National Seismological Network Research Articles

Subsurface velocity structures at the Egyptian seismological network stations retrieved by diffuse field assumption for Earthquakes

Retrieving detailed subsurface velocity structures down to the seismic bedrock at any given site is a crucial step to delineate the site amplification factors accurately and precisely. The present research work contributes first new estimations for detailed velocity structures down to the seismic bedrock beneath the Egyptian National Seismological Network (ENSN) stations, which are distributed in Egypt nationwide. We used the diffuse field assumption for earthquakes to reproduce the horizontal to vertical spectral ratios (EHVSR) at these stations. We accepted waveform database of 424 earthquakes recorded at 75 ENSN stations. After achieving the inverted subsurface velocity structures, we establish site-specific frequency-depth regression and map the VS30 and seismic bedrock depth beneath Egypt. Because of comparability regarding the seismic site class of B and C, the regression coefficients of the newly established frequency-depth regression exhibit similarity with those achieved from previous regressions in Japan. Furthermore, we observe modest consistency between the seismic bedrock depths and the various geologic features, particularly agreement between basin-shape seismic bedrock depths and the existence of Cretaceous and Jurassic extensional basins. Our findings suggest that the Precambrian basement rocks can be interpreted as the seismic bedrock in Egypt. One of the most significant obstacles in the present work is the low-dense distribution of ENSN stations nationwide. However, the achieved results raise new questions and challenges regarding precise and accurate future estimations for site amplification calculations at different localities in Egypt, particularly for urban planning by engineers.

Seismic Signature of the Super Cyclone Amphan in Bay of Bengal Using Coastal Observatories Operating Under National Seismological Network of India

AbstractWe examined the seismic noise data collected from coastal and inland observatories in India, affected by the super cyclonic storm Amphan in the Indian Ocean, to understand the storm dynamics. Prominent disturbances in the 0.05–0.50 Hz frequency range were observed at the seismic stations, arising due to ocean‐continent interactions. The coastal stations displayed more pronounced ground motions contrary to the inland stations, with spindle‐shaped seismic wave envelopes intensifying as Amphan approached. The maximum ground displacements and energy occurred hours after the cyclone's eye, with maximum wind speed, moved away from the stations and not when it was close to the station. We observed significant variations in primary (0.05–0.10 Hz) and secondary microseism (0.10–0.50 Hz) energy during Amphan's directional changes. Secondary microseisms in short and long periods were found at 0.20–0.50 Hz and 0.10–0.20 Hz, respectively. Primary microseisms exhibited a simple pattern and were the weakest among the three energy bands. The CAL seismic station's seismic wave envelope showed an en‐echelon feature with increasing amplitude as Amphan approached, indicating the influence of ocean resonance and coastal wave reflection. This study demonstrates monitoring of the tropical cyclone paths based on seismic signatures obtained using microseisms recorded at seismic stations, a cost‐effective tool. Integrating these seismic signals with atmospheric observations in near real‐time would probably enable an effective monitoring of cyclones and timely issuance of their alerts.

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.

The 2022 Seismic Hazard Model for Costa Rica

AB4STRACTCosta Rica is located at the boundary of four tectonic plates where the regularity of destructive earthquakes highlights the necessity of seismic hazard estimations. This study contains the most recent Probabilistic Seismic Hazard Assessment (PSHA) for Costa Rica, calculated with the largest and the most updated earthquake database from both—the Earthquake Engineering Laboratory and the National Seismological Network of the University of Costa Rica. For the PSHA, we updated the seismicity parameters for the upper plate, subduction interplate, and intraslab tectonic domains, characterized the upper-plate zones by percentages of fault types, and used weighted ground-motion models for each of the tectonic domains. The resulted maps of peak ground acceleration (PGA) at return periods of 475 yr (PGA-475) and 2475 yr, as well as the spectral accelerations, show geographic trends that allow for the division of the country in four seismic hazard levels: (1) extremely high for the Nicoya, Osa, and Burica peninsulas, situated directly above the subduction interplate, where the PGA-475 could be 0.55–1.20g; (2) very high for most of the Guanacaste Province, where the PGA-475 may be 0.55–0.70g; (3) high for most of the country (∼41%) with PGA-475 values of 0.40–0.55g, including Central Costa Rica and the capital city of San Jose; and (4) moderate for the Talamanca Cordillera and Northern Costa Rica, with PGA-475 up to 0.40g. These ground-motion values are 0.1–0.6g higher than the previous PSHA for the Pacific peninsulas, Guanacaste, and the southeastern Caribbean. Further, hazard curves, uniform hazard spectra, and a hazard disaggregation indicate that the seismic hazard is lower but more complex in San Jose than in Liberia—the largest city in Guanacaste.

