The mixture of natural and artificial seismic sources with random distributions cause diffuse wave field with random amplitudes and phases called noise. When noise is analyzed in a long-term process, it contains surface waves which are spread in all directions. Thus, ambient noise contains data relevant to the surface waves. In recent years, as broadband seismic networks have been distributed vastly around the world, diffuse wave fields are utilized to obtain surface waves. The data of the fields are recorded in the forms of seismic ambient noise and waveforms. Seismic waveform is created as a result of multiple diffuse seismic waves of heterogeneous areas, while seismic ambient noise is caused by many types of sources such as ocean microseisms , atmospheric turbulences (Tanimoto, 1999), storms, volcano erroptions and so on. Recent studies suggest that surface waves extracted from diffuse wave fields and seismic waveforms are according to the Green function (Wapenaar, 2004) .Although, the horizontal to vertical spectral ratio technique of microtremor measurement is widely applied in microzonation and site response studies during past two dacays. but the goal of this kind of geotchnical studies is different from seismologcal noise investigations.For the first time, Campillo and Paul (2003) have calculated group velocity of Rayleigh and Love surface waves from waveforms of 101 teleseismic earthquakes recorded in the national Mexican seismic network. After that investigation of ambient noise for Green function analysis have been continued by means of Shapiro and Campillo (2004, 2005) , Schuster et al., (2004) , Snieder (2004), Bensen et al.(2007) , Wapenaar et al.(2013), Javan and Movaghari (1392). They showed it is possible to get the Green function between stations through calculating Cross Correlation Function of recorded noise. Characteristics of seismic ambient noise are independent of occurring earthquake. That’s why ambient noise is used widely and provides the opportunity to do imaging without a source, or passive imaging in order to study crustal structure between two stations. More applications include terrestrial and solar seismology, underwater acoustics, and structural health monitoring (Larose et al., 2008).In this article, we are going to compare velocity structure created by surface waves of ambient noise and earthquake surface waves based on waveforms from IIEES broadband seismic stations. Braod band seismic stations are usually installed in quiet locations some distance from significant sources of cultural noise, such as roads, railroads, and machinery. We analyze seismic noise using continuous 50 sample/s from one year data. Using recorded ambient noise in Tabas, Sharakht (Qaenat), Zahedan, Chabahar, and Bandar Abbas broadband seismic stations, the Green function of surface waves between each pair station was obtained by cross correlation technique and dispersion curve was calculated through frequency-time analysis. According to this curve, a 1-D model of velocity structure between two stations was presented. There has been a comparison between this model and the one acquired from May 11, 2013 earthquake occurred in the north of Jask at the south of Iran. The results show that we can use the ambient noise to study crustal velocity structure and upper mantle as well. Therefore, it is necessary to record ambient noise continuously in seismic stations so as to prepare fundamental research in seismology.

The Makran zone in southeastern Iran and southern Pakistan is the result of subduction of oceanic crust of the Arabian Plate under the Eurasian Plate. From seismic behavior point of view, there is a distinct segmentation between the western and eastern parts of the subduction zone. The western part of the Makran has an abnormally very low level of deep seismicity with lack of recorded great earthquakes, while the eastern part has experienced many great earthquakes. Another difference between the western and eastern parts of the Makran region is that the distance between the Quaternary volcanic arc and fore-arc setting is larger in the east than in the west. Understanding the nature of unusual behaviors of the Makran subduction zone has long been one of the biggest challenges in seismotectonic investigations of this region. The present study aims at producing high-resolution love-wave velocity structure maps of the crust and the upper mantle in the Makran subduction zone using ambient seismic noise. To achieve this purpose, a large dataset has been provided to produce tomographic maps. Empirical Green’s functions were obtained from cross-correlations of broad-band seismic noise records at different stations inside and outside the region. Love-wave velocity dispersion curves were then extracted from the ambient noise, and finally converted into a 2D group velocity image (or tomography map) for crustal and upper mantle structures of the region.

