Institute of Earth Sciences

RESEARCH

The Institute of Earth Sciences carries out pure research to understand our Earth and its cosmic heritage. Some of our knowledge improves the preparation against natural hazards and the management of earth's resources. At present, our research effort is concentrated upon several subfields: seismology, tectonophysics, geochemistry and high pressure experiments by using fundamental physio-chemical investigative methods. These areas were selected because they do not duplicate the research at other institutions. The Institute maintains extensive domestic and international collaborations. It also contributes significantly to graduate education in earth sciences. The Institute has also offered its expertise to earthquake prevention and risk analyses of major construction projects.

SEISMOLOGY

Observational Seismology, Strong-motion Seismology, Seismic Wave Propagation, Earthquake Mechanics

Earthquake is one of the major natural hazards in Taiwan. Therefore, in addition to having basic academic sig-nificance, our seismological research also contributes its results to earthquake disaster mitigation, especially in Taiwan. The primary goals are: (1) to understand the nature of earthquakes and the causes of seismicity; (2) to study the wave propagation in the heterogeneous media; and (3) to study the characteristics of ground motions. We also provide the fundamental data for improving earthquake resistant engineering designs. We hope that these studies will help us to predict earthquakes and mitigate damage.

TECTONOPHYSICS

Gravity, Geodesy, Paleomagnetism, Geomagnetism, Tectonics

Physical properties on the surface of the Earth reflect the constitutions of the Earth's interior as well as its tectonic processes. Detailed measurements of these properties and their variations can reveal underground activities that are not directly observable. The tectonophysics group uses seismic wave propagation, gravity, magnetism, paleo-magnetism and surface deformations to investigate the sub-surface structures and their developing processes. In order to facilitate the interpretation of these observations, some efforts have also been devoted to the more fundamental studies, such as seismic wave propagations, systematic error analysis on geodetic method, and physical and chemical properties of magnetic minerals.

GEOCHEMISTRY AND HIGH PRESSURE STUDY

Stable Isotopes, Radioactive Isotopes, Biogeochemistry, High Pressure Study

Major research of this group involves the use of elemental and isotope geochemistry to understand the crustal evolution (including igneous and metamorphic processes) in the vicinity of Taiwan as well as the Asian continent, and the exogenic cycles (including water cycles and biogeo-chemical cycles) in Taiwan and its surrounding seas. In addition, this group is also engaged in studying formation of the solar system, using geochemical and theoretical approaches, and the interior of the Earth, using diamond anvil coupled with laser to simulate the extreme P-T conditions.

PERSONNEL AND FACILITIES

PERSONNEL

The Institute has 16 research fellows (including 1 distinguished research fellow and 1 joint appointment research fellow), 6 correspondence research fellows, 7 associate research fellows (including 1 adjunct researcher), 6 assistant research fellows, 4 research assistants, 6 postdoctoral fellows, 4 assistants, 10 technicians, and 4 administrative staff.

FACILITIES

This acid-resistant class 10,000 clean chemistry room is used to digest sample and separate elements.

• Strong-motion network: an islandwide strong-motion network, including analogue and digital accelerographs, and a SMART2 array located in Hualien (including surface and downhole accelerographs).
• Microtremor measurement system: four sets of WR-1wideband range and K2 digital recorders.
• Seismic Research Observatory: located at Chung-Ho, Taipei.
• Taiwan Broad Band Seismographic Network: including seven stations located in Taipei, Hualien, Taitung, Kenting, Tapu, Tachien, and Nantou.
• Gravity and magnetic instruments: two Lacoste-Romberg model D gravity meters, eight geomagnetic stations, and three portable magnetometers.
• PANDA-II system: thirty three-component digital seismic stations.
• Geodetic instruments: including twelve sets of Global Positioning (GPS) receivers, two medium range electronic distance meters, three precise levels, one tide station (located in Taitung), and one invar rod scale calibration system.
• Paleomagnetical instruments: a magnetic shielding room, thermal and alternating field demagnetizers, a spinner magnetometer, a Curie balance, and a magnetic susceptibility system.
• Geochemistry instruments: two solid source mass spec-trometers, four stable isotope ratio mass spectrometers, an electron microprobe, nine vacuum lines, a clean chemistry room (class 10,000), microscopes, two elemental analyzers, an X-ray diffractrometer, a differential thermal analyzer, several diamond anvil high pressure cells with Laser system, a Raman spectroscope, and an FT-IR microscope.

  MAT262 thermal ionization solid-source isotope ratio mass spectrometer can simultaneously detect seven ion beams of different masses.

