In this paper, concurrent/colocated measurements of seismometers, infrasonic systems, magnetometers, HF‐CW (high frequency‐continuous wave) Doppler sounding systems, and GPS receivers are employed to detect disturbances triggered by seismic waves of the 11 March 2011 M9.0 Tohoku earthquake. No time delay between colocated infrasonic (i.e., super long acoustic) waves and seismic waves indicates that the triggered acoustic and/or gravity waves in the atmosphere (or seismo‐traveling atmospheric disturbances, STADs) near the Earth's surface can be immediately activated by vertical ground motions. The circle method is used to find the origin and compute the observed horizontal traveling speed of the triggered infrasonic waves. The speed of about 3.3 km/s computed from the arrival time versus the epicentral distance suggests that the infrasonic waves (i.e., STADs) are mainly induced by the Rayleigh waves. The agreements in the travel time at various heights between the observation and theoretical calculation suggest that the STADs triggered by the vertical motion of ground surface caused by the Tohoku earthquake traveled vertically from the ground to the ionosphere with speed of the sound in the atmosphere over Taiwan.

An earthquake process includes pre-seismic stress accumulation, co-seismic rock rupture and post-seismic elastic and/or viscoelastic rebound. Although co-seismic and post-seismic deformations have been readily observed using the global positioning system (GPS), detecting pre-seismic stress accumulation hidden in time-series data remains challenging. This study applies the Hilbert-Huang transform to extract non-linear and nonstationary pre-earthquake deformation data from GPS records for central Taiwan. By converting the derived surface deformation into horizontal azimuths, the randomly oriented GPS-azimuths are reoriented in a similar direction several days before and after earthquakes due to loading and rebound stress, respectively. Analytical results demonstrate that the stress accumulation and release along the entire course of an earthquake process provide significant evidence supporting the seismic rebound theory. This finding would be applicable to areas with dense GPS networks and active plate interactions. Surface deformations detected by the proposed analytical technique have encouraging potential for mitigating future seismic hazards.

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Abstract. Unusual decreases of water levels were consistently observed in 78 % (= 42/54) of wells in the Choshuichi Alluvial Fan of central Taiwan about 250 days before the Chi-Chi earthquake (M = 7.6 on 20 September 1999) while possible factors of barometric pressure, earth tides, precipitation as well as artificial pumping were removed. Variations in groundwater levels measured on anomalous wells from 1 August 1997 to 19 September 1999, which covers the 250 day unusual decreases, were transferred into the frequency domain to unveil frequency characteristics associated with the Chi-Chi earthquake. Analytical results show that amplitudes at the frequency band between 0.02 day −1 and 0.04 day −1 generally maintained at the low stage and were apparently enhanced a few weeks before the Chi-Chi earthquake. Variations of amplitude at this particular frequency band were further examined along with other Taiwan earthquakes (M > 6) from 1 August 1997 to 31 December 2009. Features of the enhanced amplitudes at the frequency band are consistently observed prior to the other two earthquakes (the Rei-Li and Ming-Jian earthquakes) during the 12.5 yr study period. In addition, surface displacements recorded from GPS, which provides insights into understanding stress status in subsurface during the Chi-Chi earthquake, are also inspected. The result confirms that abnormal rise and fall changes in groundwater level yield an agreement with forward and backward surface displacements around the epicentre prior to the Chi-Chi earthquake.

The Parkinson vectors derived from 3-component geomagnetic data via the magnetic transfer function are discussed with respect to epicentre locations and hypocentre depths of 16 earthquakes (M ≥ 5.5) in Taiwan during a period of 2002–2005. To find out whether electric conductivity changes would happen particularly in the seismoactive depth ranges, i.e. in the vicinity of the earthquake foci, the frequency dependent penetration depth of the electromagnetic waves (skin effect) is taken into account. The background distributions involving the general conductivity structure and the coast effect at 20 particular depths are constructed using the Parkinson vectors during the entire study period. The background distributions are subtracted from the time-varying monitor distributions, which are computed using the Parkinson vectors within the 15-day moving window, to remove responses of the coast effect and underlying conductivity structure. Anomalous depth sections are identified by deviating distributions and agree with the hypocentre depths of 15 thrust and/or strike-slip earthquakes with only one exception of a normal fault event

In this study, we adopt a damping least-square inversion method to investigate the Vp structures and Vp/Vs ratios of the crust and upper mantle beneath the Chia-Nan area, Taiwan. Previous studies have shown that, velocity structure can be used as an indicator of the geometry of a fault and the general aspects of tectonics. Therefore, the first goal of this research is to analyze the degree of correlation between the velocity structure and the seismic characteristics with respect to the tectonic implications of the area. The second intention is to study the relationship between the Vp/Vs ratio and the pressure change occurring with crack opening in the deeper crust. Finally, the distribution of Vp/Vs ratios and its association with fault activities is also investigated. Our results indicate that the variations in velocity structure beneath the Chia-Nan area is caused by local geological structures, fault crossing and the existence of the Pei-Kang High Area. We also find that most earthquakes occur in areas that have Vp/Vs gradients varying rapidly. In addition, according to the distribution of the earthquakes, there seems to exist a westdipping fault west of the Chukou fault; however, this prospect needs to be investigated further in a future study.

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.