Introduction: Every year, numerous casualties and a large deal of financial losses are experienced due to earthquake events. The losses incurred by an earthquake vary depending on local site effect. Some well-known examples include the earthquake in Caracas 1967, Mexico city 1985, Kalamata 1986, Loma Prieta 1989, Roodbar 1990, Bam 2003, Jammu and Kashmir 2005, Sichuan 2008 and Haiti 2010. Therefore, in order to conquer drastic effects of an earthquake, one should evaluate urban districts in terms of the local site effect. Various methods are available for the evaluation of site effect. One of the most common methods includes ambient noise survey. Today, this approach is being used as fast, applicable, cost-effective method. Ambient seismic noise are feeble ground motions with displacement amplitudes of about 0. 1– 1 μ m and that can be detected by seismograph with high magnification. Many investigations have been conducted to determine the nature of ambient noise. One of the possible sources of ambient noise can be human activity, such as traffic, industrial noises and nature activity, such as wind, ocean waves. The Babol city is one of the largest cities in the north of Iran (Mazandaran province). It lies on alluvium beds in the region presenting a high seismic potential. Therefore, comprehensive studies are necessary to introduce suitable solutions for minimizing earthquake damage and loss of life. For this reason, in Babol city, ambient noise survey has been performed at 60 stations and the obtained data were analyzed with Nakamura or H/V method (1989). The results were compared with local geological, geotechnical and seismic data to confirm their reliability for a seismo-stratigraphic. Methodology and data collection: The analysis of ambient noise was initially proposed by Kanai and Takana (1961). Since then, many researchers have used ambient noise for site effect evaluation. As it is said before, one of the most popular techniques for estimation of site effects in the regions with low seismicity is ambient noise survey by Nakamura or H/V method (1989). Based on the literature review, the Nakamura method (1989) has been used in many places. Many theoretical and experimental studies show that, this method has the capability of estimation of fundamental frequency. Ambient noise survey was carried out at 60 stations in Babol city. Ambient noise was recorded using a velocity meter SARA. Two horizontal and a vertical components of ambient noise at each location are recorded for duration of 15 min with 100 samples per second. Because the environmental noise has an effect on ambient noise they are recoded between 10 p. m. to 6 a. m. The locations were determined by using GPS at the sites. The ambient noise survey in this study was made in compliance with the guidelines of SESAME (2004). Results and discussion: The maximum and minimum values of fundamental frequency of the present research are 11. 4 and 0. 65 Hz, respectively. Also, the maximum and minimum values of amplitude of H/V peaks have been calculated as 3. 71 and 1. 19, respectively. The most significant point is that the fundamental frequency of the major part of Babol city is smaller than 1 Hz in agreement with the previous knowledge of the city’ s geological setting. Another relevant point is the presence of some stations with very high (> 5 Hz) fundamental frequencies. In these cases, ambient noise recording and data analysis were repeated, but similar results were obtained. Considering the lack of sufficient geotechnical data (in some stations), the above phenomena need to be integrated with other methods. For this purpose, the data of electrical resistivity tomography (ERT) were used. The ERT showed that small regions of the north-west, west, and south areas have high resistance values probably related with the presence of hard deposits in the shallow subsoil. Babolrood river diversion in the west part and its return to the previous direction in the northern section is possibly due to the existence of these relatively hard deposits. By comparing these two tests, we observed that the ERT results correlate with the ambient data analysis. So Therefore, we can conclude that the high-frequency peaks measured are reliable, but we need direct investigation to associate them to a specific shallow geological layer. To validate the results, fundamental frequencies obtained from ambient noise survey were compared with geotechnical data, numerical analysis and seismic data in the study area. A general review shows that the geotechnical data, equivalent-linear analysis results and seismic data have an acceptable conformity with the results of ambient noise survey. Conclusion: The results show minimum and maximum fundamental frequencies 0. 65 and 11. 4 Hz, respectively. Assessment also reveals that the major parts of Babol city have the fundamental frequencies less than 1 Hz, which are in conformity with that of previous research. According to the results of seven cross sections, it can be concluded that fundamental frequency variations are in line with the geotechnical and geological data in the study area. It means that this method is the appropriate way to assess the local site effect in the Babol Ccity. It is also observed that besides the soil layers, the soil stiffness and its shear wave velocity are effective factors in changing the fundamental frequency. Site frequencies were also estimated by preliminary 1-D site modeling using the equivalent-linear method. In general, a reasonable correspondence between the methods was obtained. Using seismic data, the HVSR of two strong ground motions have been calculated and the results have been compared with the nearest ambient noise recording station. Analyzing the spectral ratios demonstrates that the value of the fundamental frequency obtained by the H/V method (1. 06 Hz) is very close to that of frequencies obtained by the seismic data (0. 95 and 0. 90 Hz).

