The possibility of deeper or shallower extent of the source area of Nankai Trough earthquakes based on the 1707 Hoei tsunami heights along the Pacific and Seto Inland Sea coasts, southwest Japan
© Hyodo et al.; licensee Springer. 2014
Received: 10 March 2014
Accepted: 17 September 2014
Published: 25 September 2014
To validate the abundance of scenarios of large earthquakes in the Nankai Trough, we examined the effects of both lateral and vertical expansions of the source areas on maximum tsunami heights along the Pacific coast and Seto Inland Sea. The recently proposed Nankai Trough earthquake scenario (Mw = 9) has a maximum slip of 20 m near the trough axis. However, the predicted tsunami heights exceeded those obtained from historical records of damage caused by the 1707 Hoei tsunami event at Tosa Bay and along the Pacific coastlines near the Kii Channel, owing to the large slip on the up-dip extension of fault segments off Shikoku Island. Such discrepancy indicates that for segments off Shikoku Island, the slip near the trough axis was unremarkable, even for the 1707 Hoei earthquake event, which is considered one of the larger historical Nankai Trough earthquake events. For segments east of the Kii Peninsula, the large slip on the up-dip end might be ineffective. While the proposed Mw9-class scenario also includes large slip of several meters on the down-dip side (down to about 35-km depth), coseismic crustal subsidence reached further landward than is usual for Nankai Trough earthquakes. For the Seto Inland Sea region, this resulted in maximum subsidence of about 1 m, and such crustal subsidence effectively increased the height of the tsunamis. Furthermore, simulated tsunami heights, corrected for crustal subsidence, were in good agreement with those obtained from historical records of the damage caused in the Seto Inland Sea region.
For several decades, it has been believed that large seismogenic and/or tsunamigenic slip does not occur on shallower parts of the plate interface near the trough axis, although splay faults might cause large tsunamis (Fukao 1979; Park and Kodaira 2012). Furthermore, it has been suggested that the magnitude of Nankai Trough earthquakes is controlled mainly by the number of fault segments that rupture concurrently at seismogenic depth (Ishibashi 2004). Thus, it has been believed that Nankai Trough earthquakes occur in various patterns with laterally arranged rupture units called segments (see Figure 1). In smaller Nankai events, western (A and B) and eastern segments (C, D, and E) tend not to rupture concurrently, but with temporal separation (from 30 h to 2 years), and/or seismic ruptures do not always reach segments A and E, which are located at the western and eastern edges of the Nankai Suruga Trough, respectively. Conversely, in larger Nankai events (e.g., the 1707 Hoei earthquake and tsunami events), segments A to E tend to rupture in a near-concurrent way, and even the additional rupture of the westward extension (i.e., segment Z in Figure 1) has been suggested to be concurrent with ruptures of segments A to E (Furumura et al. 2011). Accordingly, the recurrence of a Nankai Trough earthquake has been understood - and anticipated - within the traditional viewpoint of the concurrent rupture of lateral fault segments.
The disastrous Tohoku earthquake of 2011 in northeastern Japan, however, was accompanied by massive tsunamis due to large slip (approximately 50 m) near the Japan Trench (Fujiwara et al. 2011). This indicates that for other subduction zones, we cannot exclude the possibility of seismic events located at the shallowest parts of subduction plate interfaces. Actually, in the Nankai Trough region, seismic slip on the shallowest part of the decollement was confirmed by evidence from a core drilled near the Nankai Trough off Kumano basin (Sakaguchi et al. 2011). Furthermore, based on the movement of coastal rocks at the southern tip of the Kii Peninsula, Namegaya et al. (2011) guessed that tsunamis, considerably larger than those usually found in the Nankai Trough region, occur at that location. Therefore, the assessment of the damage that could result from the recurrence of a Nankai Trough earthquake in the near future was reconsidered by including the possible slip on the up-dip extension. Moreover, the different possible strong ground motions and tsunamis predicted by maximum-class earthquake scenarios with large slip (40 to 50 m) on shallow subduction interfaces - similar to the Tohoku earthquake of 2011 - have been published in an official report (Cabinet Office 2012). Based on the previous Nankai Trough earthquake scenarios (Central Disaster Prevention Council 2003), the source area used in the scenarios in the 2012 report was extended laterally westward to include the southern tip of the Kyushu region. Furthermore, the vertical range of the source area was also extended - from the usual seismogenic zone - to include both up-dip and down-dip extensions. The up-dip extension reaches the Nankai Trough axis, while the down-dip limit is shifted to the depth of slow slip events. For the extended source area, maximum slips of 40 to 50 m on the up-dip side of the seismogenic zone were kinematically assigned, in an additional slip of several tens of meters at both seismogenic and down-dip depths (Cabinet Office 2012). Consequently, the magnitudes of Nankai Trough earthquakes become Mw > 9.1 in the revised scenarios. Therefore, the estimated resultant tsunami damage could increase locally by several times compared with that estimated in previous scenarios with large slip at the usual seismogenic depth (Cabinet Office 2012).
