1 Introduction

Southwestern Taiwan locates between the deformation front and fold-and-thrust belt of Taiwan orogeny, e.g. Huang et al. (2004); Lin et al. (2009), which is one of the most active orogeny with the extremely high horizontal convergent rate (Yu et al. 1997), uplift rate (Ching et al. 2011) and erosion rate (Dadson et al. 2003). At 03:57:26 (UTC + 8) on February 6th 2016, the 2016 Meinong earthquake (ML 6.6, focal depth 14.6 km, westward oblique thrusting) occurred in SW Taiwan (Fig. 1). Several aftershocks were triggered beneath Guanmiao area (greater than ML 4.9, focal depth between 20–30 km, normal faulting) starting from 160 s after the main-shock (CWB 2016; Wen et al. 2017). This area was also struck by the disastrous 2010 Jiasian earthquake (Chang et al. 2010; Huang et al. 2011; Lin et al. 2016; Rau et al. 2012). Additionally, Tainan city, has important infrastructures including Taiwan High Speed Rail and science parks, and the population density of 861 persons / km2 (Department of Statistics 2017) which is the 14th most densely populated region compared with the other countries in the world (United Nations 2017). Therefore, the surface deformation is not only significant to better understand the structural evolution, but also a basis for assessing the earthquake hazard risks of SW Taiwan.

Fig. 1
figure 1

Geological map and cross section of the study area (modified from CPC 1989; Ho et al. 2005; Lin 2013). Red and blue arrows represent the horizontal and vertical coseismic surface displacement vectors, respectively. a Rose diagram of the horizontal displacement vectors. Frequency histograms and cumulative curves of b horizontal and c vertical displacement. Focal mechanisms of the main-shock and aftershocks are adopted from the website of Central Weather Bureau (r Bureau, Taiwan, https://scweb.cwb.gov.tw/zh-tw/earthquake/data/) and Wen et al. (2017), respectively. Black lines are geologic structures. SHF Sinhua fault, TCF Tsochen fault, LCF Lungchuan fault, PHF Pinghsi fault, GTKF Gutingkeng fault, MLF Meilin fault, LSLF Luosianliao fault, GM syncline Guanmiao syncline, CC anticline Chungchou anticline

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The coseismic surface displacements of the Meinong earthquake derived from Interferometric Synthetic Aperture Radar (InSAR) (Huang et al. 2016) and GPS observations (Tsai et al. 2017) both show that the coseismic deformed region of approximately 20 by 20 km moved toward the west. Two N-S trending linear discontinuities of displacement illuminated by InSAR (Huang et al. 2016) occurred at the eastern and western sides of the axial trace of the Guanmiao syncline, which implies that the Guanmiao syncline may reactivate. However, the causative fault did not rupture to the ground surface (CGS 2016). The geometry and slip of blind thrusts could be determined by the geometry of fault-related folds, e.g. Ishiyama et al. (2007); Philip and Meghraoui (1983); Suppe (1983). Other structures recording the seismic activities are fold-related faults including bending-moment faults and flexible-slip faults, since they are low-seismic but also coseismic (Yeats 1986). However, the fold-related faults, (e.g. Ota et al. 2006; Yeats 1986), were less reported relatively. Therefore, the secondary structures related to the folding could provide another aspect on this event.

In this paper, we present field observations on the coseismic surface deformation and report a new case of the bending-moment fault from the moderate-magnitude Meinong earthquake. We also describe observations on the preseismic deformation that give clues to high tectonic activities in SW Taiwan. Accordingly, we demonstrate the kinematic model of coseismic surface deformation and argue that the bending-moment fault could provide an opportunity understanding the recurrence interval of folding.

2 Geological setting

Southwestern Taiwan is located between the deformation front and fold-and-thrust belt of Taiwan orogeny. Balanced cross sections show that the Eocene–Oligocene syn-depositional normal faults occurred close to the deformation front (Huang et al. 2004; Lin et al. 2003). The folds and thrusts developed in the Tertiary sedimentary rocks (Suppe 1981), deeper structures may slip along the pre-existing normal faults (Huang et al. 2004; Lacombe and Mouthereau 2002; Yang et al. 2016).

