Study of Deep Long Period Events recorded at Mt. Fuji during 2000-2012

Deep long period (DLP) events are seismic events characterized by most energy content at low frequency (< 5 Hz) and sources located in the mid-to-lower crust. Although their meaning has not been clearly understood yet, numerous researchers have studied these signals investigating their spectral content and waveforms, source location and time relation with volcanic activity. DLP events have been detected in many volcanoes worldwide, such as Kilauea, Mauna Loa, Mammoth Mountain, Aleutian Arc volcanoes, Pinatubo and Fuji. In this study, we characterize the DLP activity recorded at Fuji during 2000-2012. About 1600 events, whose monthly rate varies from a few to more than 100, were detected. Their occurrence behavior, spectral features, and source location were investigated. As for this last topic, since the onset of the DLP events is emergent and then the classical location techniques based on the phase picks are sometimes difficult to be applied, grid search algorithms are tested. In particular, the location methods based on decay amplitude, on the comparison among the envelopes of the DLP events and on both the previous methods are used. Since the amplitude decay method requires the correction for the site effects, 14 regional earthquakes are used to estimate such effects in the frequency range 1-10 Hz. Finally, the location results are compared to the results obtained by manually picking the first arrival times.