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| - | **An InSAR study of the surface deformation of Duvall Mw 5.1 earthquake | + | **An InSAR study of the surface deformation of Duvall Mw 5.1 earthquake ** |
| - | ** | + | |
| - | **Abstract | + | **Abstract ** |
| - | ** | + | |
| The Duvall Mw 5.1 earthquake, the second biggest earthquake in Washington since 1966, occurred about 10 km east of Duvall on May 3, 1996, and was followed by intense aftershocks. We use four SAR scenes from ERS 2 satellite and produce four Interferometric Synthetic Aperture Radar (InSAR) images spanning the earthquake to map the surface deformation. No detectable signal clearly shows up on any one of or on the stacking of the four InSAR images. The fail to identify signal is possibly due to the data limitation, most likely the baseline decorrelation and temporal decorrelation. On the other hand, we can not rule out the possibility that the event is too deep (10 km or deeper) to generate surface deformation that is large enough to be seen by InSAR. In addition, the specific geometry of the satellite looking direction and fault slip in this study might result in subtle range change in InSAR, even though there was considerable true deformation. Investigating other available SAR data, especially with short perpendicular baseline, and other available geodetic measurements would potentially provide better constraint on the surface deformation and thus better understanding of the earthquake generation and hazard in this area. | The Duvall Mw 5.1 earthquake, the second biggest earthquake in Washington since 1966, occurred about 10 km east of Duvall on May 3, 1996, and was followed by intense aftershocks. We use four SAR scenes from ERS 2 satellite and produce four Interferometric Synthetic Aperture Radar (InSAR) images spanning the earthquake to map the surface deformation. No detectable signal clearly shows up on any one of or on the stacking of the four InSAR images. The fail to identify signal is possibly due to the data limitation, most likely the baseline decorrelation and temporal decorrelation. On the other hand, we can not rule out the possibility that the event is too deep (10 km or deeper) to generate surface deformation that is large enough to be seen by InSAR. In addition, the specific geometry of the satellite looking direction and fault slip in this study might result in subtle range change in InSAR, even though there was considerable true deformation. Investigating other available SAR data, especially with short perpendicular baseline, and other available geodetic measurements would potentially provide better constraint on the surface deformation and thus better understanding of the earthquake generation and hazard in this area. | ||
| - | **1. Introduction | + | **1. Introduction** |
| - | ** | + | |
| Interferograms from two synthetic aperture radar (SAR) scenes can be used to map the surface deformation associated with various tectonic processes (e.g., Massonnet and Feigl, 1998; Burgmann et al., 2000) as well as the topography (e.g. Burgmann et al., 2000). One important application of InSAR is to study earthquakes, including both the coseismic deformation (e.g., Massonnet et al., 1993) and the postseismic deformation (e.g., Massonnet et al., 1994; Fialko, 2004; Jacobs et al, 2002). InSAR measurements complement GPS data by providing a spatially continuous mapping of the surface deformation, and are especially useful at those regions where GPS observations are sparse (Massonnet et al., 1994; Lohman and Simons, 2005) and even the target fields are unreachable. Therefore, InSAR is useful to constrain the earthquake source parameters and to estimate the earthquake hazards. | Interferograms from two synthetic aperture radar (SAR) scenes can be used to map the surface deformation associated with various tectonic processes (e.g., Massonnet and Feigl, 1998; Burgmann et al., 2000) as well as the topography (e.g. Burgmann et al., 2000). One important application of InSAR is to study earthquakes, including both the coseismic deformation (e.g., Massonnet et al., 1993) and the postseismic deformation (e.g., Massonnet et al., 1994; Fialko, 2004; Jacobs et al, 2002). InSAR measurements complement GPS data by providing a spatially continuous mapping of the surface deformation, and are especially useful at those regions where GPS observations are sparse (Massonnet et al., 1994; Lohman and Simons, 2005) and even the target fields are unreachable. Therefore, InSAR is useful to constrain the earthquake source parameters and to estimate the earthquake hazards. | ||
