Active detachment of Taiwan illuminated by small earthquakes and
its control of first-order topography
Sara Carena 
 Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA
John Suppe 
Honn Kao Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, Republic of China
ABSTRACT
We use 110 000 small earthquakes to locate and map active
faults in three dimensions within the Taiwan arc-continent collision. The structure is dominated by a nearly horizontal band of
small earthquakes at ;10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt,
with other illuminated fault zones abutting the detachment zone.
The zone steepens below eastern Taiwan to 308–908 and reaches
depths of 30–60 km. The three-dimensional shape of the detachment
zone in relation to topography allows a new test of critical-taper
wedge mechanics and suggests that the reversal of topographic slope
across Taiwan is controlled by the shape of the detachment.
Keywords: Taiwan, earthquakes, detachment, seismotectonics, thrust
faults.
INTRODUCTION
Critical-taper wedge mechanics—the theory that compressive
mountain belts sliding above a basal detachment deform until they
reach a critical surface slope, in the same manner as bulldozer wedges—was first tested in the Taiwan arc-continent collision (Davis et al.,
1983; Dahlen et al., 1984; Barr and Dahlen, 1989; Barr et al., 1991)
and has been applied to many active and dead mountain belts and
accretionary wedges on Earth and even on Venus (Zhao et al., 1986;
Williams et al., 1994). However, the shape of the basal sliding detachment is typically poorly known in many active mountain belts, although master detachments have been well imaged seismically in a
number of active accretionary wedges and inactive mountain belts
(Cook et al., 1979; Choukroune, 1989; Moore et al., 1990; Ladd et al.,
1990; Pfiffner et al., 1997; Clowes et al., 1999). In the initial test in
Taiwan, the dip of the detachment was only known directly at the toe
of the wedge and not under the main mountain mass. Here we use the
locations of large numbers of small earthquakes to map the main detachment in three dimensions under the entire Taiwan mountain belt to
a depth of 60 km. Direct imaging of the detachment allows a most
straightforward test of critical-taper wedge mechanics and illuminates
the first-order tectonic control of the topography of Taiwan.
EARTHQUAKE DATA
More than 110 000 small earthquakes (mostly ML 5 1–4) recorded in north-central Taiwan by the Taiwan Central Weather Bureau since
1991 provide sufficiently dense acoustic emission to illuminate some
important active faults at depths of 5–60 km (Fig. 1). These earthquakes include both background seismicity and aftershocks of larger
events, especially of the 1999 Mw 7.6 Chi-Chi earthquake (Shin et al.,
2000). We applied new techniques developed to map faults based on
locations of small earthquakes in California (Carena and Suppe, 2002),
first identifying and selecting subsets of coplanar earthquakes while
viewing them in three dimensions and then mapping them by fitting
optimized three-dimensional surfaces to the earthquake locations.
The Taiwan Central Weather Bureau uses a one-dimensional velocity model to locate all events (Tsai and Wu, 1997). We use the threedimensional velocity model of Rau and Wu (1995) to relocate all
events with .10 P-wave arrival readings (Zhou, 1994). The relocated
seismicity is then clustered by using the collapsing algorithm of Jones
and Stewart (1997; also see Nicholson et al., 2000) in order to obtain
more tightly defined images of fault zones from the scattered seismicity
that surrounds them. The Gocad three-dimensional Earth-modeling
software (Gocad Consortium, 2002; Mallet, 2002) was used to isolate
dense coplanar subsets of earthquakes that have fault-like geometries
(Figs. 1 and 2) and fit optimized three-dimensional triangulated surfaces to them (Fig. 3). The three-dimensional fault surfaces fit the hypocentral locations within 61.5 km for most faults (Fig. 4), i.e., below
or equal to the uncertainty in Central Weather Bureau earthquake locations (,5 km) (Tsai and Wu, 1997). Offshore eastern Taiwan, earthquakes are less well located (;5–20 km, depending on the location).
