Abstract
In recent years, improved seismic networks in Japan have detected low-frequency events near the Mohorovicic discontinuity, but away from volcanoes. These can be classified as two types: low-frequency earthquakes (LFE) and low-frequency tremors (LFT). Source regions for LFE are distributed in isolated locations throughout Japan, while those for LFT are distributed within a belt in western Japan. Although LFE immediately below volcanoes have been previously recognized, LFE away from volcanoes represent new observations. These LFE are found specifically both in the back-arc of the volcanic front in northeastern Japan, and to the north of the LFT belt in western Japan. LFE and LFT have long durations, suggesting the involvement of fluids in their generation. This study considers the source processes that trigger LFE and LFT, using the results of high pressure and temperature experiments on water-saturated oceanic basalt and peridotite as a model. Such materials are representative of those in and around the descending slab. In the case of LFT in western Japan, the model provides, as a possible trigger, the water released by the dehydration of chlorite and forming clinopyroxene in the basalt of the descending Philippine Sea Plate. On the other hand, in the case of LFE throughout Japan, not only water from the slab but also magma generated in the slab and mantle wedge and high temperature fluid derived from the magma are possible triggers. Because LFE have clear P-wave onsets, some sort of fracture is likely to be involved in the generation of LFE. LFE and LFT are expected to be indicators useful in earthquake prediction, but the relationships between large earthquakes or earthquake swarms and the occurrence of LFE and LFT are yet to be determined.
