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The Nankai Trough earthquake tsunamis in Korea: numerical studies of the 1707 Hoei earthquake and physics-based scenarios
© Kim et al. 2016
Received: 4 February 2016
Accepted: 6 April 2016
Published: 23 April 2016
Historical documents in Korea and China report abnormal waves in the sea and rivers close to the date of the 1707 Hoei earthquake, which occurred in the Nankai Trough, off southwestern Japan. This indicates that the tsunami caused by the Hoei earthquake might have reached Korea and China, which suggests a potential hazard in Korea from large earthquakes in the Nankai Trough. We conducted tsunami simulations to study the details of tsunamis in Korea caused by large earthquakes. Our results showed that the Hoei earthquake (Mw 8.8) tsunami reached the Korean Peninsula about 200 min after the earthquake occurred. The maximum tsunami height was ~0.5 m along the Korean coast. The model of the Hoei earthquake predicted a long-lasting tsunami whose highest peak arrived 600 min later after the first arrival near the coastline of Jeju Island. In addition, we conducted tsunami simulations using physics-based scenarios of anticipated earthquakes in the Nankai subduction zone. The maximum tsunami height in the scenarios (Mw 8.5–8.6) was ~0.4 m along the Korean coast. As a simple evaluation of larger possible tsunamis, we increased the amount of stress released by the earthquake by a factor of two and three, resulting in scenarios for Mw 8.8 and 8.9 earthquakes, respectively. The tsunami height increased by 0.1–0.4 m compared to that estimated by the Hoei earthquake.
Although the previous disastrous earthquakes to Korea occurred in the backarc region, these earthquakes were all less than M8 earthquakes. However, in the forearc region there is a possibility that large M9 earthquakes may occur (Parsons et al. 2012). The most recent tsunami observed at Jeju Island is from the 2011 Tohoku-oki earthquake, which was generated in the forearc region, and tsunami heights of ~0.2 m were recorded by tidal gauges along the coast of the island. There are no instrumental records showing that Korea was affected by tsunamis from the Nankai Trough, located off the coast of southwest Japan, where large earthquakes repeatedly occur at intervals of about 100–200 years (e.g., Kumagai 1996; Ishibashi and Satake 1998). This raises an interesting question: If a large earthquake occurs in an area along the Nankai Trough, which is closer to the Korean Peninsula, what would be the size of the resulting tsunami?
Tsuji et al. (2014) reported that a historical Korean document records the 1707 Hoei earthquake tsunami, and the tsunami is also recorded in historic Chinese literature. This indicates that one of the largest earthquakes to have occurred in Japan might have reached both Korea and China. The historical Korean literature suggests the possibility that tsunamis due to potential earthquakes in the Nankai Trough can impact the Korean Peninsula in the future. However, historical records do not provide the details of the propagation process from Japan to Korea, nor do they indicate quantitatively the tsunami height near the coastline of Korea. We used numerical simulation techniques to reproduce the details of tsunamis reported in the historical documents.
We conducted numerical simulations to investigate how tsunamis generated by large earthquakes in the Nankai Trough affect the coastline of Korea. We first analyzed how the tsunami caused by the 1707 Hoei earthquake-affected Korea. We reproduced the tsunami heights and arrival times at the Korean coast using the model of the Hoei earthquake proposed by Furumura et al. (2011). Then, we estimated the tsunamis generated by possible large earthquakes in the Nankai Trough with physically reasonable earthquake scenarios proposed by Hok et al. (2011). We described the characteristics of tsunamis caused by the Nankai Trough earthquakes and the associated risks in Korea.
Description of the 1707 Hoei tsunami in historical Korean and Chinese documents
A historical Korean document called “Tamnaji” containing records of provinces of Jeju Island, located to the south of the Korean Peninsula, describes an observation of waves of abnormal amplitude. The record is dated October 29 and November 4, 1707 (Tsuji et al. 2014). Earthquakes and a volcanic eruption are also recorded around the same date. However, there is no information on the observed height of the abnormal sea waves or its location, making it difficult to assume any details from the document.
In addition, a historical Chinese document called “Huzhou Fu zhi” contains records from the city of Huzhou that state, “Suddenly, the water of the river increased” on October 28, 1707 (Tsuji et al. 2014). The city is located between Lake Taihu and the Qiantang River, and it is not near the coast, so it can be presumed that the Hoei tsunami traveled up the Yangtze River. Other historical Chinese documents from cities located near the Qiantang River also record that the water level of the river rose rapidly on the same day as the Hoei earthquake occurred, and some documents record that the wave appeared suddenly without any other accompanying event (Wang et al. 2005; Wen et al. 2014).
These historical documents indicate the possibility that a tsunami caused by a large earthquake in the Nankai Trough reached Korea and China. This also suggests potential risks from tsunamis in Korea generated by large earthquakes in the Nankai Trough.
