Naoto Ujihara

Two electrical conductors beneath Kusatsu-Shirane volcano, Japan, imaged by audiomagnetotellurics, and their implications for the hydrothermal system

Kusatsu-Shirane volcano, Japan, is known for its active phreatic eruptions. We have investigated its hydrothermal system by conducting audio-magnetotelluric soundings at 22 stations along a profile that extends across the volcano. The final two-dimensional model is characterized by two conductors. One is a 300- to 1000-m-thick conductor of 1–10 Ωm, which is located on the eastern slope and covered with 200-m-thick resistive layers of Kusatsu-Shirane lava and pyroclastics. This conductor indicates the presence of a Montmorillonite-rich layer of Pliocene volcanic rocks that may function both as an impermeable floor for the shallow fluid path from the peak to the hot springs to the east and as an impermeable cap for the deeper fluid path from the summit region to the foot of the volcano. The second conductor is found at a depth of 1–2 km from the surface, at the peak of the volcano, and its resistivity is as low as 1 Ωm or less. This low resistivity can be explained by fluids containing high concentrations of chloride and sulfate which were supplied from the magmatic gases. Micro-earthquakes cluster above this conductor, and the cut-off of the earthquakes corresponds to the top of the conductor. This conductor infers the presence of the fluid reservoir, and the upward release of these fluids from the reservoir through the conduit presumably triggers the micro-earthquakes at the peak area of the volcano. Crustal deformation modeling using GPS and leveling data of the past 10 years revealed that the center of the deflation coincides with the top of the second conductor, indicating that the fluid reservoir itself can be hosting the deformation.

Resistivity imaging across the source region of the 2004 Mid-Niigata Prefecture earthquake (M6.8), central Japan

Across the source region of the 2004 Mid-Niigata Prefecture earthquake, wideband magnetotelluric (MT) survey was performed just after the onset of the mainshock. Owing to the temporal stop of the DC powered railways around the area together with intense geomagnetic activity, we obtain MT records with excellent quality for both short and long period data, as long as 10,000 s. Two dimensional regional strike is evaluated with the aid of the Groom-Bailey tensor decomposition together with induction vector analysis. As a result, N15°W is determined for the strike. This strike is oblique to the local geological trend and also to the strike of the main shock source fault together with aftershock distribution of N35°E. Two dimensional resistivity structure is determined with the aid of an ABIC inversion code, where static shift is considered and estimated. Characteristics of the structure are as follows. (1) About 10 km thick sedimentary layer exists on the top. (2) A conductive body exists in the lower crust beneath the source region. The mainshock occurred at the boundary of the conductive sedimentary layer and a resistive basement beneath it and aftershocks occurred in the sedimentary layer. From geological studies, it is reported that the sedimentary layer was formed in the extensional rift-structure from Miocene to Pleistocene and has been thickened by compressional tectonic regime in the late Quaternary. Interstitial fluids or clay minerals, which reduce the sedimentary layer resistivity, control the reactivation of the normal fault as the mainshock thrust fault and aftershock activity. The second conductive body probably indicates existence of fluids in the depths as well. Such a conductive layer in the lower crust was also revealed by previous MT experiments along the Niigata-Kobe Tectonic Zone and probably plays a main role in concentration of strain rate along the zone.

Electric and magnetic field variations arising from the seismic dynamo effect for aftershocks of the M7.1 earthquake of 26 May 2003 off Miyagi Prefecture, NE Japan

Some examples of electric and magnetic field variations have recently been reported by Honkura and his colleagues in association with earthquakes, and these variations have been interpreted by them in terms of the seismic dynamo effect. In order to confirm that this effect is a universal phenomenon rather than a phenomenon appearing in a special local condition, we made magnetotelluric (MT) observations above the hypocentral area of the M7.1 earthquake which occurred off Miyagi Prefecture, northeastern Japan, on May 26, 2003. The MT site was selected at a location close to a seismic station belonging to the nation-wide seismic observation network called ‘Hi-net’, so that we can compare the MT signals with the seismic wave records. During the MT observation period after the mainshock, some moderate-size aftershocks of magnitudes between 2.8 and 4.1 occurred and MT signals appeared in association with all these aftershocks. In order to confirm that MT signals are not due to vibrations of MT equipment, we set up two sets ofMT equipment at the same location; in the case of electric field measurements, we used independent electrodes and arranged cables connecting electrodes on the ground for one set and in the air for the other set, and in the case of magnetic field measurements, we buried the induction coils under the ground for one set and hang them in the air for the other set. As for the electric field, the two sets showed exactly the same records. On the other hand, the magnetic field was different from one set to another, but we conclude that the induction coils buried in the ground are more likely to represent the magnetic field due to electric currents flowing in the ground as a result of the seismic dynamo effect.

Seismic dynamo effects associated with the M7.1 earthquake of 26 May 2003 off Miyagi Prefecture and the M6.4 earthquake of 26 July 2003 in northern Miyagi Prefecture, NE Japan

An earthquake of M7.1 occurred off Miyagi Prefecture, NE Japan, on May 26, 2003 at the depth of about 70 km. Just two months later, on July 26, 2003, another shallow earthquake of M6.4 occurred in northern Miyagi Prefecture. In order to investigate whether small signals precursory to the arrival of seismic wave appeared for these earthquakes, we examined the MT records which have continuously been acquired at two stations, Mizusawa and Esashi of the Geographical Survey Institute, located about 50 km northwest of the epicenter of the M7.1 earthquake and about 100 km north of the M6.4 earthquake. Unfortunately, seismometers were not available at these stations, and hence direct comparison between the MT and the seismic records were not possible. We therefore refer to the seismic record obtained at a nearby station belonging to the nation-wide seismic network called ‘Hi-net’ and also we used the arrival times estimated by Japan Meteorological Agency (JMA) for these MT stations. In conclusion, no clear signals preceding the arrival of seismic wave could be detected, although the possibility is suggested that a slight signal may have appeared in one magnetic component at the Mizusawa station before the arrival of seismic wave for the M7.1 earthquake.