Oakan volcano is one of the post-caldera volcanoes of Akan caldera, and its eruptive history has not yet been clarified well. In order to reveal the structure of volcanic edifice and eruptive history with possible age data, we carry out not only geological survey but also tephrochronological study around the volcano. We identify 10 tephra units, nine of which are wide-spread tephras from other volcanoes in Hokkaido and Baitoushan volcano. Only one tehpra unit (Oafa) from Oakan volcano has been recognized, but the other nine tephras can be used as good time markers for understanding the activity of the Oakan volcano. The volcanic activity can be divided into two main stages: the early stage (E stage) and late one (L stage). Although most of the edifice of the E stage is covered by eruptive materials of the L stage, several lava flows are distributed on the southern flank. After the formation of the edifice, sector collapse of its southern part occurred and formed a debris avalanche deposit on the flank. The edifice and the debris avalanche deposit of the E stage is covered by Meakan tephra (NaPS: ca. 1.3ka), which indicates that the activity of the E stage had terminated before 1.3ka. After a certain period of dormancy, the activity of the L stage started, which can be divided into two sub-stages: L-1 and L-2. During the L-1 stage, lava effused from four crater areas, and the Futatsudake cone was formed at one of these areas. Based on petrological features of these lavas, the L-1 stage could be divided into L-1-1 and L-1-2 groups. Oafa tephra layer recognized at the flank was derived from the cone (L-1-2) judging from its isopach and isopleth maps, and also from its petrological features. The tephra layer is sandwitched between Ma-f (ca. 6.6ka) and Ta-c2 (ca. 2.5ka) tephras. Considering thickness of soils between Oafa and these two layers, we estimate that eruption age of Oafa is about 5ka. Although Oafa can be correlated with the activity of L-1-2 group, it seems that the activity of L-stage began around 5ka, because there existed no obvious time interval between L-1-1 and L-1-2 groups. Thus, the dormancy period between the early and late stages can be estimated to be about 8000 years. In the L-2 stage, eruption centers moved northeast to construct the summit (Oakandake) pyroclastic cone, in which four craters were formed. At the same time, lava flows repeatedly effused from the cone to widely cover the north to southeast flank. Based on the location of eruption centers and the time sequence of these lava flows, the activity of L-2 stage can be divided into four groups: L-2-1 to L-2-4. The youngest crater at the summit cone of L-2 stage was formed before 1ka, because it is covered by Ma-b tephra (ca. 1.0ka). This suggests that the latest magmatic activity of L-2 stage occurred before 1ka. Although no eruptive activity was recorded, weak fumaloe activity at the north crater on the mid flank was reported. Our study reveals frequent eruptions of Oakan volcano during Holocene, and suggests that the volcano must be considered as an active volcano.

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Nantai Volcano is a symmetrical stratovolcano, situated in the southern part of the Northeast Japan arc. Many geologic studies hitherto have suggested that the stratovolcano was formed during the Main stage, and the overlying pyroclastic materials and a lava flow were formed in the Later stage. Because no sedimentary gap is found between any deposits of the Later stage, it is inferred that all of the activity in the Later stage took place successively around 12ky BP (15-14 cal ka BP) and went dormant until now. However, we found a pyroclastic flow deposit named Bentengawara Pyroclastic Flow Deposit (BPFD) at the northeastern flank of the Nantai volcano about 2km from the summit crater. This deposit overlies an 80cm thick deposit of weathered ashy sediments that in turn overlies the Arasawa Pumice Flow Deposit, a member of the Later stage. The lower half of the BPFD consists of volcanic lapilli and ash that is remarkably fine-depleted while the upper half contains abundant scoria of mainly lapilli-block sized clasts. The deposit also includes a small number of breadcrust blocks and occasional accessory lava blocks and fragments of charred wood. The breadcrust blocks consist of a dense outer crust that is significantly fractured and a vesiculated interior. It is noteworthy that the edges of the cracks are sharp and never rounded, suggesting that the vaporization of the inner magma that produced these cracks took place just before or immediately following the settlement of the blocks. Paleomagnetic data from three breadcrust block samples indicate that the magnetic vectors of high temperature components are aligned with our present-day poles. Two pieces of charred wood were measured for their ¹⁴C ages with results of 12-11 cal ka BP. The whole rock chemistry of scoria and breadcrust blocks are determined to be significantly different from any of the rocks of the Later stage, but the accessory block in the BPFD has the similar chemistry to the Osawa Lava, the last product of the Later stage. We therefore suggest that the BPFD was deposited after the Later stage with a short (～3ka) dormant period between them. Since the age is possibly around 10ka, the Nantai volcano should be counted as active volcano based on the definition provided by the Meteorological Agency of Japan.

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Sakurajima volcano experiences, in addition to repeated vulcanian eruptions, intermittent small eruptions similar in style to strombolian eruptions. These strombolian-like eruptions are associated with swarms of BL-type earthquakes dominated by low frequency components (1-3Hz). Ground deformation associated with BL-type earthquake swarms was detected by water-tube tiltmeters and extensometers in an underground tunnel. Tilt and strain records were corrected by BAYTAP-G to account for the tidal effect. Gradual tilt change of crater-side-up (20-320nrad) and extension of the ground (8-170nstrain) continued for 3-30 hours before the BL swarms. The inflation was temporarily suspended for 0.5-19 hours, and was then followed by deflation associated with BL swarms. The degree of tilt and strain change was in the same order as that for vulcanian eruptions; however the duration of inflation processes is longer than that of a vulcanian explosion (by several minutes to several hours). The inflation rates (2-28nrad/h, 2-16nstrain/h) prior to BL swarms are smaller than those prior to vulcanian eruptions (20-90nrad/h, 10-50nstrain/h). In the deflation process, tilt change of crater-side-down (40-300nrad) and contraction of the ground (20-160nstrain) continued for 1-6.5 hours and was accompanied by BL swarms. The deflation rates (17-113nrad/h, 12-57nstrain/h) accompanying BL swarms are small. The deflation rates of 1/3 of the vulcanian eruptions exceeded the upper limit of the deflation rate that accompanied the BL swarms. The depth of the source of pressure that is thought to induce the ground deformation associated with BL swarms is estimated to be 3-4km for both inflation and deflation processes (assuming the Mogi source to be horizontally located at the center of crater). No difference in depth is detected for vulcanian eruptions. Volcanic gases were emitted in the inflation process prior to BL swarms, however volcanic gas emission stopped in the inflation process prior to vulcanian eruptions. It is inferred that prior to vulcanian eruptions, the top of the conduit is plugged by a lava dome derived from cooled and degassed magma, and the internal pressure rapidly increases. In contrast, prior to BL swarms, the upper conduit is loosely choked and the internal pressure gradually increases due to the intrusion of new magma from a deeper source. The difference in inflation rates may be caused by the degree of choking of the upper conduit. BH-type earthquakes dominated by high frequency components (5-8Hz) occurred alongside inflation prior to a BL swarm. The inflation rate almost reached the maximum rate prior to BL swarms that occurred without pre-BH-type earthquakes. It is inferred that a high inflation rate due to choking of the conduit is the cause of BH-type earthquakes.

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