構造活動低地之地形與沈降速率：從其關係可以得知什麼？
Topography and subsidence rate in active lowland regions:
what can we learn from their relationship?
詹瑜璋；周素卿；李建成（中央研究院地球科學研究所）
Yu-Chang Chan; Su-Ching Chou; Jian-Cheng Lee
Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
Topography and slope in lowland regions affect sensitively the patterns of
geological flooding and sedimentation. We thus hypothesize that the present
topography in lowland regions is closely correlated with the short-term subsidence
rate of late Quaternary. The hypothesis implies that we may estimate sediment
thickness from present topography with a few thickness control points if the
topography-subsidence relation has been established in the region. In order to test the
hypothesis, we compared present topography with sediment thickness or average
subsidence rate in lowland regions to determine how they are correlated. We restricted
our study to lowland regions because lowlands are typically young and preserve much
evidence of recent tectonic activities.
We analyzed two lowland regions to evaluate the topography-subsidence
relation: the Pingtung plain and the Taipei basin. We calculated the correlation
coefficient of the topography from the 40m DEM and the extrapolated sediment
thickness from boreholes in the Taipei basin. We found high correlation between the
topography and the thickness of sediments. This indicates the pattern of topography is
related to the thickness of the Holocene sediments linearly to the first approximation.
On the other hand, the Pingtung plain has been reported to have significant subsidence
in the past few decades due to the over withdrawal of ground water. This, however,
does not affect the estimation of the average subsidence rate from Lai and Hsieh
(2003). We then calculated the correlation coefficients of the topography and the
average subsidence rate in the Pingtung plain. We also found moderate to high
correlation between the topography and the subsidence rate. This implies that the
pattern of topography may relate to the average subsidence rate in the past 20,000
years.
In general, analyses of the Pingtung and Taipei regions support the hypothesis of
topography-subsidence relationship. The topography of the Taipei basin may be used
as a factor to estimate the thickness of sediments, such as the Holocene Sungshan
formation. The topography may also be used to estimate the total sediment thickness
in the Taipei basin, although variations may occur due to complication of basement.
Because the lowland topography varies from a few to ten of meters, more accurate
digital elevation models are needed to improve the correlation results. The subsidence
resulted from cultural activities also needs to be considered and calibrated in order to
have more useful comparison.