P in the shadow zone of the earth's core—Part I

Summary Regional variations have been indicated in the slope of theP travel-time curve in the shadow zone of the earth's core. Further study is needed since the uncertainties of the slope are large, especially for the observations from North American stations. There is no significant difference between themean slope of theP travel-time curve in the 95°102.9 range and those obtained byJeffreys, andJeffreys andBullen. However, there is a significant difference between themean slope in the 103° to 135° range as obtained in this study, and those obtained byJeffreys andBullen, and in a later revision byJeffreys. Themean travel-time curve ofP in the shadow zone of the earth's core should be lowered. A trial travel-time table is given with amean slope of 4.41 sec/deg. This table is in close agreement with the times obtained byGutenberg andRichter, and with the trial travel-times ofLehmann. Under the assumption of diffraction the longitudinal wave velocity has been determined to be 13.7 km/sec at the core-mantle boundary.This paper was presented at the Annual Meeting of the Seismological Society of America Reno, Nevada, 1966.     

Continental-array measurements ofP-wave velocities in the mantle

Summary A velocity model for theP-wave is obtained for the entire mantle, by inversion ofdT/d-data from travel times of 473 earthquakes and explosions registered at 13 seismological observatories in Sweden and Finland. As a check, the travel times resulting from this model are found to be in good agreement with our observed travel times. The observations are distributed over the entire azimuth range with respect to the recording stations. The effect of the core upon the observeddT/d-values is significant from about 95° distance outwards. The estimate of the core radius from this model is 3480 km which corresponds to about 97° of epicentral distance. The resulting model has sudden changes inP-velocity gradient at about 65, 115, 400, 780, 970, 1100, 1350, 2320, 2600 and 2710 km depth, which correspond roughly to 10°, 14°, 19.5°, 29°, 40°, 46°, 54°, 80°, 86° and 90° epicentral distance, respectively.On leave from Geophysical Institute, Tehran University, Tehran, Iran     

Zur Interpretation von seismischen Refraktionsmessungen mit abgebrochenen Potenzreihen

Zusammenfassung Es wird die Interpretation der dreigliedrigen Laufzeitfunktion (1) vervollständigt und als Beispiel (P _n () nachJeffreys-Bullen im Bereich von 0° bis 19° behandelt.

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Summary The interpretation of the travel-time function (1) is completed and an example is given.

     

Pn residuals and station corrections

Summary This paper is an attempt towards determination of station adjustments for Shillong and Delly Observatories from considerations of a large number ofP _n residuals. Station adjustments toJeffreys-Bullen travel time tables for Shillong comes to about 4 seconds and for Delhi 2 seconds respectively for 20°.     

Relocation of Wyoming Mine Production Blasts Using Calibration Explosions

—?An important requirement for a comprehensive seismic monitoring system is the capability to accurately locate small seismic events worldwide. Accurate event location can improve the probability of determining whether or not a small event, recorded predominantly by local and regional stations, is a nuclear explosion. For those portions of the earth where crustal velocities are not well established, reference event calibration techniques offer a method of increased locational accuracy and reduced locational bias.¶In this study, data from a set of mining events with good ground-truth data in the Powder River Basin region of eastern Wyoming are used to investigate the potential of event calibration techniques in the area. Results of this study are compared with locations published in the prototype International Data Center's Reviewed Event Bulletin (REB). A Joint Hypocenter Determination (JHD) method was applied to a s et of 23 events. Four of those events with superior ground-truth control (mining company report or Global Positioning System data) were used as JHD reference events. Nineteen (83%) of the solutions converged and the resulting set of station-phase travel-time corrections from the JHD results was then tested. When those travel-time corrections were applied individually to the four events with good ground-truth control, the average locational error reduced the original REB location error from 16.1?km to 5.7?km (65% improvement). The JHD locations indicated reduced locational bias and all of the individual error ellipses enclosed the actual known event locations.¶Given a set of well-recorded calibration events, it appears that the JHD methodology is a viable technique for improving locational accuracy of future small events where the location depends on arrival times from predominantly local and/or regional stations. In this specific case, the International Associ ation of Seismology and the Physics of the Earth's Interior (IASPEI) travel-time tables, coupled with JHD-derived travel-time corrections, may obviate the need for an accurately known regional velocity structure in the Powder River Basin region.     

