Recurrence analysis on slow slip events in Japan using renewal processes

The study of slow slip events is essential to elucidating the slip mechanisms and frictional properties of plate interfaces. We imaged the aseismic slip at the plate interface between the subducting Philippine Sea Plate and overriding Amur Plate beneath Shikoku Island, Japan, by applying a network inversion filter to the position time series obtained from Global Navigation Satellite System data spanning January 2012 to April 2022. The results show long‐term slow slip events in the Bungo Channel, central Shikoku, and the Kii Channel, which started from 2018. We also identified 39 short‐term slow slip events associated with low‐frequency tremors. The estimated moment magnitudes of the Bungo Channel, central Shikoku, and Kii Channel slow slip events were Mw7.0, Mw6.4, and Mw6.3, respectively, with a rigidity of 30 GPa. The estimated moment magnitudes of the short‐term slow slip events ranged from Mw5.7 to Mw6.4. Most of the detected short‐term slow slip events are associated with low‐frequency tremor, and many showed propagation through the spatial domain. Events with magnitudes of ≥Mw6.30 typically consist of subevents. The slip rate of short‐term slow slip events ranges from 1 to 7 mm/day, which is considerably larger than the plate convergence rate. The short‐term slow slip events are 6 to 40 days, and follow the accepted scaling law between slip event duration and total magnitude. The short‐term slow slip event zone is consistent with a three‐segment hypothesis. The short‐term slow slip event in northwest Shikoku and long‐term slow slip event in central Shikoku may be correlated.

According to a 2013 report by the Earthquake Research Committee of Japan, it was estimated that the probability of the occurrence of the next Nankai earthquake within the next three decades is 70–80%. Therefore, to realize hazard estimation, it is crucial to estimate the state of the plate interface in the Nankai Trough subduction zone. In this study, we focused on a transient from 2018 in the subduction zone of the Nankai Trough, southwest Japan, detected by the Global Navigation Satellite System (GNSS) network. Thereafter, we estimated the spatiotemporal evolution of the slip at the plate interface by subjecting the GNSS position time series to time-dependent inversion. The results obtained showed that a long-term slow-slip event (l-SSE) possibly occurred on the Kii Peninsula in 2020. The Kii-Channel l-SSE (Mw 6.3), with an irregular recurrence interval, was observed from 2019 to 2022. Additionally, the Central Shikoku l-SSE (Mw 6.5) was observed from 2019 to 2023, and there appeared to be a correlation between the Central Shikoku l-SSE and the northwestern Shikoku short-term slow-slip event (s-SSE). l-SSE occurred from July 2018 to August 2019 in the northern Hyuga-nada and Bungo channel, and in late 2018, another l-SSE occurred in southern Hyuga-nada before the May 2019 Hyuga-nada earthquake. Further, after the 2018–2019 events, the southern Hyuga-nada l-SSE occurred from mid-2020 to early 2021 and in January 2023 and is still ongoing, consistent with the expected recurrence interval. The seemingly transient slip off the Ohsumi Peninsula, detected after the Mw 5.7 earthquake of October 2022, continued until July 2023 with Mw 6.1. Furthermore, after the 2019 Tanegashima earthquake, an l-SSE occurred for approximately 4 years. The moment magnitude (Mw) of the afterslip of the Tanegashima earthquake was estimated to be 6.7, larger than that of the main shock (Mw 6.4). We also noted that the transient slip off Tanegashima included four possible s-SSEs that occurred in 2020, 2021, 2022, and 2023. Therefore, the sporadic aseismic slips along the Nankai Trough changed the stress state of the areas neighboring the aseismic slip zones in favor of the interplate slip.Graphical

