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Using Advanced Space-borne Radar Technology for Detection and Measurement of Land Subsidence and Interseismic Slip Rates, the Case Study: NW Iran Sadra Karimzadeh1, Farshid Farnood Ahmadi *2 1
Faculty of Environmental Design, Kanazawa University, Japan
2
Department of Geomatics Engineering, University of Tabriz, Iran
1
sadrakarimzadeh@yahoo.com; *2farshid_farnood@yahoo.com
Abstract We used synthetic aperture radar interferometry (InSAR) to measure land subsidence in Tabriz Plain (TP) and strain accumulation along North Tabriz Fault (NTF). Thermal power plant of Tabriz city locats in the area called Tabriz Plain which supplies electric energy for NW Iran. Its facilities need to be constantly cool, so there are more than twenty water pumping stations in some parts of TP. Moreover, the power plant, petrochemical, refinery and Vanyar dam are located near at a hazardous tectonic structure called North Tabriz Fault. InSAR is one of satellite radar observation methods which is used in space geodesy. In this paper, we have applied twenty ASAR SLC images of Envisat satellite from descending orbits during May 2003 to July 2010. InSAR analysis shows about 20mm/yr land subsidence and about 7mm/yr slip rate for NTF. Keywords ASAR; InSAR; Fault; Slip Rate; Subsidence; Tabriz
Introduction InSAR is a radar remote sensing technique used in space geodesy (Motagh et al., 2007 and Biggs et al., 2009). This geodetic method uses at least two or more synthetic aperture radar (SAR) images to generate Digital Elevation Model (DEM) or topographic maps using phase information within interferograms (Zebker et al., 1994b). Differential SAR Interferometry (DInSAR) technique enables the measurement of surface deformation with an accuracy of a few millimeters to centimeters for wide spatial coverage. Interferograms are generated by differentiating the phase values of two co-registered SAR images acquired at different times over the same area.
Differential interferograms have significant amount of useful data, such as surface deformations due to seismic events (Massonnet et al., 1993; Jacobs et al., 2002). It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes, landslides, land subsidence and also faults (Liu et al., 2004; Pathier et al., 2003; Rosen et al., 1996; Motagh et al., 2006; Lanari et al., 2007; Cakir et al., 2007). In the recent years, other methods such as use of historical Qanat (traditional water tunnel) canals displacement for calculation of annual rate of NTF, have been used (Hessami et al., 2003). But wide spatial coverage of InSAR data and its semi continuous observations hold more advantages than the same land techniques such as conventional leveling, displacement of historical Qanats or even GPS (Motagh et al., 2006 and Anderssohn et al., 2008). North west and south east directions of NTF are good orientations for using InSAR. It can help us to detect Earth deformations caused by fault movements and land subsidence. This paper is the first attempt to detect land subsidence of Tabriz Plane. In the field of tracking fault movements and land subsidence, various researches have been done using InSAR. Some of them include investigations on North Anatolian Fault, Turkey (Wright et al., 2001, Cakir et al., 2005, Motagh et al., 2007; Walters et al., 2011), San Andreas Fault, U.S (Lyons and Sandwell 2003), Altyn Tagh Fault, Tibet (Elliott et al., 2008), the subsidence of Mexico City (OsmanoÄ&#x;lu et al., 2011), the subsidence of Mashhad Plain, Iran, (Motagh et al., 2006; Dehghani et al., 2009) and etc.
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Background In this section, we will present geological and technical characteristics of Tabriz region and Differential Synthetic Aperture Radar Interferometry (DInSAR) respectively. Geological Background of Tabriz Region Tabriz is the fourth largest city in Iran which is located in northwest of the country in East-Azerbaijan province and situated in the valley between Eynali and Sahand mountains. The valley opens out into a Tabriz Plain (TP) that slopes slowly down to the northern end of Urmieh Lake and the North Tabriz Fault (NFT) is a major feature of Iran tectonics in the surrounding of Tabriz. NFT is one of the most active faults in NW Iran that has a clear surface expression. Geodetic data is not enough along most of its length especially. So wide spatial coverage of InSAR technique and north west south east direction of NTF are good orientations for using InSAR.