Improving Regional Seismic Event Location in China

—?In an effort to improve our ability to locate seismic events in China using only regional data, we have developed empirical propagation path corrections and applied such corrections using traditional location routines. Thus far, we have concentrated on corrections to observed P arrival times for crustal events using travel-time observations available from the USGS Earthquake Data Reports, the International Seismic Centre Bulletin, the preliminary International Data Center Reviewed Event Bulletin, and our own travel-time picks from regional data. Location ground truth for events used in this study ranges from 25?km for well-located teleseimic events, down to 2?km for nuclear explosions located using satellite imagery. We also use eight events for which depth is constrained using several waveform methods. We relocate events using the EvLoc algorithm from a region encompassing much of China (latitude 20°–55°N; longitude 65°–115°E). We observe that travel-time residuals exhibit a distance-dependent bias using IASPEI91 as our base model. To remedy this bias, we have developed a new 1-D model for China, which removes a significant portion of the distance bias. For individual stations having sufficient P-wave residual data, we produce a map of the regional travel-time residuals from all well-located teleseismic events. Residuals are used only if they are smaller than 10?s in absolute value and if the seismic event is located with accuracy better than 25?km. From the residual data, correction surfaces are constructed using modified Bayesian kriging. Modified Bayesian kriging offers us the advantage of providing well-behaved interpolants and their errors, but requires that we have adequate error estimates associated with the travel-time residuals from which they are constructed. For our P-wave residual error estimate, we use the sum of measurement and modeling errors, where measurement error is based on signal-to-noise ratios when available, and on the published catalog estimate otherwise. Our modeling error originates from the variance of travel-time residuals for our 1-D China model. We calculate propagation path correction surfaces for 74 stations in and around China, including six stations from the International Monitoring System. The statistical significance of each correction surface is evaluated using a cross-validation technique. We show relocation results for nuclear tests from the Balapan and Lop Nor test sites, and for earthquakes located using interferometric synthetic aperture radar. These examples show that the use of propagation path correction surfaces in regional relocations eliminates distance bias in the residual curves and significantly improves the accuracy and precision of seismic event locations.     

Travel-time residuals and three-dimensional velocity structure of Italy

Jeffreys-Bullen P and PKP travel-time residuals observed at more than 50 seismic stations distributed along Italy and surrounding areas in the time interval 1962–1979, indicate the complex velocity pattern of this region. Strong lateral velocity inhomogeneities and low velocity zones are required to explain the observed pattern of residuals. In particular, late arrivals of about 1 sec are observed in the Apenninic mountain range, requiring both greater crustal thickness and low velocity layers, coherent with seismic refraction data and surface wave dispersion measurements. The seismic stations located in the Western and Eastern Alps indicate the presence of high velocities. In the Western Alps the strong azimuthal variation of residuals and the high values of early arrivals have a close relationship to the Ivrea body, an intrusive crustal complex characterized by a velocity as high as 7–7.2 km/sec.A travel-time inversion performed with theAki et al. (1977) block model, confirms the peculiar characteristics and the sharp variations in the lithosphere of the whole Italian region, with values of velocity perturbations between many adjacent blocks, ranging in size from 50 to 100 km, and independent from the earth parametrization chosen, reaching values up to 10% in the lithospheric part and 5% in the asthenosphere. 3-D inversion requires also high velocity along the Tyrrhenian coastal margin, equivalent to an uprise of major crustal and lithospheric discontinuities along this part of the Italian peninsula. Moreover low velocity material must be present in the northern part of the Adriatic foreland, in the lithosphere-asthenosphere system, closely related to the stress and seismicity pattern, and the lateral bending of the lithosphere in the same region.     