Recent geodetic measurements have detected recurrent slow slip events (SSEs) in many subduction zones. Numerical simulations suggest that the recurrence intervals and magnitudes of such SSEs decrease in the later stage of the interseismic period. Therefore, activities of SSEs and their temporal variations in recurrence intervals and magnitudes provide important clues for evaluating future large earthquakes. However, our knowledge of recurrent SSEs before the establishment of dense observation networks is limited. Here, we report a possible SSE in 1973 beneath the Kii peninsula along the Nankai subduction zone, southwest Japan, where magnitude 8 class earthquakes such as the 1944 Tonankai and 1946 Nankai earthquakes repeatedly occur. The possible SSE was detected by a horizontal pendulum tiltmeter installed in 1947 at the Kishu observatory. We recovered and converted the original tilt records from 1969 to 1974 drawn on bromide (light sensitive) papers to digital tilt records. In late-November 1973, there was a slow transient signal down to the northeast of about 1.4 μrad, lasting about 1–3 days. Assuming this crustal deformation was due to an SSE, we searched for a possible fault model by adopting the already-known SSE models occurring from 1996 to 2012 as template models. Among the 42 template fault models, five models were able to qualitatively explain the direction of the observed tilt change. The centroids of these fault models were located about 30–60 km to the west of the observatory. In the source region, two magnitude 5 class earthquakes occurred before the initiation of the transient signal, and thus may be related to the occurrence of the possible SSE. In contrast, the magnitude of the calculated tilt change was about 1–2 order smaller than that of the observed tilt change. This discrepancy could be partly explained by a slip amount larger than that in the template models, which suggests that the possible SSE in 1973 was larger than those detected by recent observations. Further investigation of historical data on crustal deformation will reveal more about the temporal change in the recurrence intervals and magnitudes of SSEs during earthquake cycles of large earthquakes.

We test the hypothesis that ocean seafloor pressures impart stresses that alter the initiation or termination of transient slow slip events (SSEs) on shallow submarine and near‐coastal faults, using simulated seafloor pressures and a new catalog of SSEs in the Hikurangi subduction zone. We show that seafloor pressures may be represented by an average time history over the ~100‐km dimensions of the study area. We account for SSE uncertainties and the multiplicity of processes that affect SSEs statistically by estimating the probabilities of rejecting the null hypothesis that SSE initiation or termination pressures are those to be expected by chance sampling of known (modeled) seafloor pressures, with low probabilities indicating some causal connection. No impact of ocean pressure changes on SSE initiation is detectable, but a correlation with their terminations is suggested. SSE slip that weakens the fault and makes it more sensitive to small stress changes may explain results.

Slow slip events (SSEs) regularly occur near the Boso Peninsula, central Japan. Their time of recurrence has been decreasing from 6.4 to 2.2 years from 1996 to 2014. It is important to better constrain the slip history of this area, especially as models show that the recurrence intervals could become shorter prior to the occurrence of a large interplate earthquake nearby. We analyze the seismic waveforms of more than 2,900 events (M≥1.0) taking place in the Boso Peninsula, Japan, from 1 April 2004 to 4 November 2015, calculating the correlation and the coherence between each pair of events in order to define groups of repeating earthquakes. The cumulative number of repeating earthquakes suggests the existence of two slow slip events that have escaped detection so far. Small transient displacements observed in the time series of nearby GPS stations confirm these results. The detection scheme coupling repeating earthquakes and GPS analysis allow to detect small SSEs that were not seen before by classical methods. This work brings new information on the diversity of SSEs and demonstrates that the SSEs in Boso area present a more complex history than previously considered.

In the subduction zone of southwest Japan, short‐term slow slip events (SSEs) occur with low frequency tremors (LFTs) at intervals of several months. Recently LFTs have been located with high resolution and their activities have been examined in detail. By setting the generation zones of SSEs such that these zones contain the LFT hypocenters, we simulate SSEs on a 3D plate interface beneath the Kii Peninsula and Tokai regions by using a rate‐ and state‐dependent friction law with a small cut‐off velocity for the evolution effect. Our numerical results show that recurrence intervals of SSEs in the southern and central Kii Peninsula, in the northern Kii Peninsula, and in the Tokai region are 2.5–3.0, 4.8–5.6, and 3.5–4.5 months, respectively, which are consistent with observed SSE activity. Our simulation also produces a multisegment event that propagates from the Kii to Tokai segments at a speed of 10 km/day, which is consistent with observations. The results suggest that generation zones of LFTs coincide with SSE regions and that these two events are different manifestations of the same slip process along the subduction zone. We also perform 3D modeling of faster events accompanied by short‐term SSEs by considering local circular patches with a smaller critical displacement, surrounded by SSE zones with a larger critical displacement. Local circular patches are set at the observed LFT locations. The simulation reproduces fast events that propagate at a speed of 100–200 km/day during a single SSE event.

