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Volcanic eruptions are sometimes preceded by tilt changes that are an indicator of immediate pre-eruptive processes. In this study, the tilt changes before all monitored eruptions in Japan were surveyed for the first time to address the generality and common features of these signals. An automated procedure was devised to identify tilt changes. The results showed that approximately half of the analyzed eruptions were preceded by tilt changes. Detection ratios depended on station locations, and approximately 10–20% of the total waveforms consisted of detectable tilt changes. Broadband seismic records, when integrated into displacements, showed detection ratios comparable to those of tiltmeter records. The lead times and durations of the tilt changes showed large variations. The durations correlated with amplitudes, although with considerable scatter. Both accelerating and decelerating types of tilt changes were common, and their ratios depended on the volcanoes. These are the first findings on the nature of pre-eruptive tilt changes from many volcanoes, and the waveform analysis procedure proposed in this paper is applicable to other regions. Future works involve extending this study to datasets from other regions, detecting tilt changes from continuous records, applying the proposed method to an early warning system, and detecting longer tilt changes, which require that the proposed approach be improved by subsequent studies. [Display omitted] • All pre-eruptive tilt changes in Japan were systematically surveyed. • Approximately half of the eruptions were preceded by tilt changes. • Broadband seismometers detected tilt changes at rates comparable to tiltmeters. • Durations of the tilt changes weakly correlated with amplitudes [ABSTRACT FROM AUTHOR]

We develop a centroid moment tensor (CMT) inversion method by using a densely distributed regional seismic network in order to calculate the centroid locations and MT solutions of regional earthquakes. Centroid location and time are obtained using a grid search algorithm with the new CMT inversion method. This method is applied to the earthquakes that occurred in the focal area of the 2004 mid-Niigata prefecture earthquake, central Japan. The centroid locations and focal mechanisms of the main shock and aftershocks with moment magnitudes greater than 3.4 are effectively constrained. We assess the reliability of the CMT inversion method by comparing its results with accurate hypocentres calculated with a local temporary seismic network. The accuracy of the centroid locations in the horizontal and vertical directions is approximately less than 4 km. The MT solutions are consistent with the P-wave first-motion polarities. By using the nodal planes of focal mechanisms and the centroid locations of many earthquakes, we successfully imaged the major faults distributed on a vertical cross-section across the main shock fault. Further, we evaluate the validity of the application to ‘offshore earthquakes’ by using the aftershocks that occur in the landward area, wherein the seismic network does not cover the focal area but is located in a one-sided region of the focal area. These results show that the focal mechanism and depth are very stable, although the accuracy of the horizontal distribution is reduced to half. [ABSTRACT FROM AUTHOR]

Abstract: The broadband seismic data observed at Satsuma–Iwojima volcanic island in southwestern Japan show a wide variety of interesting features. One of them is the periodic and frequency-dependent amplitude modulation of volcanic tremors. Another interesting feature is the occurrence of very long period (VLP) seismic pulses synchronized with the tremor amplitude modulation. Both the tremor amplitude modulation and the VLP pulses have fairly regular intervals of 46–50 min, suggesting a spatially fixed, non-destructive repetitive source. We performed waveform inversions to reveal the source mechanism of the VLP pulses, assuming a point source embedded in a medium that realistically represented the topography of the island. Numerical tests showed that the Green''s functions used in the inversion were significantly affected by the topography. The observed VLP pulses were well explained by the rapid expansion of an inclined crack 100 m beneath the floor of the summit crater, which had a seismic moment of 3×1010 Nm. Both the characteristic amplitude modulation and the VLP pulses are explained systematically by a model consisting of a water-filled crack and a network of gas paths. The water-filled crack was heated above the boiling temperature of the water in it. A sudden break of the unstable superheated state caused sudden vaporization and a pressure increase, which was observed as the VLP pulse. The network of gas paths consisted of swollen portions and constricted portions, which worked as tremor generators when vapor from the water-filled crack passed through. [Copyright &y& Elsevier]