Vol.44-3 ABSTRACT3


We have reexamined original leveling data associated with a possible precursory tilt change shortly before the 1944 Tonankai Earthquake in order to discuss the reliability of these data. Statistical analysis applied to closure errors of all the measurements that the entire survey was carried out with high precision and we can rely on the leveling data. There are a few cases in which a two-way closure error of a subsection was as large as that in cases identified as earthquake precursors and yet the cause of the large error was not clear. However, the leveling data previously by Sato (1977) and Mogi (1984) are consistent with one another, which is not seen for other cases. A source model proposed by Linde and Sacks (1997) for the tilt anomaly is capable of explaining the existing data in a rather natural way. However, we are still lacking both a comprehensive understanding of earthquake source process and clear evidence proving this leveling anomaly as a real precursor.