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1.
Summary A new significant correlation has been sought between high magnitude global seismicity and lateral surface wave velocity gradients. Rayleigh wave velocity divisioning of Eurasia, Africa, Pacific Ocean, Atlantic Ocean and Indian Ocean into regions of similar group velocity dispersion character of 30 sec period bySanto andSato [1]3) has been mainly used for calculating the gradients. It is quite striking to note that all earthquakes of magnitude 8.6 and above during 1897–1956 have occurred in regions having gradients of the order of 1.5·10–3 sec–1.Finally, some potential areas for high magnitude earthquake occurrences are predicted and the possible velocity gradients in regions, where division pattern is not yet investigated like South America and Australia, are also estimated.N.G.R.I. Contribution No. 70-170.  相似文献   

2.
Seismic activity that preceded, accompanied, and followed the 17–23 March 1981 Etnean eruption has been statistically analyzed.On the grounds of both time evolution of seismicity and catalogue completeness, three time intervals have been defined (12 February–2 March, 12–17 March, 19–31 March) and for each of these periods both the b coefficient of theGutenberg-Richter's (1956) relationship and the E parameter of the cluster size (Shlien andToksoz, 1970) have been calculated.No significant variations were observed between the first and third periods, while lower values of bothb andE coefficients were found in the second one. These findings might indicate that changes in the seismicity features occur just before the eruption start.Small but fast variations in the stress field acting on the volcano might originate this type of seismic activity, while the importance of the tectonic control on volcanic phenomena seems to be confirmed.  相似文献   

3.
Eruptions of the active Karymsky stratovolcano began about 5300 (6100 14C) b.p. from within a pre-existing caldera which formed 7700 14C b.p. As indicated by 32 14C determinations on buried soils and charcoal, the volcano has gone through two major cycles of activity, separated by a 2300 year period of repose. The first cycle can be divided into two stages (6100–5100 and 4300–2800 b.p.). The earlier stage began with especially intense eruptions of basaltic andesite to dacite. The later stage was characterized by moderate-strength eruptions of andesite. The second cycle, which is characterized by weak to moderate intermittent eruptions of andesite, started 500 b.p. and continues to the present. Eruptive patterns suggest that this cycle may continue for at least another 200 years with an eruptive character similar to that of the recent past.  相似文献   

4.
We establish here a comprehensive database of intraplate seismicity in the Pacific Basin. Relocation and analysis of 894 earthquakes yield 403 reliable intraplate earthquakes during 1913–1988. These numbers do not include earthquake swarms, which account for another 838 events. Most of the remainder (304 events) are actually plate boundary earthquakes that have been erroneously located in intraplate regions. A significant number occur in recent years when location capabilities should have guarded against this situation. Relocations involve a careful linear inversion ofP andS arrivals, accompanied by a Monte Carlo statistical analysis. We have also attentively removed the high number of clerical errors and nuclear tests that exist in epicenter bulletins.A geographical examination of the relocated epicenters reveals several striking features. There are three NW-SE lineaments north of the Fiji Plateau and in Micronesia; diffuse seismicity and incompatible focal mechanisms argue against the southernmost, discussed byOkal et al. (1986) andKroenke andWalker (1986), as the simple relocation of the Solomon trench to the North. Besides another striking lineament, along the 130°W meridian, there is also a strong correlation between seismicity and bathymetry in certain parts of the Basin. In the Eastcentral Pacific and Nazca plates there are many epicenters on fracture zones and fossil spreading ridges, and hot spot traces like the Louisville, Nazca and Cocos Ridges also display seismicity.  相似文献   

