IRANIAN JOURNAL OF GEOPHYSICS
Year:2019 | Volume:13 | Issue:1 |Papers Count:10

Despite the ambiguous tsunamigenic behavior of the Makran Subduction Zone (MSZ), due to the low level of offshore seismicity, historical evidences and the 1945 tsunami in Makran confirm the potential of the MSZ for generating tsunami events. Possible future tsunamis generated by the MSZ will pose the coastlines of Iran to hazard more than any other country. Probabilistic tsunami hazard assessment (PTHA) is an effective approach to assess hazard from tsunamis and help for planning for the future. In this study, we assess the probabilistic tsunami hazard along the southeastern coast of Iran considering the entire Makran, the western Makran and the eastern Makran tsunamigenic sources. Tsunami scenarios include earthquakes of magnitudes between 7. 5-8. 9 for the western and eastern Makran and between 7. 5-9. 1 for the entire Makran. Both seismicity and tsunami numerical simulation are inputs for probabilistic hazard analysis. Assuming that the tsunami sources are capable of generating tsunamigenic earthquakes, estimating the annual rate of these events is required for PTHA. The truncated Gutenberg-Richter relation (Cosentino et al., 1977 and Weichert, 1980) is used in this study to compute the annual number of the earthquakes. We model tsunamis using the COMCOT well-known algorithm (Liu et al., 1998). The distributions of tsunami heights along the coastline of Iran are used in probabilistic tsunami hazard assessment. The results of PTHA show that Konarak and Sirik coastlines are posed to the most and least hazard from tsunamis, respectively. The probability of exceeding (POE) 1 and 3 meters increases with time. The probability that tsunami wave height exceeds 3 meters in 500 years is about 0. 63 and 0 near the coastlines of Konarak and Sirik, respectively. The maximum POE for 3 meters belongs to the area between Beris and the west of Kereti. Distributions of probabilistic tsunami height along the coastline of Iran also indicate that Konarak and Sirik are the most and least vulnerable shorelines to tsunami hazard, respectively. The annual probability of exceeding 1, 2 and 3 meters are 1, 0. 4 and 0. 2, respectively. The results indicate the need of attention to tsunami long-term hazard along the southeastern coast of Iran, especially for the area between Jask and Beris. Our tsunami hazard assessment does not involve the tsunami inundation distances on dry land due to lack of high resolution site-specific bathymetric/topographic maps. Such computations are required in order to estimate the exact impacts of possible future tsunamis on the southeastern coast of Iran. High-resolution hydrographic surveys are required to be done in future for the major ports. Furthermore, future works should consider other possible near-field tsunami sources, such as the Murray Ridge, Minab-Zendan and Sonne faults and far-field tsunami sources, such as the Sumatra-Andaman subduction zone.

In this work, sensitivity and performance of the Weather Research and Forecasting (WRF) model for surface wind field simulations are evaluated under several initial and boundary conditions, along with different planetary boundary layer (PBL) schemes during several dates over the Persian Gulf region. Since there are differences between production approaches and development periods of analysis and reanalysis data (namely, assimilation system) on the one hand, and differences in applied method for PBL parameterization by any scheme on the other hand, this paper aims to identify a suitable set up among the whole configurations which are under examination. To this end, three datasets (two reanalyses and one analysis) including, ERA-Interim, NCEP-R2, and NCEP-FNL and six PBL schemes (two local and four nonlocal) including ACM2, BouLac, MYJ, MYNN, QNSE, and YSU accompanied by their relevant surface-layer schemes are used. To assess the WRF model wind simulations, available observational wind data including 22 synoptic weather stations located in the region and observations of QuikSCAT and ASCAT satellites are employed. Findings of this study indicate that when the wind simulations are compared with synoptic weather stations observations, irrespective of the type of PBL scheme, ERA-Interim and NCEP-FNL datasets exhibit better performance in comparison with the NCEP-R2 and when PBL schemes are also considered, results show that combination of YSU scheme and ERA-Interim reanalysis data leads to a better estimate of wind speed and combination of YSU and NCEP-FNL data generates less error for wind direction. Moreover, comparison of model wind simulations and observations of QuikSCAT and ASCAT satellites shows that there are no substantial differences between various configurations. However, using YSU and ACM2 scheme s, WRF model generates speed and direction of the wind close to the observations of QuikSCAT. Although all tests have almost similar results as ASCAT satellite observations, YSU scheme estimates are slightly better than other schemes. Overall, the results of this study revealed that the major difference between WRF wind simulations and measured winds arises from the choice of initial conditions data and it does not depend on different PBL schemes. Consequently, changing initial and boundary conditions data has a noticeable impact on the model wind results. Thus, in future studies, emphasis must be more on reanalysis and analysis datasets and the option of WRF PBL parameterization schemes should be the second priority. Due to the fairly good similarity of the model surface wind with QuikSCAT and ASCAT observations, the choice of WRF model simulations as offshore wind database can be a valid available alternative instead of QuikSCAT and ASCAT wind, particularly when meeting their limitation in spatial resolution (swath data) or temporal sampling.

