Slow slip events (SSEs) in subduction zones usually occur on the downdip/updip ends of the seismogenic zone or the adjacent sides (e.g., Bürgmann 2018; Rogers and Dragert 2003; Schwartz and Rokosky 2007). The close spatial distance from SSEs to the seismogenic zone raises a question: whether large earthquakes can affect the recurrence intervals of the nearby SSEs. The SSE recurrence intervals keep constant approximately between two large earthquakes and the constant increases with the duration time of SSEs (e.g., Matsuzawa et al. 2013; Obara and Kato 2016; Ozawa et al. 2007). Numerical modeling results suggest that SSE recurrence intervals will be shortened after a large earthquake and then increase gradually with time, recover to the constant (e.g., Luo and Liu 2019; Matsuzawa et al. 2010; Segall and Bradley 2012). The shortening of one SSE recurrence interval has been observed after large earthquakes immediately, triggered by coseismic dynamic stresses from the passing seismic waves (Katakami et al. 2020; Wallace et al. 2017; Wei et al. 2018). However, the observation in nature similar to those numerical modeling results is very rare. Recently, increasing recurrence intervals of SSEs are detected after the 2011 Mw 9.0 Tohoku earthquake in the Boso region, Japan (Hirose et al. 2012; Luo and Liu 2019; Ozawa et al. 2019). The recurrence intervals before the Tohoku earthquake are approximately 4 to 6 years, which shortens to 0.6, 2.2, and 4.4 years after the earthquake (Ozawa et al. 2019). Stress perturbations during coseismic and postseismic periods can be a reason for the increasing recurrence intervals of the Boso SSEs. Coseismic stress perturbations, either the dynamic or static types sustained only a short period during the coseismic slip. Afterslip usually accompanies a long-time postseismic stress perturbation. It may thus be the dominant influence in the increasing recurrence intervals of SSEs. The argument remains unsure because another SSEs sequence in Costa Rica has demonstrated a counterexample. The recurrence intervals of the Costa Rica SSEs before the 2012 Mw 7.6 Nicoya earthquake are approximately 1.8 to 2.0 years that did not change after the earthquake (Voss et al. 2017; Xie et al. 2020). Besides, the Kapiti SSEs sequence in New Zealand is well-recorded with a long history as well. The recurrence intervals before the 2016 Mw 7.8 Kaikōura earthquake are approximately 5.0 years, have been shortened to 0.5 years after the Kaikōura earthquake (Shibazaki et al. 2019; Wallace et al. 2018). This observation has a chance to support the Boso SSEs study if the future Kapiti SSEs keep the increasing recurrence intervals with time. So far, the influence of afterslip on SSEs recurrence intervals remains unclear.
The southernmost Ryukyu subduction zone has converged rapidly at 12.5 cm/yr in the N-S direction (Hsu et al. 2012), which includes a 5.0 cm/yr southward back-arc spreading from the Okinawa Trough (Nishimura et al. 2004). Whereas, magnitudes of the large earthquakes did not exceed Mw 7.7 over the last 300 years (Cheng and Yeh 1989; Theunissen et al. 2010). The most recent large one was the 2002 Mw 7.1 earthquake located in the offshore Hualien area at approximately the depth of 16 km (Fig. 1a). In this subduction, we report an observation of SSEs sequence in 2005, 2009, and 2015 following the 2002 Mw 7.1 Hualien offshore earthquake afterslip. The 2005 SSE may have nucleated near the updip region of the Mw 7.1 earthquake, and the 2009 and 2015 SSEs occurred near the downdip region (Fig. 1a). The SSEs are adjacent to the Mw 7.1 earthquake hypocenter with increasing intervals of 3.1, 4.2, and 6.2 years after the earthquake (Fig. 1b). The observation points out an important question. Whether stress perturbations during coseismic and postseismic periods of the Mw 7.1 earthquake can affect the SSEs intervals? To this end, we calculated the coseismic and postseismic slips from the Mw 7.1 earthquake in the subduction zone. Then Coulomb stress changes are estimated based on both the slips on the SSEs region. The Coulomb stress changes were examined to identify whether they were sufficient for an increase of SSE intervals. We find that SSE regions were very likely overlapped by the afterslip region of the Mw 7.1 earthquake. The 2005 SSE region has been imposed by higher positive Coulomb stresses than the 2009/2015 SSEs region. The afterslip lasted from 2002 April to at least early 2007 with a time-decaying stress rate. The continuous positive stress loads may be sufficient for the triggering of 2005 SSE and affect the 2009 SSE. Our study provides an observation of SSEs sequence following afterslip that can support the Boso SSEs case.
