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Examination of an active submarine fault off the southeast Izu Peninsula, central Japan, using field evidence for coseismic uplift and a characteristic earthquake model
© Kitamura et al. 2015
- Received: 2 September 2015
- Accepted: 28 November 2015
- Published: 9 December 2015
Detailed mapping and radiocarbon dating of emergent marine sessile assemblages show that coseismic uplift occurred at 1256–950 BC, AD 1000–1270, AD 1430–1660, and AD 1506–1815 in the southern Izu Peninsula, central Japan. Employing a characteristic earthquake model, this study reconstructed the source fault for the uplift events from the spatial distribution of coseismic vertical displacements and historical documents. The source is inferred to be a reversal fault located about 3 km off the southern Izu Peninsula that is 25 km long and 13 km wide (strike = 250°, dip = 52° to the north) and slip of 2.7 m and has generated a Mw 7 class earthquake.
- Coseismic uplift
- Late Holocene
- Izu Peninsula
- Emergent marine sessile assemblages
- Fault model
- Characteristic earthquake model
In this paper, we examine emerged marine sessile assemblages at three sites in the southern Izu Peninsula. Then we estimate coseismic vertical displacement based on a combination of previous work and new data. We propose that coseismic uplift events were caused by characteristic earthquakes, and we predict the location and geometry of the source fault using a fault model.
The coastline along the southern Izu Peninsula is characterized by a wave-dominated and microtidal regime, with a maximum tidal range of 1.6 m during spring tide. Based on geodetical records from 1896 to 1968 (Danbara and Tsuchi 1975), the study area has been subsiding at a rate of ca. 0.6 mm/year.
Ota et al. (1986) examined cored sediments from coastal lowlands and emergent sessile marine invertebrates in sea caves (large and small caves) at Kisami (Fig. 1). The authors reported that millennial-scale vertical land movements in the area changed from subsidence to uplift at ca. 3000 year BP. The uplift trend was caused by at least three coseismic uplift events (Ota et al. 1986). Taguchi (1993) reported raised benches and notches from Shirahama to Koina (Fig. 1). The elevations of the upper and lower notches are 2.5–3.0 and 1.5–2.0 m amsl, respectively. Taguchi (1993) reported no significant difference in the elevations of notches from Shirahama to Koina; coastal landforms were not observed west of Koina (Fig. 1).
Kitamura et al. (2014, 2015) investigated the faunal compositions and 14C ages of emergent sessile assemblages at five sites between the Nabeta and Kisami coasts (Fig. 1). Of these sites, the assemblages at the “large cave” on the Kisami coast, west of Shimoda, provided a complete history of coseismic uplift. The assemblages occur continuously between 0.14 and 3.40 m amsl and consist of intertidal barnacles, bivalves, and worm tubes (Fig. 2). Based on the faunal compositions and 14C ages of the assemblages in this cave, Kitamura et al. (2015) identified four coseismic uplift events at 1256–950 BC (uplift 1), AD 1000–1270 (uplift 2), AD 1430–1660 (uplift 3), and AD 1506–1815 (uplift 4). Based on a mean inter-seismic subsidence rate of 0.6 mm/year and uplift of 0.1 m resulting from the AD 1974 Off-Izu Peninsula earthquake, the estimated amounts of vertical uplift during uplifts 1–4 are 1.67–2.97, 0.01–0.67, 0.72–1.04, and 0.76–0.94 m, respectively. Emergent sessile assemblages are incomplete at the other four sites, but Kitamura et al. (2015) detected no significant difference in total uplift across the five sites.
Emerged benches and notches are observed from Shirahama to Koina (Taguchi 1993), and the 14C ages of emergent sessile assemblages have been reported from five sites between the Nabeta and Kisami coasts (Kitamura et al. 2014, 2015) (Fig. 1). Thus, we searched for emergent sessile assemblages along the coast between Shirahama and Nabeta and between Kisami and Koina. We also investigated uplifted Holocene landforms and sessile assemblages at Mikomotojima, a small (ca. 0.1 km2) previously unstudied island located 10 km off the coast of Shimoda (Fig. 1).
Results of 14C dating
Altitude (m above msl)
δ 13C (‰)
Conventional 14C age (year BP)
Calibrated age (2σ) (calendar year) (95.4 %)
Calibrated age (2σ) (calendar year BP) (95.4 %)
Lab number (Beta)
1250 ± 30
1500 ± 30
We combined new and previous data to reconstruct the deformation caused by coseismic uplift. We analyzed the geometry and deformation of the inferred source fault using the boundary element method with a Green’s function in a homogeneous elastic half-space (Okada 1992). To model the source fault, we made assumptions: (1) the fault is a reverse fault with pure dip-slip movement, to account for the large coseismic uplift; (2) the upper edge of the fault is located along the cliff near the coast; and (3) the lower limit of the fault is at 10-km depth, which roughly corresponds to the lower limit of seismogenic layer in this region (Tanaka 2004).
Distribution of coseismic vertical displacement
Global sea level during the past 5000 years has remained within 0.25 m of the present-day level (Woodroff et al. 2012). This study assumes that sea level has been stable over the past 5000 years.
