Systematic analysis of triggered and ambient tremor beneath the Central Range in Taiwan

Non-volcanic tremor triggered by teleseismic surface waves was recently discovered beneath the Central Range in Taiwan (Peng and Chao, 2008). Here we perform a systematic survey of triggered tremor for the 46 teleseismic earthquakes with Mw ≥ 7.5 and depth ≤ 100 km since 1998. Triggered tremor is identified as bursts of high-frequency, non-impulsive seismic energy that are coherent among many stations and during the passage of teleseismic body and surface waves. So far we have identified 9 teleseismic events associated with clear triggered tremor beneath the Central Range. These include the 1998/11/29 Mw 7.7 Ceram-Sea, 2001/11/14 Mw7.8 Kunlun, 2003/09/25 Mw8.3 Tokachi-Oki, 2004/12/26 Mw9.2 Sumatra, 2005/03/28 Mw8.6 Nias, 2007/01/13 Mw 8.1 Kuril-Island, 2007/04/01 Mw8.1 Solomon, 2007/09/12 Mw8.4 Sumatra, and 2008/05/12 Mw7.9 Wenchuan earthquakes. The epicentral distances between these 9 events and the broadband seismic station TPUB are from 1800 to 4000 km. The measured vertical Peak Ground Velocity (PGV) ranges from 0.1 to 2.2 cm/s, corresponding to a dynamic stress of 0.009 and 0.19 MPa (with a nominal shear rigidity of 30 GPa and a surface wave velocity of 3.5 km/s). We use the standard envelope cross-correlation techniques to locate the triggered tremor. The obtained tremor epicenters are around the location of tremor triggered by the 2001 Mw7.8 Kunlun event. Based on the depth range of 15-20 km, we infer that the triggered tremor occur on the aseismic portion of the basal detachment fault beneath the Central Range. After correcting the timing of the tremor and surface waves, we find that the triggered tremor shows correlations with both the Love and Rayleigh waves. We also identify triggered tremor in Northeast Taiwan that could be associated with the Ryukyu subduction zone. Finally, we examine 1-day seismic data around the 9 teleseismic events and identify many ambient tremor long after the passage of the teleseismic waves. Our next step is to detect low-frequency earthquakes within ambient and triggered tremor, obtain accurate locations, and examine their relationships between the triggered and ambient tremor. Our systematic survey of non-volcanic tremor in Taiwan would not only help to quantify the triggering mechanisms and necessary conditions of tremor generation in an arc-continental collision environment, but also improve our understanding of the fundamental processes of deep fault zones in active fold-thrust belts.