Detection by network inversion filter of long-term and short-term slow slip events, using a very dense GNSS Network in Shikoku, Japan, during 2018–2022

doi:10.21203/rs.3.rs-1962333/v1

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Detection by network inversion filter of long-term and short-term slow slip events, using a very dense GNSS Network in Shikoku, Japan, during 2018–2022

https://doi.org/10.21203/rs.3.rs-1962333/v1

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A study of slow slip events is important in understanding the mechanism and friction properties of plate interfaces. We imaged the state of 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 of the Global Navigation Satellite System from January 2018 to April 2022. The results of network inversion filtering show 14 short-term slow slip events associated with low-frequency earthquakes along with long-term slow slip events in the Bungo Channel, central Shikoku, and Kii-channel Japan. The long-term slow slip events in the Bungo, central Shikoku, and Kii-channels are spatially separated from each other, suggesting independent events. The recurrence interval of the central Shikoku slow slip event remains unclear. A Kii-channel slow-slip event occurs with an irregular recurrence interval. The estimated moments of the Bungo, central Shikoku, and Kii-channel slow slip events were Mw7.0, Mw6.5, and Mw6.3, respectively, with a rigidity of 30 GPa. The estimated moment magnitude of the short-term slow slip events ranges from Mw5.4 to Mw6.2. The major detected short-term slow slip events are consistent with the detected events from tilt meter analysis by the National Institute of Earth Science and Disaster Resilience of Japan. Most of the detected slow slip events are associated with low-frequency earthquakes, and some exhibited spatial propagation. Spatio-temporal relationship between low-frequency earthquakes and aseismic slip may provide us with the insight into the physical processes between them.

Recent developments in geodetic networks have revealed various slow-slip events (SSEs) worldwide (e.g., Schwartz and Rokosky 2007; Graham et al., 2016; Rogers and Dragert 2003). SSEs occurred in the transition area between the locked and creeping zones. The duration, recurrence interval, magnitude, and depth of the slip area varied widely. The properties of SSEs appear to be controlled by the friction property in a slip area (e.g. Rubin 2008). Furthermore, some SSEs trigger large earthquakes nearby (e.g., Radiguet et al., 2016; Ruiz et al., 2017). Thus, it is very important to study SSEs in detail to understand the mechanism and friction properties of plate interfaces in many places. Shikoku, in southwest Japan, is located where the Philippine Sea plate subducts beneath the Amur plate along the Nankai Trough in the direction of N57°W at a rate of 5–6 cm/year (Kreemer et al., 2014). Because of the subduction of the Philippine Sea plate, mega-thrust earthquakes have occurred off Shikoku Island with a recurrence interval of 100–150 years (e.g., Kumagai 1996). At this point, the locked zone off Shikoku Island is a well-known seismic gap (Fig. 1B). The probability of the next Nankai earthquake within 30 years is approximately 80% (CDMC, 2013). Thus, this area is quite important from the view point of hazard assessment.

Based on the duration, there are two types of SSEs at the plate interface between the subducting Philippine Sea plate and the overriding Amur plate along the Nankai Trough subduction zone in Japan. One had a duration of several months, and the other had a duration of several days. The former is called a long-term SSE (l-SSE) and the latter is called a short-term SSE (s-SSE). s-SSEs are associated with low-frequency earthquakes (LFEs) and ground displacements of several millimeters (Obara 2002; Hirose and Obara 2005; Nishimura et al., 2013). The depths of the s-SSE and LFEs ranged from 30 to 50 km. In contrast, l-SSEs occur in an up-dip extension of s-SSEs and a down-dip extension of locked zones, where mega-thrust earthquakes occur. The depth of the l-SSE ranges from 20 to 30 km on Shikoku Island (Takagi et al., 2019; Kobayashi 2012; Ozawa et al., 2020). In the Bungo Channel, l-SSEs occurred every 5 to 6 years since 1997 with moment magnitude ranging from 6.8 to 7.1 (Hirose et al., 1999; Ozawa et al., 2001; Ozawa et al., 2013; Ozawa et al., 2020). The detailed spatiotemporal evolution of Bungo aseismic slips has been reported in many studies (e.g., Ozawa et al. 2020; Yoshioka et al., 2015; Liu et al., 2010). Ozawa et al. (2019) reported the latest occurrence of a Bungo l-SSE in October 2018. The main slip of the Bungo l-SSE occurred between January and February 2019. Ozawa et al. (2019) proposed that this slow slip was delayed by the 2016 Kumamoto earthquake (Kobayashi et al., 2018). The West Shikoku l-SSE occurred with a recurrence interval of 6 years (Takagi et al., 2019). The estimated moment ranges from Mw6.2 to Mw6.4. Takagi et al. (2019) reported that the west Shikoku l-SSE tended to occur following Bungo SSEs. The Central Shikoku l-SSE occurs independently or is associated with the western Shikoku l-SSEs (Takagi et al., 2019; Kobayashi 2012). The East Shikoku l-SSE may have occurred once between 2000–2004 (Kobayashi et al., 2017). Kii-channel l-SSEs occurred in 1996, 2000–2004, and 2014–2016 (Kobayashi 2014; Kobayashi et al., 2017).

