GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L22313, doi:10.1029/2005GL024223, 2005
Seismicity increase after the construction of the world’s tallest
building: An active blind fault beneath the Taipei 101
Cheng-Horng Lin
Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
Received 27 July 2005; revised 4 September 2005; accepted 11 October 2005; published 30 November 2005.
[1] Seismic activity began to slightly increase during the
construction but rose sharply upon the completion of the
tallest building in the world, the Taipei 101, which stands at
508 m in the Taipei basin, where local seismicity had
historically very low. Besides an increase in both seismic
energy and the number of micro-earthquakes, two felt
earthquakes astonishingly occurred beneath the completed
building. The focal mechanism of the larger felt earthquake
and its aftershocks are indicative of an active blind normal
fault just beneath the building. Estimations of the vertical
loading of the Taipei 101 show that local stress on its
foundation increased at least 4.7 bars, making it seem
likely that the increased seismicity was a direct product of
the loading of the mega-structure. Further investigations in
unison with continuous seismic monitoring must be
conducted because the safety of the high-rise building in
the Taipei basin be comprehensively assessed. Citation: Lin,
C.-H. (2005), Seismicity increase after the construction of the
world’s tallest building: An active blind fault beneath the
Taipei 101, Geophys. Res. Lett., 32, L22313, doi:10.1029/
2005GL024223.
2. Earthquakes in the Taipei Basin
1. Introduction
[2] Apart from tectonic forces alone, it is well known that
earthquakes can be triggered not only by such natural forces
as local or long-distance earthquakes [Hill et al., 1993;
Seeber and Armbruster, 2000; Gomberg et al., 2001; Tibi et
al., 2003], tidal forces [Lin et al., 2003; Cochran et al.,
2004] and volcanoes but also by such human activities as
water injections [Ohtake, 1974; Raleigh et al., 1976; Zoback
and Harjes, 1997; Tadokoro et al., 2000], nuclear explosions and the construction of dams [Carder, 1970]. Yet what
has, until now, been overlooked is whether or not earthquakes
can be induced by immense, man-made mega-structures,
particularly high-rise buildings that could produce substantial
vertical loading on the ground.
[3] Recently, more and more huge high-rise buildings
have been planned, are under construction, or have just been
completed. Constructed in the eastern part of the Taipei
basin, where local seismicity used to be very low and no
active fault had ever been identified on the surface, the 508m high Taipei 101 is both one of the most newly-completed
mega-structures and, since Sept. 2003, the tallest building in
the world (Figure 1). Most unexpectedly, however, two felt
earthquakes (ML = 3.8 and 3.2) occurred on Oct. 23, 2004
and March 23, 2005 directly beneath the Taipei 101 at
depths of around 10 km. This needless to say has raised
Copyright 2005 by the American Geophysical Union.
0094-8276/05/2005GL024223$05.00
some sobering questions: Could the seismicity have been
induced by the Taipei 101 itself? Did any unnoticed active
blind fault lie beneath the Taipei basin? Is it really safe to
build such mega-structures as the Taipei 101 and Sky-city in
the future? The implications are, in a word, far-reaching.
[4] To gain a better understanding of and more insight
into the surface structures beneath the Taipei 101 and, more
specifically, the possibility of some earthquakes being
induced by this high-rise building, in this study, the spatial
and temporal variations in the seismic characteristics of the
Taipei basin over the past 15 years are investigated using
high-quality seismic data recorded by the Central Weather
Bureau (CWB), Taipei, Taiwan [Shin et al., 2000]. Interestingly enough, certain seismic features, seismicity being
one, vary considerably with time, and a blind fault delineated by the seismicity becomes most apparent. Besides
this, the amount of vertical loading from the Taipei 101 is
estimated to evaluate the possibility of earthquakes being
induced. Finally, the likely relationships between an active
blind fault and this high-rise building are discussed.