5.
The algorithm CN makes use of normalized functions. Therefore the original algorithm, developed for the California-Nevada region, can be directly applied, without adjustment of the parameters, to the determination of the Time of Increased Probability (TIP) of strong earthquakes for Central Italy. The prediction is applied to the events with magnitudeMM 0=5.6, which in Central Italy have a return period of about six years. The routinely available digital earthquake bulletins of the Istituto Nazionale di Geofisica (ING), Rome, permits continuous monitoring. Here we extend to November 1994 the first study made by Keilis-Boroket al. (1990b). On the basis of the combined analysis of seismicity and seismotectonic, we formulate a new regionalization, which reduces the total alarm time and the failures to predict, and narrows the spatial uncertainty of the prediction with respect to the results ofKeilis-Borok et al. (1990b).The premonitory pattern is stable when the key parameters of the CN algorithm and the duration of the learning period are changed, and when different earthquake catalogues are used.The anlysis of the period 1904–1940, for whichM 0=6, allows us to identify self-similar properties between the two periods, in spite of the considerably higher seismicity level of the earlier time interval compared with the recent one.  相似文献   

6.
Summary The validity ofBoltzmann's energy distribution law as applied to the charge distribution of monodisperse aerosols, i.e. aerosols which contain particles of one size only, was studied with improved equipment now available.Since monodisperse aerosols with particle size of about 10–6 cm cannot yet be produced and because aerosols in general contain particles of different sizes, polydisperse aerosols stored in a large gasometer were used for the investigation. The composition and the average radius of these aerosols heterogeneous in particle size were determined by the Exhaustion Method employing a diffusion battery without end-pieces or connecting tubing.The experimental curve found for the ratio of the uncharged (N 0) to the charged (N) nuclei versus radius (r) of the particles deviates for all investigated radii between 0.5 and 4.0·10–6 cm from the theoretical curve of a monodisperse aerosol computed according to the Boltzmann law. For radii smaller than about 1.4·10–6 cmN 0/N is smaller than that given by Boltzmann's law, for radii greater than 1.4·10–6 cm larger, or, in other words, forr<1.4·10–6 cm the number of charged nuclei found in the aerosols investigated is greater than that predicted byBoltzmann's law, and forr>1.4·10–6 cm smaller.The deviations from the theoretical curve forr>1.4·10–6 cm can be fully explained by the polydispersity of the aerosols used; forr<1.4·10–6 cm the deviations are too big to be attributed to polydispersity. From this it must be concluded thatBoltzmann's distribution law is not valid for the charge distribution of homogenous aerosols containing nuclei with radii smaller than about 1.4·10–6 cm.The equivalent radius as deduced from a substitute monodisperse aerosol in charge equilibrium for which theBoltzmann law is assumed valid, deviates in the range of 1.0·10–6<r<3.0·10–6 cm (or 90%>100N 0/Z>55%) on the average by up to 35% from the actual mean radius of the investigated polydisperse aerosols computed from the radii of their singly sized components.
Zusammenfassung Die Gültigkeit desBoltzmannschen Gesetzes der Energieverteilung für die Ladungsverteilung von monodispersen Aerosolen, das heisst Aerosolen, welche Teilchen von nur einer Grösse enthalten, wurde mit den jetzt zur Verfügung stehenden verbesserten Mitteln studiert.Da monodisperse Aerosole mit Teilchengrössen von ungefähr 10–6 cm noch nicht erzeugt werden können und Aerosole im allgemeinen Teilchen von verschiedenen Grössen enthalten, wurden für die jetzige Untersuchung polydisperse Aerosole, welche in einem grossen Gasometer gespeichert waren, benützt. Die Zusammensetzung und der mittlere Radius dieser Aerosole mit heterogener Teilchengrösse wurden nach der Exhaustions-Methode unter Benützung einer Diffusionsbatterie ohne Endstücke oder Verbindungsröhren bestimmt.Die experimentell gefundene Kurve, welche das Verhältnis der ungeladenen (N 0) und geladenen (N) Kerne als Funktion des Radius (r) darstellt, weicht für alle untersuchten Radien zwischen 0.5 und 4.0·10–6 cm von der theoretischen Kurve eines monodispersen Aerosols, wie sie nachBoltzmann's Gesetz berechnet wird, ab. Für Radien kleiner als ungefähr 1.4·10–6 cm ist das VerhältnisN 0/N kleiner als es durchBoltzmann's Gesetz gegeben wird, für Radien grössen als 1.4·10–6 cm grösser oder, mit anderen Worten, fürr<1.4·10–6 cm ist die Zahl der geladenen Teilchen, die in den untersuchten Aerosolen gefunden wurde, grösser als die vonBoltzmann's Gesetz vorausgesagte, und fürr>1.4·10–6 cm kleiner.Die Abweichungen von der theoretischen Kurve fürr>1.4·10–6cm können vollkommen durch die Polydispersität der benützten Aerosole erklärt werden; fürr<1.4·10–6 cm sind die Abweichungen zu gross, als dass sie der Polydispersität zugeschrieben werden könnten. Daraus muss geschlossen werden, dassBoltzmann's Verteilungsgesetz für die Ladungsverteilung eines homogenen Aerosols, welches Kerne mit Radien kleiner als ungefähr 1.4·10–6 cm enthält, nicht gültig ist.Der äquivalente Radius, wie er von einem im Ladungsgleichgewicht befindlichen, monodispersen Ersatz-Aerosol, für welchesBoltzmann's Gesetz als gültig angenommen wird, abgeleitet werden kann, weicht im Bereich von 1.0·10–6<r<3.0·10–6 cm (oder 90%>N 0/Z>55%) durchschnittlich bis zu 35% vom tatsächlichen mittleren Radius des untersuchten polydispersen Aerosols ab.

