Deep low-frequency earthquakes beneath Hakone volcano, central Japan, and their relation to the volcanic activity
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Abstract Deep low‐frequency earthquakes (DLFEs) are ubiquitous seismic activities in the deep parts of volcanoes. Owing to the low signal‐to‐noise ratio, the seismic activities of DLFEs have not been characterized in detail; particularly, the linkage between DLFEs and shallow volcanic activity has not been understood sufficiently. In this study, numerous DLFEs have been successfully detected beneath the Hakone volcano, central Japan, by cross‐correlating a template to the continuous seismic signals. The resulting seismic catalog reveals that DLFEs are activated prior to notable earthquake swarms in the shallow part of a volcano and to the crustal expansion caused by a pressure source at a depth of 7 km. Results indicate that the activation of DLFEs reflects the feeding of magmatic fluid from depth. The subsequent increment in the magmatic‐fluid pressure triggers shallow volcanic activities.
Fluid pressure
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Showa-Shinzan, one of th e Belonite-Volcanoes, born (1943-1945)in the eastern part of the Volcano Usu (in the South-western part of Hokkaido, Japan) is famous in the history of volcanoes in the world.This volcano erupted in a farm-land and further developed a lava dome (Spine) in the central part. It is a Pelée-type one, and its developing process was clearly observed for the whole period.Some remarkable earthquake swarms occurred during the volcanic activity, and the major ones among them were recorded by seismographs even at a station about 800 km apart from the volcano.The author investigated the relations betwee n the occurrence of earthquakes and the formation of Showa-Shinzan and tried to find some clues to the volcanic mechanism. In this first paper, after an investigation of the characteristics of earthquake swarms recorded on the seismographs in Muroran, Mori and Sapporo (epicentral distance: 25-69 km) and the relation of the volcanic activity therewith, some interesting results were obtained:1) Daily earthquake frequency diagram for the whole period of twenty three months were prepared for the first time.2) These earthquakes were clearly divided into the following three stages:( i ) Pre-volcanic stage: In this stage there was no eruption, and remarkable earthquakes occurred around Volcano Usu frequently. The relation between the maximum amplitude (A) in these earthquake swarms and their frequency (N) was expressed byNAm = const.where m is about 1.8, similar to that of the tectonic earthquake. Moreover, the migration of the hypocentres of these earthquakes showed an intimate connection with the course of the magma intrusion.(ii) Stage of eruptive activity: A violent eruption occurred in the central part of the rising area. The extremely shallow earthquakes during the period of six months including just before and through the eruptive activity were proved to have been originated at the area adjacent to the newly formed crater.(iii) Birth and developmen t of lava dome (spine): The lava dome grew and developed without eruption, accompanied with numerous earthquakes. The earthquakes, which differed greatly from those at the former two stages, began to occur almost simultaneously with the end of the eruption and the frequencies of the earthquakes during the following twelve months corresponded closely to the speed of the development of the lava dome. The value of m in this case was 3.5.3) Process of magma intrusion, increase of magma viscosity a n d their relationship with the feature change of earthquake swarms were investigated. The possibility of predicting volcanic activity Volcano Usu by seismograph at stations up to approximately sixty or seventy kilometers from the volcano is suggested.
Seismometer
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Caldera
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The relationship between volcanic and seismic activities observed around Iwate volcano in 1998 are discussed. The first subject is the relation between the seismic activity and the ground deformation, which were simultaneously observed in the period from February to August, 1998. The seismic activity is a characterized by westward extension of the focal region in the western part of the volcano. The ground deformation is explained by moving sources of tensile faults and pressure sources located near the west end of the focal region at each twomonth period considered. The synchronism of these phenomena is explained by the idea that both of the activities are two different aspects of volcanic activity caused by the westward migration of magma in the western part of the volcano. The idea is confirmed by the examination using the temporal change of the Coulomb failure function (ΔCFF). The second subject is the relation between the volcanic activity mentioned above and a moderate earthquake (M6.1) of Sept. 3, 1998, that took place at 10 km SW from the summit of the volcano. The ΔCFF calculated from the source model for the volcanic ground deformation shows no evidence that the volcanic activity triggered the earthquake. On the other hand, temporal changes in seismicity in the volcanic area are explained as the effects of the stress changes caused by the earthquake. Nevertheless, some relations between the seismic activities and the volcanic activity are explained by changes in static stress, but there remain other phenomena that were observed at the same time and whose relationships are not yet understood.
Volcanic hazards
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