- Express Letter
- Open Access
Aseismic fold growth in southwestern Taiwan detected by InSAR and GNSS
© The Author(s) 2018
- Received: 24 October 2017
- Accepted: 14 March 2018
- Published: 26 March 2018
- Aseismic deformation
Interferometric pairs used in this study
Figure 3a shows very rapid uplift in the fold and thrust belt to the east of Tainan city (Fig. 1). The uplifted area takes N–S elongated shape, which stretches ~ 25 km in the N–S direction and ~ 5 km in the E–W direction. Judging from very low seismicity in this area (Fig. 1), we may conclude that this uplift is aseismically proceeding. As an overall trend, the uplift rate increases from south to north, and it reaches ~ 37 mm/year at maximum in the northern part (Fig. 3b). To elaborate the uplift pattern, we took E–W profiles of the quasi-vertical velocity field (Fig. 3b, c). Across the southern part of the uplifted area, Ching et al. (2016) reported a one-dimensional profile of uplift rate by levelling survey (Fig. 3a), showing ~ 18 mm/year at maximum. We compared the uplift rate detected by InSAR with that measured by the levelling survey, and demonstrated that both are consistent with each other (Fig. 3d). We further found that the uplift rate in the E–W profiles indicates a sharp discontinuity at the eastern flank of the uplifted region (shading in Fig. 3c), which is also visible in the plan view (red arrows in Fig. 3a). We interpret that this uplift rate discontinuity results from a hidden active fault steadily creeping under a certain level of shear stress. The location of this fault roughly corresponds to the Lungchuan fault, which is proposed to be a thrust fault (Huang et al. 2004; Ching et al. 2016). The detailed geometry and the precise location of the Lungchuan fault are poorly known, which may be improved from combination of our interferograms and field surveys in the future.
The driving force of this rapid uplift would be the growth of mud diapirs, which is widely distributed from South China Sea to the coastal area of southwest Taiwan (Doo et al. 2015; Lin et al. 2009, Huang et al. 2004) (Fig. 1b). In the uplifted area, active mud volcanoes demonstrate the existence of mud volumes at shallow depth. Ching et al. (2016) tried to construct a fault model to explain the uplift profile obtained by the levelling survey in the southern area (circles in Fig. 3a) and found it as very difficult. Instead, they proposed the mud diapirs as main driving force. In northern part, the uplift rate detected by InSAR takes much higher value than southern part, reaching 37 mm/year (Fig. 3a). This uplift rate is higher than total convergence rate across the study area (Additional file 1: Figure S5), which is unrealistic to be solely explained by fault motion. Based on the gap in the velocity field (Fig. 3), we also constructed a two-dimensional fault model using Okada’s Green’s function (Okada 1985). For the observed horizontal velocity, we removed very long-wavelength component which directly reflects interplate convergence (note that the reference point is S01R in Fig. 1a). We fixed the top of the fault at the surface. As shown in Additional file 1: Figure S6, the optimum fault model with a slip of 55 mm/year on a fault plane dipping 65 degrees down to the west well explains the quasi-east component (Additional file 1: Figure S6a), while we see remarkable misfit (> 15 mm/year) especially at the footwall side (right) in the quasi-vertical component (Additional file 1: Figure S6b); calculated subsidence is too large at the footwall side. Looking at the plan view (Fig. 3a), the foot wall side (east) of this reverse fault is clearly uplifted, which demonstrates that the whole area is raised by a mud diapir located at depth, and the surface offset is caused by the shallow reverse faulting that we found.
It should be noted that the aseismic fault slip we found (Figs. 3, 4) only adds a short-wavelength perturbation to the whole uplift and/or uplift rate pattern. For the interseismic period, the main driving mechanism of whole surface uplift would be mud diapirs at depth. For the coseismic displacement of the Meinong earthquake, a deep-seated decollement caused long-wavelength surface deformation (Huang et al. 2016b), which, however, is not responsible for the interseismic uplift characterized by short-wavelength deformation (Fig. 3).
We detected very rapid uplift along the fold and thrust belt in southwest Taiwan by L-band SAR data. The interferograms are contaminated mainly by ionospheric disturbances, which are corrected by GNSS data. The rapid uplift is aseismically proceeding judging from the absent of earthquakes. The uplift rate increases from south to north and reaches ~ 37 mm/year in the northern part. In southern part, the uplift rate derived from InSAR is consistent with the one measured by the levelling survey which reaches ~ 18 mm/year at maximum. The main cause of uplift motion would be mud diapirs, but we also identified shallow fault motion along the eastern flank of the uplift area, which moves aseismically before and during the 2016 Meinong earthquake. The shallow fault motion adds a short-wavelength perturbation to the whole uplift pattern.
KT conducted data analysis. KT and YT wrote the manuscript. Both authors read and approved the final manuscript.
We thank Ms. Yoko Tu for discussions. Comments from Yosuke Aoki and an anonymous reviewer greatly improved the manuscript. PALSAR and PALSAR2 data were provided from PIXEL (PALSAR Interferometry Consortium to Study our Evolving Land surface) under cooperative research contract between JAXA and ERI, University of Tokyo. PALSAR data were also provided from JAXA under ALOS-2 PI project. The ownership of PALSAR data belongs to JAXA and METI, Japan. Part of the figures was created by Generic Mapping Tools (Wessel and Smith 1998).
The authors declare that they have no competing interests.
Availability of data and materials
Uplift rates estimated by InSAR are available by requesting the corresponding author by e-mail.
Ethics approval and consent to participate
This study was supported by MEXT KAKENHI Grant No. 17K05622, and partly by No. 26400454 to YT.
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