Re-visiting large historical earthquakes in the Colombian Eastern Cordillera

A re-assessment of the historic seismicity of the central sector of the Colombian Eastern Cordillera (EC) is made by revision of bibliographic sources, by calibration with modern instrumental earthquakes, and by interpretations in terms of current knowledge of the tectonics and seismicity of the region. Throughout the process we have derived an equation to estimate M_w for shallow crustal earthquakes in Colombia using the length of isoseismal VIII, L_VIII:

Rupture process of large earthquakes in the northern Mexico subduction zone

The Cocos plate subducts beneath North America at the Mexico trench. The northernmost segment of this trench, between the Orozco and Rivera fracture zones, has ruptured in a sequence of five large earthquakes from 1973 to 1985; the Jan. 30, 1973 Colima event (M _s 7.5) at the northern end of the segment near Rivera fracture zone; the Mar. 14, 1979 Petatlan event (M _s 7.6) at the southern end of the segment on the Orozco fracture zone; the Oct. 25, 1981 Playa Azul event (M _s 7.3) in the middle of the Michoacan gap; the Sept. 19, 1985 Michoacan mainshock (M _s 8.1); and the Sept. 21, 1985 Michoacan aftershock (M _s 7.6) that reruptured part of the Petatlan zone. Body wave inversion for the rupture process of these earthquakes finds the best: earthquake depth; focal mechanism; overall source time function; and seismic moment, for each earthquake. In addition, we have determined spatial concentrations of seismic moment release for the Colima earthquake, and the Michoacan mainshock and aftershock. These spatial concentrations of slip are interpreted as asperities; and the resultant asperity distribution for Mexico is compared to other subduction zones. The body wave inversion technique also determines theMoment Tensor Rate Functions; but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes. An appendix describes theMoment Tensor Rate Functions methodology in detail.The systematic bias between global and regional determinations of epicentral locations in Mexico must be resolved to enable plotting of asperities with aftershocks and geographic features. We have spatially shifted all of our results to regional determinations of epicenters. The best point source depths for the five earthquakes are all above 30 km, consistent with the idea that the down-dip edge of the seismogenic plate interface in Mexico is shallow compared to other subduction zones. Consideration of uncertainties in the focal mechanisms allows us to state that all five earthquakes occurred on fault planes with the same strike (N65°W to N70°W) and dip (15±3°), except for the smaller Playa Azul event at the down-dip edge which has a steeper dip angle of 20 to 25°. However, the Petatlan earthquake does prefer a fault plane that is rotated to a more east-west orientation—one explanation may be that this earthquake is located near the crest of the subducting Orozco fracture zone. The slip vectors of all five earthquakes are similar and generally consistent with the NUVEL-predicted Cocos-North America convergence direction of N33°E for this segment. The most important deviation is the more northerly slip direction for the Petatlan earthquake. Also, the slip vectors from the Harvard CMT solutions for large and small events in this segment prefer an overall convergence direction of about N20°E to N25°E.All five earthquakes share a common feature in the rupture process: each earthquake has a small initial precursory arrival followed by a large pulse of moment release with a distinct onset. The delay time varies from 4 s for the Playa Azul event to 8 s for the Colima event. While there is some evidence of spatial concentration of moment release for each event, our overall asperity distribution for the northern Mexico segment consists of one clear asperity, in the epicentral region of the 1973 Colima earthquake, and then a scattering of diffuse and overlapping regions of high moment release for the remainder of the segment. This character is directly displayed in the overlapping of rupture zones between the 1979 Petatlan event and the 1985 Michoacan aftershock. This character of the asperity distribution is in contrast to the widely spaced distinct asperities in the northern Japan-Kuriles Islands subduction zone, but is somewhat similar to the asperity distributions found in the central Peru and Santa Cruz Islands subduction zones. Subduction of the Orozco fracture zone may strongly affect the seismogenic character as the overlapping rupture zones are located on the crest of the subducted fracture zone. There is also a distinct change in the physiography of the upper plate that coincides with the subducting fracture zone, and the Guerrero seismic gap to the south of the Petatlan earthquake is in the wake of the Orozco fracture zone. At the northern end, the Rivera fracture zone in the subducting plate and the Colima graben in the upper plate coincide with the northernmost extent of the Colima rupture zone.     

