Advances in earthquake-swarms monitoring networks WEBNET and REYKJANET

Mapping Intimacies ◽

10.5194/egusphere-egu2020-8391 ◽

2020 ◽

Author(s):

Jakub Klicpera ◽

Jana Doubravová ◽

Josef Horálek

Keyword(s):

Main Shock ◽

Plate Boundary ◽

Earthquake Swarms ◽

Monitoring Networks ◽

Epicentral Zone ◽

Epicentral Area ◽

The North ◽

Short Period ◽

West Bohemia ◽

Reykjanes Peninsula

The IG CAS in cooperation with IRSM CAS operates two local seismic networks deployed to monitor the seismic swarms in West Bohemia/Vogtland, Czechia and Reykjanes Peninsula, Iceland.&#160;WEBNET monitors the region of West Bohemia since 1991 developing from 4 short period stations to 24 broadband stations today. The seismoactive region West Bohemia/Vogtland lies in the border area between Czechia and Germany in the western part of Bohemian Massif. It is an intra-continental area with persistent swarm-like seismicity but rarely also main-shock after-shock sequences may occur.&#160;REYKJANET local seismic network is situated in Reykjanes Peninsula on Southwest Iceland. The area is an onshore part of the mid-Atlantic plate boundary between the North America and Eurasia Plates. The seismic activity of Reykjanes peninsula is represented by typical main-shock after-shock sequences as well as earthquake swarms. The REYKJANET network was built in 2013 and it consists of 15 stations placed around the epicentral area.Both networks have been substantially upgraded during the last years. In case of REYKJANET the replacement of old sensors and digitizers with new ones made the operation easier and ready for near future plan to stream the waveform files in real time. WEBNET network which was long years divided into two subnets &#8211; on-line permanent stations and off-line autonomous stations, was recently homogenized by eco-powering and 4G LTE data connecting of the off-line stations. Additonally, the micro network HORNET was deployed within the WEBNET epicentral zone to monitor Horka water dam.Data from both above mentioned networks are automatically searched for seismic events by the neural-network-based detector designed by Doubravov&#225; et al. (2016, 2019) providing event list with completeness magnitude Mc=0 for REYKJANET and Mc=-0.5 for WEBNET. The main difference of sensitivity is given by different noise levels of the two networks.

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Aftershocks and surface faulting associated with the intraplate Guinea, West Africa, earthquake of 22 December 1983

Bulletin of the Seismological Society of America ◽

10.1785/bssa0770051579 ◽

1987 ◽

Vol 77 (5) ◽

pp. 1579-1601

Author(s):

C. J. Langer ◽

M. G. Bonilla ◽

G. A. Bollinger

Keyword(s):

West Africa ◽

Focal Mechanism ◽

Main Shock ◽

Strike Slip ◽

Fault System ◽

Epicentral Area ◽

Lateral Strike Slip ◽

Focal Mechanism Solutions ◽

Short Period ◽

East West

Abstract This study reports on the results of geological and seismological field studies conducted following the rare occurrence of a moderate-sized West African earthquake (mb = 6.4) with associated ground breakage. The epicentral area of the northwestern Guinea earthquake of 22 December 1983 is a coastal margin, intraplate locale with a very low level of historical seismicity. The principal results include the observation that seismic faulting occurred on a preexisting fault system and that there is good agreement among the surface faulting, the spatial distribution of the aftershock hypocenters, and the composite focal mechanism solutions. We are not able, however, to shed any light on the reason(s) for the unexpected occurrence of this intraplate earthquake. Thus, the significance of this study is its contribution to the observational datum for such earthquakes and for the seismicity of West Africa. The main shock was associated with at least 9 km of surface fault-rupture. Trending east-southeast to east-west, measured fault displacements up to ∼13 cm were predominantly right-lateral strike slip and were accompanied by an additional component (5 to 7 cm) of vertical movement, southwest side down. The surface faulting occurred on a preexisting fault whose field characteristics suggest a low slip rate with very infrequent earthquakes. There were extensive rockfalls and minor liquefaction effects at distances less than 10 km from the surface faulting and main shock epicenter. Main shock focal mechanism solutions derived from teleseismic data by other workers show a strong component of normal faulting motion that was not observed in the ground ruptures. A 15-day period of aftershock monitoring, commencing 22 days after the main shock, was conducted. Eleven portable, analog short-period vertical seismographs were deployed in a network with an aperture of 25 km and an average station spacing of 7 km. Ninety-five aftershocks were located from the more than 200 recorded events with duration magnitudes of about 1.5 or greater. Analysis of a selected subset (91) of those events define a tabular aftershock volume (26 km long by 14 km wide by 4 km thick) trending east-southeast and dipping steeply (∼60°) to the south-southwest. Composite focal mechanisms for groups of events, distributed throughout the aftershock volume, exhibit right-lateral, strike-slip motion on subvertical planes that strike almost due east. Although the general agreement between the field geologic and seismologic results is good, our preferred interpretation is for three en-echelon faults striking almost due east-west.

