An image of a magma intrusion process inferred from precise hypocentral migrations of the earthquake swarm east of the Izu Peninsula

Abstract

SUMMARY Earthquake swarms in the area east of the Izu Peninsula, Central Japan have been active and have been repeated intermittently since 1978 after 40 years of quiescence. The activities were always accompanied by crustal deformations, which were well modelled by dyke intrusions. To study the process of a dyke intrusion, precise hypocentres of the latest activity occurring in 1998 are obtained in this paper using waveform similarity, by which new images of volcanic processes have been successfully acquired at several volcanoes. The relocated hypocentres are mainly aligned on a thin vertical plane with a circular shape at a depth of 3–7 km, and there is an aseismic area at the centre. The normal direction to the plane coincides well with the direction of tectonic extensional stress around the hypocentral area and matches theoretical models well. At the beginning of the activity, a small fraction of the events occurred at greater depth, where hypocentres align on a vertical line and migrate upward at a rate of 1 km h−1. The migration rate in this stage agrees well with a theoretical model of buoyancy‐driven dyke propagation. After 1 day from the beginning, earthquakes began to occur around the aseismic area, and hypocentres spread on the thin circular plane described above. During this activity, the hypocentres seemed to migrate downwards and upwards from the centre. The earthquakes are caused by shear fracture at the tip of the dyke and the migration of hypocentres reflects that the dyke expands downward and upward from the neutrally buoyant layer that is located at the centre of the swarm. In this paper, we also propose a process of magma intrusion based on the precise migration of hypocentres, and show that the 1998 activity is composed of a few simple volcanic processes: magma rising by buoyancy, it staying at a neutral buoyancy point and spreading outward from the neutral point with elastic fracture by an inside excess magma pressure.

Preparatory and precursory processes leading up to the 2014 phreatic eruption of Mount Ontake, Japan

“…7a). Interestingly, these speeds are roughly comparable to those of migrating VT earthquakes triggered by dike intrusion (0.06-0.3 m/s) (e.g., Hayashi and Morita 2003;Shelly and Hill 2011). Finally, failure of the sealed surface of the vertical conduit led to the 2014 phreatic eruption.…”

Laboratory Modelling of Volcano Plumbing Systems: A Review

“…Three main behaviours were observed: (i) when the injected wax temperature was large compared to its solidus temperature, and when the heat influx was large compared to the diffusive heat loss, the dyke propagated continuously, like a non-solidifying dyke; (ii) when the injected wax temperature was close to its solidus temperature, and when the heat influx was small compared to the diffusive heat loss, the dyke did not propagate; (iii) when the wax temperature and the heat fluxes lay between the above end members, the dyke propagates in a stepwise, intermittent, manner due to local clogging of the dyke tip. This intermittent behaviour may help explain the occurrence of seismic bursts recorded during dyke emplacement in volcanoes (Hayashi and Morita 2003;) as a consequence of the thermo-mechanical interaction of the dyke and its host rocks.…”

Mechanical intrusion models and their implications for the possibility of magma-driven swarms in NW Bohemia Region

“…Small black inverted triangles indicate the duration of swarm activity. Numbers refer to references as: 2 = East Izu Bonin (Hayashi and Morita, 2003); 4 = Iceland (Dahm and Brandsdóttir, 1997); 6 = Reunion (Rivalta and Dahm, 2006); 8 = Soultz Foret (Evans et al, 2005), 9 = Cooper Basin (Asanuma et al, unpublished results); 10 = Basel (T Kraft, personal communication); 11 = KTB (Baisch and Harjes, 2003;Bohnhoff et al, 2004), 14 = Krafla (Einarsson and Brandsdóttir, 1980) 15 = Krafla (Brandsdóttir and Einarsson, 1979) 16 = Asama (Takeo et al, 2006) 17, 18, 19, 20, 21 = Iceland (Hensch et al, 2008 22 = Santorini (our own data); 23 = Texas (Rutledge et al, 2004) 30 = Afar Dyke episode (Wright et al, 2006). a slope of about 2 for reservoir induced earthquake plotted in a comparable graph.…”