We report on our observation of pulse-like electromagnetic signals in the frequency range 1∼10 kHz that we associate with earthquakes. The severest difficulty in separating earthquake-associated VLF signals from those originating in lightning discharges stems from the circumstance that the latter signals are overwhelming in number compared with the former. While claims have often been made of observation of electromagnetic signals in association with earthquakes, most of the claims, excepting a few, heavily rely on temporal correlation. By means of simple instrumentation and data processing software, our method by and large enables us to isolate VLF signals whose direction of arrival is well focussed at the epicenter direction. In this preliminary report we present several examples that positively demonstrate the existence of a class of VLF signals having a peak frequency of occurrence 1∼4 days prior to earthquakes of Magnitude 4∼6. With an accumulation of experience the technique described in this paper would seem to offer a promising approach towards earthquake prediction.
Around the Ogasawara Islands, only a few seismic stations in the area can be used to determine the hypocenters of regional earthquakes; thus, hypocenter location precision tends to be low. To more precisely determine hypocenter locations, we deployed a temporary seismic observation network of pop-up ocean bottom seismometers around the Ogasawara Islands from July to October 2015. We identified a double seismic zone in the 70-200 km depth range associated with the subducting Pacific slab. The slab-normal distance between the two planes of the double seismic zone is about 30-35 km, similar to such distances observed along the Japan and Mariana trenches. Furthermore, we found unusual seismicity in the mantle wedge at 20-50 km depth beneath the Ogasawara trough that might be related to structure formed at the onset of the oceanic slab subduction. The hypocenters determined from the ocean bottom seismometer observation were horizontally separated by a few tens of kilometers from hypocenters published by the Seismological Bulletin of Japan. USGS locations (Preliminary Determination of Epicenters) seem to be offset westward about 30 km compared with the locations determined in this study.
To investigate detailed fault distributions and shallow geological structure offshore northwestern Sumatra, we obtained high-resolution Multi-Channel Seismic (MCS) reflection data around the Sunda Trench, trench slope, and forearc high regions offshore northwestern Sumatra. In general, trench-parallel anticlinal ridges are distributed from trench slope region to forearc high region. Two kinds of different vergence systems are characterized in the Sumatra forearc region; landward vergence is dominant in the lower trench slope region, and seaward vergence is dominant in the forearc high region. Moreover, piggyback or slope basins are recognized between anticlinal ridges. Deformation in the uppermost part of these basins, that is referred to 'recent' deformation in this paper, can be identified not only along major thrusts but also between major thrusts and the lower trench slope, suggesting these are related to recently active faulting. Several but the largest number of such deformation are distributed along a major thrust located in the middle of the forearc high region, whereas few are done along other major thrusts.
We used pop-up ocean bottom seismometers deployed in 2013 and 2014 to investigate the southern limit of seismic activity within the Philippine Sea plate south of the Nankai Trough axis off the Kii Peninsula. The hypocenter distribution we determined included microearthquakes with magnitudes lower than 1.5 that were not detected by the land-based seismic network. Hypocentral depths ranged from 5 to 15 km below sea level and there were few earthquakes more than 100 km south of the axis of the Nankai Trough. We therefore infer that the southern limit of microearthquake activity in this region is about 100 km south of the trough axis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.