Assessment of Kappa Values in the Chilean Subduction Zone for Interface and In-Slab Events

Abstract One way to study the physical process that occurs behind earthquakes and how they affect different sites depending on the source type and the geological structure of the site is the shape of the Fourier spectrum. A parameter related to the shape of the spectrum is the spectral decay factor—kappa (κ), which characterizes how the amplitude of the Fourier acceleration spectrum decays at high frequencies. The parameter κ can be important in the characterization and estimation of the surface seismic demand, being useful in, for example, the adjustment of ground-motion prediction equations. We calculate the values of κ and its site component κ​0 from 36 seismic stations of the National Seismological Network of Chile to determine the dependence that this parameter has to the site conditions as well as to the properties of the path in which greater values of κ are observed for subduction earthquakes that occur in the interface between the Nazca and the South American plates, compared with the values obtained from events occurring inside the subducting plate, known as in-slab earthquakes. We find that κ​0, calculated using the hypocentral distance correlates more closely with the site fundamental frequency f​0, rather than the commonly used V​S30 (time-averaged shear-wave velocity in the top 30 m). Our kappa value results are field estimates of near-surface attenuation, which can be used to calculate the minimum site-specific damping or crustal attenuation in seismological models that have a strong impact on seismic site characterization, particularly, in subduction settings.

Organization of the stationary seismological observations point

The Institute of Geophysics of the NASU organizes and carries out continuous regional and local seismic observations on the territory of Ukraine. The article presents a universal modern model of seismic activity monitoring process, which is used in most international seismological agencies (USGS, EMSC, NEIC) and describes a typical stationary point of seismological observations of the National Seismological Network of the Institute of Geophysics of NAS of Ukraine. Seismological network of observations is a complex of systems consisting of stationary seismological points of registration of seismic waves, the distributed system of transfer and collecting of the seismological information, and also the center of operative processing of the data arriving from data registration points. The process of conducting regime seismological observations of local and remote seismic events on the territory of Ukraine and adjacent regions is described. Some important aspects of the need for comprehensive processing of registered events to identify local earthquakes and assess the current activity of tectonic structures in Ukraine are presented. The seismological network of the National Seismological Center of the Institute of Geophysics of the National Academy of Sciences of Ukraine is represented by a small number of stationary observation points: «Kiev-IRIS», «MI02-Poltava», «MI03-Skvyra», «MI04-Dnipro», «MI05-Stepanivka», «MI07-Mykolaiv», «ODS-Odesa», «MIU-Kryvyi Rih», and «MI06-Kremenchug». This number of seismological observation points does not actually provide seismic observation data to the central, eastern and southern parts of the territory of Ukraine and does not allow to reliably determine the level and quantitative characteristics of its seismic hazard. The seismic recorder Guralp CMG-40T manufactured by the British company GURALP SYSTEMS LIMITED is offered as optimal for the conditions and financial realities of Ukraine when organizing a stationary seismic observation point. It is proposed to use the seismological processing package SeisComP, which works on the SeedLink protocol, which is the basis of the data collection system by the Internet. This software product is the de facto world standard in the field of seismological data processing.

Electrical and magnetic data time series’ observations as an approach to identify the seismic activity of non-anthropic origin

In this work, the authors tried to identify a possible relationship between electromagnetic signals (EM) and seismic events in the lithospheric system in the central region of Colombia. The data, both seismic records and electromagnetic signals, were taken from the catalog of the Seismological Network of the National University of Colombia (RSUNAL) and the catalog of the National Seismological Network of Colombia (RSNC). The project included the design and instrument testing phases for recording seismic signals, electrical potential variations, and magnetic field variations to try to identify possible relationships between these signals. Possible electromagnetic precursors for seismic events were observed, mainly magnetic disturbances, but it was not possible to locate evident electrical anomalies (Seismic Electric Signals - SES). Thus, although the results are not conclusive, the magnetic disturbances identified deserve further long-term analysis.

