Wen-tzong Liang

Large earthquakes near active volcanoes, that exhibit non-double-couple source properties are usually interpreted as result the of either magma intrusion or geometrical complexity along the fault plane. Such an earthquake occurred in 1996 September 29 at Bárdarbunga volcano in central Iceland, to be followed 2 days later by a major volcanic eruption at the area between Bárdarbunga and the nearby Grimsvötn volcano. Both of these active volcanic centres lie underneath the Vatnajökull glacier, a permanent ice cap that covers a large area of central Iceland. This event was recorded by a temporary network (HOTSPOT) that consisted of 30 broad-band three-component seismometers covering most of Iceland. The waveforms of this event at all stations show an emergent, low-amplitude, high-frequency onset that is superposed on a longer-period signal. The corresponding amplitude spectra show a low-frequency content (<1 Hz) and prominent peaks around the corner frequency (~0.25 Hz) and higher frequencies. These regional waveforms were inverted in order to obtain the best-fitting deviatoric and full moment tensor using a linear, time-domain inversion method. The results for the deviatoric moment tensor indicate a large (~60 per cent) compensated linear vector dipole (CLVD) component, a hypocentral depth of 3.5 km, a moment magnitude of 5.4 and a best double-couple solution showing thrust motion in good agreement with the previously published Harvard CMT solution. The results for the full moment tensor on the other hand, indicate an implosive isotropic component of 8.5 per cent, a reduced CLVD component of 47.2 per cent and a best double-couple solution showing normal faulting. However, a statistical F-test revealed that the full moment tensor does not fit the data significantly better than the deviatoric at a confidence level of not more than 76 per cent. All of these results were found not to change substantially when a different source time function was used or when the data were weighted according to their distance from the source. The data are consistent with an earthquake of this magnitude, caused by the failure of an asperity and the formation of a tensile crack due to increasing fluid pressure. The dimensions of the crack may have been 10 × 3 km2 and 0.5 m thickness and the volume of the injected fluid was found to be 15 × 106 m3. The calculated viscosity for the fluid (0.04 Pa s) points to the possibility of water being injected rather than magma, that is also supported by the short source duration of the earthquake (~5 s). Taking into account the water saturation of the upper crust in Vatnajökull due to the presence of the glacier, this event may have been caused by increased pressure of water that was heated by magma injected through a dyke below the asperity.

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 suggestthatTVGmighthavebeenactiveduringthelast20ka.InMay2003aseismicmonitoringprojectattheTVGareawasinitiated by deploying eight three-component seismic stations some of them equipped with both short-period

Located in the southeastern periphery of the Eurasian plate, eastern Taiwan marks the collional boundary between the Eurasian plate and the Philippine Sea plate. These two plates converge at about 8 cm/yr near Taiwan and nearly half of the shortening is consumed in eastern Taiwan. There have been many studies in this area about the dynamics of the plate convergence,

We present a 2.14-Myr astronomically tuned relative geomagnetic paleointensity record from the western Philippine Sea. Pseudosingle-domain titanomagnetite is the only magnetic mineral identified and variations in titanomagnetite concentration fall well within the accepted limits for relative paleointensity variations. No significant temporally persistent periodicities are observed in wavelet analyses of the paleointensity time series or in the rock magnetic parameters used for relative paleointensity normalization. This suggests that our paleointensity record is largely free of rock magnetic or lithological artefacts and that it represents a reliable record of geomagnetic behavior with no evidence for modulation of the field at Earth orbital periods. The paleointensity record is highly coherent with the Sint-800 global paleointensity stack for the last 800 kyr and with a coeval record from the West Caroline Basin. Our record confirms that it is normal for the geomagnetic field to un...

We compile a comprehensive list of hypocentral locations and source parameters for earthquakes occurred in the Taiwan region. The collected database is used to construct 3D patterns of seismic deformation that are associated with various tectonic processes such as subduction and collision between the Philippine Sea plate and Eurasia. Specifically, we divide the entire Taiwan region into a 3D grid of 10km interval and the amount of seismic moment released at each point is the sum of all individual events in the vicinity. In case when the physical dimension of an earthquake is larger than 10km, the corresponding seismic moment is distributed across multiple grid points. This quantitative approach gives a better depiction on many first-order features in the region. The most intriguing one is that a significant deficit in the total amount of released seismic moment can be clearly identified around the Chi-Chi source region before the big earthquake occurred in 1999. The deficit trough is then filled by the seismic moment of the Chi-Chi earthquake sequence. This deficit-then-fill pattern in a region's seismic moment distribution can be used as an indicator to pinpoint the locations of large earthquakes in the foreseeable future. Following this argument, we suggest that disastrous earthquakes with magnitudes comparable to that of the Chi-Chi earthquake are due for the two regions adjacent to the Chi-Chi source area (i.e., the Miaoli-Hsinchu domain to the north and the Chiayi domain to the south).