10:45 AM - 11:00 AM
[HDS10-01] Estimating the wave source of the 1611 Keicho-Oshu tsunami by using ancient documents records and tsunami sediments
Keywords:1611 Keicho-Oshu tsunami, wave source estimation, tsunami trace data
The 1611 Keicho-Oshu tsunami caused widespread damage along the Pacific coast of Hokkaido and the Tohoku region, as recorded in ancient documents. But there is no unanimity of the wave sources. One of the reasons for this is that there are variations in the information used for estimating the wave sources. One of the main sources of information on historical tsunamis is the records of ancient documents, but the quality (reliability) and quantity of the information used for the estimation of wave sources differ among previous studies. In addition, although the sediments which are considered to be formed by the tsunami have been identified at several points along the Pacific coast of the Tohoku region, they are rarely used for wave source estimation. So, in this study, the information including the trace data of the tsunami was organized and the wave source was estimated.
Annaka et al. (1999) was used as a reference for wave source estimation. In this method, an initial wave source with some estimated parameters and trace data are set, and the slip of the source is estimated so that the residual sum of squares between the trace data and the calculated tsunami height by numerical analysis become small.
For the trace data, the "Tsunami Trace Database" and tsunami deposits were used. "Tsunami Trace Database" contains a comprehensive collection of trace data of historical tsunamis available at present, and each information is assigned a confidence level.
The confidence level is classified into A to D. Confidence level A is those whose traces are clearly indicated in ancient documents and have been resurveyed in recent years, while B have not been resurveyed. The level of C may have some variation in elevation depending on the way the representative points of tsunami height are taken, because the descriptions of the tsunami trace points are wide(ex. village). The high confidence level (A and B) is limited only to 5 points in Iwate prefecture, and it is difficult to constrain the wave source area, so the information of the confidence level A to C (39 points in total) is used in this study (A:2, B:3, C:34 points).
Previous studies have pointed out that there is a discrepancy between tsunami sediment distribution and inundation area. Goto et al. (2014) investigated the relationship between tsunami sediment thickness and inundation depth in the Sendai Plain, where tsunami sediments by the Tohoku Earthquake Tsunami were identified, and found that on average, the sediment thickness is related to about 2% of the inundation depth. In this study, tsunami sediments at 9 sites along the coast of Iwate and Miyagi prefectures, where the dating results include 1611 and the tsunami was confirmed in the vicinity, were converted from sediment thickness to inundation depth using the method of Goto et al.(2014).
As for the initial wave source, the model of the 1896 Meiji Sanriku earthquake tsunami are used, which is pointed out to be similar to the 1611 Keicho-Oshu tsunami. As a result of the analysis, the traces in the Sendai Plain could not be satisfied, so the southern fault (L=200 km) was added to the reproduced model (L=210 km) off Iwate prefecture as the initial wave source.
For the north and south faults set as initial wave sources, I divided them into 4 and 8 parts, and estimated the slip of the divided small faults by combining linear inversion and nonlinear inversion so that the sum of residual squares is minimized.
Theoretically, the reproducibility should improve as the number of partitions is increased, but for this study,
at the present stage, the smallest residual sum of squares is obtained when the number of partitions is set to 4, and no model with a smaller residual sum of squares has been obtained.
The slip of each small fault in the four sections was 21.7 m, 0 m, 1.8 m, and 48.5 m from the north, and the residual sum of squares is 3.99 (K=1.07, κ=1.94).
The slip of the central fault was 0 m, and two tsunami earthquakes occurred at the same time, which are spatially distant from each other and the maximum slip is 48.5m. It is necessary to verify whether such a large slip occurs in a tsunami earthquake from a seismological point of view.
The Keicho-Oshu tsunami is being studied interdisciplinary, and there is a theory that the deep plate boundary off the coast of Miyagi Pref. also slipped as in the 3.11 earthquake, and there is a previous study that it was an outer-rise type earthquake. Therefore, we will keep a close watch on related studies.
Annaka et al. (1999) was used as a reference for wave source estimation. In this method, an initial wave source with some estimated parameters and trace data are set, and the slip of the source is estimated so that the residual sum of squares between the trace data and the calculated tsunami height by numerical analysis become small.
For the trace data, the "Tsunami Trace Database" and tsunami deposits were used. "Tsunami Trace Database" contains a comprehensive collection of trace data of historical tsunamis available at present, and each information is assigned a confidence level.
The confidence level is classified into A to D. Confidence level A is those whose traces are clearly indicated in ancient documents and have been resurveyed in recent years, while B have not been resurveyed. The level of C may have some variation in elevation depending on the way the representative points of tsunami height are taken, because the descriptions of the tsunami trace points are wide(ex. village). The high confidence level (A and B) is limited only to 5 points in Iwate prefecture, and it is difficult to constrain the wave source area, so the information of the confidence level A to C (39 points in total) is used in this study (A:2, B:3, C:34 points).
Previous studies have pointed out that there is a discrepancy between tsunami sediment distribution and inundation area. Goto et al. (2014) investigated the relationship between tsunami sediment thickness and inundation depth in the Sendai Plain, where tsunami sediments by the Tohoku Earthquake Tsunami were identified, and found that on average, the sediment thickness is related to about 2% of the inundation depth. In this study, tsunami sediments at 9 sites along the coast of Iwate and Miyagi prefectures, where the dating results include 1611 and the tsunami was confirmed in the vicinity, were converted from sediment thickness to inundation depth using the method of Goto et al.(2014).
As for the initial wave source, the model of the 1896 Meiji Sanriku earthquake tsunami are used, which is pointed out to be similar to the 1611 Keicho-Oshu tsunami. As a result of the analysis, the traces in the Sendai Plain could not be satisfied, so the southern fault (L=200 km) was added to the reproduced model (L=210 km) off Iwate prefecture as the initial wave source.
For the north and south faults set as initial wave sources, I divided them into 4 and 8 parts, and estimated the slip of the divided small faults by combining linear inversion and nonlinear inversion so that the sum of residual squares is minimized.
Theoretically, the reproducibility should improve as the number of partitions is increased, but for this study,
at the present stage, the smallest residual sum of squares is obtained when the number of partitions is set to 4, and no model with a smaller residual sum of squares has been obtained.
The slip of each small fault in the four sections was 21.7 m, 0 m, 1.8 m, and 48.5 m from the north, and the residual sum of squares is 3.99 (K=1.07, κ=1.94).
The slip of the central fault was 0 m, and two tsunami earthquakes occurred at the same time, which are spatially distant from each other and the maximum slip is 48.5m. It is necessary to verify whether such a large slip occurs in a tsunami earthquake from a seismological point of view.
The Keicho-Oshu tsunami is being studied interdisciplinary, and there is a theory that the deep plate boundary off the coast of Miyagi Pref. also slipped as in the 3.11 earthquake, and there is a previous study that it was an outer-rise type earthquake. Therefore, we will keep a close watch on related studies.