Abstract
Plate-boundary earthquakes of magnitude 8 or greater along the Suruga–Nankai Trough subduction zone have repeated at intervals of 90–150 years, but with widely varying magnitudes and rupture areas. We propose, based on geologic data on crustal movements of the forearc wedge, that these earthquake variations are controlled by two separate locked zones in the deeper part of the plate boundary. Long-wavelength topographic undulations, composed of alternating zones of uplift and subsidence along the forearc wedge, are associated with 2000 to 3000 m of vertical relief that has accumulated during Quaternary time. We suggest that this crustal deformation in the forearc wedge is caused not by stress loading and release during earthquake cycles, but rather by vertical displacements of the plate boundary caused by the westward movement of undulations in the obliquely subducting slab of the Philippine Sea Plate as it subducts beneath Southwest Japan. Dating of emergent marine shell fossil assemblages shows that the Kii Mountains, an uplift zone at the midpoint of the trough, has undergone uplift events at intervals of 400–600 year, and the latest event of those occurred when the 1707 Hoei earthquake ruptured the entire plate boundary along the trough. We infer that the plate boundary under the Kii Mountains is a locked zone and that slips of this zone, which accompanied ruptures of the entire plate boundary, caused uplift of the mountains by decreasing the plate boundary depth. A similar locked zone is inferred under the uplift zone of the Akaishi Mountains, along the eastern margin of the trough, and the 1854 Ansei earthquake pair was presumably caused by the slip of this zone. During the two 1854 events, the locked zone under the Kii Mountains presumably restricted the rupture propagation to the eastern half of the trough, then a rupture of the western half of the trough followed within 32 h. These locked zones are inferred to slip independently every few hundred years and determine the major patterns of characteristic ruptures along the Suruga–Nankai Trough.
Highlights
The Philippine Sea Plate (PSP) has been subducting under Southwest Japan at the rate of 3–7 cm/year from the Suruga–Nankai Trough (Heki and Miyazaki 2001), and great plate-boundary earthquakes of magnitude (M) 8 and greater have been repeating at an interval of Okamura and Shishikura Earth, Planets and Space (2020) 72:5590–150 year along the trough (Ishibashi and Satake 1998; Ishibashi 2004)
We show that the crustal movements of the forearc wedge during the last 103 to 106 year have not been caused by earthquake cycles and that the uplift and subsidence of the large-scale N–S trending zones in the middle and eastern parts of the forearc wedge can be explained by passage beneath the wedge of large-scale undulations on top of the obliquely subducting PSP slab
We propose that a locked zone independent of the one under the Kii Mountains exists under the area between the Akaishi Mountains and the Suruga Trough (Fig. 6) and suggest that ruptures of this locked zone generate Anseitype earthquakes
Summary
The Philippine Sea Plate (PSP) has been subducting under Southwest Japan at the rate of 3–7 cm/year from the Suruga–Nankai Trough (Heki and Miyazaki 2001), and great plate-boundary earthquakes of magnitude (M) 8 and greater have been repeating at an interval of Okamura and Shishikura Earth, Planets and Space (2020) 72:55. Late pleistocene crustal movements In the same area where these coseismic and interseismic deformations have been observed, marine terraces of Middle-to-Late Pleistocene age that indicate coastal uplift are widespread along the Pacific coast (Fig. 2) Their elevations decrease to the north from the southern promontories of the Kii and Shikoku Mountains. Shishikura et al (2008) showed that the youngest ages in the lowest fossil assemblages range from the seventeenth to early eighteenth centuries and attributed the latest uplift to the 1707 Hoei earthquake, in which the plate boundary of the entire trough ruptured They suggested that this and earlier multi-segment earthquakes were accompanied by unusual crustal movements that were greater than those from recurring earthquakes. We infer that the plate boundary under the Shikoku Mountains is not an independent locked zone
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