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The
Earthquake of 20 September 1999 in Taiwan
George
Pararas-Carayannis
Introduction
A large
earthquake struck the island of Taiwan at approximately 1:45
a.m. (local time) on September 20, 1999. The quake killed or
injured thousands of people, left thousands more homeless and
caused extensive destruction to buildings, roads and utilities.
Most of the casualties and damage occurred in the central Nantou,
Taichung and Yunlin Counties. The quake triggered a series of
landslides.
Damage
to government building
Earthquake Epicenter,
Origin Time, Magnitude and Aftershocks
A complex,
shallow focus, earthquake, with magnitude 7.7MS struck Taiwan
on 20 September 1999. Origin time was 17:47 (09 20 UTA). The
epicenter was located 90 miles south of Taipei, near the central
city of Taichung, at 18.6 23.804N 120.958E.
Major aftershocks
measuring as much as 6.8 and 6.3 on the Richter scale occurred
the day after the major quake. More than 2,000 aftershocks were
recorded in the days following the main shock.
 Tectonic Dynamics and Plate Interaction in the
Taiwan Region
The plate
tectonic systems in the region near Taiwan where this earthquake
occurred are extremely complex and have changed considerably
throughout geologic time. The tectonic setting and the dynamics
of interaction have been extensively discussed in the literature
(see references below).
The island
of Taiwan is located on the convergent boundary between the Eurasian
and the Philippine Sea tectonic plates. Presently, and during
the Plio-Pleistocene period, principal tectonic plate interaction
in the vicinity of Taiwan, has been closely related to the Ryukyu
and Luzon arc-trench systems, characterized by subduction, convergence
and rotation, but marked primarily by the collision of the Luzon
volcanic arc with the Asian continental margin.
Also, there
has been apparent northward subduction of the Philippine Sea
plate beneath the Ryukyu arc on the Eurasian plate along the
Ryukyu Trench. The Ryukyu arc is to the east and northeast of
Taiwan while the Luzon Arc System is south of Taiwan (see figure).
Both volcanic arcs extend onto the island of Taiwan.
The tectonic
interactions are extremely complex in that they involve both
subduction and plate convergence in the vicinity of Taiwan, which
do not take place along a simple plate boundary or subduction
zone as commonly conceived, due to the difficulty of subducting
a portion of the continental crust which is markedly buoyant.
Apparently, a wide distributed shear system developed during
different stages of the arc-continent collision.
 Earlier tectonic plate
convergence in the vicinity of Taiwan was marked by an apparent
eastward subduction of the Eurasian plate underneath the Luzon
arc on the Philippine Sea plate. However, this shear motion moved
westward with time, forming a broader zone of deformation involving
subduction, collision, and plate consumption, rather than a discrete
well-defined plate boundary. On Taiwan, this wide belt of deformation
extends for about 100 km from the western to the eastern offshore
region of the island.
(U.S.G.S
map of the earthquake's epicenter, other earthquake epicenters,
and tectonic plate boundaries)
This active,
complex and ever changing tectonic interaction and collisions,
along a wide deformation belt, have affected the entire island
of Taiwan and have produced in the past the Central Mountain
Range on the island, known as the Penglai orogeny, and the Longitudinal
Valley of eastern Taiwan, both areas of high seismicity.
The Central
Range is characterized by fast uplift at a rate of 2-3 cm/yr
or more and fast tilting at both lambs of the mountains, the
west end tilting westward of about 1.0µ radian/yr while
the east of about 3.0 µ radian/yr. Both regions are characterized
by the widespread distribution of shallow-focus earthquakes.
The earthquake of September 1999, had its epicenter in this zone
of Central Taiwan.
Thus
no tsunami was observed.
Assessment of Tsunami
Generation from Earthquakes in the Taiwan and the Southern Ryukyu
Islands Region
 As already described,
Taiwan is located on the convergent boundary between the Eurasian
and the Philippine Sea tectonic plates, a region characterized
by subduction, convergence, rotation, and collision of the Luzon
volcanic arc with the Asian continental margin. BothThe Ryukyu
arc is to the northeast, and the Luzon Arc System to the south,
converge onto Taiwan.
Ishigaki,
Ryukyu Islands.
