Insar Time Series Analysis of Interseismic Deformation of Doruneh Fault

Abstract

Eastern Iran has crucial rule in accommodating N-S right-lateral shear between central Iran and Afghanistan. Part of this shear is absorbing at the North of 34º latitude on left-lateral Doruneh. Previous studies on these two major faults show discrepancies between the GPS interseismic slip rate (less than 1 mm/yr) and geological slip rates (2.4±0.3 mm/yr). This discrepancy motivated us to use ENVISAT ASAR images investigate how the deformation is distributed in the area. The differential interferogram phase is related to phase change due to the deformation signal, tropospheric delay, orbital and DEM residual and noise. Large scale seasonal atmospheric correction will be investigated using ERA-Interim meteorological model and GPS data. A twisted plane is also fitted to remove orbital errors. To investigate the long wavelength tectonic signal due to interseismic strain accumulation, a time series analysis of the selected Images has been done on a pixel basisin order to enhance signal to noise ratio affected by remaining atmospheric signal.

Key words:

Eastern Iran, interseismic deformation, InSAR Time series analysis, ENVISAT images

Introduction

Iran has been extensively studied over the last decades because it has opportunity to fully observe and quantify how the convergence between two plates –Arabia and Eurasia– has been accommodated from its onset to its present-day continuation. The convergence rate of these two plates is estimated today to 18 – 26 mm/yr from west to east. Plus, Iran is the site of regular large earthquakes. The overall distribution of the present-day deformation inside political border of Iran is roughly known due to the GPS networks over the last decades.
However, some key-pieces of knowledge are still missing. We are still lacking a kinematic model of deformation for Iran, specially its eastern half. , Eastern Iran has crucial role in accommodating N-S right-lateral shear between central Iran and Afghanistan. The tectonic deformation in eastern Iran is localized mainly on NS oriented right-lateral faults surrounding the aseismic Lut block, and EW left-lateral faults at the northern boundary of the Lut block. Different scenarios have been proposed (Walker & Jackson, 2004; Tavakoli, 2007) but data are still lacking to fully validate them.
Previous studies on these major faults in Eastern Iran show discrepancies especially between the GPS interseismic slip rate (less than 1 mm/yr on the Doruneh fault (Tavakoli et al. 2007) and geological slip rates (2.4±0.3 mm/yr Fattahi et. al, 2007). Also according to rigid block model, it is expected to accommodate near 10 mm/yr slip rate somewhere on the Doruneh and Dasht-e-Bayaz strike-slip faults. This discrepancy between the short time (GPS) and long term (geomorphological) and tectonic model motivated us to use another geodetic techniques to investigate how the deformation is distributed in the area.
The spatial coverage and precision of space-borne radar interferometry (InSAR) in measuring ground deformation makes it an appropriate geodetic tool to address the following question: Are the sparse GPS measurements representative of the regional present-day interseismic deformation or does more deformation occur either on portion of the Dorouneh and Dasht-e-Bayaz faults not well sampled by GPS or on other geological structures in the region? This is an important issue to better understand the regional tectonics and the seismic hazard in Eastern Iran.

Several InSAR studies have been already successful in measuring long-wavelength ground displacements related to interseismic fault deformation on similar continental strike-slip fault (like the North Anatolian fault or the Haiyuan fault). However, in our case, the expected slip rates of the Doruneh and Dasht-e-Bayaz faults are lower (1 to 3 mm/yr), making InSAR measurements more challenging, even if the East-West orientation of these strike-slip faults and their arid environment are favorable to it.

InSAR data

Synthetic Aperture Radar (SAR) images from the ASAR instrument (wavelength of 5.6 cm) on board the ENVISAT satellite have been used in this study to investigate interseismic deformation across the Doruneh and Dasht-e-Bayaz faults. In the “image mode” of the ASAR instrument (swath of 100km), the studied area is covered by several tracks of the satellite, amongst which one tracks have been particularly studied in this report (track 392, see location in Figure 2). On this track about 24 images, corresponding to different dates of acquisition, have been ordered at the European Space Agency (ESA). This has been done through an ESA Cat-1 project (n°7023, Principal Investigator: Zahra Mousavi). These images were acquired in descending orbit (i.e. satellite flying from North to South) from 2003 to 2010. Some of them were archived images but we also requested new acquisitions in 2010 for our project.

The raw radar images (Processing Level 0) were processed with ROI_PAC (Repeat Orbit Interferometry PACkage) which is JPL/Caltech software for processing INSAR data (Rosen et al. 2004). The topographic phase contribution was estimated from the SRTM Digital Elevation Model (DEM) at 90m spatial resolution. Precise DORIS orbital data for ENVISAT satellite provided by ESA are used for interferometric processing. The resulting differential interferogram phase is related to phase change to the deformation signal, tropospheric delay, orbital and DEM residual and noise (equation1).

We correct for the stratified part of tropospheric delay correlated with elevation using the observed phase-elevation correlation. Large scale seasonal atmospheric correction will also be investigated using ERA-Interim meteorological model and GPS data. A twisted plane is also fitted to remove orbital errors (equation2).

