Classification of submarine terraces
Around Iheya–Izena islands, the shallow seafloor above −200 m is parallel to the NW-trending islands approximately 10 km to the north and 20 km to the south (Fig. 2). Several terrace-like features divided by small steps are found on the shallow seafloor. The surface of the shallow seafloor may be divided into four terraces by means of the interpretation of the stereoscopic anaglyph image; these terraces are named T1, T2, T3, and T4, in descending order (Figs. 2B and 4).
The surface of T2 distributed at depths between − 50 m and − 80 m is the most prominent, with a maximum width of approximately 3 km; it surrounds the Iheya–Izena islands, with the exception of the northeastern part. The surface of T2 is bounded on T3 by a continuous steep slope with a height of < 10 m. The surface of T2 is relatively flat compared with modern coral reefs; however, it is characterized by topographic features, including small mounts distributed along the outer margin of the surface and a shallow depression along its inner edge. The upper heights of T2 at the west and east of Iheyajima Island are − 60 m and − 70 m, respectively (Fig. 4).
The T1 terrace is bounded on its inner edge by the indented foot of modern coral reefs at depths of approximately − 20 m. The outer margin of T1 extends intermittently around the islands. The top of this terrace reaches up to approximately − 30 m in depth and has an uneven surface. Thus, the distribution of T1 is challenging to identify. A slightly higher area among the uneven surfaces was observed along the outer margin of T1, as is typically observed at the western side of the islands. This topographical feature is similar to that seen in both T2 and the modern coral reef, suggesting that both reflect submarine coral reefs, as mentioned in preliminary reports (Yasuhara 2013).
T3 is distributed at depths between − 70 m and − 90 m, and is gently inclined toward the deep sea. It is bounded by approximately 10-m-high scarps to the east and west. Its surface is generally flat, with a few small steps such that its surface features are different from those of T1 and T2; this suggests that T3 is an erosional surface, not a submarine coral reef. The inner edges of this terrace are observed at depths of approximately − 70 m in the shallow western area and − 80 m in the east, respectively.
T4 is distributed below − 100 m and is clearly observed to the east of Iheyajima Island. Its surface has a width of less than 1 km and is gently inclined to the east. Small fragments of terraces below − 100 m remain to the west and southwest, and a well-defined step between T3 and T4 has not been recognized in the northeast, i.e., these remnants cannot be correlated to T4.
Active fault scarp on the submarine terraces and on the island of Iheyajima Island
Several active fault scarps displacing submarine terraces and onshore marine terraces are observed around Izenajima and Yanahajima islands on the anaglyph map produced in this study (Fig. 2). These NW-trending scarps are divided into two groups, named for the Izena fault zone and the Yanaha fault zone, respectively.
The Izena fault zone is composed of two strands, termed FI-N for the north and FI-S for the south (Fig. 2B). These extend from Izenajima Island to the T1, T2, and T3 submarine terraces and into the Yoron basin. Northward-facing scarps are distributed with southward convex shapes in map view. Distinct fault scarps appear on terrace T2 with a height of approximately 15 m, along the foot of which small narrow depressions are identified (Fig. 5a). Such topographic features are commonly observed along normal faults.
Three steps of marine terraces are identified on Izenajima Island according to the interpretation of topographic anaglyph from the detailed DSM; these are named for TI-1, TI-2, and TI-3, in descending order (Figs. 3 and 5b). Rounded or sub-rounded gravel beds with a thickness of 5 m or less were observed beneath these terrace surfaces (Ujiie 2000), suggesting that they represent erosional marine surfaces. Fault scarps of FI-N, of approximately 10 m height, are present on TI-2 and TI-3 terraces (Fig. 5c). The fault scarps of FI-S extending along the northeastern side of the NW-trending central mountain with a height of 15 m on TI-2 are not distinct from submarine fault scarps on terrace T2 (Fig. 5a and b). The fault scarp heights on land are almost the same as that on the seafloor, so it may be suggested that these faults have started to move in recent geological time.
The Yanaha fault zone, with a length of 20 km, is composed of two strands, named FY-N and FY-S (Fig. 2B). FY-N scarps, which face southward, are parallel to FY-S scarps facing northward, forming the narrow shallow graben in the middle part of the Yanaha fault zone. FY-N scarps are approximately 30 m high on terraces T2 and T3, and are not linear, but rather indented. The FY-S scarp on terrace T2 has a height of < 10 m, and is also indented. The inner terrace edge of T3 does not have a lateral offset along this fault, suggesting that the Yanaha fault zone is characterized by normal faulting.
The Yahanajima Island, an uninhabited island, displayed L-shape in map view and formed by an uplifted coral reef (Ujiie 2000). The linear structure may be observed from the modern coral reef to the furthest extension of the northern foot of the WNW trending tableland in the south of the Yanahajima Island in aerial photographs, suggesting that it is displaced by FY-S (Fig. 2).
Depth distribution of Paleo-shoreline on the submarine terrace
Paleo-shoreline depths were preserved in the topography of the submarine terraces during the last glacial period related to isostatic change (Yokoyama et al. 2018). Since both the T1 and T2 terraces were formed as a coral reef, the depth of the reef crest indicates the paleo-sea level at the time of terrace formation. The reef crest depth on T1, which is distributed over a limited area, does not significantly differ throughout its distribution.
On the other hand, the reef crest depth is − 60 m on the west side and − 70 m on the east side of Iheyajima Island (Figs. 2B and 4a), where distinct terrace surfaces are developed, indicating southeastward tilting. The shallow inner edges of T3 exist at a depth of − 70 m at the west and − 80 m at the east (Figs. 2B and 4a), with the exception of areas in which active faults are distributed. This also indicates southeastward tilting. The depth of the submarine terrace surfaces was displaced by these active faults, as mentioned above, for example, the inner edge of T3 was clearly offset vertically with a height of approximately 30 m along FY-N.