The rocks exposed in the coseismic deformed region include the Pliocene to early Pleistocene Gutingkeng Formation, the late Pleistocene Chiting Formations, the Pleistocene-Holocene terrace deposits, and the Holocene alluvium (CPC 1989; Ho et al. 2005; Lin 2013) (Fig. 1). The Gutingkeng Formation consists of mainly massive mudstones occasionally intercalated with thick-bedded sandstone lentils. The individual sandstone exposed on the ridge, such as the Lungchuan sandstone. The overpressure zones appearing in the Gutingkeng Formation (Yuan et al. 1987) characterize its unique mechanical property in SW Taiwan. The Chiting Formation was classified into the Erhchungchi and Liushuang Formation, which shows the relatively low velocity using ambient noise tomography (Kuo-Chen et al. 2017). The Erhchungchi Formation is comprised of inter-bedding mudstones and sandstones, intercalated with limestones. The Liushuang Formation consists of thick to massive loose fine-grained sandstones occasionally intercalated with gray mudstones. The Pleistocene-Holocene terrace deposits and the Holocene alluvium consist of clays, silts, sands and conglomerates, distributing over the terraces and coastal plain, respectively. (Yuan et al. 1987).

The coseismic deformed region shown by InSAR analysis (Huang et al. 2016) appeared to be bounded by the Sinhua fault (SHF) and Tsochen fault (TCF) to the north, Pinghsi Fault (PHF) to the east, Gutingkeng fault (GTKF) to the south, and Meilin fault (MLF) to the west. SHF is an active right-lateral fault triggering 1946 Tainan Earthquake, e.g. Chang et al. (1947), which reoccurred at least three paleo-earthquake in past 10,000 years (Lin et al. 2012). TCF is an active left-lateral fault (CPC 1989; Deffontaines et al. 1997; Lin et al. 2012) comprised of thin parallel fault gouge zones which are tightly localized. PHF is a thrust fault with left-lateral component (Lin 2013). GTKF is an oblique thrust striking NE-SW and dipping to the east. MLF is a blind fault interpreted as a steep thrust dipping toward the east (Lacombe et al. 1999). The Guanmiao syncline and Chungchou anticlines are located at the east and the west of MLF, respectively. The Guanmiao syncline is an N-S trending open fold with limbs comprise of the Liushuang formation. The Chungchou anticline was interpreted as diapiric structures by aerial photos, residual gravity, and seismic analysis (Hsieh 1972; Pan 1968; Sun 1964).

3 Methodology

The coseismic surface deformation were obtained by measuring the location, trending and displacement vectors of surface cracks using the hand-held GPS, geological compass and vernier with a precision of 0.05 mm. Due to the limitation of readability of the crack, however, 0.5 mm is the minimum record of the opening. The deformed region moved toward the west in the Meinong earthquake, so the eastern side of each crack was treated as the stationary side to describe the relative movement. It means that if the western side of the crack moved eastward or westward, it represents the contractive or tensile condition, respectively.

The limitations and solutions of field investigation are described as followings. (1) The surface cracks were still getting wider to additional several millimeters in several days following the Meinong earthquake (personal talks of residents in Guanmiao town, 2017). It indicates that the measurement of the crack opening might include both the coseismic and part of postseismic slip. We deployed the investigation from Meinong to Guanmiao town within hours of the earthquake, trying to limit the proportion of the postseismic slip. (2) The surface deformation over the hillslope might be introduced both by the reactivation of landslides and faults. The gravitational deformation was distinguished from the tectonic deformation if the cracks distributed over the landslides, such as tension cracks near the crown or compressional bulges at the toe of the landslides. (3) Surface cracks tended to be more observable on hard surfaces such as road pavements than the soft soil. This rises the controversy that if the surface cracks measured were produced from the regional tectonic deformation or the local ground shaking from the seismic waves. We ruled out the cracks occurring around the buildings or on the paved road which was detached from the ground.

In order to verify if the result was largely affected by the local ground shaking, we compared the data to the InSAR analysis (Huang et al. 2016).

