Upper crustal structure of NW Iran revealed by regional 3-D Pg velocity tomography

Abstract

SUMMARY We present the result of a 3-D Pg tomography in NW Iran to better understand the relationship between seismicity and velocity structure within the young continental collision system. In this regard, we have collected 559 07 Pg traveltime readings from 3963 well located earthquakes recorded by 353 seismic stations including 121 stations from four new temporary seismic networks. The most prominent feature of our Pg velocity model is a high correlation between the location of majority of large magnitude events and the location of low velocity regions within the seismogenic layer. The large instrumental and historical earthquakes with some limited exceptions tends to happen close to the borders of the low velocity regions. The Lorestan arc of Zagros has the thickest (∼20 km) low velocity region and Central Iran has the thinnest (less than 10 km) low velocity region where little seismicity is observed. Despite the relative increase of thickness of low velocity region in the uppermost part of the upper crust of Alborz, the average Pg velocity of the upper crust increases from Central Iran towards Alborz and reaches to its climax in the northern hills of Alborz, where the catastrophic Rudbar-Tarom 1990 event happened. The Pg velocity map shows presence of a low angle basement ramp in the Lorestan arc at the depth range of ∼10–20 km. The large low angle thrust Ezgele-Sarpolzahab 2017 earthquake and medium size high angle thrust events happened at the base and updip part of the velocity ramp, respectively. The calculated Pg velocity map shows low velocity regions at depths deeper than 11 and 20 km beneath the Sahand and Sabalan volcanoes, respectively.

Shallow Crustal Response to Arabia‐Eurasia Convergence in Northwestern Iran: Constraints From Multifrequency P‐Wave Receiver Functions

“…The thickness of overlying sediments ranges from 9 to 12 km at areas where high-velocity anomalies appear and exceeds 12 km at the surroundings. This range of sedimentary thickness is broadly consistent with that predicted from aeromagnetic data (7-16 km; Teknik & Ghods, 2017), Pg tomography (8-15 km; Maheri-Peyrov et al, 2020), and P-RF imaging (8-10 km; Motaghi et al, 2017).…”

“…Active‐source methods, despite having impressive resolution, are challenging for region‐scale imaging due to their enormous cost. Passive‐source methods based on either body wave travel time or surface wave dispersion data require a uniform and dense distribution of stations and/or events, and suffer from a low spatial resolution due to strong along‐ray averaging effects (e.g., Maheri‐Peyrov et al., 2020). The P‐wave receiver function (P‐RF), because of its excellent resolution in both horizontal and vertical directions, has been used routinely to constrain crustal structures beneath single stations or dense arrays.…”

Seismic anisotropy and mantle deformation in NW Iran inferred from splitting measurements of SK(K)S and direct S phases

“…NW Iran is a region of anomalously warm crust and upper mantle and the Sahand and Sabalan have had post-collisional magmatic activities as recently as Pliocene-Quaternary (Chiü et al 2013). The regional Pg-wave tomography of Maheri-Peyrov et al (2020) and the P-wave tomography of Bavali et al (2016) revealed low-velocity zones at mid-crustal to subcrustal mantle depths directly beneath the volcanoes, which could be related to their thermal source regions. This structural setting can create a layered anisotropy at a local scale and explain the smaller delay times in the regions of the two volcanoes.…”

Crustal density structure of the northwestern Iranian Plateau

“…Maheri-Peyrov et al, 2018) also show slow Pg velocity (i.e., not in the range of velocity of upper crustal crystalline rocks) of the upper crust within the region around Kermanshah ophiolites. This indirectly implies that the high P velocity ophiolitic rocks are too thin to increase the average velocity of the sedimentary cover.…”