Tsunami simulation of the 1707 Hoei earthquake
Numerical computation of the 1707 Hoei tsunami
Fault parameters of the 1707 Hoei earthquake deduced by Furumura et al. (2011)
Latitude (°N)/Longitude (°E)
Results of the 1707 Hoei tsunami simulation
Tsunami simulation of Nankai Trough large earthquakes
Anticipated earthquake scenarios in the Nankai Trough
The historical records imply that potential tsunamis generated in the Nankai Trough region could affect the Korean Peninsula in the future. To predict future tsunami disasters, we conducted tsunami simulations for potential Nankai earthquakes.
To assess the potential tsunami hazard, it is crucial to apply physically reliable earthquake models for tsunami simulation to obtain more realistic tsunamis. In this study, the earthquake rupture scenarios by Hok et al. (2011) are used for the prediction of the future Hoei-like earthquake. The advantage of using Hok et al. (2011) compared with other studies is that it includes the current deformation rate of the plate, and the computations include a dynamic term in the rupture propagation which is ignored in most studies.
To compute dynamic rupture scenarios, Hok et al. (2011) used a boundary integral equation method developed by Hok and Fukuyama (2011). Hok et al. (2011) used the three-dimensional geometry of the plate interface model of Hashimoto et al. (2004) to capture features of fault geometry and made a fault interface composed of 13,385 triangular elements with 7400 triangular elements at the free surface. These fault elements are used to simulate dynamic rupture earthquakes. The initial stress distribution is obtained from the slip-deficit rate. Hashimoto et al. (2009) deduced the slip-deficit rate distribution along the Nankai Trough using GPS Earth Observation Network (GEONET) data for 4 years, which Hok et al. (2011) used to calculate the accumulated slip-deficit rate for 100 years. Stress drop is estimated from the slip distribution which is derived from the slip-deficit distribution.
Parameters of anticipated scenarios
k a (%)
Name in Hok et al. (2011)
Western edge of Nankai
Hyuga-nada and Nankai
1946 Nankai hypocenter
Western edge of Nankai
Nankai and Tonankai
Eastern edge of Tonankai
Tonankai and Nankai
In scenario SC01 (Mw 8.56), the rupture area included the Hyuga-nada area and the region off Shikoku Island, and rupture started at the western edge of the Nankai area. Despite the high slip-deficit rate observed in this region, the amount of slip was lower than other models. This is because a large slip-weakening distance was assumed in Hok et al. (2011) to simulate slow slip in the Hyuga-nada segment. A large slip-weakening distance makes it difficult for coseismic slip to occur during dynamic simulations following the slip-weakening friction law. SC02 had the smallest moment magnitude (Mw 8.5) among the four models, whereas the maximum slip was much larger than for SC01. The rupture initiated at the tip of the Kii Peninsula, which is the same hypocenter as that of the 1946 Nankai earthquake. SC03 (Mw 8.61) ruptured the entire Nankai-Tonankai area as a single event with the rupture initiating at the western edge of the Nankai segment. However, in this model the rupture did not propagate to the Tokai area, which is commonly thought to be the initial rupture point of the Hoei earthquake. Hok et al. (2011) reported that they did not include the Tokai area in their study, but they considered the eastern edge of the Tonankai region as one of the initial points in their dynamic simulation. In SC04 (Mw 8.65), the rupture started from Ise Bay and propagated to the western side of the Nankai region. The dimensions of the rupture area were similar to those of SC03, but the initial rupture started at Ise Bay. In terms of the estimated rupture area and moment magnitude, we believe SC04 is the closest model to an anticipated Hoei earthquake among the four scenarios.
Before estimating the vertical seafloor deformation of each scenario, we adjusted the grid locations. The earthquake scenarios by Hok et al. (2011) are composed of 13,385 triangular fault elements whose collocation points are located at an interval of 4 km, and each element has an area of 16 km2 when projected to the free surface. However, we used the analytical solution for a rectangular fault (Okada 1985, 1992) which required us to modify the fault elements. To accommodate the differences between the fault geometry, we applied a piecewise linear interpolation with an interval of 4 km for rectangular fault elements of 4 km × 4 km. We then calculated the sea bottom deformation by representing the heterogeneous slip model by numerous small rectangular faults and used the same method for the numerical tsunami computations as that of the Hoei tsunami simulation.
Tsunamis on the Korean coast
The Hoei earthquake tsunami and historical documentation in Korea
Our simulation results using the Hoei earthquake source model produced a maximum tsunami height along the southern coast of Jeju Island of ~0.5 m, whereas it was ~0.15 m along the northern coast, the most populated area of Jeju Island. This implies that the abnormal sea wave recorded in the historical Korean document was the Hoei earthquake tsunami. However, considering that the northern coast of Jeju Island was more populated, a tsunami of ~0.15 m height might have been too small to be recognized by people. Hence, it might be difficult to conclude that the abnormal sea waves described in the historical document were in fact a tsunami originating directly from the Hoei earthquake. Our simulation indicates that the abnormal sea waves described in a historical Korean document could be the tsunami from the Hoei earthquake, or it could also be a tsunami produced from a secondary source such as a submarine landslide induced by the strong ground motion of the Hoei earthquake. Landslides are often reported when large earthquakes have occurred and can generate large tsunamis locally (e.g., Bryant 2014). However, it is uncertain that landslide-generated tsunamis occurred near Kyushu Island or Jeju Island after the Hoei earthquake. Examining the possibility of a landslide tsunami near Jeju Island is not straightforward, but presents a topic for possible future study.