4 Results

Excluding cracks of the gravitational deformation by ruling out the cracks distributing around the landslides, we obtained a total of 230 measurements related to the regional coseismic deformation in the field (Fig. 1). The surface cracks trends from the north to northeast (rose diagrams of GMWL, GMEL and Shenkeng in Fig. 2). The coseismic deformed region generally moved toward the W-WNW according to the rose diagram of the displacement vectors (Fig. 1a). The frequency and cumulative curves of the crack displacements show that the horizontal opening of cracks is within the range from -22 to 86 mm (Fig. 1b). Among all the horizontal opening data, only 4 measurements indicate the contractive movement and the others show the tensile movement. One hundred and four measurements of the horizontal opening (about 45% of the total) are 0.5 mm. Eighty-seven measurements (about 38% of the total) of the horizontal opening range between 0.8 to 6 mm. The vertical offset of cracks is within the range from -8 to 60 mm (Fig. 1c). Among all the vertical offset data, 13 and 12 measurements indicate the uplifting and subsiding movement, respectively. The other 205 measurements show no or naked-eye-unrecognizable vertical offset.

Fig. 2
figure 2

Interpolation maps of the coseismic displacement by the inverse distance weighted (IDW) function provided in ArcGIS. Positive numbers indicate westward and upward movement. Rose diagrams of the trend of ground cracks measured at Guanmiao eastern lineament (GMEL), Guanmiao western lineament (GMWL), and Shenkeng are presented. a Horizontal displacement map showing two lineaments, GMWL and GMEL, occurring at western and eastern Guanmiao, respectively. b Vertical displacement map showing a subsidence at Shenkeng. Both of the maximum horizontal and vertical displacement occurred at Luosianliao. Locations of figures are plotted in this map. Black dashed lines are lineaments of topography. The Pitou anticline and Pitou back-thrust is at the north of Guanmiao and is not shown in this map

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The spatial distribution of the coseismic displacement reveals the complex rupture process of the Meinong earthquake. Generally, the displacement of the cracks decreased from the Lungchuan ridge to Guanmiao. However, the maximum horizontal displacement of 86 mm and vertical displacement of 60 mm both occurred at the southwest of Guanmiao where is further away from the epicenter than any other deformed areas. This finding hints at the possibility that the Guanmiao area was affected by not only the Meinong main-shock but also another energy source, which could be the Guanmiao aftershocks.

We focus on the Guanmiao area to document the characteristics of the coseismic deformation. In the field, we have observed two kinds of surface breakages. The first is surface cracks distributed around the towns of Guanmiao and Shenkeng, and the length of the cracks is within several meters and the orientation is generally nonunique. The second is surface ruptures with longer length (> 10 m), and the orientation follows a N-S topographic scarp west of Guanmiao. The preseismic deformation was also discovered in this study.

4.1 Coseismic deformation—surface cracks

In order to test how the measurements were affected by ground shaking from the seismic waves, we interpolate maps of the horizontal and vertical displacement data in the Guanmiao area to compare it with InSAR analysis (Huang et al. 2016) (Fig. 2). The map of the horizontal displacement (Fig. 2a) appears two N-S trending linear gradients located at the eastern and western side of Guanmiao town, noted as Guanmiao eastern lineament (GMEL) and Guanmiao western lineament (GMWL) in this study, respectively. GMEL and GMWL are also the boundary between the hills around Guanmiao and the Guanmiao plain (Fig. 2a). It shows that the surface deformation is consistent with topographic features. The map of the vertical displacement (Fig. 2b) shows a subsidence center occurred at Shenkeng. The spatial distribution pattern shown by field measurements appears similar to the result illuminated by InSAR (Huang et al. 2016), proving that our field measurements should reflect mainly the regional coseismic deformation, not the local ground shaking effect.

The rose diagrams of the crack trending show that surface cracks trends mainly from the north to northeast (rose diagrams of GMWL, GMEL and Shenkeng in Fig. 2). The Guanmiao area generally moved toward the WNW, but 4 sites near GMEL moved toward the east. Except the contractive cracks, the horizontal openings of surface cracks range from 0.5 to 22 mm and about 50% of them are less than 1 mm at Guanmiao and Shenkeng. The vertical offset of cracks is within the range from -8.2 to 4.9 mm and more than 94% of them show no or naked-eye-unrecognizable vertical offset. At Luosianliao (also known as the location name of Wutiaoliao), the maximum horizontal and vertical displacement are 86 and 60 mm, respectively, which are much larger than the displacements at Guanmiao. Here the Guanmiao area are divided into two subareas those are Guanmiao and Shenkeng to be described their characteristics of coseismic deformation.

