Re-Examination of Seismicity Associated with Destructive Inland Earthquakes of Japan and Its Seismological Significance

Abstract

The first half of this study is dedicated to correcting the misconception that the seismometrical data obtained by the earlier seismological station network are so poor in quality that they are not available for the modern method of analysis. In the course of the study, one of the best methods to locate hypocenters using these old data was established. In the latter part of this study, we investigated seismicity associated with several destructive earthquakes which occurred in the inland area of Honshu during the period from the 1930's to the 1970's. On the basis of the examination of seismicity, common characteristics related to the faulting process of these earthquakes and the long-term variation of seismic activity around the focal area were discussed. Important conclusions derived from the review of the history of our seismological observation are as follows: 1) In spite of the gradual improvement of the network with temporary setbacks due to the social crises brought about by World War II, we could not find any distinct heterogeneity in the quality and the quantity of the observational data from the early 1930's to the late 1960's. This implies that it is possible, for all practical purposes, to apply the same data processing techniques to all the data. 2) Evaluation of earthquake location capability in the inland area for every five years in the period from 1926 to 1984 shows that most earthquakes of magnitude 4.2 or more could be located, from the early 1930's to the present, if we supplement yet unprocessed but still available data in some periods, especially during and immediately after the War. From the late 1950's, most events of magnitude 3.8 or more can be located if we reexamine all the data before 1975. This estimation will be useful for discussing the long-term variation of seismicity. 3) Since the early days of seismological observation, with respect to the earthquake location accuracy, hypocenter location using P and S arrivals has always been superior to that using S-P interval times. This latter often resulted in large location errors especially for large earthquakes which often have multiple focal processes. 4) Hypocenter location errors for earthquakes in the 1960's estimated from travel time residuals are two to three times, and those for earthquakes in the 1940's are three to four times, larger than the present level. Most of epicenter location errors for inland earthquakes in the 1940's are estimated to be within 10km. 5) Judging from the earthquake location errors, it may be possible to recognize the double seismic zone in the Kanto district from the data of the 1930's and the 1940's, if we reexamine them. 6) A joint hypocenter determination method (JHD) using mean travel time residuals as station corrections is successfully applied to the aftershocks of old earthquakes. This means that the time keeping of the old network is good enough to deduce the travel time anomaly originated in the inhomogeneous crustal structures. Results of the re-examination of seismicity associated with the destructive earthquakes occurring in the inland area of Honshu during the period from 1930 to 1970 are as follows : 1) A linear arrangement of aftershocks is usually recognized for strike slip fault earthquakes, such as the Gifuken-Chubu earthquake of 1969, the Echizenmisakioki of 1963, the Daishojioki of 1952, the Fukui of 1948 and the Tottori of 1943. 2) Density of aftershock occurrence around the hypocenter of a main shock is generally lower than that in other aftershock zones and this fact seems to be related to a faulting of large displacement on the fault plane around the hypocenter of a main shock. 3) The relative depth of the main shock to those of aftershocks suggests that a faulting of the main shock tends to start from the bottom of a fault. In the case of the reverse slip type earthquake, if faulting started near the top of a fault, a complicated focal process, such as a successive occurrence of large earthquakes of the same magnitude (Twin earthquakes) or intensive foreshock activity in the shallow depth region, will be observed as in the Imaichi earthquake of 1949, the Ogahanto of 1939 and the Mikawa of 1945. On the other hand, strike slip type earthquakes which began near the top of a fault show no complexity in the focal process. 4) Most reverse slip type faultings spread unilaterally. Strike slip type earthquakes show both unilateral and bilateral faultings. In many cases, however, the direction of rupture propagation is from east to west or from southeast to northwest, and these directions are in accordance with the relative moving direction of the Pacific plate to Eurasia and the Philippine sea plates. The rupture process of large earthquakes may be more or less affected by factors, such as the geological structure of the focal area, the variation of normal stress on the fault, the width of the seismogenic layer in the crust, the curving of the strike of fault trace, and the micro-structures of fault segments. To analyse which factor is the most important for the faulting process, it is necessary to get detailed information about these factors. 5) As a precursory phenomenon, a doughnut-shaped pattern or seismic quiescence prior to the earthquakes was recognized in several cases. Considering the short history of seismological observation and background seismicity, however, it is difficult to recognize it before an earthquake. 6) Some destructive inland earthquakes, such as the Kita-Izu earthquake of 1930, the Tottori earthquake of 1943, the Mikawa earthquake of 1945 and the Izu-Oshima-Kinkai earthquake of 1978, were preceded by intense foreshock activities by a few hours to a half year. These foreshocks tightly cluster around the hypocenter of the main shock and they are most important precursory phenomena for short-term earthquake prediction. 7) Foreshocks in the broad sense of the word, which have a magnitude 4.5 or more and occur in the focal region one to ten years prior to an earthquake, were found in several cases. Although the mechanism of the occurrence of these foreshocks is unknown so far, the occurrence of earthquakes of moderate size in an inland area is not so frequent. Thus, they will be utilized to select a plausible test site for earthquake prediction researches. 8) Micro-earthquake activity around a focal region, which is the last part of an aftershock activity, continues for a long time, and its duration length depends on the nature of the earthquake and the focal region. This implies that seismicity in an inland area may contain three currently indistinguishable types of activity, namely (1) remains of aftershock of historic or prehistoric earthquakes, (2) background seismic activity and (3) precursor of future earthquakes. In order to distinguish them for earthquake prediction, a observation of high quality over a long period should be pursued. © 1987, Japan Meteorological Agency / Meteorological Research Institute. All rights reserved.