- Open Access
Triggered tremors beneath the seismogenic zone of an active fault zone, Kyushu, Japan
© Miyazaki et al. 2015
- Received: 12 August 2015
- Accepted: 22 October 2015
- Published: 4 November 2015
Non-volcanic tremors were induced by the surface waves of the 2012 Sumatra earthquake around the Hinagu fault zone in Kyushu, Japan. We inferred from dense seismic observation data that the hypocenters of these tremors were located beneath the seismogenic zone of the Hinagu fault. Focal mechanisms of the tremors were estimated using S-wave polarization angles. The estimated focal mechanisms show similarities to those of shallow earthquakes in this region. In addition, one of the nodal planes of the focal mechanisms is almost parallel to the strike direction of the Hinagu fault. These observations suggest that the tremors were triggered at the deeper extension of the active fault zone under stress conditions similar to those in the shallower seismogenic region. A low-velocity anomaly beneath the hypocentral area of the tremors might be related to the tremor activity.
- Triggered tremor
- The Hinagu fault zone
- Intraplate earthquake
- Focal mechanism
- Polarization analysis
- Slow earthquake
Non-volcanic tremors triggered by surface waves from teleseismic events have been detected all over the world. Most studies have investigated tremors that were located near or on plate boundaries such as subduction zones (Miyazawa and Brodsky 2008; Rubinstein et al. 2009; Yabe and Ide 2013), transform boundary zones (Peng et al. 2009), and collision zones (Chao et al. 2012). However, non-volcanic tremors are rarely detected away from plate boundaries. Obara (2012) found that triggered tremors occurred near a volcano and an active fault zone in Hokkaido, Japan and Kita et al. (2014) showed through analysis of seismic attenuation that such tremors occurred in a collision zone.
According to previous studies, most non-volcanic tremors occur on or near the downward extension of seismogenic faults and are accompanied by slow earthquakes there (Obara 2011). Therefore, the study of non-volcanic tremors is important for understanding processes leading to the generation of large earthquakes not only at the plate boundary but also in intraplate regions. Iio et al. (2004) proposed a model indicating that slip on a deeper extension of an active fault is required to generate large intraplate earthquakes. These observations suggest that locations and focal mechanisms of tremors near intraplate faults can provide clues to understanding the loading process responsible for earthquakes in intraplate regions.
Chao and Obara (Active triggered tremor sources in the inland fault systems of Japan, submitted) investigated the distribution of triggered tremors in Japan. One of the events described in their study occurred beneath the Hinagu fault zone, Kyushu, Japan. However, precise locations and focal mechanisms of these tremors have not yet been determined. Therefore, we performed a detailed analysis to estimate locations and focal mechanisms of these events from data of a dense seismic network deployed around the Hinagu fault zone and discuss a possible mechanism for the loading process leading to tectonic earthquakes.
Data, methods, and results
Tremor location in the Hinagu fault zone
Focal mechanism of the tremors
Analyses using propagation velocity (Obara 2002; Katsumata and Kamaya 2003), polarization (La Rocca et al. 2005, 2009; Wech and Creager 2007), and stacked waveform (Ide et al. 2007; Shelly et al. 2006, 2007) suggested that non-volcanic tremors consist of many overlapping S-waves radiating from low-frequency earthquakes (LFEs). The waveforms of each tremor investigated in this study are characterized by obscure phase arrivals and long durations, suggesting superposition of small events.
To estimate the focal mechanisms of such very small events, we utilized an S-wave polarization analysis because we could not adopt a prevalent moment tensor analysis to such small events. Here, we assumed that slip directions of small events are constant during the period of tremor generation and that the relative amplitude of the resultant waves is characterized by a radiation pattern from the source. Under this assumption, an S-wave polarization angle at a station during a tremor could be expected to have a certain direction corresponding to its focal mechanism. Although an influence of P-waves may not be absent (La Rocca et al. 2005; Ide et al. 2007), the waveform of the horizontal component of the seismogram at the expected P-wave arrival does not reveal a distinct phase arrival. Therefore, we assumed that the contribution of P-waves to the seismogram was small enough to analyze a tremor as a single packet.
where N is the number of stations used. A grid search is performed to find the optimal parameters for ϕ, δ, and λ that minimize the function F. The definition of σ i is described below. In this study, we used the 15 stations deployed around the hypocentral area of the tremors.
where w1,w2, and w3 are minimum, moderate, and maximum eigenvalues in each time window, respectively. We extracted the data under the condition that the rectilinearity was higher than 0.9 (black line in Fig. 4c). The other criterion is that the signal-to-noise ratio was larger than 2.5 (red line in Fig. 4c). The amplitude of the signal is calculated from the waveform within the time window of 0.5 s used in the polarization analysis. The amplitude of the noise is defined as the RMS amplitude within a time window of 10 min that contains both the tremor signals and ambient noises.