As mentioned above, there is the possibility that the source area of Nankai Trough earthquakes extends to deeper or shallower extent. Here, we examine whether scenarios with up-dip or down-dip extensions of the source area are consistent with historical Nankai Trough earthquakes, by focusing particularly on consistency with the 1707 Hoei earthquake. In Japan, there exist many historical documents dating back to the 1600s, not only regarding the capital but also various other parts of the country. Thus, historical descriptions of damage by earthquakes or tsunamis are more reliable than for previous eras. Because of the collection of such reliable descriptions or instrumental records for such events, the 1707 Hoei event is generally considered the largest of the four Nankai Trough earthquakes that have occurred since 1600. Hence, we regard the Hoei event as representative of larger Nankai Trough earthquakes. For the earthquake scenarios, we used the physics-based Nankai Trough earthquake scenarios obtained from quasi-dynamic earthquake cycle simulations, as discussed in the following.
Thanks to the developments in the field of high-performance computing, simulations of large-scale earthquake-generation cycles, based on the driving forces of relative plate motion and the fault-friction law, can be realized. For the Nankai Trough region, such physics-based simulations have been conducted to mimic real-world earthquake sequences and interpret the mechanisms underlying earthquake-generation cycles along the Nankai Trough (e.g., Hori et al. 2004; Hori 2006; Hyodo and Hori 2013). This has resulted in the accumulation of many earthquake scenarios with widely varying estimates of magnitude or occurrence pattern. However, these studies have been limited to discussions of qualitative comparisons of earthquake recurrences and/or slip distributions of simulated and historical earthquake sequences (e.g., Hori 2006).
Previously, kinematic sources, represented by a few rectangular fault planes, have been used for forward modeling of seismic waves or tsunamis to improve understanding of the rupture processes and the extent of historical earthquakes. However, owing to recent advances in physics-based forward modeling, improved and more realistic simulation-based earthquake scenarios have been accumulated, which might replace these kinematic scenarios as the dominant method of forward modeling of the source and rupture processes. A considerable advantage of these simulation-based scenarios is their ability to mimic real-world rupture processes in a realistic way, through unprecedented spatiotemporal smoothness of simulated ruptures. Thus, by comparing predicted tsunami heights obtained for each scenario with historical damage records from particular historical earthquake events, we can narrow down the number of scenarios potentially suitable for the prediction of tsunami heights generated by future Nankai Trough earthquake events.
Historical tsunami damage
Here, we briefly introduce the 1707 Hoei tsunami damage that will be compared with the simulated tsunami heights in this study. Many previous studies have collated data on historical tsunami heights in southwestern Japan based on records of damage (e.g., Hatori 1974, 1985; Murakami et al. 1996). Hatori (1974) compiled pre-existing tsunami height lists of historical Nankai Trough earthquakes, and Hatori (1985) and Murakami et al. (1996) collected descriptions of historical tsunamis from unconfirmed old documents in Kyushu and Shikoku districts, respectively. Then, based on definite descriptions on tsunami heights, such as inundation up to the floorboards at a particular place, site elevations were measured through leveling surveys in field investigations; thus, the inundation height was re-affirmed for each description. From the results of above three studies, we extracted the 1707 Hoei tsunami-height data on the Pacific and Seto Inland Sea coasts and compared them with the simulated tsunami heights.
