Statistical properties of low-frequency earthquakes triggered by large earthquakes in southern Taiwan Chi-Chia Tang a,n , Zhigang Peng b,1 , Cheng-Horng Lin a,2 , Kevin Chao b,c,3 , Chau-Huei Chen d,4 a Institute of Earth Sciences, Academia Sinica,128, Section 2, Academia Road, Nangang, Taipei 11529, Taiwan b School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, USA c Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan d Department of Earth and Environmental Science, National Chung Cheng University,168 University Road, Min-Hsiung, Chia-Yi County 62102, Taiwan article info Article history: Received 5 September 2012 Received in revised form 20 April 2013 Accepted 21 April 2013 Editor: P. Shearer Keywords: Taiwan low-frequency earthquake triggered tremor non-volcanic tremor abstract The recent discovery of triggered tremors (TTs) and low-frequency earthquakes (LFEs) in various tectonic environments provides an opportunity for studying the fundamental properties and physical mechan- isms of deep tectonic tremor. Here, we quantify the relationship between TTs and LFEs beneath the Central Range in southern Taiwan and their statistical properties during the teleseismic waves of six large distant earthquakes. Using waveforms of 11 LFEs triggered by the 2005 M w 8.6 Nias earthquake as templates, we scan through 12 hours of waveform data around six mainshocks and identify a total of 783 LFEs. The LFEs were mainly located in a compact region between 12 and 36 km in depth near the Chaochou–Lishan Fault. Most of LFEs occurred within TT during the passage of large-amplitude surface waves, and the increase of the LFE rate during the surface waves is statistically significant. The LFE rates do not follow an Omori's type decay, but rather abruptly return to the background rate immediately after the surface-wave passage. These findings suggest that LFEs do not trigger any additional LFEs at later times and are primarily driven by an external forcing. Our observations are consistent with the inference that TTs consist of many reoccurring LFEs. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Tremor away from volcanoes (Obara, 2002) and triggered by seismic waves, termed triggered ‘ non-volcanic’ or ‘ deep tectonic’ tremor (TT), reflects shear slip on deep active faults driven by transient dynamic stresses (Peng and Gomberg, 2010, and references therein). Relative to regular earthquakes, TTs have been exclusively observed near major plate boundaries: California (Ghosh et al., 2009; Peng et al., 2009, 2010), Japan (Miyazawa and Brodsky, 2008; Chao et al., 2013), New Zealand (Fry et al., 2011), Vancouver Island (Rubinstein et al., 2007, 2009), Haida Gwaii (or Queen Charlotte) Island (Aiken et al., 2013), Cuba (Peng et al., 2013) and Taiwan (Tang et al., 2010; Chao et al., 2012). It is well known that the durations of triggered and ambient tremors are longer than ordinary earthquakes, and they are dominated by low-frequency seismic energy of 1–10 Hz, as compared with regular earthquakes (Nadeau and Dolenc, 2005; Peng et al., 2008). Low-frequency earthquake (LFE), a new class of seismic event, was first identified by the Japan Meteorological Agency (JMA) in their seismicity catalog in southwest Japan (Katsumata and Kamaya, 2003). Shelly et al. (2006) hypothesized that the reduction of effective stress due to elevated pore-fluid pressure might help promote LFE and tremor generation, and tremors could be consid- ered as a swarm of many LFEs (Shelly et al., 2007). Recent studies of repeating LFEs families along the Parkfield–Cholame section of the San Andreas Fault also confirmed that TTs can be largely explained by the same LFE families that occurred during ambient tremor episodes (Shelly et al., 2011). Tang et al. (2010) identified LFEs within TTs beneath the southern Central Range of Taiwan and further confirmed that TTs consist of many LFEs. The hypocentral depths of these LFEs range from 12 km to 38 km and the epicenters are near the Chaochou–Lishan Fault (CLF), a major reverse fault in Taiwan. Local seismic tomography (Wu et al., 2007) reveals a relatively high ratio of P-to-S wave velocity (V p /V s ratio) area near the hypocenter of LFEs, suggesting the existence of pore fluids. The primary objective in this study is to further investigate the statistical properties of LFEs in Taiwan around several large teleseismic mainshocks and their relationship with TTs observed in recent studies (Chao et al., 2012, 2013). Note that in this paper Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/epsl Earth and Planetary Science Letters 0012-821X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.04.039 n Corresponding author. Tel.: +886 5 2783 9910x523; fax: +886 2 2783 9871. E-mail addresses: iori897@gmail.com (C.-C. Tang), zpeng@gatech.edu (Z. Peng), lin@earth.sinica.edu.tw (C.-H. Lin), kchao@eri.u-tokyo.ac.jp (K. Chao), seichen@eq.ccu.edu.tw (C.-H. Chen). 1 Tel.: +1 404 894 0231; fax: +1 404 894 5638. 2 Tel.: +886 2 2783 9910x521; fax: +886 2 2783 9871. 3 Tel.: +81 804 447 3228. 4 Tel.: +886 5 2720 411x66202; fax: +886 5 2720 807. Earth and Planetary Science Letters 373 (2013) 1–7