Nowcasting Great Global Earthquake and Tsunami Sources

Nowcasting refers to the use of proxy data to estimate the current dynamic state of driven complex systems such as earthquakes, neural networks, or the financial markets. In previous papers, methods to nowcast earthquakes have been presented based on the natural time count of small earthquakes after the last large earthquake in a defined, seismically active geographic region. In this method, a large geographic region is identified in which a local region of interest is embedded. The primary assumption in the method is that the frequency-magnitude statistics of the local region are the same as the frequency-magnitude relation of the large region. The nowcasting technique relies on seismic catalogs that are complete in the sense that all events whose magnitude is larger than a completeness threshold have been detected. For this reason, the previous papers have been limited to nowcasting large earthquakes with magnitudes of approximately M7.5. In this article, we extend the nowcasting method to great global earthquakes of magnitudes as large as M9 by defining the surrounding large region as the entire earth. We then analyze the current hazard of a number of selected ‘‘local’’ regions and rank the regions in terms of current risk of great earthquakes. These great events also present significant hazards for generating mega-tsunamis as well as local damage due to intense shaking. We also perform a sensitivity analysis to establish the reliability of the nowcasts. One of our main results is that the eastern Aleutian Islands, site of the M8.6 earthquake of 1 April 1946, is currently the region most at current risk of a great earthquake larger than M8.0. We finish by presenting comments on the applicability of our methods for anticipating the occurrence of great destructive earthquakes and tsunamis.