Search

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 ∼5×10-4 to ∼10-3 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: A blind thrust fault with a unique strike, which is orthogonal to the strike of most tectonic structures in Taiwan, triggered the Jiashian earthquake on March 4, 2010 (M =6.4; 22.96°N, 120.70°E). This study utilizes 100 global positioning system stations to examine changes of surface displacements during the Jiashian earthquake. We mitigate effects of short-term noise and long-term plate movements from surface displacement data using a frequency dependent filter via the Hilbert–Huang transform and compute the horizontal azimuth (i.e. GPS-azimuth) using residual data at the NS component relative to residual data at the EW component. Analytical results show that orientations of horizontal azimuths were aligned and orthogonal to the strike of the blind thrust fault. Meanwhile, inverse orientations are observed before and after the earthquake that agrees well with the seismic rebound theory. As stress disturbed on strata a few days before the earthquake, an impeded region can be clearly identified by disordered orientations of horizontal azimuths for anticipating the mainshock. These results provide an additional view to explore stress disturbance associated with earthquakes and offer more information to examine diverse models of tectonic evolution in this region. [Copyright &y& Elsevier]

Non-uniqueness is a problem encountered while imaging subsurface susceptibility by inverting magnetic field surveys. We propose a novel method to construct velocity-susceptibility initial models using Vp together with the Vp/Vs ratio from seismic tomography to improve the model which satisfies more information and better identifies locations of high-susceptibility materials. For comparison, two recovered susceptibility structures are derived from two distinct initials (homogeneous and velocity-susceptibility) models through the standard inversion process. Two profiles with intense undulation of magnetic anomalies over sedimentary areas in central-west Taiwan and complex geological structures at the rim of the subduction zone in northeast Taiwan are used as examples to compare the results of the initial models. The inversion results suggest that the susceptibility structures from both models agree in terms of the location of the fault zones, specifically at depth < 10 km. Recovered susceptibility structures derived from the velocity-susceptibility initial model agree with the geological structure that reduces uncertainty at depth > 10 km. Consequently, the initial model with velocity constraints lowers the non-uniqueness associated with the inversion of magnetic anomalies. Unlabelled Image • High-susceptibility materials can be distinguished by Vp accompanying with Vp/Vs. • Velocity-susceptibility initial models can be constructed for adapting complex reality. • Recovered susceptibility distribution and geological structure yield agreements. • Non-uniqueness in the inversion of magnetic anomalies can be obviously reduced. [ABSTRACT FROM AUTHOR]