IMAGING THE DETACHMENT
The horizontal band of small earthquakes seen at ;10 km depth
(Fig. 1, profile showing ‘‘all events’’; video of earthquakes and faults
in Taiwan1) immediately suggests the presence of the main detachment
zone beneath central Taiwan, which has commonly been shown in
schematic cross sections of Taiwan (e.g., Teng et al., 2000) but has
been strongly opposed by some workers (Wu et al., 1997). This main
detachment has been inferred to exist from the presence of beddingparallel detachments of large displacement exposed in the hanging
walls of thrust ramps in the western Taiwan thrust belt (Suppe and
Namson, 1979; Suppe, 1980, 1986, 1987; Namson, 1983; Hung and
Wiltschko, 1993). The Chelungpu thrust, which slipped during the
1999 Chi-Chi earthquake, is the best known of these faults, sliding on
a detachment in the Pliocene Chinshui Shale (Davis et al., 1983; Suppe,
1987; Wang et al., 2000). A throughgoing main detachment has also
been inferred by analogy to better known mountain belts that have been
imaged by seismic reflection methods (Cook et al., 1979; Choukroune,
1989; Pfiffner et al., 1997; Clowes et al., 1999). Thus, it is reasonable
to pursue the hypothesis that this horizontal band of seismicity is associated with this proposed Taiwan main detachment zone.
The throughgoing nature of the horizontal band of seismicity is
confirmed by more detailed mapping of subregions in three dimensions, mapping that shows a three dimensionality to the detachment
(whose selected earthquakes are shown in pink in Fig. 1) that contributes to the complexity of the earthquake distribution in any twodimensional projection of a large region such as Figure 1. At present
it is not possible to effectively test this detachment interpretation by
using focal mechanisms because few are available for these very small
earthquakes (average ML 5 2), although we have used the limited
moment-tensor focal solutions on larger events (M 5 4–5) available
from the local BATS (Broadband Array in Taiwan for Seismology)
network (Kao et al., 1998) to infer the relative movements of some of
the imaged faults. It is our experience from southern California, where
tens of thousands of high-quality low-magnitude focal mechanisms are
available, that focal mechanisms of intermediate-magnitude earthquakes (ML 5 3–4) can be misleading for our purposes. They are
commonly associated with secondary faults, often located near fault
intersections, whereas the important faults are illuminated by the small1GSA Data Repository item 2002110, Earthquakes, faults, and topographic videos of Taiwan, is available on request from Documents Secretary, GSA,
P.O. Box 9140, Boulder, CO 80301, [email protected] or at
www.geosociety.org/pubs/ft2002.htm.
q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; October 2002; v. 30; no. 10; p. 935–938; 6 figures; Data Repository item 2002110.
935
Figure 1. Map and cross sections of earthquake locations in central Taiwan. Section in lower left shows all
earthquakes. In other sections, different colors indicate events belonging to different faults, selected in three
dimensions; in particular, pink refers to earthquakes of interpreted main detachment, and black refers to earthquakes not assigned to any planar structure. Black boxes on map indicate location of events projected onto
each section. Some scattering of events in cross section is due to distortion inherent in projecting a threedimensional structure onto two-dimensional plane.
er events (Carena and Suppe, 2002). Therefore abundant lowmagnitude mechanisms are required for a convincing test. Nevertheless, the detachment interpretation is supported by the observation that
other planes of seismicity that exist above and below the inferred detachment abut it but do not crosscut it (Fig. 1, where three-dimensional
fault-like planar subsets of earthquakes are shown in different colors).
A very similar distribution of earthquakes was seen locally in data from
a temporary seismic network placed to record aftershocks of the ChiChi earthquake, including a well-defined horizontal plane of events at
;10 km depth (Nagai et al., 2001). Therefore we proceed with the
hypothesis that the seismicity is emitted from the main detachment and
Figure 2. Cross section showing projected earthquakes together
with fault traces obtained from three-dimensional surfaces (in gray,
below 10 km depth) and structures obtained from surface geology,
seismic reflection profiles, and wells (in black, above 10 km depth).
(See Fig. 1 for location.)
