Abstract
LONG-TERM precursory changes of seismicity previously reported are (1) a change in the relationship between earthquake frequency and magnitude, and (2) a decrease in the frequency of micro-earthquakes. In a review of earthquake precursors Rikitake1 cites eleven examples of the former and three examples of the latter. On the dilatancy hypothesis a decrease in activity is to be expected from dilatancy hardening2; an earlier gradual increase of activity due to the secular increase of tectonic stress was also postulated2. A more distinctive sequence of seismicity fluctuations can be recognised from data in seismological bulletins. The fluctuations are large: a burst of activity marking the start of the precursory sequence is followed by a period of abnormal quiet, and this lasts until the onset of the major event. We report here characteristic changes of minor earthquake activity, preceding a number of recent major earthquakes in regions with good seismographic coverage, and suggesting a new basis for long-term prediction relying directly on the normal data output of permanent seismograph networks.
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