[5] Home to more than seven millions, the Taipei basin is
located in the northern part of Taiwan (Figure 2), and based
on the records of the Central Weather Bureau Seismic
Network (CWBSN), the western boundary of the subducted
Philippine Sea plate terminates just beneath the Taipei
basin. Understandably, seismic activity in the Taipei basin
is not very strong when compared to that in other parts of
Taiwan. In fact, from a detailed examination of seismicity, it
is clear that the number of shallow earthquakes (0 – 15 km in
depth) beneath the eastern part of the Taipei basin has been
minimal throughout the past 15 years (Figure 3).
[6] True that the background seismicity in the eastern part
of Taipei basin was, by and large, non-active, but a careful
plot of earthquake magnitude over time does provide
convincing evidence that the occurrence rate as well as
the magnitude of earthquakes were continuously on the rise
throughout the construction of the Taipei 101, and even well
beyond that (Figure 4). The background seismicity used
here was reported by the CWBSN, which have combined
both of the original seismic stations maintained by the CWB
and the Taiwan Telemetered Seismographic Network
(TTSN) operated by the Institute of Earth Sciences, Academia Sinica, and then upgraded all of 79 seismic stations to
3-component digital seismic record since 1990. In addition
to significant increase of the capability of the earthquake
detection due to dense seismic stations of the CWBSN,
estimation of the earthquake magnitude was changed from
the original Md (duration magnitude) to the ML (local
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Figure 1. The tallest building in the world, the Taipei 101,
which stands at 508 m in the Taipei basin. See color version
of this figure in the HTML.
magnitude). In this regard, three distinct periods during the
past 15 years can be readily identified after 1990.
[7] During the first period (1990 – 1997), i.e., before the
construction of the Taipei 101, seismic activity was relatively low and the seismic energy of each micro-earthquake
was small. In total, only 9 micro-earthquakes occurred, with
most magnitudes less than 2.0. On average, there was one
micro-earthquake each year during this period. Also, seismicity, as indicated by the circles in Figure 3, was widely
scattered beneath the Taipei basin.
[8] In the following period (1998 – 2003), the average
occurrence rate of micro-earthquakes rose considerably to 2
or 3 events per year, this of course representing the period of
construction of the Taipei 101. Even more alarming than
that, the magnitude of each of the 15 micro-earthquakes was
mostly in the range of ML = 2.0– 2.5, thus indicative that the
amount of seismic energy released by each one was significantly larger that any of the others in the pre-construction
period.
Figure 3. (a) Seismicity in the eastern part of the Taipei
basin (the large box), marked by circles (1990 – 1997),
triangles (1998– 2003) and diamond-shapes (2004 – 2005).
The Taipei 101 (large, dark square), the seismic stations
(open squares) and three faults (dashed lines), namely the
Taipei fault (TF), the Kanchia fault (KF) and the Chinshan
fault (CF), are marked on the map. Also the focal
mechanism of the larger felt earthquake (ML = 3.8) is
shown. (b) Seismicity is projected on the W-E cross-section
(c) Seismicity is projected on the N-S cross-section. The
blind fault (thick dashed line) is also shown. (d) Firstmotion polarization focal mechanism of the October 23,
2004 earthquake (ML = 3.8). The compressions and
dilatations recorded at the CWBSN stations are respectively
marked plus and minus in the focal mechanism. See color
version of this figure in the HTML.
[9] After 2003, once the construction of the Taipei 101
had been completed, to everyone’s surprise, two felt earthquakes with a local magnitude of ML = 3.8 and ML = 3.2
occurred directly beneath the Taipei 101 megastructure,
these on Oct. 23, 2004 and March 23, 2005, respectively.
Figure 2. Earthquake activity (M > 3.0) in the Taiwan area
during the past 15 years. (a) Distribution of the epicenters
and (b) projection of the hypocenters in the north-south
profile. The inset shows the location of Taiwan in south east
Asia. See color version of this figure in the HTML.
