April 24th, 2012

Spatial and Temporal Earthquake Clustering - Earthquake Aftershocks: EQECAT Perspective: Introduction

Posted at 1:00 AM ET

EQECAT recently released a white paper examining earthquake clustering in the context of seismic hazard and loss assessment (1). The paper, Spatial and Temporal Earthquake Clustering: Part 2 - Earthquake Aftershocks, is EQECAT’s second in a three-part series about spatial and temporal earthquake clustering. This briefing presents a summary of the paper for general interest and should not be viewed as an endorsement of EQECAT’s views.

The EQECAT paper focuses on both spatial and temporal clustering of earthquake aftershocks. Aftershocks are generally not incorporated into seismic hazard and loss analyses, and to some extent most earthquake loss estimation models require changes to incorporate the occurrence of clustered short-term and medium-term earthquakes or aftershocks.

Earthquake aftershocks sometimes can produce more severe losses than the mainshock. This depends on their depth and rupture mechanism as well as the proximity to population centers. For instance, the February 22, 2011, 6.17 Mw Lyttelton earthquake in New Zealand caused much more damage than the larger mainshock, which was the September 3, 2010, 7.1 Mw Darfield earthquake. This smaller aftershock was in very close proximity to central Christchurch at a shallow depth of six kilometers, while the mainshock was as far as six times the distance from Christchurch at a depth of 10 kilometers.

The final white paper in this series will examine fault-based models of earthquake clustering, including the elements of Coulomb stress transfer and new computer “earthquake simulators” that are currently in development.

Aftershocks are those earthquakes that occur in close spatial and temporal proximity due to local static and dynamic stress fields induced by another earthquake, meaning they are dependent on the occurrence of a so-called mainshock earthquake.

Aftershocks that occur at distances of hundreds of kilometers from a mainshock are referred to as remotely triggered earthquakes. Such far distances are well beyond the influences of static stress changes from the mainshock earthquake. Instead, remotely triggered earthquakes are likely caused by transient dynamic stresses associated with the passage of seismic surface waves generated by the mainshock. Since the wave passage effect is not associated with large damaging earthquakes, distant aftershocks are characteristically small in magnitude.

Aftershocks as discussed herein refer to both spatial and temporal local earthquake clustering that occurs in the vicinity of the mainshock.

The intensity of aftershocks decreases rapidly with increasing time and distance. In general, large aftershocks of about one magnitude unit lower than the mainshock can occur anytime within the aftershock sequence. In about 10 percent of cases, mainshocks are followed by larger-magnitude earthquakes, and the original mainshock is retrospectively labeled as a foreshock.

The relative definitions of foreshocks, mainshocks and aftershocks simply reflect the fact that earthquakes are related to each other and that there is imperfect knowledge of their physical connections in time and space.

Note:

1. Paul C. Thenhaus, Kenneth W. Campbell and Dr. Mahmoud M. Khater, “Spatial and Temporal Earthquake Clustering: Part 2 – Earthquake Aftershocks,” EQECAT, February 27, 2012. http://www.eqecat.com/global-earthquake-clustering-whitepaper-part-2-2012-02.pdf

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