We use a matched filter technique to detect 41 low‐frequency 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.

Focal mechanisms of microearthquakes ($ML<4$) are important in understanding tectonics of seismogenic structure, especially in areas with high seismic potential. However, one of the major difficulties in obtaining a reliable focal mechanism for a microearthquake is deriving sharp P‐ and S‐wave arrivals from a waveform with a low signal‐to‐noise ratio compared with those from larger earthquakes. Another challenge is the uneven distribution of seismic stations, which results in high uncertainty of focal mechanisms of microearthquakes. To address these problems, we used waveform inversion to improve the accuracy of determining focal mechanisms. High‐quality waveform data recorded by 12 temporal broadband stations deployed in the Ma River fault zone, northern Vietnam, are used to conduct a waveform inversion to obtain reliable fault‐plane solutions. In addition, a frequency–wavenumber integration method and a cut‐and‐paste method are applied to estimate the optimum focal mechanisms. This procedure is shown to effectively increase the accuracy of focal mechanisms obtained for microearthquakes. Our results are congruent with local geological structures and indicate a complex fractured crust in the Ma River fault zone. By studying local stress states, we claim that the Ma River fault zone is a wrench fault system with high seismic potential.

The primary aim of this study is to use microseismic events to perform a stress inversion, which has often been excluded in past studies. New insights on the seismogenic structures and stress state in northeastern Taiwan can be acquired by applying 3D velocity structure relocation, raytracing techniques, and stress inversion methods to an entire database, which permits objective and reliable selection of data for analysis. This aforementioned approach allows us to avoid the influence of a subjective selection of seismic events. Confidence intervals are also used to show the uncertainty in stress orientation. Our results show that the seismogenic structure in northeastern Taiwan is subject to complex influences from the subduction of the Philippine Sea plate and the ongoing opening of the Okinawa trough. In addition, we observed that the seismic activity of northeastern Taiwan is rather complicated. By incorporating microearthquakes and the zonation that is obtained from microearthquake sources, it becomes possible to thoroughly understand the spatial distribution of seismogenic structures in this region. Furthermore, our results also provide essential details on background stresses that can be used to study stress transfer in future studies.

Seismic monitoring relies on seismography. However, the high cost of seismic equipment has presented a challenge to increasing the density of seismic networks in previous decades. Due to the large station spacing and inferior coverage of stations, this situation has led to a loss of detail in many research results. Along with the improvement of technology, the problem of increasing the density of seismographic observations is no longer an impossible issue. This makes it feasible to deploy a dense seismic network for monitoring earthquakes. This study deployed a linear dense array across the Meishan Fault in west-southern Taiwan for the purpose of analyzing the shallow fault zone structure. While the 1906 Meishan earthquake occurred in a period when historic records were available, the surficial geology surveys of the Meishan Fault are challenging because farming and construction engineering have obscured the outcrop. Early surveys of the Meishan Fault were mainly seismic surveys. In recent decades, over thirty profiles have been completed. However, the reflection seismic records had poor signal-to-noise ratios because the Meishan Fault is buried under thick sediments. Thus, the shallow structure of the Meishan Fault is still not known in detail. This study applied double-beamforming tomography to a dense seismic array to obtain high-resolution images of the Meishan Fault zone. The result shows that there is a south-dipping interface near the fault trace as indicated by the Central Geological Survey of Taiwan. In addition, we observed velocity transitions of perturbation profiles that may be caused by a branch fault, the Chentsoliao Fault. This study demonstrates that the ambient noise double beamforming method is an effective tool for imaging the detailed shallow structure along with the dense seismic array.