Variation in the maximum tsunami height due to the difference in the stress drop
We consider that the physics-based scenarios in our analysis are probable in the future but do not represent the maximum earthquake that could possibly occur in the Nankai Trough region (Hyodo et al. 2014). To consider the effects of a larger magnitude earthquake, we increased the stress drop of SC03 by a factor of two and three compared with the original dynamic simulation. Hok et al. (2011) assumed that the stress accumulation could be estimated based on the distribution of the total amount of slip deficit for 100 years. Thus, if we assume that the slip deficit has accumulated for 200 or 300 years, it would create a twofold or threefold amount of slip in the same rupture area. Based on this assumption, we obtained two additional earthquake models: one for 200 years of slip-deficit accumulation characterized by the same moment magnitude as the Hoei earthquake (Mw 8.8) and the other (300 years of slip-deficit accumulation) with an Mw of 8.9. We refer to these models as SC03_A and SC03_B, respectively. Tsunami simulations were performed using these two models with the same methods employed in the previous sections.
Risks associated with Nankai Trough earthquake tsunamis on the Korean coast
Our simulation results from the Hoei earthquake model and the anticipated earthquake models showed that the maximum tsunami height along the Korean coast was <0.5 m. The tsunami would not be life-threatening, but it would disrupt economic activities such as fish farming. For example, Imai et al. (2010) report small-amplitude far-field tsunamis (<0.3 m) disrupted fish farming.
We also note that the arrival of the first tsunami does not always correspond to the maximum height. The maximum height possibly arrives 300–600 min after the first tsunami. The tsunami maintains relatively large amplitudes for a significant period from the arrival of the first peak to the arrival of the wave of maximum height (Fig. 7). It is important to take this characteristic of long duration of large-amplitude waves into account for planning tsunami hazard mitigation strategies in Korea.
Our simulation clearly indicates that tsunamis from the Nankai Trough earthquakes arrive at the Korean coast by passing offshore of Kyushu. Thus, it would be useful to install offshore tsunami gauges between Korea and Kyushu and analyze the records using a data assimilation analysis to forecast tsunami heights and arrival times on the Korean coast. However, there are also limitations in the prediction of tsunamis because we cannot neglect the possibility that strong ground motion can trigger a submarine landslide near the Korean coast that acts as a secondary tsunami source. If a landslide occurs near the coast, locally large-amplitude tsunamis can arrive at the coast earlier than we would expect from a data assimilation analysis.
We reproduced the historical tsunami generated by the 1707 Hoei earthquake using a simulation with the fault model proposed by Furumura et al. (2011), and estimated its propagation process and wave height on the coastlines of Korea. We found that the Hoei earthquake (Mw 8.8) tsunami reached Korea about 200 min after the occurrence of the earthquake. The maximum tsunami height was ~0.5 m along the southern coast of Jeju Island and the Korean Peninsula, whereas the height was less than 0.15 m along the northern coast of Jeju Island, the most populated area on Jeju Island. The maximum tsunami height arrived about 600 min after the first peak arrived, which resulted in a tsunami of long duration. In addition, we conducted tsunami simulations using scenarios of physically reasonable anticipated earthquakes in the Nankai Trough region. We obtained a maximum tsunami height of ~0.4 m among the four scenarios along the Korean coast for the anticipated earthquakes (Mw 8.5–8.6). To evaluate larger possible tsunamis, we increased the stress drop in the earthquake model by twofold and threefold and constructed the scenarios for earthquakes of Mw 8.8 and 8.9. The tsunami height increased by 0.1–0.4 m compared to the Hoei earthquake. Tsunamis produced by large Nankai Trough earthquakes would not be devastating to Korea; however, the effect of larger earthquakes still needs to be considered.
SK carried out the simulation studies and produced a draft of the manuscript. TS and EF carried out the setup of earthquake models and tsunami simulation strategy. TK conceived of the study and carried out the historical documentary studies. All authors read and approved the final manuscript.
We thank the editor and anonymous reviewers for their constructive comments for improving the manuscript. This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMIPA 2015-7120. Part of this research was supported by the National Research Institute for Earth Science and Disaster Prevention (NIED) under the project entitled “Development of the Earthquake Activity Monitoring and Forecasting.” SB is grateful to National Research Foundation of Korea (NRF), Japan International Science and Technology Exchange Center (JISTEC), and Japan–Korea Industrial Technology Co-Operation Foundation for their support in her visiting NIED.
The authors declare that they have no competing interests.
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