Nature of Macro-Anomaly Precursory to an Earthquake

Abstract

Nature of macro-anomaly precursory to an earthquake such as anomalous animal behaviour, changes inunderground water and hot springs and so on is studied based on an extensive data set related to anumber of large earthquakes in Japan. It turns out that the larger the magnitude of the main shock, the wider the area over which macro-anomalies are observed. Underground water and hot spring anomalies appear in an area where a marked crustal deformation can be expected to take place. The area for anomalous animal behaviour is slightly wider than that. No dependence of precursor time of macro-anomaly on main shock magnitude is found for an individual earthquake. However, there is a certain regularity of precursor time distribution. The maximum frequency of precursor appearance occursaround one day before the main shock. The mean precursor time takes on a value amounting to 0.42 day. It is likely that animals of smaller size such as rat, squirrel and so on become tumultuous earlier than those of larger size such as dog, pig, cow, and so on. Much macro-anomaly precursor data is spurious. Changes in synthetic probability of the main shock occurrence with time when multiple signals appear are evaluated in two ways: one is assuming the reliability, r, which is defined by the ratio of genuine data to the total (genuine and spurious) data, for all the actual data. The other is obtained from a computer simulation of random appearance. Comparing the two evaluations, it seems appropriate to assume that r amounts to 0.05 or thereabouts. © 1994, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.