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Temperature is used to trace ocean density variations, and reveals internal waves and turbulent motions in the deep ocean, called ‘internal motions.’ Ambient temperature detected by geophysical differential pressure gauges (DPGs) may provide year-long, complementary observations. Here, we use data from four DPGs fixed on the ocean bottom and a high-resolution temperature sensor (T-sensor) 13 m above the seafloor as a square-kilometer array deployed offshore ~ 50 km east of Taiwan facing the open Pacific Ocean to examine the impact of temperature on DPG signals related to internal motions. The DPG signals correlate with T-sensor temperature variations between 0.002 and 0.1 mHz, but have time shifts partially caused by slow thermal conduction from the ambient seafloor to the DPG chamber and partially by internal motion propagation time across the array. Applying beamforming-frequency-wavenumber analysis and linear regression to the arrayed T-sensor and DPG data, we estimate the propagating slowness of the internal motions to be between 0.5 and 7.4 s m−1 from the northwest and northeast quadrants of the array. The thermal relaxation time of the DPGs is within 103–104 s. This work shows that a systematic scan of DPG data at frequencies

Abundant fossil specimens of Scaphechinus mirabilis, now occurring mostly in temperate waters, have been found in the Toukoshan Formation (Pleistocene) in Miaoli County, Taiwan. Environmental changes leading to its extirpation (local extinction) have thus far been elusive. Here, we reconstruct past environmental and oceanic conditions off northwest Taiwan by analyzing clumped isotopes, as well as stable oxygen isotopes, of well-preserved fossil echinoid tests collected from the Toukoshan Formation. Radiocarbon dates suggest that these samples are from Marine Isotope Stage 3 (MIS 3). Paleotemperature estimates based on clumped isotopes indicate that fossil echinoids were living in oceanic conditions that range from 9 to 14 °C on average, comparable with the estimate derived for a modern sample from Mutsu Bay, Japan. Notably, this temperature range is ~ 10 °C colder than today’s conditions off northwest Taiwan. The substantially lower temperatures during ~ 30 ka (MIS 3) compared to the modern conditions might be due to the rerouting of surface currents off northwest Taiwan when the sea level was ~ 60 m lower than today, in addition to the cooling caused by a lower atmospheric CO2 level during the Last Glacial Period. Colder waters brought here by the China Coastal Current (CCC) and the existence of shallow subtidal zones termed “Miaoli Bay” (mainly located in the present-day Miaoli county) during MIS 3 plausibly sustained generations of S. mirabilis, yielding tens of thousands of fossil specimens in the well-preserved fossil beds. The likely extirpation driver is the drastic change from a temperate climate to much warmer conditions in the shallow sea during the Pleistocene–Holocene transition.

Acoustic-gravity waves are generally considered to be one of the major factors that drive changes of the total electron content in the ionosphere. However, causal mechanisms of couplings between sources in the lithosphere and responses in the atmosphere and the ionosphere are not fully understood, yet. A barometer in the cave of the SBCB station records an unusual phenomenon of larger amplitudes in air pressure changes inside than those at the Xinwu station (outside). Accordingly, the comparison between the recorded data at the SBCB and Xinwu station can drive investigations of potential sources of the unusual phenomenon. Analytical results of phase angle differences reveal that the air pressure outside the cave at the Xinwu station often leads air pressure changes inside at the SBCB station at relatively low frequency bands. In contrast, the larger pressure changes at frequencies > ~ 5 × 10–4 Hz inside the cave at the SBCB station lead smaller changes outside at the Xinwu station. To expose causal mechanisms of the unusual phenomenon, continuous seismic waveforms are further conducted for examination. When the horizontal and vertical ground velocities of ground vibrations yield a difference in the phase angle close to 90°, coherence values between the air pressure changes and ground vibrations become large. This suggests that the pressure-shear vertical ground vibrations can drive air pressure changes. Meanwhile, the results shed light on investigating the existence of acoustic waves near the Earth’s surface using a partially confined space underground due to that the assumptions of the waves can propagate upward into the atmosphere driving changes in the ionosphere.

The elastic response of seafloor to ocean gravity wave loading, or seafloor compliance, provides a constraint on the elastic properties of the crust. We measured seafloor compliance at three ocean bottom seismometer (OBS) sites around Taiwan—two in the southwestern (SW) Okinawa Trough and one on the Ryukyu arc—and performed inversion for crustal structures beneath them. Models best fitting the data demonstrate a decrease in upper crustal shear velocity and an increase in the compressional/shear velocity ratio from the arc site to the trough sites with increasing amount of back-arc extension. This variation suggests that the upper continental crust is highly faulted and hydrated during rifting of the Eurasian lithosphere.

Vertical ocean‐bottom seismograph (OBS) data at frequencies below 0.05 Hz are contaminated by noise induced by infragravity waves. We constructed the transfer function between pressure and velocity data from OBSs deployed in Taiwan waters to remove the wave pressure‐induced noise from seismic recordings. Data were analyzed from five portable broadband OBSs deployed each for 10 months at water depths from 1740 to 4600 m and from a cabled, shallow‐buried seismograph (EOS1) installed on the seafloor at 300 m depth. Removing long‐period noise from these OBS data improves the identification of teleseismic phases such as P , S , SS , P diff , and PKIKP that are otherwise ambiguous or unidentifiable. For EOS1, infragravity‐wave signals completely mask the P and S waveforms in the 10–50 s period band suitable for centroid moment tensor (CMT) solutions for most of the local events. Application of the transfer functions to predict and remove wave deformation yielded clean prominent P and S waveforms at these periods and aided in the CMT determination for small events jointly with land stations. The relative amplitudes of the wavenumber‐normalized transfer function for some of the OBSs are mostly determined by the thickness of the sediment at the OBS site.

