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½  ½FY»¼Source Mechanism and Seismic Velocity Structure of Source Region of Deep Low-frequency
Earthquakes beneath Volcanoes : Case Studies of Mt. Iwate and Mt. Fuji
Haruhisa NAKAMICHI+    

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216

Fig. +. Schematic volcano-magma system. Solid stars show locations of deep low-frequency
earthquakes. Crosses show locations of volcanic tremors and volcano-tectonic earthquakes.
Dashed line shows deformation of volcano ediﬁce.

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Fig. ,. Epicenters and cross-sections of hypocenters ofintermediate-depth and deep low-frequency earthquake (solid circles) and high-frequency earthquakes(light grey circles) of Mt. Iwate in the period from
December +331 to June ,***. ‘A’, ‘B’ and ‘C’ showgroups of deep low-frequency earthquakes. ‘I’ showsa group of intermediate-depth low-frequency earthquakes. The focal mechanisms of the largest deep lowfrequency earthquake (M +.3) and the largest intermediate-depth low-frequency earthquake (M +.0) arealso shown. Lower focal hemispheres are shown inequal-area projection, with compressional ﬁelds shaded.
Open and solid circles on the source mechanism
indicate inward motion and outward motion, respectively. The solid star marks the initial rupture pointof the M 0., earthquake. The dashed rectangle shows
the focal area of the M 0., earthquake. The locations of very-long-period events (Nishimura et al.,,***), dikes and Mogi sources estimated from GPSdata (Miura et al., ,***) are also shown. The
Conrad and Moho depths are shown with solid lines(Nakajima et al., ,**,). Elevation contours (dottedlines) are at -** m intervals. The cross-section isfree of any vertical exaggeration.

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Fig. -. Hypocentral distributions (black dots) of deep low-frequency earthquakes of Mt. Fuji. (a) Map view.
The source mechanism of the largest deep low-frequency earthquake (M ,.-) is also shown. Lower focal
hemisphere is shown in equal-area projection, with compressional ﬁelds shaded. Open circles on the source
mechanism indicate inward motion. (b) Cross-seciton A-A῍ in (a). (c) Cross-section B-B῍ in (a). Crosssections A-A῍ and B-B῍ include hypocenters projected from +* km either side of the cross-section line.

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Fig. .. S wave velocity perturbations obtained by thereceiver function analysis, which are projected onto
the south-north cross-section at a longitude of +.+E, crossing Mt. Iwate. Velocity perturbations are
relative the standard velocity model of Tohoku
district. Projection is along ray paths of P-to-SV
converted waves. Stars show locations of intermediate-depth and deep low-frequency earthquakes.
Red triangles denote the location of Mt. Iwate.

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Fig. /. Vertical cross-sections of (a) P wave velocity (VP) values and (c) ratio of P and S wave velocity (VP/VS)
values crossing Mt. Fuji. Contour interval for VP is *./ km/s. Tectonic earthquakes within $,* km of thesection are plotted as black dots. Red dots show the locations of deep low-frequency earthquakes.

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ῌ ῏ ῎ ῍Aki, K. and Koyanagi, R. (+32+) Deep volcanic tremor
and magma ascent mechanism under Kilauea, Hawaii. J.
Geophys. Res., 20, 1*3/ῌ1+*3.
(|)HIJ (,***) }~*60 ' ./ ,3/ῌ,33.
Hasegawa, A. and Yamamoto, A. (+33.) Deep, lowfrequency microearthquakes in or around seismic lowvelocity zones beneath active volcanoes in northeasternJapan. Tectonophys., ,--, ,--ῌ,/,.
Hasegawa, A., Zhao, D., Hori, S., Yamamoto, A. andHoriuchi, S. (+33+) Deep structure of the northeasternJapan arc and its relationship to seismic and volcanic
activity. Nature, -/,, 02-ῌ023.
Knopo#, L. and Randall, M. (+31*) The compensatedlinear-vector dipole : a possible mechanism for deep earthquakes. J. Geophys. Res., 1/, .3/1ῌ.30-.
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