Long-term Slow Slip Events Research Articles

Spatial and temporal distributions of b-values related to long-term slow-slip and low-frequency earthquakes in the Bungo Channel and Hyuga-nada regions, Japan

Long-term slow-slip events (SSEs) and low-frequency earthquakes (LFEs) repeatedly occur in the area encompassing the Bungo Channel and Hyuga-nada region in Japan. SSEs are slow earthquake events with a duration of months to years, characterized by a type of slip that is considered transitional between the fast rupture of regular earthquakes and stable sliding along a plate interface, and have characteristics sensitive to stress states near the plate interface. In addition, since LFEs are also thought to occur at the plate interface, it would be useful to infer the stress state in this region. This study focuses on the stress-dependent characteristics of the b-values of regular earthquakes, SSEs, and LFEs, as well as the spatio-temporal relationships between b-values and these events in this area, using the Japan Unified high-resolution relocated Catalog for Earthquakes (JUICE). The b-value, which represents the relative proportion of large to small earthquakes, is inversely related to differential stress or effective stress. The b-values are moderate to low (b = 0.6–1.1) in regions with a high incidence of long-term SSEs. This spatial b-value anomaly is attributed to the partial release of accumulated stress in regions with a relatively high differential stress due to SSE activity. Furthermore, the b-value distributions estimated in this study provide constraints on the mechanisms that generate LFEs. On the other hand, the b-values increase and decrease before and during periods of enhanced LFE activity in the Bungo Channel, respectively. The increase and following decrease in b-values are thought to reflect episodes of low and high shear strength along the plate interface, which were caused by the presence of abundant slab-derived fluids under undrained conditions above the plate interface, and decreased pore pressure related to fluid depletion via LFE generation, respectively.

Geodetic inversion for spatial distribution of slow earthquakes under sparsity constraints

In geodetic data inversion, insufficient observational data and smoothness constraints for model parameters make it difficult to clearly resolve small-scale heterogeneous structures with discontinuous boundaries. We therefore applied sparse modelling to geodetic data inversion. In this paper, we reported two examples; one is developed a novel regularization scheme for the inversion problem that uses discontinuity, sparsity, and smoothness constraints. In order to assess its usefulness and applicability, the proposed method was applied to synthetic displacements calculated by a ring-shaped and sharply varying afterslip distribution on the plate interface beneath the Hyuga-nada region in southwest Japan. The discontinuous boundary was sharper than that obtained by using smoothness constraint only. The other is used the fused regularization, a type of sparse modelling suitable for detecting discontinuous changes in the model parameters. We estimated spatial distribution of the long-term slow slip events beneath the Bungo channel in southwest Japan. We found that the largest slip abruptly becomes zero at the down-dip limit of the seismogenic zone, and is immediately reduced to half at the up-dip limit of the deep low-frequency tremors, and becomes zero near its down-dip limit. Such correspondences imply that some thresholds exist in the generation processes for both tremors and SSEs. These results suggest that geodetic data inversion with sparse modelling can detect such abrupt changes and discontinuous boundaries in the slip distribution of slow earthquakes.

Slow-slip events in semi-brittle serpentinite fault zones

Slow-slip events are earthquake-like events only with much lower slip rates. While peak coseismic velocities can reach tens of meters per second, slow-slip is on the order of 10−7±2 m/s and may last for days to weeks. Under the rate-and-state model of fault friction, slow-slip is produced only when the asperity size is commensurate with the critical nucleation size, a function of frictional properties. However, it is unlikely that all subduction zones embody the same frictional properties. In addition to friction, plastic flow of antigorite-rich serpentinite may significantly influence the dynamics of fault slip near the mantle wedge corner. Here, we show that the range of frictional parameters that generate slow slip is widened in the presence of a serpentinized layer along the subduction plate interface. We observe increased stability and damping of fast ruptures in a semi-brittle fault zone governed by both brittle and viscoelastic constitutive response. The rate of viscous serpentinite flow, governed by dislocation creep, is enhanced by high ambient temperatures. When effective viscosity is taken to be dynamic, long-term slow slip events spontaneously emerge. Integration of rheology, thermal effects, and other microphysical processes with rate-and-state friction may yield further insight into the phenomenology of slow slip.