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| {{:jiangang:fig1.png|}} | {{:jiangang:fig1.png|}} | ||
| - | Figure 1. (a) The earthquakes with magnitude equal to or greater than 4.0 in western Washington since 1996 (from PNSN catalog). The red, orange, yellow, blue and maybe some dark orchid dots represent continental crust earthquakes. The black and maybe some dark orchid dots represent subducting intraslab earthquakes. The Duvall Mw 5.1 earthquake is noted with a black arrow. (b) The principal Puget Sound faults are delineated with solid or dashed red lines (from http://en.wikipedia.org/wiki/Puget_Sound_faults). The orange dot around the intersection of Cherry Creek Fault Zone (CCFZ) and Rattlesnake Mountain Fault Zone (RMFZ) indicates the location of the Duvall Mw 5.1 main shock and also is noted with a black arrow. | + | **Figure 1.** (a) The earthquakes with magnitude equal to or greater than 4.0 in western Washington since 1996 (from PNSN catalog). The red, orange, yellow, blue and maybe some dark orchid dots represent continental crust earthquakes. The black and maybe some dark orchid dots represent subducting intraslab earthquakes. The Duvall Mw 5.1 earthquake is noted with a black arrow. (b) The principal Puget Sound faults are delineated with solid or dashed red lines (from http://en.wikipedia.org/wiki/Puget_Sound_faults). The orange dot around the intersection of Cherry Creek Fault Zone (CCFZ) and Rattlesnake Mountain Fault Zone (RMFZ) indicates the location of the Duvall Mw 5.1 main shock and also is noted with a black arrow. |
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| {{:jiangang:fig2.png|}} | {{:jiangang:fig2.png|}} | ||
| - | Figure 2. Duvall Mw 5.1 main shock and the aftershock sequence. (a) Spatial distribution of the seismicity in 1996. The meanings of different colors are same as those in figure 1(a). A-B is a profile with events only within the rectangular box analyzed in (b), (c) and (d). (b) A west-east depth profile shows the depth distribution of the events within the rectangular box in (a). Notice that the seismicity geometry clearly delineates an eastward dipping fault plane. (c) The timeline of the magnitude across the whole year 1996. Notice that the Duvall Mw 5.1 main shock was followed by a lot of aftershocks and the aftershocks continued at least to the end of the year 1996. (d) Cumulative quake number of the earthquake sequence within the profile box. | + | **Figure 2.** Duvall Mw 5.1 main shock and the aftershock sequence. (a) Spatial distribution of the seismicity in 1996. The meanings of different colors are same as those in figure 1(a). A-B is a profile with events only within the rectangular box analyzed in (b), ( c) and (d). (b) A west-east depth profile shows the depth distribution of the events within the rectangular box in (a). Notice that the seismicity geometry clearly delineates an eastward dipping fault plane. ('c') The timeline of the magnitude across the whole year 1996. Notice that the Duvall Mw 5.1 main shock was followed by a lot of aftershocks and the aftershocks continued at least to the end of the year 1996. (d) Cumulative quake number of the earthquake sequence within the profile box. |
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| In this work, we collect available SAR scenes from satellite ERS 2 to produce interferograms spanning the event, and map the coseismic surface deformation associated with the main shock and potentially its aftershocks. In the following sections, we first describe the data and method that we used in this study. Then, we show the InSAR images and discuss the results and make error analysis and discuss the possible causes that limit us to get a detectable signal from InSAR images. | In this work, we collect available SAR scenes from satellite ERS 2 to produce interferograms spanning the event, and map the coseismic surface deformation associated with the main shock and potentially its aftershocks. In the following sections, we first describe the data and method that we used in this study. Then, we show the InSAR images and discuss the results and make error analysis and discuss the possible causes that limit us to get a detectable signal from InSAR images. | ||
| - | **2. Data and Methods | + | **2. Data and Methods** |
| - | ** | + | |
