The Source Mechanism of the Athens Earthquake,September 7, 1999, Estimated from P Seismograms Recorded at Long Range

On 7 September, 1999, an earthquake (5.8 m_b ^USGS)took place about 20 km from the centre of Athens, until then a seismically quiet region ofeastern Greece. Considerable damage ranging from rock falls to the collapse of reinforcedconcrete structures was reported in the city and surrounding area. No surface break which couldbe directly attributed to rupture on the fault plane was mapped. We use the relativeamplitude method and forward modelling of broadband P seismogramsrecorded at long range to produce a two-dimensional model of the source. We conclude that the earthquake tookplace on a south-west dipping normal fault at a depth of 10 km. This implies that the depthof the seismogenic zone in the area is comparable to other more active regions of Greece.The rupture speed (2.1 km/s) and stress drop (0.54 MPa) are low and are typical of earthquakesin a tectonic environment dominated by high rates of extension. The estimated seismicmoment is 6.014 × 10¹⁷ N m. We have investigated reported rupture directivity and thepossibility of a circular rupture is also examined. Extrapolation of the fault plane to the freesurface suggests that the earthquake took place on a structure associated with the Fili fault.     

The Mw = 6.0, 7 September 1999 Athens Earthquake

On 7 September 1999 at 11:56 GMT a destructive earthquake (M_w = 6.0) occurred close to Athens (Greece). The rupture process is examined using data from the Cornet local permanent network, as well as teleseismic recordings. Data recorded by a temporary seismological network were analyzed to study the aftershock sequence. The mainshock was relocated at 38.105°N, 23.565°E, about 20 km northwest of Athens. Four foreshocks were also relocated close to the mainshock. The modeling of teleseismic P and SH waves provides a well-constrained focal mechanism of the mainshock (strike = 105°, dip = 55° and rake = -80°) at a depth of 8 km and a seismic moment M₀ = 1.010²⁵ dyn·cm. The obtained fault plane solution represents normal faulting indicating an almost north-south extension. More than 3500 aftershocks were located, 1813 of which present RMS < 0.1 s and ERH, ERZ < 1.0 km. Two main clusters were distinguished, while the depth distribution is concentrated between 2 and 11 km. Over 1000 fault plane solutions of aftershocks were constrained, the majority of which also correspond to N–S extension. No surface breaks were observed but the fault plane solution of the mainshock is in agreement with the tectonics of the area and with the focal mechanisms obtained by aftershocks. The hypocenter of the mainshock is located on the deep western edge of the fault plane. The relocated epicenter coincides with the fringe that represents the highest deformation observed on the differential interferometric image. The calculated source duration is 5 sec, while the estimated dimensions of the fault are 15 km length and 10 km width. The source process is characterized by unilateral eastward rupture propagation, towards the city of Athens. An evident stop phase observed in the recordings of the Cornet local stations is interpreted as a barrier caused by the Aegaleo Mountain.     

Formation of basins and mountain ranges in Attica (Greece): The role of Miocene to Recent low-angle normal detachment faults

