We report on the very-low-frequency earthquakes occurring in transition zone of subducting plate interface along Nankai subduction southwest Japan. Seismic waves generated by with seismic moment magnitudes 3.1 to 3.5 predominantly show a long period about 20 seconds. The seismicity accompanies and migrates activity deep low-frequency tremors slow slip events. coincidence these three phenomena improves detection characterization earthquakes, which are thought increase stress updip megathrust...

We estimated centroid moment tensors of earthquakes that occurred from 2003 to 2011 in and around the focal area Mw 9.0 megathrust earthquake eastern Japan. The result indicates occurring before mainshock, which included foreshocks off Miyagi, were basically interplate with thrust-type mechanisms. On other hand, aftershocks exhibited a variety Interplate thrust mechanisms did not occur within large coseismic slip GPS data but instead surrounding regions. This implies could no longer due...

Abstract National Research Institute for Earth Science and Disaster Resilience (NIED) integrated the land observation networks established since 1995 Kobe earthquake with seafloor 2011 Tohoku tsunami as MOWLAS (Monitoring of Waves on Land Seafloor) in November 2017. The purpose is to provide comprehensive, accurate, rapid monitoring earthquake, tsunami, volcano events throughout Japan its offshore areas. data are widely utilized long-term forecasting, current seismic activity, hazard...

The 2011, M w 9.0 Tohoku earthquake was followed by an abundant amount of seismicity providing a unique opportunity to analyze the triggering mechanism great earthquakes. Although occurred close dense seismic network, many aftershocks that in first few hours after mainshock are not recorded catalogs. Here we use template waveform approach recover as possible missing events 12 following mainshock. Our analysis is able detect about 1.4 times more than those listed High Sensitivity Seismograph...

Passive Image Interferometry (PII) is a seismological method employing ambient seismic noise to monitor the temporal evolution of mean shear wave velocity within Earth. First, elastic Green's tensor between two seismometers computed from cross correlation recorded during certain period. Second, constructed seismograms different time periods are treated as earthquake multiplets, and small shifts in their coda used invert relative change velocity. When applied source region 2004 M w = 6.6...

We analyze coseismic and postseismic velocity variations caused by the June 13, 2008 Iwate‐Miyagi Nairiku earthquake ( M W = 6.9) using Passive Image Interferometry (PII). Seismic noise is correlated in order to reconstruct Green's function between two sensors. Shear wave changes are determined relating coda parts of daily functions a long‐term reference function. Our study extends from January August 2010 includes correlations for 190 station pairs three different frequency ranges 0.125 1.0...

We introduce the single-station cross-correlation (SC) technique of processing ambient seismic noise and compare its results with established (CC) autocorrelation (AC) techniques. While CC is correlation signals two stations each other AC a signal itself, SC different components single three-component sensor. The comparison three techniques shows that CCs give best at frequencies below 0.5 Hz SCs higher frequencies. In all techniques, correlated in order to reconstruct Green's function...

We present a systematic study of seismic velocity changes associated with megathrust and five strong crustal earthquakes in Japan. perform both cross-correlation single-station analysis for station pairs stations, respectively. The correlation ambient noise allows us to reconstruct the Green's functions wave propagation. By relating coda parts daily long-term reference functions, shear are determined. analyse data from four areas Japan where large occurred: Iwate-Miyagi (2008 MW 6.9 Nairiku...

Abstract The spatial distributions of shallow slow earthquakes are related to stress accumulation and structural characteristics the plate boundary, which important for understanding megathrust earthquakes. To investigate spatiotemporal variation very low frequency earthquake (SVLFE) activity along Nankai Trough, we conducted centroid moment tensor inversion method incorporating effects offshore heterogeneous structures. By applying into long‐term onshore seismograms, obtained SVLFE occurred...

ABSTRACT The Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S‐net) is a novel cabled ocean‐bottom station network covering broad offshore region east of northeastern Japan. To best use S‐net data, we estimated sensor orientations all 150 stations, because without this information measurements in geodetical coordinates cannot be specified. We determined three parameters orientation at each station: tilt angle long axis cable, rotation around axis, azimuth...

Abstract Cross‐correlation analysis was applied to long‐term onshore broadband records from April 2004 March 2021 detect and relocate shallow very low frequency earthquakes (VLFEs) southeast off the Kii Peninsula, along Nankai Trough, Japan. We then determined moment rate functions of detected VLFEs using Monte Carlo‐based simulated annealing method. According this new comprehensive catalog, are widespread beneath accretionary prism toe, but with large cumulative moments localized around...

For the purpose of revealing statistical characteristics P-wave velocity, S-wave velocity and density in uppermost part crust, we analysed well-log data obtained from five deep wells different tectonic regions Japan: three through mainly sedimentary rocks Kanto plain two Kuju volcano group Kyushu Island. In plain, power spectral fractional fluctuation that are proportional to a spatial wavelength few metres 100 m, where index (slope at double logarithmic scale) is 1.1–1.3. At group, P- also...