The detection of l- and s-SSEs by the Global Navigation Satellite System (GNSS) network was successfully reported using Monte Carlo mixture Kalman filtering (MCMKF) in the Tokai region, central Japan (Sakaue et al., 2019). However, the details of s-SSEs remain unclear because of the small number of cases and the small ground displacements associated with them. In particular, the time evolution of s-SSEs is not well-documented in the Shikoku area. Furthermore, the relationship between l-SSEs and s-SSEs has not been well researched. Since 2019, l-SSEs have been occurring in the Bungo Channel area, Japan (Ozawa et al., 2020), and the examination of SSEs on Shikoku Island is a good opportunity to examine the l- and s-SSEs together. We applied the network inversion filter (NIF) (McGuire and Segall, 2003) to the GNSS position time series on Shikoku Island, Japan from 2019–2022 and estimated time evolution of the l- and s-SSEs. Furthermore, we also analyzed and estimated s-SSEs between 2012–2018, which has not been reported in previous studies.

GNSS data were analyzed daily using Bernese GPS software (version 5.2) with precise ephemeris and earth rotation parameters (Nakagawa et al., 2009). We transformed the coordinates of longitude, latitude, and height to east–west, north–south, and up–down components in a local framework, respectively. After this transformation, we removed the annual components estimated for the period between October 2019–April 2022 by regressing the trigonometric and polynomial functions (Ozawa et al., 2020). We detrended the position time series from the linear trend estimated for the period from January 2017–January 2018.

We applied NIF (McGuire and Segall, 2003; Ozawa et al., 2017) to the east–west, north–south, and up–down components at 144 selected GPS sites (see Fig. 1B) for the period between January 1, 2019–April 24, 2022. The horizontal and vertical components were weighted at a ratio of 5:1 in the following analysis, reflecting coordinate repeatability at representative sites. In addition, we analyzed the data for the period 2012–2018. We applied NIF to the data sets of one and a half years partitioning the period between 2012–2018, because analysis of all data from 2012 and 2019 is computationally difficult.

We examined the occurrence of s-SSEs by evaluating the estimated slip during a 10-day time window. After examining the occurrence of s-SSEs, we applied NIF for a short period of time spanning the s-SSEs and estimated the spatiotemporal evolution in detail.

In the NIF, we adopted an information square-root filter instead of a Kalman filter (Kitagawa and Gersch, 1996; Ozawa, 2017). We used a parallelogram consisting of two triangular elements to represent the fault interface. We adopted the geometric model of the Philippine Sea Plate proposed by Hirose et al. (2008). The range of the adopted plate interface model is shown in Fig. 1B. The grid spacing was approximately 10–14 km. Green’s function was calculated by summing the Green’s functions of two triangular elements, following the study by Meade (2007).

