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Evolution of Aseismic Slip Rate Along Plate Boundary Faults Before and After Megathrust Earthquakes ✉ Toshihiro Igarashi 1 & Aitaro Kato 1

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Evolution of Aseismic Slip Rate Along Plate Boundary Faults Before and After Megathrust Earthquakes ✉ Toshihiro Igarashi 1 & Aitaro Kato 1 ARTICLE https://doi.org/10.1038/s43247-021-00127-5 OPEN Evolution of aseismic slip rate along plate boundary faults before and after megathrust earthquakes ✉ Toshihiro Igarashi 1 & Aitaro Kato 1 Similar earthquakes that occur in approximately the same location have the potential to reveal the spatio-temporal changes in aseismic slip along plate boundaries. Here we identify similar earthquakes with moderate magnitudes that occurred worldwide between 1989 and 2016 by using seismograms recorded by the Japanese dense seismic network. The slip rate along the plate boundaries estimated from similar earthquakes increased rapidly following M > 8 megathrust ruptures and then gradually decayed over periods of ~10 years, which 1234567890():,; correlates with after-slip progressing around the source areas. More than 30 years after large megathrust earthquakes, the slip rate begins to show a gradual increase. This gradual increase in slip rate after the decay may be due to an increase in stress levels that accumulate during tectonic loading. The spatio-temporal characteristics of inter-plate aseismic slip can be used to provide a valuable framework for understanding the long-term evolution of slip-rate during megathrust earthquake cycles. ✉ 1 Earthquake Research Institute, the University of Tokyo, Tokyo, Japan. email: [email protected] COMMUNICATIONS EARTH & ENVIRONMENT | (2021) 2:60 | https://doi.org/10.1038/s43247-021-00127-5 | www.nature.com/commsenv 1 ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-021-00127-5 n earthquake is a sudden fast slip event that propagates In contrast, similar earthquakes were not detected in China, dynamically along a fault and releases elastic energy although previous studies have identified pairs of similar events in A 14 accumulated due to a locking of the fault during the inter- this region . The contrasting results of the present and previous seismic period. Growing evidence of geophysical observations has studies may have been caused by a difference in the analyzed demonstrated that subduction zone faults host a broad spectrum seismic phase and frequency band. of slip modes, from slow to fast slip1,2. This recent view is quite different from the classic notion of allowing only two discrete states of fault: locking and fast slip. Average slip rate along the subducting plate boundary. Based Earthquakes with similar waveforms are typically considered to on the spatio-temporal evolutions of similar earthquakes that occur at approximately the same location but at different times, occurred during the analysis period, we demonstrate that inter- and are referred to as similar earthquakes. Seismic observations plate similar earthquake sequences occurred more frequently after made over several decades in Japan have shown that many similar megathrust earthquakes. Other sequences of similar earthquakes earthquake sequences in the subducting plates are caused by have a constant recurrence interval of several years to several repeated slips in small patches surrounded by creeping aseismic decades. Pairs of similar earthquakes with the longest recurrence – regions along with the plate boundary3 5. These sequences are interval (~20 years) occurred in the Aleutian Islands. These also referred to as repeating earthquakes due to the recurring sequences may reflect spatio-temporal changes in inter-plate nature of seismic energy release, and they yield information on aseismic slip, and we, therefore, attempt to analyze such changes the spatio-temporal evolution of fault slip rate (i.e., creeping rate). in subduction zones around the Pacific and Indian oceans based Recently, similar earthquakes have been detected in many on similar earthquakes extracted in this study. regions around the world, based on similarities in seismogram To estimate the aseismic slip history, we assume that the slip characteristics and the inter-event distance of each earthquake rate during the inter-seismic period of similar earthquake pair. This includes subducting plate boundaries in Japan4, the sequences is equal to the slip rate of surrounding creeping areas Kuril–Kamchatka–Aleutian Islands, and the South Sandwich (see ‘Methods’). If the moderate-sized similar earthquakes Islands6, the Tonga–Fiji–Solomon region6,7, Sumatra8, and analyzed in this study have recurrence intervals that relate to Chile9,10. In addition, similar earthquakes have been detected in the plate convergence rate, the recurrence interval at the plate the plate collision zone in Taiwan11, along with transform faults boundaries with low average slip rates will exceed 10 years. The – in