台灣地震地質研究-臺灣西南部活斷層研究─台灣西南部活動斷層活動性近斷層監測之研究 (II)
Other Title
Land subsidence in a mountain front as revealed by GPS measurements
in the Pingtung area, southwestern Taiwan
in the Pingtung area, southwestern Taiwan
Date Issued
2005
Date
2005
Author(s)
DOI
932116M002008
Abstract
GPS measurements with three campaigns in southwestern Taiwan have been
carried out by the Central Geologic Survey since 1996 for four years. The
Pingtung-Kaohsiung network, which consists of 48 stations, was established to
investigate crustal deformation and land subsidence. These GPS surveys mapped the
complete velocity field of the Pingtung area in southwestern Taiwan relative to the
Paisha station (S01R) of the Penghu islands (located in the Taiwan Strait), a
recognizably stable continent shelf. GPS displacement velocities display the trends of
variation in the investigated area. The horizontal station velocities varied from 32
mm/yr to 54 mm/yr in azimuths ranging from 247.2 o to 272.6 o with respect to the
permanent station S01R. The horizontal station velocities decrease westwards and
increase somewhat southwards. In terms of horizontal velocity vector trends, there is a
contrast in kinematics in the study area. In the central and western part of the study
area, GPS stations move generally toward the west, whereas in the
Kaohsiung-Pingtung coastal area, the displacement vectors demonstrate a clear
counter-clockwise deviation toward SW. The southern part of coastal area its offshore
area show remarkable extension rates of 0.6-2.0 µstrain/year in an azimuth 015°-020°.
This significant southward increasing of extensional strain rates is attributed to the
lateral extrusion of blocks bounded by major discontinuities in the study area. For the vertical movement, the station velocities are from ~13 mm/yr to -25 mm/yr. There are
20 stations with measured velocities pointing to subsidence in the southern part of the
Pingtung plain. The significant subsidence rates from ~ 11 mm/yr to 25 mm/yr have
been observed. Our results clearly demonstrate the existence of transtensional
deformation which increases southward in the along-strike direction. They are
consistent with the Holocene subsidence rate pattern in this region. The long-term
subsidence rate is about 4 mm/yr near the coast of the southeastern part of the
Pingtung plain. These data are highly consistent with the depositional isopach of
fine-grained sediments in the subsidence area. The comparison with Holocene
subsidence and isopach of fine-grained sediments suggest that about 75% of the
present-day subsidence may result from decreasing groundwater level induced by
over-pumping, adding much to the natural risk resulting from tectonic subsidence. We
attribute this regional subsidence to be caused by an appreciable drop in groundwater
level as revealed by rapid subsidence in the coastal area of the Pingtung plain, as well
as the transtentional deformation associated with tectonic extrusion.
Publisher
臺北市：國立臺灣大學地質科學系暨研究所
Type
report
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