After selection of the data (blue circles in Fig. 4b), we obtained the azimuthal distribution of the directions of particle motions within the target lapse time range for each station (blue bars in Fig. 4d). To evaluate the stability in the azimuthal direction, we counted the cumulative frequency of the azimuthal distribution for every ±2° in the azimuthal range. The frequency distribution was normalized by the total number of samples (orange line in Fig. 4d). Finally, we obtained the observed S polarization angle as the angular difference between the azimuthal direction with the maximum relative frequency (orange line in Fig. 4b, e) and the direction from each station toward the epicenter (red line in Fig. 4b, e). We used the value of the maximum relative frequency as the weighting factor (σ i ) for the following grid search. Even if several peaks emerged in the smoothed azimuthal distribution, the influence on the focal mechanism estimation was restrained because the weighting factor became small in that case.
Discussions and conclusions
In this study, we estimated the locations and focal mechanisms of the triggered tremors in the Hinagu active fault zone. We found that five tremors were located beneath the seismogenic zone and seem to align the strike direction of the fault. These results suggest that the tremors occurred near the deeper extension of the Hinagu fault zone. The Japan Meteorological Agency detected low-frequency events around the Hinagu fault zone in 2004 and 2005 (Fig. 3). However, they were not triggered by the surface wave arrivals shown in this study and were located far from the tremors, and therefore, it is assumed that these events were not related to the tremors.
Therefore, we can compare the two optimal focal mechanisms of the tremors to the focal mechanisms of shallow crustal earthquakes studied by Matsumoto et al. (2013). We found that one of the nodal planes has a similar strike angle to that of the shallow earthquakes. Considering the principal stress directions estimated by Matsumoto et al. (2013) shown in Fig. 6a, the similar nodal planes of the tremors and crustal earthquakes are favorably oriented under the current stress field. This could imply that both the shallow earthquakes and the tremors in this region are generated under the same tectonic stress conditions. In addition, the Hinagu fault zone at the surface has similar strike direction to one of the nodal planes of optimal focal mechanisms estimated in this study. This implies the possibility that the tremors occurred on a fault plane forming a downward extension of the Hinagu fault zone. The similarity of strike angles of the fault planes suggests that the active fault, the shallow earthquakes, and the tremors represent a fault system reaching from the surface to the lower crust as described in Iio et al. (2004). This geometrical relationship indicates that the tremors are a phenomenon reflecting the loading process of the crustal earthquakes.
We also investigated the relationship between the locations of the tremors and the velocity structure (Fig. 6b), estimated by seismic tomographic inversion (Matsubara and Obara 2011). In the seismogenic zone, a region characterized by high P-wave velocities (Vp) exists parallel to the strike direction of the Hinagu fault zone. A low Vp region is seen below the tremor zone and in the northeastern part of the fault zone. The distribution of the tremors seems to indicate the possibility that the tremor is triggered in the transient zone between the seismogenic zone and the ductile lower crust.
In the Nankai trough region, Shelly et al. (2006) showed that the Vp/Vs ratio around the tremor source was over 1.8, which was interpreted as the existence of high pore-fluid pressure. This is also supported by the relationship between the volumetric strain change generated by surface waves and amplitudes of triggered tremors (Miyazawa and Brodsky 2008). Thomas et al. (2012) showed that high pore-fluid pressure exists in the brittle-ductile transient zone of a transform boundary in California via a study of tidal triggering of tectonic tremors. In contrast, the Vp/Vs ratio near the source region of the Hinagu fault is approximately 1.7 to 1.8 and seems to be less influenced by the fluid pressure. According to Matsubara and Obara (2011), the resolution of the velocity structure in the horizontal direction is 20 km. Therefore, there is a possibility that a high Vp/Vs region indicating high pore-pressure is concentrated in a limited region, smaller than the resolution of the velocity tomography.
We are thankful to two anonymous reviewers for useful comments and suggestions to improve this manuscript. In this study, we used the seismic observation data recorded by various organizations, including the Japan Meteorological Agency (JMA), National Research Institute for Earth Science and Disaster Prevention (NIED), and Kagoshima University. Figures are generated by Generic Mapping Tools.
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- Chao K, Peng Z, Wu C, Tang C, Lin C (2012) Remote triggering of non-volcanic tremor around Taiwan. Geophys J Int 188:301–324. doi:10.1111/j.1365-246X.2011.05261.x View ArticleGoogle Scholar
- Chouet B, Saccorotti G, Martini M, Dawson P, DeLuca G, Milana G, Scarpa R (1997) Source and path effects in the wave fields of tremor and explosions at Stromboli Volcano, Italy. J Geophys Res. doi:10.1029/97JB00953.
- Efron B (1979) Bootstrap methods: another look at the jackknife. Ann Statist 7:1–26. doi:10.1214/aos/1176344552 View ArticleGoogle Scholar
- Hirasawa T (1970) Focal mechanism determination from S wave observations of different quality. J Phys Earth 18:285–294. doi:10.4294/jpe1952.18.285 View ArticleGoogle Scholar
- Ide S, Shelly DR, Beroza GC (2007) Mechanism of deep low frequency earthquakes: further evidence that deep non-volcanic tremor is generated by shear slip on the plate interface. Geophys Res Lett. doi:10.1029/2006GL028890 Google Scholar
- Iio Y, Sagiya T, Kobayashi Y (2004) Origin of the concentrated deformation zone in the Japanese Islands and stress accumulation process of intraplate earthquakes. Earth, Planets and Space 56:831–42. doi:10.1186/BF03353090 View ArticleGoogle Scholar
- Katsumata A, Kamaya N (2003) Low-frequency continuous tremor around the Moho discontinuity away from volcanoes in the southwest Japan. Geophys Res Lett. doi:10.1029/2002GL015981.