• The Institute's library currently includes holdings of: (1) 7,500 volumes of books, mainly on subjects of seismology, gravity, geomagnetism, paleomagnetism, geo-desy, tectonics, geology, isotope geochemistry, hydro-logy, environmental sciences, and earthquake engineering, and partially on mathematics, physics, astronomy, and electronics, (2) 120 professional journals, (3) over 2,500 volumes of research reports and theses from related institutions, (4) about 700 titles of research reports and publications of the Institute, (5) over 200 various kinds of maps, (6) about 2,000 sheets of microfiches, most are back issues of journals. Most of our collection can be accessed through Academia Sinica's online library system.
• A Sun Server 690, and twenty-three workstations are located in-house. We have one HP workstation, and two ALPHA workstations. About seventy desktop com-puters (386 and 486 compatibles, Pentium, and Macintosh) are accommodated to use. About sixty desktop computers are accessible via remote link.

MAJOR RESULTS OF RESEARCH

• Based on the seismicity data in the Taiwan area, two subduction zones were clearly delineated, a northward subduction in the northeast and an eastward subduction in the south.
• Several downhole accelerograph arrays were installed in the Taipei basin. Amplitude variations of seismic wave from bedrock to ground surface in the Wuku area were analyzed. The characteristics of ground motion during earthquakes can be realized through the study. The results will be very helpful to the engineering construction and seismic resistant design in the Taipei basin.
• A state-of-the-art seismic data acquisition and pro-cessing system has been developed. This system provides automatic ability to detect and locate earthquakes. The location and wave form of earth-quakes are also displayed automatically on a workstation monitor via local network. The system improves the interaction between the scientists and the database.

  The peak ground acceleration(PGA) contour map in the Taipei basin of theJune 25, 1995 earthquake (epicenter in Ilan southwest, magnitude=6.1). The black dashed lines show the base rock surface contours. The red and purple solid lines are the contours of the PGA. Black solid triangles show the stations which recorded this event.

• The SMART-2 array with different configurations has been deployed on a stiff-soil site since 1991. The data from this array have been used to study the near-field ground motion, site amplification and spatial variation of ground motion on a stiff-soil site. The current research of this array is aimed at the study of earthquake rupture process that provides a key information to understanding the nature of an earthquake.
• Dynamical modeling of earthquakes occurrence: the scaling of seismicity was studied through numerical simulations based on a one-dimensional dynamical lattice model. The major results are: (1) velocity-weakening friction causes three types of ruptures, which lead to different scaling laws; (2) the stiffness and stress drop are significant factors in controlling the scaling; and (3) the fractal dimension of the distribution of the breaking strengths and the level of ambient stress are minor factors in influencing the scaling.
• Fractal characterization of earthquakes: The spatial distribution and temporal variation of earthquakes showed multifractality. For aftershocks, the Gutenberg-Rich-ters b value related negatively to the Omoris p value.

  Free-air anomaly map in and around Taiwan. Both the marine data and athe 640 free-air anomaly measurements on Taiwan and its offshore islands are interpolated into 5 minute spaced girds using a minimum curvature technique.

• A high resolution seismic network of 21 PANDA II stations has been installed at the Taitung, Hualien, Lanyu and Lutao areas of eastern Taiwan. Three component digital data transmitted by a flexible telemetry system will improve the capability of detecting earthquakes as well as understanding seismic activity and general tectonics in eastern Taiwan.
• To explore the deep crustal structures beneath Taiwan, we deployed three seismic profiles to record signals released by the powerful airguns in the R/V Ewing around the island in the summer of 1995. The general features of crustal shape and its thickness have been delineated from a preliminary study of analyzing the wide-angle refracted and reflected signal. This result will improve our understanding of the Taiwan orogeny.
• To delineate the tectonic character of the subduction-collision zone in the marine area near Taiwan, we have constructed a regional gravity map by reconciling the shipboard data of 15 cruises over the past 20 years, and 640 gravity measurements on Taiwan and its offshore islands. On the map, the free-air anomaly ranges from -225 to 350 mgal, primarily reflecting the topography, with the maximum and minimum in the high mountains of Taiwan and the Ryukyu Trench-Taiwan intersection, respectively.
• Using a unique, new seismology method, we are now able to see the boundary between the earth's mantle and core at the depth of 2,890 km. The new global map reveals systematic high velocity distribution that mir-rors the Pacific fire rim on the surface. This finding encourages the hypothesis that the subducting plates initiated around the Pacific rim may have made their way into the deep mantle, then dismembered at the surface of the core.

  Field work of GPS satellite survey for crustal deformation study.