In seismically active regions, active low-angle detachment faults are probably more frequent as is commonly thought and may play an important but still underestimated role in the evolution of landforms and basins. We investigate the tectonically active region of Attica (Greece) in the Aegean back arc as a model region to show how basins and mountain ranges commonly thought to be formed by movements on high-angle normal faults in fact reflect the surface expression of displacements on yet undetected, deep-seated, active low-angle normal detachment faults. Inferences are made based on an integrated study of Attica linking the petrology of clastic sediments with geomorphology and structures, and including few new palynological data. From the Miocene to Recent, three sets of normal detachment fault systems were successively active. Shear zones of the 1st (Early Miocene) stage emplaced rocks of the Attic Cycladic high-P metamorphic belt (AC-HP-belt) from depth corresponding to greeschist facies conditions in the brittle, upper crust. In the 2nd stage the WNW dipping Attica low-angle normal detachment fault system between the AC-HP-belt and the un- or weakly metamorphosed rocks of the sub-Pelagonian Zone (SPZ) was active. Clastic sedimentation started in the Late Miocene, during the 2nd stage. Late Miocene and Early Pliocene clastic sediments reveal that during the 2nd stage many areas that presently expose the AC-HP-belt were still covered by the overlying SPZ. Also, now uplifted areas such as the Parnitha mountain range that currently undergo strong erosion were then the sites of sedimentary sinks. The 3rd stage (Late Pliocene through Recent) is associated with dramatic changes in the morphology and recurring steepening of the relief. Reversal of the Parnitha area from the site of deposition into the site of erosion is associated with deposition of coarse conglomerates to the SE of the Parnitha Mt. and S of the Penteli Mt. Sediments of the 3rd stage reflect activity of the here newly described, SSE-dipping Penteli—Athens low-angle detachment fault (PADF) system formed at a high angle to the Attica detachment fault. The outcome of this study is that the present-day geomorphology is to a high degree related to the operation of the PADF system. Steep fault bounding the Athens and Mesogea basins as well as the mountain ranges (Parnitha, Penteli, Hymittos mounts) belongs to its breakaway zone or root into the PADF. Ongoing tectonic movements related to this fault system were responsible for the 1999 Athens (Mw = 6.0) earthquake. We particularly discuss how the PADF may continue into greater depth, the translation magnitude, and how the PADF fits into the wider kinematic framework of the Aegean region.     

Brittle deformation events at the western border of the Bohemian Massif (Germany)

Investigations of brittle deformation structures, present within the crystalline rocks of the Bavarian Oberpfalz, reveal a complex late to post-Variscan crustal evolution. Upper Carboniferous (mainly Westphalian) granites were emplaced into semibrittle to brittle rocks of the ZEV (zone of Erbendorf-Vohenstrauß) and the EGZ (Erbendorf greenschist unit), respectively. From both the alignment of the granites and the direction of granite-related tension gashes a north-east-south-west extension must be assumed for the period of magmatic activities. Apart from the granite intrusions, rapid crustal uplift (about 1.5 km/my) led to an increase in the geothermal gradient from < 30 °C/km (late Variscan pre-granitic) to > 40 °C/km (late Variscan post-granitic). The increased geothermal gradient persisted during the succeeding reverse faulting which results from late Carboniferous (probably Stephanian) east-west and northeast-south-west compression. Although not evidenced directly in the area considered, strike-slip faults seem to have played an important part during the late Variscan crustal evolution, particularly in the Early Permian. The strike-slip events indicate further crustal shortening and indentation under north-south compression.A similar indentation was present in Cretaceous time. After a weak phase of Early Cretaceous reverse faulting, which results from north-south compression, strike-slip faults formed under north-west-south-east and north-south compression. All these faults, in particular the strike-slip faults, seem to be related to the Cretaceous and lowermost Tertiary convergence of the Alpine/Carpathian orogeny.A late stage of crustal extension, characterized by a radial stress tensor (₂ = ₃), is indicated through high angle normal faults which probably formed during the subsidence of the adjacent Neogene Eger Graben.     