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The 2020 volcano-tectonic unrest at Reykjanes Peninsula, Iceland: stress triggering and reactivation of several volcanic systems

10.5194/egusphere-egu21-7534 ◽

2021 ◽

Author(s):

Halldór Geirsson ◽

Michelle Parks ◽

Kristín Vogfjörd ◽

Páll Einarsson ◽

Freysteinn Sigmundsson ◽

...

Keyword(s):

Earthquake Swarm ◽

Plate Boundary ◽

Tectonic Activity ◽

Volcanic System ◽

Multiple Systems ◽

The North ◽

Stress Changes ◽

Uplift Rates ◽

Source Models ◽

Reykjanes Peninsula

The Reykjanes Peninsula in south-west Iceland straddles the North-America - Eurasia plate boundary and hosts several active volcanic systems, including the Svartsengi volcanic system. The last eruption in this area took place around 1240 CE, with eruptive episodes recurring every 800-1000 years, affecting one volcanic system at a time, but spanning multiple systems &#160;with activity spaced ~100 to 200 years. In January 2020, unrest was identified in Svartsengi, characterized by intense seismicity and inflation at a rate of 3-4 mm per day. This area is located within 5 km of several important infrastructures: a) the town of Grindav&#237;k; b) the Svartsengi geothermal power plant; c) and the Blue Lagoon geothermal spa, which had over a million annual visits before the Covid pandemy.&#160;Two continuously recording GNSS stations were installed in the Svartsengi geothermal area in 2013-2015 to monitor geothermally-induced subsidence.&#160; Coinciding with the onset of an earthquake swarm starting on January 21 (M<4), uplift of about 3-4 mm/day was noticed in automated GNSS and InSAR results. The uplift rates in this first inflation phase decreased after January 31 and reverted to slight subsidence in early February. Interestingly, the most intense seismicity was offset from the uplift center by about 2-4 km to the southeast. Geodetic source models from the initial two weeks indicate the deformation is the result of a sill intrusion at a depth of about 4 km &#160;with a volume change of approximately 3 &#160;million m3. The resulting stress changes from this intrusion act to increase seismicity at the sill edges, thus offering an explanation for why the seismicity is offset from the center of uplift. The location of the sill coincides roughly with a crustal volume with a high Vp/Vs ratio.&#160;Two more inflation-deflation episodes have occurred at Svartsengi in 2020 and the total uplift amounts to approximately 12 cm. Additionally, at least one inflation episode occurred in the Reykjanes system, in February 2020, and inflation started in the Kr&#253;suv&#237;k system in mid-July 2020, culminating in a M5.6 earthquake on October 20. The Fagradalsfjall system, between Kr&#253;suv&#237;k and Svartsengi, has shown high seismicity in 2020, but does not display detectable inflation nor deflation. Therefore, the volcano-tectonic activity in 2020 spans the entire western part of the Reykjanes Peninsula. The stress changes for each of these events are too small to explain the cross-system activity, hence we suggest the entire unrest is &#160;by deep magma migration beneath the entire western Reykjanes Peninsula.&#160;&#160;

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Earthquake swarms in West Bohemia-Vogtland and South-West Iceland: are they of similar nature?

