地震波からみた東北日本の火山の深部構造と内陸地震の発生機構(<特集>火山のモデル(II))

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Possible evidence for magmatic activity in the crust and uppermost mantle beneath the northeastern Japan arc has been obtained from microearthquake observations. Tomographic images of the 3-D P-wave velocity structure estimated from more than 20,000 arrival time data clearly delineate low-velocity zones, which are continuously distributed from the uppermost mantle to the upper crust beneath active volcanoes. Exceptionally deep (25〜40 km) microearthquakes are found at 9 locations around the low-velocity zones beneath volcanoes, and all these events have extremely low predominant frequencies (1.5〜3.5 Hz for P-waves), suggesting a close relation to the magmatic activity in this depth range. At shallower depths (10〜15 km) in or around the low-velocity zones, there exist distinct reflectors of S-waves from shallow events. Reflection point locations and the calculated reflection coefficients suggest that the reflectors depict the upper surfaces of magma bodies. A detailed seismicity study reveals that focal depths of shallow crustal microearthquakes in the inland of northeastern Japan are shallower than about 15km, with exceptions of a small number of deep (25〜40km) low-frequency events. Cut-off depth for this shallow seismicity, which is sharply delimited, can be understood as a transition from brittle to ductile deformation or a transition from stick-slip to stable sliding due to increasing temperature. The cut-off depth changes with the location and becomes shallow beneath actitve volcanoes. This depth decrease can be attributed to the local elevation of temperature caused by magma coming up from below. Under the tectonic stress field of horizontal compression beneath this volcanic arc, stress concentrations will arise around the regions where the base of the brittle seismogenic zone is locally elevated. This interpretation seems to be supported by the fact that large crustal earthquakes are apt to occur around the low-velocity zones of P-wave. These observations appear to shed some light on the state magma at depths and its relation to shallow seismic activity beneath northeastern Japan.
特定非営利活動法人日本火山学会の論文
1991-07-15

地震波からみた東北日本の火山の深部構造と内陸地震の発生機構(<特集>火山のモデル(II))

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