4.1.1 Guanmiao

In Guanmiao town, the surface cracks distributed at the limbs of the Guanmiao syncline. A series of NNE striking surface cracks with the crack density of 0.03/m occurred on the nearly E-W striking road from GMWL to the Meilin fault. The cracks on GMWL formed a one-hundred-meter-wide graben consisted of small normal faults with the opening of 15–17 mm and the maximum vertical offset of 4.9 mm (Fig. 3a). The cracks close to the Meilin fault showed the western-side-down movement with the maximum vertical offset of 3.5 mm. The section between GMWL and Meilin fault indicates the uplift movement and possibly reflects the reactivation of the Pitou anticline (Le Béon et al. 2017).

Fig. 3
figure 3

a Open surface cracks formed a one-hundred-meter-wide graben at GMWL. b Stretching of the wall with 0.95 at downtown Guanmiao. c Stretching of the pavement with 0.99 at GMEL

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From the axial trace of the Guanmiao syncline to GMEL, a series of NNE striking surface cracks with the crack density of 0.05 / m distributed on the nearly E-W striking road. Although most of the surface cracks were tensile, but there were four compressional cracks occurring, which makes it different from the deformation at GMWL. For examples, the wall of the elementary school was subjected to nearly E-W compression with the stretch of 0.95 by measuring the initial length (L0 = 21.1 cm) and elongated length (L = 20 cm) of the buckled rebar (Fig. 3b). Additionally, the road close to GMEL was subjected to E-W compression with the stretch of 0.99 by measuring the initial length (L0 = 155 cm) and elongated length (L = 152.8 cm) of the wavy pavement orthogonal to the crack (Fig. 3c).

4.1.2 Shenkeng

The surface cracks occurred mostly at the east limb of the Guanmiao syncline at Shenkeng (“deep pit” in Chinese), which is different from the pattern in downtown Guanmiao. The rose diagrams of the crack trending show that surface cracks trends to the ENE and NW (Fig. 4b). Shenkeng generally moved toward the NW with the horizontal displacement of 0.5 to 22 mm (Fig. 4c). In a total of 14 measurements, the vertical offset of cracks is mostly zero. One measurement of 2.7 mm indicates the uplifting and one of − 8.2 mm indicates the subsidence.

Fig. 4
figure 4

a Conjugate surface cracks on the road in Shenkeng. b Rose diagram of the strikes of coseismic displacement. Reds are data of westward cracks, and blues are data of eastward cracks. c North-westward slip vectors of surface cracks. d Conjugate surface cracks indicate a NNE striking maximum principle stress. e NNW-trending minor folds. f Lateral view of minor folds. Tension fractures occurred on the crest of the anticline

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The conjugate fractures and minor folds were shown on the paved road at central Shenkeng (Fig. 4). The conjugate fractures consist of NW striking fractures with dextral-slip and NE striking fractures with sinistral-slip, indicating the NNE-SSW maximum principal stress (Fig. 4a, d and e). The minor folds occurred locally at the SE quadrant of the conjugate fractures (Fig. 4f). The stress condition of the folds is different with the conjugate fractures, so they would form in different phase during the Meinong earthquake.

4.2 Coseismic deformation—surface ruptures

At about 4 km southwest of Guanmiao, we observed a series of coseismic shallow fault ruptures along the range-facing scarp striking NNW-SSE to N-S with the length of about 5 km. Base on the field observations and topographic analysis described in the following sections, we argue that the Meinong earthquake produced shallow ruptures on the pre-existing fault, the Luosianliao fault. Luosianliao means “the fallen and subsided hut” in Chinese.

4.2.1 Field observations

There were two sites of shallow ruptures of the Luosianliao fault observed in the field. The largest ruptures of the Luosianliao fault located at Hudi (“lake bottom” in Chinese) (Fig. 5). A series of en-echelon vertical and open ruptures extended at least 17 m long, with their tips overlapping each other (Fig. 5a). These ruptures primarily stroke northwest to north (rose diagram in Fig. 5a) and were nearly vertically. The slip vectors, including (090, 35) and (065, 20) (trend, plunge) with the displacement of 10.5 cm and 11.5 cm, respectively (Fig. 5b), show the dextral-slip with the western-side-up displacement. The northern end of the ruptures extended through the retaining wall (Fig. 5c). The apparent vertical western-side-up displacements were 4–19 cm (Fig. 5d). The vertical and open ruptures appear to be associated with the near-surface normal fault as demonstrated by the earlier study (Philip and Meghraoui 1983) (Fig. 5e).

