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

Because of a lack of suitable instruments, rotational ground motions have not been observed until the last decade. Rotational measurements in the near field of earthquakes in Japan (Takeo, 1998) indicate that rotational ground motions are many times larger than expected from the classical elasticity theory. After failing to obtain useful rotational ground motions (using similar rotational sensors as Takeo did), we deployed a far more sensitive rotational velocity sensor (R-1) at the HGSD station in eastern Taiwan. From 7 December 2004 to 12 November 2006, several hundreds of earthquakes were recorded during our Phase 1 operation. This was mostly a learning exercise to solve field operation problems; Phase 1 operations ended when our two R-1 sensors ceased to operate. A K2 R1 instrument was deployed in the spring of 2007 to start our Phase 2 operation.

The additional observation of three components of rotational ground motions has benefits for tilt-seismometer coupling (e.g., ocean-bottom seismometry and volcano seismology), local site characterization, wavefield separation, source inversion, glacial and planetary seismology, as well as the monitoring of structural health. Field applications have been mostly hampered by the lack of portable sensors with appropriate broadband operation range and weak-motion sensitivity. Here, we present field observations of the first commercial portable broadband rotation sensor specifically designed for seismology. The sensor is a three-component fiber-optic gyro strictly sensitive to ground rotation only. The sensor field performance and records are validated by comparing it with both array-derived rotation measurements and a navigation-type gyro. We present observations of the 2018 Mw 5.4 Hualien earthquake and the 2016 central Italy earthquake sequence. Processing collocated rotation and classical translation records shows the potential in retrieving wave propagation direction and local structural velocity from point measurements comparable to small-scale arrays of seismic stations. We consider the availability of a portable, broadband, high sensitivity, and low self-noise rotation sensor to be a milestone in seismic instrumentation. Complete and accurate ground-motion observations (assuming a rigid base plate) are possible in the near, local, or regional field, opening up a wide range of seismological applications.

S U M M A R YA broad-band ocean-bottom seismometer (OBS) deployed ∼180 km east of Taiwan provides a first glimpse into the upper mantle beneath the westernmost section of the Philippine Sea or the Huatung basin (HB). We measured interstation phase velocities of Rayleigh waves between the OBS and stations on the eastern coast of Taiwan. The phase velocities show smooth variations from 3.8 to 3.9 km s −1 for periods of 25-40 s. In this short period range, phase velocities are comparable to those characterizing the 15-30 Ma Parece-Vela basin of the Philippine Sea. Modelling of the finite-frequency effect proves the validity of the measurement for the average HB. The shear-wave velocity models inverted from the 25 to 40 s dispersion show a velocity at lithospheric depths about 0.1 km s −1 lower than that of the west Philippine Sea, which agrees with the age effect derived from the Pacific pure-path model. Inversions incorporating the less reliable data above 40 s yield a shear velocity <4.0 km s −1 below 150 km, an unrealistic value even for a hotspot plume environment. The seismological evidence, together with the correlation in seafloor depth, suggests that the HB and the Parece-Vela basin may have a similar age. This is at odds with the previous geochronological study suggesting an early-Cretaceous age for the HB. Thermal rejuvenation of the lithosphere was examined as a potential solution to reconciling the two age models.