Yoshiaki Orihara

Co‐seismic geoelectric potential changes observed in Japan

Co-seismic signals of geoelectric potential have been observed for several earthquakes (EQs) in Japan. It has been argued that if there are precursory changes, stronger signals should be at the time of EQ. Our results prove that co-seismic signals do exist, but they start with the arrival of seismic waves and not at the origin time of EQs. Their amplitude does not scale with the dipole length. Both facts indicate that the changes are local effects of passing seismic waves and not due to electric signals emitted from the seismic source. Observed changes contain two components; oscillatory and offset/decay ones. The former is synchronized with seismic waves, whereas the latter, with a rise time of a few seconds and a decay time of 0.1–1 hour, is suspected to be related to EQ induced subsurface hydrological change. In both cases, the origin of electric potential variations may be electrokinetic.

Preseismic anomalous telluric current signals observed in Kozu-shima Island, Japan

Monitoring of telluric current, which is practically a synonym for geoelectric potential difference, was conducted on Kozu-shima Island about 170 km south of Tokyo from May 14, 1997 to June 25, 2000. During the monitoring period, 19 anomalous telluric current changes (ATCs) were observed. Their possible correlation with nearby earthquakes was statistically examined by assuming various lead times for different ranges of magnitude and focal distance. The best correlation may be obtained for earthquakes with a magnitude greater than 3.0 occurring within 20 km of focal distance. There were 23 such earthquakes, of which 11 were preceded by ATCs within 30 d. Of these 11 earthquakes, preceding ATCs of 5 and 6 were positive and negative polarities of telluric current, respectively. Their epicenters were spatially well clustered in the east and west of the island. These facts were clearly beyond those expected by chance and led to a simple speculative model.

A possible case of SES selectivity at Kozu-shima Island, Japan

Abstract Geoelectric potential difference monitoring has been conducted in Kozu-shima Island about 170 km south of Tokyo. During the monitoring period (14 May 1997–30 November 1999), there were 21 anomalous geoelectric changes that could have been VAN-type seismic electric signals (SES) and 26 M>3 earthquakes within about 20 km. Possible correspondence between the geoelectric changes and earthquakes has been examined. There seems to be some systematic tendency regarding the dependence of appearance/non-appearance of changes and their polarity on epicentral location. This observation, however, is preliminary and admittedly inconclusive because of small number of cases and non-uniqueness in signal-earthquake correlation.

Preseismic Changes of the Level and Temperature of Confined Groundwater related to the 2011 Tohoku Earthquake

Anomalous groundwater changes started three months before the 2011 M9.0 Off the Pacific coast of the Tohoku Earthquake (Tohoku EQ), Japan. Groundwater level and temperature decreased almost simultaneously in a 2000-m well at a spa, Goyo-onsen, in Iwate Prefecture, 155 km northwest of the epicenter. Since the source of Goyo-onsen, located above the edge of a coseismic rupture area of the Tohoku EQ fault, is probably confined, the observed anomalies were caused probably by preseismic crustal deformation. Preseismic groundwater anomalies have been observed prior to similar large subduction EQs such as the 1946 M8.1 Nankai EQ. Thus, monitoring confined groundwater may be useful to identify precursors of large subduction EQs.

Does an ionospheric hole appear after an inland earthquake

Ionospheric disturbances occurred as a result of the tsunami associated with the 2011 M9.0 off the Pacific Coast of the Tohoku Earthquake (EQ). The ionospheric disturbances propagated radially from the tsunami source area, termed the traveling ionospheric disturbance. In addition to the traveling ionospheric disturbance, an ionospheric plasma depression lasting for approximately 1 h occurred above the tsunami source area, called a tsunami ionospheric hole. In this study, we compare the ionospheric disturbances caused by large inland and submarine EQs to investigate whether an ionospheric plasma depression only occurs in association with a tsunami. Note that we term an EQ with a tsunami a submarine EQ. To investigate the presence of a plasma depression, i.e., an ionospheric hole, associated with an inland EQ, data on total electron content between the global positioning system satellite and its receivers were used. Comparison of two inland and two submarine EQ events with similar magnitudes around 7 showed that ionospheric holes were observed only for the submarine EQs. This discrepancy might be attributed to the different excitation amplitudes of the atmospheric acoustic waves between the unidirectional fault displacement and the tsunami uplift/depression, corresponding to half- and one-period variation. From this hypothesis, we predicted that an ionospheric hole could be observed after a significantly large inland EQ with a sufficiently large vertical ground displacement. In fact, we recognized the ionospheric hole generated by the large inland EQ that recently occurred in the Nepal with the magnitude of 7.8 on April 25, 2015.

