Mehrdad Mostafazadeh

Seismic strain and b value are used to quantify seismic potential in the Zagros region (Iran). Small b values (0.69 and 0.69) are related to large seismic moment rates (9.96×10¹⁷ and 4.12×10¹⁷) in southern zones of the Zagros, indicating more frequent large earthquakes. Medium to large b values (0.72 and 0.92) are related to small seismic moment rates (2.94×10¹⁶ and 6.80×10¹⁶) in middle zones of the Zagros, indicating less frequent large earthquakes. Small b value (0.64) is related to medium seismic moment rate (1.38×10¹⁷) in middle to northern zone of the Zagros, indicating frequent large earthquakes. Large b value (0.87) is related to large seismic moment rate (2.29×10¹⁷) in northwestern zone, indicating more frequent large earthquakes. Recurrence intervals of large earthquakes (M > 6) are short in southern (10 and 14 years) and northwestern (13 years) zones, while the recurrence intervals are long in the middle (46 and 114 years) and middle to northern (25 years) zones.

To achieve a comprehensive attitude about seismicity, it is necessary to consider spatial and magnitude distributions of earthquakes. Earthquake distributions in space and magnitude can be quantified by means of spatial fractal dimension D(s) and Gutenberg–Richter b value. In this paper, b value and D(s) are used to evaluate seismicity of the Zagros zone (Iran) in time interval 1964 to 2012. Seismic catalog of the Zagros zone is extracted from unified seismic catalog of the Iranian Plateau. The b value and D(s) are estimated using frequency–magnitude distribution, Kijko-Sellevoll (Bull Seism Soc Am 79(3):645–654, 1989) and correlation integral methods. Correlations between spatial variations in b value and D(s) along individual profiles across the North Zagros and the Central Zagros indicate different stress release regimes for North and Central parts of the Zagros zone. Evaluation of b value with respect to depth along the profiles indicates larger b values at shallower depths. Temporal variations in b value and D(s) are also obtained from background seismicity to evaluate seismicity behavior of the Zagros zone. Our results indicate high b values and moderate D(s) for seismicity of the North Zagros, while seismicity of the Central Zagros has low b values and high D(s) during time interval 1964–2012. Asperities of the Main Zagros Thrust are also located by investigating ratio D(s)/b along the Main Zagros Thrust.

In this study, the teleseismic body waveform modeling of the far-field P- and SH-waveforms as well as spectral analysis of P-waves are used to determine the source parameters of March 14, 1998 Fandoqa earthquake (Ms 6.9). Its epicenter is located southeast of Iran at Kerman province on the Gowk fault system. Previous focal mechanism solutions indicate motion on right-lateral strike slip faults. Based on waveform modeling, the results of this study represent a right-lateral strike slip motion on a NW-SE striking fault with parameters: Fault plane (strike=158 o , dip=54 o and the rake of 200 o ) and auxiliary plane (strike=58 o , dip =76 o , rake =-35 o ), depth of 4km and seismic moment of 1.32E+19N.m. The spectral analysis of the far-field P-wave pulses resulted in a fault length L ~ 20-26km, stress drop Δσ ~ 23 bars and average displacement u ~ 1m. In this study, the variant models are also examined to determine the source dimension and it is found that both the Madariaga and Sato-Hirasawa models are more consistent with the surface faulting in this area.

The occurrence of the western Iran earthquake of 31 March 2006 provided an important opportunity to study the source properties of earthquakes in this region. Although moderate in size (ML = 6:1, IIEES), this earthquake was the largest to have occurred in the region since the deployment of the Global Digital Seismograph Network. The far--eld data determination of body wave (P) spectra, interpreted in terms of the circular seismic source model, are used to estimate the parameters seismic moment (M O), corner frequency (f 0) , source radius (r) and stress drop (P waves recorded at teleseismic distances can be obtained from stations of this network that are to displacement, in a frequency range of 0.19 to 0.32 Hz. The average seismic moment (M O = 14:92 10 19 N-M) and source radius (r = 9281 m) were calculated from the long period spectral levels, which were corrected for the radiation pattern of a double couple point source. In addition, the stress drops (= 87 6 N/m 2) of this event have been calculated by using an average seismic moment and source radius. Additional errors in the stress drop determination are produced by uncertainty in the seismic moment. Scatter in the seismic moment values is caused by such factors as site condition and errors in the radiation pattern.

The newly installed broadband seismic network of Iran, at present, consists of 12 stations distributed across the country. We combine these high-quality data with data from IRIS broadband stations in neighboring countries to perform full-waveform regional moment tensor inversions of Iranian earthquakes. The station distribution is relatively sparse. We thus use data up to 1200 km epicentral distance and perform low frequency (f < 0.033 Hz) inversions. Preliminary results indicate that the use of regional data lowers the earthquake analysis threshold to about magnitudes 4-4.5, which is an order of magnitude less than routine teleseismic inversions. We currently derive suitable crustal models for various source-receiver geometries before analyzing the large number of moderate-to-strong earthquakes that occur in Iran. Within a short time span, we expect to build a large moment tensor database, which significantly expands the existing databases and provides improved constraints for seismotectonic studies, earthquake size calibration and seismic hazard assessments.