936
its surrounding zone of deformation. In a following section we see that
the observed geometry is consistent with critical-taper wedge
mechanics.
The inferred main detachment beneath the Taiwan mountains is
subhorizontal and at an average depth of 10–15 km under most of
west-central Taiwan, but it dives steeply down under eastern Taiwan
(Figs. 3 and 5; video of topography of Taiwan, see footnote one). The
maximum depth at which it can be recognized because of abundant
earthquakes is 50 km at the northern end of the examined area and 35
km at the southern end. The surface becomes progressively steeper
Figure 3. View of three-dimensional spatial relationship between topography and main detachment.
GEOLOGY, October 2002
Figure 4. Fit of main detachment surface
in three dimensions: 98% of main detachment subset events fall within 3 km
(2s) of our modeled surface. s 5 standard deviation, N 5 number of samples.
from south to north, where it is close to vertical (Figs. 1 and 5). Faults
above and below the inferred main detachment abut it but do not cross
it (Figs. 1 and 2), showing that it is a throughgoing tectonic feature.
Active, high-angle faults exist in its footwall, but they do not significantly deform the overlying detachment. (Note that the presence of
faults characterized by small earthquakes says little about total displacements, given the very small rate of seismic moment release.) The
hanging wall above the detachment contains mainly low-angle thrust
faults that ramp up either directly from the main detachment or from
higher level ones. However, in interpreting these patterns of seismicity,
we emphasize that not all active faults are illuminated faults; e.g., only
the creeping parts of the San Andreas fault in California are illuminated. Several major events in Taiwan, including the 1999 MW 7.6
Chi-Chi earthquake and the 1998 ML 6.2 Rueyli earthquake, occurred
on thrust faults above the detachment that were not illuminated by
small earthquakes before or after the main events.
Figure 5. Taiwan drainage divide regionally is parallel to inflection
in main detachment, as expected by wedge theory (see Fig. 6). Black
contours show main detachment, color contours show topography.
TESTING WEDGE MECHANICS
Our mapping of the inferred main detachment in three dimensions
with microseismicity allows us to directly measure the taper of the
actively deforming mountain belt in many locations and thus make a
new test of critical-taper wedge mechanics. According to wedge theory,
the condition for sliding on the detachment is that the taper (a 1 b,
Fig. 6) be at or above some critical value controlled by the strengths
of the wedge and detachment. The regional topography of Taiwan
should be close to this critical surface slope in most regions, given the
very high present and past erosion rates (5–6 km/m.y.) and the evidence for an erosionally steady-state mountain belt (Li, 1976; Suppe,
1981; Dahlen and Suppe, 1988; Willett et al., 2001). The only exception is the western half of section B (Fig. 1), where low and heterogeneous erosion rates suggest that taper may not be critical.
For a narrow-taper, weak-base cohesionless wedge, the criticaltaper equation can be approximated (Dahlen, 1990) by
b 1 mb (1 2 lb )
a1b5
1 1 2(1 2 l)
5
11 2 sin w2
sin w
b 1 m b∗
,
1 1 2m*
(1)
where l and lb are the fluid-pressure ratios within the wedge and on
the base, w is the angle of internal friction, m*
b 5 mb(1 2 lb) is the
effective coefficient of friction on the base, and
m* 5
11 2 sin w2 (1 2 l)
sin w
(2)
is the effective coefficient of internal friction. Thus low effective basal
GEOLOGY, October 2002
Figure 6. Observed Taiwan wedge tapers (topographic slope a
and detachment dip b) show good agreement with best-fitting
solution for homogeneous, brittle, cohesionless critical-taper
wedge (Dahlen, 1990) for depths of 0–15 km, expected zone of
brittle behavior. Higher than brittle tapers are expected at greater
depths as wedge passes through brittle-plastic transition, whereas detachment remains brittle due to its higher strain rate (Williams et al., 1994). Cross section shows best-fitting wedge for
section D. Transition to negative topographic slope a is caused
by steepening detachment dip b. Sections C and D are shown in
Figure 1.