Some results of short wave insolation measurements during the IGY along the 10° E meridian and a comparison with theoretical values

Summary The observational material of 138 stations, which had during the July 1957 to December 1958 IGY period made obervations of global radiation along the longitude 10°E (±10°) between the latitudes 47°S and 70°N, had been evaluated to obtain mean monthly and annual totals of global radiation for 12 different latitudes along thea/n-meridian. The sums are compared with theoretical results obtained byBudyko, Black, andBernhardt andPhilipps. These theoretically computed values are drawn graphically or numerically from charts or tables of thea/n-authors for the coordinates corresponding to the means of the relevant IGY stations. In hardly a case the measured annual totals deviate by more than 10% from theoretical values. In hardly a case the measured annual totals deviate by more than 10% from theoretical values. TheBudyko data are the nearest approximation to the measured ones;Black's computation shows the greatest deviation. In the 0 to 47° S zone theoretical values are actually exceeded by to 20%.The annual variation is marked by the following deviations as compared to the theoretical results: In the region north to 50° latitude in October, November and January only 80% of the theoretically computed amount of radiation was observed, while in the remaining months the monthly totals are within a ±10% boundary of the theoretical values; in March about 20% above normal global radiation was observed. Between 30° and 50°N the measured data in autumn, winter and spring 10 to 20% below the theoretical values, in the remaining time of the year the deviation is less than ±10%. A comparison with cloud conditions (relative sunshine duration) suggests that deviations cannot be interpreted merely by deviating cloud conditions of this zone. Between 0° and 30°N only January and February are 10% below theoretical values, while in August to October the totals were 10 to 15% above. South of the equator from October to May (southern summer) the radiation totals were too high by up to 25%, while in the remaining time of the year the data were about 10% below theoretical values; it should be noted that deviation increases with increasing southern latitude. For comparison theAshbel IGY global radiation charts were used which indicate similar deviations from theoretical values; there are also some differences to microcard values.There is reason to suggest that there is a complex relation between the observed deviations and variation of the large-scale circulation during IGY. The results of the small number of available turbidity measurements (Central European area only) can be interpreted in the same way. To what extent the observed deviations from theoretical values are due to weather conditions deviating during the IGY period from normal or originate from inconsistencies in the theoretical consideration cannot definition be said until inspection of the complete IGY cloud and turbidity data and the availability of the climatological material which served for the theoretical work. The latter was only the case in theBernhardt andPhilipps paper.

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Zusammenfassung Aus 138 Stationen auf dem Meridian 10°E (±10°) zwischen –47° und +70° Breite, die während der IGJ-Periode die Globalstrahlung beobachtet haben, wurden mittlere Monats-und Jahressummen der Globalstrahlung für 12 verschiedene Breiten auf diesem Meridian berechnet. Diese Summen wurden mit den theoretischen Werten vonBudyko, Black, undBernhardt undPhilipps verglichen. Die gemessenen Summen weichen meist um höchstens 10% von den theoretischen Werten ab. Die Werte vonBudyko kommen den gemessenen am nächsten, die vonBlack zeigen die grössten Abweichungen. Für die Zone zwischen 0° und 47° südlicher Breite liegen die gemessenen Werte jedoch durchwegs um 8 bis 20% über den theoretischen. Im jahreszeitlichen Verlauf traten folgende bemerkenswerte Abweichungen gegen die theoretischen Werte auf: Nördlich 50° Breite erhielten die Monate Oktober, November und Januar nur etwa 80% der theoretischen Strahlungssummen, in den anderen Monaten stimmten die Summen innerhalb 10% mit den theoretischen Werten überein, der März erhielt etwa 20% übernormale Globalstrahlung. Zwischen 30° und 50° nördlicher Breite lagen die gemessenen Werte besonders im Herbst, Winter und Frühling 10 bis 20% unter den theoretischen. Die Abweichungen können nicht allein der Bewölkung in dieser Zone zugeschrieben werden. Südlich des Äquators wiesen die Monate Oktober bis Mai (Südsommer!) bis zu 25% zu hohe Strahlungssummen auf. Die zum Vergleich herangezogenen Karten vonAshbel ergeben ähnliche Abweichungen gegen die theoretischen Werte, wenn auch einige Unterschiede gegen die hier veröffentlichten Summen auftreten.Ein komplexer Zusammenhang zwischen diesen Abweichungen und der grossräumigen Zirkulation im IGJ scheint zu bestehen. Auch die wenigen Trübungsmessungen (nur mitteleuropäische Stationen) können im gleichen Sinne gedeutet werden. Inwieweit die beobachteten Abweichungen der Strahlung von den theoretischen Werten auf den im Mittel abweichenden Witterungsverhältnissen der IGJ-Periode beruhen oder auf Unstimmigkeiten der theoretischen Werte, lässt sich aus dem vorliegenden Material noch nicht definitiv entscheiden.

