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The Kopeh-Dagh zone of NE Iran is dominated by active strike-slip and thrust faults that accommodate a part of the convergence between the Arabian and Eurasian Plates. The Ashkhaneh thrust fault zone with approximately 80 kmlong is one of the main accommodative structures, which has been dissected by a number of strike-slip tear faults. Tectonic geomorphology, satellite-based Global Positioning System (GPS), and seismic data imply that the development of tear faults is one of the main controlling factors in structural deformation and related seismic activity along the Ashkhaneh thrust fault zone. The tectonic activity of the Ashkhaneh fault zone is mainly due to the E-Wtrending range-parallel reverse faults and NE-SWto ENEWSW trending range-crossing left-lateral strike-slip tear faults coming from two stages. In the first stage, the major E-Wtrending Ashkhaneh thrust fault zone has been developed in response to the collision of Central Iran with the South Caspian Basin. In the second stage, the progressive N-S shortening resulted in mountain curvature in eastern Alborz and the formation of strike-slip tear faults in response to the differential shortening along the Ashkhaneh fault zone. The sense of slip and geometry of the tear faults and the Ashkhaneh thrust fault seems to provide insights into faults interaction, so that the likely movement along one of these faults may cause reactivation of the other fault(s); similar to the earthquake occurrences on the Shalgun-Yelimsi tear fault (2019/11/07, Mw 5.9) and South Bozgush thrust fault (2019/11/10, Mw 4.4) in northwestern Iran. [ABSTRACT FROM AUTHOR]

The 1641 Dehkhwargan-Tabriz earthquake (Ms 6.8) is the only destructive earthquake to have occurred in Azarshahr-Khosrowshahr-Osku region of northwestern Iran. Ambiguities regarding the causative fault of this earthquake were a motivation to someway declare its mechanism. In this study, by conducting a field survey in an area between Tabriz and Azarshahr, and use of satellite imagery and aerial photographs, we provide geologic and geomorphic indications to identify the poorly known Azarshahr-Tabriz fault zone (ATF), which passes through the meizoseismal region of the 1641 earthquake. Our observations highlight the presence of a sinistral active fault zone that extends ~40 km in length from south of Tabriz to the northeast and north of Azarshahr to the southwest. The ATF is a structural assemblage of several fault strands with a variety of extensional and compressional mesoscale structures. We estimated slip rates ranging from ~0.01 mm yr-1 to <1 mm yr-1, based on drainage offset measurements, which demonstrate a relatively stable tectonic environment located immediately south of the well-known seismically active dextral North Tabriz fault. [ABSTRACT FROM AUTHOR]

Spectral parameter kappa ([Math Processing Error]) is widely used to model the decay slope of the Fourier acceleration spectrum at high frequencies. This study presents estimates of [Math Processing Error], often referred to as the site-related contribution in [Math Processing Error], for the Iranian strong-motion dataset. For our analysis, we use 2798 acceleration time histories from 325 earthquakes (between 1976 and 2020, inclusive) in the magnitude range [Math Processing Error] 3-7.4 and recorded over 285 stations at distances up to 100 km. The main objective of this study is to estimate [Math Processing Error] and associated uncertainties at Iranian strong-motion stations. We investigate different sources of uncertainty that contribute to the higher scatter in the station-specific measurements of [Math Processing Error]. According to our results, the choice of frequency range used to approximate linear decay of acceleration spectrum contributes significantly to measurement-related uncertainty. Moreover, our measurements exhibit regional variations in (average) [Math Processing Error] within Iran; in particular, for Zagros, the average [Math Processing Error] is found to be lower than that for northern Iran ([Math Processing Error]) and central-eastern Iran ([Math Processing Error]). In terms of between-station variability, there is a weak negative correlation between [Math Processing Error] and [Math Processing Error]. The within-station uncertainty (i.e., standard deviation of [Math Processing Error] values at individual stations) is also found to be decreasing with increasing [Math Processing Error]. The fundamental frequency of the site ([Math Processing Error]) also appears to affect both measurement-related uncertainty and between-station variability at stations that exhibit significant 1D soil response behavior. [ABSTRACT FROM AUTHOR]