We adopted the following state in the NIF (Ozawa et al., 2020):

x(t) = (u₁, v₁, --, u_k, v_k, --u_L, v_L, r₁, --,r_k –r_P, e_EW, e_NS, e_ud) (1)

where u_k indicates the slip and v_k indicates the slip velocity at the kth grid on the plate interface with the total grid of L. r_k represents a random walk motion at the kth site with total sites of P. e_EW, e_NS, and e_UD indicate common mode errors in the position time series for the east–west, north–south, and up–down components (Fukuda et al., 2018). We incorporated spatial roughness in the transition equation as follows (Ozawa, 2017):

λ (M u_n|n−1- M u_n−1|n−1)/Δt = e₁

λ M v_n|n−1 = e₂ (2)

where M is the roughness matrix, and λ is the hyperparameter of spatial smoothing. u_n|n−1 and u_n−1|n−1 are the predicted slip at time n based on the data until time n–1, and the estimated slip at time n-1 based on the data until time n–1, respectively. Δt is the time lapse. v_n|n−1 is the predicted slip velocity at time n, based on the data until time n–1. e₁ and e₂ are Gaussian noises with a mean of 0 and standard deviation of 1. As a roughness matrix, M is generated to represent u ^TM^TMu as the integral of the square of the Laplacian of parameter u.

We constrained the aseismic slip vector and slip velocity vector on the plate surface to the direction opposite to the motion of the Philippine Sea plate, relative to the Amur plate, using the method of Simon and Simon (2006). The direction of the Philippine Sea Plate relative to the Amur Plate was obtained from Kreemer et al. (2014).

The hyperparameter l is taken as a value that produces consistent results as seen with that of previous studies (e.g., Ozawa et al., 2020). The temporal hyperparameter was taken at a value that produces s-SSEs, consistent with that of other studies (e.g., Nishimura et al., 2013).

We conducted a checkerboard test by inverting the synthesized data with Gaussian errors, with 1 mm standard deviation (SD) added for horizontal components and 5 mm SD added for vertical components. The resolution near Shikoku in our inversion method is shown in Supplementary Figure S1, indicating that the resolution is sufficient to resolve slips near Shikoku.

The s-SSE causes very small displacements. Thus, we regressed a ramp function on the position time series without common-mode errors to obtain step-like displacements associated with s-SSEs. Common-mode errors were estimated using NIF. To fit the position time series, the time constant and start time of the ramp function, were determined by considering the NIF results. The estimated displacements were used for comparison with the calculations by the model.

Figure 2 shows the position time series at the selected stations until April 2022 (Fig. 1B). We observed long-term southeastward transients with uplifting in 950437, 051142, which is associated with the l-SSEs in this area. The Bungo l-SSE occurred in late 2018 and subsided in August 2019 (Ozawa et al., 2020). We observed long-term transients at stations 940083 and 021051 in central Shikoku, which also suggests an l-SSE in this area. 940083 shows an eastward transient from early 2019, with uplifting. 021051 showed a northeastward transient with uplift from early 2019. The 131198 and 03P216 sites in eastern Shikoku showed eastward transients from early 2019, suggesting an l-SSE in the Kii-channel.

Figure 3 shows the spatial pattern of the transient associated with the l- and selected s-SSEs. We can see a southeastward transient in the Bungo channel and an eastward transient in central Shikoku associated with the l-SSEs. The horizontal transient in the Bungo Channel amounted to 5 cm for the period between January 2018 and April 2022, whereas that in central Shikoku reached up to 1 cm, indicating SSEs in the Bungo Channel and central Shikoku (Fig. 3A). Northeast Kyushu showed subsidence of up to 1 cm, and southwest Shikoku showed an uplift of up to 5 cm, which indicates SSEs in the Bungo-channel area (Fig. 3B). The central Shikoku area showed an uplift of approximately 1 cm, suggesting the occurrence of l-SSEs in this area (Fig. 3B). We can also see eastward displacements of up to 1 cm in east Shikoku adjacent to the Kii-channel, indicating an l-SSE in the Kii-channel (Fig. 3B).

Associated with the s-SSE, we can see relatively clear displacements in a selected case, which are estimated by regressing a ramp function to the position time series without common-mode errors. As mentioned before, we estimated short-term SSEs by using a position time series spanning the period of each case, which was first examined using a long period of position time series. As shown in Fig. 3E, a southeastward transient of up to 5 mm was observed in the case of the January 2021 s-SSE. These southeastward transients indicate that the continental plate slipped southeastward at the plate interface for a short period of time.