Objective detection of long-term slow slip events along the Nankai Trough using GNSS data (1996\u20132016)

This paper presents a method for objective detection of long-term slow slip events with durations on the order of years, on the plate boundary along the Nankai Trough, relying on global navigation satellite system daily coordinate data. The Chugoku region of Japan was held fixed to remove common mode errors, and a displacement component was calculated relative to the direction of plate subduction. Correlations were then calculated between this displacement component and a 3-year ramp function with a 1-year slope. Nearly all periods of strong correlation coincide with periods of previously reported long-term slow slip events. A period of strong correlation around the Kii Channel in 2000–2002 is attributed to a previously undocumented long-term slow slip event beneath the Kii Channel and the eastern part of Shikoku Island with an equivalent moment magnitude of 6.6. This detection method reveals variation among long-term slow slip events along the Nankai Trough.

Discontinuous boundaries of slow slip events beneath the Bungo Channel, southwest Japan

The down-dip limit of the seismogenic zone and up-dip and down-dip limits of the deep low-frequency tremors in southwest Japan are clearly imaged by the hypocentre distribution. Previous studies using smooth constraints in inversion analyses estimated that long-term slow slip events (L-SSEs) beneath the Bungo Channel are distributed smoothly from the down-dip part of the seismogenic zone to the up-dip part of the tremors. Here, we use fused regularisation, a type of sparse modelling suitable for detecting discontinuous changes in the model parameters to estimate the slip distribution of L-SSEs. The largest slip abruptly becomes zero at the down-dip limit of the seismogenic zone, is immediately reduced to half at the up-dip limit of the tremors, and becomes zero near its down-dip limit. Such correspondences imply that some thresholds exist in the generation processes for both tremors and SSEs. Hence, geodetic data inversion with sparse modelling can detect such high resolution in the slip distribution.

Three‐dimensional numerical modeling of temperature and mantle flow fields associated with subduction of the Philippine Sea plate, southwest Japan

AbstractWe investigated temperature and mantle flow distributions associated with subduction of the Philippine Sea (PHS) plate beneath southwest Japan, by constructing a three‐dimensional parallelepiped model incorporating a past clockwise rotation, the bathymetry of the Philippine Sea plate, and distribution of the subducting velocity within its slab. The geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3‐D simulation. Using the model, we estimated a realistic and high‐resolution temperature field on the subduction plate interface, which was constrained by a large number of heat flow data, and attempted to clarify its relationship with occurrences of megathrust earthquakes, long‐term slow slip events (L‐SSEs), and nonvolcanic low‐frequency earthquakes (LFEs). Results showed that the oblique subduction coupled with the 3‐D geometry of subducting PHS plate was a key factor affecting the interplate and intraplate temperature distributions, leading to a cold anomaly in the plate interface beneath western Shikoku, the Bungo Channel, and the Kii Peninsula. Temperatures in the slab core in these regions at a depth near the continental Moho were nearly 200°C lower than that in eastern Shikoku, indicating a high thermal lateral heterogeneity within the subducting plate. The geothermal control of the LFEs beneath western Shikoku was estimated to be within a range from 400 to 700°C, and the interplate temperature for the L‐SSEs with a slip larger than 15 cm beneath the Bungo Channel was estimated to be approximately 350–500°C. A large horizontal temperature gradient of 2.5 ~ °C/km was present where the LFEs occurred repeatedly. The steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid‐ocean ridge basalt of the subducting PHS plate.