Northward
subduction of the Philippine Sea plate beneath the Ryukyu arc
on the Eurasian plate along the Ryukyu Trench can generate, large
potentially tsunamigenic earthquakes. According to old Japanese
records, on April 24, 1771, a large earthquake (with an estimated
Richter magnitude of 7.4) occurred near the southermost Ryuku
Islands, just south of Ishigaki Island, an area controlled then
by the Japanese Satsuma samurais.
A tremendous
tsunami was generated according to old records. Claims of maximum
runup of 50m to 85m have veen made. Tsunami devastated the islands
of this group and the more distant islands of the Miyako group.
Huge blocks of coral were carried by wave action. A large coral
block was found 2.5 km inland. Claims were made that the block
was not a remnant of erosion but that it was deposited by a tsunami
wave. Records of the Satsuma samurais indicate that about 11,000
people were killed in the southern Ryukyu islands. The tsunami
effects in Taiwan, Hong Kong and elsewhere in the region are
not known.
Assessment of Tsunami
Generation from Earthquakes in the Taiwan Strait
None of
the earthquakes with epicenter on the island of Taiwan are not
known to have generated tsunamis. However, earthquakes with epicenters
near or in the sea can generate tsunamis along the coast of Taiwan
and of mainland China. The most recent large earthquake (mb=6.5),
in the Taiwan Strait occurred on 16 September 1994 in the western
part of the Tainan basin, at a very shallow depth of 13 km. It
was named Peng-Hu Earthquakesk. The quake's focal mechanism indicated
a sea floor movement consistent with normal faulting, with its
axis along a N-S direction.
The occurrence
of this earthquake indicates that the Taiwan Strait is seismically
active and capable of generating earthquakes with Richter magnitudes
greater than 6.0. Although it is not believed that this particular
earthquake generated a tsunami, earthquakes with magnitude greater
than 6 on the Richter scale have the ability to trigger tsunamis,
while earthquakes with magnitude greater than 7 and with a large
vertical component, have the ability to generate destructive
tsunamis.
The orientation
of the crustal strain beneath the Taiwan Strait appears to be
dominated by North-South extension rather than the East-West
compression that is observed along eastern Taiwan. Given the
complex tectonic interactions of the region, tsunami generation
is possible. The tsunami risk for the Taiwan Strait requires
careful evaluation and study.
REFERENCES AND FURTHER
READING
Angelier
J., Bergerat F., Chu H.-T., Juang W.-S. and Lee T.-Q. (1990).
Tectonic-paleomagnetic
analyses and the evolution of a curved collision belt : the Hsueshan
Range, northern Taiwan. Tectonophysics, 183, 77-96.
Angelier,
J., H.T. Chu and J.C. Lee (1997). Shear concentration in a collision
zone: kinematics of the active Chihshang Fault, Longitudinal
Valley, eastern Taiwan: Tectonophysics, 274, 117-144.
Chen, W.-P.
and H. Kao (1996). Seismotectonics
of Asia: some recent progress: in The Rubey Volume IX: Tectonic Evolution of Asia, edited by A.Yin and
M. Haarrison, 37-62, Cambridge Univ Press.
Chen, K.J.,
Y.H. Yeh, H.Y. Yen and C.H. Lin (1995). Seismological studies in the Chinshan
fault area:
J. Geol. Soc. China, 38(4), 335-354.
Davis, D.,
J. Suppe, and F.A. Dahlen (1983). Mechanics of Fold-and-Thrust Belts
and Accretionary Wedges, J. Geophys. Res., 88, 1153-172.
Delcaillau,
B., J. Deramond, P. Souquet, J. Angelier, H.T. Chu, J.C. Lee,
T.Q. Lee, T.F. Lee, P.M. Liew and T.S, Lin (1994). Enregistrement tectono-sedimentaire
de deux collisions dans l'avant pays nord-occidental de la chaine
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Deramond,
J., Delcaillau, B., P. Souquet, J. Angelier, H.T. Chu, J.F. Lee,
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Hu, J.C.,
J. Angelier, J.C. Lee, H.T. Chu and D. Byrne (1996). Kinematics of convergence,
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Kao, H.
(1998). Can
great earthquakes occur in the southernmost Ryukyu arcúTaiwan
region?:
TAO, 9(3),487-508.
Kao, H.,
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Kao, Honn
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Wang, C.Y.,
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