937
friction m*
b makes for low taper (a 1 b). We measured the taper (a,
b) for 12 segments of sections C and D in Figure 1 (6–25 km long,
chosen to minimize errors) using three-dimensional best-fitting detachment and smoothed topographic surface.
The measured wedge tapers for depths ,15–20 km (Fig. 6) show
the characteristic linear decrease in topographic slope with increasing
detachment dip that is predicted by a variety of homogeneous brittlewedge theories (Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1990).
This observation constitutes a strong indication that (1) Taiwan is close
to critical taper and (2) the physical properties that control taper in the
brittle upper crust are relatively homogeneous. This agreement provides further encouragement for the detachment interpretation of the
horizontal zone of seismicity under Taiwan. Below depths of 15–20
km, observed tapers deviate from this linear relationship in the expected way for a wedge undergoing a brittle-plastic transition, which
weakens the wedge, leading to higher tapers (note that we expect the
interior of the wedge to undergo the brittle-plastic transition before the
detachment because of the higher strain rates on the detachment; see
Williams et al., 1994).
The observed tapers are substantially less than those previously
assumed for Taiwan on the basis of extrapolation of detachment dip at
the toe (Davis et al., 1983; Dahlen et al., 1984; Barr and Dahlen, 1989;
Barr et al., 1991). The lower observed tapers imply a substantially
weaker base; the best fit to our data by using the approximated criticaltaper equation results in m* 5 0.29 and m*
b 5 0.08. The best-fitting
cohesionless Coulomb wedge is shown in Figure 6. This very weak
computed apparent basal friction for Taiwan strengthens the observation that very large faults such as the San Andreas are apparently very
weak, for reasons that are as yet unknown (e.g., Mount and Suppe,
1987; Zoback et al., 1987; Townend and Zoback, 2001).
TECTONIC CONTROL OF TOPOGRAPHY
This linear relationship between detachment dip and surface slope
(Fig. 6) extends to negative values of the surface slope, implying that
the regional reversal in topographic slope across the Taiwan mountain
belt is controlled by the increasing dip of the detachment as it steepens
under eastern Taiwan. This change is consistent with the rather close
spatial correlation of the regional drainage divide or topographic crest
with the location of bending of the underlying detachment to a steeper
dip (Figs. 3 and 5). These observations suggest that the first-order
topography of the active Taiwan mountain belt is controlled by the
shape of the bending detachment through the mechanism of criticaltaper wedge mechanics.
ACKNOWLEDGMENTS
We are grateful to the Taiwan Central Weather Bureau for access to their comprehensive catalog of Taiwan earthquakes and to our colleague F.A. Dahlen for insightful
suggestions on wedge theory. We thank Tim Byrne and Paul Segall for thoughtful reviews.
REFERENCES CITED
Barr, T.D., and Dahlen, F.A., 1989, Brittle frictional mountain building: 1. Deformation and
mechanical energy budget: Journal of Geophysical Research, v. 94, p. 3906–3922.
Barr, T.D., Dahlen, F.A., and McPhail, D.C., 1991, Brittle frictional mountain building: 3.
Low-grade metamorphism: Journal of Geophysical Research, v. 96,
p. 10 319–10 338.
Carena, S., and Suppe, J., 2002, Three-dimensional imaging of active structures using
earthquake aftershocks: The Northridge thrust, California: Journal of Structural Geology, v. 24, p. 887–904.
Choukroune, P., 1989, The ECORS Pyrenean deep seismic profile reflection data and the
overall structure of an orogenic belt: Tectonics, v. 8, p. 23–39.
Clowes, R., Cook, F.A., Hajnal, Z., Hall, J., Lewry, J., Lucas, S., and Wardle, R., 1999,
Canada’s LITHOPROBE Project. Collaborative, multidisciplinary geoscience research leads to new understanding of continental evolution: Episodes, v. 22, p. 3–20.