Hyodo and Hori (2013) proposed a model of Mw9-class Nankai Trough earthquakes based on quasi-dynamic earthquake-cycle simulations. In their simulated earthquake sequence, the Mw9-class Nankai Trough earthquake and a smaller one occurred alternately. In Hyodo and Hori (2013), the slip distributions of simulation-based earthquakes are defined through the selection of a slip-velocity threshold of V > 1 cm/s, and the inclusion of slip above this threshold in the distribution during unstable slip events. Such ‘velocity-based’ slip distributions might depend on the choice of the threshold value. Hence, for the initial conditions of tsunami simulations in this paper, we adopt another slip distribution, defined by selecting a specific period during which all slip is included in the distribution. Thus, if we select the specific period of 1,000 s for the ‘duration-based’ slip distribution, the total amount of fault slip from the onset of the unstable slip to 1,000 s afterwards is extracted from the simulation result as the initial condition of the tsunami calculation.
Evidently, uncertainty could also be introduced into the duration-based slip distribution through the selection of this time interval. However, to some extent, the slip distribution must become smooth if the time interval is longer because spatiotemporal propagation of slip is equal to that of the raw simulation result during the selected time interval. Actually, we confirmed that slip distribution is smooth and not largely dependent on the value of the selected time interval, if a time interval of between several hundreds to 1,000 s is selected. However, for the velocity-based slip distribution, spatiotemporal smoothness might not be guaranteed because the resultant slip distribution might have unphysical gaps, depending on the value of the slip-velocity threshold.
To examine the possibility of the deeper or shallower extent of the source areas in Nankai Trough earthquakes, we compared tsunami heights predicted in the scenario with Mw = 9.03 to real damage caused by the 1707 Hoei earthquake event, i.e., we evaluated the degree to which the newly proposed Mw9-class Nankai Trough earthquake scenario was consistent with reality. For the comparison of predicted and real damages, we considered damaged sites, not only along the Pacific coastline but also in the Seto Inland Sea region.
JAGURS code for tsunami simulation
To compute the predicted tsunami heights based on simulated-earthquake scenarios, 9-h tsunami simulations were performed using the recently developed parallel finite-difference tsunami simulation code JAGURS (Baba et al. 2014), which solves nonlinear shallow-water equations numerically using the initial submarine deformation resulting from earthquake scenarios. To assess tsunamis resulting from Nankai Trough earthquakes, 9-h simulations are sufficiently long to evaluate the maximum heights of tsunamis at target regions because tsunamis reach the Pacific coast and the Seto Inland Sea region within about 30 min and 4 h, respectively, after the occurrence of Nankai Trough earthquakes.
Computational domain and grid spacing
Sources and initial conditions
For the duration-based coseismic slip distributions, we were able to calculate the corresponding surface vertical deformation using the slip response in the homogeneous elastic half-space. The source area of the larger earthquake of Hyodo and Hori (2013) was extrapolated from previous estimates of Nankai Trough earthquakes in the westward and up- and down-dip directions, similar to the maximum-class model of Cabinet Office (2012).
It should also be noted that the southwestern bound of large slip in the Hyuga-Nada region was moved to around 32°N for the three sub-slip distributions in Figure 6. This is because Furumura et al. (2011) confirmed that the southwestern extension of the source area around 32°N is necessary for the reproduction of tsunami heights observed during the 1707 Hoei tsunami event along the eastern coast of Kyushu Island. Therefore, in this study, we focused on the effects of up-dip and down-dip extensions of the source region on tsunami height along the Pacific coasts and in the Seto Inland Sea region.
Results and discussion
Throughout this study, in assessing simulated tsunami heights, we added the crustal vertical deformation - due to the various earthquake scenarios - to the calculated tsunami height with respect to the level of seawater at rest. Thus, the coastal tsunami height was measured from the vertical position of the site that was moved by coseismic crustal deformation. Consequently, coastal subsidence due to coseismic crustal deformation increases tsunami height, whereas uplift acts to decrease coastal inundation.
