IRANIAN JOURNAL OF GEOPHYSICS
Year:2009 | Volume:2 | Issue:2|Papers Count:9

SKS phase passes through the mantle as an S wave; it converts to a P wave at the core-mantle boundary (CMB) beneath the source, travels through the fluid outer core, and then converts back to an S wave at the CMB on the receiver side. When a shear wave (SKS) propagates through an anisotropic media, it is partitioned into orthogonally polarized fast and slow shear waves. This property is referred to as shear wave splitting. In this study we used shear wave splitting of SKS phase from teleseismic events to identify and estimate the shear wave anisotropy in the upper mantle of the Alborz region.In this study, to find stable splitting measurement with small error, we selected the automate analysis window by performing a grid search over 210 windows. The selected teleseismic events(M³5.5 and D³65were recorded by 3 broadband seismic stations, DAMV, CHTH and THKV (belongs to IIEES network) and by 5 short period seismic stations, FIR, DMV, AFJ, MHD and GZV (belongs to IGTU network). They used to determine anisotropy parameters in upper mantle and crust beneath the Alborz region.In this study the average direction of anisotropy in mantle and crust of the Alborz region is estimated about 48±12 degree by study on shear wave splitting of SKS phase. Thus, the fast azimuth of anisotropy oriented approximately in NE-SW direction too. Moreover our results showed that the magnitude of anisotropy is about 1.5±0.4 seconds.

The aim of the study is to evaluate the return period and risk of drought duration in five ancient and unique stations of Iran, having more than one-hundred annual precipitation data. For determining the return period and risk of drought in the stationsof interest (including Jask, Bushehr, Isfahan, Mashhad and Tehran) the following stages were performed: a) Applying a homogeneity test (i.e., Run test) on annual precipitation series was indicated the Jask station unlike other stations which were homogenous, is nonhomogenous, b) Using a goodness-of-fit test (c2), the most suitable distribution function for annual precipitation data was selected from among fourteen functions, c) Based on the best-fit function obtained for each station, the amount of precipitation for seven probability levels (Thresholds) from 0.1 to 0.7 (with step 0.1) were determined, d) With respect to the precipitation of each probability level and subtracting it from annual precipitation data during record period, dry periods were indicated and drought duration for each dry period was computed, and e) The two statistical characteristics of drought including return period and risk were calculated with different mathematical equations. The results based on the annual precipitation series of four aforesaid stations showed: a) For a given drought duration, as the probability of occurrence of precipitation increases the return period decreases, or with a certain probability of occurrence of precipitation, as increasing drought duration the return period also increases, b) For the probability of occurrence of drought 30 %, the longest drought in Isfahan (i.e., six-year duration) and other stations (i.e., three-year duration) occur once each 2000 and 51 years, respectively, and d) for a certain probability of drought occurrence, as increasing the basic period, the risk of drought increases. For instance, for the probability of occurrence of drought 50 %, it was identified that the drought risk in Bushehr and Tehran station is more than Isfahan and Mashhad.

An analysis of annual rainfall over Iran for a period of 30 years (1971-2000) shows a significant variability in spatial and temporal distributions of rainfall as well as its frequency and intensity. In this paper, the likely effects of ENSO on the rainfall anomalies are investigated. The accumulated monthly and annual rainfalls from 43 synoptic stations are taken through Islamic Republic of Iran Meteorology Organization (IRIMO) and the ENSO activity years are obtained from NOAA. Three cases of strong cold phase (La-Nina) and three cases of strong warm phase (El-Nino) of ENSO in the period of this study havebeen selected. Then, the distribution of annual rainfall anomalies for these cases is studied. Also, the relation between the annual rainfall anomalies and Oceanic Nino Index (ONI), from the sign and magnitude view point, for the selected cases is examined. These investigations have been done for both solar and agriculture (from autumn of one year to the next) years. In addition, the correlation between the ONI and the annual rainfall of the 43 stations for the total 30 year period of study is calculated.The results of the distribution of annual rainfall anomalies for the above cases indicate that there is no certain relation between the rainfall anomalies over Iran and the different phases of ENSO activity. The comparison of annual rainfall anomalies and the ONI anomalies shows that although there is consistency between the signs of anomalies in five of the cases, but the magnitudes are substantially inconsistent. For example, the most intense El-Nino is associated with the least positive rainfall anomalies and the most intense La-Nina is associated with positive rainfall anomalies, instead of a negative anomaly. The weak correlation between the ONI and the annual rainfall of the 43 stations for the period of 30 years confirms the above results.The key result is that it is not possible to predict the sign and magnitude of annual rainfall anomalies having the phase of ENSO activity. The possible reason being that ENSO has a complex and nonlinear interaction with other phenomena such as Monsoon, North Atlantic Oscillation (NAO), and Madden- Julian Oscillation (MJO).

