A Study on Crustal Deformation Analysis of the Alborz Region: GPS Versus Seismological Observations

The Alborz range of northern Iran is a region of active deformation within the broad Arabia–Eurasia collision zone. The range is also an excellent example of coeval strike-slip and compressional deformation, and as such can be an analogue for inactive fold and thrust belts thought to involve a component of oblique shortening. It is roughly 600 km long and 100 km across, running along the southern side of the Caspian Sea. Several summits are 4000 m in altitude. Damavand, a dormant volcano, reaches 5671 m. The highest non-volcanic summit is Alam Kuh, at 4830 m.
The Alborz range, northern Iran, deforms by strain partitioning of oblique shortening onto range-parallel left-lateral strike-slip and thrust faults. Deformation is due to the north–south Arabia–Eurasia convergence, and westward motion of the adjacent South Caspian relative to Iran. The occurrence of moderate to large earthquakes in the Alborz suggests an important deformation regime in this mountain belt. This belt has been responsible for several catastrophic earthquakes in the past. The Manjil earthquake of 20 June 1990, which is the most disastrous Iranian earthquake in the twentieth century, occurred in this belt. Both thrust and strike-slip faulting have been reported in this belt.
By the knowledge of the crustal deformation characteristics in areas with active tectonics we can realize the style, direction and magnitude of the deformation in the area, it can contribute to the deeper understanding of the underlying tectonic processes and to the improvement of the seismic hazard assessment.
In this contribution strain rate fields (using geodetic data vs. seismic data) calculated over the three different parts of Alborz regions (Western, Central, and Eastern Alborz). Eigenspace components of seismic strain tensor (seismic events with Ms ≥ 4.0 in the time interval 1900–2010) over three zones revealed crustal shortening over all three zones. Namely, the results of seismic data showed left-lateral strike-slip faulting in the Eastern Alborz, the right-lateral strike-slip motion in the Western Alborz and compression mechanism in the Central zone. The highest compressional rate in the Western Alborz probably represent the high seismicity rate of this region. In comparison, eigenspace components of geodetic strain tensor (during time interval from 2005 to 2009) illustrated high rate of compressional components in the Central and Western zones. Comparison of seismic and geodetic strain rates in the Western Alborz indicates the deformation rate over this region is associated with seismic activities. However, in Central and Western regions the geodetic strain rates appeared remarkably larger than seismic strain rates. May, this fact illustrated the deformation pattern in these regions are related to the local aseismic creep of those segments. The difference between eigendirection of both kind of tensors (geodestic vs. seismic) in segments probably were related to the short period of GPS data with respect to the seismic data. However, it is a fact that early historical data are incomplete. So, the catalog completeness is crucial for estimating reliable seismicity strain rates and, consequently, for use in seismic hazard assessments. Hence, the performing the repeated geodetic data over the Alborz region is proposed to investigate the reliable estimating of the strain rate.