Jaromír Janský

LAYERED VELOCITY MODELS OF THE WESTERN BOHEMIA REGION

A new robust and effective optimization algorithm – isometric algorithm – was used for the inversion of layered velocity models, with constant gradient in each layer, to find suitable 1-D models for the location of microearthquakes in the individual four subregions of the West Bohemian earthquake swarm region. Models which are considered as optimal yield the minimum sum of the absolute values of the travel-time residua in locating the whole group of earthquakes in the given subregion. The results obtained from the inversion of P and S waves and from P waves only are shown. For comparison, optimum homogeneous models derived by the grid search method, again using both P and S waves and P waves only, are given. The computations indicate that the models for the individual subregions differ from each other. For layered models the differences are more pronounced, as expected, in the upper parts, down to depths of about 5 km. In comparison with the subregions Nový Kostel and Plesná, the P and S wave velocities for subregion Lazy are relatively higher and the P and S velocities for subregion Klingenthal relatively lower. In the lower parts the differences are smaller and the velocities have practically identical gradients. The highest velocities were obtained for subregion Lazy and the lowest velocities for subregion Klingenthal, as well for the homogeneous models. The model that represents the whole swarm region was determined in a similar way. This model is compared with the previously published velocity-depth distribution, obtained from DSS profile VI/70 in the vicinity of the area under study.

Ray amplitudes of seismic body waves in inhomogeneous radially symmetric media

РезюмеИсхо¶rt;я uз сnецuaльноŭ annроксuмaцuu скоросmного рaзрезa, nолучены nросmые aнaлumuческuе вырaженuя ¶rt;ля лучевых uнmегрaлов,геомеmрuческого рaсхож¶rt;енuя u лучевых aмnлumу¶rt; обьёмных волн, рaсnросmрaняющuхся в сферuческu сuммеmрuчноŭ мо¶rt;елu Землu. Эma annроксuмaцuя обесnечuвaеm неnрерывносmь скоросmu u её nервоŭ u вmороŭ nроuзво¶rt;ных, не обрaзуеm слоев nонuженноŭ скоросmu u uскуссmвенных aномaлuŭ aмnлumу¶rt;ных крuвых. Пре¶rt;ложеннaя annроксuмaцuя maкже знaчumельно nовышaеm усmоŭчuвосmь aмnлumу¶rt;ных крuвых.

Homogeneous Velocity Models of The West Bohemian Swarm Region Obtained By Grid Search

West Bohemian earthquake swarms are used to determine the parameters of simple homogeneous velocity models of the individual subregions of the given area, based on a group of earthquakes that occurred in these subregions. The grid search method is used for location. Models yielding the minimum sum of squares of the travel-time residua in locating the whole group of earthquakes in the given subregion are considered suitable. Relocation obtained by grid search is compared with that obtained by the FASTHYPO method. The computations indicate that the subregions under consideration can be, from the point of view of earthquake location, sufficiently represented by homogeneous models, but the models differ for the data from different subregions. The velocities of these models are given. The models under consideration are compared with some of the previously published 1-D models of the broader region of Western Bohemia.

Earthquake location from P-arrival times only: problems and some solutions

Selected problems related to accurate hypocenter locations are discussed in the difficult case that only reliable P-wave readings are available. Near stations are usually only few, and often have a poor azimuthal coverage. As such, they are insufficient because the inversion is highly ill-posed, and the epicenter position strongly trades-off with depth. Thus more distant stations are also needed to obtain the correct epicenter. However, joint use of near and distant stations present another difficulty; it may yield a significantly incorrect depth estimate in case that the crustal model is not fully appropriate. In practice, the erroneous depth often remains unrecognized. An indication of the depth problem can be obtained by analyzing the travel-time residuals at individual stations. It is also useful to check fully independent depth estimates, for example those from the centroid-moment-tensor analysis. If the problematic crustal model is detected, and it is not easy to find a better one, the near- and distant station effects should be decoupled (a two-step location): the epicenter is calculated from all stations, kept fixed, and the source depth is grid-search beneath the epicenter by means of the near stations. The ideas are applied to the Mw 5.2 Efpalio (Western Greece) earthquake of January 18, 2010, and the following aftershock sequence.

Magnitude determination based on short-period core waves

SummaryPKP-amplitudes of about 400 earthquakes from the SW Pacific Ocean observed at three stations in Central Europe were used to obtain the calibration function qPKP(D, h) based on the NEIS mb magnitudes. The calibration function allows us to determine the magnitudes for week earthquakes from the Pacific, for which the magnitudes from the P-waves cannot be derived.

A recent deep earthquake doublet in light of long-term evolution of Nazca subduction

Earthquake faulting at ~600 km depth remains puzzling. Here we present a new kinematic interpretation of two Mw7.6 earthquakes of November 24, 2015. In contrast to teleseismic analysis of this doublet, we use regional seismic data providing robust two-point source models, further validated by regional back-projection and rupture-stop analysis. The doublet represents segmented rupture of a ∼30-year gap in a narrow, deep fault zone, fully consistent with the stress field derived from neighbouring 1976–2015 earthquakes. Seismic observations are interpreted using a geodynamic model of regional subduction, incorporating realistic rheology and major phase transitions, yielding a model slab that is nearly vertical in the deep-earthquake zone but stagnant below 660 km, consistent with tomographic imaging. Geodynamically modelled stresses match the seismically inferred stress field, where the steeply down-dip orientation of compressive stress axes at ∼600 km arises from combined viscous and buoyant forces resisting slab penetration into the lower mantle and deformation associated with slab buckling and stagnation. Observed fault-rupture geometry, demonstrated likelihood of seismic triggering, and high model temperatures in young subducted lithosphere, together favour nanometric crystallisation (and associated grain-boundary sliding) attending high-pressure dehydration as a likely seismogenic mechanism, unless a segment of much older lithosphere is present at depth.