Tsunami heights along the Pacific coastline of Kyushu region
Regional K and κ values for five earthquake scenarios
Tsunami heights along the Pacific coastline of Shikoku region
Tsunami heights along the Pacific coastline of Kinki region
From the predictions of sub-slip models for the larger-earthquake scenario (Figure 9b), it can be observed that sub-slip model 3, which was assumed to have slip only on the seismogenic and down-dip extensions, resembles the large-tsunami scenario in Figure 9a, and K-value for the sub-slip model 3 is 0.90. Conversely, sub-slip models 1 and 2 indicate considerable mutual correlation with poor K-values (0.66 and 0.69, as shown in Table 1). Small K-values for these two models indicate that the up-dip extensions of segments B and C have little effect on damage done to sites throughout the Kinki region. Thus, the Pacific coastal tsunami damage sustained by the Kinki region is compatible with large slip to the up-dip end on the segment off the Kii Peninsula, as suggested by Sakaguchi et al. (2011). Moreover, it is interesting that sub-slip model 1, which included rupture of the seismogenic zone only, predicted greater tsunami heights than sub-slip model 3, which included the seismogenic and down-dip extensions. This feature is similar to the Pacific coast of Shikoku region. As shown in Figure 6, slip on the usual seismogenic zone causes subsidence of up to several meters along the Pacific coastline of Shikoku and Kinki regions, while inclusion of the down-dip extension makes this coastal subsidence shift landward. Because of this shift, crustal vertical deformation decreases considerably along the coastline. This explains why increasing the source area goes hand-in-hand with predictions of decreasing tsunami height.
Tsunami heights along the Pacific coastline of Tokai region
Tsunami height in the Seto Inland Sea region
For Pacific coastal tsunami heights, we confirmed the rupture effect of the nearest segment to be dominant in reproducing tsunami heights comparable with historical damage records. Furthermore, we also examined the critical factor for generating large tsunami heights in the Seto Inland Sea. Figure 11c shows the tsunami-height profiles obtained using the sub-slip models for the larger-earthquake scenario. From the results of sub-slip models 1 and 2 (red and blue lines in Figure 11c, respectively), the up-dip extension of the seismogenic zone can be seen to have almost no effect on tsunami height in the Seto Inland Sea, and the total tsunami heights are the same as those of the smaller-earthquake scenario. The K-values for sub-slip models 1, 2, and the smaller-earthquake scenario are poor with values of 2.34, 2.31, and 1.96, respectively (Table 1). Conversely, the down-dip extension shifts the subsidence associated with the down-dip edge of the slip landward (as mentioned in the sub-section associated with the Pacific coastal tsunami in Shikoku region) and raises tsunami heights up to about 1 m for the Seto Inland Sea (green line Figure 11c). The resultant profile is almost the same as that of the larger-earthquake scenario, except for the western edge of the Seto Inland Sea. The K-value for sub-slip model 3 is improved to be 1.37, and this value is a little larger than that for larger-earthquake scenario.
Other factors that might affect inland tsunami height include ocean currents or earth tides. For tsunamis in the Seto Inland Sea, Miyamoto et al. (2006) discussed the possibility of amplification of tsunami heights by earth tide. Owing to the narrow straits located at the western and eastern edges of the Seto Inland Sea, earth tides cause sharp differences in water level (with a maximum gap of several meters) between the Seto Inland Sea and the open sea, resulting in local changes of flow velocities of ocean currents through these straits. As the Nankai Trough earthquake tsunamis surge through the Kii or Bungo Channels, such changes in flow velocities of ocean currents might affect their velocities or heights. Increases in tsunami heights due to tidal coupling were estimated to be about 0.5 m at most by Miyamoto et al. (2006), and through calculation of tsunami flows in the Seto Inland Sea - assuming an initial tidal gap of a few meters between the inland and open seas - they concluded that tidal effects were practically negligible. However, they adopted a minimum spatial grid size of 450 m for the simulation of the Seto Inland Sea region, which is about ten times larger than that adopted in this paper. Such a coarse grid size cannot resolve the complex bathymetry of the Seto Inland Sea and might result in large underestimations of tsunami heights. Hence, the coupling of tsunamis, tidal effects, and ocean currents should be reexamined using finer grid sizes for better and more localized understanding of the historical records of severe damage to the central part of the Seto Inland Sea region caused by the 1707 Hoei tsunami event. Such work would improve the assessment of the potential for disaster from future tsunamis in the Seto Inland Sea.