Figure 4. Changes in seismicity during the 1990 – 2005
period. Seismic activity is separated into three periods, as
indicated by the circles (1990 – 1997), the triangles (1998–
2003) and the diamond-shapes (2004 – 2005). The period of
construction of the Taipei 101 is shown by the bold, dashed,
horizontal arrow. See color version of this figure in the
HTML.
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Subsequently, 3 aftershocks, as marked by the triangles in
Figure 3c, were triggered by the larger felt earthquake (ML =
3.8). In short, seismic activity had not only increased in
some 7 years, rather than a co-incidence, it seemed unambiguous that it was associated with the construction of the
Taipei 101.
[10] To account for the stress pattern that generated the
earthquakes beneath the Taipei basin, in general, and
beneath the Taipei 101, in particular, the fault plane solution
of the larger felt earthquake (ML = 3.8) is determined by the
first-motion polarizations recorded at the CWBSN stations
(Figure 3d). The results unquestionably show that the focal
mechanism of the larger felt earthquake belonged to a
typical normal faulting. The strike and dip of two fault
planes were (N108°E, 57°S) and (N85°E, 32°N), respectively. For the most part, the two fault planes were well
constrained by the first-motion polarizations recorded at the
CWBSN stations.
[11] Combining the focal mechanism of the larger felt
earthquake (ML = 3.8) with the seismic pattern of its
aftershocks and that of the background earthquakes, it is
worth noting that an active normal fault dipping to the south
is detected just beneath the Taipei 101 (Figure 3c). This
fault is located at depths of roughly 10 km beneath the
Taipei 101, and its dipping angle gradually decreases with
depth. Basically, this fault is consistent with the southdipping fault of the focal mechanism of the larger felt
earthquake (ML = 3.8), as determined by the first-motion
polarizations recorded by the CWBSN.
3. Discussion
[12] The normal faulting mechanism of the larger felt
earthquake (ML = 3.8) can be considered a typical blind
fault for the very reason that it is not associated with any
known surface fault across the Taipei basin. Granted there
are 3 known faults, namely the Taipei, Kanchiao and the
Chinshang faults, across the Taipei basin in the northern
area of Taiwan (Figure 3a), but none can explain the
occurrence of the larger felt earthquake and its aftershocks.
First, the three faults are, in essence, reverse faults striking
in the NE-SW directions, and they are largely associated
with the convergence of the Eurasian and Philippine Sea
plates [Ho, 1988]. They are markedly different from the
focal mechanism of the larger felt earthquake which had
normal faulting and a strike mostly in the W-E direction.
Furthermore, though the epicenters of the felt earthquakes
were very close to the Taipei fault on the surface, based on
the report of the Central Geological Survey [Lin et al.,
2000], this fault is, for all intents and purposes, considered a
non-active one. And, the hypocenter of the larger felt
earthquake was not located on the Taipei fault because the
fault is dipping in the southeast direction. On the weight of
this evidence, it seems to conclude that the normal fault
delineated by the focal mechanism of the main shock (ML =
3.8) and by the seismic pattern of its aftershocks must be an
active blind fault beneath the Taipei 101, but it would have
been difficult to identify this fault simply on the basis of the
surface geology alone.