In this research, we measured underwater ambient noise in Nowshahr Port using standard devices and analyzers during a 24 hours period. Proper processing capabilities of analyzers made some comparisons between obtained noise level spectrum and noise generating sources through related frequency range. Then analytic evaluation of importance of these sources in generating noise and its transmission was performed. Results showed that although Nowshahr Port is not a busy area in Caspian Sea referring to statistical reports, but measured underwater noise levels showed the contrary, a high shipping traffic. The relatively high level of noise compared to shipping traffic, can be because this port is located in shallow waters and affected by coastal boundaries that made resonance in underwater sound transmission.

The crustal structure study based on ambient noise measurements has become a popular, fast and reliable method in earthquake seismology in recent years around the world. Generally, not only in seismology but also in other applications which deal with signals, accept noise as an undesired component of the signal. It is believed that noise obscures data and does not contain useful information. Ambient noise measurements promise significant improvements in the resolution and accuracy of crustal and upper mantle images. Traditional dispersion analysis, however, does not yield reliable estimates of the structure in the shallow crust because of strong scattering at short-periods (T<30). Recent advances in surface-wave ambient noise tomography (e. g., Shapiro et al., 2005; Sabra et al., 2005; Yao et al., 2006; Yang et al., 2007; Lin et al., 2007) greatly enhance our ability to resolve the shallow crustal structure. In this study, we apply ambient noise tomography to image and investigate the shallow shear velocity structure of the upper crust beneath south west of Tehran area. Data from seven stations of Iranian Long Period Array (ILPA) are acquired from IRIS free data center. These data were recorded during 1975 to 1977 in corporation of the FDSN with the Institute of Geophysics at University of Tehran. At the moment, all continuous seismic waveforms are available for researchers. After obtaining the continuous waveforms, we preprocessed and segmented the data into one hour time windows. Hourly cross correlation of ambient noise between all station pairs were calculated and group velocity of Rayleigh waves dispersion curve in periods between 3 to 10 seconds are measured from the Green’ s function resulting cross correlations. To determine dispersion curves of surface waves we have used the Frequency-Time Analysis technique (FTAN). Because of using borehole seismometer in ILPA array and our new Gaussian noise selection proposed method; all Green functions had acceptable SNR ratio and greater than 10. Therefore, we predict the suitable and reliable result of Green’ s functions in comparison with ambient noise of free surface seismometers. Then using dispersion map in each period, we extracted a local dispersion curve for each grid point. Finally, the quasi-3D shear wave velocity model in the study area provided using nonlinear inversion procedure for each grid point of local dispersion curves by means of Shapiro et al., (2005) technique. By preparing different shear wave velocity profiles in the direction of NESW, of the studied area, we try to image the changing velocity variations and trends along the profiles, which can indicate the existence of one of the branches of the IPAK fault, or the existence of an anticline with the axis in this direction and the slope to mards the northeast direction. The results indicate a reliable image from upper crust of south east of Tehran region in consistent with the results of Doloei and Roberts (2003) from teleseismic P-waveform time domain receiver function (RF) method. Moreover, the upper crustal structural model proposed for this area is in agreement with surface geological setting. Therefore, we suggest that for isolating the ambient noise from temporary and very local conditions, the digging and covering the seismic stations prepares a suitable noise level for the crustal structure studies.