- Kita S, Nakajima J, Hasegawa A, Okada T, Katsumata K, Asano Y, Kimura T (2014) Detailed seismic attenuation structure beneath Hokkaido, northeastern Japan: arc-arc collision process, arc magmatism, and seismotectonics. J Geophys Res. doi:10.1002/2014JB011099.
- La Rocca M, McCausland W, Galluzzo D, Malone S, Saccorotti G, Pezzo ED (2005) Array measurements of deep tremor signals in the Cascadia subduction zone. Geophys Res Lett. doi:10.1029/2005GL023974.
- La Rocca M, Creager KC, Galluzzo D, Malone S, Vidale JE, Sweet JR, Wech AG (2009) Cascadia tremor located near plate interface constrained by S minus P wave times. Science 323:620–3. doi:10.1126/science.1167112 View ArticleGoogle Scholar
- Matsubara M, Obara K (2011) The 2011 off the Pacific coast of Tohoku Earthquake related to a strong velocity gradient with the Pacific plate. Earth, Planets and Space 63:663–7. doi:10.5047/eps.2011.05.018 View ArticleGoogle Scholar
- Matsumoto S, Chikura H, Ohkura T, Miyazaki M, Shimizu H, Abe Y, Inoue H, Yoshikawa S, Yamashita Y (2013) Spatial heterogeneities of deviatoric stress and pore-pressure in Kyushu, Japan, and their implication for seismic activity. In: EGU general assembly 2013, Vienna, Austria, 7-12 April 2013. http://meetingorganizer.copernicus.org/EGU2013/posters/12580
- Meng L, Ampuero JP, Stock J, Duputel Z, Luo Y, Tsai VC (2012) An earthquake in a maze: compressional rupture branching during the 2012 Mw8.6 Sumatra earthquake. Science 337:724–6. doi:10.1126/science.1224030 View ArticleGoogle Scholar
- Miyazawa M, Brodsky EE (2008) Deep low-frequency tremor that correlates with passing surface waves. J Geophys Res. doi:10.1029/2006JB004890.
- Nishimura T (2014) Crustal block movements of southwestern Japan estimated by fault-block modeling of GNSS data (In Japanese). In: Programme and Abstracts, the Seismological Society of Japan, 2014, fall meeting, Seismological Society of Japan, Niigata, Japan, 24-26 November 2014Google Scholar
- Nuttli O (1961) The effect of the Earth's surface on the S wave particle motion. Bull Seismol Soc Am 51:237–46Google Scholar
- Obara K (2002) Nonvolcanic deep tremor associated with subduction in southwest Japan. Science 296:1679–81. doi:10.1126/science.1070378 View ArticleGoogle Scholar
- Obara K (2011) Characteristics and interactions between non-volcanic tremor and related slow earthquakes in the Nankai subduction zone, southwest Japan. J Geodyn 52:229–48. doi:10.1016/j.jog.2011.04.002 View ArticleGoogle Scholar
- Obara K (2012) New detection of tremor triggered in Hokkaido, northern Japan by the 2004 Sumatra-Andaman earthquake. Geophys Res Lett. doi:10.1029/2012GL053339.
- Peng Z, Vidale JE, Wech AG, Nadeau RM, Creager KC (2009) Remote triggering of tremor along the San Andreas Fault in central California. J Geophys Res. doi:10.1029/2008JB006049.
- Rubinstein JL, Gomberg J, Vidale JE, Wech AG, Kao H, Creager KC, Rogers G (2009) Seismic wave triggering of nonvolcanic tremor, episodic tremor and slip, and earthquakes on Vancouver Island. J Geophys Res. doi:10.1029/2008JB005875.
- Shelly DR, Beroza GC, Ide S, Nakamula S (2006) Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip. Nature 442:188–191. doi:10.1038/nature04931 View ArticleGoogle Scholar
- Shelly DR, Beroza GC, Ide S (2007) Non-volcanic tremor and low-frequency earthquake swarms. Nature 446:305–307. doi:10.1038/nature05666 View ArticleGoogle Scholar
- Thomas AM, Bürgmann R, Shelly DR, Beeler NM, Rudolph ML (2012) Tidal triggering of low frequency earthquakes near Parkfield, California: implications for fault mechanics within the brittle-ductile transition. J Geophys Res. doi:10.1029/2011JB009036.
- Wech AG, Creager KC (2007) Cascadia tremor polarization evidence for plate interface slip. Geophys Res Lett. doi:10.1029/2007GL031167.
- Yabe S, Ide S (2013) Repeating deep tremors on the plate interface beneath Kyushu, southwest Japan. Earth, Planets Space 65:17–23. doi:10.5047/eps.2012.06.001 View ArticleGoogle Scholar