Fig. 5
figure 5

a Coseismic dextral-slip fault ruptures with length of 17 m. Rose diagram shows N-NW striking ruptures. b Slip vectors showing that the western side of the ruptures moved upward and toward the west. c Retaining wall was offset by the ruptures. d The western side of ruptures uplifted 19 cm. e Western-side-up rupture and the schematic diagram (modified from Philip and Meghraoui 1983) shows the possible geometry of the fault plane in depth

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To the north of Hudi, we observed the grayish sandblasting in the bamboos garden and the western-side-up cracks on the paved road along the scarp. However, the dipping of the fault plane is not determined yet without further excavation.

4.2.2 Topographic features

Two lineaments were recognized from the 1-m digital terrain model that gave the topographic features of the Luosianliao fault. The western lineament is a NNW-SSE to N-S striking scarp forming the straight river bend perpendicular to the Er-Ren River. The flow direction of the rivers turning from the west to the south at the range-facing scarp indicate that it is a tectonic scarp. The height of the scarp increasing from 1.6 m to 12 m from the north to the south (Fig. 6a, red arrows) could be produced by the repeat activity of the Luosianliao fault. However, the erosion rate of the rivers should not be neglected.

Fig. 6
figure 6

Topographic evidence of the Luosianliao fault. a Shallow fault ruptures occurred along a range-facing scarp (red arrows). To the east of the scarp, the slope change of the terraces formed a lineament (blue arrows). b Schematic diagram shows these two lineaments as the conjugated normal faults above the neutral surface of the folds and the Luosianliao fault is the well-developed one (modified from (Currie et al. 1962)

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The eastern N-S striking lineament is recognized according to the slope change of the terraces (Fig. 6a, blue arrows), but the dipping of the plane related to the lineament could not be determined yet. Because the Luosianliao fault and the N-S striking lineament are located between the Guanmiao syncline and Chungchou anticline, a possibility is that these two lineaments served as the conjugated normal faults above the neutral surface. The Luosianliao fault is the well-developed one (Fig. 6b).

4.3 Preseismic deformation

We found preseismic deformation on the culture features such as concrete retaining wall and old brick house at Guanmiao and Shenkeng, which reveal the highly creeping activity of the Guanmiao syncline. On the axial trace of the Guanmiao syncline, the bulge of the retaining wall indicates a horizontal contraction (Fig. 7). The conjugated fracture dips to the east at 35 degrees with sinistral-slip (view to the north) and the opening of ~ 3 cm (Fig. 7a). Another conjugated fracture dips to the west at 30 degrees with dextral-slip and the opening of 2–3 cm (Fig. 7b, c). The extensional fracture is horizontal and break up along the interface of concrete blocks. The bulged block above the extensional fracture uplifted 4–5 cm. Therefore, we argue that downtown Guanmiao is located above the neutral surface of the Guanmiao syncline and this area is subjected to on-going tectonic deformation.

Fig. 7
figure 7

The bulge of the retaining wall on the axial trace of the Guanmiao syncline indicates a horizontal contraction. a The sinistral-slip fracture (top to the west). b The deformed pipe shows dextral-slip. c The concrete was subjected to the contraction and bulged

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Another case demonstrating the activity of the Guanmiao syncline is the damaged floor of an old brick house at Shenkeng (Fig. 8). The house owner had cemented the surface crack produced by continuous creeping before the Meinong earthquake. But the coseismic surface crack occurred over the same cemented pre-existing crack, indicating the rapid and continuous deformation of the Guanmiao syncline.

Fig. 8
figure 8

The damaged floor of an old brick house at Shenkeng. Coseismic surface crack (red arrows, bright orange crack) occurred over the same cemented pre-existing crack (blue arrow, light gray cement), indicating the rapid and continuous deformation of the Guanmiao syncline

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5 Discussion

5.1 High structural activity in Guanmiao area

The 2016 Meinong earthquake provided an excellent opportunity to examine the activities of the shallow geological structures and their response to the seismicity in SW Taiwan. In the Guanmiao area, coseismic surface cracks were distributed at the limbs of the Guanmiao syncline, forming two fold-axis-parallel lineaments GMEL and GMWL (Fig. 2). The contractive movement occurred at GMEL and the tensile movement occurred at GMWL (Fig. 3). We interpret that the reactivation of the Guanmiao syncline may cause by the Meinong main- shock and aftershocks.

