Imaging active faulting in the western Taiwan Strait

Large, destructive historical earthquakes off the coast of China’s Fujian Province point to important tectonic activity in the western Taiwan Strait that, until recently, has received little attention. We present newly acquired reflection seismic data that is used to study the shallow crustal structure of the western Taiwan Strait. With these data we map the location of the Benhai fault for the first time, describe its upper crustal geometry and, in combination with seismicity and earthquake focal mechanisms, interpret its kinematics. These new data demonstrate that there is wide spread evidence of faulting that reaches the sea floor in the western Taiwan Strait, clearly indicating that the Benhai fault is active. Faults that cut up section from steep basin sidewalls to form flower structures or terraced sidewall fault zones, together with the fault and basin map pattern, are consistent with this fault zone being in the early developmental stages of a dextral strike-slip system. Earthquake focal mechanisms, although not definitive, support the model of an active dextral strike-slip fault system in the western Taiwan Strait.

Large, destructive historical earthquakes off the coast of China's Fujian Province point to important tectonic activity in the western Taiwan Strait that, until recently, has received little attention. We present newly acquired reflection seismic data that is used to study the shallow crustal structure of the western Taiwan Strait. With these data we map the location of the Benhai fault for the first time, describe its upper crustal geometry and, in combination with seismicity and earthquake focal mechanisms, interpret its kinematics. These new data demonstrate that there is wide spread evidence of faulting that reaches the sea floor in the western Taiwan Strait, clearly indicating that the Benhai fault is active. Faults that cut up section from steep basin sidewalls to form flower structures or terraced sidewall fault zones, together with the fault and basin map pattern, are consistent with this fault zone being in the early developmental stages of a dextral strike-slip system. Earthquake focal mechanisms, although not definitive, support the model of an active dextral strike-slip fault system in the western Taiwan Strait.
Despite a number of large, destructive historical earthquakes having occurred in the western Taiwan Strait (Fig. 1) (see Methodology for the data base of historical events used in this study), little is known about its geological structure or the potential risk that this area could pose for people and infrastructure along the coast of China's Fujian Province. In large part, this paucity of knowledge is because little geoscientific attention has focused on the Taiwan Strait since the 1970's and 1980's when extensive reflection seismic and drilling campaigns 1-3 failed to identify significant petroleum potential. This lack of attention was further compounded by the fact that much of the data collected during this time has not been made available to the general scientific community for more detailed studies, and undertaking new studies has been complicated by territorial disputes. This meant that until recently 4,5 , little scientific research was carried out there. Nevertheless, based on historical and recent seismicity, sparse shallow reflection seismic data, refs. [6][7][8] have suggested that there is a roughly NE-striking fault system, called the Binhai fault, that skirts the coast of China from the east of the island of Hainan, along the western margin of the Pearl River Basin, and through the western Taiwan Strait into the East China Sea Basin (Fig. 1). Yet, the existence of this fault and its possible location, geometry, and kinematics are still not certain.
The part of the continental margin that comprises the Taiwan Strait consists of a broad shelf area with a number of deep, fault-bounded basins [9][10][11][12][13][14][15] . The extensional tectonic history of this area began with rifting during the Early to Late Eocene and, by the late Early Oligocene, had evolved to sea-floor spreading and eventually the formation of the South China Sea ocean basin to the south 2,14 . The late Early Oligocene change from rifting to sea-floor spreading in the South China Sea is recorded in the stratigraphy of the Taiwan Strait by what is commonly called the break-up unconformity 2,3,14 . Further extension during the Middle and Late Miocene affected much of the margin to various degrees, but was particularly felt on the outer part of the shelf, facing the South China Sea 2 . On the shelf, the Miocene extension was accompanied by the eruption of extensive intraplate flood basalts, forming the Penghu Archipelago [16][17][18] . Since the Late Miocene onset of arc-continent collision in Taiwan, a westward advancing foreland basin has been forming in the strait, and its sediments now reach to approximately 120° E 1,19 .
Far-field stress from the Taiwan arc-continent collision is causing reactivation of Cenozoic faults and widespread, but moderate, low magnitiude seismicity in the eastern and central part of the Taiwan Strait [20][21][22] . Yet, much of the western part of the strait is thought to be an area of relative tectonic quiescence despite widespread evidence for active faulting to the south, in the Pearl River Mouth Basin 7,8,[23][24][25][26] and to the north, in the Taibei Depression 27,28 . Ref. 28 shows a marked crustal-scale P-wave velocity anomaly just off the southeast coast of Fujian  Stratigraphic ages are derived from borehole DW5 whose location is shown in Fig. 1. Sediments below the break-up unconformity are termed syn-rift and those above it are termed post-rift.