Fresnel volumes corresponding to PKP waves in the IASP91 model

Fresnel volumes provide a suitable tool for estimating the resolution limits or accuracy of tomographic methods based only on the arrival times of these seismic waves, which penetrate the deep interior of the Earth. The standard definition of the Fresnel volume was modified to include effects of multipathing, i.e., multivalued travel times. Computations were performed in the IASP91 model for three representative epicentral distances and two prevailing signal periods. Preliminary results have been obtained. They place a rather strong restriction on the resolution limits of tomographic methods, depending on the prevailing period, due to the relatively large fatness of PKP Fresnel volumes.

Fast computation of ray synthetic seismograms in vertically inhomogeneous media

РезюмеПре¶rt;лaгaеmся меmо¶rt; бысmрого рaсчеma сuнmеmuческuх лучевых сеŭсмогрaмм объгемных волн в верmuкaльно нео¶rt;норо¶rt;ноŭ сре¶rt;е, uсхо¶rt;я uз сnецuaльноŭ annроксuмaцuu скоросmного рaзрезa, коmорaя обесnечuвaеm неnрерывносmь скоросmu u еге nервоŭ u вmороŭ nроuзво¶rt;ных u не nрuво¶rt;um к обрaзовaнuю uскуссmвенных слоев nонuженноŭ скоросmu. Крamко оnuсaн aлгорumмгруnы nрогрaмм ZESY82, основaнноŭ нa эmом меmо¶rt;е. Точечныŭ uсmочнuк с любоŭ хaрaкmерuсmuкоŭ uзлученuя рaсnоложен в любоŭ mочке мо¶rt;елu; nрuгемнuкu ¶rt;олжны быmь рaсnре¶rt;елены в¶rt;оль nрофuля, со¶rt;ержaщего эnuценmр. Прuве¶rt;ен nрuмер сuнmеmuческuх сеŭсмогрaмм ¶rt;ля о¶rt;ноŭ мо¶rt;елu земноŭ коры u верхнеŭ мaмmuu.SummaryA procedure of fast computation of body-wave ray synthetic seismograms in vertically inhomogeneous media is suggested. The procedure uses a special approximation of the velocitydepth distribution which guarantees continuity of the first and second derivatives of velocity and does not generate false low-velocity layers (oscillations in the velocity-depth function). The ZESY82 program package, which is based on the suggested procedure, is described. The point source with an arbitrary radiation pattern may be situated at any points of the model, the receivers are situated regularly or irregularly along any profile on the Earths surface, containing the epicentre. Numerical examples of the synthetic record sections for a model of the Earths crust and the uppermost mantle are given.

Refracted wave in a horizontally stratified medium with constant velocity gradients

ZusammenfassungDie Arbeit befasst sich mit der Berechnung der Laufzeit- und Amplitudenkurve einer Refraktionswelle im horizontal geschichteten Medium mit konstanten Geschwindigkeitsgradienten in einzelnen Schichten. Die Laufzeitkurve wird auf Grund geometrischer Seismik, die Amplitudenkurve auf Grund der Null-Annäherung der Strahlentheorie berechnet. Ausser den Formeln wird eine kurze Beschreibung des für die Berechnung und Analyse des Verhaltens der Refraktionswelle in Abhängigkeit vom Geschwindigkeitsschnitt zusammengestellten Programms angeführt.

Analysis of the source scanning algorithm with a new P-wave picker

We analyze the location of earthquakes in near regional networks using complete seismic records. The method is based on the source scanning algorithm (SSA) of Kao and Shan (2004), but similarly to Grigoli et al. (2013), seismograms are substituted by a P-wave picker trace. The picker traces in a network are repeatedly stacked using grid of trial source positions, and hypocenter is identified with the point providing the best stack (the largest brightness). The first innovation of this paper is a new picker, measuring the ratio of the summed absolute values of seismogram in the right and left part of a moving time window, the RPA/LPA picker. The brightness maps based on this picker are clearer than those based on the STA/LTA picker. The second innovation is a simple theoretical model of the brightness maps. It makes it easy to identify how individual stations contribute to form the brightness spot. It is shown on synthetic tests that the performance of the method depends on focal mechanism, progressively improving from normal to reverse and strike-slip events. The method is successfully applied to four events of different mechanisms and depths, recorded at different ranges of epicentral distance by either broad-band sensors or accelerographs. The events have been located close to previously published epicenters. The brightness maps provide an estimate of the relative uncertainty of the (non-linear) location problem. The uncertainty estimate is also applicable without measured arrival times, “without earthquakes”, thus useful when designing or upgrading seismic networks for better location performance.