The research reported in this article has been supported in part by the Geophysics Research Directorate of the Air Force Cambridge Research Laboratories, through the European Office of the Air Force Research Division, United States Air Force under Contract AF 61(052)-26, by the United States Department of Army, through its European Research Office, Contract DA-91-591-EUC-1282 & 1657 and by the Instrumentation Engineering Physics & Analysis Laboratory of the General Electric Co., Schenectady, New York under Retainer Agreement.  相似文献   

7.
Earthquake activity in the Aswan region,Egypt   总被引：3，自引：0，他引：3  
The November 14, 1981 Aswan earthquake (M L= 5.7), which was related to the impoundment of Lake Aswan, was followed by an extended sequence of earthquakes, and is investigated in this study. Earthquake data from June 1982 to late 1991, collected from the Aswan network, are classified into two sets on the basis of focal depth (i.e., shallow, or deeper than 10 km). It is determined that (a) shallow seismicity is characterized by swarm activity, whereas deep seismicity is characterized by a foreshock-main shock-aftershock sequence; (b) the b value is equal to 0.77 and 0.99 for the shallow and deep sequences, respectively; and (c) observations clearly indicate that the temporal variations of shallow seismic activity were associated with a high rate of water-level fluctuation in Lake Aswan; a correlation with the deeper earthquake sequence, however, is not evident. These features, as well as the tomographic characteristics of the Aswan region (Awad andMizoue, this issue), imply that the Aswan seismic activity must be regarded as consisting of two distinct earthquake groups.We also relocated the largest 500 earthquakes to determine their seismotectonic characteristics. The results reveal that the epicenters are well distributed along four fault segments, which constitute a conjugate pattern in the region. Moreover, fault-plane solutions are determined for several earthquakes selected from each segment, which, along with the 14 November 1981 main shock, demonstrate a prominent E-W compressional stress.  相似文献   

8.
Summary Many writers treated on the problem of dynamic instability of westerly flow due to the excessive horizontal shear, and the present author discusses the corresponding dynamic instability due to the vertical shear. The critical vertical shear in indifferent stratification is given by the condition — the meridional component of absolute vorticity vanishes, — and is an approximate negative valueof 10–4 sec –1 in middle latitude. However the critical vertical shear in normal stable stratification is a fairly large negative value of 2 sec–1. It might be emphasized that the problem of this study differs fromRichardson's criterion of turbulence, for the present author discusses the condition under which the zonal flow is dynamically stable, whileRichardson expressed the condition under which the turbulence will decrease.  相似文献   

