Application of an Objective Detection Method of Long-Term Slow Slip Events using GNSS Data: Detection of Short-Term Slow Slip Events and Estimation of Moment Magnitude of Long-Term Slow Slip Events

Numerous deep short-term slow slip events (S-SSEs) and long-term slow slip events (L-SSEs) have been detected in the Nankai Trough subduction zone in southwestern Japan based on Global Navigation Satellite System (GNSS) data. Recently, slow slip events (SSEs) lasting several months, which are significantly longer than typical deep S-SSEs but similar to or shorter than L-SSEs, have been reported near the trench, particularly off the Kii Peninsula in the central Nankai Trough. To investigate whether similar SSEs occur in the shallow region of the Suruga Trough, the easternmost segment of the Nankai Trough, we analyzed changes in GNSS baseline lengths between stations on the upper and lower plates across the Suruga Trough. To identify the timing of baseline shortening, we first converted the GNSS baseline length time series into velocity time series. We then compared these velocity time series with the correlation coefficients computed between the observed data and a template function representing baseline shortening associated with shallow SSEs. As a result, a signal corresponding to a plausible SSE in late 2019, whose duration can be approximately represented by 118 days, was detected at multiple GNSS stations near Omaezaki. A Bayesian inversion of the fault slip distribution estimated a maximum slip on the order of 10 mm at depths shallower than ~ 10 km on the shallow plate boundary off Omaezaki. This result was consistent across all three assumed plate interface models. Furthermore, slip extending to the trench was also inferred. The estimated moment magnitude (Mw) was approximately 5.5–5.6. This SSE coincided with an increase in small repeating earthquake activity on the deeper side. The discovery of a months-long SSE in the shallow region of the Suruga Trough suggests that SSEs with significantly longer durations than typical deep S-SSEs may be a widespread characteristic of the shallow Nankai Trough subduction zone. Moreover, by relaxing the detection criteria and considering smaller possible SSEs, the results further imply that such events may occur at intervals of approximately 500 days.

We detected short‐term slow slip events (SSEs) previously observable only with tilt and strain data along the Nankai Trough, southwest Japan, using GNSS (Global Navigation Satellite System) data. Offsets detected in GNSS time series using Akaike's information criterion helped automatically identify 207 episodes with a motion direction opposite to that of the relative plate motion from June 1996 to January 2012. By nonlinear inversion of the detected displacement, we estimated rectangular fault models for 133 probable and 25 possible short‐term SSEs over 15 years. The SSE moment magnitudes range from 5.5 to 6.3. Most SSE fault models are located in a narrow band of non‐volcanic tremors on the interfaces of the subducting Philippine Sea Plate. Large SSEs (moment magnitude, Mw, ≥6) often occur in western and central Shikoku. The cumulative slip is distributed heterogeneously along the strike, generally decreasing eastward with the maximum slip (~50 cm) in western Shikoku. No definite short‐term SSEs were found in the Kii Channel, but several short‐term SSEs occurred in Ise Bay. Both regions are known as tremor gaps. The local maximum of the cumulative slip fills in the tremor gap located in Ise Bay. The long‐term rate of short‐term SSE cumulative moment increased by threefold around 2003 in eastern Shikoku, whereas it was almost constant in other regions. Comparison with short‐term SSE catalogues using tilt data suggests that both this study and previous studies missed some SSEs along the Nankai Trough. A combination of geodetic data is important in the monitoring of the spatiotemporal distribution of short‐term SSEs.