On the efficiency with which aerosol particles of radius larger than 0.1 μm are collected by simple ice crystal plates

A theoretical model is presented which allows computing the efficiency with which aerosol particles of radius 0.1r10 m are collected by simple ice crystal plates of radius 50a _c 640 m in air of various relative humidities, temperatures and pressures. Particle capture due to thermophoresis, diffusiophoresis and inertial impaction are considered. It is shown that the capture efficiency of an ice crystal in considerably affected by phoretic effects in the range 0.1r1 m. For aerosol particles ofr>1 m the efficiency is strongly controlled by the flow field around the crystal and the density of the aerosol material. Trajectory analysis also predicts that aerosol particles are preferentially captured by the ice crystal rim. Our theoretica results are found to agree satisfactorily with the laboratory studies presently available. Comparison shows that for the same pressure, temperature and relative humidity of the ambient air ice crystal plates are better aerosol particle scavengers than water drops.     

Attenuation and dispersion ofSH waves due to scattering by randomly distributed cracks

The effect of randomly distributed cracks on the attenuation and dispersion ofSH waves is theoretically studied. If earthquake ruptures are caused by sudden coalescence of preexisting cracks, it will be crucial for earthquake prediction to monitor the temporal variation of the crack distribution. Our aim is to investigate how the property of crack distribution is reflected in the attenuation and dispersion of elastic waves.We introduce the stochastic property, in the mathematical analysis, for the distributions of crack location, crack size and crack orientation. The crack size distribution is assumed to be described by a power law probability density (p(a) a ^– fora _minaa _max according to recent seismological and experimental knowledge, wherea is a half crack length and the range 13 is assumed. The distribution of crack location is assumed to be homogeneous for the sake of mathematical simplicity, and a low crack density is assumed. The stochastic property of each crack is assumed to be independent of that of the other cracks. We assume two models, that is, the aligned crack model and the randomly oriented crack model, for the distribution of crack orientation. All cracks are assumed to be aligned in the former model. The orientation of each crack is assumed to be random in the latter model, and the homogeneous distribution is assumed for the crack orientation. The idea of the mean wave formalism is employed in the analysis, and Foldy's approximation is assumed.We observe the following features common to both the aligned crack model and the randomly oriented crack model. The attenuation coefficientQ ^–1 decays in proportion tok ^–1 in the high frequency range and its growth is proportional tok ² in the low frequency range, wherek is the intrinsic wave number. This asymptotic behavior is parameter-independent, too. The attenuation coefficientQ ^–1 has a broader peak as increases and/ora _min/a _max decreases. The nondimensional peak wave numberk _p a _max at whichQ ^–1 takes the peak value is almost independent ofa _min/a _max for =1 and 2 while it considerably depends ona _min/a _max for =3. The phase velocity is almost independent ofk in the rangeka _max<1 and increases monotonically ask increases in the rangeka _max>1. While the magnitude ofQ ^–1 and the phase velocity considerably depend on the orientation of the crack in the aligned crack model, the above feature does not depend on the crack orientation.The accumulation of seismological measurements suggests thatQ ^–1 ofS waves has a peak at around 0.5 Hz. If this observation is combined with our theoretical results onk _p a _max, the probable range ofa _max of the crack distribution in the earth can be estimated for =1 or 2. If we assume 4 km/sec as theS wave velocity of the matrix medium,a _max is estimated to range from 2 to 5 km. We cannot estimatea _max in a narrow range for =3.     

Types of focal mechanism solutions and parameter consistency of the sub-blocks in Sichuan and Yunnan Provinces

Based on P- and S-wave amplitudes and some clear initial P-wave motion data, we calculated focal mechanism solutions of 928 M≥2.5 earthquakes （1994-2005） in four sub-blocks of Sichuan and Yunnan Provinces, namely Sichuan-Qinghai, Yajiang, Central Sichuan and Central Yunnan blocks. Combining these calculation results with those of the focal mechanism solutions of moderately strong earthquakes, we analyzed the stress field characteristics and dislocation types of seismogenic faults that are distributed in the four sub-blocks. The orientation of principal compressive stress for each block is： EW in Sichuan-Qinghai, ESE or SE in Yajiang, Central Sichuan and Central Yunnan blocks. Based on a great deal of focal mechanism data, we designed a program and calculated the directions of the principal stress tensors, σ1, σ2 and σ3, for the four blocks. Meanwhile, we estimated the difference （also referred to as consistency parameter θ^- ） between the force axis direction of focal mechanism solution and the direction of the mean stress tensor of each block. Then we further analyzed the variation of θ^- versus time and the dislocation types of seismogenic faults. Through determination of focal mechanism solutions for each block, we present information on the variation in θ^- value and dislocation types of seismogenic faults.     

