Computation Of Seismic Hazard Research Articles

Testing the applicability of ground motion prediction equations for the Hainaut region (Belgium) using intensity data

In regions where strong earthquakes occurred before the deployment of dense seismic and accelerometric networks, intensity datasets can help select appropriate ground motion prediction equations (GMPEs) for seismic hazard studies. This is the case for the Hainaut seismic zone, which was one of the most seismically active zones in and around Belgium during the twentieth century. A recent reassessment of the intensity dataset of the area showed that intensities in this region attenuate much faster with distance than in other parts of northwestern Europe. Unfortunately, this characteristic has not yet been taken into account in current hazard maps for Belgium and northern France. Based on this dataset, we evaluate the goodness of fit of published GMPEs with intensities in Hainaut by means of a ground-motion-to-intensity conversion equation (GMICE) and according to different metrics (Likelihood, Log-likelihood and Euclidean-based Distance Ranking) published in literature. We also introduce a new measure to specifically evaluate the distance trend. Our results show that none of the tested GMPEs convincingly fits the intensity dataset, in particular the fast attenuation with distance. Nevertheless, applying the few GMPEs that show a reasonable fit in seismic hazard computations, we observe a decrease of the influence of the Hainaut seismicity on hazard maps for Belgium and northern France. This result is compatible with the earthquake intensity observations for the last 350 years in this part of Europe.

Probabilistic seismic hazard assessment in Namibia

Namibia is located in Southern Africa on a Stable Continental Region (SRC). Despite the fact that a number of earthquakes have been observed or recorded in Namibia since 1910, with the July 31, 2009 and the April 4, 2021 earthquakes both of moment magnitude (Mw) of 5.6, no up-to-date country-wide seismic hazard study has been conducted. As a result, the purpose of this research is to conduct a comprehensive Probabilistic Seismic Hazard Assessment (PSHA) for the entire country in order to aid in planning and seismic risk mitigation. The earthquake catalogue developed for this study spans the years 1910–2021 and includes both historical and instrumental events. Data from the International Seismological Centre (ISC), the Advanced Network of Seismological System (ANSS), and Namibia's National Seismological Network (NSN) were used to create the catalogue. The catalogue was then declustered to remove fore- and aftershocks as well as similar events; the resulting catalogue was used in conjunction with available geological evidence to identify, delineate, and characterise 11 seismic source zones as well as one fault source, the Hebron/Dreylingen fault. Four Ground Motion Prediction Equations (GMPEs) were carefully selected and used in the seismic hazard computations. The GMPEs were implemented in the calculations using a logic tree formalism with equal weights, which assisted in addressing the uncertainties associated with both the seismic sources and the ground motion models. Peak Ground Accelerations (PGA) obtained for a 10% chance of exceeding in 50 years ranged from 0.017 g to 0.149 g. The highest levels of hazard were observed in Namibia's north-western, north-central, central and southern regions. This study also produced seismic hazard maps that show the distribution of acceleration at different response periods (0s (PGA), 0.1s, 0.15s, 0.2. s, 0.3s, 0.5s and 1.0s) computed for a 10% probability of exceedance in 50 years. The findings from this Namibia's first large-scale seismic hazard study are expected to make a significant contribution for future land use planning in facility and infrastructural development in the country.

Damping‐dependent correlations between response spectral ordinates

AbstractCorrelations between response spectral ordinates at different periods are used in several seismic hazard computations, such as for the construction of conditional mean spectra and conditional spectra. Conventionally, these correlations have been computed and reported only for a damping ratio of 5%; however, structures may have damping ratios substantially lower or higher than 5%. Therefore, in those cases, one requires correlations of spectral ordinates at different periods but having the same damping ratios that are different than 5%, correlations of spectral ordinates at the same period but having different damping ratios, or the general case of correlation between spectral ordinates of two oscillators having different damping ratios and different periods. This work computes such damping‐dependent correlations by using the NGA‐West2 ground motion database. In general, it is found that correlations increase as the damping ratio of any of the two spectral ordinates increases and as the ratio of periods of vibration of the two oscillators departs from one. A nonlinear regression model is fitted to the resulting damping‐dependent correlations to simplify future computations. Finally, the use of the new damping‐dependent correlations is illustrated by computing example conditional spectra for damping ratios differing from 5%. The results show that using 5%‐damped correlations for the construction of condition mean spectra, overestimates spectral ordinates for damping ratios lower than 5% and underestimates spectral ordinates for damping ratios higher than 5%.