Uncertainties in differential thermodynamic (Gibbs' method) P-T paths

A combined analytical and numerical evaluation of the uncertainties in P-T paths is made for three assemblages that propagates errors in the parameters: initial pressure, initial temperature, initial composition, change of composition of monitor parameters, endmember entropy, and endmember volume. Propagated errors along an isobaric heating path (T=77°C) for assemblage 1 (Grt-Bt-Pl-Qtz-Ms-Chl-H₂O), using as monitor parameters the mole fractions of almandine, spessartine, grossular, and anorthite, and ignoring uncertainties in thermodynamic properties, are approximately ±320 bars (1) and ±8.3°C (1) if rim compositional uncertainties of 5% in major cations are assumed, or ±2.5°C (1) and ±50 bars (1) if electron microprobe analytical uncertainties are assumed for compositions. The largest source of uncertainty is from the errors in the monitor parameters, and P-T path uncertainties can depend critically on which monitor parameters are used. If the mole fraction of annite is used as a monitor parameter in place of the anorthite content of plagioclase, then propagated uncertainties are worse than ±29°C (1) and ±5800 bars (1). P-T path uncertainties also depend on assemblage. 1 precisions in assemblages 2 (Grt–Bt–Pl–Qtz–Ms–Sil–H₂O) and 3 (Grt–Bt–Qtz–Kfs–Sil–H₂O) using as monitor parameters the mole fractions of almandine, spessartine, grossular, and anorthite are calculated to be ±267 bars and ±43.4°C and ±372 bars and ±8.2°C respectively. Estimates of the accuracies in P-T paths that include potential errors in endmember entropy of ±1 J/mol·K and in endmember volume of ±1 cm³/mol are: ±324 bars and ±8.5°C (assemblage 1), ±341 bars and ±48.6°C (assemblage 2), and ±388 bars and ±9.7°C (assemblage 3). Use of different garnet, plagioclase, and muscovite activity models can change the length of a P-T path by as much as 15%, but does not typically change directions in P-T space significantly. Models that incorporate changes of fluid composition shorten P-T paths in assemblages 1 and 3 but do not change trajectories significantly. Assemblage 2 is virtually unaffected by fluid phase models. For the mineral assemblages considered here and using appropriate monitor parameters, propagated errors are small compared to the total path length, suggesting that the differential thermodynamic approach is a precise and accurate method for determining amounts of heating or thickening during metamorphism, and hence for interpreting orogenic processes.     

Deformation und Gefügeregelung von Steinsalz im Temperaturbereich 20–200° c

Cubic specimens of fine-grained artifical polycristalline rock salt were deformed at different principal stresses in the range of 20–200° C. The data from different types of tests are compared by plotting ₁–₃ against ₁, ₂, ₃. The deformation behavior is highly influenced by the intermediate principal stress ₂. An increase of the intermediate stress from compression (₁>₂=₃) to extension (₁=₂>₃) lowers the rate of deformation. An increase of temperature always gives rise to an increase of deformation rate.Translation gliding on (110) is assumed to be the dominant flow mechanism. Textural changes and development of preferred orientation were investigated by means of the X-ray diffractometer. Intensities were measured for (110), (111), and (100). Compression leads to girdle occupations about the unique principle compressive axis, extension to girdles about the unique principle axis of extension. When all principle stresses are different the polfigures tend to a position intermediate between the two occupations.     

Dreiaxiale verformungen an Solnhofener Kalkstein im Temperaturbereich von 20–650° C. Röntgenographische Gefügeuntersuchungen mit dem Texturgoniometer

Axial-symmetric and real triaxial deformation tests on Solnhofen limestones were made in the range of 20 to 650° C. Stress, strain and time observations as functions of temperature and the ratios of the external stresses are plotted as stress-strain and strain-time curves. Mechanical twinning on e (01¯12) and syntectonic recrystallization (at temperatures above 500° C) is assumed to be the dominant flow mechanism. On samples of high permanent strain the textural changes and development of preferred orientations were investigated by means of the X-ray diffractometer. In all tests the symmetry of the polefigures is exactly identical with the symmetry of the external stress-field.In tests with ₁>₂=₃ the poles of the planes investigated (10¯11; 10¯12; 11¯20) occupy girdles about the unique principle compressive axis. When all principle stresses are different (₁>₂>₃) the poles of the planes investigated tend to maxima (in the case of plane strain) ore to girdle-occupations about the principle axis of maximum extension. The deduced c-diagrams show a fairly similar tendency of the spacial orientations.The pole figures (as well as the calculated orientation of the c-axis) must be explained by the superposition of reflected intensities on planes in twinned and untwinned grains. In every diagram partial occupations—possibly representing the orientations of twinned grains-are in fairly good agreement with the orientation predicted by the theory of Brace-MacDonald for the stable orientation of anisotropic minerals in a non-hydrostatic stress-field.

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Herrn Prof. Dr. F. Karl danke ich sehr herzlich für zahlreiche Diskussionen sowie für die kritische Durchsicht des Manuskripts. Mein Dank gilt auch der Deutschen Forschungsgemeinschaft, welche die Untersuchung durch eine Sachbeihilfe unterstützte.     