Some comparisons between mining-induced and laboratory earthquakes

Although laboratory stick-slip friction experiments have long been regarded as analogs to natural crustal earthquakes, the potential use of laboratory results for understanding the earthquake source mechanism has not been fully exploited because of essential difficulties in relating seismographic data to measurements made in the controlled laboratory environment. Mining-induced earthquakes, however, provide a means of calibrating the seismic data in terms of laboratory results because, in contrast to natural earthquakes, the causative forces as well as the hypocentral conditions are known. A comparison of stick-slip friction events in a large granite sample with mining-induced earthquakes in South Africa and Canada indicates both similarities and differences between the two phenomena. The physics of unstable fault slip appears to be largely the same for both types of events. For example, both laboratory and mining-induced earthquakes have very low seismic efficiencies where _a is the apparent stress and is the average stress acting on the fault plane to cause slip; nearly all of the energy released by faulting is consumed in overcoming friction. In more detail, the mining-induced earthquakes differ from the laboratory events in the behavior of as a function of seismic momentM ₀. Whereas for the laboratory events 0.06 independent ofM ₀, depends quite strongly onM ₀ for each set of induced earthquakes, with 0.06 serving, apparently, as an upper bound. It seems most likely that this observed scaling difference is due to variations in slip distribution over the fault plane. In the laboratory, a stick-slip event entails homogeneous slip over a fault of fixed area. For each set of induced earthquakes, the fault area appears to be approximately fixed but the slip is inhomogeneous due presumably to barriers (zones of no slip) distributed over the fault plane; at constant , larger events correspond to larger _a as a consequence of fewer barriers to slip. If the inequality _a / 0.06 has general validity, then measurements of _a =µE _a /M ₀, where is the modulus of rigidity andE _a is the seismically-radiated energy, can be used to infer the absolute level of deviatoric stress at the hypocenter.     

Source characteristics of the 1992 Nicaragua tsunami earthquake inferred from teleseismic body waves

We analyzed the broadband body waves of the 1992 Nicaragua earthquake to determine the nature of rupture. The rupture propagation was represented by the distribution of point sources with moment-rate functions at 9 grid points with uniform spacing of 20 km along the fault strike. The moment-rate functions were then parameterized, and the parameters were determined with the least squares method with some constraints. The centroid times of the individual moment-rate functions indicate slow and smooth rupture propagation at a velocity of 1.5 km/s toward NW and 1.0 km/s toward SE. Including a small initial break which precedes the main rupture by about 10 s, we obtained a total source duration of 110 s. The total seismic moment isM _o =3.4×10²⁰ Nm, which is consistent with the value determined from long-period surface waves,M _o =3.7×10²⁰ Nm. The average rise time of dislocation is determined to be 10 s. The major moment release occurred along a fault length of 160 km. With the assumption of a fault widthW=50 km, we obtained the dislocationD=1.3 m. From andD the dislocation velocity isD=D/0.1 m/s, significantly smaller than the typical value for ordinary earthquakes. The stress drop =1.1 MPa is also less than the typical value for subduction zone earthquakes by a factor of 2–3. On the other hand, the apparent stress defined by 2E _s /M _o , where andE _s are respectively the rigidity and the seismic wave energy, is 0.037 MPa, more than an order of magnitude smaller than . The Nicaragua tsunami earthquake is characterized by the following three properties: 1) slow rupture propagation; 2) smooth rupture; 3) slow dislocation motion.     