California12, from plate interiors13 19, in mines20, and at resolvable lower limit of the slip rate that can be estimated from intermediate-deep depths in subducting slabs6,21. the similar earthquakes in the 27-year interval used in this study The recurrent nature of similar earthquakes over relatively long is about 20–30 mm yr−1, depending on the recurrence time and periods of time can be used to directly analyze spatio-temporal magnitude of the similar earthquakes. changes in slip rate along inter-plate boundaries at depth4. This is Figure 2a shows the average slip rates in subduction zones normally difficult to achieve from surface geodetic measurements, estimated from similar earthquakes that occurred at depths of especially in offshore regions that contain few geodetic stations. <100 km. The average slip rates vary laterally between different In addition, changes in seismogram characteristics at the time of subduction zones, and in some cases along the length of the same recurrence can be used to investigate temporal changes in seismic subduction zone. Figure 2b shows the relationship between the velocity within the Earth22,23. In this study, we detected relative velocity of the two converging plates, taking into account moderate-sized similar earthquakes around the world by calcu- back-arc spreading24; the estimated slip rates in each region are lating the cross-correlation coefficients of bandpass-filtered seis- shown in Fig. 2a. The estimated slip rates in several areas are mograms recorded by the Japanese dense seismic network. Based higher than the relative velocity of the two converging plates. on these data, we investigated the spatial distributions and tem- These areas correspond to regions in which postseismic slip has poral characteristics of inter-plate aseismic slip at a number of occurred recently following giant earthquakes, including the 2004 subduction zones around the world. Sumatra–Andaman earthquake (Mw9.1) in area 12, the 2005 northern Sumatra earthquake (Mw8.6) in area 13, the 2006 Kuril earthquake (Mw8.3) in area 2c, the 2007 southern Sumatra Results earthquake (Mw8.4) in area 14, and the 2011 Tohoku-Oki Spatial distribution of similar earthquakes. We used seismic earthquake (Mw9.1) in area 2f. These areas show large error bars waveform data collected by dense nationwide seismic networks due to the spatio-temporal variation of the slip rates linked to the located across the Japanese islands (Fig. 1). To extract similar postseismic slip (Fig. 2b). The increase of slip rate in area 5 near earthquakes, we calculated the cross-correlation coefficient of the the Kanto District in Japan was induced by the 2011 Tohoku-Oki vertical component in bandpass-filtered seismograms at each earthquake25. The recurrence intervals of similar earthquake station (see ‘Methods’). The hemisphere map centered on Japan in sequences decreased significantly following these recent giant Fig. 1 shows the distribution of the cross-correlation coefficients earthquakes, resulting in an increase in the average slip rate. obtained in this study. We identified numerous similar earth- In contrast, we recognized several areas where the slip rates are quakes that occurred along subducting plate boundaries, including much lower than the relative plate velocities, including the the Andaman–Sumatra–Java, the Solomon–Vanuatu–Tonga, Kuril–Kamchatka–Aleutian (areas 1–2), Nankai–Ryukyu (areas and Japan–Kuril–Kamchatka–Aleutian subduction zones. These 6–7), Manila (area 9), Philippines (area 10), and Solomon (areas similar earthquakes are linked to aseismic fault slip along shallow 16–20) subduction zones. These observations indicate that the inter-plate boundaries at depths of <100 km. The similar plate boundary faults in these regions are strongly locked, and earthquakes are distributed heterogeneously, even within single that elastic strain energy has been accumulating within each of subduction zones that host megathrust earthquakes. Similar these subduction zones. earthquakes were also identified at depths of >100 km in regions Slip rates in some areas are almost equal to the rate of relative where intra-slab earthquakes occur frequently (e.g., the Solomon, plate motion, including the northern extents of the 1952 and 2011 Tonga, and Izu–Ogasawara regions). These deeper events might M9 earthquakes, the Izu–Bonin–Mariana region (areas 3–4), and have been detected due to relatively simple source processes, the parts of the Solomon subduction zone (areas 16–20). These relatively homogeneous structure near the source, and a nearly regions may host episodic aseismic slip events, including slow slip identical path to radiated seismic waves. events and/or creeping movement. The Tonga subduction zone 2 COMMUNICATIONS EARTH & ENVIRONMENT | (2021) 2:60 | https://doi.org/10.1038/s43247-021-00127-5 | www.nature.com/commsenv COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-021-00127-5
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