Spatial and Temporal Earthquake Clustering: An Overview of EQECAT’s Perspective - Part II: Temporal Global Earthquake Clustering
Figure 2 shows the temporal chart of the mega-thrust earthquakes since 1900. The data show dramatic spikes in the global occurrence of these earthquakes. Clusters are schematically indicated by the green dashed curves.
The 1964 Alaska earthquake was the last giant earthquake in a cluster of three that began in 1952 with the 9.0 Mw mega-thrust earthquake off the Kamchatka Peninsula, Russia in the northwestern Pacific Ocean. The second earthquake in this sequence was the 9.5 Mw Valdivia, Chile earthquake, which is the largest earthquake ever recorded by modern seismograph networks.
There is not enough knowledge to fully understand the global effects and physical connectedness of megathrust earthquakes. While classical seismological models based on elastic Earth properties have been developed and applied very successfully to model local and regional seismic sources, these models breakdown at global scales due to the immense distances involved.
Nonetheless, empirical evidence over the last seven years suggests that rare, giant earthquakes of magnitude near 9.0 Mw and greater may cluster temporally over time periods of perhaps 15 years. The low annual probability of these earthquakes of 4.6 percent per year makes it highly unlikely that the temporal clustering occurs on a random basis.
Since the 2004 9.1 Mw Andaman-Nicobar (Sumatra) earthquake, both the 8.8 Mw Maule, Chile and 9.0 Mw Tohoku-oki, Japan earthquakes have occurred over a short time span of seven years.
Formal statistical analyses have shown the time period between 1952 and 1964 as being highly significant in the context of global earthquake occurrence. The 15-year time interval between the years 1950 and 1965 contains seven of the nine greatest earthquakes occurring before 2005. That is also statistically significant with only a 0.5 percent chance of being a random occurrence. Other random simulation tests suggest that the outbreak of giant earthquakes between 1950 and the mid-1960’s, and the 36-year period of global quiescence that followed, were highly unusual, and not easily relegated to chance.
The clustered periods of mega-thrust earthquakes stand out even more if the magnitudes of the earthquakes are transformed into seismic moment and accumulated since 1900 as shown in Figure 3.
It is easy to see that the huge increases in global seismic moment release correspond to the periods containing the clustered occurrence of giant earthquakes.
Recent research shows that deep seismic tremor activity is triggered on some fault zones by the wave passage effect. The deep tremor activity persists long after the surface waves have passed. Over time, the resulting deep stresses can propagate up-dip to shallow fault levels which would serve to trigger shallow large earthquakes.
The most viable mechanism may be post-seismic viscoelastic strains that develop in the deep ductile layers of the Earth, for example, the asthenosphere. These very deep layers react slowly to induced strains but are capable of transmitting stresses over long distances during time periods of years to decades and can influence regional tectonic stressing rates. More speculatively, perhaps, global strains generated by oscillations of the Earth caused by these giant earthquakes serve to stress other large faults that are critically stressed in other regions of the world. It is possible that more than one mechanism is responsible for the observed global correlation among giant earthquakes.
Paul C. Thenhaus, Kenneth W. Campbell and Dr. Mahmoud M. Khater, “Spatial and Temporal Earthquake Clustering: Part 1 - Global Earthquake Clustering,” EQECAT, October 14, 2011. http://www.eqecat.com/pdfs/global-earthquake-clustering-whitepaper-part-1-2011-10.pdf