Determining earthquake epicentre from a single 3-component seismological station

An attempt has been made in this study to determine the epicentres by trigonometrical method from a single station using the three components of the ground's motion recorded by the Benioff Short Period Seismograph of the World Wide Standardized Seismograph Network (WWSSN) at Central Seismological Observatory (C.S.O.), Shillong. The trigonometrical method is based on the angle of bearing obtained from the ratio of the amplitudes of waves recorded on the horizontal components of the WWSSN seismograms. A comparison of recorded epicentres from the I.M.D. Seismological Network of Seismo, New Delhi with that of a single station i.e., C.S.O., Shillong has been made to estimate the accuracy of the method. The characteristics of the epicentral variations obtained from the Seismological Network of Seismo, New Delhi with that of C.S.O., Shillong are relatively small for local earthquakes compared to regional earthquakes with C.S.O., Shillong as the reference station. The characteristics of the epicentral variations particularly in the Shillong plateau are observed to be minimum. Strong correlations of epicentres are observed for both local as well as regional earthquake events.

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.

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.

Results of seismological observations on the territory of the Republic of North Ossetia-Alania and adjacent areas in 2020

North-Ossetian Division of the Geophysical Survey of the Russian Academy of Sciences (NOD GS RAS) carries out the continuous seismological observations of the central part of the North Caucasus region. The branch's seismological network includes 12 observation points located on the territory of the Republic of North Ossetia-Alania. The paper provides basic information about the places of installation of seismic stations and their equipment, and notes the registration possibilities of the network. The total number of registered seismic events and separately within the administrative territories in 2020 is given. The total released seismic energy is calculated. A comparison of the energy indicator with the previous year is given. The schedule of repeatability is calculated and constructed, which indicates the level of reliability of event registration. The analysis of the earthquakes foci distribution with ref-erence to geotectonic structures is presented on the map of the epicenters of recorded seismic events with an indication of the distribution against the background of the focal zones. Iden-tified and described the swarm sequences of seismic events, which occurred in the territories of the Republics of North Ossetia-Alania and Kabardino-Balkaria, as well as in the Stavropol Territory. The mechanisms of the foci for the three most powerful earthquakes are deter-mined and their stereograms are given. The obtained results will allow us to study the chang-es in the seismic regime of the North Caucasus region more effectively.

TO STUDY THE POSSIBILITIES OF USING THE SEISMOLOGICAL NETWORK OF THE ALTAI-SAYAN REGION FOR REGIME VIBRO-SEISMIC OBSERVATIONS

The results of experiments on the study of the radiation characteristics of powerful vibrators and reception conditions at a number of seismic stations in the Altai-Sayan region in the range of distances of 50-450 kilometers in the problem of selecting points for active vibro-seismic monitoring are presented. It is shown that the real effective range of the CV-40 vibrator can be significantly increased by using multiple and semi-multiple harmonics. A method for assessing the applicability of stations for vibration monitoring using special programs for assessing the seismic background and seismic resonances within the boundaries of the location of seismic stations is proposed and tested.

National Seismological Network in India for Real-Time Earthquake Monitoring

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.

Micro-seismicity, seismic-wave velocity model and earthquake clustering in the Akarnanian region (western Greece)

<p>Characterized for the first time in 2013, the Island Akarnanian Block (IAB) is a micro-plate located in the western Greece. This micro-plate accommodates the deformation in between larger scale tectonic structures as the Gulf of Corinth (South-East), the Hellenic subduction (South) and the Apulian Collison (West).</p><p><span>W</span><span>e </span><span>start</span><span>ed a micro-seismic </span><span>survey</span><span> (MADAM) at the end of 2015 with a dense seismological network </span><span>over the area, between the Gulf of Patras and the Gulf of Amvrakikos. </span><span>In order t</span><span>o obtain precise locations of the recorded events, we better constrained the local velocity model. In fact, </span><span>s</span><span>everal velocity models </span><span>(local or regional) </span><span>have been proposed for this area. </span><span>H</span><span>owever,</span><span> the velocity model generally used by the scientific community remains the Hasslinger 98 velocity model. This model, nevertheless, raises some questions about its physical meaning, mainly due to a low velocity layer bet</span><span>w</span><span>ee</span><span>n </span><span>4 and 7 km-depth. </span></p><p><span>Thanks to our seismological network and permanent networks of the Corinth Rift Laboratory and the Hellenic Unified Seismic Network, w</span><span>e colle</span><span>c</span><span>ted and anal</span><span>y</span><span>s</span><span>ed</span><span> a huge quantity of data </span><span>a</span><span>c</span><span>quired </span><span>between </span><span>O</span><span>ctober 2015 and </span><span>D</span><span>ecember 2017. </span><span>Those analyses of </span><span>more than </span><span>10,000</span><span> events allowed us to </span><span>develop </span><span>a new </span><span>and robust </span><span>local velocity model, </span><span>wh</span><span>ich</span><span> is consi</span><span>s</span><span>tent with the seismic data and </span><span>the </span><span>geophysical obse</span><span>r</span><span>vations</span><span>.</span></p><p>The observed seismic activity is characterized by the presence of numerous clusters. The clusters are analysed in detail by relative relocations in order to appraise their physical processes and their possible implications in the fault activity to finally have a better understanding of the deformation mode(s) of the IAB micro-plate.</p>

METEOR: Online Seismic Metadata Builder

METEOR is an online tool dedicated to the generation of instrumental responses for seismological stations. This tool builds the complete response of the acquisition chain, while keeping the number of fields to be filled in very reduced. METEOR queries a very rich response databank, designed for the management of the French broadband seismological network (RESIF) and for many research projects involving velocimetric and accelerometric sensors over the last 20 yr. This tool is not intended to allow the management of metadata of an entire network, but to quickly build the instrumental responses of temporary stations. Information about the measuring instruments and their settings can be entered directly in the field, or later, back in the office. Instrument responses can be produced in the International Federation of Digital Seismograph Networks Station Extended Markup Language format (FDSN StationXML) and the metadata-only form of the Standard for Exchange of Earthquake Data (dataless SEED). METEOR is publicly available (see Data and Resources).