Volcanic eruptions are usually done by increasing magma pressure. Monitoring this process in real-time can provide useful information for predicting eruptions. The cross-correlation function of ambient seismic noises has been used many times to monitor the activity of volcanoes around the world. Still, this method is usually limited to volcanoes equipped with large networks and broadband stations.   In this article, a technique has been proposed that automatically and without the need for advanced equipment or lots of data can calculate a cross-correlation function of seismic waves, then using the calculated cross-correlation function, it analyzes and examines the temporary changes in the relative velocity of seismic waves as well as the anomaly of the amplitude ratio of continuous data recorded in all pairs of stations as two eruption attributes.The SARA (Seismic Amplitude Ratio Analysis) method was used to investigate the changes in the amplitude ratio, and the MWCS (Moving-Window Cross-Spectral) method was used to calculate the relative speed of environmental seismic wave data. Both methods have been implemented using MSNoise software package.   In order to validate these methods, the continuous data of 5 seismic stations near Etna volcano in Italy were used. First, in order to process the data, daily vertical recordings of all stations were divided into 30-minute segments. Then, the segments were demeaned, tapered and normalized to three times the root-mean-square (RMS). Next, the daily cross-correlation between all pairs of stations was calculated and the cross-correlation function was filtered in different frequency ranges. Finally, the velocity variations were measured with the MWCS method. The results showed a decrease of about 0.2 percent in the velocity before the main eruption and an increase rapidly after the eruption.   In the second step, the amplitude ratio of all pairs of stations was calculated with the SARA method. The increasing trend of the amplitude ratio was observed from three hours before the main eruption. To quantify the results, the Mann-Kendall trend analysis test was used for all pairs of stations. By using Sen's slope estimation test, the slope value of each figure was calculated separately. The results showed a temporary increase in the seismic amplitude ratio in 90 percent of pairs of stations before the main eruption. Automatic and continuous measurement of these attributes and combining their results can show the potential of this method to improve volcano monitoring and eruption early warning for active volcanoes.

In recent years, Vibration Based System Identification (VBSI) as a powerful tool to disclosure a mathematical expression of dynamic behaviors of structures, is taken into consideration for structure engineers. Among developed strategies for VBSI, the strategies identifying under ambient vibration tests without using input data, with no limitation in serviceability and no need to complex excitation tools, have been more desirable. In some cases, regarding to high numbers of Degrees Of Freedom (DOFs) and impossibility of recording in whole DOFs, it is necessary to identify physical characteristics beside modal parameters with recording in limited numbers of DOFs. Among those physical characteristics, stiffness parameter is more important. The main goal of this paper is to present a method for identification of story stiffness in shear type buildings using incomplete structural responses. At the first, the sub matrix of structural stiffness matrix is identified by the proposed method based on the structural dynamics theory and the realization theory-based Stochastic Subspace Identification (SSI) method and then story stiffness will be available. Since the presence of noise is imaginable in ambient vibration tests, effects of noise also been investigated. To evaluate the proposed method, a five-story analytical shear building is studied. Extensive analysis show the high ability and accuracy of proposed method in correct identification of story stiffness from incomplete output records even in presence of noise.

