Repeating deep tremors on the plate interface beneath Kyushu, southwest Japan
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 2 March 2012
Accepted: 11 June 2012
Published: 19 February 2013
In the subduction zone south of Kyushu Island, at the western extension of the Nankai subduction zone, southwest Japan, the age of the oceanic crust increases toward the south across the subducting Kyushu-Palau ridge. While tremor activity is very high in Nankai, tectonic tremors have only recently been discovered in Kyushu. In this study, we examined tremors beneath Kyushu using an improved version of the envelope correlation method. In doing so, we distinguished tremors from normal earthquakes and background noise using the criteria of source duration and the spectrum ratio between low and high frequencies. Accurate measurement of S-P times, using cross-correlation between vertical and horizontal seismograms, constrains the tremor depth precisely. Tremor activity is low and within a small region in southern Kyushu, where thick crust of the Kyushu-Palau ridge is being subducted, at depths between 35 and 45 km (i.e., shallower than intra-slab earthquakes by about 20 km), which is consistent with the location of the plate interface within uncertainties proposed in previous studies. Establishing precise depth estimates for tectonic tremors beneath Kyushu, which results from shear slip along the plate interface, is useful in defining the plate interface within the Nankai subduction zone.
1. Introduction and Tectonic Setting
Tectonic tremors within the Nankai subduction zone were first documented by Obara (2002), and they are often accompanied by slow slip events (SSEs) and considered to represent shear slip within the transition zone between stick-slip and stable slip along the plate interface, based on their locations and focal mechanisms (e.g., Shelly et al., 2006, 2007; Ide et al., 2007). Fluids released by the dehydration of a subducting slab are thought to occur at high pore pressures in areas of tremor sources (Shelly et al., 2006), thereby reducing the effective normal stress and enhancing the sensitivity of tremor activity to tidal stresses and large surface waves. For recent reviews of tremor characteristics, see Rubinstein et al. (2010), Obara (2011), and Beroza and Ide (2011).
The distribution of tremor in the Nankai subduction zone terminates at the Bungo channel, across which several characteristics of the subduction zone change dramatically, including seismicity, volcanic activity, and the geometry and age of the subducting plate. The relatively young and warm PHS plate within the Nankai subduction zone must exist at the required conditions to enable tremor activity, because both slip transition and active dehydration in the subducting slab occur at shallower regions of the plate interface. In contrast, the occurrence of tremor activity is not favored by the relatively old plate being subducting south of Kyushu. Nevertheless, the Japan Meteorological Agency (JMA) detected and located two low-frequency earthquakes (LFEs) at a forearc region in the southern Kyushu area (Fig. 1(b)) during a routine analysis, implying the existence of tectonic tremor activity in the area. Ide (2012) reported such tremor activity in this region. The main focus of the present study is to conduct a more detailed investigation of tremor activity in the southern Kyushu area and to accurately constrain the depth of these seismic events.
2. Detection and Hypocenter Determination of Tremor Events
2.1 Detection and location of tremors by automated analysis
The 1D velocity structure used in this study, following Saiga et al. (2010).
S-wave velocity (km/s)
P-wave velocity (km/s)
As an extension of this method of tremor detection and location, we also compared the power spectrum ratios observed for each set of data—i.e., the ratio of the mean power at low frequencies (2–8 Hz) to the mean power at high frequencies (10–20 Hz) calculated from the original 100 sps data—based on the fact that tremors have a stronger power at low frequencies (Shelly et al., 2006). If the average of the spectrum ratios for all stations within 50 km of each epicenter is higher than 10, the signal is ultimately considered to be a tremor. As a result of employing this new criterion, most of the detected events were rejected as either earthquakes or artifacts of background noise. However, 488 ‘genuine’ tremor events were successfully picked up (Fig. 1(b)). By comparing the red dots and grey dots in Fig. 1(b), the scattered events that are usually considered to represent either normal earthquakes occurring outside of the station distribution or analytical artifacts originating from background noise are removed, whereas clustering events, which usually represent genuine tremor events, are not removed by this method.
2.2 Accurate estimation of tremor depths
Although the automated detection method described above can be used to estimate the depth of tremor events, it uses only S-waves, and, ultimately, these absolute depth determinations are relatively imprecise. A better constraint on the absolute depth of tremor events is provided by the S-P times for events occurring immediately beneath the observation network, as demonstrated by La Rocca et al. (2009) in the Cascadia subduction zone. At some stations in Kyushu, P-waves are often visible in the vertical component, while S-waves are dominant in all components. Therefore, we can measure S-P times from cross-correlation functions between vertical and horizontal seismograms. At one station, two 40 s velocity seismograms, in both vertical and horizontal directions, are bandpass filtered between 2 and 8 Hz, converted into envelope waveforms, and smoothed by using a running average over 15 samples. The direction of the horizontal waveform, which is N20°W in this study, is chosen as P-S peaks, explained in the next paragraph, can be seen most clearly by testing some directions. In addition, we calculated a cross-correlation function for each dataset, changing the time lag between two envelope waveforms from —20 to 20 s.