Probabilistic characterization of the directionality of horizontal earthquake response spectra

AbstractEarthquake response spectral ordinates vary significantly with changes in orientation within the horizontal plane. This variation is characterized probabilistically in this study using a large database of recorded earthquake ground motions. For each ground motion record, response spectral ordinates are computed in all horizontal orientations as a function of the rotation angle with respect to the azimuth of maximum response and then normalized by (1) the maximum and (2) the median spectral ordinate from all these orientations. Nonlinear regression models are then fitted to the means, standard deviations, and correlations of both ratios, as a function of rotation angle. To achieve a more complete probabilistic description, probability distributions are fitted to both ratios at each rotation angle. These results can be used for several probabilistic seismic hazard computations, such as the sampling of response spectral ordinates at specific orientations within the same site.

Seismic hazard analysis using simulated ground-motion time histories: The case of the Sefidrud dam, Iran

This study aims at conducting probabilistic seismic hazard analysis for the Sefidrud dam located near Rudbar City. For this purpose, firstly, the characteristic earthquake recurrence model for major earthquakes on individual main faults have been combined with the smaller (smoothed) background seismicity of the region. Then, appropriate ground motion prediction equations were applied to estimate hazard values. Finally, in order to obtain reliable estimation of seismic hazard due to sources close to the dam site and to investigate near-field characteristics of motion, the Rudbar fault as the most prominent earthquake source in the immediate vicinity of the site is considered in seismic hazard computation using hybrid broadband simulation based ground motion parameters.The results of this method with different declustering schemes are reported for two level of seismic hazard analysis (i.e. return periods of 475 and 2475 years). Best estimate seismic hazard maps of PGA and PGV values obtained from the logic tree method is presented. By inclusion of simulation results for the Rudbar fault in the probabilistic seismic hazard analysis (PSHA), maximum PGA and PGV for 475 years return period obtained around 340cm/s/s and 25cm/s, respectively. For classic PSHA without including simulation the maximum PGA and PGV for 475 years return period obtained around 450cm/s/s and 32cm/s, respectively. With the simulation-based PSHA for a 2475 years return period a maximum PGA of 650cm/s/s and PGV of 50cm/s have been estimated. Classic PSHA (without simulation) for a 2475 years return period has resulted a maximum PGA of 850cm/s/s and PGV of 65cm/s.

A Statistical Investigation on a Seismic Transient Occurred in Italy Between the 17th and 20th Centuries

According to the historical earthquake catalog of Italy, the country experienced a pulse of seismicity between the 17th century, when the rate of destructive events increased by more than 100%, and the 20th century, characterized by a symmetric decrease. In the present work, I performed a statistical analysis to verify the reliability of such transient, considering different sources of bias and uncertainty, such as completeness and declustering of the catalog, as well as errors on magnitude estimation. I also searched for a confirmation externally to the catalog, analyzing the correlation with the volcanic activity. The similarity is high for the eruptive history of Vesuvius, which agrees on both the main rate changes of the 17th and 20th centuries and on minor variations in the intermediate period. Of general interest, beyond the specific case of Italy, the observed rate changes suggest the existence of large-scale crustal processes taking place within decades and lasting for centuries, responsible for the synchronous activation/deactivation of remote, loosely connected faults in different tectonic domains. Although their origin is still unexplained (I discuss a possible link with the climate changes and the consequent variations of the sea level), their existence and long lasting is critical for seismic hazard computation. In fact, they introduce a hardly predictable time variability that undermines any hypothesis of regularity of the earthquake cycle on individual faults and systems of interconnected faults.