Characterization of siliciclastic reservoirs from seismic data is very sensitive to their clay and shale content. Clay can affect P-and S-wave velocities through its type and shape as well as location. Such an elastic behavior of clay infers that reservoir properties can be overlooked if their clay content is not understood and interpreted adequately. Clay can exist in different types with their specific shape and even can be distributed within siliciclastic rocks in various forms-structural, laminar, interstitial and dispersed clay-with different velocity responses. The mixture of the various clay types and forms can make their velocity interpretation for reservoir properties more complicated. Therefore, a proper strategy to separate the effects of clay types and clay forms is necessary for any seismic reservoir characterization on siliciclastic reservoirs with high clay content. Rock physics is a bridge between seismic and reservoir properties. An important goal of this branch of science is to understand the physical properties of the reservoir, so it is important for this kind of study. This study integrates rock physics modeling and simultaneous seismic inversion in order to find different clay distribution (forms) in one of the oilfields in the western part of the Persian Gulf. The well log data (wells A, B, and C) from this field show how the reservoir quality varies within the field with no obvious relationship to their shale content. This independent behavior of shale content and reservoir properties could be an indication that clay distribution may vary and clay type is not the only parameter for clay effects on the reservoir properties. Therefore, Thomas-Stieber rock physics template, first, is used to characterize shale distribution at well location and then the same template is applied on the reservoir properties derived from simultaneous seismic inversion to understand clay distribution in the whole area. Our results confirm that at wells, clay distribution is varying from top to the bottom of the reservoir. We find out that reservoir quality is not changed within the bottom part of the reservoir with high clay content (due to structural clay) while the same clay content reduced reservoir quality in the top and middle parts of the reservoir (due to the dispersed and layered clay). In order to do reservoir characterization, a map of shale content from top Ghar and top lower Asmari is generated. This generated map differentiates proper reservoir interval from the non-reservoir interval. Therefore, by using the proposed method in this study, one can delineate the potential zones of the reservoir for the future plan of drilling and production.

The study of variations of damage intensity for different earthquakes indicates the importance of the site effects on earthquake induced damage and ground motion parameters such as peak ground acceleration and induced amplification. In the occurrence of an earthquake, local site conditions such as soil characteristics, dimension of topographic irregularities, seismic bedrock depth, etc. as well as characteristics of incident wave have important e ects on seismic ground responseLocal site effects play an important role in earthquake-resistant designs and should be assessed carefully. By assessment of induced damages to structures and major infrastructures, seismic geotechnical researchers have concluded that the site conditions significantly influence on the failure distribution in urban and rural areas. Consequently, to determine the characteristics of seismic motions of the ground, it is essential to study the effective geotechnical factors. Consideration of the effects of the site response in the design of civil structures systems is of important to mitigate the damages to a certain extent on structures and the environment. Hence, it is relatively crucial to reliably attain the dynamic soil parameters of an earthquake-prone city/state. The nature of local site effects can be explained by using different methods such as simple theoretical analysis of ground response, measuring of surface and subsurface actual motion at the same site, and measuring the movements of the ground in sites with different conditions in comparison with the proposed site. In this paper, the effective factors on ground motion and site response in the soil classes II and III for Tehran city under the influence of Tabas and Bam earthquake were reviewed with numerical method and by using the finite difference software (FLAC 2D). Some of the effective parameters of the site dynamic characteristics on the seismic response of the ground surface have been studied and the response of peak ground acceleration and the resonance function has been compared. The results of this study show that at both sites, the horizontal maximum acceleration of the ground surface increases with respect to the bedrock shear velocity, reflecting the influence of the site on ground movement. At both sites, with increasing soil internal friction angle, the maximum acceleration at ground level increases and the acceleration response decreases. Also, as the shear wave velocity increases, the horizontal maximum acceleration at ground level for both structures decreases. Increasing the shear wave velocity in the surface layer, assuming the thickness of the layer is constant, results in a decrease in the natural frequency, which is far from the normal frequency range of conventional structures in Tehran. This indicates a decrease in the earthquake's dynamic force on surface structures. The natural frequency of five to seven story buildings in Tehran is about 0. 5 to 0. 7 Hz. As the surface layer thickness and bedrock depth increase, the horizontal maximum acceleration of the ground surface also decreases, meaning that the structural response can also be reduced.

