Jan Zedník

EARTHQUAKES IN THE CZECH REPUBLIC AND SURROUNDING REGIONS IN 1995-1999

In the time span from January 1995 to December 1999 the Czech National Seismological Network (CNSN), consisting of ten permanent digital broadband stations, several local networks and two data centers, detected and recorded 9530 regional natural seismic events, 27 greater than magnitude 2. Most of these events were located by the Czech Seismological Service (CSS), and the most prominent of them were analyzed in detail. A large number of quarry blasts were recorded as well but were not included in the analysis. We provide basic information on the configuration of the CNSN and on the way of routine data processing employed by the CSS in this paper. The over-all regional seismicity monitored by the CNSN in 1995–1999 is briefly reviewed. The main results of observations and evaluation of the local (NW-Bohemia/Vogtland, South Bohemia, Sudeten) and induced (Kladno, Příbram, Upper Silesia, Lubin/Poland) seismic activity within this period are presented in a condensed form. Finally, a summary on macroseismic observations on the territory of the Czech Republic in 1995–1999 is also presented.

Contribution of Local Seismic Networks To The Regional Velocity Model of The Bohemian Massif

The routine location of regional seismic events using data from the Czech National Seismological Network (CNSN) is based on Pn, Pg, Sn, Sg phases. A simple velocity model derived from Kárníks (1953) interpretation of an earthquake in Northern Hungary in 1951 has hitherto been used. At present, numerous local seismic networks record and locate local events, which are occasionally recorded at regional distances as well. Due to the relatively small dimensions of local networks, hypocenters (and origin times) determined by a local network might be considered as nearly exact from the point of view of regional-scale CNSN. The comparison of common locations performed by CNSN and by a local network enables us to estimate the accuracy of CNSN locations, as well as to optimize a simple velocity model. The joint interpretation of the CNSN bulletin and the catalogues of four local seismic networks WEBNET, OSTRAVA, KLADNO and LUBIN produced a new ID velocity model. The most frequent epicentral error in this model is less than 5 km, and most foci lie up to 15 km from the true position. The performed analysis indicates bimodal distribution of Sn residuals.

Theoretical pv-magnitude calibrating curves for deep foci

РезюмеРaссчumaны mеореmuческuе кaлuбровочные крuвые крuвые ¶rt;ля оnре¶rt;еленuя мaгнumу¶rt;ы Р-волн nо сре¶rt;неnерuо¶rt;ноŭ annaрamуре ¶rt;ля мо¶rt;елu Землu РREМ в шuроком ¶rt;uanaзонеглубuн очaгa. Полученные крuвuе оmрaжaюm рaсnре¶rt;еленuе naрaмеmров мо¶rt;елu без учеma uзлученuя uз uсmочнuкa. Шuроко nрuменяемые эмnuрuческuе кaлuбровочные крuвые Гуmенбергa былu срaвнены с mеореmuческuмu крuвымu. Былu нaŭ¶rt;ены чеmкuе рaзлuчuя nо форме u уровням крuвых. Тaкuм обрaзом, еслu nрuняmь мо¶rt;ель РREМ nре¶rt;сmaвumельноŭ ¶rt;ля сmроенuя Землu, mо необхо¶rt;uмо nроверumь эмnuрuческuе кaлuбровочные крuвые σ(D, h) ¶rt;ля оnре¶rt;еленuя мaгнumу¶rt;ы, коmорые в нaсmоящее время uсnользуюmся в сеŭсмологuческоŭ nрaкmuке.SummaryThe theoretical medium period PV-magnitude calibrating curves were computed for the Earth model PREM and a wide range of focal depths. The calculated set of curves reflects the distribution of model parameters, the influence of source radiation was not taken into account. The widely used Gutenbergs empirical calibrating curves were compared with the theoretical ones. Pronounced deviations in the shape and differences in the level of isolines were found. Thus, if model PREM is considered to be representative of the Earths structure, the empirical calibrating curves σ(D, h) for magnitude determination currently used in seismological practice, have to be verified.