Results
Reflection seismic data. The age of the various stratigraphic units and their contacts have been correlated between our profile 3 and profile 65-1D-33 of ref. 2 , with constraints provided by borehole DW5 located in the Penghu Basin (Figs. 1 and 2). Two surfaces are of particular importance: (1) the top of the basement and, (2) the break-up unconformity (TB and BU, respectively in Fig. 2). According to ref. 2 , the ages of TB and BU are ~66-58 Ma and ~37-30 Ma, respectively, which separate three episodes (pre-rift, syn-rift, and post-rift) of the rifting processes related to the opening of the South China Sea. Because these two unconformities are the most pronounced unconformities shown in all the seismic profiles ( Fig. 3), we are able to use them to identify the cross-cutting relationship between the unconformities and the faults. Based on this, faults that do not cut the break-up unconformity (shown in black) range from Eocene to Early Oligocene in age, whereas those that do cut it (shown in red) are younger. If a fault reaches the sea floor, it is currently active (although faults that do not breach the surface may also be seismically active). Below, we present the interpretation of profiles 1 through 6 (Fig. 3). The un-interpreted versions are presented in Supplementary Figure S1. Profiles 7 and 8 provided tie profiles that helped with the correlation of the structure between profiles 1 through 6. Interpreted versions of these are given in Supplementary Figure S2. To further aid with the structural interpretation, fault maps with depth contours of the top of the basement and the break-up unconformity reflections have been constructed (Fig. 4).
In all profiles, the top of the basement (TB in Fig. 3) coincides with a high-amplitude reflection below which the crust is overall acoustically transparent. Only locally is there sufficient reflectivity to allow minimal interpretation of the basement structure. In profiles 1, 2, and 3, the top of the basement has an overall domed shape, and it is cut by numerous faults. A northeast-striking basement high extends from profile 4 to profile 1 and, locally, the post-rift sequences unconformably overlie the basement (Figs. 3 and 4A). In both the west and the east, the top of the basement reflection reaches to between 1 km and 3 km depth. Up to 2 km thick packages of inclined reflections with, commonly, a fan-like geometry, correlate with the Eocene and Early Oligocene syn-rift sediments. Reflections that are attributed to the syn-rift sediments generally terminate against the basement along the steeply inclined basin bounding faults. In profiles 4, 5, and 6, the top of the basement reflector reaches much shallower depths, being less than a hundred meters deep in the west (e.g., profile 6). In profiles 5 and 6, syn-rift basins are filled with up to a kilometer of sediments (Fig. 5). In profile 6, the top of the basement reflector deepens toward the east, where it is offset by several faults with asymmetric packages of syn-rift sediments forming distinct basins.
The seismic characteristics related to the syn-rift sediments from the tie line of Fig. 2 are also observed in profiles 1, 2, and 3. The syn-rift sediments were truncated at the top by a high-amplitude, gently eastward dipping reflection that can be correlated with the regional break-up unconformity (BU in Fig. 3). Throughout the study area, the break-up unconformity reflection often terminates against basement highs that are generally, but not always (see profile 6), fault bounded. The reflection package above the break-up unconformity is related to the Miocene and younger post-rift sediments 2 (Fig. 3). The thickness of this package of reflections increases from around 100 meters in the west to more than 1000 meters in the east.
Throughout the study area, a number of faults cut both the top of the basement and the break-up unconformity, and extend to the surface. In many cases, such as in the western part of profiles 1, 2, and 3, and the basin in profile 4, these faults often display convex upward or concave downward warping of reflections that are crosscut by a system of faults that merge at depth (Fig. 5A,B). The geometry of these reflections and the associated fault systems is suggestive of positive and negative strike-slip flower structures. In profile 4, a roughly one km deep fault-bounded basin with nearly horizontal reflections within the basin sedimentary sequence is suggestive of   (Fig. 5B). We interpret this basin to strike northeast, appearing in the eastern end of profile 4 and also in profile 6 (Fig. 4). In all three profiles, there are also faults that clearly cut through the break-up unconformity. Several of these reach the surface indicating that they are currently active. In profile 4, the western bounding fault of the basin also offsets the break-up unconformity, extending upward into the post-rift sequences, suggesting it has been reactivated (Fig. 5A).
Seismicity data. Seismicity is predominantly clustered in the central part of the study area, with two well-defined clusters to the southeast and southwest (Fig. 6A). Elsewhere, seismicity is evenly scattered throughout. Focal mechanisms were determined for 10 events of between Ms 3 and 5.2 (Ms is the magnitude determined from surface waves) using P-wave first motion polarity. These show a range of fault types, from oblique extension and thrusting through to strike-slip (Fig. 6B), indicative of the complex fault types commonly found in strike-slip fault systems. It is interesting to note, however, that a composite focal mechanism for 10 events gives a strike-slip fault sense with a P-axis that plunges gently toward 98.8° (Fig. 6C), sub-parallel to the relative motion vector between the Philippine Sea and Eurasian plates (306°) (Fig. 1). Throughout the study area, the majority of seismicity has been in the upper 10 km of the crust, with some events reaching as deep as 30 km (Fig. 7). Along profiles 1 and 2, seismicity is mostly diffuse, with weak clustering between kms 10 to 20 in profile 2 where faults are interpreted to breach the break-up unconformity, and from kms 30 to the end of the profile where a number of faults are interpreted to reach the surface. In profiles 3 and 4, there is clustering of seismicity from c. km 35 to the www.nature.com/scientificreports www.nature.com/scientificreports/ eastern end of the profiles. This clustering coincides with the western flank of the Penghu Basin where numerous faults are interpreted to reach the surface. There is very little seismicity associated with the area beneath the basin imaged in the central part of profile 4. Only a small number of events were recorded beneath the basin imaged in the west of profiles 1, 2, and 3, although there is a small cluster between 0 and 20 km in profile 2. The western part of profile 5 displays sparse events, although there is a small cluster at around km 10 in profile 6. Eastward, both profiles 5 and 6 have clustering of events between km 50 and km 60 that coincide with active faults interpreted in this area.
In 2018, a series of earthquakes occurred in the Taiwan Strait, with the Mw 5.7 (USGS) main shock being near the study area (Fig. 1). We have relocated (see Methodology for how the relocation was carried out) the main shock and aftershocks by jointly using the data from both sides of the Taiwan Strait in order to have a better constraint on the event locations. In total, we relocated 101 events with depths that range from 5 to 20 km. Based on the focal mechanism from the USGS and the distribution of the aftershocks, the sesimogenic zone is a dextral fault system with the P-axis plunging gently toward 308°, parallel to the relative motion vector between the Eurasia and the Philippine Sea plates (Fig. 6D).  www.nature.com/scientificreports www.nature.com/scientificreports/