1) Subsidence Phenomena at Tabriz Plain Land subsidence is the motion of a surface part as it moves downward relative to a datum. It may occur for different reasons such as: earthquake; over capacity extraction of oil, natural gas and groundwater; some mining and urban activities such as: coal extraction, metro and tunnel constructions and etc. It has been recognized as a serious environmental problem in many areas (Motagh et al., 2006; Dehghani et al., 2009 and OsmanoÄ&#x;lu et al., 2011). Some vital structures are located in the vicinity of TP such as: thermal power plant, petrochemical and refinery center. This paper is the first attempt to detect land subsidence of TP. Iran's climate is arid and semiarid, therefore rain rate of this country and natural recharge are lesser than global average (Sedighi et al., 2010). Thus groundwater resources are needed for expansion of cities, agriculture and industrial uses, (Dehghani et al., 2009). Also, the thermal power plant which supplies electrical energy for NWof Iran needs to be constantly cool. So, there are more than twenty deep or semi deep wells in some parts of TP. The amount of water taken from the wells is about 5 million cubic meters per year, while allowable amount is 3 million cubic meters for each year (Razzaghmanesh et al., 2006). FIG. 1 shows some of water pumping stations in the study area.
2
FIG. 1 OUTLINE OF TABRIZ PLAIN. SOME WATER PUMPING STATIONS ARE SHOWN BY TRIANGULARS
2) Strain Accumulation on North Tabriz Fault As shown in FIG. 2 the fault has an average strike of NW-SE over a length of about 150 km (Hessami et al., 2003) and which extends from Misho mountain in the west to Bostanabad city in the east (Vafaie et al., 2008). It is located within 5km of Vanyar dam (Mozaffari et al.,). “NW Iran is a region with intense deformation and seismicity situated between two thrust belts, the Caucasus to the north and the Zagros mountains to the south (Hessami et al., 2003). The NTF is one of the most active faults in NW of Iran which has a clear surface expression. FIG. 1 shows its trace. The NTF did not generate large earthquakes during the last 200 years. The recurrence interval of the NTF is calculated for 300 years approximately based on the historical movements such as surface displacements of old Qanat lines (Hessami et al., 2003). Right-lateral movement along this fault has been suggested from aerial photographs (Berberian and Arshadi, 1976; Berberian et al., 1999). This can be clearly confirmed observing the field (Karakhanian et al., 2004). Through wide cover InSAR observations, we can quantify NTF trends. The seismic risk for the built environment and the population is estimated to be high and alerting which has gradually gained significant scientific and public interest. One major factor affecting the level of high risk is that the area is expanding rapidly with vulnerable infrastructure development and constructions. As an example, the petrochemical plant or the Tabriz-Miandoab water distribution project are exposed directly to the potential NTF earthquakes. Furthermore, Vanyar dam can induce stress changes and intensify the movement (Moghaddam and Saghafi, 2006).
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FIG. 2 A) SATELLITE IMAGE AROUND NTF FROM MISHO MOUNTAIN IN THE WEST TO BOSTANABAD CITY IN THE EAST. B) THE IMAGE SHOWS WATERWAYS OFFSET CAUSED BY NTF CREEP ON ONE HILL BETWEEN SOFIAN AND TABRIZ CITY. C AND D, TWO IMAGES ARE TAKEN FROM ROSHDIYE TOWN IN NORTH OF TABRIZ (JULY 2010) THAT SHOW THE FAULT IS ACTIVE
Technical Background
is related to the deformation of the surface
Synthetic Aperture Radar Interferometry is a technique which measures the phase difference between two radar images taken from two slightly different positions. The formation of an interferogram can be expressed as: (1) where
denotes interferogram,
across and along-track image coordinates, are the complex master and slave images,
are the and denotes
is the pixel the complex conjugate operator, and wise multiplication operator. The interferometric phases reflect the range changes in the radar line of sight (LOS) direction between the two SAR passes (Zebker et al., 1994a,b). Different types of errors are presented in the interferograms that need to be estimated and, if possible, corrected. Often ionospheric error is presented, which also manifests itself as a linear trend such as ramp of colors from red to blue across interferograms. However, ionospheric phase contributions become more evident with L- and P-band (3 GHz-300 MHz) SAR systems. The ionospheric effects for C-band (5.4 GHz) Envisat data are negligible (Lyons and Sandwell 2003; Takada et al., 2009). Equation 2 presents the several important components of an interferogram for each pixel. (2)
between the two acquisitions,
is related to the
difference of atmospheric conditions,
is
related to errors in the topographic model, represents the errors in orbital information, represents phase changes caused by relates to the temporal decorrelation, and phase changes due to thermal noise (Zebker et al., 1992, 1997; Gatelli et al., 1994). Nine interferograms exhibiting lower level of noises were selected among fifty interferograms produced from a pool of twenty SAR complex images acquired from May 2003 to July 2010. Analyzing the interferograms which are near the faults indicates traceable phase difference patterns. Analysis & Results Interferograms are generated by differentiating the phase value of two coregistered radar images acquired from different times over the same area. Some of these interferograms were select to contribute to making a rate map of displacements by stacking approach (Wright et al., 2001, Motagh et al., 2006; Walters et al., 2011). DInSAR Results DInSAR technique allows monitoring inter-seismic motion in order to better assess the seismic risk of
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regions. We chose the so-called two-pass technique to generate nine interferograms obtained from twenty Cband (5.6cm) Envisat ASAR descending images track 49 acquired from May 2003 to July 2010. FIG. 3 displays the location of the SAR scenes.