Vertical derivative is used in interpretation of potential field anomalies extensively. This filter enhances the details and sharpens anomalies. However difficulty is that by this filter the noises are increased as signals are enhanced. This filter is used with first and second orders normally. However recently vertical derivative is applied with non – integer order, in order to produce a good equilibrium between signal and noise. This method is preferred when anomaly variation throughout the survey area is uniform. In this regard in areas with different types of data compiling a map with different order of vertical derivative is the best methodology. This method is based on calculating standard deviation on a moving window over the study area. In this paper the application of this method on vertical prismatic model and over aeromagnetic data of Abadeh region are discussed.

This paper is devoted to the study of the differences in distribution of some important meteorological quantities over the South West of Asia (SWA) between the positive and negative phases of the North Atlantic Oscillation (NAO). The data used are winter time gridded fields of wind as well as geopotential height at different levels and mean sea level pressure (MSLP), each at four times a day, taken from the NCEP/NCAR reanalysis data sets for a 55-year period from 1950 to 2005. The spatial resolution of datais 2.5×2.5 in longitudinal and latitudinal directions covering a grid from 90W to 90E in longitude and from 20N to 80N in latitude. The 168 months of this period are divided into two categories of critical months: 29 positive and 33 negative NAO months. For a selected number of meteorological quantities, the ensemble mean of the anomaly fields at critical positive and negative months are calculated. A lowpass filter is applied to the 500hPa geopotential height to filter disturbances with periods of less than 10 days and then the ensemble mean of the anomaly of the filtered 500hPa geopotential height as well as the outgoing long-wave radiation are calculated for the two above groups of months.For the selected quantities, the results show that the differences between the two phases of NAO take their maxima over the North Atlantic Ocean and fall to much lower values over the SWA. In critical positive (negative) months there is a cold (warm) air anomaly over the North Atlantic that is due to the differences of flow anomaly between the two phases of NAO. The mean wind-speed of the subtropical jet at 300hPa level over the SWA in critical positive months is about 8 ms-1 greater than that in critical negative months. The intensification of the subtropical jet at positive months is expected to lead to the formation of stronger than normal cyclones over the SWA. The corresponding differences between the patterns of outgoing long-wave radiation indicate a southward displacement of the Atlantic storm track in negative phase associated with which there is a noticeable transport of warm and humid air to the Mediterranean region and south of Europe. In most of the SWA, the cloudiness is higher over the negative phase than the positive phase. With regard to streamline maps at 700hPa, the sources of the observed changes in cloudiness in the south and north of the SWA seem to be the Indian and Atlantic oceans, respectively.

During the past two decades, marine researches have focused on gas hydrates because of many reasons that the most important of them is future energy resource potential. Gas hydrates are ice-like crystals that form a rigid cage of water molecules and entrap hydrocarbon and non-hydrocarbon gas by hydrogen bonding. There are many methods for prospecting gas hydrates. Seismic methods are the most applicabletools to study them. The presence of the suitable thermodynamic conditions, gas and water in appropriate amount and gas migration pathway from depth to sea floor sediment are necessary for growth and stability of gas hydrates and evaluation of these conditions are necessary for prospecting gas hydrates. The occurrence of gas hydrates and free gas in host sediments changes the elastic properties and makes them detectable with seismic methods. The important seismic indicators of gas hydrates are bottom simulating reflector, flat spot and bright spot. Therefore, the presence of the gas hydrates in host sediments are detectable with seismic indicators and the study of the gas hydrate anomalies are applicable with seismic attributes. In this article, the appropriate conditions for growth and stability of gas hydrates in Oman Sea were examined with 2D seismic sections, the occurrence of the gas hydrates in sediments was proven with seismic indicators and the properties of their indicators were studied with reflection strength and apparent polarity attributes. Also the effect of the gas hydrates in attenuation of seismic wave amplitude was studied with spectral decomposition attribute. The results of this study prove the presence of gas hydrates in Oman Sea.

In this paper, seismic zonation of peak ground acceleration (PGA) in the damaged areas of 2005 February 22 Dahooiyeh-Zarand earthquake is presented. The PGA is evaluated in the damaged areas with both interpolation technique and attenuation relationships. In the interpolation method, PGA is estimated in different regions using inverse distance weighting (IDW) method and recorded PGA at the stations located in meizoseismal area of Dahooiyeh-Zarand (2005) earthquake. The optimum power of IDW method for best estimation of PGA is decided from the root mean squared error statistic. Then, IDW is applied to estimate PGA in the damaged areas where no records of PGA were available. In the second step, The PGA is estimated in the damaged areas using attenuation relationships of both Campbell and Bozorgnia (2008) and Fukushima (2003) attenuation relationships. Furthermore, the attenuation relationships were corrected by minimizing the difference between estimated and recorded PGA at the recording stations, where the PGA of mainshock was available. The results revealed that the estimated PGAs from the Fukushima (2003) attenuation relationship provides more accurate results than those derived from the other methods.