     

The effect of the descending lithosphere beneath the Tonga island arc on P-wave travel-time residuals at the Warramunga Seismic Array

P-wave travel-time residuals at the Warramunga Seismic Array (WRA) in the Northern Territory, Australia, have been studied from 49 earthquakes with epicenters south of 19°S in the Fiji-Tonga region. Focal depths are between 42 and 679 km as determined from pP-P. Using the Jeffreys-Bullen and the Herrin travel-time tables the epicentral parameters have been redetermined by considering only “normal” seismic stations in the location procedure. These are those stations where P-wave travel times are probably not affected by lateral heterogeneities caused by the lithosphere descending beneath the Tonga trench. Epicenters of deep earthquakes below 300 km have been relocated by using stations at Δ > 25° only. Epicenters from shallower-depth earthquakes have been recalculated without using stations between 35 < Δ < 75° epicentral distance. In both cases focal depths were determined from pP-P times. The resulting pattern of P-residuals at WRA does not show any significant change with depth below 350 km. The residuals become more negative for shallower earthquakes above about 250 km. P-waves to WRA are advanced by approximately 2 s compared with those from deep earthquakes. The results do not essentially differ for the two different travel-time tables used. The observations can be interpreted by P-wave velocities that are higher in the sinking slab down to 350–400 km by 5±2% than in both the Jeffreys-Bullen and Herrin models. Without considering possible elevations of phase boundaries this estimate yields a temperature contrast of 1000±450°C between slab and normal mantle material in this depth range.     

Average crustal structure of Sweden

Summary Records obtained at the permanent stations of the Swedish seismograph network from explosions carried out in Scandinavian waters in June 1969 are evaluated. The study includes determination of velocities for all crustal phases observed, furthermore of layer thicknesses, Poisson ratios and amplitude ratios. The purpose of the study is partly to provide a first approximation to the crustal structure in Sweden, partly to provide regional data for location of earthquakes and explosions in the area in the future. Average velocities (km/sec) are forPn 7.88±0.05,Pg1 6.25±0.08,Pg2 5.70,Sn 4.58±0.04,S ^* 3.70±0.04,Sg1 (Lg1) 3.58±0.03,Sg2 (Sg) 3.40±0.03,Rg 3.02±0.07. The average thickness is 12 km for the granitic layer, and 23 km for the basaltic layer, thus making the average crustal thickness equal to 35 km. Relative amplitudes plotted versus distance complete the dynamical side of the study and they are useful for identification of waves. A regional travel-time table is presented for the distance range 0°–10° with entries for each 0.1° and including all crustal phases read.     

Luni-solar 18.6-year signal in tree-rings from Argentina and Chile

Spectrum analysis of 32 tree-ring chronologies from Argentina and Chile yields evidence for two peaks with periods 19.2±1.6 years (30 out of 32 records) and 10.5±0.4 years in 22 instances. Tests by thet-statistic show that the long-period peak is significant at a confidence level of 99%. This signal is identified as the luni-solar 18.6-year M _n term reported earlier byCurrie (1983) in two treering chronologies from the same region, and later in tree-rings from North America, Tasmania, New Zealand, and South Africa (Currie, 1991a-c). Amplitude and phase of the M _n signal are nonstationary with respect to both time and geography. In particular, abrupt 180° phase changes in wave polarity are often observed.     

