Introduction to the special issue on the 2018 Hualien, Taiwan, earthquake

Full text

doi: 10. 3319/TAO.2019.05.31.01
* Corresponding author
E-mail: raurj@mail.ncku.edu.tw
Introduction to the special issue on the 2018 Hualien, Taiwan, earthquake
Ruey-Juin Rau1, * and Tai-Lin Tseng 2
1 Department of Earth Sciences, National Cheng Kung University, Tainan City, Taiwan
2 Department of Geosciences, National Taiwan University, Taipei City, Taiwan
Received 29 May 2019, Accepted 31 May 2019
Keywords: 2018 Hualien earthquake
Citation: Rau, R.-J. and T.-L. Tseng, 2019: Introduction to the special issue on the 2018 Hualien, Taiwan, earthquake. Terr. Atmos. Ocean. Sci., 30, 281-283,
doi: 10.3319/TAO.2019.05.31.01
Exactly two years after the 2016 Mw 6.5 Meinong event,
an Mw 6.4 earthquake occurred slightly offshore Hualien at
about 16 km NNE of the Hualien city with a focal depth
of 6.3 km on 6 February 2018 (23:50:41.6 local time). It
is a moderate-sized event, however, produced strong shak-
ing in the Hualien city, triggered and ruptured the Milun
fault, which was previously activated during the 1951 M
7.3 Hualien-Taitung earthquake sequence (e.g., Chen et al.
2008). The 2018 event caused several buildings along the
Milun fault collapsed and 17 deaths. At the end of 2015,
the Taiwan Earthquake Model (TEM) announced a seismic
hazard map of Taiwan indicating a relatively high seismic
hazard in both Tainan and Hualien (Rau and Ma 2016;
Wang et al. 2016). The occurrences of the 2016 Meinong
event and the 2018 Hualien event validate and strengthen
the importance of the seismic hazard map proposed by TEM
(Rau and Liang 2017). With the extremely high strain rate,
10-7 - 10-6, and therefore short earthquake recurrence inter-
vals in Taiwan, reactivations of any pre-existing structures
in this highly deformed crust are immensely anticipated in
the foreseeable future.
The 6 February 2018 Hualien earthquake main shock
(Mw 6.4) was preceded by prominent foreshocks, which
considerably overlapped with the 4 February (Mw 6.1) earth-
quake sequence in the offshore region northeast of Hual-
ien city. The close distribution in space and time between
the two sequences had made it difficult to clearly define
their relation, whereas the overall length of the foreshocks-
mainshock-aftershocks sequence is approximately 50 km
with complicated rupture. For the special issue of this 2018
Hualien event, we have collected 13 papers focused on field
investigations, GPS and InSAR analyses, rupture models,
seismicity, earthquake early warning, and integrated geo-
physical observations. The research presented here provide
valuable constrains to the rupture behavior and geologic
structures in the region. It is also an important and yet a
great challenge to evaluate the forecast models based on
precursory events or foreshock-aftershock properties.
To begin with Tung et al. (2019) determined the co-
seismic deformation of the 2018 Hualien event using the
continuous GPS measurements and InSAR analyses and
they found that the main deformation is concentrated on the
Milun and the Lingding faults. Moreover, they suggested
that an unknown west-dipping fault close to the Lingding
fault was triggered during the earthquake sequence. Tung
et al. (2019) also obtained 1-Hz GPS positions peak ground
displacement to evaluate the earthquake damages of the
Hualien area. Wu et al. (2019), on the other hand, inves-
tigated and re-measured more than 100 benchmarks set up
by various agencies by using network RTK. They found
70 and 50 cm coseismic left-lateral motion for the Milun
and the Lingding faults, respectively, yet the Milun Terrace
showing uplifting motion and the northern Coastal Range
indicating subsidence.
Three studies focused on field investigations of the
surface coseismic ruptures using field geology, stress analy-
ses and unmanned aerial systems photogrammetry. Huang
et al. (2019) investigated the coseismic rupture patterns of
the 2018 Hualien earthquake using the analyses from field
survey and drone-based image results. They found that ma-
jor ruptures occurred as arrays of Riedel shears and formed
right-stepping step-overs and restraining bends in the linking
damage zones along the Milun fault, where the rupture traces
repeat the surface breaks of the 1951 M 7.3 Hualien-Taitung
earthquake sequence. Characteristics of the 2018 coseismic
ruptures apparently closely related to the near-surface geol-
ogy structure along the Milun fault and such features provide
important information for earthquake hazard assessment on
this extremely active Milun fault in eastern Taiwan.
Independently Hsu et al. (2019) conducted field surveys
along the Milun fault and examined the coseismic ruptures
of the 2018 Hualien event. In addition to the rupture geom-
etry, they determined the principal displacement zone and
Terr. Atmos. Ocean. Sci., Vol. 30, No. 3, 281-283, June 2019