Relations entre le creusement et le comblement des perturbations et leur déplacement

Résumé On commence par définir le creusement et le comblement d'une fonctionp(, t) du tempst et des points (, ) d'une surface régulière fermée en se donnant, sur cette surface, un vecteur vitesse d'advection ou de transfert tangent à . Le creusement (ou le comblement) est la variation dep sur les particules fictives se déplaçant constamment et partout à la vitesse , A chaque vecteur et pour un mêmep(, ,t) correspond naturellement une fonction creusementC (, ,t) admissible a priori; mais une condition analytique très générale (l'intégrale du creusement sur toute la surface fermée du champ est nulle à chaque instant), à laquelle satisfont les fonctions de perturbation sur les surfaces géopotentielles, permet de restreindre beaucoup la généralité des vecteurs d'advection admissibles a priori et conduit à des vecteurs de la forme: , oùT est un scalaire régulier, () une fonction régulière de la latitude , le vecteur unitaire des verticales ascendantes etR/2 une constante. Ces vecteurs sont donc une généralisation naturelle des vitesses géostrophiques attachées à tout scalaire régulier. Dans le cas oùp(, ,t) est la perturbation de la pression sur la surface du géoïde, le vecteur d'advection par rapport auquel on doit définir le creusement est précisément une vitesse géostrophique: on a alors ()=sin etT un certain champ bien défini de température moyenne.On déduit ensuite une formule générale de géométrie et de cinématique différentielles reliant la vitesse de déplacement d'un centre ou d'un col d'un champp(, ,t) à son champ de creusementC (, ,t) et au vecteur d'advection correspondant. Cette formule peut être transformée et prend la forme d'une relation générale entre le creusement (ou le comblement) d'un centre ou d'un col et la vitesse de son déplacement, sans que le vecteur d'advection intervienne explicitement. On analyse alors les conséquences de ces formules dans les cas suivants: 1^o) perturbations circulaires dans le voisinage du centre; 2^o) perturbations ayant, dans le voisinage du centre, un axe de symétrie normal ou tangent à la vitesse du centre; 3^o) évolution normale des cyclones tropicaux.Finalement, on examine les relations qui existent entre le creusement ou le comblement d'un champ, le vecteur d'advection et la configuration des iso-lignes du champ dans le voisinage d'un centre.Ces considérations permettent d'expliquer plusieurs propriétés bien connues du comportement des perturbations dans différentes régions.

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Summary The deepening and filling (development) of a functionp(, ,t) of the timet and the points (, ) of a regular closed surface is first of all defined, in respect to a given advection or transfer velocity field tangent to , as the variation ofp on any fictitious particle moving constantly and everywhere with the velocity . For a givenp(, ,t) and to any there corresponds a well defined development fieldC (, ,t). All theseC fields are a priori admissible, but a very general analytical condition of the perturbation fields in synoptic meteorology (the integral of the development fieldC (, ,t) on any geopotential surface vanishes at any moment), leads to an important restriction to advection vectors of the form: , whereT is any regular scalar, () any regular function of latitude, the unit vector of the ascending verticals andR/2 a constant. These vectors are a natural generalisation of the geostrophic velocities attached to any regular scalar. Whenp(, ,t) is the pressure perturbation at sea level, its development must be defined in respect to a geostrophic advection vector belonging to the above defined class of vectors with ()=sin andT a well defined mean temperature field.A general formula of the differential geometry and kinematics ofp(, ,t) is then derived, giving the velocity of any centre and col of ap(, ,t) as a function of the advection vector and the corresponding development fieldC (, ,t). This formula can be transformed and takes the form of a general relation between the deepening (and filling) of a centre (or a col) of ap(, ,t) and its displament velocity, the advection vector appearing no more explicitly. A detailed analysis of the consequences of these formulae is then given for the following cases: 1^o) circular perturbations in the vicinity of a centre; 2^o) perturbations having, in the vicinity of a centre, an axis of symmetry normal or tangent to the velocity of the centre; 3^o) normal evolution of the tropical cyclones.Finally, the relations between the developmentC (, ,t) of a fieldp(, ,t), the advection velocity vector and the configuration of the iso-lines in the vicinity of a centre are analysed.These theoretical results give a rational explanation of several well known properties of the behaviour of the perturbations in different geographical regions.

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Communication à la 2ème Assemblée de la «Società Italiana di Geofisica e Meteorologia» (Gênes, 23–25 Avril 1954).     