Cook, F.A., Albaugh, D.S., Brown, L.D., Kaufman, S., Oliver, J.E., and Hatcher, R.D., Jr.,
1979, Thin-skinned tectonics in the crystalline southern Appalachians: COCORP seismic-reflection profiling of the Blue Ridge and Piedmont: Geology, v. 7, p. 563–567.
Dahlen, F.A., 1990, Critical taper model of fold-and-thrust belts and accretionary wedges:
Annual Review of Earth and Planetary Sciences, v. 18, p. 55–99.
Dahlen, F.A., and Suppe, J., 1988, Mechanics, growth and erosion of mountain belts, in Clark,
S.P., Jr., et al., eds., Processes in continental lithospheric deformation: Geological Society of America Special Paper 218, p. 161–178.
938
Dahlen, F.A., Davis, D., and Suppe, J., 1984, Mechanics of fold-and-thrust belts and accretionary wedges: Cohesive Coulomb theory: Journal of Geophysical Research,
v. 89, p. 10 087–10 101.
Davis, D., Suppe, J., and Dahlen, F.A., 1983, Mechanics of fold-and-thrust belts and accretionary wedges: Journal of Geophysical Research, v. 88, p. 1153–1172.
Gocad Consortium, 2002, http://www.ensg.u-nancy.fr/GOCAD/consortium.html (June 2002).
Hung, Jih-Hao, and Wiltschko, D., 1993, Structure and kinematics of arcuate thrust faults in
the Miaoli-Cholan area of western Taiwan: Petroleum Geology of Taiwan, v. 28,
p. 59–96.
Jones, R.H., and Stewart, R.C., 1997, A method for determining significant structures in a
cloud of earthquakes: Journal of Geophysical Research, v. 102, p. 8245–8254.
Kao, Honn, Jian, Pei-Ru, Ma, Kuo-Fong, Huang, Bor-Shouh, and Liu, Chun-Chi, 1998,
Moment-tensor inversion for offshore earthquakes east of Taiwan and their implications to regional collision: Geophysical Research Letters, v. 25, p. 3619–3622.
Ladd, J.W., Westbrook, G.K., Buhl, P., and Bangs, N., 1990, Wide-aperture seismic reflection profiles across the Barbados Ridge Complex, in Mascle, A., Moore, J.C., et al.,
Proceedings of the Ocean Drilling Program, Scientific results, Volume 110: College
Station, Texas, Ocean Drilling Program, p. 3–6.
Li, Yuan-Hui, 1976, Denudation of Taiwan Island since the Pliocene Epoch: Geology, v. 4,
p. 105–107.
Mallet, J.L., 2002, Geomodeling: New York, Oxford University Press, 624 p.
Moore, G.F., Shipley, T.H., Stoffa, P.L., Karig, D.E., Taira, A., Kuramoto, S., Tokuyama,
H., and Suyehiro, K., 1990, Structure of the Nankai Trough accretionary zone from
multichannel seismic reflection data: Journal of Geophysical Research, v. 95,
p. 8753–8765.
Mount, V.S., and Suppe, J., 1987, State of stress near the San Andreas fault: Implications
for wrench tectonics: Geology, v. 15, p. 1143–1146.
Nagai, S., Hirata, N., Sakai, S., and Huang, B.S., 2001, Distribution and structural details
of the aftershocks of the 1999 Chi-Chi, Taiwan earthquake: Eos (Transactions, American Geophysical Union), v. 82, p. 1177.
Namson, J., 1983, Structure of the Western Foothills belt, Miaoli-Hsinchu area, Taiwan: II.
Central part: Petroleum Geology of Taiwan, v. 19, p. 51–76.
Nicholson, T., Sambridge, M., and Gudmundsson, O., 2000, On entropy and clustering in
earthquake hypocentre distributions: Geophysical Journal International, v. 142,
p. 37–51.
Pfiffner, O.A., Lehner, P., Heitzman, P., Mueller, S., and Steck, A., edtiors, 1997, Deep
structure of the Swiss Alps: Results of NRP 20: Basel, Birkhäuser, 347 p.