In this study, new empirical relationships to estimate key parameters in Earthquake Early Warning (EEW) system including magnitude, epicentral distance and Peak Ground Acceleration (PGA) are introduced based on features of the initial portion of P-wave's amplitude envelope curve. For this purpose, 226 time series recorded by bore-hole accelerometers of Japanese KiK-net are processed for earthquakes with magnitudes from 3 to 7.2 MJMA and epicentral distances of less than 50 km. Hereby, an improved single station method for estimation of epicentral distance and two new methods for estimation of magnitude and amplitude of PGA are proposed based on exponentially envelope curve as B x t x e-Axt in known (B - Δ) method. A scaling relationship of B × Tr - Δ is proposed to estimate epicentral distance which is well correlated for larger earthquakes (M > 6), with results more robust and reliable than the previous method (B - Δ). Non-dimensional parameter, Tr depends on earthquake magnitude and A parameter in the above-mentioned function. By using the features of acceleration envelope curve and peak amplitude of P-waves, scale parameter 𝑆􀯔 is proposed that is well correlated with magnitude and has capability of estimating magnitude with standard deviation of less than ±0.77 magnitude unit. Sa is proportional to a part of area under envelope curve as a function of magnitude. Moreover, it is indicated that PGA in a single station can be estimated by using envelope curve characteristics of initial P-waves portion. [ABSTRACT FROM AUTHOR]

Transition boundary between Zagros continental collision and Makran oceanic-continental subduction can be specified by two wide limits: (a) Oman Line is the seismicity boundary with a sizeable reduction in seismicity rate from Zagros in the west to Makran in the east; and (b) the Zendan-Minab-Palami (ZMP) fault system is believed to be a prominent tectonic boundary. The purpose of this paper is to analyze the stress field in the Zagros-Makran transition zone by the iterative joint inversion method developed by Vavrycuk (Geophysical Journal International 199:69-77, 2014). The results suggest a rather uniform pattern of the stress field around these two boundaries. We compare the results with the strain rates obtained from the Global Positioning System (GPS) network stations. In most cases, the velocity vectors show a relatively good agreement with the stress field except for the Bandar Abbas (BABS) station which displays a relatively large deviation between the stress field and the strain vector. This deviation probably reflects a specific location of the BABS station being in the transition zone between Zagros continental collision and Makran subduction zones. [ABSTRACT FROM AUTHOR]

The spatial autocorrelation (SPAC) method is a useful tool for interpreting ambient noise, and several theoretical tools based on this method are available for data collection, processing, and analysis. The method uses the three components of seismic microtremors recorded simultaneously with arrays of different configurations and yields dispersion curves of Rayleigh and Love waves. Existing theories of the SPAC method are based on an idealized circular array with an infinite number of stations (the continuous model). Errors due to the use of a finite number of stations have been derived theoretically for vertically polarized ambient noise. This paper presents an extension of that error theory to horizontally polarized ambient noise so that it is valid for a circular array with M uniformly spaced stations. The quantities that are obtained as the output of the SPAC method, however, depend on the ratio of the power spectral densities (PSDs) of Rayleigh and Love waves, which are difficult to identify beforehand. Instead we introduce a quantity, n, that does not depend on the PSD ratio and therefore is more convenient for separating the properties of Rayleigh and Love waves. We derive an expression of n for a finite, M-station circular array (κM) and compare it with what it should be in the continuous model (κT). We illustrate κM and κT using field data from a site in Tehran and discuss their discrepancies. Finally, we demonstrate how we have estimated Love-wave phase velocities using the calculated κM. [ABSTRACT FROM AUTHOR]