Tectonic background of the southernmost Ryukyu subduction zone
In the southernmost Ryukyu subduction zone, the Philippine Sea Plate subducts northward beneath the Eurasian Plate to form the back-arc rifting (Sibuet et al. 1987). At the southernmost end, the Eurasian Plate subducts eastward beneath the Philippine Sea Plate at the Taiwan orogen; the convergence rate is approximately 8.0 cm/yr in the 310° direction (e.g., Chen et al. 2014, 2017; Hsu et al. 2009). There is an additional 5.0 cm/yr southward spreading from the Okinawa Trough. Thus, the N-S direction convergence rate reaches 12.5 cm/yr between the Ryukyu Trench and the Yonaguni Island (Fig. 1a). Three areas have been proposed on this subduction interface: (1) the Interplate Seismogenic Zone (ISZ; Kao 1998), (2) Ryukyu Fault (Hsu et al. 2012), and (3) Slow Slip Zone (SSZ; Chen et al. 2018). The ISZ is a seismogenic zone that many earthquakes are with magnitudes greater than Mw 5.0. This zone is between the Ryukyu Trench and the Yonaguni Island at depths of approximately 20‒40 km (Fig. 1a). The 2002 Mw 7.1 Hualien offshore earthquake was situated at the western end of the ISZ and above its interface in the overriding plate (Fig. 1a). The Mw 7.1 earthquake was less studied due to the lack of near-field observations, only for the coseismic displacement (Chen et al. 2004), seismic wave propagation (Lee et al. 2009), and afterslip (Nakamura 2009). The afterslip may have lasted over five years and leads to long stress perturbations in the subduction zone. The Ryukyu Fault is a locked region where megathrust earthquakes might occur (Hsu et al. 2012). The locked dimension remains unclear, estimated by only the Global Navigation Satellite System (GNSS) observations in the eastern Taiwan orogen. If the dimensions reached the maximum value (Fig. 1a), the magnitude of the earthquake would be 7.5 ≤ Mw ≤ 8.7 (Hsu et al. 2012). The longer the earthquake cycle is, the larger the earthquake size will be. The SSZ is an SSE region next to the Taiwan orogen where SSEs occurred in 2005, 2009, and 2015 at depths of approximately 15‒45 km (Chen et al. 2018). SSEs in this region were difficult to be identified before the 2002 Mw 7.1 Hualien offshore earthquake because the onshore GNSS observations were limited (Fig. 2). Current GNSS observations revealed that 2005 SSE might have occurred on the Ryukyu Fault. Some SSEs have been observed coexisting in the seismogenic zone or the locked region (e.g., Dixon et al. 2014; Ito et al. 2013). The 2009 and 2015 SSEs recurred on the same patch near the downdip end of Ryukyu Fault as regarded as repeating SSEs. The three SSEs lasted from 2 to 4 months with a potential maximum size, Mw 6.4 to 6.6. The SSEs were likely originated from a high VP/VS ratio zone on the subduction interface (Huang et al. 2014) and are accompanied by overriding plate seismicity with maximal magnitudes greater than Mw 5.0. The peak slip of the three SSEs is adjacent to a high b-value region in the northeastern Taiwan orogen (Wu et al. 2018). A state of low differential stress may thus appear in that region (Scholz 2015). These observations agree with a broad consequence of SSEs that usually occur in the state of rich high-pressure fluids, low effective stress, and transitional friction (e.g., Bürgmann 2018; Saffer and Wallace 2015; Schwartz and Rokosky 2007). The close distances from the SSEs to the Mw 7.1 earthquake allow us to investigate the relation between afterslip and SSE intervals.