Choice of a teleseismicP-wave travel-time model and the associated source corrections

Summary Among various trial models ofP-wave travel times, there exists at least one model which best predicts the times of first arrivals from a certain region at a set of recording stations even if no attempt is made to correct the travel-time standards against known station errors and source bias. In teleseismic distance range (25°100°) and surface-focus case, the possibility of statistically establishing which of the twoP models, viz. Jeffreys-Bullen and Herrin, is more appropriate for each of the four source regions, viz. Southern Nevada, Aleutian Islands, Eastern Kazakh and Novaya Zemlya, is discussed in this paper. Data corresponding to a set of underground explosions from these regions form the basis of such an analysis. The Herrin model is found to be better applied to Aleutian Islands region while the Jeffreys-Bullen model seems more appropriate for each of the remaining three regions.As a result of the study of the travel-time models, numerical estimates of space and time corrections pertaining to the above mentioned regions, based on the most appropriate model and directly applicable to the computed source parameters, are obtained. On applying these corrections, the size of source location error ellipse and the source-time error reduce to very small representative values, viz. 4 km×6 km (area 75 km²) and ±0.2 sec respectively.     

Lunar and solar barometric tides at Rarotonga,Cook Islands

Lunar and solar atmospheric tidal oscillations have been determined with reasonable accuracy from a ten-year record of hourly mercury-barometer readings, corrected to mean-sea-level, at Rarotonga (Cook Islands), 21.2°S. For the lunar semidiurnal tide, the annual determination shows an amplitude (56 b) slightly lower and a phase (51°) much smaller than the values (58 b, 72°) that would be derived, for the position of Rarotonga, from the spherical harmonic analysis given byHaurwitz andCowley (1969). The seasonal variation of this oscillation, as given by the monthly and J, E, D values, shows most of the characteristic features found in world-wide determinations. In particular, the near equality of the J, D amplitudes at Rarotonga tends to support theHaurwitz andCowley (1969) suggestion of negative J-D values in southern middle latitudes. For the solar tides, the semidiurnal and terdiurnal oscillations at Rarotonga are similar to those found at other stations in the south-west Pacific region. However, for the diurnal oscillation, the annual amplitude (232 b) is only about half the value (465 b) indicated for the position of Rarotonga by the world maps of theS ₁(p) annual harmonic coefficients given byHaurwitz (1965). It thus seems likely that the relatively small area of lowS ₁(p) annual amplitude in the eastern part of the south Pacific, as indicated by these maps, is much more extensive than formerly supposed.     

Some space gravity formulas and the dimensions and the mass of the Earth

Summary Adopting thePizzetti-Somigliana method and using elliptic integrals we have obtained closed formulas for the space gravity field in which one of the equipotential surfaces is a triaxial ellipsoid. The same formulas are also obtained in first approximation of the equatorial flattening avoiding the use of the elliptic integrals. Using data from satellites and Earth gravity data the gravitational and geometric bulge of the Earth's equator are computed. On the basis of these results and on the basis of recent gravity data taken around the equator between the longitudes 50° to 100° E, 155° to 180° E, and 145° to 180° W, we question the advantage of using a triaxial gravity formula and a triaxial ellipsoid in geodesy. Closed formulas for the space field in which a biaxial ellipsoid is an equipotential surface are also derived in polar coordinates and its parameters are specialized to give the international gravity formula values on the international ellipsoid. The possibility to compute the Earth's dimensions from the present Earth gravity data is the discussed and the value ofMG=(3.98603×10²⁰ cm³ sec^–2) (M mass of the Earth,G gravitational constant) is computed. The agreement of this value with others computed from the mean distance Earth-Moon is discussed. The Legendre polinomials series expansion of the gravitational potential is also added. In this series the coefficients of the polinomials are closed formulas in terms of the flattening andMG.Publication Number 327, and Istituto di Geodesia e Geofisica of Università di Trieste.     

