Horng-Yuan Yen

Two‐dimensional crustal structures of Taiwan from gravity data

To delineate the tectonic character of Taiwan, an island-wide gravity survey of Taiwan was conducted between 1980 and 1987. The Bouguer anomaly map shows that, in general, isogals trend NNE in consonance with the overall structural trend of the island. With seismic and other geophysical data as constraints, the subsurface density structures were modeled along three profiles across the major structural trends. The gravity data are consistent with average continental Moho depths of 26 km in the Coastal Plain and the Western Foothills, 28 km underneath the Coastal Range in eastern Taiwan, and 33 km under the Central Range. A lack of significant correlation of the Bouguer anomalies with the topography implies dynamic, rather than isostatic, support of topography in the Taiwan region.

Geomagnetic fluctuations during the 1999 Chi-Chi earthquake in Taiwan

On 21 September 1999 (Taiwan local time), a major earthquake measuring M7.3 occurred near the town of Chi-Chi in central Taiwan. After the Chi-Chi earthquake, geomagnetic data recorded by a network of 8 stations equipped with continuous recording systems was analyzed. The results revealed that the total geomagnetic intensity of the Liyutan station, about 8 kilometers from the northern end of the Chelungpu fault (considered to be related to the earthquake), fluctuated significantly for more than a month prior to the earthquake. The fluctuation features continued and then stopped after the Chia-Yi earthquake (M6.2) occurred near the southern end of the Chelungpu fault on 22 October 1999. The variation of intensity reached 200 nTs. Geomagnetic fluctuations were also found at the Tsengwen station, located about 42 kilometers from the southern end of the Chelungpu fault and 30 kilometers from the Chia-Yi earthquake. These geomagnetic disruptions with highly anomalous amplitudes associated with the Chi-Chi and Chia-Yi earthquakes appear to have been the result of the accumulation and release of crustal stress that led to the subsequently severe surface rupture at the time of the earthquakes.

Three-dimensional elastic wave velocity structure of the Hualien region of Taiwan: Evidence of active crustal exhumation

The Hualien region of Taiwan is located at a complex transition of the boundary between the Eurasian and Philippine Sea Plates. To the southwest, the mountains of Taiwan are uplifting rapidly as a consequence of an ongoing arc-continent collision, while to the east the oceanic Philippine Sea Plate is subducting northward beneath Eurasia. We investigated the structure and dynamics of this region by analyzing seismograms of local earthquakes recorded during a deployment of the Portable Array for Numerical Data Acquisition II network. P and S wave velocity structures deduced from travel time tomography analysis show that the collisional suture to the south of Hualien is characterized by a narrow (< 10 km width), near vertically dipping zone of low velocities that extends to depths in excess of 20 km. Velocities in the Eastern Central Range west of the suture zone are significantly higher and define a feature 10–15 km wide that appears to be continuous from the near surface to depths as great as 40 km. Farther to the west beneath the Western Central Range, the velocities again decrease. Focal mechanisms of local earthquakes show that while thrust faulting is the predominate mode of deformation throughout the region, normal faulting occurs as well beneath the Eastern Central Range. Thus the rapid uplift of the mountains of Taiwan may be a result not only of compressional shortening but also of an excess of positive buoyancy. We suggest that the higher velocities and extensional mechanisms in the Eastern Central Range are caused by the ongoing exhumation of previously subducted continental crust, while the lower velocities to the west reflect continued underthrusting of the crust beneath the Eastern Central Range.

Lateral variations of Pn velocity and anisotropy in Taiwan from travel-time tomography

In this study, the P arrivals of 1726 ray paths with the epicentral distances longer than 200 km from 539 earthquakes that occurred in Taiwan are used to investigate the variation of Pn velocity. The results show that the lateral variation of Pn velocity in Taiwan is quite similar to the pattern of gravity anomaly and strongly related to several features of Taiwan’s geological structure. The crust is a little thicker (about 39 km) in the Central Range and somewhat thinner toward the east and west. A relatively thin crust (about 35 km) is obtained in the area of Peikang High; however, an unexpected thick crust of greater than 40 km is also found at the corner of southwestern Taiwan. The degree of anisotropy of Pn velocity is found less than 10% and the fast direction is generally in the EW direction. This direction is parallel to the axis of compressional stress or the direction of plate motion, indicating that the anisotropy results from the deformation of the upper mantle.

