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On the basis of Akaike's Bayesian information criterion (ABIC), a new method is proposed for detecting a temporal change in a seismic velocity in a source region. The method of joint hypocenter determination was modified in order to determine a seismic velocity in a source layer as a function of time together with hypocenters and station corrections. Arrival times of initial waves of shallow earthquakes in a small area are analyzed in this method. The smoothness of the estimated temporal variation in the velocity is guaranteed by the introduction of a prior distribution of the parameter. The hyperparameter of the prior distribution of the velocity, the reading error of arrival times, and the initial velocity in the source layer are chosen to minimize ABIC. This procedure was applied to the 1983 eastern Yamanashi M = 6.0 earthquake in central Japan. We analyzed P arrival times of 374 earthquakes observed at 12 stations in the network of the National Research Center for Disaster Prevention by dividing the whole period (from October 1981 to May 1987) into 12 six‐month subperiods. Calculating ABICs for different combinations of the three parameters above, we searched for the minimum value of ABIC and found two minima. The first one corresponds to a model of a constant velocity in time, and the other corresponds to a model of a variable velocity with 5% velocity change at maximum. However, since ABIC in the former is 10 smaller than that in the latter, the former constant velocity model is statistically more suitable than the latter. Furthermore, generating artificial data with the same reading errors as the actual data, we used computer simulation to examine the lower limit of the velocity change detectable for this data set. In conclusion, the velocity in the source region is 6.24±0.18 km/s, and the velocity change exceeding 6–7% at maximum did not exist during the 6 years before and after the M = 6.0 earthquake.
Earthquake prediction research on the recent anomalous crustal activity in the Izu Peninsula is summarized. Swarm activity of shallow microearthquakes in the eastern Izu Peninsula began in August 1975 and became more active in October. The epicenters clustered near Togasa-yama at first but spread over other places in the eastern part of and east off the peninsula in early 1976. Releveling carried out in January-April 1976 has disclosed crustal uplift of 15cm centered at Hiekawatoge, several kilometers north of Togasa-yama. The uplift area, more than 30km in diameter, covered the eastern half of the peninsula. Leveling, gravimetric, and tidal data showed that this uplift had developed only during the preceding 1-1.5 year period. Rapid changes in length of base-lines were also detected by repeated geodimeter measurements.These phenomena were noted by the Coordinating Committee for Earthquake Prediction and observations were intensified. The information about the crustal activity was made public by the Committee in May 1976.On August 18, the Kawazu earthquake (M=5.4) occurred in the southern part of the uplift area. Short-term precursors except for foreshocks were not so clear in spite of the intensified observations.In the eastern part of the Izu Peninsula, Central Honshu, remarkable crustal uplift has developed since around the end of 1974, with accompanying microearthquake swarms. An indication of anomalous crustal activity was firstly noticed in increasing microseismicity in August 1975, and the crustal uplift of 15cm was found by releveling in early 1976. Rapid changes in sea level, gravity and base-line length were also found. These facts were noted by the Coordinating Committee for Earthquake Prediction (CCEP) and various observations were intensified by its cooperating institutions. The information about this anomalous activity was made public by the CCEP in May 1976. On August 18, 1976, the Kawazu earthquake (M=5.4) occurred in the southernmost part of the uplift area, and caused some property damage. Short-term precursors were not clear in spite of the intensified observations, except for the foreshock sequence during about 1.5hr prior to the main shock and slight change in groundwater temperature. The process of the research and the response of the CCEP for this anomalous activity are reviewed chronologically as an actual example of the Japanese efforts to predict earthquakes.S 51
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