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.

Most earthquakes are considered to be caused by stress accumulating in and subsequently releasing from the crust. To extract non-linear and non-stationary earthquake-induced signals associated with stress accumulation, the Hilbert–Huang transform was utilized to filter long-term movements, short-term noise, and frequency-dependent (annual and semi-annual) variations from surface displacements measured by the global positioning system (GPS) in Taiwan. Earthquake-related surface displacements were expressed as horizontal directions (i.e., GPS azimuths) using the north–south and east–west components of residual GPS data to bypass influences resulted from the inhomogeneous nature of the crust. Analytical results showed that the relationships between earthquake occurrence and the aligned GPS azimuth passed the statistical test of the Molchan’s error diagram. Aligned GPS azimuths were in agreement with direction of earthquake-related P axes for 81% (26/32) studied events. Areas with the highest paralleling orientations of GPS azimuths appeared around epicenters several days to weeks before earthquake occurrence. Durations from aligned GPS azimuths to earthquake occurrence are roughly proportional to earthquake magnitude. Similar variations of the GPS azimuths were observed in GPS data containing or excluding co-seismic dislocation (i.e., one day before) in the temporal and spatial domain. These suggest that the aligned GPS azimuth could be a promising anomalous phenomenon for studying crustal deformation before earthquakes.

Scientists demystify stress changes within tens of days before a mainshock and often utilize its foreshocks as an indicator. Typically, foreshocks are detected near fault zones, which may be due to the distribution of seismometers. This study investigates changes in seismicity far from mainshocks by examining tens of thousands of M≥2 quakes that were monitored by dense seismic arrays for more than 10 years in Taiwan and Japan. The quakes occurred within epicentral distances ranging from 0 to 400 km during a period of 60 d before and after the mainshocks that are utilized to exhibit common behaviors of seismicity in the spatiotemporal domain. The superimposition results show that wide areas exhibit increased seismicity associated with mainshocks occurring more than several times to areas of the fault rupture. The seismicity increase initially concentrates in the fault zones and gradually expands outward to over 50 km away from the epicenters approximately 40 d before the mainshocks. The seismicity increases more rapidly around the fault zones approximately 20 d before the mainshocks. The stressed crust triggers ground vibrations at frequencies varying from $\sim \mathrm{5}\times {\mathrm{10}}^{-\mathrm{4}}$ to $\sim {\mathrm{10}}^{-\mathrm{3}}$ Hz (i.e., variable frequency) along with earthquake-related stress that migrates from exterior areas to approach the fault zones. The variable frequency is determined by the observation of continuous seismic waveforms through the superimposition processes and is further supported by the resonant frequency model. These results suggest that the variable frequency of ground vibrations is a function of areas with increased seismicity leading to earthquakes.

The 2013 ML 6.4 Ruisui earthquake struck a seismic gap around the northern part of the Longitudinal Valley where rare earthquakes have occurred for at least two decades. Seismic data with different frequency bands connote a diverse scale of source characteristics. The slip model derived by a previous study using long-period seismic data provided us with the preliminary earthquake source properties in low-frequency energy radiation. However, the high-frequency part of the seismic energy needs to be considered for the strong motions in the comprehensive frequency band. As a case study, we carried out broadband strong motion simulations through the hybrid scheme, which simulated waveforms combining the low-frequency motions (frequency-wavenumber integration method) and high-frequency motions (empirical Green’s function method) to reveal the strong ground motion and source characteristics of the 2013 Ruisui earthquake. The results show that even though the local structure is complicated this hybrid simulation can effectively rebuild the overall broadband strong ground motion characteristics for the 2013 Ruisui earthquake. Additional study issues are required to better understand the nature of localized high peak ground acceleration and the related seismic hazards for future earthquakes.\u

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.