The vibration magnetometer and the processing of its output signal strongly affected by noise

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Defining a model of 3D seismogenic sources for Seismic Hazard Assessment applications: The case of central Apennines (Italy)

Geology-based methods for Probabilistic Seismic Hazard Assessment (PSHA) have been developing in Italy. These methods require information on the geometric, kinematic and energetic parameters of the major seismogenic faults. In this paper, we define a model of 3D seismogenic sources in the central Apennines of Italy. Our approach is mainly structural-seismotectonic: we integrate surface geology data (trace of active faults, i.e. 2D features) with seismicity and subsurface geological–geophysical data (3D approach). A fundamental step is to fix constraints on the thickness of the seismogenic layer and deep geometry of faults: we use constraints from the depth distribution of aftershock zones and background seismicity; we also use information on the structural style of the extensional deformation at crustal scale (mainly from seismic reflection data), as well as on the strength and behaviour (brittle versus plastic) of the crust by rheological profiling. Geological observations allow us to define a segmentation model consisting of major fault structures separated by first-order (kilometric scale) structural-geometric complexities considered as likely barriers to the propagation of major earthquake ruptures. Once defined the 3D fault features and the segmentation model, the step onward is the computation of the maximum magnitude of the expected earthquake (M _max). We compare three different estimates of M _max: (1) from association of past earthquakes to faults; (2) from 3D fault geometry and (3) from geometrical estimate corrected by earthquake scaling laws. By integrating all the data, we define a model of seismogenic sources (seismogenic boxes), which can be directly used for regional-scale PSHA. Preliminary applications of PSHA indicate that the 3D approach may allow to hazard scenarios more realistic than those previously proposed.     

Source spectra and RMS acceleration of Mexican subduction zone earthquakes

Strong motion (SM) data of six Mexican subduction zone earthquakes (6.4M _S8.1) recorded near the epicentral zone are analyzed to estimate their far-field source acceleration spectra at higher frequencies (f0.3 Hz). Apart from the usual corrections such as geometrical spreading (1/R), average radiation pattern (0.6), free surface amplification (a factor of 2), and equal partitioning of the energy into two orthogonal horizontal components (a factor of 1/ ), the observed spectra are corrected for a frequency dependentQ(Q=100f), a site dependent filter (e ^–kf ), and amplification ofS waves near the surface (a factor of about 2 atf2Hz). We takeR as the average distance from the rupture area to the site. If we model the high frequency plateau (f1 Hz) of the source spectra, by a point source ^–2-model, and interpret them in terms of Brune's model we obtain between 50 and 100 bars for all earthquakes. The low-frequency broadband teleseismicP wave spectra, corrected witht ^*=1.0 s, agrees within a factor of two with SM source spectra near 1 Hz. The ^–2-model is inadequate to explain the observed source spectra in a broad frequency range; these resemble spectra given byGusev (1983) with some differences.SM source acceleration spectra require significant corrections to explain observed spectra and RMS acceleration (a_rms) (a) at farther coastal sites for extended sources due to directivity effect and (b) at inland sites (100R200 km) because of unaccounted path and site amplification and/or invalidity of body-wave approximation. The observed spectra and a_rms at these sites are significantly greater than the predicted values from the estimated source spectra.     

The periodicity of aurorae in the years 1001–1900

Summary The sequence of aurorae, observed at latitudes up to 55° between the years 1001 and 1900 was processed by methods of spectral analysis. The same methods were applied to parts of various duration of this interval. The periods predominant in the time series under investigation were determined. In all the selected parts of the interval, these periods are always located within the same frequency band. Their position is related to the periods corresponding to mutual conjunctions of the large planets.

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Asymmetry in a nearly symmetric dynamo

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Determination of source duration and moment of earthquakes in the Indian Peninsula using love waveforms