In the southwest Japan subduction zone, short‐term slow slip events (SSEs) accompanying nonvolcanic tremor (ETS: episodic tremor and slip) occur repeatedly with recurrence intervals of several months. It is important to know the detailed slip areas of the SSEs because short‐term SSEs occur at the downdip extension of a megathrust earthquake rupture zone, and hence, the SSEs play a key role in the stress buildup processes of the megathrust earthquakes. Although most of the previous studies model a SSE fault motion with uniform slip on a rectangular fault, this may lead to a bias in estimating the slip area. In this study we estimate a spatial slip distribution for each of the short‐term SSEs from tilt offset measurements. In total, the slip distributions of 61 SSEs in Shikoku from January 2001 to March 2019 are obtained. The ETS zone in Shikoku is divided into three segments in terms of slip history of the SSEs. Before 2011, the slip areas of most of the SSEs are limited in any one of the three segments. After 2012, however, the number of SSEs having a slip area that extends to multiple segments increases. The slip histories at each position in the ETS zone suggest that the short‐term SSE activity is affected by the occurrence of nearby long‐term SSEs. The average slip rate over 18 years suggests that an estimated slip deficit in the ETS zone in western Shikoku could be released by long‐term SSEs, or coseismic slip or afterslip of the megathrust earthquakes.

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

Along the Nankai Trough subduction zone, southwest Japan, short-term slow slip events (SSEs) are commonly detected in strain and tilt records. These observational data have been used in rectangular fault models with uniform slip to analyze SSEs; however, the assumption of uniform slip precludes the possibility of mapping the slip distribution in detail. We report here an inversion method, based on the joint use of strain and tilt data and evaluated in terms of the Akaike’s Bayesian information criterion (ABIC), to estimate the slip distributions of short-term SSEs on the plate interface. Tests of this method yield slip distributions with smaller errors than are possible with the use of strain or tilt data alone. This method provides detailed spatial slip distributions of short-term SSEs including probability estimates, enabling improved monitoring of their locations and amounts of slip.

We analyzed Global Navigation Satellite System (GNSS) data from 1 January 2008 to 31 January 2016 for the Tokai region of the Nankai subduction zone, in order to study long‐ and short‐term slow slip events (SSEs) on the subduction interface. The dataset included our new temporary stations to obtain high resolution observations with station spacing of about 5 km. Long‐ and short‐term SSEs have been reported in the Tokai region, over the past two decades. It is difficult to directly compare those SSEs because long‐term SSEs were observed by GNSS, while short‐term SSEs were mainly observed by tiltmeters and strainmeters. We applied a time‐dependent inversion with improved temporal resolution to GNSS data and successfully obtained the spatiotemporal evolution of long‐ and short‐term SSEs. Our results show the very slow and stable slip of a long‐term SSE with Mw 6.6, along with several episodes of accelerated slip associated with short‐term SSEs (Mw 6.0–6.1). Our results showed the details of the migrating slip patterns and slip rates for the short‐term SSEs and possible relation with low frequency tremor (LFT). LFT is more associated with smaller magnitude short‐term SSEs rather than the larger long‐term SSEs. However, the maximum daily slip rates for short‐term SSEs are an order of magnitude larger than those for the long‐term SSEs. This suggests an explanation that the LFT may be directly related to the loading of the plate interface by the short‐term SSEs.

<p>The Kanto region, central Japan situated in the complex tectonic region where two oceanic plates subducts from the Japan trench and Sagami trough. Although many previous studies clarified repeated Mw~6.6 Slow Slip Events (SSEs) with a duration of a week in an offshore region of the Boso Peninsula along the Sagami trough, the number of the detected SSEs are limited and overall activity of SSEs have not been fully understood in these regions. We, here, applied our SSE detection in these regions to the whole available GNSS dataset for a quarter century spanning from 1994 to 2019 and clarify the relation between SSE and tremor distribution.</p><p>We use daily coordinates at 291 GNSS stations using a precise point positioning strategy of the GIPSY 6.4 software. We apply the method of Nishimura et al. (2013) and Nishimura (2014) to detect a jump associated with short-term SSEs in GNSS time-series and estimate their fault models from observed displacements. A rectangular fault on the Philippine Sea or the Pacific plates is assumed for each SSE. The stacking of GNSS time-series based on the displacement predicted by the fault model [Miyaoka and Yokota, 2012] enable us to estimate duration of SSEs.</p><p>  We detected ≥ 150 possible SSEs along both the Japan trench and Sagami trough but we here focus on SSEs along the southernmost part of the Japan trench. Total slip distribution of the detected possible SSEs shows that large slip (≥ 0.3 m) is limited near the trench. A comparison with low-frequency tremors (LFTs) along the Japan trench (Nishikawa et al., 2019) suggests SSEs occur in the same depth range (10-20 km) of LFTs but their distribution is rather complimentary whereas a minor tremor activity exists at the edge of the SSE total slip. This complimentary distribution is contrast to overlapping distribution of SSEs and LFTs observed in a deep episodic and tremor region in the other subduction zones including southwest Japan. Another distinctive feature is that SSEs continuously occur from the trench to a depth of ~60 km only at ~ 35.5ºN. Because the subducted seamounts locate at this latitude, geometry of plate interface may control a genesis of SSEs in these regions.</p>