Earth gravitational field interacts closely with a variety of natural phenomena. Therefore, studying and modeling the gravitational field and keeping tracks of its changes over time can make a huge contribution to the related geophysical and environmental researches. A well-proven method which is used in dynamic estimation of time varying state parameters is "Kalman filter". Kalman filter exploits dynamical properties of the states and noisy measurements to estimate the optimal state for every time epoch, in its filtering scheme. Smoothed properties of the involved dynamics, the accuracy of the noisy measurements, pay for noises in measurements and incompleteness of the interchangeable dynamics provide optimal estimates of state for the dynamic system. "Standard Kalman Filter" is performed to acquire the optimal solution when dealing with linear dynamics with white noises in both dynamics and measurements while a non-linear system cannot be dealt with in the same way. Various versions of Kalman filter have been introduced so far, including Extended Kalman Filter (EKF) and Unscented Kalman Filter (UKF). The former uses linearization to derive locally-linear dynamics while the latter is the approximation of the probability distribution of the states using sampling points of the state vector which are commonly referred to as "sigma points". Due to the linearization involved in EKF, the method does not give precise solutions when there are non-linearities of high orders while UKF solution is significantly better in the sense of precision, accuracy and optimality. In this paper, the 9-points Newton numerical differentiation is applied on the range values calculated from the relative state vector to reconstruct the so-called KBR range-rate observations and EKF is performed along with positions of observations to give an optimal estimate of the state vector at each time epoch. The calculated range-rates from both methods are then compared statistically. Then, UKF is used to fuse position and range-rate observations and to estimate state vector. Finally, the residuals of the reconstructed range-rate observations of three methods are compared statistically. The results show a relatively significant improvement in KBR observations reconstructions for UKF estimated states. To re-evaluate the superiority of UKF from the practicality point of view, the reconstructed range-rates are used to estimate the harmonics coefficients of geopotential model in a field recovery problem using "acceleration method" in a closed-loop simulation for EGM96. The degree-variances and differences in geoid heights are calculated and plotted. The results show a near 5 to 10-times fold improvement in gravitational field recovery using UKF results compared to the other two mentioned methods.

The global gravity models (GGM) are combined with the surface gravity data to geoid determination in removerestore scheme. In the remove step, the residual gravity anomalies are computed by subtracting the long wavelength signal of gravity anomalies, computed from GGM, as well as the gravitational effect of topographic masses. In next step, the residual anomalies are downward continued (DWC) into the geoid/ellipsoid surface for solving the Stokesian boundary value problem. In restore step, the long wavelength of geoid and indirect effect of topography are restored. The main goal of the present paper is to study the truncation error of spheroidal Poisson’ s integral. The comparison of truncation coefficient of full and spheroidal kernel shows that the truncation error of spheroidal kernel is at least 500 times of full kernel. As a result, modification of the kernel using spheroidal Poisson kernel is vital for DWC. Since the Poisson kernel depends on height, the modification must be computed for individual observation height. The computation of modification coefficients for all observations needs long computational time. To overcome this problem, they can be interpolated using suitable pre-computed coefficients of few reference altitudes. To escape from time consuming modification process, we proposed a fast and accurate method based on the full kernel. This method uses the orthogonal property of Legendre polynomial. For numerical test, the proposed method was applied in Iran within latitude band of [25∘ , 40∘ ] and longitude band of [45∘ , 60∘ ]. To test the effect of the truncation error on DWC accuracy, Helmert gravity anomalies corresponding to spherical degree 281-2160 were synthesized using EGM2008 and spherical harmonics of the topography on both Earth’ s surface and geoid. The truncation error of full, spheroidal and modified spheroidal (using Molokensij method) were evaluated for integration radius 𝜓 􀬴 = 0. 5 and 1 arc-deg. Our results show that for both radii, truncation error of full and modified kernel is about hundreds 𝜇 Gals, whereas these values can reach to several mGals for spheroidal kernel. Numerical results show that large truncation error yields the wrong results of DWC with spheroidal Poisson kernel. Also, the results show the good performance of proposed method in comparison with Molodenskij modified kernel.

Changes in seismic signal properties related to the production of hydrocarbon, substituted fluid and reservoir conditions, suggest the seismic reacquisition for monitoring of the hydrocarbon fields during the injection scenarios. Due to the fact that many Iranian hydrocarbon reservoirs are in their second half of the production life, this possibility must be investigated at certain time intervals (in order to carry out 4D seismic survey). Therefore, the rock physics model is constructed using the petrophysical data to calculate the elastic properties of the rock and then, using seismic forward modeling, the calculated elastic properties are related to seismic properties of the reservoir. Finally, the possibility of the seismic reacquisition will be examined based on the seismic properties changes in the reservoir. In this paper which investigates one of the Iranian sandstone oil reservoirs, two rock physics models of Gassmann and the Hertz-Mindlin using the lower limit of Hashin-Shtrikman were used to calculate the elastic properties of the dry rock. Regarding the most significant changes in the seismic amplitude, replacing 30% of oil with gas is considered as a selected scenario for fluid substitution. Compared to the in-situ conditions, after replacing the fluid, the Gassmann model with homogeneous saturation and the Hertz-Mindlin using the lower limit of Hashin-Shtrikman show compressive wave velocity reduction of 17% and 13%, respectively, while the reduction of compression wave velocity in heterogeneous saturation is about 3%. The results of seismic forward modeling show that if the distribution of fluid in the reservoir is homogeneous and about 30% gas is substituted by oil, the change of seismic signal in the reservoir and the time delay created in the underlying reservoir layers will be observable and seismic monitoring will be applicable. However, if the distribution of the fluid is heterogeneous, this will be possible by replacing more oil with gas.