Love waves recorded by a long-period seismograph at New Delhi (NDI) from seven earthquakes of magnitude 4.3 to 5.2 in Koyna and Bhatsa on the western coast and one earthquake in Ongole on the eastern coast of the Indian Peninsula have been used to determine the seismic moment for each of the earthquakes by waveform modeling. Transverse component of the synthetic seismogram shows that the maximum amplitude of waveform decreases with an increase of source duration. Thus for an evaluation of the seismic moment by equating the amplitude level of the observed and synthetic waveforms, we must know the source duration. The synthetic seismogram also indicates that a short source duration gives rise to a small but sharp pulse and this pulse is interpreted as anLg wave. Comparison of the observed and synthetic waveforms has been used for a simultaneous evaluation of the source duration and seismic moment. The source durations are found to vary between 2.2 and 4.4 s; for earthquakes with a magnitude range between 4.3 and 5.2 these durations are slightly higher than normal. We obtain moment (M ₀) of Ongole earthquake (M _L =5.1)as 1.7×10²⁴ dyne-cm; moments of Koyna and Bhatsa earthquakes (4.3M _L 5.2) on the western coast lie between 0.7×10²³ and 3.6×10²³ dyne-cm. Moment (M ₀)-magnitude (M _L ) relation logM ₀=1.5M _L +16.0 for the western United States region agrees as well, in general, with the results for the earthquakes in the Indian Peninsula.     

Case 25 application of the concentration parameter of seismoactive faults to Southern California

In this paper we evaluate the present state of the seismic regime in Southern California using the concentration parameter of seismogenic faults (K _sf ,Sobolev andZavyalov, 1981). The purpose of this work is to identify potential sites for large earthquakes during the next five or ten years. The data for this study derived from the California Institute of Technology's catalog of southern California earthquakes, and spanned the period between 1932 to June 1982. We examined events as small asM _L 1.8 but used a magnitude cutoff atM _L =3.3 for a detailed analysis. The size of the target earthquakes (M _M ) was chosen as 5.3 and 5.8.The algorithm for calculatingK _sf used here was improved over the algorithm described bySobolev andZavyalov (1981) in that it considered the seismic history of each elementary seismoactive volume. The dimensions of the elementary seismoactive volumes were 50 km×50 km and 20 km deep. We found that the mean value ofK _sf within 6 months prior to the target events was 6.1±2.0 for target events withM _L 5.3 and 5.41.8 for targets withM _L 5.8. Seventy-three percent of the targets withM _L 5.8 occurred in areas whereK _sf was less than 6.1. The variance of the time between the appearance of areas with lowK _sf values and the following main shocks was quite large (from a few months to ten years) so this parameter cannot be used here for accurate predictions of occurrence time.Regions where the value ofK _sf was below 6.1 at the end of our data set (June, 1982) are proposed as the sites of target earthquakes during the next five to ten years. The most dangerous area is the area east of San Bernardino whereK _sf values are presently between 2.9 and 3.7 and where there has been no earthquake withM _L 5.3 since 1948.     

Estimate of earthquake hazard in the Vrancea (Romania) region

A maximum likelihood method is used to estimate the earthquake hazard parameters maximum magnitudeM _max, annual activity rate , and theb value of the Gutenberg-Richter equation in the Vrancea (Romania) region. The applied procedure permits the use of mixed catalogs with incomplete historical as well as complete instrumental parts, the consideration of variable detection thresholds, and the incorporation of earthquake magnitude uncertainty.Our imput data, comprises 105 historical earthquakes which occurred between 984 and 1934, and a complete data file containing 1067 earthquakes which occurred during the period 1935–30 August, 1986. The complete part was divided into four subcatalogs according to different thresholds of completeness. Only subcrustal events were considered, and dependent events were removed.The obtained value (=0.65) is at the lower range of the previously reported results, but it appears concurrent with conceptual and observational facts. The same concerns inferred value of _max = 7.8 and activity rate _4.0 = 5.34.     