Seismic Hazard Analysis for the Albertine Region, Uganda – A Probabilistic Approach

The Albertine region of Uganda is characterized by high levels of seismic activity and by many active normal faults. The aim of this work is to assess the potential hazard accruing from earthquakes in the Albertine region, using the probabilistic approach to assess the seismic hazard for, specifically, the Hoima and Buliisa districts. The input parameters for the seismic hazard computation were obtained using the earthquake catalogue compiled for the period 1900 to 2012, including earthquakes within an area bounded by latitudes 1.5°S to 4°N and longitudes 29°E to 33°E. All magnitudes were homogenized to moment magnitude scale. The study area was divided into two seismic source zones based on seismotectonic analysis. The updated ground motion prediction equation by Pankow and Pechmann was used in the computations. Seismic hazard maps in terms of the Peak Ground Acceleration (PGA) for 10%, 5% and 2% probabilities of exceedance in 50 years were derived. The results show that the PGA values for both districts fall in the range of 0.152 to 0.162 g, 0.187 to 0.195 g and 0.248 to 0.256 g (where g is the acceleration due to gravity), for return periods of 475, 975 and 2,475 years, respectively. The highest levels of seismic hazard were found in the southwestern part of the Hoima district, where PGA values were in excess of 0.162 g, 0.195 g and 0.256 g for 10%, 5% and 2% chances of exceedance in 50 years, respectively. We conclude that in the Albertine region, the return period for an earthquake capable of causing damage to engineering structures is, on average, 30 years. It is thus recommended that all important engineering structures in the region should be designed for earthquake resistance.

Non-Poisson probabilistic seismic hazard assessment

The development of the new seismic hazard map of metropolitan Tehran is based on probabilistic seismic hazard computation using the non-Poisson recurrence time model. For this model, two maps have been prepared to indicate the earthquake hazard of the region in the form of iso-acceleration contour lines. They display the non-Poisson probabilistic estimates of peak ground accelerations over bedrock for 10 and 63 % probability of exceedance in 50 years. To carry out the non-Poisson seismic hazard analysis, appropriate distributions of interoccurrence times of earthquakes were used for the seismotectonic provinces which the study region is located and then the renewal process was applied. In order to calculate the seismic hazard for different return periods in the probabilistic procedure, the study area encompassed by the 49.5–54.5°E longitudes and 34–37°N latitudes was divided into 0.1° intervals generating 1,350 grid points. PGA values for this region are estimated to be 0.30–0.32 and 0.16–0.17 g for 10 and 63 % probability of exceedance, respectively, in 50 years for bedrock condition.

Seismic hazard and site-specific ground motion for typical ports of Gujarat

Economic importance of major ports is well known, and if ports are located in seismically active regions, then site-specific seismic hazard studies are essential to mitigate the seismic risk of the ports. Seismic design of port sites and related structures can be accomplished in three steps that include assessment of regional seismicity, geotechnical hazards, and soil structure interaction analysis. In the present study, site-specific probabilistic seismic hazard analysis is performed to identify the seismic hazard associated with four typical port sites of Gujarat state (bounded by 20°–25.5°N and 68°–75°E) of India viz. Kandla, Mundra, Hazira, and Dahej ports. The primary aim of the study is to develop consistent seismic ground motion for the structures within the four port sites for different three levels of ground shaking, i.e., operating level earthquake (72 years return period), contingency level earthquake (CLE) (475 year return period), and maximum considered earthquake (2,475 year return period). The geotechnical characterization for each port site is carried out using available geotechnical data. Shear wave velocities of the soil profile are estimated from SPT blow counts using various empirical formulae. Seismicity of the Gujarat region is modeled through delineating the 40 fault sources based on the seismotectonic setting. The Gujarat state is divided into three regions, i.e., Kachchh, Saurashtra, and Mainland Gujarat, and regional recurrence relations are assigned in the form of Gutenberg-Richter parameters in order to calculate seismic hazard associated with each port site. The horizontal component of ground acceleration for three levels of ground shaking is estimated by using different ground motion attenuation relations (GMAR) including one country-specific GMAR for Peninsular India. Uncertainty in seismic hazard computations is handled by using logic tree approach to develop uniform hazard spectra for 5% damping which are consistent with the specified three levels of ground shaking. Using recorded acceleration time history of Bhuj 2001 earthquake as the input time motion, synthetic time histories are generated to match the developed designed response spectra to study site-specific responses of port sites during different levels of ground shaking. It is observed that the Mundra and Kandla port sites are most vulnerable sites for seismic hazard as estimated CLE ground motion is in order of 0.79 and 0.48 g for Mundra and Kandla port sites, respectively. Hazira and Dahej port sites have comparatively less hazard with estimated CLE ground motion of 0.17 and 0.11 g, respectively. The ground amplification factor is observed at all sites which ranges from 1.3 to 2.0 for the frequency range of 1.0–2.7 Hz. The obtained spectral accelerations for the three levels of ground motions and obtained transfer functions for each port sites are compared with provisions made in Indian seismic code IS:1893-Part 1 (2002). The outcome of present study is recommended for further performance-based design to evaluate the seismic response of the port structures with respect to various performance levels.