Petrographic and geochemical evidence for hydrothermal evolution of the North Deposit,Mt Tom Price,Western Australia

High-grade iron mineralisation (>65%Fe) in the North Deposit occurs as an E-W trending synclinal sheet within banded iron formation (BIF) of the Early Proterozoic Dales Gorge Member and consists of martite-microplaty hematite ore. Three hypogene alteration zones between unmineralised BIF and high-grade iron ore are observed: (1) distal magnetite-siderite-iron silicate, (2) intermediate hematite-ankerite-magnetite, and (3) proximal martite-microplaty hematite-apatite alteration zones. Fluid inclusions trapped in ankerite within ankerite-hematite veins in the hematite-ankerite-magnetite alteration zone revealed mostly H₂O–CaCl₂ pseudosecondary and secondary inclusions with salinities of 23.9±1.5 (1, n=38) and 24.4±1.5 (1, n=66) eq.wt.% CaCl₂, respectively. Pseudosecondary inclusions homogenised at 253±59.9°C (1, n=34) and secondary inclusions at 117±10.0°C (1, n=66). The decrepitation of pseudosecondary inclusions above 350°C suggests that their trapping temperatures are likely to be higher (i.e. 400°C). Hypogene siderite and ankerite from magnetite-siderite-iron silicate and hematite-ankerite-magnetite alteration zones have similar oxygen isotope compositions, but increasingly enriched carbon isotopes from magnetite-siderite-iron silicate alteration (–8.8±0.7, 1, n=17) to hematite-ankerite-magnetite alteration zones (–4.9±2.2, 1, n=17) when compared to the dolomite in the Wittenoom Formation (0.9±0.7, 1, n=15) that underlies the deposit. A two-stage hydrothermal-supergene model is proposed for the formation of the North Deposit. Early 1a hypogene alteration involved the upward movement of hydrothermal, CaCl₂-rich brines (150–250°C), likely from the carbonate-rich Wittenoom Formation (¹³C signature of 0.9±0.7, 1, n=15), within large-scale folds of the Dales Gorge Member. Fluid rock reactions transformed unmineralised BIF to magnetite siderite-iron silicate BIF, with subsequent desilicification of the chert bands. Stage 1b hypogene alteration is characterised by an increase in temperature (possibly to 400°C), depleted ¹³C signature of –4.9±2.2 (1, n=17), and the formation of hematite-ankerite-magnetite alteration and finally the crystallisation of microplaty hematite. Late Stage 1c hypogene alteration involved the interaction of low temperature (~120°C) basinal brines with the hematite-ankerite-magnetite hydrothermal assemblage leaving a porous martite-microplaty hematite-apatite mineral assemblage. Stage 2 supergene enrichment in the Tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.Editorial handling: B. Lehmann     

Paleomagnetic constraints on Zn–Pb ore genesis of the Pillara Mine, Lennard Shelf, Western Australia

The Pillara Zn–Pb deposit is the largest of several known Mississippi Valley-type (MVT) deposits in the Lennard Shelf of the Canning Basin. Paleomagnetic and rock magnetic measurements are reported for 294 specimens from 23 sites in mineralization and its carbonate host rocks from the deposit as well as on 15 artificial specimens of zinc and lead concentrate and of tailings. Pyrrhotite carries the characteristic remanent magnetization (ChRM) in nearly all specimens. The ChRM postdates most faulting as shown by breccia tests and most minor regional tilting as shown by the degraded fit on tilt correction. The mean ChRM direction for all sites is D=20.6°, I=–27.5° (N=23, ₉₅=5.3°, k=34.1), yielding an age of 358±5 Ma (2) that is similar to the comparable age of 354±8 Ma (2) for the Kapok MVT deposit. Host rock diagenesis with attendant secondary remagnetization yields an age of 361±5 Ma (1) and the MVT mineralization with a primary chemical remanent magnetization gives an age of 356±3 Ma (1), co-eval with a published Rb–Sr sphalerite age of 357±3 Ma. Interpretation of this temporal data suggests that the MVT deposits of the southeastern Lennard Shelf originated during extension, probably in response to rift-related topography-driven fluid flow.Editorial handling: C. Brauhart     

Did the 7/9/1999 M5.9 Athens Earthquake Come with a Warning?