Heterogeneous electrical structure of Kozu-shima volcanic island, Japan

Nearly twenty anomalous geoelectric field changes were observed before earthquakes at Kozu-shima Island, Japan, from 1997 to 2000. In order to help locating the current sources of the observed anomalous changes, a bipole-dipole resistivity survey was conducted. From the resistivity survey, including current injection into the ground, it was found that various features of the anomalous changes were systematically different from those of changes caused by artificial sources and induction of geomagnetic disturbances. Moreover, it is suspected that the currents of anomalous changes were generated not near the ground surface but deep under the ground.

Electric field polarization around Ioannina VAN station, Greece, inferred from a resistivity mapping

Abstract In the summer of 1997, we made a bipole–dipole mapping survey around Ioannina station of VAN (Varotsos, Alexopoulos, and Nomicos), where detection of the pre-seismic electric signal (SES) has been repeatedly reported. Since we had found the characteristic directional properties of the electric field in the previous study, the present study was aimed to examine it by investigating the shallow electric structure around the station. The apparent resistivity tensor was derived from two sets of measured voltages at each receiver position. From a rough sketch of the resistivity tensor distribution, we found that the electric field was enhanced along the direction parallel to the trend of the basin at receivers located in the conductive basin, and perpendicular to it at receivers in the resistive mountainside. Conductance distribution models with thin plates were constructed by using the measured voltages. The results showed that the VAN station is located on the resistive portion near the contact between the conductive and the resistive part. Furthermore, we simulated the apparent resistivity tensor near the VAN station on the inferred conductance distribution model. Although the directional property similar to those of magnetotelluric (MT) and lightning electric field was not reproduced there, we found that the electric field polarization is affected by heterogeneous structure not only around receivers but also around the source.

A possible space-based tsunami early warning system using observations of the tsunami ionospheric hole

Ionospheric plasma disturbances after a large tsunami can be detected by measurement of the total electron content (TEC) between a Global Positioning System (GPS) satellite and its ground-based receivers. TEC depression lasting for a few minutes to tens of minutes termed as tsunami ionospheric hole (TIH) is formed above the tsunami source area. Here we describe the quantitative relationship between initial tsunami height and the TEC depression rate caused by a TIH from seven tsunamigenic earthquakes in Japan and Chile. We found that the percentage of TEC depression and initial tsunami height are correlated and the largest TEC depressions appear 10 to 20 minutes after the main shocks. Our findings imply that Ionospheric TEC measurement using the existing ground receiver networks could be used in an early warning system for near-field tsunamis that take more than 20 minutes to arrive in coastal areas.

Diurnal variation of atmospheric electric field at the summit of Mount Fuji, Japan, distinctly different from the Carnegie curve in the summertime

In fair weather, a clear diurnal variation of atmospheric electric field (AEF), which is distinctly different from the global diurnal variation, i.e., Carnegie curve, was observed during the summer time (July and August) of the year 2010 and 2011 at the summit of Mount Fuji, Japan (3776 m in altitude). The variation of the AEF at the summit showed a local time dependent feature, which means that the AEF increased at sunrise and decreased at sunset. This local diurnal variation is known as a mountain variation. The intensity of AEF in the daytime reached 1.5–3 times larger than that in the nighttime. From the multipoint observations of cloud images and AEF, the mountain variation was found to be attributable to the AEF generated by the positively charged top of a horizontally extensive sea of clouds below the summit. Since the sea of clouds grows with the temperature rise, the AEF variation follows local time.

Subterranean electrical structure of Kozu-shima volcanic island, Japan

Following the electric current injection experiment carried out in 2009, a VLF-MT (Very Low Frequency Magnetotelluric) survey has been conducted in Kozu-shima Island to obtain further information on the subterranean electrical structure that might help understanding the results of our monitoring of geoelectric potentials. A number of VAN-type pre-seismic geoelectric potential anomalies were observed in 1997–2000, even showing a remarkable “Selectivity”. However, similar pre-seismic anomalies were not observed during the Izu-Island volcano-seismic swarm 2000. All these observations would require extremely high degree of heterogeneity in the subterranean electrical structure of the volcanic island and its possible time changes. Several correlations between the results of this survey and the volcanic geology of the island and ground water distribution were found. Further investigation is needed for a complete explanation of the observed phenomena.