Preliminary Results of an Earthquake Early Warning System in Costa Rica

We analyze the performance of a prototype earthquake early warning system deployed at the National Seismological Network of Costa Rica in collaboration with the Swiss Seismological Service by presenting the real-time performance during six earthquakes (Mw 5.1-6.4) that took place during 2018 and 2019. We observe that, despite only limited efforts to optimize the existing network of 158 stations, for EEW purposes, the network density allows fast determination of source parameters using both the Virtual Seismologist and the Finite Fault Rupture Detector algorithms. Shallow earthquakes on or near-shore are routinely identified within 11–20 s of their occurrence. The warning times for the capital city of San Jose are of 43 s for epicenters located at 220 km, like for the Mw 6.4 Armuelles earthquake. On the other hand, during the time analyzed, the EEW system did not provide positive warning times for earthquakes at distances less than 40 km from San Jose. Even though large (Mw > 7) distant historical earthquakes have not caused heavy damage in San Jose, there is potential for developing an EEW system for Costa Rica, especially for the purposes of rapid earthquake notifications, disaster response management, and seismic risk mitigation.

Orogenic Collapse and Stress Adjustments Revealed by an Intense Seismic Swarm Following the 2015 Gorkha Earthquake in Nepal

The April 25, 2015 Mw 7.9 Gorkha earthquake in Nepal was characterized by a peak slip of several meters and persisting aftershocks. We report here that, in addition, a dense seismic swarm initiated abruptly in August 2017 at the western edge of the afterslip region, below the high Himalchuli-Manaslu range culminating at 8156 m, a region seismically inactive during the past 35 years. Over 6500 events were recorded by the Nepal National Seismological Network with local magnitude ranging between 1.8 and 3.7 until November 2017. This swarm was reactivated between April and July 2018, with about 10 times less events than in 2017, and in 2019 with only sporadic events. The relocation of swarm earthquakes using proximal temporary stations ascertains a shallow depth of hypocenters between the surface and 20 km depth in the High Himalayan Crystalline slab. This swarm reveals an intriguing localized interplay between orogenic collapse and stress adjustments, involving possibly CO2-rich fluid migration, more likely post-seismic slip and seasonal enhancements.

Spatial variability of the b-value and seismic potential in Costa Rica

Costa Rica is a seismically active region located in a subduction zone. Combining earthquake data from the National Seismological Network and several Central American catalogs from 1522 to 2020, we present a new estimation for the Gutenberg-Richter relationship. The seismic catalog compiled with ~122,000 earthquakes was studied by calculating the magnitude of completeness (Mc) and by applying a space-time window declustering method. The residual catalog and a previously proposed seismic zonation were used to determine the b-value, the maximum magnitude (Mmax), and the mean recurrence interval (MRI). Our results show a temporal and geographic variation in the Mc, decreasing from 7.0 since 1793 to 3.0 since 1995, and with the lowest estimate (2.5) in Central Costa Rica. The b-values of 0.85 for the entire catalog and ~0.83 for the interplate zones, are similar to other regions worldwide with young subducting slabs. There is a general trend of higher and more variable b-values among the upper-plate (average 0.90) and intraslab (1.14) zones as compared to the interplate regions (0.85). The upper-plate variability is explained in terms of the diversity in geological units and faulting style, whereas in the interplate and the subducting slab is connected to the stress level imposed by different seafloor morphologies and hydration states along the margin. Our data suggest a seismic potential of moment magnitude (Mw) 7.9–8.0 for the Nicoya and Southern Nicaragua interplate zones and for the Limon region. The Gutenberg-Richter distribution shows that a Mw 7.0 has a longer MRI for the intraslab zones (~72 years) than for the subduction interplate region (~15 years) and plate and microplate boundaries (~40–45 years). Interplate earthquakes have occurred recently (1991 Mw 7.7 and 2012 Mw 7.6) but no major intraslab since 1948 (Mw 7.0).

National Seismological Network in India for Real-Time Earthquake Monitoring

Abstract The National Seismological Network (NSN) of India has a history of more than 120 yr. During the last two decades, the NSN has gone through a significant modernization process, involving installation of seismic stations equipped with a broadband seismograph (BBS) and a strong-motion accelerograph (SMA). Each station has a very-small-aperture terminal connectivity for streaming data in real time to the central receiving station (CRS) in New Delhi. Seismic data recorded by the network are analyzed continuously on 24×7 basis to monitor the earthquakes in India and its adjoining regions. In this article, we present details of BBS and SMA network configurations; data streaming from the field seismic stations to the CRS for analysis; and the automatic and manual publication of the earthquake parameters including location coordinates, focal depth, time of occurrence, and magnitude, etc. Details of historically significant analog seismic charts and the seismic catalog, which includes more than 34,000 events with magnitude Mw 1.7–9.3 since 1505, are provided. The national network of India has been strengthened over the years and is now capable of estimating the main earthquake source parameters within ∼5–10min with an average of about 8.0 min. The spatial analysis of minimum magnitude of completeness further indicates a significant enhancement in minimum threshold magnitude detection capability of the network in recent decades.