Prior to the 7/9/1999 M_S = 5.9 Athens earthquake, regional seismicity has exhibited a power-law increase, of the form = K+A(t_c - t)ⁿ, where is estimated using an expression log = cM + d and t_c is the time of the culminating event. Such changes appeared after the 17/8/1999 M7.4 Izmit event. We quantified the performance of the power law vs. the null hypothesis of constant seismic release rates, by defining the curvature C as the ratio of the power law fit RMS/linear fit RMS, so that the smaller C is, the better the power law behaviour. By mapping C, we have established a critical radius of 110 km and observed that the region of correlated accelerating seismic release extended from the N. Aegean, through Euboea and Attica to the SW Peloponnese. A few days prior to the Athens event, min(C) was centred at the epicentral area and numerical simulation yielded t_c = 1999.676 and predicted M_S = 5.77. Seismicity rates returned to normal (quasi-constant) after the Athens event. We interpret this effect as critical point behaviour, following remote excitation of a broad area by stress redistribution due to the Izmit event which, at Athens, has triggered `premature' failure of a fault nearing its load bearing capacity. If this is correct, we have documented a case of remote earthquake triggering by another earthquake, as well as insight into the mechanisms producing it. As a corollary, we note that a large event may beget another large event in its broader region of interaction, which may be preceded by characteristic precursory seismicity changes.     

Deformationsmechanik eines offenen Konglomeratgefüges—Ein Beitrag zur statischen Tektonik

Zusammenfassung Offene Faltung in proterozoischen Tilliten der Huronian Supergroup führte zu geringen Differentialverschiebungen zwischen Matrix und Grobkomponenten innerhalb von konglomeratischen Schichtgliedern. Einmessung der aus der Bewegung resultierenden Striemungen und feiner Risse erlaubt die Bestimmung der regionalen Spannungsachsen (₁ > ₂ > ₃).

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Deformation mechanics of an open conglomerate fabric—A contribution to static tectonics

Summary Open folding of Proterozoic tillites (Huronian Supergroup) caused minor differential displacements between matrix and clasts within conglomeratic units. Detailed measurements of striations and ruptures along the clast surface permitted the deduction of regional stress axes (₁ > ₂ > ₃).

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Mit 6 Abbildungen

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Herrn Prof. Dr. Dr. h. c.B. Sander zum 90. Geburtstag gewidmet.     

Analysis of the variability of unconfined compression tests of rock

Summary The laboratory testing program reported herein describes the statistical correlations of unconfined compressive strength ( _c ), Young's modulus (E _t50), and modulus ratio (MR) to their coefficients of variation (CV( _c ), CV(E _t50), and CV(MR)). Negative correlation of both _c andE _t50 to CV( _c ) indicates that data scatter tends to decrease with increasing strength and stiffness. A positive correlation between MR and CV( _c ) indicates that stiffer rocks are more variable for a given compressive strength.     