| The Duvall earthquake occurred about 10 km east of Duvall, Washington on May 3, 1996. The quake area was imaged both before and after the main shock by satellites ERS 1 and ERS 2. We collect four SAR scenes spanning the earthquake, two of which are about one month before the quake and the other two scenes are about one week after the quake. Because the event is located at the edge of the frame, we merged two frames to achieve scenes continuously mapping the Duvall earthquake. The details of the four scenes are shown in table 1. We make a combination of the different SAR scenes before and after the earthquake, and get four date pairs scenes to process for potential interferograms. The date pair scenes must share the same track and frame. These date pairs are shown in table 2. Notice that the perpendicular baseline for each scene pair might be too big to achieve a highly correlated interferograms. We will analyze the baseline effect on the correlation of the phase later in the discussion section. | The Duvall earthquake occurred about 10 km east of Duvall, Washington on May 3, 1996. The quake area was imaged both before and after the main shock by satellites ERS 1 and ERS 2. We collect four SAR scenes spanning the earthquake, two of which are about one month before the quake and the other two scenes are about one week after the quake. Because the event is located at the edge of the frame, we merged two frames to achieve scenes continuously mapping the Duvall earthquake. The details of the four scenes are shown in table 1. We make a combination of the different SAR scenes before and after the earthquake, and get four date pairs scenes to process for potential interferograms. The date pair scenes must share the same track and frame. These date pairs are shown in table 2. Notice that the perpendicular baseline for each scene pair might be too big to achieve a highly correlated interferograms. We will analyze the baseline effect on the correlation of the phase later in the discussion section. | ||
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| + | {{:jiangang:Slide1.png|}} {{:jiangang:Slide2.png|}} | ||
| To obtain the interferogram for each date pair, we perform standard processing both for the master scene and the slave scene with the Repeat Orbit Interferometry Package (ROI_PAC). The interferometric phase contains a topography component that needs to be corrected to get a differential interferogram for deformation interpretation. We first construct digital elevation model (DEM) for the study area using data from Shuttle Radar Topography Mapping (SRTM) mission (http://dds.cr.usgs.gov/srtm). Then we use this DEM and the ROI_PAC to estimate and remove the topography contribution to the interferometric phase. | To obtain the interferogram for each date pair, we perform standard processing both for the master scene and the slave scene with the Repeat Orbit Interferometry Package (ROI_PAC). The interferometric phase contains a topography component that needs to be corrected to get a differential interferogram for deformation interpretation. We first construct digital elevation model (DEM) for the study area using data from Shuttle Radar Topography Mapping (SRTM) mission (http://dds.cr.usgs.gov/srtm). Then we use this DEM and the ROI_PAC to estimate and remove the topography contribution to the interferometric phase. | ||
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| For all of these four date pairs, we perform the same processing and finally obtain four interferograms with amplitude and wrapped phase. Because there are low correlations in all of the four interferograms, we are unable to wrap the phase. We will show the interferograms with wrapped phase in the next section. | For all of these four date pairs, we perform the same processing and finally obtain four interferograms with amplitude and wrapped phase. Because there are low correlations in all of the four interferograms, we are unable to wrap the phase. We will show the interferograms with wrapped phase in the next section. | ||
| - | **3. Results | + | **3. Results** |
| - | ** | + | |
| After the process of the SAR data, we finally obtain four interferograms for the Duvall earthquake (see table 2 for date pair details). The wrapped phases for these four interferograms are shown in figure 3. In spite of some coherent phase in some part of InSAR images, there are low correlations near the epicenter of the Duvall shock. No detectable signal associated with the earthquake can be identified on these four interferograms. The phases near the earthquake location is very noisy, indicating decorrelation from data limitation and/or the surface deformation associated with the main shock is not large enough to be seen by InSAR. | After the process of the SAR data, we finally obtain four interferograms for the Duvall earthquake (see table 2 for date pair details). The wrapped phases for these four interferograms are shown in figure 3. In spite of some coherent phase in some part of InSAR images, there are low correlations near the epicenter of the Duvall shock. No detectable signal associated with the earthquake can be identified on these four interferograms. The phases near the earthquake location is very noisy, indicating decorrelation from data limitation and/or the surface deformation associated with the main shock is not large enough to be seen by InSAR. | ||