Intraslab events recorded by ocean-bottom seismometers in the Okinawa trough provide an extended depiction of shear wave splitting in the southwest section of the Ryukyu subduction zone. At 100–200 km from the western edge of the subduction system, we observed trench-normal fast polarization direction in the back arc compatible with 2D slab- or rifting-driven corner flow. Towards the edge, the fast directions are sub-parallel to the trench in the arc—back arc region, and rotate to trench-normal within 50 km of the edge. Splitting constrained by land stations with paths mostly in the mantle wedge exhibits similar trench-normal fast directions in the subduction edge zone. Further inland, the dominant component of fast directions becomes roughly parallel to the Taiwan orogenic fabric. Splitting of P-to-S phases converted at the Moho of the Okinawa trough and of S phases from shallow events suggest that crustal anisotropy may affect the measured splitting, but the observed pattern reflects predominantly mantle anisotropy. The variation in splitting along the Okinawa trough cannot be explained by a B-type–A-type olivine fabric transition in the mantle wedge. It may indicate the presence of an along-arc flow in the mantle wedge towards the edge where it is blocked and deflected by the Eurasian lithosphere. This scenario bolsters previous studies suggesting a significant impact of Eurasian lithosphere on the dynamics of the Ryukyu subduction system.

Typhoons generate ocean waves, which induce ground motions on the seafloor. Such processes can generate microseismic signals observed at far distances (Longuet-Higgins 1950; Tabulevich 1971; Sutton 1996; Bromirski et al. 1999; Tolman 1999; Grevemeyer et al. 2000; Kedar and Webb 2005; Webb 2007; Gerstoft et al. 2008; Kedar et al. 2008; Wilson and Makris 2008; Koper and de Foy 2008). However, direct observations on the deep seafloor under a typhoon were not available until now, due to the difficulty of field operations and the lack of proper instruments installed under the path of a typhoon. Recently we have set up a portable wideband oceanbottom seismology network in Taiwan. We have deployed broadband ocean-bottom seismographs (OBSs) to collect direct observations. For the engineering parameters of the OBS, see Table 1. Such datasets were then combined with data from many permanent land-based seismic stations for analyses. View this table: TABLE 1 Technical Specifications of the Ocean Bottom Seismographs Used for This Study The combined OBSs and land seismic stations have captured a special “seismic event” associated with Typhoon Shanshan— …

SUMMARY A broad-band ocean-bottom seismometer (OBS) deployed ∼180 km east of Taiwan provides a first glimpse into the upper mantle beneath the westernmost section of the Philippine Sea or the Huatung basin (HB). We measured interstation phase velocities of Rayleigh waves between the OBS and stations on the eastern coast of Taiwan. The phase velocities show smooth variations from 3.8 to 3.9 km s −1 for periods of 25–40 s. In this short period range, phase velocities are comparable to those characterizing the 15–30 Ma Parece-Vela basin of the Philippine Sea. Modelling of the finite-frequency effect proves the validity of the measurement for the average HB. The shear-wave velocity models inverted from the 25 to 40 s dispersion show a velocity at lithospheric depths about 0.1 km s −1 lower than that of the west Philippine Sea, which agrees with the age effect derived from the Pacific pure-path model. Inversions incorporating the less reliable data above 40 s yield a shear velocity

A comparative numerical modeling using single component (measuring the vertical component of the fleld, Hz) time-domain electromagnetic measurements (TEM) receiver and three component (measuring Hx, Hy and Hz) TEM receiver was undertaken. A forward mod- eling approach was used to compute the voltage response of half-space containing one or more conductive bodies excited by a bi-polar square waveform. Although this was a conductor scatter- ing, it was particularly useful as a practical use for the unexploded ordnance (UXO) detection. Unlike the single component data, results from the three component data are unambiguous as to the location and orientation of conductors. Measuring adding of the horizontal components of the secondary magnetic fleld leads to not only provide the best indication of target location, but also can be used to determine size, orientation, and characteristics of the targets, especially for the horizontal extending target. A three-component TEM fleld experiment at a well-documented wells site, NCU campus, were consistent with the efiects predicted by our theoretical modeling. As a result, the 3-component TEM survey is a must for the high resolution EM in the engineering purpose.

Taiwan is located near the edge of the continental Eurasia plate and the Philippine Sea plate, a quake-prone zone. Averagely speaking, out of 1000 sensible quakes detected per year, more than 50% of the epicenters are located in the surrounding waters. Therefore, there is a great need from our seismology community to deploy ocean bottom seismometer (OBS) to have high fidelity data, and widen the aperture of the observation as well. In this paper, we report the development of a “midweight” OBS called Yardbird which has one year deployment duration. We utilized the newly available ultra-low power MCU (micro control unit) and SD card to design a compact data logger. The response of 4.5-Hz geophones was extended to 3 s and used as the seismic sensors. With the compactness of the sensor module, it enables us to design a compact dual-axis motor-driven stage to level the vertical component to be less than 0.1 degree with respect to the gravity. Currently, the total power consumption of Yardbird OBS is less than 0.2 mW for three channels. A pilot experiment and was conducted in the south western Okinawa trough in 2010. During the three months experiment, an array of five Yardbirds captured and deep quakes from the subducting oceanic plate. The results prove that the performance of Yardbird OBS suffices the basic needs of seismic research.

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°.

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.