Two decades of spatiotemporal variations in subduction zone coupling offshore Japan

Spatial patterns of interplate coupling on global subduction zones can be used to guide seismic hazard assessment, but estimates of coupling are often constrained using a limited temporal range of geodetic data. Here we analyze ∼19 years of geodetic observations from the GEONET network to assess time-dependent variations in the spatial distribution of coupling on the subduction zones offshore Japan. We divide the position time series into five, ∼3.75-year epochs each decomposed into best-fit velocity, annual periodic signals, coseismic offsets, and postseismic effects following seven major earthquakes. Nominally interseismic velocities are interpreted in terms of a combination of tectonic block motions and earthquake cycle activity. The duration of the inferred postseismic activity covaries with the linear velocity. To address this trade-off, we assume that the nominally interseismic velocity at each station varies minimally from epoch to epoch. This approach is distinct from prior time-series analysis across the earthquake cycle in that position data are not detrended using preseismic velocity, which inherently assumes that interseismic processes are spatially stable through time, but rather the best-fit velocity at each station may vary between epochs. These velocities reveal significant consistency since 1996 in the spatial distribution of coupling on the Nankai subduction zone, with variation limited primarily to the Tokai and Bungo Channel regions, where long-term slow slip events have occurred, and persistently coupled regions coincident with areas that slipped during historic great earthquakes. On the Sagami subduction zone south of Tokyo, we also estimate relatively stable coupling through time. On the Japan–Kuril Trench, we image significant coupling variations owing to effects of the 1994 MW=7.7 Sanriku-oki, 2003 MW=8.2 Tokachi-oki, and 2011 MW=9.0 Tohoku-oki earthquakes. In particular, strong coupling becomes more spatially extensive following the 1994 event until 2011, coseismic-sense slip precedes the Tohoku-oki event, and coupling offshore northern Honshu is reduced after the 2011 earthquake. Despite the occurrence of the 2003 Tokachi-oki earthquake, persistent coupling offshore Hokkaido suggests ongoing seismic hazard, possibly similar to past MW∼9-class earthquakes interpreted from coastal paleoseismic records. This time-dependent analysis of interseismic deformation illuminates rich diversity in the distribution of subduction zone coupling, including spatiotemporal stability in coupling, effective reduction in strongly coupled regions due to aseismic thrust-sense slip events, and broad changes in the distribution of coupling following major earthquakes.

Temporal change in plate coupling and long-term slow slip events in southwestern Japan

In the southwestern part of Japan, many large earthquakes have been reported and many geodetic or seismic observations have been performed to monitor crustal deformation due to the interaction between the subducting Philippine Sea plate and the overriding continental plate along the Suruga–Nankai trough. These precise observations provide insight into aseismic stress-releasing processes such as slow slip events (SSEs). In this study, I focus on the effect of SSEs on interplate coupling to reveal the overall temporal evolution of interplate states along the trough in southwestern Japan. I focus mainly on the western part of the subduction zone, called the Nankai region. In this part, three SSEs (in 1997, 2003, and 2010) were detected during the period in which observations are available. Interplate coupling recovered quickly after the SSEs terminated. The eastern part of this region shows consistently strong coupling whether SSEs occur or not. Long-term SSEs are distributed between the coupling area and the area where tremors occur. I also examine the balance between stress accumulation and release at four points in the region. Below the eastern part of the strongly coupled area, at a depth of ∼15 km, constant coupling of about 7 cm/yr takes place. In the center of the SSE region, at a depth of ∼25 km, ∼40% of the accumulated stress is released through SSEs, with the rest contributing to the stress accumulation process.