Geometría de la zona sismogénica interplacas en el Sureste de Costa Rica a la luz de la secuencia de Golfito del 2018

Between August and November 2018, a seismic sequence took place in the vicinity of Golfito, a city in the Dulce Gulf in Southeastern Costa Rica. The main shock had a moment magnitude (Mw) of 6.1 and was widely felt in Costa Rica and Western Panama, with maximum Modified Mercalli intensities of VI. In this region, the oceanic Cocos Ridge, riding on top of the Cocos Plate, subducts beneath the Panama Microplate. Using the seismic records from the National Seismological Network of Costa Rica, in this work the seismicity is relocated using the double-difference technique, and an analysis of its temporal and geographic distribution together with the focal mechanism and intensities of the strongest events are presented. The results show that the sequence occurred at the interplate seismogenic zone, within the rupture area of the 1983 Golfito earthquake (7.4 Mw), between 12 and 27 km depth, in a cluster dipping 35º northeast underneath the Dulce Gulf. Based mainly on these results and on previous seismic sequences, it is here proposed that the seismogenic zone in Southeastern Costa Rica has an extension of ~160 x 45 km. Further, during the Golfito sequence, the rupture of an inverse fault (5.9 Mw) took place within the Cocos Plate beneath the Dulce Gulf, as well as of dextral strike-slip faults (4.6-5.6 Mw) in the Panama Microplate, 50 km away of the Dulce Gulf. The analysis of the interseismic interplate seismicity contributes to a better understating of the dynamics of the seismogenic zone. This is of particular relevance in Southeastern Costa Rica, where at least six damaging earthquakes of Mw > 7 have occurred since 1803, implying the impending risk of the next big earthquake in this region.

Present-day stress state in northwestern Syria

Northwestern Syria is a key area in the eastern Mediterranean to study the active tectonics and stress pattern across the Arabia-Eurasia convergent plate boundary. This study aims to outline the present-day stress regime in this region of Syria using the fault plane solutions of the largest events recorded by the Syrian National Seismological Network from 1995 to 2011. A dataset of fault-plane solutions was obtained for 48 events having at least 5 P-wave polarities. The tectonic regime for most of these events is extensional and produces normal mechanisms in agreement with the local configurations of the seismogenic faults in the region. Strike-slip mechanisms are more scarce and restricted to certain areas, such as the northern extension of the Dead Sea fault system. The results of the current study reveal the spatial variations of SHmax orientation across the northwestern Syria region. This spatial variation of the present-day stress field highlights the role of main geometrically complex shear zones in the present-day stress pattern of northwestern Syria. However, these results show, regardless of the relatively small magnitudes of the studied events, they provide a picture of the local stress deviations that have currently been taking place along the local active faults.

Seismological aspects of the 15 November 2019 earthquake sequence, Kuwait

A sequence of small-to-moderate terrifying earthquakes occurred in the northern part of the State of Kuwait on November 15, 2019. These earthquakes have certainly been recorded by the Kuwait National Seismological Network (KNSN), where the magnitudes of the earthquakes ranged from 0.8 to 4.1 having focal depths ranging from 2 to 15 km. This sequence of the earthquakes continued to occur for 2 days in a row, where a total of 56 earthquakes were recorded. The objective of this work is to use the available data to determine the seismic source characterizations including fault plane solutions and stress pattern in the vicinity of earthquake sequence area. Detecting and recording the digital waveform of these earthquakes by the KNSN have given us an opportunity to analyze the data of these earthquakes using waveform inversion, focal mechanism, and stress inversion techniques. Using the waveform inversion method, the seismic moment (Mo) and moment magnitudes (Mw) of the two largest earthquakes in this sequence were determined to be 0.1532E+16 and 0.9578E+15 equivalent to Mw 4.1 and 3.9, respectively. The focal mechanism solution of all earthquakes of this sequence indicated that the type of fault was normal faulting with strike-slip component and took a northeast-southwest (NE-SW) direction, and seemingly being compatible with the structure setting of the region and also the direction of earthquakes sequence clustering. Using the multiple inverse method (MIM), the mean stress difference was essentially calculated and found to be 0.549 and sigma1 axis = 76.3/38.0 and sigma3 axis = 189.6/26.9. The acquired results give a clear picture about the structure of the region and may be used later to get acquainted with the type of earthquakes, whether they are of tectonic type, triggered, or induced.