9.
Interplate coupling plays an important role in the seismogenesis of great interplate earthquakes at subduction zones. The spatial and temporal variations of such coupling control the patterns of subduction zone seismicity. We calculate stresses in the outer rise based on a model of oceanic plate bending and coupling at the interplate contact, to quantitatively estimate the degree of interplate coupling for the Tonga, New Hebrides, Kurile, Kamchatka, and Marianas subduction zones. Depths and focal mechanisms of outer rise earthquakes are used to constrain the stress models. We perform waveform modeling of body waves from the GDSN network to obtain reliable focal depth estimates for 24 outer rise earthquakes. A propagator matrix technique is used to calculate outer rise stresses in a bending 2-D elastic plate floating on a weak mantle. The modeling of normal and tangential loads simulates the total vertical and shear forces acting on the subducting plate. We estimate the interplate coupling by searching for an optimal tangential load at the plate interface that causes the corresponding stress regime within the plate to best fit the earthquake mechanisms in depth and location.We find the estimated mean tangential load over 125–200 km width ranging between 166 and 671 bars for Tonga, the New Hebrides, the Kuriles, and Kamchatka. This magnitude of the coupling stress is generally compatible with the predicted shear stress at the plate contact from thermal-mechanical plate models byMolnar andEngland (1990), andVan den Buekel andWortel (1988). The estimated tectonic coupling,F tc , is on the order of 1012–1013 N/m for all the subduction zones.F tc for Tonga and New Hebrides is about twice as high as in the Kurile and Kamchatka arcs. The corresponding earthquake coupling forceF ec appears to be 1–10% of the tectonic coupling from our estimates. There seems to be no definitive correlation of the degree of seismic coupling with the estimated tectonic coupling. We find that outer rise earthquakes in the Marianas can be modeled using zero tangential load.  相似文献   

10.
Numerical modelling ofSH wave seismograms in media whose material properties are prescribed by a random distribution of many perfectly elastic cavities and by intrinsic absorption of seismic energy (anelasticity) demonstrates that the main characteristics of the coda waves, namely amplitude decay and duration, are well described by singly scattered waves in anelastic media rather than by multiply scattered waves in either elastic or anelastic media. We use the Boundary Integral scheme developed byBenites et al. (1992) to compute the complete wave field and measure the values of the direct waveQ and coda wavesQ in a wide range of frequencies, determining the spatial decay of the direct wave log-amplitude relation and the temporal decay of the coda envelope, respectively. The effects of both intrinsic absorption and pure scattering on the overall attenuation can be quantified separately by computing theQ values for corresponding models with (anelastic) and without (elastic) absorption. For the models considered in this study, the values of codaQ –1 in anelastic media are in good agreement with the sum of the corresponding scatteringQ –1 and intrinsicQ –1 values, as established by the single-scattering model ofAki andChouet (1975). Also, for the same random model with intrinsic absorption it appears that the singly scattered waves propagate without significant loss of energy as compared with the multiply scattered waves, which are strongly affected by absorption, suggesting its dominant role in the attenuation of coda waves.  相似文献   

11.
Based on previous observations of the phenomenon of precursory seismic quiescence before crustal main shocks and recent results that indicate an increase in the occurrence of main shocks in the next years, we focus this study on the detection of the seismic quiescence situation in Greece in the beginning of 1999. We use the declustered seismicity catalogue of the Institute of Geodynamics, National Observatory of Athens (NOA) from 1968–1998, to investigate the significance of seismic quiescence for the region 19°–29°E and 34°–42°N. We searched for seismicity rate changes at every node of the grid by a moving time window and we present the results for the beginning of 1999. The results map four (4) areas having a quiescence which duration ranges from 3.8 to6 years in the beginning of 1999. Three of these areas have been devestated by catastrophic earthquakes 17–21 years ago and significant quiescence also preceded those main shocks. Based on these results, an estimate of the future seismic hazard of these areas is made.  相似文献   