10.5194/egusphere-egu2020-7904 ◽

2020 ◽

Author(s):

Josef Horálek ◽

Hana Jakoubková ◽

Jana Doubravová ◽

Martin Bachura

Keyword(s):

Seismic Moment ◽

Earthquake Swarm ◽

Aftershock Sequence ◽

Earthquake Swarms ◽

A Value ◽

South West ◽

Aftershock Sequences ◽

West Bohemia ◽

Reykjanes Peninsula ◽

Seismic Moment Release

Earthquake swarms occurred worldwide in diverse geological units, however, their origin is still unclear. West Bohemia-Vogtland represents one of the most active intraplate earthquake-swarm areas in Europe, South-West Iceland is characterized by intense interplate earthquake swarms. Both these areas exhibit high activity of crustal fluids.We investigated earthquake swarms from W-Bohemia and from different areas in SW-Iceland: the Hengill volcanic complex, &#214;lfus transition zone (the edge of the SISZ), and Reykjanes Peninsula, from the perspective of their magnitude-time development, seismic moment release with time, the magnitude-frequency distribution and distribution of the inter-event times, and the space and time distribution of the foci. The aim was to determine the swarm characteristics that are dependent or vice-versa independent on the tectonic environment, and also the characteristics which should help us to distinguish more precisely earthquake swarms from mainshock-aftershock sequences.We found that the frequency-magnitude (b-values) and inter-event-time distributions are similar for both areas, while total seismic moment release and its rate are much larger for the SW Icelandic activities compared to the W-Bohemia ones. One dominant short-term swarm phase with one or a few dominant events in which significant part of M0tot released, is typical of the SW Icelandic swarms, whereas the W-Bohemia swarms are characterised by stepwise seismic moment release, which is manifested by several swarm phases. MFDs of the SW-Iceland swarms indicate significantly lower a-value (number of ML > 0 evens), particularly of those on the Reykjanes Peninsula, compared to W-Bohemia swarms; it is due to the fact that considerable amount of M0tot released in quasi-mainshocks and the rest in aftershocks; lower a-value was also found for the W-Bohemian mainshock-aftershock sequence in 2014. The W-Bohemian swarms took place in a bounded focal zone consisting of several fault segments but the SW-Icelandic swarms correspond well to tectonic structures along the Mid Atlantic Ridge. We conclude that most of the W-Bohemia earthquake swarms were series of subswarms with one or more embedded mainshock-aftershock sequences, while the SW-Icelandic swarms (particularly those on the Reykjanes Peninsula appear to be a transition between earthquake swarm and mainshock-aftershock sequence. The W-Bohemia and SW-Iceland focal zones are characterized by complex system of short, differently oriented faults/fault segments; interestingly, the W-Bohemia and some SW-Icelandic focal zones exhibit coexistence of faults susceptible to earthquake swarms and differently oriented faults predisposed to common earthquakes (mainshock-aftershocks).

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TECTONIC POSITION, SISMOLOGICAL AND GEOLOGICAL MANIFESTATION OF 7 SEPTEMBER 2009 ONI?II EARTHQUAKE FOCUS ON THE SOUTHERN SLOPES OF THE GREAT CAUCASUS

Геология и геофизика Юга России ◽

10.23671/vnc.2017.4.9530 ◽

2017 ◽

Author(s):

Е.А. Рогожин

Keyword(s):