Preferred orientation in experimentally deformed limestone

Over sixty syntectonic deformation experiments in uniaxial compression have been done on fine-grained limestones in the stability fields of calcite I, calcite II and aragonite. X-ray techniques and spherical harmonic analysis of the data were used to determine preferred orientation quantitatively, and inverse pole-figures were derived for these axially symmetric specimens. They display in most cases strong preferred orientation which varies as a function of the experimental conditions, mainly temperature and pressure. At temperatures below 350° C recrystallization is lacking and flattened grains indicate that translation, twin gliding and kinking have been the dominant deformation mechanisms. The inverse pole-figure shows a maximum at c with a shoulder towards or a second maximum at e. This is in agreement with preferred orientation observed in experimentally deformed Yule marble and can be explained as the product of dominant twin gliding on e and translation gliding on r (Turner et al., 1956). At high temperatures (900–1000° C) strong grain growth (from 4 to 50 microns) indicates that the fabric recrystallized. Grains are equidimensional and clear with a marble-like texture. The inverse pole-figure shows a single maximum at r, and c-axes are oriented in a small circle around the axis of compression, ₁. Such a pattern of preferred orientation would be expected on thermodynamic grounds assuming that recrystallized grains will be oriented in such a way that the strain energy is a maximum (e.g. MacDonald, 1960). Decrease in confining pressure caused a decrease of the maximum at c and the formation of a secondary maximum at highangle positive rhombs in the inverse pole-figure. This can be interpreted as r translation dominating over e twinning. In all deformation experiments an equilibrium in preferred orientation was reached after 20 percent shortening. The strength of preferred orientation decreased with increasing temperature. Aragonite was produced within its hydrostatic stability field at temperatures above 500° C. Close to the phase boundary, coarse-grained textures showed preferred orientation with poles to (010) parallel to ₁. At higher pressures the fabric is fine-grained and [001] is aligned parallel to ₁. Evidence is given that the phase change from calcite to aragonite in these deformation experiments is a diffusive and not a martensitic transformation.Publication No. 1043, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California.     

Teleseismic finite-fault inversion of two <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> = 6.4 earthquakes along the East Anatolian Fault Zone in Turkey: the 1998 Adana and 2003 Bingöl earthquakes

The East Anatolian Fault Zone is a continental transform fault accommodating westward motion of the Anatolian fault. This study aims to investigate the source properties of two moderately large and damaging earthquakes which occurred along the transform fault in the last two decades using the teleseismic broadband P and SH body waveforms. The first earthquake, the 27 June 1998 Adana earthquake, occurred beneath the Adana basin, located close to the eastern extreme of Turkey’s Mediterranean coast. The faulting associated with the 1998 Adana earthquake is unilateral to the NE and confined to depths below 15 km with a length of 30 km along the strike (53°) and a dipping of 81° SE. The fixed-rake models fit the data less well than the variable-rake model. The main slip area centered at depth of about 27 km and to the NE of the hypocenter, covering a circular area of 10 km in diameter with a peak slip of about 60 cm. The slip model yields a seismic moment of 3.5?×?10¹⁸ N-m (M_w???6.4). The second earthquake, the 1 May 2003 Bingöl earthquake, occurred along a dextral conjugate fault of the East Anatolian Fault Zone. The preferred slip model with a seismic moment of 4.1?×?10¹⁸ N-m (M_w???6.4) suggests that the rupture was unilateral toward SE and was controlled by a failure of large asperity roughly circular in shape and centered at a depth of 5 km with peak displacement of about 55 cm. Our results suggest that the 1998 Adana earthquake did not occur on the mapped Göksun Yakap?nar Fault Zone but rather on a SE dipping unmapped fault that may be a split fault of it and buried under the thick (about 6 km) deposits of the Adana basin. For the 2003 Bingöl earthquake, the final slip model requires a rupture plane having 15° different strike than the most possible mapped fault.     

Ground failure around buried tubes

Summary The hoop forces which develop in circular tubes buried in elastic-plastic ground are investigated. A closed form solution is used to determine the hoop forces which develop when the field stresses in the elastic-plastic ground are initially uniform. The finite element method is used to solve the problem for biaxial field stress. A parametric study is undertaken to assess the influence of tube stiffness and ground strength on the hoop forces, and use is made of elastic stress contours to predict the likely extent of material failure around tubes buried in ground with biaxial prestress.Notation a tube radius - c cohesion - D flexural stiffness of the structure - E _i Young's modulus of structure - E _s Young's modulus of ground - F hoop force (compression positive) - G _s shear modulus of ground - H hoop stiffness of the structure - K coefficient of lateral pressure - N tan²(45+/2) - q ( ₁– ₁)/2 - S _f relative flexural stiffness of the structure - S _h relative hoop stiffness of the structure - t structural thickness - v circumferential displacement - w radial displacement - v _l Poisson's ratio of structure - v _s Poisson's ratio of ground - normal traction acting on the structure - _d deviatoric component of field stress - _h horizontal field stress - _m uniform component of field stress - _v vertical field stress - ₁ major principal stress - ₃ minor principal stress - tangential traction acting on the structure - angle of internal friction of the ground - angle of dilation of the ground     

