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The solar (S) and lunar (L) variations of geomagnetic fields at the horizontal (H), declination (D), and the downward vertical component data (Z) are modeled by the Chap man-Miller method with four order harmonics. In this paper, we compare S and L variations of the geomagnetic total intensity field using a consistent method with 3-component data for seasonal variations (summer, winter, and equinox) for three distinct phases during the years 1988 - 2007. The results show that consistency in the S and L variations for geomagnetic total intensity indicates normal stations and discrepancies are occurred due to data quality. In application, consistent results also prove that the function of the magnetometers at TW was normal and that large anomalies were certainly in existence during the Chia-Yi earthquake.

During exploration using electromagnetic methods, secondary electro magnetic (EM) fields due to induced currents from conductors, together with the primary EM field, are re corded with a suit able receiver at various points in space. In general, the secondary EM field at the receiver, which contains all the in for mation regarding under ground conductors, may be several orders of magnitude smaller than the primary field. Under these conditions the separation of the measured total EM field into its primary and secondary parts is difficult. For the purpose of measuring the secondary fields in the absence of the primary field, time-do main electromagnetic measurements, often referred to as transient electromagnetic (TEM) techniques are employed. Comparative numerical modeling using a single component (measuring the vertical component of the field, Hz) TEM receiver and a three-component (measuring Hx, Hy, and Hz) TEM receiver was under taken. A for ward modeling approach was used to compute the voltage response of half-space containing one or more conductive bodies ex cited by a bi-polar square wave form. Al though this method utilizes conductor scattering, it is particularly useful as a practical use for unexploded dordnance (UXO) detection. Un like single component data, results from the three-component data are unambiguous as to the location and orientation of conductors. Measurements with the addition of horizontal- component data for secondary magnetic fields lead a better indication of tar get location, and tar get size determination, orientation, and characteristics, especially for the tar gets in the horizontal plan. A three-component TEM field experiment at a well-documented well site (NCU cam pus) was con sis tent with the effects predicted by our the oretical modeling. As a result, the three-component TEM survey is an essential element for high-resolution EM engineering survey.

Broadband records from ocean-bottom seismometers deployed in the Okinawa trough and the Huatung basin were analyzed to provide seafloor noise characteristics and the detection thresholds for teleseismic body and surface waves. Ambient noise levels on the horizontal components are 10-40 dB higher than on the vertical component, with the sensor seated on the surface of the sediment. On the vertical components, infragravity waves are 10-30 dB more energetic at the shallower Okinawa trough sites (≤2000 m depth) than at the deeper Huatung basin site (~4700 m). From 0.03 to 0.2 Hz, the Huatung basin noise levels are comparable to that of the broadband stations in Taiwan on a quiet day. The microseism peaks (~0.2-0.5 Hz) of OBSs reach or exceed the high noise model of continental stations. At regional distances M(subscript w) 6.5 is required for recording prominent Rayleigh waves if the source radiation is unfavorable, but 6.2 is sufficient for a favorable focal mechanism. Several tens to over one hundred high-fidelity P, P(subscript diff) and PKP waveforms have been recorded per year by OBSs at high corner frequency of 0.1 Hz with a minimum M(subscript w) 5.3-6.0. The number of recording drops to less than 5 per year at 1 Hz with M(subscript w)≥6.4 and distances less than 90°.

To achieve an improved over all under standing of the tectonics of Tai wan, an is land-wide magnetic survey, especially in the mostly inaccessible mountain ranges, was carried out from July 2003 to February 2004. In total, 6063 magnetic points were surveyed at an average spacing of 2 km. After data reductions, a new magnetic anomaly map of Taiwan was constructed. The map shows the range of magnetic anomalies mostly distributing between -400 to 400 nT. Anomalies in the southern part of Taiwan are much higher than those in the north whilst anomalies in the western plains area are higher than those of the mountain ranges.

On 17 July 1998, an earthquake of magnitude 6.2 (M(subscript L)) occurred near Chiayi, southwestern Taiwan. Two days after the occurrence of the mainshock, a temporary digital seismic network was deployed near its epicenter for 24 days to monitor the aftershock activity. The events within and near the network were first located using a traditional single-event location and then relocated using a joint hypocentral determination (JHD) technique to determine the significance of lateral velocity variations and to improve earthquake locations. The station corrections obtained from the JHD analysis vary from 0.30 to -0.18 sec for the P-waves and from 0.51 to -0.58 sec for the S-waves. The patterns of station corrections suggest that upper crustal velocities on the eastern side of the study area are relatively higher than those on the western side. The depth cross-section of the relocated aftershocks shows a clear pattern of southeast-dipping distribution of the hypocenters. The focal mechanisms and hypocentral distribution of the relocated aftershocks reveal a close relationship between seismicity and the known faults in the study area.

A high-resolution seismic array experiment using a 30-station second-generation of Portable Array for Numerical Data Acquisition (PANDA Ⅱ) array was deployed in the Hualien area, Taiwan from late July 1993 to September 1995. This array covered an area of roughly 60 km x 30 km extending from an oceanic crust (the Coastal Range) across a suture zone (the Longitudinal Valley) into a continental crust (the Central Range) with 5- to 10-km interstation spacing. Each station was equipped with a 2-Hz three-component seismometer and had four channels with three data channels and one gain channel. This experiment making use of the PANDA Ⅱ system was the first of its kind in the world. The PANDA Ⅱ is a second generation of PANDA with major improvements in its dynamic range and telemetry scheme. It has a maximum of 132 dB dynamic range by using a five-step gain ranging system (18 dB each). Seismic signals and gain information were telemetered by radio to a recording center where two PC-486 computers were used for on-line digital data acquisition and off-line data analysis. More than 1,600 local and regional earthquakes with magnitudes up to 6.2 were recorded on-scale in the first five months. This experiment shows that both the monitoring capability and the locating reliability of earthquakes by the PANDA Ⅱ array are better than those by the regional seismic network. This indicates that the PANDA Ⅱ system is a better seismographic system for studying seismicity and related tectonics in great detail. The experiment also reveals that seismicity in the Hualien area is characterized by an abundance of shallow crustal earthquakes.