Figure 4 shows the main l- and s-SSEs detected. The l-SSEs were observed in the Bungo channel, central Shikoku, and Kii-channel (Fig. 4A). Since the Bungo l-SSE was reported by Ozawa et al. (2020) in detail, we briefly describe this study’s results. The estimated moment of the Bungo l-SSE was approximately Mw7.0 with a rigidity of 30 GPa. The Bungo l-SSE was detected in late 2018 and subsided around August 2019, as reported by Ozawa et al. (2020) (see Supplementary Figure S2).

Hereafter, we report on the central Shikoku SSE. The central Shikoku l-SSE was reported to have occurred between 1977 and 1980 in a leveling survey (Kobayashi 2018). After the GNSS network installation, Takagi et al. (2019) reported cases in 2010, 2012, and 2015, with events extending to western Shikoku in 2010 and 2015. Thus, the recurrence interval is not well determined, and details of the central Shikoku SSE remain elusive. The 2020 event was consistent with past events. The Central Shikoku SSE started in early 2019 and continued until April 2022, with fluctuations over time (Supplementary Figure S3). The slip area moved southwest from 2019 to 2022 (Supplementary Figure S4). It remains unclear whether the slip area extends to the western Shikoku SSE patch. The moment magnitude of the central Shikoku SSE was estimated to be 6.5 with a rigidity of 30 GPa. The model of Kobayashi (2011) indicates Mw6.8 with a rigidity of 30 GPa, at the time of the 1977–1980 event. Takagi et al. (2019) reports Mw6.6 for the 2010 event, which extends to west Shikoku, Mw6.3 for the 2012 event and Mw6.2 for the 2015 event, respectively, which all extend to west Shikoku. The 2010 event reported by Takagi et al. (2019) is relatively similar to the event in this study.

The Kii-channel l-SSE started in early 2019 and continued until April 2022 (Supplementary Figure S5). The slip magnitude decreased from 2019 to 2020, and increased from 2021 to 2022 (Supplementary Figure S4). The Kii-channel l-SSE appears to occur at irregular recurrence intervals. The estimated moment magnitude of the Kii-channel l-SSE was 6.3 with a rigidity of 30 GPa. Because the Kii-channel l-SSE is near the edge of the analysis area, we cannot describe its features in detail, and it remains to be studied in a future study.

In comparison, our optimal l-SSE model reproduces the detected transient well, as shown in Figs. 2 and 3A-D.

We detected 14 s-SSEs associated with LFEs between 2018 and 2022. The selected detected short-term SSEs are shown in Figs. 4(B)–(L).

The slip area of the s-SSEs mostly coincided with that of the LFEs area. From February to March 2018, an Mw 6.2 s-SSE occurred in northwest Shikoku. At this time, the slip area of the s-SSE propagated eastward over time. Figures 4(B)–(C) show the slip propagation, which coincided with the propagation of LFEs. An Mw6.1 s-SSE occurred in northwest Shikoku from September to October 2018, associated with LFEs (Fig. 4D). An Mw6.0 s-SSE occurred in north central Shikoku from October to November 2018 (Fig. 4E). LFEs coincidentally occurred in the slip area. An Mw6.0 s-SSE occurred from northwest to northcentral Shikoku from February to March 2019 (Fig. 4F). In August 2019, an Mw6.2 s-SSE occurred northwest to north central Shikoku (Fig. 4G). In September 2019, an Mw5.9 s-SSE occurred in northwest Shikoku (Fig. 4H). In February 2020, an Mw6.0 s-SSE was centered in northwest Shikoku (Fig. 4I). The main slip area of the January 2021 s-SSE is similar to that of the March 2019 event. The July 2021 s-SSE shows a slip extending to the long-term SSE area, which remains to be examined in a future study. The April 2022 s-SSE centers in an area similar to that of the March 2019 event. Thus, the maximum slip area of the March 2019 event slips frequently.

Figure 3D and some cases in Supplementary Figure S6 show clear ground displacements, which are in good agreement with the computation (see Fig. 3E and Supplementary Figure S6).

The propagation of the slip area and LFE, as shown in Figs. 4B-C, has been reported in many cases (e.g. Sakaue et al 2019). The 2021 Mw6.1 January event also shows eastward propagation, as shown in Supplementary Figure S6. It appears that the direction of propagation depends on each case. Several other propagation cases are presented in Supplementary Figure S6.

