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Simulated Aftershock Sequences for an M 7.8 Earthquake on the Southern San Andreas Fault

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Aftershock activity constitutes one of the largest risks in the aftermath of an earthquake. Aftershocks shake already weakened structures, and if an aftershock is closer to a population center than the original rupture it may cause even more severe local shaking. The 1992 M 6.4 Big Bear aftershock, for example, which occurred several hours after and 40 km to the west of the 1992 M 7.3 Landers, California, mainshock, caused substantially more damage to the city of Big Bear than the Landers earthquake. Even more damaging was the 22 August 1952 M 5.8 Bakersfield, California, aftershock of the M 7.5 Kern County earthquake, which occurred about a month after the mainshock. Due to the proximity of the aftershock to Bakersfield and the weakened condition of the buildings, this aftershock killed two, injured 35, and caused $10 million in property damage. More recently the 12 May 2008 M 7.9 Sichuan Province, China, earthquake has been followed, as of 1 August 2008, by five aftershocks that caused significant additional injuries, fatalities, and/or major damage.

Given the danger posed by aftershocks it is important to model the type of aftershock sequence that might follow the next large earthquake in southern California. One of the potential large earthquakes that may threaten southern California is an M 8 on the southern San Andreas fault; a statewide simulation exercise called ShakeOut was held in California in November 2008 to practice response to such a quake. Here I present the 10 different random simulations of the first week of aftershocks that could have accompanied the earthquake modeled for the preparation exercises. Simulation #10 was used for the actual exercises.

No physics is used in the modeling here because aftershock physics are both very complex and controversial. Instead the aftershocks are generated stochastically using established empirical relationships for . . . [Full Text of this Article]

U.S. Geological Survey
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kfelzer@gps.caltech.edu