December 27th, 2011

Spatial and Temporal Earthquake Clustering: An Overview of EQECAT’s Perspective - Part I: Summary, Mega-thrust Earthquakes

Posted at 1:01 AM ET

The recent major earthquakes that have occurred in Chile (February 2010), New Zealand - Canterbury (September 2010), New Zealand - Christchurch (June 2011) and Tohoku (March 2011) have raised questions such as: Do earthquakes of major intensity cluster around the world? Have there been other, previous series of major earthquakes around the world? Are other major earthquakes more likely to occur in the very near future, if so where? These are all very challenging questions. This briefing begins to answer some of them.

This briefing presents a summary of the white paper recently released by EQECAT, Spatial and Temporal Earthquake Clustering: Part 1 - Global Earthquake Clustering. It is EQECAT’s first paper about both spatial and temporal clustering of mega-thrust earthquakes (1). This briefing is presented for general interest and should not be viewed as an endorsement of EQECAT’s views.

The paper reports that the statistics on giant earthquake occurrences show that the historical temporal clustering of these earthquakes on a global scale cannot be attributed to chance. However, the physical processes producing this global temporal clustering are unknown.

The paper indicates that, on a global scale, effects of giant earthquakes are likely transmitted over large distances through post-seismic relaxations and strain transfer mechanisms in the deep ductile layers of the Earth. The transmission happens in relatively short time intervals. On the other hand, global temporal earthquake clustering appears not to be related to shallow co-seismic effects of elastic earth properties associated with near-field and far-field stress models.

If the current earthquake cycle of mega-thrust earthquakes, which started in 2004 with the December 24, 2004 9.1 Mw Andaman-Nicobar (Sumatra) earthquake, follows the one that occurred between 1950 and 1965, then we may expect that the cycle is only half complete. The largest earthquake in the current cluster has yet to occur.


Table 1 chronologically lists the world’s largest mega-thrust events with magnitudes greater than 8.5 Mw, according to the U.S. Geological Survey (USGS).

Table 1


The locations of these earthquakes are shown in Figure 1 (Page 3) by using the USGS identification number assigned to each earthquake in the table. The colors indicate different time periods of occurrence.

Figure 1


Mega-thrust Earthquakes

The December 24, 2004 Andaman-Nicobar (Sumatra) earthquake/tsunami has been referred to as the first “global” earthquake. The label is appropriate in terms of the number of lives lost, the number of countries affected and the global effort required for recovery. The moment magnitude 9.1 Mw earthquake and its tsunami caused the death of approximately 280,000 people. This earthquake was the largest to occur worldwide in 40 years.

The previous earthquake of similar magnitude was the 9.0 Mw Good Friday, Alaska earthquake of March 27, 1964. Both of these earthquakes are members of a class of “giant” earthquakes, which are only those earthquakes with magnitudes 9.0 Mw and greater.

On a worldwide scale, while earthquakes on the order of 8.0 Mw occur on average once per year, giant earthquakes of 9.0 Mw and greater occur far less frequently.

The Andaman-Nicobar earthquake was only the fourth giant earthquake to be recorded since 1900. The probability of such an earthquake occurring in any given year is about 4.6 percent, or about once every 22 years.


1. 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.

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