The preseismic surface deformation also shows that the Guanmiao area is highly tectonic-active. The interseismic creeping at the axial trace of the Guanmiao syncline proves the on-going contraction of the Guanmiao syncline (Fig. 7). The damaged old brick house at Shenkeng (Fig. 8) also gave us an excellent case showing the rapid and continuous deformation of the Guanmiao syncline. In addition, the traditional place names, such as Shenkeng—“deep pit”, Luosianliao—“fallen and subsided hut”, Hudi—“lake bottom”, reminded us that early immigrants had already recognized the nature of the ground deformation in the Guanmiao area.

Accordingly, we argue that the Guanmiao syncline is an active fold with both coseismic activity and interseismic creeping. The on-going creeping of the ground surface largely and widely affected residents’ properties and lives. People have to repair numerous tiny and thin fractures on the floor or in the house frequently but apparently the effort is in vain (Fig. 8). The on-going creeping also could induce the long-lasting landform changes, causing the damage of the underground facilities and/or infrastructures. The “micro-geohazards” usually were neglected, but continual loss would make it become the most important factor on evaluating the earthquake risks in SW Taiwan.

5.2 Luosianliao fault: recording the joint effect of Meinong main-shock and Guanmiao aftershocks

The Meinong earthquake induced a series of shallow fault ruptures of the Luosianliao fault along the range-facing scarp (Fig. 5). The ruptures with the length of 17 m and the displacement of 19 cm were way larger than any surface cracks in the other areas in this event. However, the Luosianliao fault is further away from the Meinong main-shock than the surface cracks distributed over the Guanmiao area. It implies that the Meinong main-shock did not likely triggered the Luosianliao fault.

The eastern-side-subsided Luosianliao fault is consistent with the Guanmiao aftershocks which seem to trigger the paleo-normal faults existing in the lower crust (Wen et al. 2017), rather than the Meinong main-shock with the low-angle westward thrusting. It implies that Guanmiao aftershocks should contributed considerably to the occurrence of the Luosianliao fault.

The GPS stations around the Luosianliao fault also recorded that the ground uplifted when the main-shock arrived and subsided when aftershock occurred in 160 s following the main-shock (Fig. 9). Based on the high-rate GPS results, the GPS station all showed some uplift during the main shock, and the stations close to the fault (SHWA, GS31, and GS35) showed downward movements when the aftershock occurred. The surface rupture we observed could be the combination result of the coseismic displacements of the main shock and aftershock. Since the surface rupture follows the topographic scarp, therefore, we argue that the Luosianliao fault is a previous unknown shallow fault, which records the joint effect of Meinong main-shock and Guanmiao aftershocks in 2016. However, the linkage between the shallow Luosianliao fault and the deep causative fault of Guanmiao aftershocks are not known yet.

Fig. 9
figure 9

High-rate GPS time-series of stations GS31、SHWA、GS34 and GS35. Gray line and dash line represent occurring time of Meinong main shock and aftershock, respectively

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The Luosianliao fault is located between the Chungchou anticline and Guanmiao syncline, as the east-dipping normal faults (Fig. 6b). Another interpretation is that the Luosianliao fault is a west-dipping back-thrust as the Pitou back-thrust (Le Béon et al. 2017). However, the trace of Pitou back-thrust does not connect to the Luosianliao fault and the Pitou anticline seems to die out to the south. If the Pitou back-thrust is equal to the Luosianliao fault, then the Pitou anticline should be equal to the Chungchou anticline, which is not true (Fig. 1 of Le Béon et al. 2017). Another possibility is that the Luosianliao fault is a back-thrust, and an E-W striking tear fault separating the Pitou back-thrust and Luosianliao back-thrust. Without further observations, the most likely interpretation is that the Luosianliao fault is the bending-moment fault (Currie et al. 1962), which is the secondary structures at the inflection point above the neutral surface between these anticline and syncline. The Meinong earthquake triggered the Guanmiao syncline and/or the Chungchou anticline then reactivated the shallow pre-existing Luosianliao fault.