Ground-Motion Hazard Values for Northern Algeria

This study examines distinctive features of ground motion parameters in northern Algeria. An initial computation of seismic hazard in terms of horizontal peak ground acceleration (PGA) and spectral acceleration (SA) at different periods, damped at 5%, is carried out for three different types of soils (rock, stiff soils and soft soils) for return periods of 100 and 475 years. In addition, uniform hazard spectra (UHS) are computed for these two return periods at several locations in the region. Then, the UHS computed for different soil types are proposed as a starting point to define elastic design spectra for building-code purposes. We have used the well-known Newmark-Hall approach. As proposed in the most recent International Building Codes, the SA (0.2 s) value is used to establish the spectral region for lower periods (region controlled by acceleration), whereas the SA (1.0 s) value is used to establish the spectral region for intermediate periods (region controlled by velocity). We also obtained important relations, dependent on site condition, between SA (0.2 s), SA (1.0 s) or SAmax values, and the PGA, for both return periods of 100 and 475 years. Other relationships between PGA or SAmax values have also been derived for return periods of 100 and 475 years, in this case independent of site condition.

Modular Filter-based Approach to Ground Motion Attenuation Modeling

Previous-generation ground-motion attenuation relations ( e.g. , Abrahamson and Silva 1997; Boore et al. 1997; Campbell 1997; Sadigh et al. 1997) were designed to predict ground motion at distances less than 100 km from an earthquake fault. As probabilistic and deterministic seismic hazard analysis concepts have become more prevalent in performance-based seismic design, these relations have been used in practice much beyond their distance limitation. Realizing the engineering needs for predicting ground motions at distances farther than 100 km, the Next Generation Attenuation (NGA) relations project targeted a distance range up to 200 km (Power et al. 2006). For seismic hazard analysis in the western United States (WUS), 200 km is a sufficiently large distance to quantify design-basis hazard level from known active faults due to relatively fast attenuation of ground motion. For the central and eastern United States (CEUS), ground motion attenuates more slowly, yet design of critical infrastructures in the CEUS ( e.g. , nuclear power plants) requires computation of seismic hazard from distant large events. Thus, the recently initiated NGA-East project for CEUS set a goal to design ground-motion attenuation models applicable to 1,000 km. The objective of this article is to revisit the commonly used empirical approach to ground-motion attenuation modeling and compare it with an alternative modular filter-based approach that can be effectively used for predicting ground motion at near- ( 100 km). In this latter approach, each filter is calibrated separately to represent a certain physical phenomenon affecting seismic radiation from the source. We demonstrate that the modular filter-based approach provides accurate (that is, expected median prediction without significant bias) and efficient (that is, relatively small standard error of prediction) predictions. We also present our peak ground acceleration (PGA) based predictive model for …

Deterministic seismic hazard assessment of the inner town of Budapest

Deterministic seismic hazard computations were performed along four different profiles across the downtown of Budapest. Synthetic seismograms were computed by the so called “hybrid technique”. By applying the hybrid technique it is possible to take into account the focal source, the path and the site effect together. Four independent computations have been performed using the same seismic source but different profiles. The parameters of the seismic source were adopted from the parameters of the well-known 1956 Dunaharaszti earthquake. The focal mechanism and the homogeneous and heterogeneous parts of the profiles are known from geophysical and geological data of the investigated area.As the results of the computations PGA (peak ground acceleration) grid maps of the downtown of Budapest for the three different components came into existence. Furthermore spectral acceleration (response spectra, SA) and RSR charts of the synthetic seismograms for the four different profiles were created. The PGA grid maps show that the maximal PGA values are situated at the eastern (Pest) part of the downtown, and their values are 50–200 cm/s 2 .For the downtown of Budapest a special seismic risk map have been prepared. This special seismic risk map were created on the basis of the difference between the maximal amplitude frequencies of SA of synthetic seismograms and the building’s eigenfrequencies at every 0.1 km 2 of the downtown. In order to determine the building’s eigenfrequencies microseismic noise measurement were performed at 6 different buildings in the downtown. The special seismic risk map shows that the buildings situated at the hilly western section of the downtown have higher seismic risk than the ones at the flat eastern part.

Probabilistic Seismic Hazard Assessment for Izmir, Turkey

Izmir, the third largest city and one of the major economic centers in Turkey, has more than three million residents and one-half million buildings. The city, located in a seismically active region in western Anatolia, was a subject of the 1997 RADIUS (Risk Assessment Tools for Diagnosis of Urban Areas against Seismic Disaster) project. In this paper, the seismic hazard of Izmir is investigated through probabilistic seismic hazard assessment. First, the seismic setting of Izmir is presented. Considering the statistics of earthquakes that took place in the region during the period 1900–2005, a simple seismic hazard model is used to facilitate the assessment. To account for modeling uncertainties associated with the values of seismicity parameters, a logic tree procedure is employed in carrying out the seismic hazard computations. The resulting weighted average seismic hazard, presented in terms of peak ground acceleration and associated probability of exceedence, could be considered the “best estimate” of seismic hazard for Izmir. Accordingly, for a return period of 475 years, for rock sites, a PGA value of 0.34 g is calculated. This PGA hazard estimate is close to the current code-recommended design acceleration level for Izmir.

Layered Seismogenic Source Model and Probabilistic Seismic-Hazard Analyses in Central Italy

We defined a seismogenic model for central Italy based on three layers of sources and computed the relative seismic-hazard maps. One layer is constituted by individual structures liable to generate major earthquakes (M 5.5). We defined them as seismogenic boxes by using geological information in terms of plan projec- tion of active faults; the seismicity rates associated with an individual source are based on the geometry and kinematics of the fault; the recurrence model is controlled by the earthquake-source association, and, when possible, we defined the occurrence time of the last major event, using it in a time-dependent approach. Another layer is given by the instrumental seismicity analysis of the past two decades, which allows us to evaluate the background seismicity (M �� 5.5); using a sliding-window selec- tion of events, we defined a model of regular adjacent cells of variable a and b values of the Gutenberg-Richter relation. The last layer utilizes all the instrumental earth- quakes and the historical events not correlated to known structures (4.5 � M �� 6), by separating them into seismotectonic provinces shaped on a geological-structural basis. The seismic-hazard computations first use this layered model in a traditional probabilistic scheme. The results indicate a narrow belt of peak ground acceleration (PGA) higher than 0.30g (with standard deviation in attenuation functions) in the axial part of the Apennine chain, with a maximum spot of PGA � 0.40g southeast of the area damaged by the 1997-1998 Umbria-Marche sequence (PGA expected not to be exceeded in 50 years at 90% probability level). The background seismicity gives a nonnegligible contribution to the hazard, at least for first damage levels. Then, a simplified time-dependent hypothesis has been introduced for the individual sources alone, computing the conditional probability of occurrence of characteristic earth- quakes for each source by Brownian passage time distributions. Adopting equivalent fictitious seismicity rates, we obtained maps referring to the next 50 years by using traditional codes. These results show that the contribution of the recently active sources vanishes, and the most hazardous sites are now located south of L'Aquila and in the Sulmona area. We consider that the methodology and results obtained are useful for seismic risk reduction strategies.

Single-Station Sigma

Is the random variability of ground motions for a single station less than that obtained by regression analyses of ground-motion data recorded over a broad network of sites? This question has important implications for seismic design of critical facilities because of the influence of this variability (commonly referred to as “sigma”) on probabilistic seismic-hazard computations at low probabilities. I address this question using ShakeMap data recorded at a group of 21 stations, all in the Los Angeles region, for which the shear-wave velocity in the upper 30 m ( V 30) is known. Ground-motion prediction equations are derived from a database of site- corrected amplitudes compiled for the group of stations as a whole. The standard deviation of residuals (sigma) for the regression of the entire database is then compared with the standard deviation of residuals at individual stations. Regressions of single-station databases are also performed. The sigma for an individual station is less than the overall sigma. The results of this study suggest that when computing hazard at a specific site for which the site amplification has been estimated based on either an empirical correction or on V 30, the site sigma can be taken as 90% of the corresponding sigma for the applicable ground-motion prediction equation, if the problem under consideration is one of multiple earthquake sources. If hazard from a single source at a fixed azimuth is considered (such as a single fault), the site sigma is 60% of the corresponding sigma for regional ground-motion relations. Further study with additional datasets is warranted to determine whether these results apply to hazard computations in a general sense, beyond the limited range of conditions studied here.