Due to low slip rate of Doruneh fault it seems the atmospheric perturbation is the most important source of error in this area. The stratified part of tropospheric delay is corrected using the observed phase-elevation correction. To increase the signal to noise ratio, another strategy could help to mitigate the atmospheric perturbation using other measurements based on external data (pressure, temperature and humidity) and extrapolated at higher elevations, such as Modis, Moris, and GPS network, meteorological models (like ERA).
GPS total tropospheric delay can be expressed as wet (ZWD) and dry (ZHD) components. Dry delay contributes to InSAR atmospheric phase variability and manifests as slowly-varying phase trends similar to signatures caused by orbit inaccuracies. Water vapor, in contrast, varies significantly both spatially and temporally. Iranian GPS permanent GPS data are processed with GAMIT/GLOBK software to estimate total zenith delay. Dry zenith delay can be calculated from the local surface pressure. Wet component yields form subtracted Dry zenith delay from Total Zenith delay.
ERA-Interim covers the 1989–2001 time period with 4 time steps per day (0am, 6am, 12pm, 18pm) and a spatial resolution of 0.75°. It includes a stratification layers based on 37 pressure levels with 25hPa interval.
In this study the global meteorological models like ERA-Interim is used to produce the tropospheric delay map for each date and interferogram (e.g. Doin et al, 2009, Jovilet et al. 2011). The availability of GPS measurements of the tropospheric delay in the area makes to valid this approach. Figures 5 and 6 indicate the tropospheric delay map for ERA-Interim (Jovilet et al. 2011) and discontinuous GPS point for two dates and one interferogram, respectively. This ERA-Interim map delay is consistent with GPS tropospheric delay in the global view. Figure 8 present the ratio between this GPS and ERA-Interim tropospheric delay.

A careful visual inspection of the corrected interferograms is performed to investigate short wavelength signals (1-10 km scale) that could be related to superficial creep along faults. Local deformations related to subsidence phenomena were found in some valleys but there is no evidence over the 2003 to 2010 period for any sharp creep deformation signal located along fault.

Time series Analysis

To investigate the long wavelength tectonic signal due to interseismic strain accumulation, a time series analysis is done based on the phase accommodation through time of the selected interferograms (Lopez-Quiroz et al. 2009). The constraint related to perpendicular baseline is introduced to take into account possible DEM errors (equation 3, 4).

As the expected tectonic signal is a North-South ground displacement gradient across the fault, to enhance the deformation signal with respect to the local variations, an North-South averaged profile is created along track (perpendicular to the Doruneh fault, see Figure 4). A weighted averaging (the weight of each point is given by the cumulated time from stack counting) along the profiles is done for bins of 1800m length computed every 900m (Figure 4)

Conclusion

In this study, we use ENVISAT ASAR images from 2003 to 2010, in descending orbits. The 400 by 400 km studied area that includes the eastern part of the Dorouneh fault is covered by 7 satellite tracks (Dec 120,163,392,435 and 206, Asc A385 and A156). The raw radar images are processed with ROI_PAC to construct the interferograms and unwrapped them. The topography phase contribution is estimated from the SRTM Digital Elevation Model (DEM) at 90 m spatial resolution. We first performed a careful visual inspection of interferograms, looking for short wavelength signals (1-10 km scale) that could be related to superficial creep along faults. We found deformation related to subsidence phenomena in some valleys but no evidence over the 2003-2010 periods for a sharp creep deformation signal located along fault.
Then, we started to analyze the long wavelength (30-300 km scale) ground deformation looking for interseismic strain accumulation related to elastic deformation caused by fault creep at depth (above which the fault is locked). We correct for the stratified part of tropospheric delay correlated with elevation using the observed phaseelevation correlation and for a twisted plane to remove orbital errors. Large scale seasonal atmospheric corrections are also investigated using the ERA-Interim meteorological model and GPS data. To investigate the long wavelength tectonic signal due to interseismic strain accumulation, a time series analysis of the selected images based on the small base line method (SBAS) has been done on a pixel basisin order to enhance the signal to noise ratio affected by a remaining atmospheric signal. The selection and the weighting of the interferograms are based on a noise energy function that measures the quality of each interferogram. The resulting displacement time series and a mean velocity map can be compared to GPS data.
The time series analysis reveal that the slip rate on the Dorouneh fault is less than 2mm/yr, this result does not explain the inconsistent between geodetic and long-term slip rate difference in this part of Iran, thus it seems that the strain is accommodating in other way in this area. Peltzer et al 2001 suggested that the transient strain accumulation is happening along the Blackwater–Little Lake fault system within the Eastern California shear zone. They used the Satellite synthetic aperture radar interferometry and produce mean velocity map by averaging eight years (1992-2000) of Earth Resource Satellite (ERS) radar data. The area is sustained by right lateral shear, but the InSAR result does not show any evidence of left-lateral slip. Then they found that a rate 2-3 times greater than the geologic rates estimated on northwest-trending faults in the eastern Mojave area. This transient slip rate and the absence of resolvable slip on the Garlock fault may be the manifestation of an oscillatory strain pattern between interacting, conjugate fault systems (Peltzer et al 2001). Now we will do more investigate to find that does the same scenario happen in Doruneh fault and surrounding area? Does the deformation transfer to other fault in surrounding area?

Reference

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Acknowledgment

We would like to thank the European Space Agency (ESA) for providing ENVISAT images. This work has been supported by National Cartography Center (NCC), geodynamic department.