Ruey-Juin Rau and Tai-Lin Tseng
282
the regional stress directions of the surface ruptures. They
suggested that the coseismic rupture forms a horsetail struc-
ture at Qixingtan in the northernmost part of the Milun fault,
and a fault splays trending 170° in the central segments. The
coseismic surface rupture of the Milun fault and the surface
trace of the Beipu fault to its left form a macro-scale Riedel
shear model with a maximum horizontal compressive stress
directing NW-SE. Results of this study offer an extraordi-
nary example to understand the linkage between the outcrop
scale and the macro-scale of the Riedel shear model.
Lin et al. (2019) used the unmanned aerial systems
photogrammetry to collect aerial images and to map surface
fractures of the 2018 Hualien event. They showed that sur-
face ruptures follow the traces of the Milun fault and north-
ern Linding fault. The mapped surface ruptures are typically
appeared in en échelon arrays or distributed fractures rather
than a through-going fault, which are comparable to the re-
sults of Hsu et al. (2019) and Huang et al. (2019). They con-
sidered that the appearances of the along-strike variations
of the surface rupture for the 2018 Hualien earthquake are
different from the surface rupture patterns documented for
the 1951 Hualien-Taitung event.
For the earthquake source model, Hwang et al. (2019)
conducted a simple forward analysis to constrain the rupture
of the main shock of 2018 Hualien earthquake, which can be
decomposed into six sub-events with the maximum moment
rate at 6.9 s and the focal depth is 9 km. Along the west-dip-
ping fault, the overall duration and velocity of rupture are
~11 s and < 2.0 km s-1 (~0.6 Vs), respectively. The total fault
length derived from moment (Mo) is about ~21 km. Two
stages of rupture are proposed with the changes on stress
drop (Δσs), energy moment ratio (Es/Mo) and speed across
the sea-land boundary. The results are comparable to other
source studies (e.g., Jian et al. 2019; Wen et al. 2019b).
In Wen et al. (2019b), multi-segment rupture is con-
strained by finite fault inversion of far-field seismic data
and forward modeling of local GPS deformation. The pre-
ferred model consists of two fault planes: a west-dipping
main fault dominant in the initial rupture and a shallow east-
dipping fault taken over in the later phase, with a rupture
speed of 2 km s-1. After examining the possible trade-off
and ambiguities in solutions, the results show that the main
asperity, with strong left-lateral slip, is right underneath Mi-
lun Fault where the major damage and the centroid location
of regional moment tensor located.
Wen et al. (2019a) focused on the local background
structure of the region and its relation with the 2018 Hual-
ien earthquake. This article presents a comprehensive seis-
mological analysis including 3-D tomographic inversion,
earthquake relocation, and focal mechanism determination
in the Hualien region. A concentration of low velocity and
high Vp/Vs is found at shallow depth near Meilun fault
which could be passively ruptured (Wen et al. 2019b). On
the contrary, high Vp and low Vp/Vs appear at deeper depth
on a west dipping seismic zone further north where the 2018
event initiated. The high Vp/Vs & seismicity is interpreted
as the presence of migrating fluid. The low velocity near
Meilun seems correlated well with the drop of rupture speed
observed by other studies (Hwang et al. 2019).
Two studies concern the seismicity rate of the Hualien
sequences that are related to the hazard potential. Chen et al.
(2019c) explored the temporal evolution of the 2018 Hual-
ien earthquake sequence and other major events previous
occurred in the nearby region based on the CWB seismicity
catalogues. For both foreshocks and aftershocks of the 2018
Haulien sequence, the seismicity rates (K-values) are rela-
tively high among all 11 studied events, while the b-values
are almost the lowest (0.63 and 0.68), which is also sig-
nificantly below the long term background average (0.99).
Comparisons lead to the conclusions that the high seismic-
ity rate accompanied with a low b-values is indicative of
higher potential for events with larger magnitude.
Chen et al. (2019d) estimated “time-varying” b-values
and seismicity rates after the Hualien main shock based on
generalized Reasenberg-Jones statistic models. The study
attempt to forecast, in near real-time manner, the forthcom-
ing aftershock hazard based on 6- and 12-hr windows after
the main shock. They found the double-sequence model,
which comprise main shock and the second large aftershock
(M 5.4 on 7 February), can better predict the aftershock oc-
currence rate with magnitude above 3. It not only shows the
improvements on short-term hazard evaluation but also in-
dependently supports the multi-sequence nature of the 2018
Hualien earthquake. This study also confirmed the low b-
value obtained in 72 hr after Hualien main shock in Chen
et al. (2019c).
Chen et al. (2019b) demonstrates the efficiency of the
CWB Early Warning System on the 2018 Hualien earthquake
sequence. The study implemented the “effective epicenter,”
a new approach based on triggered stations to estimate in-
tensity. The results show that the time for issuance warning
can be shortened by 2 - 9 s, comparing with the original
system. The improvement on the prediction of maximum
intensity is however less satisfying because the values pre-
dicted for the event sequences are generally underestimated
with respect to the actual observations. Furthermore, Chang
et al. (2019) presented a new method to automatically iden-
tify P and S phases and therefore the event detection, with
an algorithm computationally efficient and suitable for the
high seismicity rate of the dense seismic network in Taiwan.
The algorithm was applied near real-time to the foreshock-
mainshock sequences of the 2018 Hualien earthquake, and
proven to provide timely information for the estimation of
seismic hazard assessment and source characteristics.
Finally, Chen et al. (2019a) reported coseismic data of
the 2018 Hualien earthquake observed from the induction
magnetometers, geophones, infrasound systems, tiltmeters,
micro-barometers, and fluxgate magnetometers established

Introduction 283
in Taiwan. Long-lasting coseismic geomagnetic fluctua-
tions were recorded by both induction and fluxgate magne-
tometers, and about 15 - 45 s delayed seismo-traveling at-
mospheric disturbances were observed in infrasonic waves
and micro-barometers. The long-lasting coseismic geomag-
netic fluctuations are likely resulted from the interaction of
surface waves and groundwater oscillations.
Acknowledgements We wish to thank all the authors who
contributed to this special issue and we express our gratitude
to many reviewers who provided thorough and insightful
comments to the manuscripts. This preface was supported
by Ministry of Science and Technology under grants MOST
108-2119-M-006-004.
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