in DEM can propagate into the deformation phase measurement. The topographic phase error is:
In order to avoid snow cover and temporal variability, we selected data pairs within the same season. Images taken at 2 May 2003 and 9 July 2010 were removed due to large baseline, undesired Doppler centroid and excessive noise. Interferograms with temporal baselines of up to 5 years were generated to detect and measure the amount of seismic slip. The fault creep in interferograms appears as a shift in phase at the surface trace of the fault. In other words, fault-creep gives rise as a discontinuity in phase across the fault if it reaches to the surface. Therefore, the amount of phase shift defines the creep rate (Cakir et al., 2005). The images were processed using the DORIS software on ADORE console (Kampes et al., 2007; OsmanoÄ&#x;lu 2010). Precise orbits were used to estimate the exact position of the satellite during the image acquisition (Otten et al., 2010). Shuttle Radar Topography Mission (SRTM http://dds.cr.usgs.gov/srtm) and Digital Elevation Model (with resulotion of 90m) data were
where
used to remove the topographic phase. An error of
(3) is the slant range from the sensor to the
is ground, is the local radar incidence angle and the length of the perpendicular baseline. It is clear that approaches to zero when approaches to zero. Therefore short perpendicular baselines were used to obtain higher accuracy. In top and bottom areas of most of the interferograms shown in FIG. 4, phase variation is observed. For the top part of the images, the phase variation is caused by dense and tall vegetation. Envisat uses C-band (5.6 cm wavelength) that exhibits lower penetration into vegetation in comparison with other sensors like Lband (15-30 cm wavelength). In the bottom part of the above mentioned images, no vegetation is observed. The resulted dominate phase signature on the bottom of the images may be related to the land subsidence of TP and movements of NTF. White arrows indicate the location of subsidence area which is observed on TP. FIG. 5 shows ground subsidence around some of the water pumping stations.
FIG. 3 A) OVERVIEW OF ACQUIRED SAR DATA FRAME. B) THE FOOTPRINT OF THE ACQUIRED ASAR DATA OVER TABRIZ VIA BLACK BOX. C) TECTONIC MAP OF IRAN. IT SHOWS NW OF IRAN AFFECTED BY COLLISION OF ARABIA AND EURASIA PLATES. BLACK RECTANGULAR SHOWS INSET B. YELLOW STAR SHOWS LOCATION OF VANYAR DAM. CS = CASPIAN SEA, TP = TABRIZ PLAIN, NTF = NORTH TABRIZ FAULT
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FIG. 4 SOME DIFFERENTIAL INTERFEROGRAMS (WRAPPED) WITH TEMPORAL AND SPATIAL BASELINE FOR TABRIZ REGION. THE INTERFEROGRAMS ARE CORRECTED ACCORDIN TO THE TOPOGRAPHY. WHITE ARROWS ILLUSTRATE SUBSIDENCE AREA. EACH COLOR CYCLE REPRESENTS ABOUT 28MM FOR THE ENVISAT RADAR SENSOR
FIG. 5 GROUND SUBSIDENCE AROUND SOME OF THE WATER PUMPING STATIONS IS EVIDENT (SEPTEMBER OF 2011)
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TABLE 1 DIFFERENTAL INTERFERO GRAMS WITH TEMPORAL AND SPATIAL PERPENDICULAR BASELINES.
