2017 年 69 巻 p. 87-98
The Ecuador-Colombia earthquake that occurred on 31 January 1906 has been considered as one of the largest thrust-fault earthquakes, with magnitudes of Mt 8.7 and Mw 8.8, and the largest event ever recorded in the subduction zone off the Ecuador-Colombia region. The value of Mt 8.7 was derived mainly by the tsunami height data in Hilo, Hawaii where the tsunami was reported as high as 12 feet (3.6 m) on a local newspaper. The earthquake was followed by three large earthquakes in 1942 (Ms 7.9), 1958 (Ms 7.8) and 1979 (Ms 7.7) in the same area. We reexamined tsunami records of the 1906 earthquake from the newspaper articles in Hawaii, wave heights in tsunami catalogues, and tidal gauge records in Japan. Ratios of tsunami heights of Hilo to Kahului, both in the Hawaii Islands, are in a same order for all the tsunamis generated by earthquakes in the western coast of South America since 1920’s. This contradicts with the 1906 values of 3.6 m in Hilo and 30 cm in Kahului. We re-evaluated the tsunami magnitude Mt of the 1906 earthquake to be much less than 8.7. Next, we compared synthetic tsunamis of three fault models (Mw 8.5, 8.6 and 8.8) with the observed tsunami heights at several stations. The model of Mw 8.5 was found to be most appropriate. These results are consistent with the long-period seismic magnitude (Mw smaller than 8.5) because of the lack of major arc wave (G2) in the record of Uppsala, Sweden. The previous seismic estimation of Mw 8.8 may be due to the over-estimation of its fault extent, judging from nearby coastal uplift records, which would include after-slips and/or slow slips near its source region.