Spatiotemporal slip distributions of three long-term slow slip events beneath the Bungo Channel, southwest Japan, inferred from inversion analyses of GPS data

Spatiotemporal slip distributions of three long-term slow slip events beneath the Bungo Channel, southwest Japan, inferred from inversion analyses of GPS data

Two types of antigorite serpentinite controlling heterogeneous slow-slip behaviours of slab–mantle interface

It is known that plate boundaries in subduction zones show heterogeneous slip nature with strongly coupled seismogenic zones and various types of episodic tremor and slip (ETS) zones. In order to examine the petrological controls on the large-scale structure, we compared recent geophysical observations in the Shikoku area, southwest Japan with petrological models of the hanging wall mantle wedge. As a result, we found a close relationship between mineral assemblages in the mantle wedge and the characteristics of slow slip behaviour recorded in the Shikoku area: Short-term ETSs take place in the antigorite + olivine stability field and silent long-term slow slip events (SSEs) take place in the antigorite+brucite stability field. Based on observations of natural antigorite serpentinites, we propose a model that the dominant serpentinization reaction in the mantle wedge changes with increasing depth resulting in variable extents of pore fluid pressures along slip planes. The serpentinization reaction in the antigorite+brucite stability field (olivine + H2O → antigorite + brucite) proceeds at the expense of water. This is consistent with moderately elevated pore pressures inferred for long-term SSEs. The existence of weak brucite enhances the development of shear zones oblique to the main foliation. The resultant anastomosing network provides fluid pathways that may help reduce pore pressures on slip planes. In contrast, progress of the serpentinization reaction in the antigorite + olivine stability field (olivine + H2O + SiO2 → antigorite) results in a large amount of residual water that contributes to further increase pore fluid pressures on slip planes of short-term SSEs. Our results imply that understanding of serpentinization reactions and their contributions to fluid networks in mantle wedge is important in constructing quantitative 3-D models for strain evolutions along plate boundaries.

A long-term slow slip event from 1996 to 1997 in the Kii Channel, Japan

We present a long-term slow slip event from 1996 to 1997 in the Kii Channel along the Nankai Trough in southwestern Japan that has not been reported previously. The long-term slow slip event had a moment magnitude Mw 6.7 and a duration of 1 to 1.5 years. The magnitude of the event was smaller than those of the Tokai (Mw 7.1) and the Bungo Channel (Mw 7.0 to 7.1) events along the Nankai Trough. The slip was located slightly shallower than the depth of the low-frequency earthquakes around the Kii Channel. Long-term slow slips have been identified in various regions along the Nankai Trough, except in the Kii Peninsula. Unveiling the history of long-term slow slips is expected to contribute to an understanding of plate boundary characteristics as well as to the prediction of large earthquakes along the Nankai Trough.

Seismic quiescence and activation anomalies from 2005 to 2008 beneath the Kanto district, central Honshu, Japan

In the present study, an earthquake catalog is used that lists 1,197 earthquakes with M ≥ 3.9. All of the earthquake waveforms were recorded by the Earthquake Research Institute, University of Tokyo. These waveforms have been manually re-examined, and hypocenters and magnitudes re-calculated. A detailed analysis of the re-determined earthquake catalog between 1996 and 2007, using a gridding technique (ZMAP), shows a pair of seismic quiescence and activation anomalies that start around the middle of 2005, and last about 30 months. The pair of quiescence and activation anomalies are located very close to each other, and the Z-values are +5.0 and −3.8 for a time window of T w = 1.5 years, using a sample size of N = 100 earthquakes. The anomaly pair is not a coincidence as is confirmed by a numerical simulation with the assumption of random seismicity. One possible hypothesis is presented to explain the seismicity anomaly: a long-term slow slip event (LSSE) occurs on the upper boundary of the subducting Pacific plate, and the seismic quiescence and activation anomalies are caused by the Coulomb failure stress change associated with the LSSE.