Rau, R.J., and Wu, F.T., 1995, Tomographic imaging of lithospheric structures under Taiwan: Earth and Planetary Science Letters, v. 133, p. 517–532.
Shin, T.C., Kuo, K.W., Lee, W.H.K., Teng, T.L., and Tsai, Y.B., 2000, A preliminary report
on the 1999 Chi-Chi (Taiwan) earthquake: Seismological Research Letters, v. 71,
p. 24–30.
Suppe, J., 1980, A retrodeformable cross section of northern Taiwan: Geological Society
of China Proceedings, v. 23, p. 46–55.
Suppe, J., 1981, Mechanics of mountain building and metamorphism in Taiwan: Geological
Society of China Memoir 4, p. 67–89.
Suppe, J., 1986, Reactivated normal faults in the western Taiwan fold-and-thrust belt: Geological Society of China Memoir 7, p. 187–200.
Suppe, J., 1987, The active Taiwan mountain belt, in Schaer, J.P., and Rodgers, J., eds.,
Anatomy of mountain chains: Princeton, New Jersey, Princeton University Press,
p. 277–293.
Suppe, J., and Namson, J., 1979, Fault-bend origin of frontal folds of the western Taiwan
fold-and-thrust belt: Petroleum Geology of Taiwan, v. 16, p. 1–18.
Teng, Louis-S., Lee, C.-T., Tsai, Y.-B., and Hsiao, Li-Yuan, 2000, Slab breakoff as a mechanism for flipping of subduction polarity in Taiwan: Geology, v. 28, p. 155–158.
Townend, J., and Zoback, M.D., 2001, Implications of earthquake focal mechanisms for
the frictional strength of the San Andreas fault system, in Holdsworth, R.E., et al.,
eds., The nature and significance of fault zone weakening: Geological Society [London] Special Publication 186, p. 13–21.
Tsai, Yi-Ben, and Wu, Hsin-Hung, 1997, A study on the errors in locating earthquakes due
to the geometry of the Taiwan seismic network: Terrestrial, Atmospheric and Oceanic
Sciences, v. 8, p. 355–370.
Wang, Chien-Ying, Chang, Chien-Hsin, and Yen, Horng-Yuen, 2000, An interpretation of
the 1999 Chi-Chi earthquake in Taiwan based on the thin-skinned thrust model:
Terrestrial, Atmospheric and Oceanic Sciences, v. 11, p. 609–630.
Willett, S.D., Slingerland, R., and Hovius, N., 2001, Uplift, shortening, and steady state
topography in active mountain belts: American Journal of Science, v. 301,
p. 455–485.
Williams, C.A., Connors, C., Dahlen, F.A., Price, E.J., and Suppe, J., 1994, Effects of the
brittle-ductile transition on the topography of compressive mountain belts on Earth
and Venus: Journal of Geophysical Research, v. 99, p. 19 947–19 974.
Wu, F.T., Rau, R.J., and Salzberg, D., 1997, Taiwan orogeny: Thin-skinned or lithospheric
collision: Tectonophysics, v. 274, p. 191–220.
Zhao, W.L., Davis, D., Dahlen, F.A., and Suppe, J., 1986, Origin of convex accretionary
wedges: Evidence from Barbados: Journal of Geophysical Research, v. 91,
p. 10 246–10 258.
Zhou, H.W., 1994, Rapid three-dimensional hypocentral determination using a master station method: Journal of Geophysical Research, v. 99, p. 15 439–15 455.
Zoback, M.D., Zoback, M.L., Mount, V.S., Suppe, J., Eaton, J.P., Healy, J.H., Oppenheimer,
D.H., Reasenberg, P.A., Jones, L.M., Raleigh, C.B., Wong, I.G., Scotti, O., and Wentworth, C.M., 1987, New evidence on the state of stress of the San Andreas fault
system: Science, v. 238, p. 1105–1111.
Manuscript received March 15, 2002
Revised manuscript received June 24, 2002
Manuscript accepted July 3, 2002
Printed in USA
GEOLOGY, October 2002