10:45 AM - 12:15 PM
[SSS13-P03] Caldera boundary fault reactivation due to regional tectonics during the post-caldera stage at the Akaigawa caldera, western Hokkaido, northeastern Japan
Keywords:Ring faults, Caldera collapse, Trapdoor faulting
Understanding the reactivation processes of caldera boundary (ring) faults during post-caldera stages is necessary for both reconstructing the geomorphological history of calderas and assessing the volcanic and seismic hazards. Although magmatism induced reactivation events have received much attention, those caused by the regional stress have not. The Akaigawa Fault, which is located in the early Pleistocene Akaigawa Caldera, western Hokkaido, is likely to be a part of the boundary fault of Akaigawa Caldera. However, whether the Akaigawa Fault is active during the post-caldera stage is controversial because of a lack of a detailed investigation. Therefore, we here conduct a tectonic geomorphological survey on the Akaigawa Caldera to clarify whether the fault is active, relationships between the fault activity and the regional tectonic stress, and how they contribute to the geomorphological evolution of calderas.
Two lacustrine terraces (Lacustrine terraces I and II) and two fluvial terraces (Middle terraces I and II) are developed in the Akaigawa Caldera (see Figure). Because Lacustrine terrace II is remarkably dissected and is covered by the Toya Ignimbrite (106 ka), the terrace is inferred to have emerged before MIS 6. Middle terraces I and II were formed after 46 ka, because these terrace deposits contain reworked volcanic glass shards of Sp-1 tephra (46 ka). The Akaigawa Fault extending along the northwestern rim of the caldera floor deforms these terraces with a vertical downthrow to the southeast. Considering the trend and heights of scarplets, and cumulative vertical displacement, we assert that the Akaigawa Fault is active. The fault has been active since at least 46 ka, and has increased the relative height between the caldera floor and the somma.
The distribution of the Akaigawa Fault and the post-caldera vent strongly implies that the fault originates from the caldera boundary fault (see Figure). Therefore, the activity of the Akaigawa Fault is interpreted as the reactivation of the boundary fault (i.e., trapdoor faulting) of the Akaigawa Caldera. The post-caldera volcanism is unlikely to have contributed to the reactivation because the volcanism occurred before it. On the other hand, the σ1 axis in the western Hokkaido during the last 6 Ma has been perpendicular to that of the Akaigawa Fault, indicating that the reactivation is induced by the regional compressive stress. Our findings suggest that the regional stress-induced reactivation is common during post-caldera stages, and it should be considered both when reconstructing the geomorphological evolution of calderas and when assessing seismic hazards.
Two lacustrine terraces (Lacustrine terraces I and II) and two fluvial terraces (Middle terraces I and II) are developed in the Akaigawa Caldera (see Figure). Because Lacustrine terrace II is remarkably dissected and is covered by the Toya Ignimbrite (106 ka), the terrace is inferred to have emerged before MIS 6. Middle terraces I and II were formed after 46 ka, because these terrace deposits contain reworked volcanic glass shards of Sp-1 tephra (46 ka). The Akaigawa Fault extending along the northwestern rim of the caldera floor deforms these terraces with a vertical downthrow to the southeast. Considering the trend and heights of scarplets, and cumulative vertical displacement, we assert that the Akaigawa Fault is active. The fault has been active since at least 46 ka, and has increased the relative height between the caldera floor and the somma.
The distribution of the Akaigawa Fault and the post-caldera vent strongly implies that the fault originates from the caldera boundary fault (see Figure). Therefore, the activity of the Akaigawa Fault is interpreted as the reactivation of the boundary fault (i.e., trapdoor faulting) of the Akaigawa Caldera. The post-caldera volcanism is unlikely to have contributed to the reactivation because the volcanism occurred before it. On the other hand, the σ1 axis in the western Hokkaido during the last 6 Ma has been perpendicular to that of the Akaigawa Fault, indicating that the reactivation is induced by the regional compressive stress. Our findings suggest that the regional stress-induced reactivation is common during post-caldera stages, and it should be considered both when reconstructing the geomorphological evolution of calderas and when assessing seismic hazards.