Magnitude of completeness for the Northern stations of the Egyptian National Seismological Network

After the occurrence of an earthquake near Cairo on October 1992 (Mw 5.8), the Egyptian authorities established the Egyptian National Seismological Network (ENSN), which was targeted to monitor the seismic activity in and around Egypt. The ENSN started operation in 1997 with 66 stations installed by the National Research Institute of Astronomy and Geophysics (NRIAG). Almost 55,000 events were recorded up until 2018 since the time of ENSN’s operation. After more than 20 years of establishing ENSN, a scientific project was funded to review the detectability of the stations by different methods; so, the goal of this research is to evaluate the magnitude of completeness (Mc) which represents an important parameter for instrumental catalog studies using different catalog-based methods. The Mc is calculated based on deflection from the linear relation of frequency-magnitude of the recorded events in ENSN’s catalog. Calculation of Mc is indispensable for knowing and clarifying the seismicity parameters such as the b value in the Gutenberg-Richter equation. Three different methods were used, i.e., maximum curvature method (MAXC), b value stability approach (MBS), and entire magnitude range method (EMR). The results determine the variations among different methods, up to 0.5 magnitudes unit is observed. The importance of this study comes from its concentrate on the detectability of ENSN’s stations to show their efficiency for recording seismic events with high accuracy and for contributing in evaluation of seismic hazard in Egypt, in seismic engineering applications to mitigate earthquake risk, and in changing the position of bad recording stations to a quieter place with minimum noise level.

Statistical seismic quiescence evaluation in the Gulf of Aqaba source zone

The seismicity rate variations prior to the 1993 and 1995 Gulf of Aqaba mainshocks have been investigated using statistical analysis, focusing on the precursory phenomena of the seismic activity progression. A statistical estimation of the seismic quiescence was applied to detect a significant reduction in the rate of seismic activity, using the normal standard deviate Z-test. The catalogue data both from the Egyptian National Seismological Network (ENSN) and the International Seismological Center (ISC) which cover the interval from 1983 to 2003 was used in this analysis. For our computations, only events with completeness magnitude ≥3.5 in the source area of the two Gulf of Aqaba mainshocks were considered. We investigated seismicity rate variations as a function of time and space in terms of the Z value preceding the 1993 and 1995 Gulf of Aqaba mainshocks respectively, using a declustered catalogue. Mapping the spatial changes of the seismicity over the entire period of catalogue manifests clear seismic quiescence anomalies related to the epicenters of the 1993 and 1995 Gulf of Aqaba mainshocks. The long-term average and the corresponding cumulative number of events versus time plot for anomalous areas have been constructed as a way of determining the starting time of the seismic quiescence and its duration for both mainshocks. The plot of the 1993 mainshock shows a decrease in the seismicity rate with a maximum Z value level (Zmax) equal to 5.4 with an observed seismic quiescence starting two years before its occurrence while the 1995 mainshock plot gives a Zmax = 4.5 with 1.3 seismic quiescence duration before it. The estimated periods of quiescence are consistent with the duration of the second type seismic gap derived from the detailed investigations of the spatio-temporal variations of the seismicity pattern which precedes the 1993 and 1995 Gulf of Aqaba main earthquakes. Earthquakes of magnitudes smaller than those which have shown the second type of seismic gap reflect pattern similarities of the activity before both 1993 and 1995 earthquakes. Both mainshocks are preceded by anomalous patterns of activity, which include both swarm type activity and short-term seismic gap, respectively.

A review of the Egyptian National Seismological Network after 20 years of operation

ABSTRACT On 12 October 1992, a moderate earthquake with magnitude, Mw 5.8 was occurred in Cairo; the epicentre is located 25 km south west of Cairo, resulted in great damage in the surrounding area and extended 300 km away from the epicentre. Subsequently, the government established Egyptian National Seismological Network (ENSN) under the supervision of National Research Institute of Astronomy and Geophysics (NRIAG) with 66 stations of different recording bands (e.g. short period, broadband) for monitoring the seismic activity in and around Egypt. The network started operation since mid-1997 up till now, consider revising 20 years of operation, a lot of changes were done (sensors, digitisers, station locations and even stations name), there is still the absence of adequate documentation of the type of changes made. This article is the first attempt to study the recorded data to track all changes done in the network, as a first step towards a different studies regarding determination of the detection capabilities of ENSN’s stations and proposed an optimisation solutions for the current faced problems. A software code was designed under Matlab programme to read around 55,000 recorded events and track different changes that took place during the operational time. The output of this study will affect all related research work including magnitude determination and hazard analysis.

