Seismogenic structure framework in a tectonic suture zone: with new constraints from 2006
Mw6.1 Taitung earthquake
Yih-Min Wu, Yue-Gau Chen, Chien-Hsin Chang, Ling-Ho Chung, Ta-Liang Teng, Francis T.
Wu, and Chien-Fu Wu
Abstract
A Mw 6.1 earthquake occurred on April 1st, 2006 near Taitung, eastern Taiwan. It produced
significant coseismic ground displacements and a large number of aftershocks in the ensuing
month. This event provided an opportunity to diagnose the seismogenic structure framework in the
southern Longitudinal Valley (LV) of eastern Taiwan, long viewed as one of the collision sutures between
the Philippine Sea and the Eurasian plates. With more precisely relocated main- and aftershock
hypocenters, focal mechanisms for M  3.8 events, coseismic ground displacements from strong motion
records, we determined a main shock dislocation model. Our results indicate that the main shock is
generated from a high angle fault (azimuth 198, dip 77). The fault appears to be divided into two
segments; the main shock and a greater number of aftershocks are associated with the northern
segment and it is dominated by left-lateral strike-slip fault, agreeing closely with the P-wave first motion
and waveform (USGS) solutions. The southern segment has a slightly larger thrust component agreeing
more closely CMT solutions. Tectonically this event highlights a NNE-trending fault on the west side of
the LV, which is strike-slip motion dominant. We interpret this belongs to the western boundary fault (the
Central Range fault system) of LV. The aftershocks with mainly thrust faulting located in the east of the
main shock associated fault represents shortening is still acting on the sedimentary materials deposited
prior to collision between two mountain ranges.
Figure 1 Map showing the epicenter and focal mechanisms of 2006 Taitung earthquake, geomorphic
features, major structures, locations of seismic stations, and interseismic ground slip. Inset shows that
the current tectonic environment surrounding Taiwan is composed of two arc-trench systems.
Figure 2 The epicenter distribution of the 2006 Taitung earthquake sequence are shown by
circles. The circle size is proportion to the magnitude. Also shown are focal mechanisms of the
aftershocks greater than 4.0, which are numbered by the order of the occurrence time. Three clusters
are clearly identified. There is one located offshore. On-land northern one is the most active cluster,
which can be further subdivided into western and eastern two similar trending but separated sub-clusters
(also see Fig. 3). The main shock is located in the western sub-cluster. On the other hand, the on-land
southern one is sparse and seems the southern counterpart of the western sub-cluster of the northern
one. Based on the occurrence time, the main shock-included western sub-cluster of the northern
on-land cluster leads the sequence. The eastern sub-cluster was triggered right after the main
shock. The offshore one came days later in the sequence.
Figure 3 Profiles show the vertical distributions of the clusters. (A) The main shock is located within
the western sub-cluster. The eastern sub-cluster of the northern cluster is dominated by seismicities
with evident thrust motion. The offshore cluster is spatially related to the Coastal Range fault as
presented in Wu et al., 2006 and Kuochen et al., 2006. (B) The southern on-land cluster shares the
orientation and similar focal mechanism of the western sub-cluster in the north. Red line represents the
modeled the seismogenic fault of 2006 Taitung earthquake. Red dashed lines are faults proposed by
dominant focal mechanisms. Blue kink-line is the subsurface projection of the Coastal Range fault.
Figure 4 The co-seismic ground displacements derived from records of strong motion stations and
modeled by the supposed fault plane (white rectangles) are shown as vectors in blue and red
respectively for horizontal (4A) and vertical components (4B). Blue circles are the seismicties occurred
in a few hours after main shock, delineating an area for the modeled fault plane. Modeled the rupture
slips and moment magnitudes for different segments are shown in 4C.