地核反射之剪力波触发余震:以四川地震为例(英文)

Since the Earth outer core is liquid, the shear waves that are generated by a large earthquake and traveling to the core (ScS) will totally reflect with strong seismic energy.     

Three-dimensional shallow velocity structure beneath Taal Volcano,Philippines

Based on its numerous historical explosive eruptions and high potential hazards to nearby population of millions, Taal Volcano is one of the most dangerous “Decade Volcanoes” in the world. To provide better investigation on local seismicity and seismic structure beneath Taal Volcano, we deployed a temporary seismic network consisting of eight stations from March 2008 to March 2010. In the preliminary data processing stage, three periods showing linear time-drifting of internal clock were clearly identified from noise-derived empirical Green’s functions. The time-drifting errors were corrected prior to further data analyses. By using VELEST, 2274 local earthquakes were manually picked and located. Two major earthquake groups are noticed, with one lying beneath the western shore of Taal Lake showing a linear feature, and the other spreading around the eastern flank of Taal Volcano Island at shallower depths. We performed seismic tomography to image the 3D structure beneath Taal Volcano using the LOTOS algorithm. Some interesting features are revealed from the tomographic results, including a solidified magma conduit below the northwestern corner of Taal Volcano Island, indicated by high Vp, Vs, and low Vp/Vs ratio, and a large potential hydrothermal reservoir beneath the center of Taal Volcano Island, suggested by low Vs and high Vp/Vs ratio. Furthermore, combining earthquake distributions and tomographic images, we suggest potential existence of a hydrothermal reservoir beneath the southwestern corner of Taal Lake, and a fluid conduit extending to the northwest. These seismic features have never been proposed in previous studies, implying that new hydrothermal activity might be formed in places away from the historical craters on Taal Volcano Island.     

Seismicity characteristics of a potentially active Quaternary volcano: The Tatun Volcano Group, northern Taiwan

The Tatun Volcano Group (TVG) is located at the northern tip of Taiwan, near the capital Taipei and close to two nuclear power plants. Because of lack of any activity in historical times it has been classified as an extinct volcano, even though more recent studies suggest that TVG might have been active during the last 20 ka. In May 2003 a seismic monitoring project at the TVG area was initiated by deploying eight three-component seismic stations some of them equipped with both short-period and broadband sensors. During the 18 months observation period local seismicity mainly consisted of high frequency earthquakes either occurring as isolated events, or as a continuous sequence in the form of spasmodic bursts. Mixed and low frequency events were also present during the same period, even though they occurred only rarely. Arrival times from events with clear P-/S-wave phases were inverted in order to obtain a minimum 1D velocity model with station corrections. Probabilistic nonlinear earthquake locations were calculated for all these events using the newly derived velocity model. Most high frequency seismicity appeared to be concentrated near the areas of hydrothermal activity, forming tight clusters at depths shallower than 4 km. Relative locations, calculated using the double-difference method and utilising catalogue and cross-correlation differential traveltimes, showed insignificant differences when compared to the nonlinear probabilistic locations. In general, seismicity in the TVG area seems to be primarily driven by circulation of hydrothermal fluids as indicated by the occurrence of spasmodic bursts, mixed/low frequency events and a b-value (1.17 ± 0.1) higher than in any other part of Taiwan. These observations, that are similar to those reported in other dormant Quaternary volcanoes, indicate that a magma chamber may still exist beneath TVG and that a future eruption or period of unrest should not be considered unlikely.     

Active continental subduction and crustal exhumation: the Taiwan orogeny

ABSTRACT A tectonic model of active continental subduction followed by crustal exhumation is proposed to explain the orogeny in Taiwan. The subducted crust is represented by a low-velocity zone dipping eastwards beneath the major part of Taiwan, while the exhumed crust is marked by a high-velocity bulge, high heat flow and absence of seismicity beneath the eastern Central Range. The boundary between the subducted and exhumed crust has been identified from surface geology and analyses of thermal history across the Central Range. The dynamic force that has been driving the exhumed crust is identified by results from focal mechanisms, structural geology and geodetic survey in the eastern Central Range. Such a tectonic model may provide a good explanation for the evolution of the Taiwan Orogeny, as well as an active case for studying other long-extinct systems of continental subduction and exhumation.     

Geomagnetic anomaly at Lunping before the 1999 Chi-Chi earthquake (M w = 7.6) in Taiwan

A strong earthquake with a magnitude of 7.6 (M _L  = 7.3) occurred on September 21, 1999, in central Taiwan. In order to discern any potential precursors before this earthquake, geomagnetic data at Lunping (LNP), Taiwan, Geomagnetic Observatory situated 100 km from the epicenter are examined using two methods, i.e., the traditional induction arrows and complex demodulation. Our results show that the remarkable temporal variation of real induction arrows appear to be strong prior to the great earthquake over the previous 24 months. After the great earthquake, the magnitudes of induction arrows decreased to the normal (mean of 8 years) levels. In other words, the direction of real induction arrows of the periods 30 and 20 min rotated 85° and 40° anticlockwise, respectively, before the Chi-Chi earthquake and returned to mean direction of last 10 years after the earthquake. A horizontal source field model using the finite difference method for 3-D shows that the variation of the real induction arrows might be ascribed to the conductivity variation body, which is 5 km buried at the epicenter area of the Chi-Chi earthquake, changing its conductivity from 0.002S/m to 0.06 S/m. The ratios of modulus (demodulated by using the complex demodulation method) over a period 12, and 8 h relative to the period of 24 h reveal a remarkable change that appeared 4–5 months prior to this strong earthquake. They increased gradually from the beginning of 1999 to August 1999 and decreased again to a (8 years) mean level after the strong earthquake occurrence. We consider that the variation of the induction arrow might be ascribed to the conductivity anomaly, which is buried 5 km at the south-east side of LNP with a conductivity change of 0.06 S/m. We propose that this elevation might be related to the preparation process of the great earthquake.     

Tomographic image of crustal structures across the Chelungpu fault: Is the seismogenic layer structure- or depth-dependent?

In 1999, a large earthquake (M_w = 7.6) occurred along the Chelungpu fault in the fold-and-thrust belt of western Taiwan. To shed more light on the subsurface structures and the seismogenic layers, three-dimensional velocity structures were inverted by using the travel times of both P- and S-waves from 2391 aftershocks recorded by the Central Weather Bureau during the 15 months that followed. From tomography, a typical image of the large-scale thrusting structures in the upper crust across the Chelungpu fault was obtained. In general, high velocities beneath the Western Foothills and Central Ranges are separated from low velocities beneath the Coastal Plain by an east-dipping boundary that is roughly consistent with the Chelungpu fault on the surface. The contrast in velocity on either side of the Chelungpu fault is indicative of about a 7- to 9-km vertical offset in the upper crust. The relocated hypocenter for the Chi-Chi earthquake shifts by 2.2 km toward the northwest, and its focal depth decreases by 0.7 km. A plot of focal depths versus rock velocities where the aftershocks occurred shows earthquakes are more inclined to occur in rock with a velocity of around 5.6 km/s. This strongly suggests the seismogenic layer in the fold-and-thrust belt of Taiwan is more structure-dependent than depth-dependent.