| 1. |
- Abdetedal, Mahsa, et al.
(författare)
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Ambient noise surface wave tomography of the Makran subduction zone, south-east Iran : Implications for crustal and uppermost mantle structures
- 2015
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Ingår i: Earthquake Science. - : Elsevier BV. - 1674-4519 .- 1867-8777. ; 28:4, s. 235-251
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Tidskriftsartikel (refereegranskat)abstract
- Seismic ambient noise of surface wave tomography was applied to estimate Rayleigh wave empirical Green’s functions (EGFs) and then to study crust and uppermost mantle structure beneath the Makran region in south-east Iran. 12 months of continuous data from January 2009 through January 2010, recorded at broadband seismic stations, were analyzed. Group velocities of the fundamental mode Rayleigh wave dispersion curves were obtained from the empirical Green’s functions. Multiple-filter analysis was used to plot group velocity variations at periods from 10 to 50 s. Using group velocity dispersion curves, 1-D v S velocity models were calculated between several station pairs. The final results demonstrate significant agreement to known geological and tectonic features. Our tomography maps display low-velocity anomaly with SW-NE trend, comparable with volcanic arc settings of the Makran region which may be attributable to the geometry of Arabian Plate subducting beneath the overriding the Lut block. The northward subducting Arabian Plate is determined by high-velocity anomaly along the Straits of Hormuz. At short periods (<20 s), there is a sharp transition boundary between low- and high-velocity transition zone with the NW trending at the western edge of Makran which is attributable to the Minab fault system.
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| 2. |
- Abdollahi, Somayeh, et al.
(författare)
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3D joint inversion of gravity data and Rayleigh wave group velocities to resolve shear-wave velocity and density structure in the Makran subduction zone, south-east Iran
- 2019
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Ingår i: Journal of Asian Earth Sciences. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1367-9120 .- 1878-5786. ; 173, s. 275-290
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we developed a method to invert jointly Rayleigh wave group velocities and gravity anomalies for velocity and density structure of the lithosphere. We applied the method to the Makran accretionary prism, SE Iran. The reason for using different data sets is that each of these data sets is sensitive to different parameters. Surface wave group velocities are sensitive mainly to shear wave velocity distribution in depth but do not well resolve density variations. Therefore, joint inversion with gravity data increases the resolution of density distribution. Our approach differs from others mainly in the model parameterization: Instead of subdividing the model into a large number of thin layers, we invert for the properties of only four layers: thickness, P- and S-wave velocities and densities and their vertical gradients in sediments, upper-crust, lower-crust and upper mantle. The method is applied first to synthetic models in order to demonstrate its usefulness. We then applied the method to real data to investigate the lithosphere structure beneath the Makran. The resulting model shows that Moho depth increases from Oman Sea (18-33 km) and Makran fore-arc (33-37 km) to the volcanic-arc (44-46 km). The crustal density is high in the Oman Sea as should be expected for the oceanic crust. We also find a high-velocity anomaly in the upper mantle under the Oman Sea corresponding to the subducting slab. The crust under the fore-arc, volcanic-arc and back-arc settings of Makran subduction zone is characterized by low-velocity zones.
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| 3. |
- Abdollahi, Somayeh, et al.