2022年1月8日青海门源MS 6.9地震地表破裂考察的初步结果及对冷龙岭断裂活动行为和区域强震危险性的启示

2022年1月8日青海门源M_S 6.9地震发生在青藏高原东北缘的祁连山断块内部,仪器震中位于海原活动断裂系西段的冷龙岭断裂带上,是该断裂系自1920年海原8.5级大地震后再次发生M>6.5的强震。考察结果的初步总结表明,此次门源地震产生了呈左阶斜列分布、总长度近23 km的南北两条破裂,在两者之间存在长约3.2 km、宽近2 km的地表破裂空区。南支破裂(F₁)出现在托来山断裂的东段,走向91°,长约2.4 km,以兼具向南逆冲的左旋走滑变形为主,最大走滑位移近0.4 m。北支主破裂(F₂)出现在冷龙岭断裂的西段,总长度近20 km,以左旋走滑变形为主,呈整体微凸向北东的弧形展布,包含了走向分别为102°、109°和118°的西、中、东三段,最大走滑位移出现在中段,为3.0±0.2 m。此外,在北支主破裂中—东段的北侧新发现一条累计长度约7.6 km、以右旋正断为主的北支次级破裂(F₃),累计最大走滑量约0.8 m,最大正断位移约1.5 m。综合分析认为,整个同震破裂以左旋走滑变形为主,具有双侧破裂特点,宏观震中位于北支主破裂的中段,其地表走滑位移很大可能与震源破裂深度浅有关,其中的右旋正断次级破裂可能是南侧主动盘向东运移过程中拖曳北侧块体发生差异运动所引起的特殊变形现象。印度与欧亚板块近南北向强烈碰撞挤压导致南祁连断块沿海原左旋走滑断裂系向东挤出,从而引发该断裂系中的托来山断裂与冷龙岭断裂同时发生破裂,成为导致此次强震的主要动力机制。在此大陆动力学背景下,以海原左旋走滑断裂系为主边界的祁连山断块及其周边的未来强震危险性需得到进一步重视。      

Statistical Properties of Ideal Rock Textures: Relationship Between Crystal Size Distribution and Spatial Correlation of Minerals

The concept of an ideal rock texture, in which crystals are distributed randomly in space, is proposed for use in general analysis of rock textures. The spatial correlation function for the ideal rock texture was examined and the function, a specific kind of spatial correlation function, is related to crystal size distribution and to some extent to crystal boundaries. The function is unity at distance zero, and monotonically decreases with increasing distance for the ideal texture. This behavior of the function is observed for any size distribution. In an ideal texture, the function is directly related to crystal size distribution and crystal shape. It is important in stereology because the crystal size distribution in three dimensions may be deduced from analyzing a function that is obtained from analyzing two-dimensional section images. Crystal shape is also related to the function. If crystals are concave in shape, or have inclusions of other phases, the function may show a hump or plateau when plotted against distance. However, the crystal shape effect cannot produce values smaller than zero. If values become negative, the texture is no longer considered ideal. The ideal textures for two model size distributions—step and delta functions—are considered. The rate of decrease of values is more strongly dependent on size distribution than on system dimension.     

Preliminary analysis of the first digital recordings obtained in the Portuguese seismographic network — Attenuation studies

During experiments with digital stations in the period 1985–1987, twenty five earthquakes with magnitudes m _b in the range 2.9 to 4.8 and epicentres located within the area 36°–42.3° (N) and 4.5°–13.6° (W) were recorded at Montemor (MOE) and Montachique (MTH). The three-component recordings were obtained by Geotech S13 instruments with 1 second period. A preliminary analysis of the recordings consisted in the determination of amplitudes and spectral contents of P and S waves, and led to the following observations: (1) The attenuation of waves is expressed by the equation V = exp(C ₂).R ^C ₁. exp(C ₃.M), where V stands for acceleration, velocity or displacement; M-magnitude; R-focal distance; C ₁, C ₂ and C ₃ are constants to be obtained by least square fitting. The application of this equation led to C ₁ of the order 1.7 for displacement, 1.8 for velocity and 2.0 for acceleration, with an average mean square error 0.8. (2) The ratios L/T (longitudinal/transversal amplitudes), for velocity and displacement, showed a tendency to reduce with increasing focal distance, being 2 for short distances (<50 km) and 1 for long distances (400 km). (3) The ratios S/P (S-wave/P-wave amplitudes), although with a large dispersion, showed a slight tendency for increasing with focal distance. (4) The predominant frequencies also showed a slight tendency to decrease with increasing focal distance and with magnitude. (5) The dependence of C ₁ with frequency (3 to 12 Hz) is well behaved from 0.95 to 1.75 (for the velocity trace).     

