INTRODUCTION

Many species of animals in a number of localities reportedly behave unusually before earthquakes [Kilian, 1964; Lee et al., 1976; Rikitake, 1976; Academia Sinica, 1977a, b; Toksöz, 1977; Tributsch, 1978; Lott et al., 1979a, b]. Few of the reports are more than anecdotes, and some may be due to retrospective enthusiasm of anxious human observers. In addition, some of the strange behavior apparently coincides approximately with the P arrival of the main earthquake and may not present true precursors. However, numerous stories do report abnormal behavior which occurs hours to weeks before the event, at times when humans do not sense anything unusual in the environment.

Several problems complicate an analysis of reports concerning anomalous animal behavior prior to earthquakes: the variability of animal behavior, the unreliability of human observations, and the existence of uncontrolled physical factors such as weather [McClellan, 1980]. These problems are so significant that scientists have taken a serious look at the phenomena only recently. At present, several types of investigations are underway to document the nature of animal behavior precursors. These investigations include systematic postearthquake interviews [Lott et al., 1979a, b], a network of observers reporting by telephone [Otis and Kautz, 1980], and activity monitors on individual animals under controlled conditions in a field laboratory [Kenagy and Enright, 1980; Skiles et al., 1980].

Our approach is entirely different from other recent studies. Rather than evaluating the reliability of the anecdotes, we hope to provide a framework for geophysicists to evaluate the behavioral phenomena. Our review concentrates on comparing available data on geophysical precursors to the sensory thresholds of humans and other animals, particularly those species mentioned in the anecdotes. Because much of the biological research is published in literature that is unfamiliar to most geophysicists, we include in this paper a comprehensive review of the pertinent biological literature. Although some geophysicists may wish to look at the original references, the information in this review should be sufficient for most purposes for evaluating the plausibility of anecdotal reports. As in most scientific fields, research concerning animal sensory equipment and animal behavior is highly variable in depth and in quality. This review generally reports only the most reliable available research; however, we indicate in the text wherever we report results that may be less reliable.

TABLE 1. Abnormal Animal Behavior Prior to Earthquakes and Other Contexts in Which Similar Behavior Has Been Observed

*Reported by Lee et at. [1976] and Academia Sinica [1977a, b].

A number of geophysical phenomena that have been observed prior to earthquakes might also stimulate unusual animal behavior, including sound with an intensity and frequency outside the range of human perception [Armstrong, 1969; Hill et al, 1976], variations in local magnetic or electric fields [Bufe and Nanewicz, 1976; Johnson, 1978], ground vibrations or foreshocks [Jones and Molnar, 1979], changes in groundwater level [Rikitake, 1976; Raleigh et al., 1977], electromagnetic waves [Derr, 1973], and the release of gases usually trapped beneath the surface [King, 1978]. Animals probably do not sense some types of reported geophysical precursors, including ground tilt [Mortensen and Johnston, 1976], variations in P and S velocities [Ward, 1979], and gravity anomalies [Brown et al., 1977].

In this paper we shall further review the reported geophysical stimuli in terms of published data on animal sensory capabilities. From this comparison we shall dismiss some precursors which are unlikely to affect animal behavior and designate others which seem good candidates for causing unusual animal behavior. This analysis will also allow us to identify some phenomena which are not commonly measured in earthquake-prone areas but which could possibly explain unusual animal behavior. We are restricted because of the paucity of geophysical field data recorded in the epicentral region of earthquakes prior to their occurrence. Installing instruments that measure these phenomena may provide useful information to geophysicists trying to predict earthquakes.