(författare)
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Crustal and upper mantle structures of Makran subduction zone, SE Iran by combined surface wave velocity analysis and gravity modeling
- 2018
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Ingår i: Tectonophysics. - : ELSEVIER SCIENCE BV. - 0040-1951 .- 1879-3266. ; 747, s. 191-210
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Tidskriftsartikel (refereegranskat)abstract
- The inversion of Rayleigh wave group velocity dispersion curves is challenging, because it is non-linear and multimodal. In this study, we develop and test a new Rayleigh wave dispersion curve inversion scheme using the Shuffled Complex Evolution (SCE) algorithm. Incorporating this optimization algorithm into the inverse procedure not only can effectively locate the promising areas in the solution space for a global minimum but also avoids its wandering near the global minimum in the final stage of search. In addition, our approach differs from others in the model parameterization: Instead of subdividing the model into a large number of thin layers, we invert for thickness, velocities and densities and their vertical gradients of four layers, sediments, upper-crust, lower-crust and upper mantle. The proposed inverse procedure is applied to non-linear inversion of fundamental mode Rayleigh wave group dispersion curves for shear and compressional wave velocities. At first, to determine the efficiency and stability of the SCE method, two noise-free and two noisy synthetic data sets are inverted. Then real data for Makran region in SE Iran are inverted to examine the usage and robustness of the proposed approach on real surface wave data. In a second step, we applied 3D Gravity Modeling based on surface wave analysis results to obtain the density structure and thickness of each layer. The reason for using both types of data sets, is that gravity anomaly has a bad vertical resolution and surface wave group velocities are good for placing layer limits at depth, but they are not very sensitive to densities. Therefore, using gravity data increases the overall resolution of density distribution. In a final step, we used again the SCE method to invert the fundamental mode Rayleigh wave group dispersion curves based on the gravity results. Gravity results like thicknesses and sediment densities have been used to constrain the limit of search space in the SCE method. Results show a high shear and compressional velocity under the Gulf of Oman which reduce to the North of the Makran region. The Moho depth of the Oman Gulf is about 18-28 km and it increases to 46-48 km under the Taftan-Bazman volcanic-arc. The density image shows an average crustal density with maximum values under the Gulf of Oman decreasing northward to the Makran.
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| 4. |
- Amini, Samar, et al.
(författare)
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Fault slip and identification of the second fault plane in the Varzeghan earthquake doublet
- 2018
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Ingår i: Journal of Seismology. - : SPRINGER. - 1383-4649 .- 1573-157X. ; 22:4, s. 815-831
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Tidskriftsartikel (refereegranskat)abstract
- An intraplate earthquake doublet, with 11-min delay between the events, devastated the city of Varzeghan in northwestern Iran on August 11, 2012. The first Mw 6.5 strike-slip earthquake, which occurred after more than 200 years of low seismicity, was followed by an Mw 6.4 oblique thrust event at an epicentral separation of about 6 km. While the first event can be associated with a distinct surface rupture, the absence of a surface fault trace and no clear aftershock signature makes it challenging to identify the fault plane of the second event. We use teleseismic body wave inversion to deduce the slip distribution in the first event. Using both P and SH waves stabilize the inversion and we further constrain the result with the surface rupture extent and the aftershock distribution. The obtained slip pattern shows two distinct slip patches with dissimilar slip directions where aftershocks avoid high-slip areas. Using the estimated slip for the first event, we calculate the induced Coulomb stress change on the nodal planes of the second event and find a preference for higher Coulomb stress on the N-S nodal plane. Assuming a simple slip model for the second event, we estimate the combined Coulomb stress changes from the two events on the focal planes of the largest aftershocks. We find that 90% of the aftershocks show increased Coulomb stress on one of their nodal planes when the N-S plane of the second event is assumed to be the correct fault plane.
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| 5. |
- Basir, Hadi Mahdavi, et al.
(författare)
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Acoustic wave propagation simulation by reduced order modelling
- 2018
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Ingår i: Exploration Geophysics. - : CSIRO PUBLISHING. - 0812-3985 .- 1834-7533. ; 49:3, s. 386-397
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Tidskriftsartikel (refereegranskat)abstract
- Wave propagation simulation, as an essential part of many algorithms in seismic exploration, is associated with high computational cost. Reduced order modelling(ROM) is a known technique in many different applications that can reduce the computational cost of simulation by employing an approximation of the model parameters. ROM can be carried out using different algorithms. The method proposed in this work is based on using the most important mode shapes of the model, which can be computed by an efficient numerical method. The numerical accuracy and computational performance of the proposed method were investigated over a simple synthetic velocity model and a portion of the SEG/EAGE velocity model. Different boundary conditions were discussed, and among them the random boundary condition had higher performance for applications like reverse time migration (RTM). The capability of the proposed method for RTM was evaluated and confirmed by the synthetic velocity model of SEG/EAGE. The results showed that the proposed ROM method, compared with the conventional finite element method (FEM), can decrease the computational cost of wave propagation modelling for applications with many simulations like the reverse time migration. Depending on the number of simulations, the proposed method can increase the computational efficiency by several orders of magnitudes.