Appraisal of seismic noise scenario at national seismological network of India in COVID-19 lockdown situation

We evaluated seismic background noise at national network in India using PSD, Fourier spectra, Spectrogram, and HVSR approach, before and during the nationwide lockdown declared due to COVID-19 pandemic. The analyses were performed to understand characteristics of noise wave-field in such unprecedented situation and its effect on site response at the station. SBN in long period (> 20 s), primary microseism band (10–20 s) and secondary microseisms (1–10 s) performed well and the noise levels found within the new LNM and HNM. However, in short period (< 1 s) the variation in SBN performance found significant before and during the lockdown. We observed that the SBN at each site in short period (< 1 s) is found to be about 10–12 dB noisier in the time segment prior to the lockdown. The HVSR analysis of SBN at recording sites clearly indicates that the predominant frequency for the entire region remains stable and independent of seismic noise generated before or during lockdown. A substantial variation in amplification factor, however, observed in either situation. Most of the stations across the country experienced diminished cultural noise subsequent to declaration of lockdown on 25 March 2020. Such drastic decrease in cultural noise significantly enhanced the performance of noisy stations, and the best recording stations picked the seismic phases originated from micro to small earthquakes. We suggest installation of seismometers at some depth below the surface, particularly at disturbed sites, may substantially reduce short period noise in earthquake recording.

Decontamination of earthquake catalog from quarry blast events in northern Egypt

Earthquake catalogs are often contaminated by man-made crustal activities such as quarry blasts. That is the case in northern Egypt, where many of cement quarries were working so long time ago since the beginning of the twentieth century (1911). Therefore, it is necessary and essential to separate these quarry blast events from the earthquakes for real seismicity and hazard analysis. Generally, discrimination of quarry blasts from tectonic events often done through different techniques and methods. In this study, the time and frequency-domain analyses of the Egyptian National Seismological Network (ENSN) digital seismograms have performed using three techniques and methods (S/P wave amplitude peak ratio, complexity, and spectral ratio). Moreover, the power of event (Pe) analysis comprising three techniques (S/P wave amplitude peak ratio, complexity, spectral ratio) was also used together to extend the discrimination accuracy as well. Finally, we have used a linear discrimination function (LDF) analysis. A total of 1473 seismic event waveforms (ML ≤ 3.0) from the ENSN digital database have been investigated. The obtained results indicate that for West of Nile Dislocation (zone no. 1) 78.1% of the investigated seismic events are tectonic events; however, 21.1% are quarry events and 0.8% are misclassified events. For the East Nile Dislocation (zone no. 2) 36.2% and 63.7% are earthquakes and quarry blast events, respectively and 0.1% are misclassified events. This means that the earthquake catalog for zone no. 2 in northern Egypt is highly contaminated and needs very high concern during the further treats especially with microearthquakes dataset for actually real seismicity and hazard analysis.

Review of seismological investigations related to 1998 Mw5.6 and 2004 Mw5.2 earthquakes in Krn Mountains

Overview of extensive seismological studies of Krn Mountains earthquakes performed in two decades is given. Detailed macroseismic studies by using a new European Macroseismic Scale EMS-98 showed large variations in damage to buildings due to the influence of very heterogeneous sediments and partly also due to the differences in source radiation pattern. Site effects were carefully studied and it was proven by microtremor HVSR method that soil-structure resonance effects severely enhanced the damage in many places. Particularly important were seismotectonic studies based mainly on focal mechanisms and distribution of aftershocks. Combined with geological data these studies pointed to the complex structure of segmented Ravne fault, which is growing by interactions between individual fault segments. A wider area is characterised by a kinematic transition between Dinaric (NW-SE) strike-slip faults in W Slovenia and E-W trending Alpine structures with predominantly reverse faulting in Friuli. Other investigations included static stress changes on neighbouring faults, analyses of the time decay of extensive aftershock sequences and magnitude-frequency relations. All these studies have significantly fostered seismological research in Slovenia and have enhanced international cooperation. Following the 1998 earthquake a modern national seismological network was built composed of 26 stations equipped with broadband sensors, accelerometers and high-resolution digitizers. Together with cross-border exchange of real-time data the seismological monitoring has been significantly improved.