• The 131 stations of Taiwan GPS Network were surveyed 4-6 times from 1990 to 1995 with a dual-frequency geodetic receiver. The average rates of length change for all baselines of the network and that from nine continuous monitoring permanent stations were used in a least squares adjustment to estimate the velocities of GPS stations relative to Paisha, Penghu, situated in the Chinese continental margin. To the south of Fengping in the northern Coastal Range, the velocity vectors of stations in Lanyu, Lutao, and the Coastal Range trend in the directions of 306 -322 with rates of 56-82 mm/yr. In contrast, there are dramatically decreasing rates to the north of Fengping. This may be caused by the motion along the NE-SW thrusts which cut obliquely through the northern Coastal Range. A discontinuity of about 30 mm/yr on rates and remarkable changes in the directions of station velocity have been observed across the Longitudinal Valley, then the moving directions gradually to the west for the stations in the Western Foothills. In the Kaohsiung-Pingtung coastal area the station velocities are even directed toward the southwest. To the north of Peikang High, the velocity vectors of stations change the directions from the west gradually to the north, finally to the east and southeast. Significant NW-SE extensional deformation is found in the Ilan Plain and northern Taiwan. In general, the pattern of the velocity field for GPS stations in the Taiwan area is quite consistent with the directions of present-day tectonic stress.
• By the integration of magnetostratigraphy and biostra-tigraphy (planktonic foraminifera and calcareous nannoplankton), a total of twelve quantitative ages have been obtained in the Madagida-chi section, southern Coastal Range. The section ranging from about 3.40 Ma to 1.15 Ma is composed of four reversed and one normal (Olduvai) magnetic polarities. Paleomagnetic data from 106 sites of the section further reveal a coherent ~33^o clockwise rotation throughout the sequence in the age from 3.18 to 1.66 Ma and a ~20^o clockwise rotation in the strata from 1.66 to 1.15 Ma. We interpret these records due to two major tectonic phases which result in block rotation: one is in the age about 1.60 Ma and the other is later than 1.15 Ma.
• The precipitation pattern of Taiwan for the past 100 years shows a gradually decreasing trend since 1950, presumably due to the influence of global warming. Four severe negative precipitation anomalies in Taiwan are found to be highly correlated with S-rich volcanic activities of the world.
• The record-breaking of precipitations in February and March 1983, are due to the 1982-1983 El Nino event, which incurred a sea-surface temperature rise to 1-2 degrees for the Kuroshiro Current in the winter of early 1983. This phenomenon presents the direct evidence linking the precipitation of Taiwan with the El Nino event. Similar studies are in progress to investigate various mechanisms affecting the precipitation of Taiwan.

  **rations for I-Lan groundwaters. A two-endmember mixing hyperbolic curve is placed, showing the conceptual mixing trajectory between normal groundwater and local sea water (SW) with each 10% increment. Wells P4 contain significant portions of sea water.

• Groundwaters from wells near the coastal areas of northern Lan-Yang and Chia-Nan Plains, show clear isotopic signals of sea waters. Some wells are shown to be significantly contaminated by sea water up to 90%. Overpumping the groundwater by the aquaculture is the main reason for the sea-water encroachment. To alleviate the salinization of coastal groundwaters and prevent further vitiation, it has been suggested that the pumping of groundwaters in the salinization area be strictly banned until the quality and water table of ground-waters reach safe levels.
• The magma generation along Japan-Taiwan-Philippine volcanic chain is being studied. The isotopic results indicate a positive relationship between the eruption volume and Nd isotopic composition of rock. From the historical eruption record and isotopic study of an active volcano, Sakurajima, it is suggested that the magma chamber of Sakurajima may have reached a steady state condition, and therefore that the probability of the large-scale eruption in Sakurajima should be low in the near future.
• Taking advantage of skeletal Sr/Ca rations of living coral and the continuous sea surface temperature (SST) records at Kenting, we have calibrated the Sr/Ca-SST thermometer to < precision of ~ 0.2^oC Applying the newly developed paleo-thermometer to core samples (>=100 yrs) from Kenting and Lutao, we will compare the marine proxy records with direct on-land instrumental observation in order to investigate the correlation between SST and climate. For thousand-year time scale, several dated coral fossils have been analyzed for Sr/Ca with monthly resolution to study seasonal variation of paleo-climate change in a few time windows in the past.
• We have developed the gem anvil cells (cubic zirconia and sapphire anvil cells) using the anvils designed by us and made in Taiwan. The maximum pressure achieved up to 25.8 GPa, which is almost 13 times more than the highest pressure (2 GPa) achieved in the world using the similarly designed zirconia anvil cell earlier. Using this sapphire anvil cell, we found that the high pressure phase of graphite (about 18 GPa) at room temperature is, indeed, not a hexagonal diamond. This understanding is significant because of the increasing importance of the diamond industry in the world.
• Based on the paleomagnetism study of lacustrine sedi-ments sampled from Yuan-Yang Lake in northern Taiwan, we set up the first paleo-secular variation pattern of the earth magnetic field for the last four thousand years. This pattern can be used as a time reference of stratigraphic correlation.

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