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| 6. |
- Basir, Hadi Mahdavi, et al.
(författare)
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Modified imaging condition for reverse time migration based on reduction of modelling time
- 2018
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Ingår i: Exploration Geophysics. - : CSIRO PUBLISHING. - 0812-3985 .- 1834-7533. ; 49:4, s. 494-505
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Tidskriftsartikel (refereegranskat)abstract
- Reverse time migration (RTM) is considered as a high-end imaging algorithm due to its ability to image geologically complex environments. However, this algorithm suffers from very high computational costs and low-frequency artefacts. The former drawback is the result of the intensive computations and huge memory allocation involved in RTM. Wave propagation modelling, as a kernel of RTM, demands intensive computations, and conventional imaging conditions are associated with huge memory allocation. In this paper, a modification of imaging condition is proposed that improves the efficiency of RTM as a reduction of computational cost, memory (RAM) allocation and low-frequency artefacts. The proposed imaging condition is similar to the conventional imaging condition but with the reduction of modelling time to near half the maximum time of recording. As the main idea of the proposed imaging condition, the impact of wave propagation modelling time is investigated on the quality of RTM and illumination of reflectors. The performance of the proposed method is considered using two synthetic models (SEG/EAGE and BP) and a real dataset from an Iranian oilfield in the south of Iran. Results showed that the new imaging condition can properly image the reflectors and enhance the efficiency of RTM. By using the proposed imaging condition, we achieved similar to 25% increase in CPU performance and 50% decrease in the memory allocation. Despite the improvement of the performance, results showed that the proposed imaging condition had no significant effect on the illumination.
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| 7. |
- Basir, Hadi Mandavi, et al.
(författare)
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Reverse time migration by Krylov subspace reduced order modeling
- 2018
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 151, s. 298-308
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Tidskriftsartikel (refereegranskat)abstract
- Imaging is a key step in seismic data processing. To date, a myriad of advanced pre-stack depth migration approaches have been developed; however, reverse time migration (RTM) is still considered as the high-end imaging algorithm. The main limitations associated with the performance cost of reverse time migration are the intensive computation of the forward and backward simulations, time consumption, and memory allocation related to imaging condition. Based on the reduced order modeling, we proposed an algorithm, which can be adapted to all the aforementioned factors. Our proposed method benefit from Krylov subspaces method to compute certain mode shapes of the velocity model computed by as an orthogonal base of reduced order modeling. Reverse time migration by reduced order modeling is helpful concerning the highly parallel computation and strongly reduces the memory requirement of reverse time migration. The synthetic model results showed that suggested method can decrease the computational costs of reverse time migration by several orders of magnitudes, compared with reverse time migration by finite element method.
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| 8. |
- Karkooti, Ehsan, et al.