Discussion
The western Taiwan Strait has been typically shown on maps as tectonically quiescent, with often little more interpretation than the presence of rift-related Paleogene basins 3,6,24,29 or, at the most, with strike-slip faults on land along the coast of Fujian Province 2,4,30 . Nevertheless, several seismicity studies present data that indicate that this area is seismically active 4,6,8 although, to date, no reflection seismic data has provided unequivocal images of active faults. The historical large earthquakes (Fig. 1) likely have large uncertainties since they occurred before the use of modern seismometers, but their locations coincide with those of the background seismicity (Fig. 6A) and the many active faults shown in the seismic profiles (Fig. 3). It is reasonable to infer that the seismogenic zones in this region are highly active and are related to those large earthquakes that occurred during the period 1600-1918. Our new reflection seismic profiles provide important new data that allows us to place constraints on the location and structural style of the Benhai fault system in the western Taiwan Strait. It shows that, unlike the discrete fault interpreted by previous authors [6][7][8] (Fig. 1), the Benhai fault system is a more than 75 km wide diffuse zone of faulting. Seismicity suggests that certain parts of the fault system are active to depths of greater than 20 km, but that the majority of activity occurs in the upper 10 km of the crust (Fig. 7).
Our data clearly shows a number of faults that cut the break-up unconformity and, in many cases, reach the sea floor. In several cases, these active faults cut up section from steep basin sidewalls to form flower structures or terraced sidewall fault zones (Fig. 5), forming structures that are similar to those described from strike-slip pull-apart basins 31,32 . Furthermore, the fault and basin map pattern (Fig. 4) is also consistent with that expected for the early stages of development of a dextral strike-slip system 32 . Based on the comparison of our reflection seismic data interpretation with the analog models of ref. 32 and with reflection seismic profiles in areas of strike-slip faulting [33][34][35] , we interpret the structure of the Binhai fault in the western Taiwan Strait to be that of the dextral strike-slip system. Furthermore, in a number of cases (e.g., profiles 3 and 7; Fig. 3) these active strike-slip faults at depth form basin-bounding faults to syn-rift basin, providing strong evidence that, at least locally, they are reactivating pre-existing faults. Finally, growth strata and truncation reflection geometries high up within the post-rift sedimentary packages (Fig. 5C,D) suggests that the Binhai fault system has been active since the Miocene.

Methodology
Reflection seismic data. In May 2017, eight multichannel seismic reflection (MCS) profiles were acquired in the western Taiwan Strait with the express purpose of locating and imaging the active Benhai fault system (Fig. 1). A GGUN array, comprised of 2 air guns with 520 CI and 2 with 150 CI was used, providing a total compressive pressure of 2000 PSI. A 1350 meter-long Sentinel streamer containing 108 hydrophones spaced at 12.5 m was towed at 8 meters depth below sea level. The shooting interval was every 37.5m. The CDP (Common Depth Point) interval and the maximum fold are 6.25m and 18, respectively. All profiles were recorded for 3.5 seconds, except profile 5 which was recorded for 3.2 seconds. The Seal 408XL recording system with a standard SEGD