12.
Summary Using the fromulae given byGutenberg andRichter, the writer has computed the magnitude and energy of 1804 earthquakes which occurred in Turkey during the period 1850–1960. For drawing the Isenerget, the formula =log10 S has been used in accordance with the definitions given byToperczer andTrapp, whereS=e i/F·p represents the energy in erg/m2 h corresponding to the surface element of 0.5° Lat. x 0.5° Long. Also the relationship between the seismicity and the tectonics of Turkey has been studied by drawing the maps of the epicenters, the focus-depths and the frequences of the earthquakes with various intensities.  相似文献   

13.
Some months prior to the 1995 eruption of Mt Ruapehu (New Zealand), a series of shallow earthquake swarms occurred about 15–20 km west of the summit of Ruapehu. Several earthquakes in these swarms were felt, and the largest event was ML 4.8. Crustal earthquakes of ML≥3.0 within 20 km of the summit of Ruapehu have been rather uncommon in recent years. Furthermore, the two periods of strongest activity were both just before times when the temperature of Crater Lake showed rapid increases. The second of these rapid heating phases was immediately followed by increases in the Mg2+ ion concentration in Crater Lake, indicating that chemical interactions were occurring between fresh magmatic material and the lake water. The coincidence between seismicity and lake changes suggested a link with the following eruption. A 1-D simultaneous inversion to locate the earthquakes more accurately showed that most of the earthquakes fell into three spatial clusters, each cluster having a small horizontal cross-section. The predominant depth was about 10–16 km. The b-value of this swarm was 0.74, quite compatible with ordinary tectonic earthquakes. Each cluster of earthquakes lies close to the normal Raurimu Fault which runs predominantly north–south to the west of Ruapehu, with an east-trending branch splaying off near its northern end (see Fig. 1b). Composite focal mechanisms of events in the two more southern clusters are oblique-normal, while the other cluster to the north has an oblique-reverse mechanism. The two oblique-normal mechanisms suggest that extension has occurred on part of the fault. This stress pattern was also observed in the focal mechanism solutions of events that occurred after the eruption, when a denser network of portable seismographs covered the region. Although we cannot definitely connect the occurrence of these swarms to the eruptions later in 1995, there is a strong suggestion that the seismicity was connected to the process of magma movement, which temperature and chemical changes in Crater Lake suggest was occurring during the first half of 1995.  相似文献   

14.
The method of relative seismic moment tensor determination proposed byStrelitz (1980) is extended a) from an interactive time domain analysis to an automated frequency domain procedure, and b) from an analysis of subevents of complex deep-focus earthquakes to the study of individual source mechanism of small events recorded at few stations.The method was applied to the recovery of seismic moment tensor components of 95 intermediate depth earthquakes withM L=2.6–4.9 from the Vrancea region, Romania. The main feature of the obtained fault plane solutions is the horizontality ofP axes and the nonhorizontal orienaation ofT axes (inverse faulting). Those events with high fracture energy per unit area of the fault can be grouped unambiguously into three depth intervals: 102–106 km, 124–135 km and 141–152 km. Moreover, their fault plane solutions are similar to ones of all strong and most moderate events from this region and the last two damaging earthquakes (November 10, 1940 withM W=7.8 and March 4 1977 withM W=7.5) occurred within the third and first depth interval, respectively. This suggests a possible correlation at these depths between fresh fracture of rocks and the occurrence of strong earthquakes.  相似文献   

15.
Summary ApplyingWiener-Hopf techniques the diffraction ofSH-waves originating from a moving point source due to the presence of a step change in the elevation of the surface of a homogeneous half-space have been obtained. An interesting feature of resonance associated with the motion of a load at a velocityv 0 –1 approachingv 0 –1 has been observed.  相似文献   

