We perform a systematic detection of deep tectonic tremor beneath the southern Central Range in Taiwan using a dense, small-aperture seismic array. Deployed in February 2011, the array has been recording tectonic tremor nearly continuously for 134 days, including tremor triggered by the 2011 M w 9.0 Tohoku-Oki earthquake. We use broadband frequency-wavenumber beamforming and moving-window grid-search methods to compute array parameters for all continuous data and identify tremor as coherent nonimpulsive seismic signals with deep incidence angles. The obtained array parameters closely match with those of relocated local earthquakes and triggered tremor bursts located by the waveform envelope correlation and clustering (WECC) method, indicating the robustness of our array technique. During the 134-day study period, we detect tremor for 44 days, with a total duration of 1481 min, three to six times as long as the detection of tremor by the WECC method. We find a relative quiescence in ambient tremor activity for about 20 days following the 2011 Tohoku-Oki earthquake, suggesting that dynamic stresses from the distant mainshock triggered most tremor close to failure, resulting in a temporary lack of tremor activity. In some cases, we observe rapid tremor migration with a speed at the order of 40-50 km=hr that is similar to the speed of fast tremor migration along dip on narrow streaks in Japan and Cascadia. Our results suggest that dense array techniques are capable of capturing detailed spatiotemporal evolutions of tremor behaviors in southern Taiwan.

M w 6.4 Meinong earthquake occurred on 6 February 2016 in southern Taiwan, resulting in more than one hundred casualties and several collapsed buildings. The aftershocks occurred mostly at mid-to-lower crustal depths (10-30 km), related to a blind fault system. However, several centimeters of cosesimic surface uplift within the Liushuang, Erhchungli, and GutingKeng Formations, composed mainly of mudstone, was recorded from the InSAR results. The uplifted pattern is similar to that of GPS and leveling data from 2000-2010, which indicates the deformation is accomplished by creeping due to the shallow mudstone structure related to mud diapir. Previous studies have shown limited information about the shallow structure in this region due to few deployed seismic stations. We deployed 36 temporary seismic stations (~5 km spacing) in this study around one month after the main shock to obtain a 3-D shear wave shallow crustal velocity structure using ambient noise tomography. The reliable periods of group and phase velocities from Rayleigh waves were 0.6-5 s, corresponding to around 0-5 km at depths. As a result, the low S-wave pattern speeds at 0-4 km depths correspond to the uplift region from both InSAR data for the coseismic period and GPS and leveling data for the interseismic period. The results from this study are compatible with the reflected seismic profile. The results show that with dense seismic array deployment we can obtain high subsurface image resolution to link the relationship between the surface observations to the subsurface structures.

The February 2018 M w 6.4 earthquake in eastern Taiwan caused extensive damage in Hualien City. Although damaging earthquakes are common in this region, there are relatively few permanent seismic stations deployed. Two days after the mainshock, we deployed 70 temporary seismic stations around Hualien City for 12 days (8-19 February), and station spacing was 1-5 km. During this time, 2192 aftershocks were located from which 580 focal mechanisms were determined. The aftershock sequence extended about 25 km southwestward from the epicenter of the mainshock into the Longitudinal Valley and to depths between 5 and 15 km. Earthquake hypocenters indicate that the aftershocks took place along a near-vertical to steeply west-dipping plane in the north that becomes more diffuse in the south. Focal mechanisms are predominantly extensional, different than the left-lateral strike slip with thrust-component faulting of the mainshock. Very few events occurred in the uppermost crust at depths of less than 5 km, and their focal mechanisms are left-lateral strike-slip faulting. From the mainshock to the aftershocks, the stress changed rapidly from a north-northwest-oriented compressional axis (P axis) to the same direction for the extensional axis (T axis). Electronic Supplement: Tables of aftershocks and focal mechanisms obtained in this study and figures of M w determination via manual spectral analysis and result of automatic spectral analysis.

<p><span>The Longitudinal Valley in eastern Taiwan, the arc-collision boundary between the Eurasian and Philippine Sea plates, is one the most seismic active areas in the world. On September 18, 2022, the Mw 6.9 Chihshang earthquake struck the south half of the valley and caused severe damage. Since November 2021, we have installed a five-station permanent broadband seismic array with station spacings of 10-20 km around the Chihshang area, and right after the Mw 6.5 foreshock occurred, we further installed a 46-station temporary dense array of nodal seismometers with station spacings of 2-5 km for 35 days. We use SeisBlue, a deep-learning platform/package, to extract the whole earthquake sequence including the Mw 6.5 foreshock, the Mw 6.9 main shock, and over 5,000 aftershocks from the broadband array, and to obtain over 40,000 aftershocks from the dense nodal array. With the high quality and quantity of P- and S-wave arrival times, we apply the finite difference travel time tomography, developed by Roecker et al. (2006). The improved resolution at the shallow part of the crust (at depth < 10 km) provides new constraints to get detailed (with grid spacing 1 km) and reliable Vp, Vs, and Vp/Vs velocity models at the local scale for the first time. Combined with the high-resolution velocity models and the much more complete seismicity, our results clearly depict not only the Central Range fault and the Longitudinal fault but also several local, shallow tectonic structures that have not been observed along the southern Longitudinal Valley.</span></p>

The empirical attenuation functions for Local Magnitude (M L) currently used in Taiwan have been known for overestimated magnitudes around 0.2 compared with moment magnitude (M W) for shallow earthquakes (depths ≤ 35 km). Moreover, for deep earthquakes (depths > 35 km), M L (> 6) can be larger than M W around 0.5. Based on global observations and seismological theory, M L is equal to M W for magnitudes 4-6, whereas M L is smaller than M W for magnitudes > 6. This indicates that the attenuation functions for the Taiwan region need to be recalculated. In this study, we used the new data set collected at the upgraded Central Weather Bureau Seismic Network (CWBSN24) from January 2012 to April 2019 and totally there are 692 events with M L ≥ 3 and 35228 amplitude data used for analysis. To accommodate the complicated tectonic environment in Taiwan, there are four attenuation functions, logA 0 (Δ), based on the focal depths and hypocentral distances, which are: