Analysis of Registration and Processing of Seismological Data in the Republic of Armenia from 1991 to 1998 ()
1. Introduction
It is well known that for the successful forecast of strong earthquakes, seismic zoning and earthquake-resistant construction, it is necessary to have an accurate understanding of the distribution of earthquake hypocenters in the study area.
For this purpose, when processing the initial seismological information, this work uses the averaged velocity curve given by [1] [2] and the HYPOBUR algorithm.
To determine the coordinates of earthquakes, currently, mainly variations of the Geiger method are used [3] [4] . When determining the coordinates of earthquake hypo-centers, the arrival times of P- and S-waves, seismic wave hodographs, or velocity columns for the region being studied are used as initial data.
To determine the location of earthquake sources with high accuracy, it is necessary to have a fairly detailed understanding of the deep structure of the earth’s crust and upper mantle, where processes associated with the preparation of an earthquake occur. Therefore, errors in determining the coordinates of earthquake hypocenters also depend on the choice of the velocity model of the region being studied.
Despite the fact that a number of works have been devoted to the structure of the earth’s crust and upper mantle in the Caucasus, and in particular on the territory of Armenia, existing hodographs do not allow determining the coordinates of earthquake hypocenters with the required accuracy [5] [6] [7] . Until recently, to determine the coordinates of earthquake hypocenters, the Levitskaya hodograph [8] or the Jeffery’s Bullen hodograph was used with station corrections, which should take into account the conditions under the stations [9] [10] [11] .
According to the authors themselves, they are local in nature and cannot generally ensure the necessary accuracy of data processing.
The work [12] constructed a velocity curve for the crust of Armenia. For this purpose, all available data on velocities were used, obtained on the basis of materials from regional profiles of the ECWM (Earthquake Conversion Wave Method) and DSS (Deep Seismic Sounding) [12] . Each point of the studied environment is mainly selected in such a way that its location coincides with the intersection of the DSS and ECWM pro-files.
To study the deep structure of the territory of the Republic of Armenia, in addition to data obtained using the methods of converted waves and deep seismic sounding, it is advisable to use information about nearby earthquakes. Such data were accumulated in sufficient quantities in 1971-1990 when the network of seismic stations was in the RA Academy of Sciences system.
A seismological observation system that plays a decisive role in seismically active zones for the period 1971-1990. Functioned normally, providing real-time monitoring. Almost 1.500 earthquakes were recorded using 13 stations, i.e. there is an average of 75 earthquakes per year.
From 1991 to 2010, the number of seismic stations increased to 34. From 1991 to the present, bulletins and catalogs have not been published in Armenia. Data for processing was taken from the international website https://isc.ac.uk/.
2. Distribution of Epicenters
Despite the increase in the number of seismic stations during the period 1991-1998, the system recorded 209 earthquakes, of which only 65 occurred on the territory of the Republic of Armenia, i.e. in 8 years there were only 65 earthquakes.
It should be noted that out of 209 recorded earthquakes, according to the bulletins, it is impossible to determine the coordinates of 85, but they are indicated in the catalogs. It is not clear how they were determined. A study of the catalogs showed that for 1991, 1992, 1993 only one earthquake was presented. No data is available for 1994. The catalogs present basic data on earthquakes from 1995 to 1997. It should be noted that monthly data is presented only for 1995 and 1996, and 1997 is represented by only two months (January and February), and in 1998 there were no earthquakes. It is obvious that the observation system did not function fully. Seismic events were recorded by only 3 - 4 stations out of 34.
Figure 1 shows the distribution of epicenters of 124 earthquakes, according to the National Seismic Protection Service of the Republic of Armenia (NSPS), data were taken from the international website https://isc.ac.uk/, the results are shown in Table 1.
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Figure 1. Distribution of epicenters of 124 earthquakes according to catalog data.
As can be seen from Figure 1, the epicenters of earthquakes have a widespread, a significant part of which is located outside the territory of the Republic of Armenia, particularly, in Georgia, Iran, Azerbaijan, Turkey, South Ossetia, and Russia.
Table 1 presents a consolidated catalog of 124 earthquakes according to the NSPS data. The indicated 124 earthquakes are characterized by M ≤ 4.5.
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Table 1. Consolidated catalog of 124 earthquakes based on data NSPS.
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Figure 2. Distribution of epicenters of 124 earthquakes after recalculation.
Figure 2 shows the distribution of epicenters of 124 earthquakes after recalculation. The results were obtained using a new averaged velocity curve [12] .
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Table 2. Results of 124 earthquakes after recalculation using the velocity curve.
3. Distribution of Focal Depths
To compare the results of the focal depths, the distribution of hypocenters in the latitudinal (Figure 3(a)) and longitudinal planes (Figure 3(b)) was constructed according to the catalog data for 1991-1998.
As can be seen from Figures 3(a)-(b), earthquake hypocenters, as one would expect, are mainly distributed at depths with discrete values of 0, 5, 10, 15, 20, 25 km. Such a pattern in the distribution of focal depths has not been established. Consequently, the depths of the lesions were not determined accurately. The main reason for this distribution is that the location of the global minimum in this case is sought for a local minimum at the interval [0; 25] km.
In Figures 4(a)-(b) shows the distribution of earthquake hypocenters after recalculation for 1991-1998.
From Figures 4(a)-(b) it is clear that the depths of the over-determined earth-quake hypocenters are not located discretely, as presented in the NSPS results. These results indicate that the depth hotspots cover a larger area in depth, up to 140 km, rather than 25 km as indicated in the NSPS results.
Most of the earthquakes listed in Table 1 (124 earthquakes according to the NSPS data) were recorded by 3 or 4 stations. With such data, it is difficult to guarantee high accuracy of processing results.
Wadati plot for 124 earthquakes for the period 1991-1998 according to the NSPS data and the results after recalculation are given in Figures 5(a)-5(b).
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Figure 3. (a) Distribution of hypocenters in the latitudinal plane for 1991-1998 according to catalog data; (b) Distribution of hypocenters in the longitudinal plane for 1991-1998 according to catalog data.
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Figure 4. (a) Distribution of epicenters in the latitudinal plane after recalculation for 1991-1998; (b) Distribution of hypocenters in the longitudinal plane after recalculation for 1991-1998.
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Figure 5. (a) Wadati plot for 124 earthquakes for the period 1991-1998 according to the NSPS; (b) Wadati plot for 124 earthquakes for the period 1991-1998 after recalculation.
In Figure 5(a) there is a large scatter, which indicates that the times at the source were obtained inaccurately. Comparing Figure 5(b) with Figure 5(a) it is clear that after recalculation more reliable results were obtained, i.e. the environmental model was chosen more accurately.
4. Conclusions
The results of processing for 1991-1998 indicate that the registration and processing of source data was not carried out properly. When determining the main parameters of earthquake hypocenters, in addition to random errors, systematic errors caused by an inaccurate choice of the velocity structure of the studied region have a great influence.
The use of the average velocity curve, as shown by the results obtained, makes it possible to determine the coordinates of earthquake hypocenters, including the depth of the hypocenter, which can reach 60 km. It is shown that during the study period, the distribution of hypocenters obtained using the averaged velocity curve turns out to be less compact, and earthquake epicenters have a smaller scatter in the study area than those obtained from the catalog of the Armenian seismic service. Finally, to create an accurate and reliable seismological information base, it is necessary to redefine existing data using new methods for processing a more accurate model of the structure of the earth, as well as improve the efficiency of the existing observing system.