Seismic evidence of lithospheric slab segmentation, dehydration, and anisotropic rocks beneath the Kii Peninsula, central Japan, is detected with receiver function (RF) analyses. By stacking radial transverse RFs moveout perturbations, we estimate downdip azimuth, dip angle, depth Moho each seismic station. Comparing azimuth to convergence can classify into three segments. The segments correspond both fault zones past megathrust earthquakes spatial distribution intraslab seismicity. lies...

SUMMARY Many earthquakes with various types of focal mechanisms occur along the subducting Philippine Sea slab (PHS) beneath southwest Japan, and hypocentre distribution is quite inhomogeneous. In this region, distinct non-volcanic deep low-frequency tremors are often observed. To clarify cause these apparently complicated peculiar seismic activities, we first construct a detailed configuration oceanic Moho (slab Moho) within PHS based on receiver function (RF) analysis. convert time-domain...

We construct a detailed image of velocity discontinuity beneath southwest Japan by using receiver functions. first propose an improved function estimation method based on the statistical multivariate autoregressive model. Then we apply new to teleseismic waveform data recorded high‐density seismograph network in Japan. The results show clear at 30 km depth southern Shikoku region. This discontinuity, corresponding boundary between oceanic crust and high‐velocity layer (the Moho) Philippine...

The shape of the Philippine Sea Plate subducting beneath western Japan is a crucial factor in understanding earthquakes and volcanic activity. We propose that subducted plate was split along an extinct ridge due to abrupt change subduction direction, followed by elastic deformation accumulation stress near ridge. This consistent with receiver function images, distribution deep tremor sources, seismicity focal mechanisms intraplate earthquakes, anomalous ratios helium isotopes ground water....

Research Article| October 31, 2018 Implementation of a Real‐Time System for Automatic Aftershock Forecasting in Japan Takahiro Omi; Omi aInstitute Industrial Science, The University Tokyo, 4‐6‐1 Komaba, Meguro‐ku, Tokyo 153‐8505, Japan, omi@sat.t.u-tokyo.ac.jp, aihara@sat.t.u-tokyo.ac.jp Search other works by this author on: GSW Google Scholar Yosihiko Ogata; Ogata bThe Institute Statistical Mathematics, 10‐3 Midori‐cho, Tachikawa, 190‐8562, ogata@ism.ac.jp Katsuhiko Shiomi; Shiomi cNational...

Abstract The spatiotemporal characteristics of shallow slow earthquake activity are linked to the tectonic environments plate boundaries. In this work, variations very low frequency (SVLFE) along Nankai Trough were investigated using a cross‐correlation analysis. SVLFEs migrated or spread eastward strike direction trench during large SVLFE episodes. Migrations and clusters suggest occurrence slip events. observed lateral in patterns reflect heterogeneous distributions effective strengths at...

SUMMARY Due to complex 3-D heterogeneous structures, conventional 1-D analysis techniques using onshore seismograms can yield incorrect estimation of earthquake source parameters, especially dip angles and centroid depths offshore earthquakes. Combining long-term seismic observations numerical simulations wave propagation in a model, we conducted moment tensor (CMT) inversions earthquakes along the Nankai Trough between April 2004 August 2019 evaluate decade-scale seismicity. Green's...

Abstract We detected shallow very low frequency earthquakes (VLFEs) off the Cape Muroto and Kii Channel in Nankai subduction zone estimated their moment rate functions. Combining new previously catalogs, we obtained comprehensive catalog of VLFE functions along Trough. defined swarms investigated scaling relationships cumulative moments, activity area, durations each region. Detected were considered candidates for slow slip events. A similar relationship was observed between moments areas,...

The reconstruction of surface waves from the cross-correlation random wavefields has recently been extensively inspected by theoretical and experimental approaches. However, body not studied. In this study, we present a method for extracting waves, that is, direct P S reflected Philippine Sea slab. We use wavefield generated teleseismic observed Hi-net tiltmeters with passband 0.07–0.5 Hz. To enhance contribution coda, reduced source-time function deterministic phases sS, ScS SS, wave in...

Although no deterministic and reliable earthquake precursor is known to date, we are steadily gaining insight into probabilistic forecasting that draws on space–time characteristics of clustering. Clustering-based models aiming forecast earthquakes within the next 24 hours under test in global project 'Collaboratory for Study Earthquake Predictability' (CSEP). The 2011 March 11 magnitude 9.0 Tohoku-Oki Japan provides a unique opportunity existing 1-day CSEP against its unprecedentedly active...

Real‐time aftershock forecasting is important to reduce seismic risks after a damaging earthquake. The main challenge prepare forecasts based on the data available in real time, which many events, including large ones, are missing and hypocenter determination errors present due automatic detection process of earthquakes before operator inspection manual compilation. Despite its practical importance, forecast skill aftershocks such real‐time still developmental stage. Here, we conduct test...

Abstract We conducted centroid moment tensor inversions of shallow very low frequency earthquakes (SVLFEs) that occurred around off the southeast Kii Peninsula, Japan. A set Green's functions was evaluated using numerical simulations seismic wave propagation in a three‐dimensional velocity structure model, including subducting Philippine Sea slab and accretionary prism. used observed seismograms for periods 20–50 s at an onshore broadband network, which enable us to investigate long‐term...