Over the past decade, short-term slow slip events (S-SSEs) have been detected along the entire Nankai Trough using Global Navigation Satellite System (GNSS) data. To enhance the detection of S-SSEs, we focused on the spatial and temporal coincidence of tremors and S-SSEs, a phenomenon known as episodic tremor and slip. We developed a machine learning-based method to detect S-SSEs directly from continuous seismic waveforms and applied it to seismic and geodetic data in western Shikoku, Japan. We trained a random forest regression model using statistical features extracted from continuous seismic waveforms as input variables and GNSS-derived displacement rates as target outputs. We predicted the GNSS displacement rate over a period of ~ 6 years and defined S-SSEs as periods when the predicted GNSS displacement rate increased sharply. We then estimated fault models for each detected S-SSE. The predicted displacement rates were correlated strongly with the observed displacement rates, and we identified a total of 23 S-SSEs, including 5 previously unrecognized events. The results demonstrate the effectiveness of machine learning using continuous seismic waveforms for improving S-SSE detection along the Nankai Trough. Graphical Abstract

In order to develop new tools or techniques to detect short‐term slow slip events (S‐SSEs) along subduction zones, we attempted to detect S‐SSEs by conducting groundwater pressure observations. At ANO station, which is a groundwater observation station operated by the Geological Survey of Japan, the National Institute of Advanced Industrial Science and Technology, for earthquake prediction research, groundwater pressures changed due to six S‐SSEs that occurred near ANO from June 2011 to April in 2013. The fault models of these S‐SSEs, which were estimated mainly by observing the crustal strains and tilts, explained the changes in the groundwater pressures. If the strain sensitivity of the observed groundwater pressure or level is larger than 1 mm/nstrain and the noise level is smaller than 50 mm/day, it is possible to detect S‐SSEs that occur in southwest Japan by conducting groundwater pressure or level observations.

High- VP/ VS zone accompanying non-volcanic tremors and slow-slip events beneath southwestern Japan

In this study, short-term slow slip events (SSEs) along the Ryukyu Trench, southwestern Japan were systematically examined using continuous global navigation satellite system (GNSS) data. In total, 130 probable and 93 possible short-term SSEs in the moment magnitude (M w ) range of 5.6 to 6.8 were identified from January 1997 to November 2013 by GNSS time series offset monitoring and elastic dislocation modeling with a rectangular fault located on the subducting Philippine Sea Plate. The detected short-term SSEs were found to have a variety of characteristic recurrence intervals, magnitudes, durations, and coincidental seismic activities. Short-term SSEs without identified low-frequency earthquakes (LFEs) and low-frequency tremors (LFTs) were found to be common along the Ryukyu Trench. The total slip distributions and SSE numbers were heterogeneous and mostly between 10 and 60 km in depth. Although shallow (depth ≤20 km) short-term SSEs have never been detected along the Nankai Trough, it was notable that such SSEs often occur on the shallow plate interface along the Ryukyu Trench. This may be related to the incomplete interplate locking estimated by various geodetic studies. A band of short-term SSEs in the 20 to 40 km depth range extends from west Shikoku through the Bungo Channel to mid-Kyushu and then fades away around the subducted Kyushu-Palau Ridge. SSEs with accompanying LFEs and LFTs were found to be limited to western Shikoku and the Bungo Channel. We found several distinctive clusters of short-term SSEs, in addition to a cluster previously identified in the Yaeyama Islands. The study also identified a cluster northeast of Kikaijima consisting of 20 repeated SSEs at depths in the vicinity of 10 km near the trench where the Amami Plateau subducts, as well as another cluster southeast of Okinawa Island consisting of 29 M w ≤ 6.0 SSEs. The results suggest that the distribution of short-term SSEs, as well as that of large earthquakes, is affected by the topography of the subducting plate.

