paleo-seismic data at 32 sites compiled for the Uniform California Earthquake Rupture Forecast reveal that the elapsed time since the last paleo-event exceeds the mean recurrence time at many of the most active sites. Since 1918 there have been no paleo-events on any of the 32 sites. Many interpret those observations to mean that the major fault systems are overdue for large earthquakes. The continuing paleo-hiatus is problematical because the reported statewide rate of paleo-events for 1000 years up to 1918 was over 4 per century. At that rate the random chance of such a hiatus is about 1%. A similar paleo-hiatus occurs in New Zealand, but the calculated probability is not quite so low. In 2014 I suggested three possible explanations for the California hiatus: extreme luck, distant correlations not explained by current theories, or “false positives” in the paleo record caused by mis-identifying non-seismic displacements. Only the first supports traditional use of the data, but it begs the question of why other data are judged by the “5%” rule. The second is still awaiting a theory. By the last explanation, paleo-events are not all earthquakes; there is not an earthquake hiatus but rather an overestimate of the previous paleo-event rate. The quandary persists. Many subsequent hazard models and physics-based simulations imply we are overdue based on single-site quasi-periodic recurrence at the paleo sites, even though such models conflict with the paleo-hiatus.

Seismic and macro-seismic data tell a much different story. The Comcat catalog shows about 9 magnitude 7 earthquakes in California for both the last century and the previous one. Only a few of those events could have contributed to the paleo-seismic record. Thus, the apparent paleo-hiatus is best explained by earlier false positives, and single-site recurrence estimates don’t reflect actual earthquakes. California is seismically quite active, as before, but not “overdue.”