Characterizing Near‐Field Surface Deformation in the 1990 Rudbar Earthquake (Iran) Using Optical Image Correlation

Abstract

The 1990 Rudbar earthquake (Mw 7.3) broke along three right‐stepping segments of the left‐lateral Rudbar fault, in the western Alborz mountains (Iran), producing ∼80 km of surface rupture. Previously reported horizontal (∼60 cm) field offsets were surprisingly low, and the vertical (∼95 cm) unexpected, given the relatively large moment release, shallow source depth, vertical fault geometry, and left‐lateral rake. We characterize the surface displacement from optical image correlation of satellite and aerial images. A maximum displacement of ∼6.5 m is observed in the eastern part of the rupture consistent with both a sharp pulse in the moment release rate ∼10 s after the rupture initiation, and the macroseismic isoseismal curves. We find ∼2.2 m mean surface slip, larger than field measurements, but consistent with empirical scaling laws based upon the seismological moment release. Estimates of fault zone width based on correlation of 1 m resolution aerial photos (and corrected for smoothing bias) are <200 m; these values increase artificially with lower resolution imagery, which is more affected by smoothing bias. The large mismatch between field and correlation displacements suggests the Rudbar earthquake probably featured significant off‐fault deformation, consistent with its low structural maturity. Finally, we document a surface rupture along a structural continuation of the Kabateh segment, east of the Sefid Rud river. Slip along this segment (1–2 m), is less than the neighboring Zard Geli segment (1.5–6 m). The two faults converge at Sefid Rud, which appears to represent a major structural feature controlling fault segmentation in this region.

Complex 3‐D Surface Deformation in the 1971 San Fernando, California Earthquake Reveals Static and Dynamic Controls on Off‐Fault Deformation

“…Our results are consistent with previous studies in that offset measurements are generally highest and vary more smoothly on simpler, straighter segments of the main fault, and lower and more heterogeneous where there are small wavelength variations in fault trace orientation and where slip is partitioned onto secondary faults (Figures 2c and 4a; e.g., ; Ajorlou et al., 2021; Bruhat et al., 2020; Klinger et al., 2006; Manighetti et al., 2007; Milliner et al., 2015; Perrin et al., 2016). This suggests that structural complexity has a primary control on slip distribution.…”

“…Once the pre‐earthquake and post‐earthquake orthomosaics are created using ASP, we measured the lateral coseismic displacements using COSI‐Corr (Ajorlou et al., 2021; Leprince et al., 2007; Milliner et al., 2015). We used a multiscale sliding correlation window of 256 by 256 pixels to 32 by 32 pixels for the correlation, with a step size of eight pixels, resulting in 8 m‐resolution images that represent the eastward and northward components of displacement.…”

Signature of transition to supershear rupture speed in the coseismic off-fault damage zone

“…Recent developments in satellite optical image analysis and sub-pixel correlation methods allow for detecting displacement variations due to an earthquake down to sub-metric resolutions (e.g. [75,[77][78][79]). These methods enable characterizing the surface rupture geometry, the amount of surface displacement and the width of the zone affected by this displacement after an earthquake, also referred to as the 'fault zone width' [80].…”

Revisiting the 1959 Hebgen Lake Earthquake Using Optical Image Correlation; New Constraints on Near‐Field 3D Ground Displacement

“…The scale of the images is 1:48,000 (ground footprint ≈ 11 × 11 km, resolution ≈ 1.1 m/px) for the pre-earthquake images and 1:28,000 (ground footprint ≈ 6 × 6 km, resolution ≈ 0.6 m/px) for the post-earthquake images; the resolution of the image scans is 1,000 dpi. Since the historical images suffer from film distortions, scanning artifacts, and image defects (Hollingsworth et al, 2012;Michel & Avouac, 2006), we built on the method described in Ajorlou et al (2021) and Bhushan et al (2021) and created high-resolution DEMs from the pre-and post-earthquake stereo-image pairs which were used to precisely orthorectify the pre-and post-earthquake aerial photographs at 1 m/px. Precise orthorectification is crucial to remove topographic distortion and to avoid the interference of residual stereoscopic signal with the tectonic displacement in the final correlation.…”

“…Since the historical images suffer from film distortions, scanning artifacts, and image defects (Hollingsworth et al., 2012; Michel & Avouac, 2006), we built on the method described in Ajorlou et al. (2021) and Bhushan et al. (2021) and created high‐resolution DEMs from the pre‐ and post‐earthquake stereo‐image pairs which were used to precisely orthorectify the pre‐ and post‐earthquake aerial photographs at 1 m/px.…”