S-SSEs are mostly accompanied with LFEs. We used the catalog of the Meteorological Agency of Japan (JMA). The catalog of JMA is slightly different from that of National Research Institute for Earth Science and Disaster Resilience (NIED). Thus, we cannot describe the relationship between aseismic slip area and LFEs in detail. The physical mechanism of LFEs and s-SSEs remains unresolved. One explanation is that the stress change in the s-SSE drives LFEs. Other explanation is that the LFEs are the results of aseismic slip of the s-SSEs. The detailed spatial relationship between s-SSEs and LFEs may provide insight into the physical processes of s-SSE and LFEs.

The relationship between l- and s-SSEs remains unclear. The s-SSEs detected in northwest Shikoku seem to be correlated with the slip of the central Shikoku l-SSE, as shown in Supplementary Figure S3. When an s-SSE occurs in northeast Shikoku, central Shikoku slips simultaneously, in many cases. Whether this is a mere mixing of the s-SSE area into an l-SSE by spatial smoothing or real correlation between s-SSEs and l-SSEs remains to be determined in a future study.

Supplementary Figure S7 shows the s-SSE events beneath Shikoku for the period 2012–2022. Our analysis demonstrates the frequent occurrence of s-SSEs in west to central Shikoku, whereas the number of s-SSEs in east Shikoku is small in comparison. The estimated moment ranges from 5.4 to 6.4 for the entire period. The recurrence interval is not clear in Supplementary Figure S7. The number of events detected by NIF is small compared with those of other methods, such as Nishimura et al. (2013) or tilt meter analysis by the NIED. However, the main detected events are mainly consistent with the results of the tilt meter results by the NIED (see Supplementary Figure S8). Thus, we believe that ordinary NIF is successful in detecting major s-SSEs in the Shikoku area based on GNSS data. Supplementary Figure S6 shows the detected s-SSEs for the entire period together with the detected displacements and model computation.

These detected events may lead to an understanding of the friction property at the plate interface and the dynamics of SSEs.

We applied ordinary NIF to the position time series and successfully detected major s- and l-SSEs together. Three l-SSEs occurred along the Nankai Trough and were spatially isolated from each other. Except for the Bungo l-SSE, recurrence interval of central Shikoku and Kii-channel l-SSEs is unclear. S-SSEs are accompanied with LFEs in most cases. The detailed spatial and temporal evolution of s-SSEs show slip propagation in some cases. These findings may provide us with insight into the relationship between s-SSEs and LFEs and their physical processes. The physical mechanism behind s-SSEs remains to be elucidated in future studies.

GNSS

Global Navigation Satellite System

l-SSE

long-term slow slip event

s-SSE

short-term slow slip event

LFE

low-frequency earthquake

NIF

network inversion filter

JMA

Meteorological Agency of Japan

NIED

National Institute of Earth Science and Disaster Resilience

Availability of data and materials

The data used in this paper are available by contacting [email protected].

Competing interests

We have no competing interests to declare.

Funding

This research is funded by the special research project of the Geospatial Information Authority of Japan.

Authors’ contributions

S.O. and H.M. analyzed the data. S.O. wrote the manuscript.

Acknowledgements

We are grateful to our colleagues for their helpful discussion. Low-frequency earthquake data were provided by the Meteorological Agency of Japan. We also used the catalog of National Institute of Earth Science and Disaster Prevention. We used fault parameters of s-SSE by the NIED published in the Coordinating Committee of Earthquake Prediction.

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Graph2.png
suppfig2.pdf
Supplementary Figure S1. Checker board test. (A) Synthetic slip (B) Inverted slip.Supplementary Figure S2. Time evolution of slip on grid in the Bungo Channel.Supplementary Figure S3. Time evolution of slip on grid in central Shikoku. Red vertical lines indicate the dates of s-SSE in northwest Shikoku.Supplementary Figure S4. Slip distribution from 2019–2022.Supplementary Figure S5. Time evolution of slip on grid in the central Kii-channelSupplementary Figure S6. Detected s-SSEs and observed and computed displacements.Supplementary Figure S7. Detected s-SSEs in this study.Supplementary Figure S8. Detected s-SSEs in this study (black) and those by the NIED based on tilt meter (red).

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Detection by network inversion filter of long-term and short-term slow slip events, using a very dense GNSS Network in Shikoku, Japan, during 2018–2022

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