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| {{:jiangang:fig3.png|}} | {{:jiangang:fig3.png|}} | ||
| - | Figure 3. Interferogram wrapped phase using (a) SAR scenes of April 4, 1996 and May 9, 1996; (b) SAR scenes of April 4, 1996 and May 10, 1996; (c) SAR scenes of April 5, 1996 and May 9, 1996; (d) SAR scenes of April 5, 1996 and May 10, 1996. All of the four interferograms show low phase corrlation at the epicenter region. | + | **Figure 3.** Interferogram wrapped phase using (a) SAR scenes of April 4, 1996 and May 9, 1996; (b) SAR scenes of April 4, 1996 and May 10, 1996; (c) SAR scenes of April 5, 1996 and May 9, 1996; (d) SAR scenes of April 5, 1996 and May 10, 1996. All of the four interferograms show low phase corrlation at the epicenter region. |
| {{:jiangang:fig4.png|}} | {{:jiangang:fig4.png|}} | ||
| - | Figure 4. Interferogram amplitude and phase. (a) The amplitude stacking of the four interferograms. The orange dot represents the Duvall earthquake location. (b) The phase stacking of the four interferograms in fiure 3. Notice that the correlation is still low, at least near the epicenter of the earthquake. (c) Interferogram phase using SAR scenes of 1996.04.04 and 1996.04.05. For this date pair the correlation is high, probably because of the short perpendicular baseline (only 66 m) and short time span (only one day). (d) Interferogram phase using SAR scenes of 1996.05.09 and 1996.05.10. The correlation for this scene pair is also high, with same reason of short perpendicular baseline (about 143 m) and short period (only one day). | + | **Figure 4.** Interferogram amplitude and phase. (a) The amplitude stacking of the four interferograms. The orange dot represents the Duvall earthquake location. (b) The phase stacking of the four interferograms in fiure 3. Notice that the correlation is still low, at least near the epicenter of the earthquake. (c) Interferogram phase using SAR scenes of 1996.04.04 and 1996.04.05. For this date pair the correlation is high, probably because of the short perpendicular baseline (only 66 m) and short time span (only one day). (d) Interferogram phase using SAR scenes of 1996.05.09 and 1996.05.10. The correlation for this scene pair is also high, with same reason of short perpendicular baseline (about 143 m) and short period (only one day). |
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| To check the corresponding causes for the low correlation in the interferograms, we also produce two interferograms using SAR scenes both before (April 4, 1996 and April 5, 1996) and after (May 9, 1996 and May 10, 1996) the Duvall earthquake (May 3, 1996). The wrapped phase of these two interferograms are shown in figure 4(c) and figure 4(d). Different from the four previous interferograms shown in figure 3, these two interferograms have a fairly good correlation. The perpendicular baseline for the Interferogram using scenes of April 4, 1996 and April 5, 1996 is 66 m, and the perpendicular baseline for the interferogram using scenes of May 9, 1996 and May 10, 1996 is 143 m. These short baselines are likely to contribute to the high correlation in the interferometric phase. | To check the corresponding causes for the low correlation in the interferograms, we also produce two interferograms using SAR scenes both before (April 4, 1996 and April 5, 1996) and after (May 9, 1996 and May 10, 1996) the Duvall earthquake (May 3, 1996). The wrapped phase of these two interferograms are shown in figure 4(c) and figure 4(d). Different from the four previous interferograms shown in figure 3, these two interferograms have a fairly good correlation. The perpendicular baseline for the Interferogram using scenes of April 4, 1996 and April 5, 1996 is 66 m, and the perpendicular baseline for the interferogram using scenes of May 9, 1996 and May 10, 1996 is 143 m. These short baselines are likely to contribute to the high correlation in the interferometric phase. | ||
| - | **4. Discussion | + | **4. Discussion** |
| - | ** | + | |