The interactions between aseismic slip and seismicity is mostly studied during postseismic afterslip associated with the generation of aftershocks following large earthquakes. However because of the large stress perturbations produced by the coseismic rupture, it remains difficult to distinguish the contribution of the coseismic stress perturbation and the effects of stresses induced by the afterslip on the triggering of aftershocks. Studying seismicity triggered by slow slip events enables us to understand the direct effect of aseismic slip on the generation of the seismicity. While most subduction slow slip events are deep-seated, at the down-dip edge of seismogenic zones accompanied by tectonic tremors, some are also observed at shallower depths associated with seismic swarms. Among them are the well-studied Boso slow slip events located on the Sagami trough, between 10 and 20 km depth. Recorded every ~4 years since 1996, they are always accompanied by swarms of Mw 1 to 5 earthquakes on their northern and western flanks. Being located right beneath the Boso Peninsula coastline, the kinematics of these slow slip events is particularly well imaged by dense GNSS and tiltmeter networks. In this study, we model the time dependent aseismic slip of the 2018 Boso slow slip event, with the largest moment released of all Boso slow slip events, by inverting time step by time step the slip on the fault with joint GNSS and tiltmeter data. We do not impose arbitrary temporal smoothing in the inversion and find that the well constrained fault slip is first growing and then migrating southwestward with a migration speed of ~ 2 km/day. In order to model the interaction with the seismicity, we compute the Coulomb stress change due to the transient slip on receiver faults located in 9 cubes centered in the seismicity swarm and parallel to the subduction interface. From June 2nd to June 18th, the seismicity is migrating up-dip at a rate of 1 km/day. This migration period coincides with the peak slip rate and with Coulomb stress produced by the slow slip migrating updip together with the seismicity, indicating a causal relationship. Adopting a rate and state friction formalism to explain the nucleation of the seismicity, we finally investigate the ensemble of parameters, in particular the constitutive parameter that relates changes in stress to logarithmic changes of slip velocity, the effective normal stress and the tectonic stressing rates, that can explain the seismicity rate. 

Long-term slow slip events (L-SSEs) have repeatedly occurred beneath the Bungo Channel in southwestern Japan with durations of several months to a couple of years, with a recurrence interval of approximately 6 years. We estimated the spatiotemporal slip distributions of the 2018–2019 Bungo Channel L-SSE by inverting processed GNSS time series data. This event was divided into two subevents, with the first on the southwest side of the Bungo Channel from 2018.3 to 2018.7 and the second beneath the Bungo Channel from 2018.8 to 2019.4. Tectonic tremors became active on the downdip side of the L-SSE occurrence region when large slow slips took place beneath the Bungo Channel. Compared with the previous Bungo Channel L-SSEs, this spatiotemporal slip pattern and amount were similar to those of the 2002–2004 L-SSE. However, the slip expanded in the northeast and southwest directions in the latter half of the second subevent. The maximum amount of slip, the maximum slip velocity, the total released seismic moment, and the moment magnitude of the 2018–2019 L-SSE were estimated to be 28 cm, 54 cm/year, 4.4 times 10^{19} Nm, and 7.0, respectively, all of which were the largest among the 1996–1998, 2002–2004, 2009–2011, and 2018–2019 L-SSEs.

