S U M M A R YWe perform a systematic survey of triggered deep 'non-volcanic' tremor beneath the Central Range (CR) in Taiwan for 45 teleseismic earthquakes from 1998 to 2009 with M w ≥ 7.5 and epicentral distance ≥1000 km to the broad-band station TPUB. Triggered tremors are visually identified as bursts of high-frequency (2-8 Hz), non-impulsive and long-duration seismic energy that are coherent among many seismic stations and modulated by the teleseismic surface waves. Out of the 45 earthquakes, we identified nine teleseismic events associated with nine tremor sources in the southern and five in the northern CR. Most of the tremor sources are located within the depth range of 15-25 km in the lower crust above the Moho. We find that the amplitudes of the surface waves play an important role in determining the triggering potential, and the apparent triggering threshold is ∼0.1 cm s −1 , or 7-8 KPa. However, such threshold is partially controlled by the background noise level, which could prevent weaker tremor triggered by surface waves with smaller amplitudes from being identified. The amplitudes of the triggered tremor show a positive correlation with the amplitudes of the triggering surface waves, consistent with the predictions by the 'clock-advance' model. In addition to amplitudes, other factors, such as frequency contents and incidence angles, also affect the triggering potential. We find that intermediate-period (30-10 s) surface waves could trigger/modulate tremors, suggesting that long-period (>30 s) surface waves are not always required in long-range triggering. Tremors appear to be triggered by both Love and Rayleigh waves. When the incidence angles are parallel to the strike of the CR, all six events triggered tremor primarily during the Rayleigh waves. For strike normal incidence, only the 2001 M w 7.8 Kunlun earthquake showed predominant Love-wave triggering. This observation can be qualitatively explained by a simple Coulomb failure for a left-lateral shear on the low-angle detachment fault beneath the southern CR.

[1] We use a matched filter technique to detect 41 lowfrequency earthquakes (LFEs) within 700-s of triggered tremor signals in the Southern Central Range in Taiwan during the surface waves of the 2005 Mw8.6 Nias earthquake off the coast of northern Sumatra. The depth distributions of LFEs after double-difference relocations concentrate at the depth range of 12-38 km below the background seismicity and above the Moho depth inferred from receiver function studies. The locations of LFEs are close to the downward extension of the steep-dipping Chaochou-Lishan fault with only modestly high Vp/Vs ratios (1.75-1.85). Our observation indicates that at least portions of triggered tremor consists of many LFEs, similar to ambient tremor observed at other major plate boundary faults. Citation: Tang, C

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Broadband seismometers, ground-based Global Navigation Satellite Systems (GNSS), and magnetometers that were located within an epicentral distance of approximately 150 km consistently observed the novel anomalous behaviors of the common-mode ground vibrations approximately 5–10 days before the M6.6 Meinong earthquake in Taiwan. The common-mode ground vibrations with amplitudes near 0.1 m at frequencies ranging from 8 × 10−5 to 2 × 10−4 Hz were generated near the region close to the epicenter of the impending earthquake. The common-mode vibrations were consistently observed in seismic and GNSS data associated with five other earthquakes in four distinct areas. The results reveal that the common-mode vibrations could be a typical behavior before earthquakes. The causal mechanism of common-mode vibrations can be attributed to crustal resonance excitations before fault dislocations occur. Potential relationships with other pre-earthquake anomalies suggest that the common-mode vibrations could be ground motion before earthquakes, which was investigated for a significant length of time.