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.
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 re-examined 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.
Abstract
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.
Key Points
We investigated episodic tremor activity near the Nankai Trough axis by using DONET data
Migration and triggering properties similar to those of deep tremor were observed
Source locations were near the initiation points of past megathrust earthquakes near the Nankai Trough
A tsunami earthquake is an earthquake event that generates abnormally high tsunami waves considering the amplitude of the seismic waves. These abnormally high waves relative to the seismic wave ...amplitude are related to the longer rupture duration of such earthquakes compared with typical events. Rapid magnitude estimation is essential for the timely issuance of effective tsunami warnings for tsunami earthquakes. For local events, event magnitude estimated from the observed displacement amplitudes of the seismic waves, which can be obtained before estimation of the seismic moment, is often used for the first tsunami warning. However, because the observed displacement amplitude is approximately proportional to the moment rate, conventional magnitudes of tsunami earthquakes estimated based on the seismic wave amplitude tend to underestimate the event size. To overcome this problem, we investigated several methods of magnitude estimation, including magnitudes based on long-period displacement, integrated displacement, and multiband amplitude distribution. We tested the methods using synthetic waveforms calculated from finite fault models of tsunami earthquakes. We found that methods based on observed amplitudes could not estimate magnitude properly, but the method based on the multiband amplitude distribution gave values close to the moment magnitude for many tsunami earthquakes. In this method, peak amplitudes of bandpass filtered waveforms are compared with those of synthetic records for an assumed source duration and fault mechanism. We applied the multiband amplitude distribution method to the records of events that occurred around the Japanese Islands and to those of tsunami earthquakes, and confirmed that this method could be used to estimate event magnitudes close to the moment magnitudes.
Temporary slip speed increases with durations of 1–3 h were identified during short-term slow slip events in records of borehole and laser strainmeters in the Tokai region, Japan. They were found by ...searching for peaks of correlation coefficients between stacked strain data and ramp functions with rise times of 1 and 2 h. Although many of the strain steps were considered due to noise, some strain steps occurred with simultaneous activation of the deep tectonic tremors and shared source areas with the tremors. From 2016 to 2022, we observed five strain steps with simultaneous activation of tectonic tremors and coincidence of source locations with the tremors. Those strain steps occurred during short-term slow slip events and were temporary slip speed increases of the slow slip events. Those strain steps seemed to be related to successive occurrences with source migration of short-term slow slip events. The detrended strain steps corresponded to plate boundary slip events of moment magnitude around 5, which was consistent with the scaling law of slow earthquakes.
Graphical Abstract
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.
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.
The
M
w
6.9 earthquake off Fukushima Prefecture, Japan, of 22 November 2016 was followed by a tsunami that struck the Japanese coast from Hokkaido in northern Japan to Wakayama Prefecture in western ...Japan. We compared the performance of a seismologically deduced single-fault model, a seismologically deduced finite fault slip model (FFM), an optimized single-fault model based on tsunami data, the FFM with horizontal shift, and the tsunami waveform inversion models of the previous studies considered for this earthquake regarding reproduction of tsunami waves by tsunami computations. It is important to discuss how these models work well because it is sometimes desirable to obtain an earthquake source model to estimate tsunami waves with a simple process obtained with limited data from the viewpoint of tsunami prediction. The seismologically deduced FFM has an advantage in terms of the information of slip regions of fault plane and was superior to the seismologically deduced single-fault model, especially in predicting amplitudes of tsunami waves. This means that when only with seismic data, the FFM could narrow the range of forecast of tsunami amplitude. In the comparison of models optimized with tsunami data, the single-fault model showed the almost equivalent performance of the tsunami waveform inversion models of previous studies regarding the waveform coincidence with observations and the horizontal location at the negative peak of the initial sea surface displacement. In case the main generation region of the tsunami is concentrated in one place, the tsunamis can be expressed by a single-fault model by conducting the detailed grid search. We also confirmed that the centroid location of centroid moment tensor (CMT) solution and the absolute location of the FFM were not necessarily suitable to express tsunamis, while the moment magnitude, the focal mechanism, the centroid depth of CMT solution, and the relative slip distribution of the FFM were effective to represent tsunamis. Since this event occurred at the shallow depth, the speed of tsunami wave is particularly slow. Therefore, it would be advisable to pay attention to the horizontal uncertainty to apply seismologically obtained solution to tsunami forecast, especially when a tsunami occurs in shallow water.
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.
The rapid determination of its magnitude soon after a great earthquake is necessary for the issuing of effective tsunami warnings, as demonstrated in the great earthquake off Tohoku district in Japan ...on March 11, 2011. The earthquake magnitude for the first tsunami warning was underestimated due to magnitude saturation. This paper proposes a method to determine magnitude rapidly from peak velocity and displacement of long-period seismic waves up to 100 seconds at local stations. When waveform data at local stations are available, the magnitude from
S
-wave peaks is expected to be determined faster than that from only
P
-wave peaks. It takes about 140 seconds to estimate a magnitude of about 9 for the March 11, 2011, earthquake, which would enable us to issue the first tsunami warning within three minutes after the same type of earthquake.
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