The vertical propagation of disturbances triggered by seismic waves of the 11 March 2011 M9.0 Tohoku earthquake over Taiwan

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 Earths 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.

Joint Passive/Controlled Source Seismic Experiment Across Taiwan

Scientists who study Earth structures and interactions at tectonic plate boundaries illuminate the three-dimensional (3-D) subsurface primarily through passive or active source seismic studies. In passive source studies, natural earthquakes are observed through arrays of seismic stations, allowing seismologists to image structures at depth and infer physical properties of the Earth. In active (controlled) source studies, scientists provide an artificial seismic source to be recorded by seismic networks, allowing more detailed views of fine-scale structures buried within plate boundaries. The use of “continuous” modes on large-storage-capacity seismic recording systems enables seismologists to blur traditional lines between active and passive source seismology in regions of high earthquake activity. In addition to generating seismograms from controlled sources, a deployed dense array can record local and teleseismic earthquakes with exceptionally coherent signals across its aperture. As a result of such joint recording, the scientific value of the collected data increases. Moreover, whether or not the seismic sources are natural or engineered becomes less of a factor for data storage, sorting, and analysis methods. Local earthquakes can supplement controlled sources, multichannel processing and imaging methods traditionally used in active seismic studies become amenable to dense sets of passive source data, and active source data with known source origin times and locations are easily incorporated into regional seismic 3-D tomography and earthquake location studies.

Comparisons Between Air and Subsurface Temperatures in Taiwan for the Past Century: A Global Warming Perspective

Air and sea surface temperature increases due to global warming have been widely observed around the world at various rates. This temperature rising has also been documented in many subsurface records recently. The air-ground temperature coupling system introduces an important factor in disturbing the original thermal balance and provides a new dimension to comprehend the effects of global warming on the Earth system. Ten meteorological stations of Central Weather Bureau in Taiwan that have been routinely measured for air (1.5 m above the ground) and subsurface (at depths of 0, 5, 10, 20, 30, 50, 100, 200, 300 and 500 cm below the ground) temperatures are used for in-depth comparison in this study. These stations have a mean observation period of 82 years (as of 2008) to provide good coverage for a preliminary examination of air-ground temperature coupling relationship in a century scale. Results show that patterns and variations of air and subsurface temperature are quite different among stations in Taiwan. In general, air and subsurface temperatures exhibit consistent linear trends after 1980 due to accelerating global warming, but display complex and inconsistent tendencies before 1980. When surface air temperature is subtracted from subsurface one, the differences in the eastern Taiwan are generally larger than those in the western Taiwan. This observation is possibly caused by (1) heat absorption of dense high-rise buildings, and/or (2) cut off heat propagating into deep depths in the urban area of western Taiwan. By comparing temperature peaks at various layers from shallow to deep, rates of thermal propagation can be estimated. The distinct time shifts among stations suggest that thermal propagations have to be taken into account when constructing historical temperature records.

Crustal‐Scale Weak Zone along a Collisional Suture Revealed by Spatial Variations in Velocity Structures and Seismicity

Abstract High‐quality first‐arrival data collected with a high‐density temporary seismic array and regional seismic network were used to construct a P ‐wave velocity model and identify the precise location of earthquakes in the active collision zone of southeastern Taiwan. A crustal‐scale weak zone, defined by high seismicity, is characterized by a steeply east‐dipping lower‐velocity anomaly and represents a boundary between an uplifted upper‐mantle and the Luzon volcanic arc. The main features of the weak zone, with its associated low‐velocity zone and seismicity pattern, vary significantly along the collisional boundary of southeastern Taiwan. Along the weak zone, unlithified sediments or highly fractured materials are compacted as the Philippine Sea plate moves toward the Eurasian plate. In areas of more advanced collision, soft material is more compacted and can therefore store significantly greater amounts of strain energy. This energy is released through episodic earthquakes.