Simulation of recurring earthquakes along the Nankai trough and their relationship to the Tokai long‐term slow slip events taking into account the effect of locally elevated pore pressure and subducting ridges

AbstractMany earthquakes of moment magnitudes greater than 8 associated with subduction of the Philippine Sea plate under Japan have occurred along the Nankai trough with a recurrence interval of 90–150 years. A large‐scale rupture of the Tokai fault segment is believed to be imminent because that segment did not rupture during the most recent Tonankai earthquake in 1944. Recurring long‐term slow slip events (LSSEs) have recently been observed in the Tokai region. To investigate the current stress state in the Tokai region, we numerically simulated earthquake cycles using a three‐dimensional model consisting of triangular cells based on the rate‐ and state‐dependent friction law. Our numerical simulations successfully modeled the Tokai segment not ruptured during the 1944 Tonankai earthquake as well as the recurring LSSEs in the Tokai region. We found that a large characteristic displacement (L) and effective normal stress (σ) in areas of subducted ridges are essential to avoid rupture of the Tokai segment during the 1944 Tonankai earthquake. We also demonstrated that we can reproduce the recurring LSSEs by assigning low values of σ and L to the area beneath Lake Hamana without introducing a velocity‐strengthening friction law at high slip rate. Our simulation showed that the amplitudes of the LSSEs increased following the earthquakes that did not rupture the Tokai segment, which suggests that the recent LSSEs may be related to the accumulation of stresses in the Tokai segment because it did not rupture during the 1944 earthquake.

高知市付近で１９７７〜１９８０年頃に発生した長期的スロースリップ

We extracted unsteady vertical crustal deformations in the Shikoku region before GPS deployment using leveling and sea-level data. GPS-derived, steady-state vertical displacements related to ongoing subduction of the Philippine Sea plate were subtracted from the vertical displacements observed by leveling surveys. Monthly mean sea-level data were corrected for atmospheric pressure and hydrographic effects. As a result, we found an upheaval of 4-5cm near Kochi City from 1977 to 1980 in both the leveling and sea-level data. This may represent a crustal deformation caused by a long-term slow slip event on the plate boundary.

Repetitive long-term slow slip events beneath the Bungo Channel, southwestern Japan, identified from leveling and sea level data from 1979 to 2008

[1] Leveling and sea level data for the period from 1979 to 2008 around the Bungo Channel, southwestern Japan, were investigated to characterize vertical deformation patterns. We found evidence of discrete events that may have occurred beneath the Bungo Channel before the two most recent long-term slow slip events (SSEs) (1996–1997 and 2003) detected from GPS data. GPS-derived steady state vertical displacements related to ongoing subduction of the Philippine Sea plate were subtracted from the vertical displacements observed by leveling surveys. The spatial pattern of the residual vertical displacements observed by leveling was similar to that observed by GPS in the periods including recent long-term SSEs. This suggests that discrete vertical displacements, which might represent long-term SSEs, have occurred in each of four intervals between leveling surveys before GPS deployment. We calculated differences of sea level between tidal stations near the Bungo Channel after some corrections and then cross-correlated them with the GPS-derived vertical displacements of the recent long-term SSE to estimate the timing of the events. These cross correlations and the discrete vertical displacements derived from leveling suggest that long-term SSEs may have occurred around 1980, around 1985–1986, and around 1991.

Gravity changes observed between 2004 and 2009 near the Tokai slow-slip area and prospects for detecting fluid flow during future slow-slip events

Slow-slip events (SSEs) have been observed in many plate-boundary zones along the circum-Pacific seismic belt. Previous studies have revealed that high-pressure fluids supplied from the subducted oceanic plate can generate SSEs. However, the behavior of these fluids during an SSE has not been fully elucidated. This paper discusses possible fluid migration along the plate boundary on the basis of spatiotemporal gravity changes observed by absolute and relative gravimeters during a long-term SSE in the Tokai district, Japan. Relative-gravity data are sometimes unreliable because of limited observation accuracies and possible noise produced by groundwater. Nevertheless, the observed gravity changes show a systematic pattern of spatial changes over the slow-slip area. This pattern can be explained by a poroelastic model assuming fluid migration along the plate interface, for which an inversion indicates a permeability of about 10−15 m2. This lies within the range of permeability values inferred by other studies in slow-slip areas. Long-term SSEs have occurred repeatedly in the Tokai district. If the permeability remains greater than 10−15 m2 during a future SSE, it will be possible to detect fluid migration by improving the observation accuracy to the 1-μGal level and accurately evaluating groundwater-related noise.