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

The Nankai Trough in Southwest Japan exhibits a wide spectrum of fault slip, with long-term and short-term slow-slip events, slow and fast earthquakes, all associated with different segments down the plate interface. Frictional and viscous properties vary depending on rock type, temperature, and pressure. However, what controls the down-dip segmentation of the Nankai subduction zone megathrust and how the different domains of the subduction zone interact during the seismic cycle remains unclear. Here, we model a representative cross-section of the Nankai subduction zone offshore Shikoku Island where the frictional behavior is dictated by the structure and composition of the overriding plate. The intersections of the megathrust with the accretionary prism, arc crust, metamorphic belt, and upper mantle down to the asthenosphere constitute important domain boundaries that shape the characteristics of the seismic cycle. The mechanical interactions between neighboring fault segments and the impact from the long-term viscoelastic flow strongly modulate the recurrence pattern of earthquakes and slow-slip events. Afterslip penetrates down-dip and up-dip into slow-slip regions, leading to accelerated slow-slip cycles at depth and long-lasting creep waves in the accretionary prism. The trench-ward migrating locking boundary near the bottom of the seismogenic zone progressively increases the size of long-term slow-slip events during the interseismic period. Fault dynamics is complex and potentially tsunami-genic in the accretionary region due to low friction, off-fault deformation, and coupling with the seismogenic zone.

In the Bungo Channel along the Nankai Trough, southwest Japan, long-term slow slip events (LSSEs) of about Mw 6–7 and lasting 6–12 months occur at several-year intervals at the base of the seismogenic zone. In sync with these LSSEs, deep tectonic tremors are activated at the base of the LSSE zone. The tidal response of tectonic tremors during short-term slow slip events lasting several days has been extensively investigated, but variations of the tidal correlation of tectonic tremors over periods on the order of years remain poorly understood. Here, we investigated long-term temporal changes in the correlation between deep tectonic tremors and tides in the Bungo Channel. We found that tectonic tremors along the base of the LSSE region (region Ba) are strongly correlated with tidal shear stress and/or Coulomb failure stress under a very small apparent friction coefficient, and that these tremors are more likely to occur when the tidal stress acting in the direction promoting fault slip is large. We also found that tidal sensitivity is relatively high during LSSE periods and low during other periods. Because the LSSE region largely includes region Ba, fault coupling in region Ba during LSSE periods is undoubtedly smaller than during other periods. As a result of this low friction state, even tidal stresses much smaller than the lithostatic stress can affect the generation of deep tectonic tremor, and tidal sensitivity during LSSE periods seems to be higher than during other periods.Graphical abstract

A strong correlation exists between the average slip rate by short‐term slow slip events (SSEs) and changes in the slab geometry in Cascadia and Nankai. The generation of short‐term SSEs is generally assumed to be related to the presence of fluids and we investigate the hypothesis that fluids released by metamorphic dehydration reactions migrate in 3‐D due to complex slab geometry. The associated along‐arc focusing of fluid flux is likely to cause higher average slip rate in certain patches. To test this hypothesis, we investigate how fluid migration is modified by along‐strike changes in slab geometry. We use a numerical model of two‐phase flow in subduction zones. In this model fluids migrate subparallel to the slab surface due to the anisotropic permeability inside a serpentinite layer just above the slab. In 3‐D, we find that fluids migrate in the maximum‐dip direction of the slab, rather than subparallel to the plate motion. As a result fluid paths concentrate with increasing porosity where the slab has a convex shape (and diverge with decreasing porosity where it has a concave shape). These results suggest that regions with a high average slip rate by short‐term SSEs in Cascadia and Nankai can be explained by 3‐D focusing of fluid migration. We predict a defocusing of fluids below the Kii Channel, Nankai, which may be the reason for the observed small slip by short‐term SSEs in this location.