16.
Crustal neon: a striking uniformity   总被引：1，自引：0，他引：1  
By combining data from a diverse suite of crustal fluid samples representing a broad geographical distribution, we have identified a well-defined nucleogenic (crustal) neon component. The neon is produced from (α, n) and (n, α) nuclear interactions involving nuclei of O, Mg, and F [1]. In the limiting case of 20Ne/22Ne = 0, the composition is: 21Ne/22Ne = 0.47 ± 0.01 and 21Ne/4He = (0.46 ± 0.08) × 10−7. A crustal O/F ratio of 110 (atomic) calculated from the 21Ne/22Ne ratio is 4–10 times less than the average crustal O/F ratio. The discrepancy can be accounted for by an enhanced O/F ratio within the 10–40 μm range of the U-Th-generated α-particles.  相似文献   

17.
Summary Using the concerning data of the Bulgarian earthquakes with magnitude greater 4 the seismicity of the period 1901–1965 was computed after a method byUllmann andMaaz and represented cartographically by isolines. The map lets us interpret the relation of the seismicity to the tectonic situation of Bulgaria. Further, the annual frequency of earthquakes of different magnitude basing on the same data is given.  相似文献   

18.
Two uplift episodes have recently been recorded at Phlegraean Fields, a volcanic region near Naples (south-central Italy). The first episode occurred in 1970 and lasted up to 1972; the second has begun at the end of 1982 and is still in progress.An attempt to model the ground deformations which occurred during the 1970–1972 event is made in this paper. The source has been assumed to be a two-dimensional fluid-filled fracture, similar to a sill. A good fit with experimental data has been obtained for a short (1–2 km long) shallow (3 km deep) source and a driving pressure ranging from 60 to 210 bars. Rigidity values have been fixed at 3.5–4.0 × 1010 c.g.s., with Poisson ratio equal to 0.3.This solution which differs from previous approaches byMogi (1958) andWalsh andDecker (1971) is non-unique, but the present results are in good agreement with observed horizontal and vertical displacements. Notwithstanding the oversimplification made in assuming a homogeneous elastic semi-infinite medium, the predicted stress field seems to be in agreement with the fault-plane solutions and the pattern of epicenters determined for the uplift-seismic swarm episode that is still in progress.  相似文献   

19.
Summary The measurement of condensation nucleus concentration from an aircraft is considered and preliminary observations made with a photo-electric condensation nucleus counter discussed. It is concluded that at heights up to 6000 ft concentrations of condensation nuclei vary, from 200 cm–3 in clean maritime air to 5 × 103 cm–3 generally over large industrial areas and 35 × 103 cm–3 downwind of specific industrial sources. It appears that sea spray makes a contribution to the nucleus population.Mr. G. J.Day, B. Sc., Meteorological Research Flight, Royal Aircraft Establishment,Farnborough, Hants. (Great Britain).  相似文献   

20.
Major and trace element and Sr–Nd–Hf–Pb isotopic data for the most primitive Tertiary lavas from the Veneto region (South-Eastern Alps, Italy) show the typical features of HIMU hotspot volcanism, variably diluted by a depleted asthenospheric mantle component (87Sr/86Sri=0.70306–0.70378; Ndi=+3.9 to +6.8; Hfi=+6.4 to +8.1, 206Pb/204Pbi=18.786–19.574). P-wave seismic tomography of the mantle below the Veneto region shows the presence of low-velocity anomalies at depth, which is consistent with possible upwellings of plume material. Between the depths of 100–250 km the velocity anomalies are approximately 2–2.5% slower than average, implying a temperature excess of about 220–280 K, in agreement with estimates for other mantle plumes in the world. In this context, the Veneto volcanics may represent the shallow expression of a mantle upflow. The presence of a HIMU-DM component in a collision environment has significant geodynamic implications. Slab detachment and ensuing rise of deep mantle material into the lithospheric gap is proposed to be a viable mechanism of hotspot magmatism in a subduction zone setting.  相似文献   

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