東海地域の長期的スロースリップイベントおよび地震サイクルの再現の試み

Long-term slow slip events with the recurrence period of about 10-30 years have been observed beneath Lake Hamana in the Tokai district where historically great interplate earthquakes occurred repeatedly. We intend to simulate the recurrent slow slip events before the occurrence of cyclic great earthquakes using a three-dimensional earthquake cycle model based on the rate- and state-dependent friction law with heterogeneous friction parameters on the plate interface. In our model we can simulate recurring slow slip events with the period of about 30-40 years near beneath Lake Hamana, by applying small negative values to frictional parameter (a-b)and small values to characteristic distance L for the western region of the Tokai district, and large values to L for the regions off Tokai district where seismic structure surveys reveal the existence of a subducting ridge.

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

In southwestern Japan, the Philippine Sea plate subducts beneath the Eurasian plate. Non-volcanic tremors and slow slow-slip events are observed beneath southwestern Japan. Seismic tomography based on the data obtained from the dense seismic network revealed the velocity structure in and around the subducting Philippine Sea plate. The oceanic crust of the Philippine Sea plate subducting from the Suruga and Nankai troughs can be imaged clearly beneath the Tokai region and the Kii peninsula. Non-volcanic tremors and short-term slow-slip events may occur in the high- V P/ V S zones owing to the fluid dehydrated from the subducting plate where the oceanic crust of the Philippine Sea plate encounters the serpentinized wedge mantle of the Eurasian plate. These tremors and short-term slow-slip events occur in the high- V P/ V S zone at depths of 30–40 km beneath the Tokai region, the landward edge of the high- V P/ V S zone beneath the Kii peninsula and eastern Shikoku region, and the seaward edge of the high- V P/ V S zone beneath the western Shikoku region. Long-term slow-slip events were observed beneath the Tokai region and the Bungo channel and both these events are located on the seaward side of a high- V P/ V S zone. These long-term slow-slip events are considered to be caused at the plate boundary between the oceanic crust of the Philippine Sea plate and the lower crust of the Eurasian plate by the high pore-fluid pressure caused by the fluid dehydrated from the subducting plate. The existence of the high- V P/ V S lower crust of the Eurasian plate may control the occurrence of the long-term slow-slip events.

Long-term slow slip events are not necessarily caused by high pore fluid pressure at the plate interface: an implication from two-dimensional model calculations

A recent tomographic study proposed that high-pore pressure in the deeper portion of the locked zone of a subduction thrust resulting from metamorphic dehydration reactions may cause long-term slow slip events. The study used the concept of ‘critical fault stiffness’, which derives from laboratory-derived rate- and state-dependent friction laws. To test the proposition, we execute 2-D model calculations using laboratory-derived rate- and state-dependent friction laws. Our numerical result is against the proposition, but it can also be explained by the concept of the critical fault stiffness. We agree that metamorphic dehydration reactions definitely produce a bulk property of high fluid saturation, but we caution that they do not necessarily lead to high-pore pressure in the fault zone.

Repeating short- and long-term slow slip events with deep tremor activity around the Bungo channel region, southwest Japan

AbstractWe report the repeating occurrence of short- and long-term slow slip events (SSE) which are accompanied by deep tremor activity around the Bungo channel region, southwest Japan. Both of these activities are detected by NIED Hi-net, which is composed of densely distributed observatories equipped with a set of tiltmeter and a high-sensitivity seismograph. Since the short-term SSE is small in magnitude, GPS can detect only the long-term SSE. Some of these episodes have nearly the same surface deformation pattern. This shows the existence of ‘slow slip patches’ on a plate interface, where the episodic slow slip is the characteristic slip behavior. We observe a change in periodicity and size of the short-term episode after the onset of the long-term SSE. Moreover, the long-term slow slip accelerates when the short-term activity takes place. This suggests that there is an interaction between these two types of SSEs.