Stacking Results A stack rate of interferograms can be calculated for each pixel through the following equation: (4) where
is the average deformation velocity
measured in rad/yr and is the phase of th interferogram (radians) calculated over a time period (years). We tried to reduce another error sources in DInSAR processes. Therefore, we can assume
~
(Samsonov, 2010). We applied 6 differential unwrapped interferograms from descending path frame 2830 to generate stacked map over Tabriz area. TABLE 1 shows detail of used interferograms. Stacking is an effective method to reduce residual noises and errors of interferograms providing average rate of displacements (Wright et al., 2001; Walters et al., 2011). For revealing land subsidence and fault rates, we create three profiles along TP and NTF. FIG. 6 shows stack map and A, B and C transects. We considered a weighted value for pixels and fitted curves for better assessment.
No
Master
Slave
T.Baseline (D)
P.Baseline (m)
1
2003.11.28
2004.04.16
140
184
2
2003.11.28
2004.05.21
175
222
3
2004.05.21
2006.12.22
945
51
4
2004.06.25
2008.04.25
1400
93
5
2006.02.10
2007.01.26
350
23
6
2007.05.11
2008.04.25
330
-108
FIG. 6 shows general land subsidence in the research area whose rate is about 20mm/yr. Also, fitted curves indicate about 7 mm/yr slip rate between south and north part of NTF. This rate is consistent with the results of the similar research in which GPS measurement was used for slip rate partitioning (Rastbood and Voosoghi 2011). FIG. 7 shows InSAR image frame and GPS network of NW Iran. Further analysis is required to obtain independent slip rates from InSAR dataset. As an alternative, we will utilize time series analysis methods such as persistent scatterer InSAR (PSI), Small Baselines Interferometry (SBAS) or Stanford Method for Persistent Scatterers (StaMPS) (Motagh et al., 2007; Lanari et al., 2007; Hooper et al., 2010).
FIG. 6 LEFT IS THE STACK MAP OF INTERFEROGRAMS WHICH MENTIONED IN TABLE 1. RIGHT IS WEIGHTED PIXELS OF A, B AND C TRANSECTS AND SHOWS YEARLY DISPLACEMENTS IN TABRIZ REGION RESPECT TO NTF. A AND B TRANSECTS SHOW LAND SUBSIDENCE PHENOMENA IN SOUTH PART OF NTF. CURVES ARE ILLUSTRATED WITH CONFIDENT OF 95%. YELLOW STAR SHOWS THE LOCATION OF VANYAR DAM. RED AND GREEN POINTS ARE OUTLIER DATA AND THEY HAVE NOT BEEN USED...
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pattern controlled by old alpine basement faults in the Kashmar Valley, northeast Iran: results from InSAR and levelling. Geophysical Journal International. Berberian M., Arshadi S., 1976, On the evidence of the youngest activity of the North Tabriz Fault and the seismicity of Tabriz city, Geol. Surv. Iran., 39, 397-418. Berberian M., Yeats R.S., Patterns of historical earthquake rupture in the Iranian Plateau, 1999, Bulletin of the Seismological Society of America, 89, 120–139. Biggs J., Burgmann R., Freymueller J.T., Lu Z., Parsons B., Ryder I., Schmalzle G., Wright T., 2009, The postseismic response to the 2002M 7.9 Denali Fault earthquake: constraints from InSAR 2003–2005. Geophysical Journal International. 353–367. FIG. 7 INSAR IMAGE FRAME AND GPS NETWORK OF NW IRAN (MASSON ET AL.,2006). UL = URMIEH LAKE
Cakir Z., Akoglu A.M., Belabbes S., Ergintav S., Meghraoui M., 2005, Creeping along the Ismetpasa section of the North Anatolian fault (Western Turkey): Rate and extent
Conclusions This paper is a feasibility study in the field of detecting hydrogeological and seismological phenomenon such as land subsidence and interseismic slip rates detection using InSAR technique in Tabriz Plain and North Tabriz Fault. The results of the research show that stacking is an effective method to reduce residual noises and errors of interferograms providing average rate of displacements. This method illustrates that the rate of general land subsidence in the research area is 20mm/yr. Meanwhile, fitted curves indicate about 7 mm/yr slip rate between south and north part of NTF. Increasing density of GPS stations near NTF, precise leveling process around TP, taking more SAR images which cover wide domain of the study area and combination of ascending and descending data for extracting 3D velocity rate of NW Iran by InSAR, are recommended for future studies. In the last months of 2012, several earthquakes occurred around Tabriz fault. The earthquakes with high intensity and frequency show that this area is very active from geodynamical point of view. The rate of land slip and land subsidence measured in this research using space-borne radar technology, confirm this activity. So, the methodology of this investigation can help researchers for earthquake prediction.
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