Magnitude estimation of Koyna-Warna earthquakes, India

The paper presents the current state of magnitude estimation for Koyna earthquakes exceeding magnitude 3.0. We estimate coda duration magnitude from analogue seismograms recorded on the short period vertical (SPZ) seismometer at Hyderabad seismic observatory HYB and determine moment magnitude using very broad-band (VBB) data from the Geoscope station (HYB)and short period digital data from the local seismic network of NationalGeophysical Research Institute (NGRI) around the Koyna and Warna reservoirs.Firstly, the seismograms of 97 Koyna earthquakes exceeding magnitude 4.0 havebeen used to establish a new empirical coda duration magnitude scale which includes the higher order terms of log₁₀, where is the coda length in seconds. Four background noise levels (1, 2, 6 and 10 mm) areconsidered to estimate the coda duration. We found that the duration magnitudes for 1 mm background level are more stable than those for 2, 6 and 10 mm. The new coda duration magnitude (M_dnew) scale, for 1 mmlevel, is:M_dnew = –0.594 + 2.04 log₁₀ – 0.0435 (log₁₀)²The estimated M_dnew are compatible with the reported M_S values of the NGRI observatory and the m_b values of the United States Geological Survey (USGS). These magnitudes can be obtained within the standard deviation of ± 0.26 units of M_S (NGRI). A new relatively homogeneous catalog for Koyna earthquakes of M_dnew 4.0 is obtained. The momentmagnitudes for 34 Koyna-Warna events of M_dnew ranging from 3.0 to 5.4 have been estimated using two techniques. The first utilizes amplitudes of band-pass filtered (between 15 and 30 sec) velocity traces of moderate Koyna-Warna earthquakes of M_W} 4.4 to 5.4, we abbreviate the magnitude using M_A. The second is based on the S-wave spectrum of short period seismograms of local earthquakes (M_W < 3.8). Moment magnitudes estimated by spectral analysis mainly depend on the estimation of event's long-period spectral level and appears to saturate for moderate Koyna-Warnaearthquakes (M_W > 3.8). We recommend the use of both techniques whenever possible. The estimated moment magnitudes and M_dnew show an almost linear relationship with a standard deviation of ± 0.05.     

First Results of Hydrogeochemical Measurements in the Soos Area,Czech Republic

For some years, the Saxon Academy of Sciences at Leipzig (SAW) is engaged in the research project Contributions to Environmental Research by means of Radiometric-Geochemical Methods in the Vogtland area (Germany). The main goal is to investigate the hydrogeological and hydrochemical parameters of mineral springs in that region and their interrelations. Here, the high CO ₂ degassing rates are of particular interest. The isotopic signature of these gases indicates an anomalous high mantle/crust contribution (Weinlich et al., 1999). A further geoscientifically important phenomenon of this region is the high seismicity, in particular the Vogtland/NW Bohemian earthquake swarms. Therefore, we have tried to use the mantle-originated fluids as transport media for information from the seismogenic horizons. The variations of the parameters recorded continuously at the springs for years show a connection to the seismic events of the epicentral area of Nový Kostel in the Czech Republic. In particular, numerous anomalous degassing intervals were recorded at the mineral spring Wettinquelle (formerly: Radonquelle), Bad Brambach, prior to earthquakes occurred in the region of Nový Kostel.     

An investigation of analysis of variance as a tool for exploring regional differences in strong ground motions

The statistical technique known as analysis of variance is applied to a large set of European strong-motion data to investigate whether strong ground motions show a regional dependence. This question is important when selecting strong-motion records for the derivation of ground motion prediction equations and also when choosing strong-motion records from one geographical region for design purposes in another. Five regions with much strong-motion data (the Caucasus region, central Italy, Friuli, Greece and south Iceland) are investigated here. For the magnitude and distance range where there are overlapping data from the five areas (2.50 M_s 5.50, 0 d 35 km) and consequently analysis of variance can be performed, there is little evidence for a regional dependence of ground motions. There is a lack of data from moderate and large magnitude earthquakes (M_s > 5.5) so analysis of variance cannot be performed there. Since there is uncertainty regarding scaling ground motions from small to large magnitudes whether ground motions from large earthquakes are significantly different in different parts of Europe is not known. Analysis of variance has the ability to complement other techniques for the assessment of regional dependence of ground motions.     

The probability of occurrence of largest earthquakes in the European area

Summary TheGumbel's theory of largest values has been applied to the estimation of probability of occurrence and of return periods of largest earthquakes in the European area. For this study shallow shocks from the period 1901–1955 and from 15 earthquake zones were used. For each zone the largest magnitudes corresponding to one-year intervals were arranged in order of increasingM, grouped in classes and then the probabilitiesF(x _j) were calculated. The data plotted on the probability paper fit a straight line fairly well. The extrapolated lines yield the possibility of estimating large magnitudes which will be exceeded with a given probability, e.g. 1%. Such values were compared with largest magnitudes observed during the period 1901–1955. Their return periods indicate that in most regions the largest probable shock already occurred. Following the procedure ofEpstein-Lomnitz the coefficients and were calculated and compared with corresponding values ofa andb of the magnitude-frequency relation.