Sistema probabilístico de información geográfica de Amenaza sísmica de Colombia

Computation of seismic hazard at rock or firm sites has been a well established technique for many years. Hazard is generally expressed in terms of exceedance rates (ER), defined as the mean annual number of times in which a given value of intensity is exceeded. The inverse of the ER of some intensity is called its mean return period. Usually, the practice was to compute seismic hazard in terms of ER of peak motion values, such as peak ground acceleration, velocity and displacement,. More recently, uniform-hazard response spectra (UHS) have been constructed in a more direct and precise way, by means of attenuation equations that relate, typically, magnitude and distance with response spectral values for a range of periods. By performing a conventional hazard analysis on a period by period basis, it is possible to construct response spectra whose ordinates are, all, associated to the same return period, thus constituting a UHS. In order to compute approximate uniform-hazard response spectra at places affected by local soil amplifications, several approaches have been adopted in the past to overcome analytical difficulties. We present a coherent approach to construct UHS of different intensities at soft soil sites by means of interactive Internet-software named Zp, which will be illustrated with deterministic validations on Armenia city during the 25 January Armenia Earthquake and probabilistic examples for Bogota city.

Seismic hazard maps of Italy including site effects

The seismic hazard of Italy was mapped adopting multiple models of seismic sources, recurrence rates and attenuation relationships, and investigating the influence of site effects on assessing the earthquake hazard. This result was achieved by placing all the alternatives in a logic-tree diagram, and performing an uncertainty analysis of the main epistemic variables involved in probabilistic seismic hazard computations. Several ground motion parameters such as peak ground acceleration (PGA), velocity (PGV) and displacement (PGD), spectral accelerations at 0.2 and 1.0s periods, and site intensities were computed. Maps of PGA and PGV values with a 10% chance of being exceeded in 50years are shown as an illustration of the seismic hazard of Italy.

Synthetic Seismogram Based Deterministic Seismic Zoning for the Hungarian Part of the Pannonian Basin

—Deterministic seismic hazard computations have been done for the Hungarian part of the Pannonian basin within the framework of a cooperation of five countries. Synthetic seismograms have been computed by the modal summation method up to 1 Hz in order to determine the expected maximum displacement (DMAX), velocity (VMAX) and the design ground acceleration (DGA) on a 0.2°× 0.2° grid. DGA values have been estimated from the seismograms by using the standard.¶This investigation justified the suspicion that a considerable part of seismic hazard of Hungary comes from the seismogenic zones of the neighbouring countries. The highest DGA reaches a value as high as 0.14 g (which corresponds approximately to the VIII intensity degree in the MSK-64 scale). Among the six largest cities of Hungary, three art particularly subject to a high seismic risk. Greater acceleration values have been found for the cities of Szeged and Debrecen than was expected before this study.

Seismic hazard assessment of Iran

The development of the new seismic hazard map of Iran is based on probabilistic seismic hazard computation using the historical earthquakes data, geology, tectonics, fault activity and seismic source models in Iran. These maps have been prepared to indicate the earthquake hazard of Iran in the form of iso-acceleration contour lines, and seismic hazard zoning, by using current probabilistic procedures. They display the probabilistic estimates of Peak Ground Acceleration (PGA) for the return periods of 75 and 475 years. The maps have been divided into intervals of 0.25 degrees in both latitudinal and longitudinal directions to calculate the peak ground acceleration values at each grid point and draw the seismic hazard curves. The results presented in this study will provide the basis for the preparation of seismic risk maps, the estimation of earthquake insurance premiums, and the preliminary site evaluation of critical facilities.

Seismic hazard maps for Jordan and vicinity

Probabilistic methods are used to quantify the seismic hazard in Jordan and neighbouring regions. The hazard model incorporates the uncertainties associated with the seismicity parameters and the attenuation equation. Seven seismic sources are identified in the region and the seismicity parameters of these sources are estimated by making use of all the available information. Seismic hazard computations and the selection of peak ground acceleration and modified Mercalli intensity values at the nodes of a 25 × 25 km mesh covering the region under study are carried out by two different computer programs.