[13] Alternatively, the occurrence of the earthquakes
along the blind fault beneath the Taipei 101 might be
attributed to vertical loading that accompanied during and
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after the construction of this massive, towering structure in
the Taipei basin. As the tallest building in the world, it
should not at all seem surprising that the Taipei 101 has
huge vertical loading on its foundation. To protect its
exterior and interior super-structural framework from the
effects of earthquakes or fire, hybrid structures made of both
concrete and steel had to be used in the construction. In this
sense, the structural framework of the Taipei 101 differs
from that of many other high-rise buildings in low seismic
areas, such as the World Trade Center in New York,
which was primarily built of steel, and because of that,
collapsed due to terrorist attacks on September 11, 2001. In
the case of the Taipei 101, concrete was used to encase
structural steel columns and sometimes beams, yet this
gives the building considerably greater mass. As a consequence, structural loading on the ground is substantially
increased. All in all, for the Taipei 101, the steel that was
used weighed 122,288 tons, while total concrete volume
was 242,852 cubic meters (KTRT, Taipei Financial Center
Project Summary, http://www.ktrt.com.tw/english/
index.htm, 2005). If the density of concrete is assumed to
have the standard value of 2,400 kg/m3 [Richard, 1996],
then the weight of the concrete must have been
582,844 tons. This means that the total mass of both the
concrete and steel was probably about 705,132 tons. The
surface area of the foundation of the Taipei 101 covers
15,081 square meters. Thus, the total stress from vertical
loading on the ground is 0.47 MPa (or 4.7 bars), without
considering a lot of furniture and electronic equipments
installed later. Such a huge amount of local stress is not only
drastically higher than the daily tidal stress fluctuations
(0.01 bar) required to trigger an earthquake [Lin et
al., 2003; Cochran et al., 2004], but also considerably
greater than the generally inferred static Coulomb Failure
Function (DCFF) triggering thresholds of about 0.1 bar
[Harris, 1988; Reasenberg and Simpson, 1992]. Although
the total vertical loading (4.7 bars) was not directly acting
on the blind fault beneath the Taipei 101, considerable
stress might be transferred into the upper crust due to the
extremely soft sedimentary rocks beneath the Taipei
basin.
[14] In addition to significant vertical stress on the
ground, the expectation of brittle failure under vertical
loading is highly consistent with one of the fault planes in
the focal mechanism of the larger felt earthquake
(Figure 3d). It is a fact that brittle failure under vertical
loading would be favored on a normal fault with a dip of
about 60 degrees. This is in accordance with the fault plane
with a dip of about 57 degrees toward the south for the focal
mechanism of the larger felt earthquake (ML = 3.8) and the
seismic zone delineated by the background seismicity and
its aftershocks. The dipping angle of the observed fault
which gradually decreases with depth is also similar to the
diagram of the principal stress trajectories due to a point
load on a half-space [Jaeger, 1962].
[15] On account of the consistencies in the increase in
seismicity after the construction of the Taipei 101 and the
normal fault generated by significant vertical loading
(4.7 bars), the construction of this mega-structure might
very well have triggered the earthquakes beneath the building. Yet, worth bearing in mind is that no solid, substantive
evidence has been found to substantiate that the changes in
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the seismic characteristics of the Taipei basin were a direct
product of the construction of the massive high-rise building. For this reason, a call is made here for immediate,
extensive research into this issue. In the meantime, careful,
thorough seismic monitoring must be continued in the
Taipei basin. Should seismic activity remain high or, even
worse, should it significantly increase, then the possibility
of earthquakes being triggered by the high-rise building will
become ever so much more of a reality.
4. Conclusions
[16] There is a distinct possibility of earthquakes being
triggered by the recent construction of the world’s highest
building, the imposing Taipei 101. This is consistently
suggested by the following credible evidence: (1) the
observation of significant increases in both the seismic
energy and the number of micro-earthquakes in the Taipei
basin since the completion of the Taipei 101; (2) the
presence of an active normal fault along the expectation
plane with a dip of about 60 degrees of brittle failure under
vertical loading; and (3) the estimation of huge vertical
loading (4.7 bars) from the Taipei 101. To examine the
possibility of earthquakes being triggered by this massive
skyscraper and to evaluate the safety of the Taipei 101, a
great deal of more extensive investigative research coupled
with continuous seismic monitoring of the Taipei basin have
to continuously be carried out in the future.
[17] Acknowledgments. The author would like to thank two
reviewers (R. Tibi and the other anonymous) for constructive comments.
Thanks are also extended to the Central Weather Bureau in Taipei for
providing the earthquake catalogue data. Discussions with K. Konstantinou
have been very constructive. This research has been funded in part by the
National Science Council, Taipei, Taiwan.
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