The precursory earthquake swarm and the inferred precursory quarm

Abstract

The earthquake swarm as a long-term precursor to major earthquakes is being studied empirically by the method of hypothesis formulation and testing. Results suggest that swarms occur as an early stage of the seismogenic process for major earthquakes, but that the phenomenon is limited to shallow subduction regions and other environments where high fluid pressure provides a triggering mechanism for swarms. An inference from a proposed seismogenic model is that, if swarms are not triggered, the same precursory earthquakes should occur, though in a more protracted manner. A cluster of four major earthquakes (ML 6.7, 6.3, 6.0, 5.8) occurred without precursory swarms in the area around Arthur's Pass, New Zealand, during 1992–95. This area is in the region of continental collision which lies between the Hikurangi and Fiordland subduction regions. A prior cluster of seismicity anomalies has now been identified which displays the same precursory relations as have been established for swarms. These anomalies are termed quasi-swarms, or quarms, and the swarm hypothesis is now being extended to include the precursory quarm. © 1999 Taylor & Francis Group, LLC.