Recent enhancement of seismic networks in the Japan Islands revealed occurrence of low‐frequency continuous tremors of a beltlike distribution in the southwest Japan, where the subducting Philippine Sea plate reaches depths of 30–40 km. Source depth of the tremor is estimated by selecting tremor segments with relatively clear P‐wave onsets. The source region of the tremor is assumed to correspond to lowermost parts of crust close to a triple boundary of the crust, mantle wedge, and the subducting slab. The long duration of the phenomenon indicates existence of fluid relating to the generation of the tremor. The most probable fluid is considered to be water produced by dehydration process of chlorite and amphibole in the slab on the basis of data from high pressure and temperature experiments. The northern border of the beltlike distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water.
[1] The crustal structure beneath the Japanese islands, including depth distributions of the Conrad and Moho discontinuities, was estimated using a tomographic inversion of regional body wave arrival times. Depth distributions of the bottom of the surface layer, the Conrad, and the Moho were modeled with two-dimensional B spline functions, while velocity distributions in layers were expressed by three-dimensional B spline functions. The depth of the discontinuities and the velocity in the layers were estimated simultaneously by the least squares method. The velocity structure was sequentially estimated from shallower parts to deeper parts to avoid correlation between them. This sequential analysis provided improved depth resolution. The deepest region of the Moho discontinuity was located in central Honshu, reaching about 40 km. The Moho discontinuity was generally deep in the central part of the islands, whereas it was relatively shallow in the Kanto, southwestern Chubu, and Chugoku districts in Honshu and in northern Kyushu. Some of the shallow Moho regions would be related to graben formation due to tensile tectonic stress since the Miocene. The results were compared with those of seismic refraction surveys and receiver function analyses, and it was found that the obtained model was consistent with many of these studies.Citation: Katsumata, A. (2010), Depth of the Moho discontinuity beneath the Japanese islands estimated by traveltime analysis,
Previous studies have suggested submarine landslides as sources of the tsunami that damaged coastal areas of Palu Bay after the 2018 Sulawesi earthquake. Indeed, tsunami run-up heights as high as 10 m determined by field surveys cannot be explained by the earthquake source alone although the earthquake is definitely the primary cause of the tsunami. The quantitatively reexamined results using the earthquake fault models reported so far showed that none of them could fully explain the observed tsunami data: tsunami waveforms inferred from video footage and the field survey run-up tsunami height distribution. Here, we present probable tsunami source models including submarine landslides that are consistent with the observed tsunami data. We simulated tsunamis generated by submarine landslides using a simplified depth-averaged two-dimensional model. The estimated submarine landslide model consisted of two sources in the northern and southern parts of the bay, and it explained the observed tsunami data well. Their volumes were 0.02 and 0.07 km 3. The radius of the major axis and the maximum thickness of the initial paraboloid masses and the maximum horizontal velocity of the masses were 0.8 km, 40 m and 21 m/s in the northern bay, and 2.0 km, 15 m and 19 m/s in the southern bay, respectively. The landslide source in the northern bay needed to start to move about 70 s after the earthquake to match the calculated and observed arrival times.
We analyzed long‐term continuous seismic records (from September 2015 to April 2016) of Dense Ocean‐floor Network System for Earthquake and Tsunamis, an ocean‐floor observation system deployed around the fore‐arc slope of the Nankai subduction zone to investigate shallow tremor near the trough axis. We found that the activity of shallow tremor was concentrated in two time periods: 6 days in October 2015 and 2 weeks in April 2016. During the episode in April 2016, migration and triggering of tremor were observed. These characteristics are similar to those of tremor in the deeper part of the subduction zone. The triggering of tremor indicates that the tremor activity is very sensitive to nearby stress perturbation in the area of this study, which is near the initiation points of past large earthquakes along the Nankai Trough. Therefore, it is very important to monitor tremor activity in this region for understanding the stress accumulation process of megathrust earthquakes.
We discovered a secular change in the travel time of direct S-waves over a 10-year observation period by means of continuous operation of an artificial and stable seismic source, called Accurately Controlled Routinely Operated Signal System (ACROSS), which is deployed in the central part of Japan along the Nankai Trough. We used 13 High Sensitivity Seismograph Network Japan (Hi-net) stations around the ACROSS source to monitor the temporal variation in travel time. Green's functions were calculated for each station daily from March 29, 2007, through October 30, 2017. Secular advance in the temporal variation in travel time was seen for the whole operation period, in addition to a steplike delay associated with the 2011 Tohoku earthquake. We estimated the rate of secular change and the amount of coseismic step by modeling the transfer function of S-waves with a linear trend and the coseismic step of the 2011 Tohoku earthquake. Distance dependences of the travel time changes can be explained as a combination of common bias and dispersion for each station, for both the secular and coseismic changes. This can be interpreted as a randomly distributed change in seismic velocity over the range of the observation region. An azimuthal dependence exists for both changes and shows larger changes in the NE-SW direction than in the NW-SE direction from the ACROSS source.
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