We have developed a broadband ocean bottom seismometer (BBOBS) and its new generation (BBOBS-NX) with the penetrator sensor system since 1999. With them, we performed many practical observations to create a new research category of ocean bottom broadband seismology. As the next step in seafloor geophysical observation, the BBOBS and the BBOBS-NX can be a breakthrough in realizing a geodetic observation network on the seafloor. Although vertical displacement observation by the absolute pressure gauge has been widely conducted in recent years, other geodetic observations are rarely performed. A few trials to measure the seafloor tilt were performed, but those looked inadequate for practical observations. Note that the broadband sensor in our BBOBSs has a mass position signal output, which can be used to measure the tilt change. As the horizontal component noise level of the BBOBS-NX is good at a long period range, we expected it to be adequate for the tilt measurement. At the first evaluation, we performed a comparison with a water-tube tiltmeter. The result was comparable with a resolution of better than 1 µ radian. A practical observation at the south of Boso Peninsula (KAP3 site) was conducted as thein-situstudy from April, 2013. In January, 2014, a slow slip event (SSE) occurred near this site. The tilt data were processed by removing steps, mechanical relaxation, and tides. The results show a clear peak started from late December 2013. Two more 2 year-long tilt observations began in 2015: one was at the KAP3 site and another was off the Miyagi Prefecture at the slope to the Japan Trench. The latter was recovered in 2017 with about 1.5 years of data, which indicate a large continuous tilt up to several tens of µ radian. This amount of tilt can be explained by a similar already estimated SSE. Mobile tilt measurement at the seafloor can be a powerful tool to study SSEs, as they can be located above the source area and also possible to build an observation array for a practical study because of its low cost and ease of deployment compared with a seafloor borehole site.

Episodic tremor and slow-slip events are sensitive to the exogenous stress perturbations process. Although tidal and remote triggering phenomena of tremors are well-established facts; however, the triggering mechanism induced by low-barometric pressure of typhoons or larger storms remains poorly addressed. In this paper, a time-synchronous tremor modulation is presented from the northern Kii Peninsula in western Japan, associated with a large Super Typhoon Jelawat, which occurred on September 30, 2012. It has been argued that such tremor excitation may not be correlated with other types of signal, such as the short-term or long-term slow-slip events, tidal effect, or remote triggering. The atmospheric low-pressure condition during the passage of super typhoon Jelawat causes vertical crustal uplift by a negative load and hence possibly enhances the thrust motion due to unclamping effects on the fault. Therefore, it is suggested that the synchronous tremor modulation process in the northern Kii Peninsula is a real consequence induced by super typhoon Jelawat.

The 1923 Taisho Kanto earthquake—an interplate event along the Sagami trough where the Philippine Sea plate subducts beneath the Kanto region—produced ground shaking, crustal deformation, landslides, and tsunamis, and caused the worst earthquake disaster in Japan. Based on seismological and geodetic data, many fault models have been proposed, extending ∼100 km from the epicenter, with a moment magnitude (Mw) of 7.8–8.2, and large slips of ∼8 m located near the epicenter and beneath the Miura Peninsula. The penultimate 1703 Genroku Kanto earthquake produced similar macroseismic effects around Sagami Bay and the Miura Peninsula, but larger coastal uplift and tsunami in the Boso Peninsula. The proposed fault models extend off the Boso Peninsula with Mw of 8.1–8.5. In 2004, the Earthquake Research Committee (ERC) classified the Kanto earthquakes as “Taisho type” and “Genroku type” with recurrence intervals of 200–400 yr and 2300 yr, respectively. In 2014, the ERC revised the long-term evaluation to a recurrence interval of 180–590 yr and a 30 yr probability of 0%–5% based on the Brownian passage time model. With the Cabinet Office, the ERC considered the source area of the maximum possible earthquake of Mw 8.6–8.7. The recent historiographical and paleoseismological studies have identified other candidates for the past Kanto earthquakes in 1495, 1433, 1293, and 878. Various combinations of these candidates give a mean recurrence interval of 210–315 yr, an aperiodicity parameter of 0.04–0.76, and a 30 yr probability of 0.0%–19%. The Cabinet Office has calculated the seismic intensity and tsunami heights of various types of Kanto earthquakes. National and local governments estimate the damage from these hazards. For the Tokyo metropolitan area, the estimated damage and occurrence probability are more significant for M ∼7 earthquakes with various types and depths, and most mitigation efforts are directed at such events.