We believe that the collection of various earthquake scenarios, consistent with historical earthquakes, is important for preparing against the next Nankai Trough earthquake. Of the historical Nankai Trough earthquakes, the 1707 Hoei earthquake is the largest that has many available descriptions in old documents. Therefore, by making full use of the historical data associated with the Hoei earthquake, we can estimate the characteristics of larger Nankai Trough earthquakes.
Therefore, the available damage records from the Pacific and Seto Inland Sea were used to explore the source scenarios for the 1707 Hoei earthquake and tsunami event. As a reference scenario of the 1707 event, we first selected the simulation-based Mw9-class earthquake of Hyodo and Hori (2013), which has a similar source area to the maximum models of Cabinet Office (2012); although the maximum slip at the up-dip end is less than half. Then, by decomposing this Mw9-class scenario into sub-slips, we calculated the various resultant tsunami profiles and determined the most appropriate scenario among them to represent the 1707 Hoei tsunami damages in the Pacific and Seto Inland Sea coasts. By excluding the large slips both near the up-dip end off Shikoku and the southward extension of large slip beyond 32°N from the Mw9-class scenario, the consistency with the 1707 Hoei tsunami heights was improved considerably from the original Mw9-class scenario.
As a first step in validating the many Nankai Trough earthquake scenarios available, we examined the effects of both lateral and vertical expansions of the segments on the maximum tsunami heights along the Pacific and Seto Inland Sea coasts for the proposed Mw9-class Nankai Trough earthquake scenario (Hyodo and Hori 2013). Comparing the Mw9-class Nankai Trough earthquake scenario with the historical 1707 Hoei earthquake and tsunami events, we found the following similarities and differences between the simulated and historical events:
The westward extension of rupture segments with about 10-m slip is indispensable for explaining the historical damage along the Pacific coastal region of Kyushu, which is similar to the findings of Furumura et al. (2011).
The up-dip extension of large slip on segments off Shikoku Island predicts tsunamis much larger than the 1707 Hoei tsunami, resulting in far more damage to regions along the Pacific coast, such as Tosa Bay, southeastern Shikoku Island, and the southwestern Kii areas. Conversely, the assumption of source areas with usual up-dip limits (approximately 10-km depth) of large slip provides coastal profiles of maximum tsunami heights that are more consistent.
Owing to the associated crustal vertical deformation, the down-dip extension of large slip on segments beneath Shikoku Island induces tsunamis higher than a few meters in the Seto Inland Sea region. These tsunami heights are comparable with those observed during the 1707 Hoei tsunami event, except for in the central part of the Seto Inland Sea where tsunami heights higher than in the surrounding areas were observed.
For the Pacific coastal tsunamis affecting the Shikoku to Kii regions, a down-dip extension of large slip is necessary, down to about 30 km or deeper. In other words, a shallower down-dip limit causes subsidence of the coastline from the Shikoku to Kii regions, and maximum tsunami heights become significantly larger than the 1707 Hoei tsunami.
We thank the editor and anonymous reviewer for their constructive comments for improving the manuscript. This research was implemented in the Strategic Programs for Innovative Research, Field 3, and part of the results were obtained using the K computer at the RIKEN Advanced Institute for Computational Science (Proposal number hp120278). We thank Dr. Ritsuko Matsu’ura for providing us with important information on tsunami damage records of the Seto Inland Sea region. We thank Dr. Kentaro Imai for his cooperation and for providing us with the maximum tsunami-height data compiled from existing studies (Hatori 1974, 1985; Murakami et al. 1996).
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