Because of the use of seismic ambient noise as a low-cost tool for researching subsurface structure and hazard assessments in recent decades, urban seismology has become an active research subject both with seismological objectives, as obtaining better microzonation maps in highly populated areas, and with engineering objectives, as the monitoring of traffic or the surveying of historical buildings. As a result, urban seismology has been used in the metropolis of Tehran, which is one of the world's most populated cities. The city is situated on the southern slopes of the Central Alborz Mountains on Quaternary alluvial deposits, and its southern section is situated on the northwest side of Central Iran's Great Desert, surrounded by active faults such as the Mosha Fault, North Tehran Fault and South Rey Fault. By using cultural noises caused by human activities such as traffic, subway, concerts, and rituals in metropolitan settings, urban seismology explains underlying structures, enhances seismic hazard management, and zoning. As a result of the high level of noise created by human activities in Tehran, as well as the presence of subterranean structures in this metropolis, the necessity of urban seismology and seismic ambient noise approaches is clear. The data from Tehran's accelerometer networks that named Tehran Disaster Mitigation and Management Organization (TDMMO) and the Road, Housing and Urban Development Research Center (BHRC) which are equipped with a Guralp CMG-5T three component accelerometer and the power spectral density-probability density function (PSD-PDF) approach by PQLX software, which was made available to the seismologist community in June 2004 were utilized in this study to analyze the spatial and temporal fluctuations of seismic ambient noise in the frequency domain. For this purpose, the period of 10 December to 24 December 2020 was investigated. In addition, the effect of the Corona virus pandemic on the frequency and time domain level of seismic ambient noise was explored in period of 16 March to 26 March 2019 and 15 March to 25 March 2020. This period was chosen because the ancient Nowruz festival is in this period. Finally, considering the energy level of cultural noise denoted by the large daytime/nighttime variation with large energy during working hours and much less during nighttime and weekends, most of the stations have been experiencing the lowest level of cultural noise between the hours of 3: 30-4: 30 AM across all three frequency ranges that include human movement, traffic and subway, and in terms of spatial variation, D011 station has been experiencing the lowest noise levels in all three frequency ranges due to its construction on a stone structure. Furthermore, investigations on the change of noise levels during the Corona epidemic revealed a drop of 1-4% and a rise of 1-15% for certain stations depending on their location in all three frequency bands.

Horizontal to vertical spectral ratio technique on single station ambient noise data, is a well-known technique in study of site effect. Recently, this technique is introduced as a tool for identification of shear wave velocity profile of soil beside its normal usage.Many studies in recent years showed that the ellipticity of the fundamental mode of Rayleigh waves can be obtained by reducing the Love and Body waves effects from the H/V spectral ratio. Based on the relation of the H/V curves and the ellipticity of Rayleigh waves and dependency of ellipticity to the shear-wave, this method can retrieve the S-velocity structure in a thick alluvial deposit.In this paper, HVTFA and RayDEC methods are used to retrieve the ellipticity curves for more than 140 single-station ambient noise measurements. The HVTFA technique based on time-frequency analysis with Continuous Wavelet Transform tries to reduce the SH-wave influence that is possible by identifying P-SV wavelets along the signal and computing the spectral ratio from these wavelets. It is assumed that the energetic points in time-frequency representation of the vertical signal is related to a single Rayleigh wave wavelet. The average over all wavelets defines as ellipticity.Based on random decrement technique, the Ray DEC method uses the vertical component as a master trigger and stacks a large number of horizontal and vertical signals from three-component records of seismic noise to obtain ellipticity curves.The right flank of ellipticity curves (from the first peak of curves to the next trough) were used in inversion, because numerical studies show that the right flank is the most reliable part of ellipticity, and the energy of the Rayleigh-wave fundamental mode strongly dominates in these frequency ranges.In the following, ellipticity curves were classified based on the f0 peaks and the right flanks in two ways; visual observation of similarities and k-means clustering statistical approach.Inversions process performed using the Neighborhood Algorithm based on the partition of the parameter space into Voronoi cells. The Voronoi decomposition of the parameter space is the base of an approximation of the misfit function, which is progressively refined during the inversion. The method uses prior information (initial parameterizations) and try to optimize the computation at the different stages of inversion.The results of inversion show the existence of the thick alluvial deposits in the northern and eastern parts of the city. For the southern parts, the method shows higher velocity and lower depth of bedrock. These results are in agreement with geological situation of the region, existence of mountainous area at the southern and western parts, and extensive alluvial plains at northern and eastern parts.