Both F. albicostatus and H. mutabilis inhabit in intertidal zone from −0.8 to 0.8 m amsl (Okutani 2000; Yamaguchi and Hisatsune 2006). Their fossils occur at 1.5 m amsl at the Kujyuhama coast and at 1.9 m amsl at Tarai Cape, indicating that 1.5 and 1.9 m of relative sea level fall has occurred at the Kujyuhama coast and Tarai Cape, respectively. Figure 2 compares our 14C data with those of Kitamura et al. (2015). The ages and elevations of the specimens at Tarai Cape and the Kujyuhama coast match those of zones B and C at Kisami, respectively. This indicates that their emergence was related to uplifts 2 and 3, respectively, and that there is no significant spatial difference in total uplift in the coastal area between Tarai Cape and Kujyuhama.
As noted above, barnacle individuals have been reported at 1.8–2.2 m amsl along the Koina coast (Fig. 1) (Taguchi 1993), and their elevation corresponds to that of zone B at Kisami. We therefore used the values of total coseismic uplift deduced from the large cave at Kisami to infer the total coseismic uplift in the coastal area between Koina and Kujyuhama.
Based on previous studies of the sedimentary facies in cored coastal plain deposits of the southeastern Izu Peninsula, reliable constraints on the age and amount of coseismic uplift were obtained at site 7 near Minami Izu (Fig. 1). Here, Kitamura et al. (2013) determined that the uppermost tidal deposits were deposited at 4530–4430 calendar year BP, at −0.7 m amsl. Using the above constraints, the mean coseismic uplift at site 7 is estimated to be 0.3–0.7 m (Fig. 6).
Taguchi (1993) reported that the elevations of emerged notches, which form in the intertidal zone (from −0.8 to 0.8 m amsl), are up to 3.0 m amsl at Shirahama. Given that the rate of subsidence during the past 3000 years has been the same as that between 1896 and 1968 (0 mm/year; Danbara and Tsuchi 1975), the net uplift is calculated to be 2.2–3.8 m. At Shirahama, the mean coseismic uplift is estimated to be 0.6–1.0 m (Fig. 6).
Fault source model
As noted above, there is no significant spatial difference in total uplift in the coastal area between Tarai Cape and Kujyuhama, indicating that the fault is a west-dipping reverse fault and strikes NNE–SSW, parallel to the coastline of the Izu Peninsula. Since emerged coastal landforms are not observed west of Koina (Fig. 6), the western edge of the fault seems to be located off Koina.
Kanamori and Anderson (1975) proposed a typical stress drop value of 10 MPa for intraplate earthquakes. Using this value and a scaling relation between stress drop and fault area (Kanamori and Anderson 1975), we estimate that the fault area of the targeted Mw 7 class earthquake was several hundred square kilometers.
Given these constraints on the fault, and the assumptions described in the methods, we constructed two models for estimating the average coseismic uplift. A submarine fault in model 1 is located 15 km off the coast of Shimoda, based on topographic analysis of the seabed inferred by Kim et al. (2012) (Fig. 1), and is located 5 km south of Mikomotojima. Although there is absence of emergent marine sessile assemblages and landforms at the island, the area is very erosional all around, so that emerged evidences might disappear. A submarine fault in model 2 is located at the base of a steep slope at 1 km off the Suzaki Peninsula (Fig. 6b).
Second, we set another fault for model 2. The eastern corner of the top edge (0 km depth) is at 34.6882° N, 139.1364° E, and the western corner is at 34.6165° N, 138.8905° E. The fault strike is approximately 250°. Figure 8b shows the grid search result. The best-fit case is a dip angle of 52° and a slip amount of 2.7 m. The fault is 25 km long and 13 km wide. The moment magnitude is 6.9 with a rigidity of 30 GPa. The stress drop is approximately 11 MPa.
When comparing two models, model 2 is more realistic than model 1 in the following respects: (1) the sum of squared residuals between the observed and calculated values for model 2 is far smaller than the sum of squared residuals for model 1 (see Fig. 8). (2) The vertical deformation at Mikomotojima (34.5754° N, 138.9416° E) for model 2 is nearly zero, but that for model 1 is over 1.8 m. The slight vertical deformation for model 2 may correspond to the absence of emergent marine sessile assemblages and landforms at Mikomotojima.
In contrast, the dip angle for model 2, 52° (>45°), seems peculiar as a reverse fault in terms of Anderson’s theory. One can consider that the high-angle reverse fault was a reactivated one (Jackson 1980). Or we confirmed that the dip angle for model 2 can be <45° when the eastern corner of the top edge is near the coastline, although this setting leads to a smaller fault and higher stress drop deviating from the scaling relation.
On the basis of the distribution and radiocarbon dating of emergent marine sessile assemblages, we determined that coseismic uplift occurred at 1256–950 BC, AD 1000–1270, AD 1430–1660, and AD 1506–1815 in the southern Izu Peninsula, central Japan. We constructed source fault models from the spatial distribution of coseismic vertical displacement and from historical documents. The results indicate that a reverse fault (strike = 250°, dip = 52° to the north) is located about 3 km off the southern Izu Peninsula and is 25 km long and 13 km wide, records a total slip of 2.7 m, and has caused a Mw 7 class earthquake.
We thank Takafumi Imai and Youki Takikawa for the help in field work. We also thank A. Stallard for improving the English of the manuscript. This study was funded by Grants-in-Aid (26287126) awarded by the Japan Society for Promotion of Science. This study was conducted under the permit from the Agency for Cultural Affairs, Kanto Regional Ministry of the Environment.
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