The Rupture Process of the Manjil, Iran Earthquake of 20 June 1990 and Implications for Intraplate Strike-Slip Earthquakes

In terms of seismically radiated energy or moment release, the earthquake of 20 January 1990 in the Manjil Basin-Alborz Mountain region of Iran is the second largest strike-slip earthquake to have occurred in an intracontinental setting in the past decade. It caused enormous loss of life and the virtual destruction of several cities. Despite a very large meizoseismal area, the identification of the causative faults has been hampered by the lack of reliable earthquake locations and conflicting field reports of surface displacement. Using broadband data from global networks of digitally recording seismographs, we analyse broadband seismic waveforms to derive characteristics of the rupture process. Complexities in waveforms generated by the earthquake indicate that the main shock consisted of a tiny precursory subevent followed in the next 20 seconds by a series of four major subevents with depths ranging from 10 to 15 km. The focal mechanisms of the major subevents, which are predominantly strike-slip, have a common nodal plane striking about 285°–295°. Based on the coincidence of this strike with the dominant tectonic fabric of the region we presume that the EW striking planes are the fault planes. The first major subevent nucleated slightly south of the initial precursor. The second subevent occurred northwest of the initial precursor. The last two subevents moved progressively southeastward of the first subevent in a direction collinear with the predominant strike of the fault planes. The offsets in the relative locations and the temporal delays of the rupture subevents indicate heterogeneous distribution of fracture strength and the involvement of multiple faults. The spatial distribution of teleseismic aftershocks, which at first appears uncorrelated with meizoseismal contours, can be decomposed into stages. The initial activity, being within and on the periphery of the rupture zone, correlates in shape and length with meizoseismal lines. In the second stage of activity the aftershock zone expands and appears to cluster about the geomorphic and geologic features several tens of kilometres from the rupture zone. The activity is interpreted as a regional response to quasistatic stress migration along zones of tectonic weakness. The radiated energy of the main shock and the estimate of seismic moment yields an apparent stress of 20 bars. High apparent stress may be typical of strike slip earthquakes occurring in intracontinental environments undergoing continental collision.

Source radiation pattern andPKP amplitudes from the Tonga region

РезюмеС¶rt;елaно срaвненuе оmношенuŭ aмnлumу¶rt; короmкоnерuо¶rt;ных волн PKP1 u PKP2 оmглубокuх землеmрясенuŭ Тонгa, зanuсaнных о¶rt;ноŭ сеŭсмuческоŭ сmaнцuеŭ Ценmрaльноŭ Евроnы, с сооmвеmсmвующuмu mеореmuческuмu велuчuнaмu, рaссчumaннымu ¶rt;ля рaзных naрaмеmров uзлучaющего uсmочнuкa uглубuн очaгa. В рaсчеmaх uсnользовaнa мо¶rt;ель PREM. Нaблю¶rt;енные u рaссчumaнные велuчuны ¶rt;aюm хорошее кaчесmвенное соглaсuе ¶rt;ля очaгового uзлученuя, сооmвеmсmвующего верояmноŭгеомеmрuu зоны суб¶rt;укцuu в рaŭоне Тонгa. Обсуж¶rt;aеmся влuянuе вaрuaцuŭ naрaмеmров uсmочнuков u мо¶rt;елu нa велuчuны рaсчеmных оmношенuŭ aмnлumу¶rt; PKP1 u PKP2.SummaryThe relation of short period PKP1 and PKP2 amplitudes from deep earthquakes in the Tonga region observed at a seismological station in Central Europe, is compared with the theoretical relations computed for different source radiation parameters and depths. The Preliminary Earth Model was used in the computations. Qualitatively good agreement of observed and calculated values was found for the radiation pattern corresponding to the likely geometry of the Tonga subduction zone. The influence of the variation of some source and model parameters on the calculated PKP1 and PKP2 relation is discussed.

Observed and calculated short-periodPKIKP/PKP1 amplitude ratio

РезюмеС¶rt;елaно срaвненuе оmнощенuŭ aмnлumу¶rt; короmкоnерuо¶rt;ных волн PKIKP u PKP1 оmлубокuх землеmрясенuŭ Тонa, зanuсaнных сеŭсмuческоŭ сmaцuеŭ HKC, с сооmвеmсmвующuмu mеореmuческuмu велuчuнaмu, рaссчumaннымu ¶rt;ля верояmноŭеомеmрuu зоны суб¶rt;укцuu в рaŭоне Тонa u мо¶rt;елu PREM. Нaблю¶rt;енные u рaссчumaнные велuчuны ¶rt;aюm колuчесmвенно хорощее солaсuе. Обсуж¶rt;aеmся влuянuе вaрuaцuŭ nерuо¶rt;ов Т u уклонa л нa велuчuны рaссчеmных aмnлuму¶rt; PKIKP u PKP1.SummaryThe ratio of short-period PKIKP and PKP1 amplitudes of deep earthquakes in the Tonga region observed at the seismological station KHC is compared with the theoretical ratio computed for the probable geometry of the Tonga subduction zone and the Preliminary Reference Earth Model. Quantitatively good agreement of observed and calculated values was attained. The influence of the period of the signal and of a broad range of the source parameter rake on the calculated PKIKP/PKP1 amplitude ratio is investigated.