(författare)
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Investigating the role of source mechanism, surface topography, and attenuation on the observed PGA pattern in May 28, 2004, Mw 6.2 Baladeh earthquake (Iran)
- 2016
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Ingår i: Journal of Seismology. - : Springer Science and Business Media LLC. - 1383-4649 .- 1573-157X. ; 20:2, s. 495-510
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we use seismic waveform simulation to investigate the influence of source mechanism complexity, surface topography, and quality factor on the observed peak ground motions in May 28, 2004, moment magnitude (Mw) 6.2 Baladeh earthquake. The observed peak ground acceleration (PGA) pattern in this event, which is the biggest earthquake to hit the Central Alborz Mountains of Iran in modern instrumental era, is irregular in some respects. First, the observed PGA contours are elongated toward north-west and, second, the maximum observed PGA value of 1049 cm/s(2) on the horizontal component of Hasan Keyf station 50 km away from the epicenter is quite high and irregular for an earthquake of this magnitude, at such long distance. In this study, we employ the spectral element method, implemented in SPECFEM3D software package to simulate the 3D wave propagation from several source models in the area. Our results suggest directivity effect is the main cause of the anomalous observations in this earthquake and could account for the elongation of PGA contours and also the anomalous maximum PGA value observed at Hasan Keyf strong motion station. We show that the surface topography has minor effect on the observed peak ground acceleration and the resulting PGA maps. Also by finding the bounds of seismic quality factor effect on the peak ground acceleration values, we show that this factor could not account for the elongation of iso-acceleration contours in the north-west direction.
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| 9. |
- Nazeri, Sahar, et al.
(författare)
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Magnitude Estimation Based on Integrated Amplitude and Frequency Content of the Initial P Wave in Earthquake Early Warning Applied to Tehran, Iran
- 2017
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Ingår i: Bulletin of The Seismological Society of America (BSSA). - : SEISMOLOGICAL SOC AMER. - 0037-1106 .- 1943-3573. ; 107:3, s. 1432-1438
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Tidskriftsartikel (refereegranskat)abstract
- To optimize magnitude estimation for the earthquake early warning system around the Tehran region, different amplitude-and frequency-based parameters, that is, predominant period (tau(max)(p)), characteristic period (tau(c)), log-average period (tau(log)), and peak displacement (P-d) were analyzed in this article. All parameters were calculated directly from seismic records, with an epicentral distance less than 150 km, and within the initial 3 s of the P waves. The analysis of earthquakes in the 2.4 < M-L< 4.9 magnitude range verified that the result of tau(max)(p) showed a consistent trend as compared with the global observations, and provided a robust estimate of magnitude for the dataset used in this research. In comparison with worldwide observations, the calculated P-d and tau(c) were underestimated, and there was no scaling relationship with the tau(log) parameter. When combined with the global observations from Japan, Taiwan, and Italy, the results of P-d and tc for the Tehran region produced optimized results.
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| 10. |
- Nazeri, Sahar, et al.
(författare)
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Rapid Estimation of the Epicentral Distance in the Earthquake Early Warning System around the Tehran Region, Iran
- 2019
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Ingår i: Seismological Research Letters. - : SEISMOLOGICAL SOC AMER. - 0895-0695 .- 1938-2057. ; 90:5, s. 1916-1922
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Tidskriftsartikel (refereegranskat)abstract
- The estimation of epicentral distance is a critical step in earthquake early warning systems (EEWSs) that is necessary to characterize the level of expected ground shaking. In this study, two rapid methodologies, that is, B-Delta and C-Delta , are evaluated to estimate the epicentral distance for use in the EEWSs around the Tehran region. Traditionally, the B and C coefficients are computed using acceleration records, however, in this study, we utilize both acceleration and velocity waveforms for obtaining a suitable B-Delta and C-Delta relationships for the Tehran region. In comparison with observations from Japan, our measurements fall within the range of scatter. However, our results show a lower trend, which can strongly depend on the few numbers of events and range of magnitude (small-to-moderate) of earthquakes used in the current research. To improve our result, we include some large earthquakes from Iran, Italy, and Japan with magnitude larger than 5.9. Although the optimal trend is finally obtained by fitting a line to the distance-averaged points, we conclude that the same trend and relationship as Japan can be used in Tehran early warning system. We also found that B and C parameters are strongly compatible to each other. As time windows of 3.0 and 0.5 s after the P onset are chosen respectively to compute the B and C values, so by selecting the C parameter as a proxy of B parameter to estimate the epicentral distance, we may save significant time in order of about 2.5 s in any earthquake early warning applications.
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