Main Shock ◽

Focal Depth ◽

Seismic Source ◽

Epicentral Area ◽

Strong Earthquakes ◽

North East ◽

The North ◽

Tectonic Position ◽

South Slope ◽

Racha Earthquake

В статье приведены сейсмологические и сейсмотектонические материалы о главном толчке и афтершоках Онийского-II землетрясения 7 сентября 2009 г. с Мs = 5,8 на южном склоне Большого Кавказа. Положение облака эпицентров основного толчка и афтершоков совпадает с северной ветвью очаговой зоны Рачинского землетрясения 29.04.1991 г. с МS = 7,0, I0 = 7-8. Глубина гипоцентра основного толчка составляет 8?15 км. В качестве действующей в очаге принята пологая плоскость, погружающаяся в север – северо-восточном направлении. Тип подвижки по такой плоскости – надвиг с компонентами правостороннего сдвига. Сейсмодислокации носили вторичный, гравитационный характер. Результаты палеосейсмологические исследований, проведенных в восточной части эпицентральной области, Рачинского землетрясения, показали, что в этом сейсмической очаге и раньше происходили сильне сейсмические толчки. Согласно полученным данням возраст предыдущего сильного землетрясения в Рача-Джавской зоне (т. е. до 1991 г.) – около 2000 лет назад. Еще одно, болем древнее событие произошло около 6000 лет назад. Период повторяемости сильних землетрясений, подобных катастрофе 1991 г., таким образом, составляет в среднем 2000-3000 лет. The article provides seismological and seismotectonic materials about the main shock and aftershocks of the Oni-II earthquake of 7 September 2009, with MS = 5,8 on the South slope of the Greater Caucasus. The position of the cloud of epicenters of the main shock and aftershocks coincides with the northern branch of the focal zone of 29.04.1991 Racha earthquake, MS = 7,0, I0 = 7?8. The focal depth of the main shock is 8 to 15 km. As the active in the focus adopted the sloping plane, plunging to the North – North-East direction. Type progress on such a plane – thrust with component of right-lateral strike-slip. Seismodislocationswere of secondary gravitational nature. The results of paleoseismological studies conducted in the Eastern part of the epicentral area of the Racha earthquake, showed that this seismic source the strong seismic shocks happened before. According to the obtained data, the age of the previous strong earthquake in the Racha – Dzhava zone (i.e., before 1991) – about 2000 years ago. Another, more ancient event occurred about 6,000 years ago. The recurrence period of strong earthquakes, similar to the disaster of 1991, thus, is an average of 2000?3000 years.

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Complexity measures and variability of the seismicity monitored whithin the Mexican flat slab before the main shock occurred on 19 September 2017

10.5194/egusphere-egu2020-6184 ◽

2020 ◽

Author(s):

Alejandro Ramírez-Rojas ◽

Elsa Leticia Flores-Márquez

Keyword(s):

North America ◽

Subduction Zones ◽

Main Shock ◽

Plate Boundary ◽

North American Plate ◽

Cocos Plate ◽

Complexity Measures ◽

Flat Slab ◽

The North ◽

Natural Time

Several subduction zones exists in Earth, which have a more or less known dynamic, however each of them has its particularities, as in the case of the Mexican subduction zone, where the flat slab is of special interest. The present flat-slab area is located along the central part of the Cocos-North America plate boundary that the convergence rate between Cocos and North America. The Cocos plate is a remnant of the large Farallon plate, which began to split into smaller plates since 28 Myr ago approximately, when the East Pacific Rise began to interact with the North American Plate. Within such flat slab could be trigger large and destructives earthquakes like the main shock occurred close to Mexico City on September 19, 2017. In this work, we analyze, under the natural time domain, the seismicity registered within the Mexican flat slab since 1995 until the main shock occurred on September 19, 2017. We analyzed the fluctuations of order parameter for seismicity in order to provide some complex measures defined on natural time. Our analysis reveals a possible precursor measure switching on a few weeks before the main shock.&#160; Also we have observed that in the flat slab region the number of earthquakes recorded is lesser than those observed along the total south Pacific Mexican coast.