In current study, earthquakes occurred in northwest of Iran in the past 20 years were relocated using JHD algorithm. A dense seismic network is usually required to produce a stable and precise earthquake location, i. e., improved quantitative and qualitative earthquake data are achieved due to various parameters including low amount of azimuthal gap. Combining data from different seismic networks in the study area can be effective and useful to improve earthquake locations. Thus, a list of earthquakes in the northwest region of Iran was developed using data available from different seismic networks. The recorded earthquakes data by the seismic network stations of Iran and neighboring countries were collected and a complete data set of earthquakes in the region was prepared. Then, the information from 30, 000 recorded and reported earthquakes in the area was compiled. This information includes more than 400, 000 seismic phases from databases of International Seismological Center (ISC), Iranian Seismological Center (IRSC), Broadband Iranian National Seismic Network Center (IIEES) and National Seismic Network of Turkey (DDA). This dataset was sorted and the outliers were taken off based on analyzing the travel-time residuals. In the next step, the recorded phases of an earthquake from different stations were combined and a homogeneous catalog was established. This catalog is more complete than the original catalog, e. g., because of increasing the number of phases, azimuthal gap of earthquakes has decreased. In the next step, after the preparation of the data, the hypocenter relocation was done using single event and the JHD algorithms in VELEST computer program. Comparison of RMS of earthquakes indicates a decrease in the RMS due to relocation process. The average value of RMS for the entire earthquakes reduced from 0. 69 s to 0. 41 s at the final stage, and this shows a reduction in difference between calculated and real earthquake location. The RMS value for more than 90% of earthquakes is less than one second. Thus, a homogeneous catalog of earthquakes in the region was prepared and new results were evaluated and compared with the initial information. Finally, the seismicity patterns in the region were investigated by drawing the hypocenter depth sections in the active seismic areas. Based on the obtained results in many sections, the removal of unrealistic structures due to systematic errors in earthquake location was evident, and in some sections, scattering of the earthquake hypocenter locations reduced and the existing structures were more obvious.

Providing reliable monthly to seasonal forecasts of the climate system using regional climate models with a relatively high spatial resolution is essential to reduce the socio-economic impacts of extreme climate events. In this study, performance of the Regional Climate Model version 4 (RegCM4) with four different sub-grid scale parameterization schemes in simulating 2-m temperature and precipitation over Iran against the observational Climate Research Unit (CRU) dataset in 2010 is evaluated. The climate forecast system version 2 (CFSv2) data are used as initial and boundary conditions for RegCM4. Analysis of 2-m temperature biases of these four simulations by RegCM4 indicated that the largest negative bias is located in the southern coastal plains of the Caspian Sea and over the Alborz Mountain, while the largest positive bias is located over Dast-e Lut and southern Iran. It is found that the largest temperature biases over Iran mostly occur in late spring, during summer and early autumn. Analysis of absolute monthly mean 2-m temperature biases indicated that in two of the conducted simulations the bias is lower, in both of which the Holstlag boundary-layer scheme is used. The root mean square errors (RMSE) of 2-m temperature for these four simulations are also examined and it is found that the Holstlag boundary-layer scheme and UW PBL perform better in cold and warm months, respectively. Using a combination of the Tiedtke convection scheme and the Holstlag boundary-layer scheme significantly reduces the RMSE of 2-m temperature in warm months of the year and leads to the least bias during the whole year over Iran. Thus, to conduct simulations with RegCM4 over Iran, the Holstlag boundary-layer scheme in cold months of the year and the same boundary-layer scheme along with the Tiedtke convection scheme during the whole year are recommended in order to have the least 2-m temperature biases. In the conducted four simulations, positive precipitation biases are observed over Iran in most months of the year, suggesting that RegCM4 generally overestimates precipitation over Iran. Results also indicated that the largest correlation between the observed and simulated precipitation is seen over southeastern, eastern, central, western and northwestern Iran, while the least correlation is seen over the Alborz Mountain, western foothills of the Zagros Mountains, western parts of southern Iran and some parts of Dasht-e Kavir. It is also found that in the RegCM4 simulation with the Tiedtke convection scheme, there are larger spatial and temporal correlations between the observed and simulated precipitation.