5 research outputs found
The Volcanic Earthquake Swarm of October 20, 2009 in the Tatun Area of Northern Taiwan
On October 20, 2009, a series of felt earthquakes with local magnitudes ranging from 2.8 - 3.2 occurred in the Tatun volcanic area off the northern tip of Taiwan. Although there was no damage caused by those earthquakes, many residents in the Taipei metropolitan area, particularly for people who live near the Yangminshan National Park, felt strong ground shaking. In order to know what the possible mechanisms were that generated those earthquakes, we carefully examined seismic data recorded by a dense seismic array in the Tatun volcanic area. During the period between October 18 and 22, 2009 we detected at least 202 micro-earthquakes. Most of the earthquakes were relocated using the double-difference method and were clustered in the shallow crust beneath the Dayoukeng area, which is the strongest fumarole in the Tatun volcanic area. Among these earthquakes, 72 focal mechanisms were determined by polarizing the first P-wave motion. Most earthquakes belonged to normal faulting. An extremely high b-value of 2.17 was obtained from those earthquakes. Based on the seismic variations in both the temporary and spatial distribution as well as an extremely high b-value, we conclude that the earthquake sequence on October 20, 2009 was a typically seismic swarm associated with possible active volcanism in the Tatun volcanic area
Studies on Aftershocks in Taiwan: A Review
We reviewed studies on aftershocks in Taiwan for the following topics: the spatial-temporal distributions and focal-plane solutions of aftershocks fromof thirty larger earthquakes with magnitudes > 5; the correlations between the mainshock and the largest aftershock based on dependence of the differences in magnitudes (ΔM), occurrence times (ΔT), epicenters (ΔH), and focal depths (ΔD) upon the mainshock magnitude, Mm; magnitude-dependence of p-value of Omori’s law of aftershocks; the correlation between the b-value of the Gutenberg-Richter’s frequency-magnitude law and the p-value; application of the epidemic-type aftershock sequences (ETAS) model to describe the aftershock sequence; the mechanisms of triggering aftershocks; and dynamic modeling of aftershocks. The main results are: (1) The spatial distribution of aftershocks for some earthquakes is consistent with the recognized fault; (2) Unlike Båth’s law, ΔM slightly increases with Mm; (3) ΔT does not correlate with Mm; (4) ΔD does not correlate with Mm; (5) ΔT somewhat increases with ΔD; (6) The p-value slightly increases with Mm; (7) There is a negative correlation between the b- and p-values. (8) There was seismic quiescence over a broader region of Taiwan before the 1999 Chi-Chi earthquake; (9) Both the static and dynamic stress changes trigger aftershocks; and (10) Dynamic modeling shows that a decrease in elastic modulus is a significant factor in triggering aftershocks
The Geophysical Database Management System in Taiwan
The Geophysical Database Management System (GDMS) is an integrated and web-based open data service which has been developed by the Central Weather Bureau (CWB), Taiwan, ROC since 2005. This service went online on August 1, 2008. The GDMS provides six types of geophysical data acquired from the Short-period Seismographic System, Broadband Seismographic System, Free-field Strong-motion Station, Strong-motion Building Array, Global Positioning System, and Groundwater Observation System. When utilizing the GDMS website, users can download seismic event data and continuous geophysical data. At present, many researchers have accessed this public platform to obtain geophysical data. Clearly, the establishment of GDMS is a significant improvement in data sorting for interested researchers
Continuous CWB GPS Array in Taiwan and Applications to Monitoring Seismic Activity
GPS observations have revealed important information for studying active tectonics and plate motion and are a useful tool for monitoring crustal deformation. The CWB continuous GPS array consists of approximately 150 stations with dense spatial coverage throughout Taiwan and can be used not only to monitor crustal deformation and seismic activity, but also to analyze the earthquake precursors in Taiwan
Velocity Field Derived from Taiwan Continuous GPS Array (2007 - 2013)
Data were collected from 281 Taiwan continuous Global Positioning System (cGPS) Array sites from 2007 - 2013 and processed with GAMIT/GLOBK software. Power spectral density stacking from cGPS position time series in Taiwan found the spectral index as -0.72, -0.77, and -0.57 for the E, N, U components, respectively. This indicates the cGPS data errors can be described as a combination of white noise and flicker noise. The common-mode errors are removed by stacking data from 50 cGPS sites with data periods greater than 5 years. By removing the common-mode errors the GPS data precision is further improved to 2.3, 1.9, and 6.9 mm in the E, N, U components, respectively. After strict data quality control, time series analysis and noise analysis, we derive a new Taiwan velocity field using cGPS data from 2007 - 2013. The general pattern of the newly derived 2007 - 2013 velocity field is quite similar to that from previous studies, but the station density is much larger and spatial coverage better. About 80 mm yr-1 plate convergence rate is observed, half of the rate is accommodated on the fold and thrust belt of western Taiwan and another half is taken up in the Longitudinal Valley and Coastal Range in eastern Taiwan. The velocities in western Taiwan generally show a fan-shaped pattern, consistent with the maximum compression tectonic stress direction. In northern Taiwan the velocity vectors reveal clockwise rotation, indicating the on-going extensional deformation related to the back-arc extension of the Okinawa Trough. In southern Taiwan, the horizontal velocity increases from about 40 mm yr-1 in the Chia-Nan area to 55 mm yr-1 in the Kao-Ping area with a counterclockwise rotation