5.3 Kinematic model of coseismic deformation

As a bending-moment fault associated with folding, the Luosianliao fault is unlikely to be of major seismogenic significance (Yeats 1986). The overpressured Gutingkeng Formation (Yuan et al.1987) and the low-strength Chiting Formation underneath Guanmiao may contribute on the amplification of ground motion observed in the Meinong earthquake (Kanamori et al. 2017; Lee et al. 2016). After 160 s, Guanmiao aftershocks induced the Luosianliao faults. If it is the case, we may look into the recurrence interval of folding through studying the activity of the Luosianliao fault (Additional file 1).

We propose a kinematic model of coseismic deformation induced by the Meinong earthquake as follows (Fig. 10):

  1. (A)

    The geology of SW Taiwan is characterized by the ~ 3-km-thick Gutingkeng Formation (Fig. 10a, light blue) with high fluid pressure underlying the low-strength Chiting Formation (green), which formed the mud diapirs or mud-core anticlines. These two formations do not accumulate strain due to weak mudstones and loose sandstones so that they can easily undergo the aseismic creeping in the collisional tectonic setting.

  2. (B)

    The Meinong main-shock occurred and produced the upward and westward movement, which triggered the nearby faults and folds. The Guanmiao syncline was reactivated to produce GMEL and GMWL. Since GMWL is located at a compressional area and GMEL is located at an extensional area (Fig. 2a), GMWL could be a bending-moment fault directly related to the Guanmiao syncline and GMEL may be a bending-moment fault associated with another anticline to the east, which require further investigation. The Guanmiao syncline and/or Chungchou anticline were amplified by seismic energy through overpressured mudstones.

  3. (C)

    The Meinong main-shock triggered the pre-existing normal faults at depth of 20–30 km beneath the Guanmiao area. The ground surface subsided when aftershock occurred in 160 s following the main-shock. Though the linkage between the shallow Luosianliao fault and the deep causative fault are not known yet. They both showed normal faulting and GPS stations also recorded the rupture process from the main-shock uplift to aftershock subsidence. Hence, we confidently argue that the Luosianliao fault worked as the joint effect of Meinong main-shock and Guanmiao aftershocks.

Fig. 10
figure 10

Kinematic model of the coseismic deformation induced by the Meinong earthquake. A Weak mudstones and loose sandstones can easily undergo the aseismic creeping. B Meinong main-shock triggered the nearby faults and folds. The Guanmiao syncline and/or Chungchou anticline were amplified by seismic energy through overpressured mudstones, producing GMEL and GMWL. C Guanmiao aftershocks occurred in 160 s following the main-shock and triggered the Luosianliao fault that the rupture process from the main-shock uplift to aftershock subsidence was recorded by GPS stations. SHF Sinhua fault, TCF Tsochen fault, LCF Lungchuan fault. PHF Pinghsi fault, MLF Meilin fault, LSLF Luosianliao fault, GM syncline Guanmiao syncline, CC anticline Chungchou anticline

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6 Conclusion

The 2016 Meinong earthquake is a moderate-magnitude earthquake but significant for providing an excellent opportunity to better understand the activities of the shallow geological structures and their response to the seismicity in SW Taiwan. We conclude that:

  1. 1.

    The Guanmiao area in SW Taiwan possess high structural activity. We argue that the Guanmiao syncline is an active fold with both coseismic activity and interseismic creeping. The micro-geohazards induced by the on-going creeping should not be neglected because the continual loss, such as house repairing, damage of the underground facilities and/or infrastructures, would make it become the most important factor on evaluating earthquake hazards in this folding-active area of SW Taiwan.

  2. 2.

    We found the Luosianliao fault which recorded the joint effect of Meinong main-shock and Guanmiao aftershocks. We argue that the Luosianliao fault served as the bending-moment fault associated with folding amplified by seismic energy through overpressured mudstones (Huang et al. 1998; Yuan et al. 1987). Therefore, we may look into the recurrence interval of folding through studying the activity of the Luosianliao fault in the next study.

  3. 3.

    We propose the kinematic model of coseismic deformation during the Meinong earthquake, to demonstrate that seismic potential in SW Taiwan needs to be re-evaluated. The earthquake could be triggered the near-by faults and folds. The mechanism of earthquake-induced folding amplification through high fluid-pressure rocks may play a critical role in assessing earthquake hazard risks in regions with similar geology to SW Taiwan.