The source mechanism derived from the inversion of long-period body waves revealed that the earthquake occurred on a north-south trending strike-slip fault with a thrust component. According to the source model estimated in this study, the 2003 Bam, Iran, earthquake was a multiple event formed by two subevents. The rupture following subevent one started at a depth of about 8 km. However, the depth of subevent two is about 10 km. The total seismic moment estimated from inversion processes is 8.34×1018 Nm. The pulse duration of subevent one and subevent two was determined from source time function as 1.7 s and 0.8 s, respectively. Corner frequency and source radius have been calculated by using major pulse duration. The corner frequency and source radius are 0.187 Hz and 5.47 km, respectively. The aftershock events distributed along a 30 km north-south striking fault. The focal depths of aftershocks distribution show a nearly vertical alignment of aftershocks located between 6 and 20 km depth. The focal mechanism solutions of aftershocks indicate right-lateral strike-slip faulting on a north-south trending fault, parallel to the previously known Bam fault trace in the east of Bam

The generalized linear inverse technique has been applied to the problem of determining earthquake source models for five moderately large earthquakes in the Zagros Fold Thrust Belt (ZFTB). These earthquakes occurred in the southeastern () parts of the ZFTB. Propagation and instrumental effects are deconvolved from the long period P and SH wave records of GDSN stations to obtain the teleseismic source time function using a damped least squares inversion. The inversion has the additional constraint that the source time function is positive everywhere. A comparison of source time function of these events show that the average source duration of earthquake along the strike-slip Kazerun fault is greater than the other parts of the region. Generally slow earthquakes have long source duration. Slow risetime presumably results from a very low stress drop. Focal mechanisms of the earthquakes are thrust in the southeastern ZFTB, and have strike-slip components in the central and northwestern ZFTB. Depth values of these events are estimated to vary between 5-15km. Considering the depths and observed emergent moment release, we speculate that these events occurred beneath the sedimentary sections near to the detachment horizon. The slip vectors along the Zagros zone that are computed on the basis of compiled focal mechanism solutions in this and previous studies show a similarity with direction of convergence between the Arabian plate and the Iranian plateau.

The south Caspian Basin is a seismic block within the Alpine-Himalayan Belt. The source time function of 31 events of Caspian earthquakes have been obtained from teleseismic body waveform modeling. The duration of each subevent with magnitude larger than 5 (M w >5.0) and depth between km h 76 4 ≤ ≤ was determined from source time function. Corner frequency and stress drops have been calculated for each of 31 events by using pulse duration from source time function.When viewed over the entire depth range, the total duration ( ) t τ is related to Mo by log t τ = (0.2642 ± 0.001) log M O - 8.9119 ( ± .0.194). Corner frequencies range is from 0.038Hz to 0.16Hz. Static stress drops calculated from the pulse shapes for each event studied in this paper changed between 0.07 bars to a maximum of 46 bars. Minimum and maximum displacements are 0.79m and 3.3m respectively. The variation in stress drop is considerable, but no evidence is seen for a scaling relation in which stress drop increase with moment. These relative source durations do not show any clear depth dependence.

The Alborz region is a relatively seismic block within the Alpine-Himalayan Belt. We have compiled source time function of 31 events of Caspian earthquakes that is obtained from teleseismic body waveform modeling. The duration of each subevent with magnitude larger than 5 (M w >5.0) and depth between 4≤ h≤76 km was determined from source time function. When viewed over the entire depth range, the total duration (τ t) is related to M o by log τ t = (0.2642 ± 0.001) log M O _ 8.9119 (±.0.194).

A waveform inversion algorithm, based on least square method, has been applied to the P and S waves of the 26 December 2003 Bam earthquake. The aftershocks of this event distributed along a narrow zone (approximately 20km) in N-S direction. In this research, estimates of centroid depth, seismic moment, and source mechanism have been obtained. The source mechanism derived from the inversion of long period body waves revealed that two events occurred on N-S trending strike-slip fault with a thrust component. According to the source model estimated in this study, the Bam earthquake was a multiple event. The rupture following the first event started at a depth of about 8km. However depth of the second event is about 10km. The total seismic moment estimated from inversion processes is 8.34×10 18 Nm. The seismic moment of the second event is less than the first one (the seismic moment of second event is calculated as 2.34×10 17 Nm). The pulse duration of main shock and the second event was determined from source time function and it is 1.7s and 0.8s respectively. Corner frequency and source radius have been calculated for main shock and the second event by using pulse duration. The range of corner frequency and source radius are from 0.187Hz -0.397Hz and 5.47km-2.57km for main shock and second event, respectively.

Regional and teleseismic determination of body wave (S) spectra, interpreted in terms of the circular seismic source model, are used to estimate the parameters of March 13, 2005 earthquake that is occurred in south east of Iran. The results as follows: the seismic moment (Mo = 9.30 e +25 dyn-cm) , corner frequency (fo= 0.35hz), source dimension (R=3.9 km) and stress drop (Δσ= 0.26 ×10 4 bar). Comparison between our results and Harvard CMT solution data show that it has some difference between common parameters. Especially the depth value reported by Harvard is 24 km greater than our result (CMT depth =56 km). Besides we can see some discrepancy between seismic moment parameters (Harvard data is Mo =1.17 e +25 dyn-cm). Scatter in the seismic moment values is caused by such factors as the site conditions and errors in the radiation pattern corrections.