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Aftershocks of the 13 December 1982 North Yemen earthquake: Conjugate normal faulting in an extensional setting

Bulletin of the Seismological Society of America ◽

10.1785/bssa0770062038 ◽

1987 ◽

Vol 77 (6) ◽

pp. 2038-2055

Author(s):

C. J. Langer ◽

G. A. Bollinger ◽

H. M. Merghelani

Keyword(s):

Main Shock ◽

Aftershock Sequence ◽

Focal Mechanisms ◽

Data Set ◽

Epicentral Area ◽

Focal Mechanism Solutions ◽

The North ◽

Composite Focal Mechanism ◽

Good Agreement ◽

Extensional Setting

Abstract The North Yemen epicentral locale in the southwestern part of the Arabian Peninsula is 200 to 300 km landward from the active rifting of the Red Sea and Sea of Aden. The magnitude 6.0 (MS and mb) main shock of 13 December 1982 locally caused considerable death, injury, and damage and was followed by an extensive aftershock sequence. A 12-day study employing a 10-station portable seismograph network was conducted between 29 December 1982 and 9 January 1983. Hypocentral locations were determined for 230 shocks selected from the thousands of recorded events (duration magnitudes between 1.8 and 4.6). These aftershocks define a source volume that is roughly 20 × 20 × 10 km. From that volume, about half (∼110) of only the best-constrained hypocenters with depths greater than 3 km were selected for detailed analysis. The 110 aftershock data set was divided into subsets according to geographic position (northern and southern) and temporal sequencing (a distinct aftershock sequence late in the monitoring period). A series of composite focal mechanisms show the aftershocks are dip-slip faulting (normal) on planes with north-northwest to northwest strikes and with dips that are variable in amount (∼30° to ∼80°) and direction (southwest and northeast). The strike and extensional nature of these composite focal mechanism solutions are in good agreement with the main shock focal mechanisms, the surficial and bedrock geology of the epicentral area, and the linear surface cracks observed in the field there following the December main shock. We interpret the spatial distribution of our results to describe conjugate faulting episodes associated with north-northwest striking faults.

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THE EMPIRICAL GREEN’S FUNCTION TECHNIQUE MODIFICATION FOR ACCELEROGRAM SYNTHESIS IN LOWSEISMICITY REGIONS

Engineering survey ◽

10.25296/1997-8650-2018-12-5-6-72-80 ◽

2018 ◽

Vol 12 (5-6) ◽

pp. 72-80

Author(s):

A. A. Krylov

Keyword(s):

Green’S Function ◽

Green's Function ◽

Main Shock ◽

Amplitude Spectrum ◽

Strong Motion ◽

Function Technique ◽

Focal Region ◽

Empirical Green’S Function ◽

Epicentral Zone ◽

Empirical Green's Function

In the absence of strong motion records at the future construction sites, different theoretical and semi-empirical approaches are used to estimate the initial seismic vibrations of the soil. If there are records of weak earthquakes on the site and the parameters of the fault that generates the calculated earthquake are known, then the empirical Green’s function can be used. Initially, the empirical Green’s function method in the formulation of Irikura was applied for main shock record modelling using its aftershocks under the following conditions: the magnitude of the weak event is only 1–2 units smaller than the magnitude of the main shock; the focus of the weak event is localized in the focal region of a strong event, hearth, and it should be the same for both events. However, short-termed local instrumental seismological investigation, especially on seafloor, results usually with weak microearthquakes recordings. The magnitude of the observed micro-earthquakes is much lower than of the modeling event (more than 2). To test whether the method of the empirical Green’s function can be applied under these conditions, the accelerograms of the main shock of the earthquake in L'Aquila (6.04.09) with a magnitude Mw = 6.3 were modelled. The microearthquake with ML = 3,3 (21.05.2011) and unknown origin mechanism located in mainshock’s epicentral zone was used as the empirical Green’s function. It was concluded that the empirical Green’s function is to be preprocessed. The complex Fourier spectrum smoothing by moving average was suggested. After the smoothing the inverses Fourier transform results with new Green’s function. Thus, not only the amplitude spectrum is smoothed out, but also the phase spectrum. After such preliminary processing, the spectra of the calculated accelerograms and recorded correspond to each other much better. The modelling demonstrate good results within frequency range 0,1–10 Hz, considered usually for engineering seismological studies.

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