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

Abstract

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.

Keywords

• aerial photo
• fault width zone
• Iran
• KH9
• off‐fault deformation
• Rudbar earthquake