Probabilistic seismic hazard analysis (PSHA) is a methodology with a long history and has been widely implemented. However, in the conventional PSHA and sequence-based probabilistic seismic hazard analysis (SPSHA) approaches, the occurrence of mainshocks is modeled as the homogeneous Poisson process, which is unsuitable for large earthquakes. To account for the stationary occurrence of small-to-moderate (STM) mainshocks and the nonstationary behavior of large mainshocks, we propose a time-dependent sequence-based probabilistic seismic hazard analysis (TD-SPSHA) approach by combining the time-dependent mainshock probabilistic seismic hazard analysis (TD-PSHA) and aftershock probabilistic seismic hazard analysis, consisting of four components: (1) STM mainshocks, (2) aftershocks associated with STM mainshocks, (3) large mainshocks, and (4) aftershocks associated with large mainshocks. The approach incorporates an exponential-magnitude, exponential-time model for STM mainshocks, and a renewal-time, characteristic-magnitude model for large mainshocks to assess the time-dependent hazard for mainshocks. Then nonhomogeneous Poisson process is used to model the occurrence of associated aftershocks, in which the aftershock sequences can be modeled using the Reasenberg and Jones (RJ) model or the epidemic-type aftershock sequence (ETAS) model. To demonstrate the proposed TD-SPSHA approach, a representative site of the San Andreas fault is selected as a benchmark case, for which five time-dependent recurrence models, including normal, lognormal, gamma, Weibull, and Brownian passage time (BPT) distributions, are chosen to determine the occurrence of large mainshocks. Then sensitivity tests are presented to show the effects on TD-SPSHA, including (1) time-dependent recurrence models, (2) mainshock magnitude, (3) rupture distance, (4) aftershock duration, (5) escaped time since the last event, and (6) future time interval. Furthermore, the bimodal hybrid renewal model is utilized by TD-SPSHA for another case site. The comparison results illustrate that the sequence hazard analysis approach ignoring time-varying properties of large earthquakes for long periods and the effects of associated aftershocks will result in a significantly underestimated hazard. The TD-SPSHA-based hazard curves using the ETAS model are larger than those of the RJ model. The proposed TD-SPSHA approach may be of significant interest to the field of earthquake engineering, particularly in the context of structural design or seismic risk analysis for the long term.

Earthquake catalogue declustering is a critical preprocessing step in time-dependent seismicity analyses (Gardner and Knopoff, 1974; Reasenberg, 1985), yet its systematic influence on conditional earthquake probability estimates remains insufficiently quantified, particularly in tectonically complex continental collision zones such as the Himalayas (Bungum et al., 2017). Renewal-based recurrence models typically assume that declustered catalogues isolate tectonically driven mainshock recurrence by removing dependent events. However, recent advances in declustering theory demonstrate that methodological choices, ranging from fixed spatio-temporal windows to adaptive and stochastic approaches, can substantially modify inter-event time statistics and inferred recurrence memory (Zaliapin et al., 2008; Zaliapin & Ben-Zion, 2020; Teng & Baker, 2019). Despite these developments, the implications of declustering-induced variability for time-dependent conditional probabilities remain underexplored in active orogenic belts.In this study, we explicitly quantify how alternative declustering strategies influence time-dependent recurrence behavior and conditional rupture probabilities across selected Himalayan seismic source zones. Inter-event time series were constructed for moderate-to-large earthquakes (M ≥ 4.0) using both raw (non-declustered) and declustered catalogues derived from regional earthquake compilations. Declustering was performed using commonly applied fixed-window and adaptive approaches to capture epistemic variability associated with catalogue preprocessing. The resulting inter-event times were analyzed within renewal process models, including Brownian Passage Time (BPT), Lognormal, Weibull, and Gamma distributions, to estimate conditional probabilities as functions of elapsed time since the most recent major event.Results show that declustered catalogues consistently yield smoother initial probability gradients and delayed probability peaks relative to raw catalogues, reflecting reduced short-term temporal clustering in inter-event time distributions. These shifts correspond to systematic changes in inferred renewal memory parameters, with declustering suppressing short-term contagion effects while largely preserving long-term mean recurrence intervals. In the Himalayas, collision-driven aftershock swarms and spatially heterogeneous fault interactions amplify these effects, introducing substantial epistemic uncertainty in early-time conditional probabilities, which can locally exceed factors of two to three depending on the declustering strategy employed. In contrast, long-term probability remains comparatively robust across declustering scenarios, consistent with steady-state tectonic strain accumulation.These findings identify catalogue declustering as a dominant and often underappreciated source of uncertainty in time-dependent seismic probability modelling, reinforcing recent calls for ensemble-based and transparent pre-processing strategies in probabilistic seismic hazard workflows. This study advances a methodological framework for interpreting renewal-based conditional probabilities in clustered tectonic regimes. The Himalayas emerge as a natural laboratory where combined raw and declustered analyses can yield more resilient probabilistic interpretations.