Here, it is important to highlight that the seismic tomographic image of Hirose et al. (2008) shows a sharp boundary between high- and low-V p /V s regions (thin dashed line in Fig. 5(b)), located close to our estimated tremor sources and again about 20 km above intra-slab earthquakes. Therefore, we consider this boundary to represent the plate interface beneath Kyushu Island, with all intra-slab earthquakes occurring on, or below, the oceanic Moho. The separation of up to 20 km between the plate interface and the oceanic Moho appears to be somewhat large, but this may reflect the subduction of the anomalously thick crust of the KPR. Another relevant observation is that a region with a high Poisson ratio occurs near the tremor sources identified in this study (see Fig. 5(a)), which is interpreted as evidence for the existence of fluids. In addition, Yoshioka et al. (2008) estimated the thermal structure of the Kyushu subduction zone, indicating that temperatures are about 350–400°C near the tremor sources, which is typical of the conditions required for tremor generation.
In the Hyuga-nada region, offshore of the main location of Kyushu tremor activity, the afterslip of the 1996 (Yagi et al., 1999) Hyuga-nada earthquakes and faint SSEs have both been observed, and their source region is estimated to be at slightly shallower depths than the tremor events beneath Kyushu (Figs. 1(a), 5(a)) (Yagi and Kikuchi, 2003; Yarai and Ozawa, 2010). The amplitude of these Hyuganada SSEs is smaller than the amplitude of the SSEs observed in the Nankai region; i.e., it is similar to the observed differences in tremor activities between the two regions. However, the time constants of SSEs, and the ∼2 year recurrence interval and ∼1 year duration of SSEs, are not consistent with comparable data on tremors detected in this study, which revealed an 8 month recurrence interval and a duration of only a few days. Nevertheless, based on all of these arguments and geophysical constraints, we conclude that the plate boundary beneath Kyushu exists at a depth of 40 km, and the tremor activity recorded in this study is interpreted to represent shear slip along the plate interface.
Also of note is the fact that tremor activity detected in Kyushu is spatially localized. The narrowness of the tremor source along the dip direction could be related to the steepness of the subducting PHS plate, which results in a rapid increase in temperature and a corresponding rapid transition from stick-slip to stable slip along the plate interface. The distribution of tremors along strike seems to coincide with the location of the KPR, which is a remnant arc that is considered to have undergone a comparatively small amount of dehydration during subduction compared with the surrounding oceanic plate. Within the Nankai subduction zone, a relatively inactive tremor zone identified previously is interpreted as a result of ridge subduction (Matsubara et al., 2009). Therefore, the coincidence between tremor activity and the KPR determined in this study appears to contradict some previous views and geophysical models of the region. However, additional studies are needed to examine the effects of ridge subduction on fluid distribution, thermal structure, and other important geological and geophysical factors linked with tremor generation.
In southern Kyushu, we recorded tectonic tremor events that exhibit a recurrence interval of about 8 months, spaced closely along the down-dip direction of small SSEs and their associated source fault (Yarai and Ozawa, 2010), and laterally distributed across the region where the KPR is being subducted (Park et al., 2009). The depth of this tremor activity is estimated to be 35–45 km, based on tremor S-P times, which is consistent with the location of the plate boundary estimated by Yagi and Kikuchi (2003) and is coincident with a sharp boundary in the V p /V s structure reported by Hirose et al. (2008). The existence of fluids near our tremor sources has also been suggested (Hirose et al., 2008; Tahara et al., 2008), and the temperature in this region is 350–400°C (Yoshioka et al., 2008); these conditions are considered to be essential for tremor generation. Based on these geophysical observations, the tremor activity in Kyushu probably represents shear slip along the plate interface, akin to tremor activity recorded in Nankai and other subduction zones. Ultimately, by determining the depth of such tremor activity accurately in this study, we were able to constrain the location and geometry of the plate interface in the Kyushu region. In future geophysical studies of the area, it will be important to consider the effects of subduction of the KPR (a remnant arc) on the mechanisms of tremor activity. In this regard, it seems clear that suitable conditions for tremor activity in southern Kyushu would locally be realized due to ridge subduction, although the influence of subduction of the KPR on the process of tremor generation in the region has yet to be explored.
All of the data in this study were obtained from the NIED Hi-net data server. We thank H. Yarai for SSE data. This manuscript has been improved by helpful comments of two anonymous reviewers. This work was supported by JSPS KAKENHI (23244090) and MEXT KAKENHI (21107007).
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