| Using four SAR scenes from the ERS 1 and ERS 2 satellites, we produce four interferograms to image the surface deformation associated with Duvall earthquake. All the resultant phase of the four interferograms, however, show little correlation at least in the eipicenter location of the event. There are several possible reasons that might account for the decorrelation in the interferometric phase. | Using four SAR scenes from the ERS 1 and ERS 2 satellites, we produce four interferograms to image the surface deformation associated with Duvall earthquake. All the resultant phase of the four interferograms, however, show little correlation at least in the eipicenter location of the event. There are several possible reasons that might account for the decorrelation in the interferometric phase. | ||
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| {{:jiangang:fig5.png|}} | {{:jiangang:fig5.png|}} | ||
| - | Figure 5. Forward modeling of the interferograms for Duvall earthquake at different depth. The strike, dip, rake, slip and length of the fault for the two modeling are set to be the same, but with different depth range. (a) The earthquake fault is put at a depth between 8.7-11.1 km. (b) The earthquake fault is put at a depth between 2.6-5.0 km. | + | **Figure 5.** Forward modeling of the interferograms for Duvall earthquake at different depth. The strike, dip, rake, slip and length of the fault for the two modeling are set to be the same, but with different depth range. (a) The earthquake fault is put at a depth between 8.7-11.1 km. (b) The earthquake fault is put at a depth between 2.6-5.0 km. |
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| One thing should be mentioned is that the surface displacement vector for the Duvall earthquake maybe has a considerable component that is perpendicular to the looking direction of the satellite. This specific looking-displacement geometry could also limit us to achieve InSAR images with detectable signals. So exploration of more available ascending scenes (we use descending scenes in this study) with short perpendicular baseline would help to produce a high quality InSAR image. | One thing should be mentioned is that the surface displacement vector for the Duvall earthquake maybe has a considerable component that is perpendicular to the looking direction of the satellite. This specific looking-displacement geometry could also limit us to achieve InSAR images with detectable signals. So exploration of more available ascending scenes (we use descending scenes in this study) with short perpendicular baseline would help to produce a high quality InSAR image. | ||
| - | **5. Summary | + | **5. Summary ** |
| - | ** | + | |
| The 1996 Duvall Mw 5.1 earthquake is the biggest one in Washington since 1966 and was followed by intense aftershock seismicity. We use four SAR scenes from ERS 1 and ERS 2 satellites to map the surface deformation of the earthquake. We produce four interferograms spanning the main shock, but they all show very low correlated interferometric phase. On one hand, the decorrelation might be from the long perpendicular baseline and/or temporal change of the scatter properties. On the other hand, using forward modeling we find that if the focal depth of Duval shock is 10 km or even deeper the corresponding signal on InSAR is 0.5 cm or smaller, which is might be at the edge of InSAR detectability. Given the distribution of the aftershocks, the Duvall earthquake might be a shallow event. Check of other available scenes with short perpendicular baseline would be key to give a better constraint on the earthquake location and a better understanding of the seismogenesis and earthquake hazard analysis in this area. | The 1996 Duvall Mw 5.1 earthquake is the biggest one in Washington since 1966 and was followed by intense aftershock seismicity. We use four SAR scenes from ERS 1 and ERS 2 satellites to map the surface deformation of the earthquake. We produce four interferograms spanning the main shock, but they all show very low correlated interferometric phase. On one hand, the decorrelation might be from the long perpendicular baseline and/or temporal change of the scatter properties. On the other hand, using forward modeling we find that if the focal depth of Duval shock is 10 km or even deeper the corresponding signal on InSAR is 0.5 cm or smaller, which is might be at the edge of InSAR detectability. Given the distribution of the aftershocks, the Duvall earthquake might be a shallow event. Check of other available scenes with short perpendicular baseline would be key to give a better constraint on the earthquake location and a better understanding of the seismogenesis and earthquake hazard analysis in this area. | ||