The 1854 Ansei Nankai Earthquake caused widespread damage from Kii peninsula to Kyushu region with Japanese seismic intensities of V to VI. A large amount of descriptions related to this earthquake are found in the archives “New Collection of Material for the History of Japanese Earthquakes”. The no. 5 of supplements to vol. 5 has collected documents related to the Ansei Tokai Earthquake (Nov. 4th, Ansei 1 in the old (lunisolar) calendar, or Dec. 23rd, 1854 in the new (Gregorian) calendar), Ansei Nankai Earthquake (Nov. 5th), and the largest aftershock in the Bungo channel (Nov. 7th), whose total number of pages amounts to 2,528. These collection books have quoted from many historical documents published by local governments, and reprinted from original historical material related to the earthquakes. However, when the published books were used, no reprint of original material was made. Purpose of this study is to reprint photocopy of an original private record handed down to Warabioka family who was a head of Masaki village of Ainan-town in the southernmost of Ehime Prefecture in Southwestern Japan. Furthermore, we compare our reprinted text with published ones, and make detailed investigation on natural phenomena and damage by the Ansei Nankai earthquake. It is identified that a lack of description about natural phenomena and damage and an error of the number of the dead in the texts adopted in the “New Collection of Material for the History of Japanese Earthquakes”. In particular, it is clear that the number of the dead in Fukaura village is not 101 but only one. This is consistent with the record of the necrology and gravestone. Preliminary estimation for the crustal deformation indicates a lowering of groundwater level in Masaki region by volume dilatation associated with the earthquake. This result is consistent with the record that the groundwater level was lowered. Furthermore, we compared our reprinted text with seven historical documents of Uwajima-Date family who had governed this area at that time in order to know the difference of information in the historical documents of the two families. As a result of comparison, there is no contradiction about descriptions of tsunami damage. However, the comparison shows no description about inland natural phenomena and damage such as well water dried up and rock fall in the historical documents of Uwajima-Date family, whereas it is written in the record of Warabioka family. These results show that the private record written by the village head has also valuable information on the earthquake.

• NMR-based K prediction using the SDR model for unconsolidated geological materials. • Covers both organic & inorganic geological units from multiple sites. • Assesses K estimation using both field-based and laboratory-based datasets. • Tests general, site-specific, and geology-specific SDR calibration approaches. • Reducing K estimation uncertainty in sediments by excluding porosity from SDR model. Accurate hydraulic conductivity ( K ) estimation is essential for effective groundwater resource management, yet conventional methods struggle with spatial heterogeneity and scale disparities. This study evaluated nuclear magnetic resonance (NMR)-based K prediction using the Schlumberger-Doll Research (SDR) model across diverse unconsolidated environments. We analyzed 92 laboratory samples from nine sites and 198 borehole screens at eight sites in Denmark and Germany, encompassing sediments (sand, clay, till) and organic deposits (peat, gyttja). Bootstrapping and statistical analysis revealed that general calibration of the SDR model was successful for inorganic sediments but failed when organic geological units were present in the dataset. No correlation was found between the mean logarithm of the NMR transverse relaxation time distribution ( T 2ML ) and K in peat soils, making reliable K estimation impossible in these materials. However, gyttja – despite being an organic soil – allowed for reliable K estimation using geology-specific calibration, although with different SDR constant coefficients than those for inorganic sediments. Generally, geology- or site-specific calibrations significantly improved K accuracy for inorganic geological units when sufficient data points were available for the curve fitting, and T 2ML and K values spanned a sufficiently broad range (e.g., at least 1 to 1.5 orders of magnitude). We demonstrated that using a simplified SDR model – excluding porosity ( K = b T 2 M L n ) – for unconsolidated inorganic geological materials reduces uncertainty in the free-fitting coefficient parameters ( b and n ). Moreover, it avoids significant errors arising from magnetic susceptibility-induced porosity underestimation, which reached up to 70 % in our samples.