Modal and vectorial analysis for determination of stress axes associated with fault slip data

Based on Anderson's faulting theory, for a given set of fault slip data including the sense of slip on faults and fault planes, this paper provides two possible methods to reconstruct the principal stress axes using vectorial and modal analysis procedure. The vectorial analysis consists of computing eigenvectors of the orientation matrices defined by axes P, B, and T (P, B, and T being geometrical axes parallel to axes ₁, ₂, ₃ associated with a single striated fault) which are determined geometrically knowing the slip vector S, the normal to fault plane N, and the dihedral angle 2. A parameter R, related to the maximum eigenvalue of the orientation matrix and the size of data sample, is shown to be a good test value for the homogeneity of the data. A process of refinement of this parameter enables bad data (representing faults generated under different tectonic events) to be ignored in the final computation of the principal stress axes; results are thus significantly improved and the vectorial analysis procedure enables the orientation of stress to be numerically determined. On defining probability density regions around each P, B, and T axes that incorporate the variation of fault geometry, the modal analysis is carried out in two density functions established by a convolution process to locate modes which represent optimal locations of the principal stress axes in spherical space. Splitting of heterogeneous data samples into homogeneous subsets is achieved by a dynamical cluster procedure which enables the principal stress axes associated with each subset to be determined separately.     

Sm-Nd dating of Seve eclogites,Norrbotten, Sweden — Evidence for early Caledonian (505 Ma) subduction

The Seve Nappes consist of long-transported thrust sheets belonging to the Upper Allochthon within the Scandinavian Caledonides. Eclogites from two different megalenses within the Seve Nappe Complex in Norrbotten (northern Sweden) have been dated with the Sm-Nd method. The two eclogite-bearing megalenses have been subjected to different pressure and temperature conditions during the high-pressure metamorphism. Garnet + omphacite + whole-rock from TS2 within the Tsäkkok Lens give an age of 505±18 Ma (2), I=0.512231±0.000024, MSWD=0.10. Garnet and whole-rock from TS5 and whole-rock TS3 also fall on the above isochron. The composite isochron gives an age of 503±18 Ma (2). These results are interpreted to date the peak of the high-pressure metamorphism (500–630° C and 12–15 kbars) for the Tsäkkok Lens. The eclogites in the Vaimok Lens (at Grapesvarre) were subjected to higher pressure and temperature conditions and more extensive reequilibration during the early cooling stages. Retrograde breakdown-reactions accompanied by retrograde zoning of relict garnet seems to be associated with disturbance of the Sm-Nd isotopic systems. In contrast, a sample with unzoned garnet and well preserved high-pressure mineralogy from Grapesvarre gives a Sm-Nd garnet + omphacite age of 503±14 Ma (2), I = 0.512010±0.000038 (2). The ages for the Seve eclogites are significantly older than the Sm-Nd eclogite dates from the Western Gneiss Region of Norway (WGR), suggesting the existence of at least two eclogite-forming events in the Caledonide Orogen. The younger event has been related to the main continent-continent collision stage of the Caledonian Orogeny, while the older event that led to the production of the Norrbotten eclogites must have taken place several hundred kilometres to the north and in a different tectonic setting more oceanward to the WGR. It appears that the older event (ca. 505 Ma) was restricted to the subduction of dyke-intruded sedimentary cover rocks which are thought to represent the rifted edge of the Baltic continent.