Relocation and assessment of seismicity in the Iran region

Abstract

S U M M A R YMore than 2000 instrumentally recorded earthquakes occurring in the Iran region during the period 1918-2004 have been relocated and reassessed, with special attention to focal depth, using an advanced technique for 1-D earthquake location. A careful review of starting depths, association of teleseismic depth phases, and the effects of reading errors on these phases are made and, when necessary, waveforms have been examined to better constrain EHB focal depths. Uncertainties in EHB epicentres are on the order of 10-15 km in the Iran region, owing to the Earth's lateral heterogeneity and uneven station distribution. Uncertainties of reviewed EHB focal depth estimates are on the order of 10 km, as compared to about 4 km for long-period P and SH body-waveform inversions. Nevertheless, these EHB depth estimates are sufficiently accurate to resolve robust differences in focal depth distribution throughout the Iran region and, within their errors, show patterns that are in agreement with the smaller number of earthquakes whose depths have been confirmed by body-wave modelling or local seismic networks. The importance of this result is that future earthquakes with apparently anomalous depths can easily be identified, and checked, if necessary. Most earthquakes in the Iranian continental lithosphere occur in the upper crust, with the crustal shortening produced by continental collision accommodated entirely by thickening and distributed deformation. In the Zagros Mountains nearly all earthquakes are confined to the upper crust (depths <20 km), and there is no evidence for a seismically active subducted slab dipping NE beneath central Iran. By contrast, in southeastern Iran, where the Arabian seafloor is being subducted beneath the Makran coast, low-level earthquake activity occurs in the upper crust as well as to depths of at least 150 km within a northward-dipping subducting slab. Near the Oman Line, a region transitional between the Zagros and the Makran, seismicity extends to depths of up to 30-45 km in the crust, consistent with low-angle thrusting of Arabian basement beneath central Iran. In north-central Iran, along the Alborz mountain belt, seismic activity occurs primarily in the upper crust but with some infrequent events in the lower crust, particularly in the western part of the belt (the Talesh), where the South Caspian basin underthrusts NW Iran. Earthquakes that occur in a band across the central Caspian, following the Apscheron-Balkhan sill between Azerbaijan and Turkmenistan, have depths in the range 30-100 km, deepening northwards. These are thought to be connected with either incipient or remnant northeast subduction of the South Caspian basin basement beneath the east-west trending Apscheron-Balkhan sill. Curiously, in this region of genuine mantle seismicity, there is no evidence for earthquakes shallower than 30 km.

Estimates ofTefor continental regions using GOCE gravity

“…In the other regions the depths listed in the catalogue of Engdahl et al (1998 and later additions) were used. Engdahl et al (2006) compared the depths from the catalogue with those from waveform modelling, and showed that, if the catalogue depths were restricted to those listed with the code 'DEQ', they were within ±10 km of those from waveform modelling. In N Africa, S. India and Antarctica there are no earthquakes with well constrained depths .…”

Active accommodation of plate convergence in Southern Iran: Earthquake locations, triggered aseismic slip, and regional strain rates

“…The SFB is a salt-detached fold-and-thrust belt wherein a thick (8-10 km) package of sedimentary rocks lies above crystalline, Proterozoic Arabian shield, with an intervening 1-2 km-thick infra-Cambrian Hormuz Salt unit [Falcon, 1974;Stocklin, 1974;Colman-Sadd, 1978]. Uncertainties in teleseismic earthquake depths in this region are generally too large to allow resolution of whether the events nucleated in the sedimentary section or underlying crystalline basement [Maggi et al, 2000;Engdahl et al, 2006]. Recent geodetic work on two SFB earthquakes (M w 5.9) combined with observations of aftershocks from dense, local seismic networks suggests that these events likely occurred in the crystalline basement and triggered a similar magnitude of aseismic fault slip in the overlying sedimentary section [Barnhart and Lohman, 2013].…”

New kinematic constraints of the western Doruneh fault, northeastern Iran, from interseismic deformation analysis

“…We fixed the fault location and strike angle as the WFZ, the locking depth to 12 km (average depth of the regional seismicity according to Engdahl et al 2006), and simulated an infinitely long and deep fault. Using the Okada (1985) analytical solutions, we jointly inverted all profiles of the three data sets for the fault dip, rake and slip rate by means of a non-linear, least-squares inversion algorithm based on the Levemberg-Marquardt approach, as described by Atzori et al (2009); the convention for dip, rake and strike angles is the same as Okada (1985).…”