Further evidence for radial velocity anomalies in the lower mantle

Summary Wright andLyons (1981) used a least-squares matching technique (LMM) and an adaptive processing method (ADP) to study the behaviour of slowness and azimuth measurements made on two synthetic interfering wavelets having different arrival vectors and onset times. We have applied these results to the analysis of real array seismograms. Some of the effects generated synthetically are frequently observed on real seismograms of earthquakes recorded at Yellowknife at distances close to 50° and 90°. We have also processed sufficient data to illustrate how the interference phenomenon can be used to confirm the presence of radial velocity anomalies in the lower mantle. NumerousP arrivals from South American earthquakes at distances between 78° and 98° suggest the presence of two radial velocity anomalies at depths close to 2400 and 2730 km below the Caribbean region; these anomalies also appear to vary laterally.Contribution No. 864 from the Earth Physics Branch.     

P-wave velocity anomalies in the earth's mantle from the Uppsala array observations

Summary Distribution of compressional-wave velocities in the mantle is determined fromdT/d measurements using the Uppsala seismograph array station (UPSAS). Short-period vertical-component seismograms from 181 events in the epicentral distance range 16°–100° have been used. The velocity distribution shows anomalous variations at depths of 750, 1500, 1800, 2300 and 2550 km. Evidence of lateral heterogeneity beneath the northern part of the Asian continent, in the depth range 1700–2300 km, is discussed. Computed travel times, based on this velocity-depth relation, are tested by an examination of travel-time residuals from the Long Shot and Milrow explosions on Amchitka, Aleutian Islands.     

Magnetospheric currents and estimation of theSq focal latitude

The average latitude of theSq(X) focus along the longitude 75°E has been estimated using the Tsyganenko model (Tsyganenko, 1981) of the external magnetic field for storm time conditions of the magnetosphere. It is observed that the shift of the focal latitudes, due to magnetospheric currents, is only about 1 to 2° even during strong storms. It is also shown that the shift is asymmetric about the equator and longitude dependent. The day to day changes in observed focal position are much larger and the magnetospheric currents cannot, therefore, be regarded as a dominant mechanism of focal movement.The paper was presented at the IAGA General Assembly meeting held in Prague, Czechoslovakia, during August 1985.     

HYPOSAT – An Enhanced Routine to Locate Seismic Events

—?A program package, called HYPOSAT, has been under development that attempts to use the maximum information possible to estimate the hypocenter of a seismic source. The standard input parameters can be used: arrival times of first and later onsets with backazimuths and ray parameters (or apparent velocities). In addition, travel-time differences between different phases observed at the same station can be optionally used. The observed standard deviations are used to weight all input parameters and the inversion is done with a generalized matrix inversion code.¶A starting solution with a priori uncertainties can be calculated as the intersection of all backazimuth observations. If S observations are also available, a preliminary origin time is estimated using Wadati's approach to estimate a source time.¶Global earth models and user-defined horizontally layered local or regional models can be used alone or together to locate seismic events. To gain the best result from all input data, observations of all seismic phases as defined in the IASPEI91 tables can be inverted. Station corrections and corrections for phases with reflection points at the earth's surface can be applied by using local velocity structures.     

Regional travel-time and slowness measurements for P-waves in the distance range 40–57°

A method for analysing travel times measured at a large array or a network of seismographs from many earthquakes within a specific region has been developed. Approximate relative station corrections are calculated from the residuals on a least-squares line or least-squares quadratic form fitted through the times for each earthquake, and may be improved by iteration after a preliminary travel-time curve has been derived. Accurate relative baseline corrections for each earthquake are also calculated iteratively, and an optimum slowness-distance curve is determined from the combined corrected travel times from all earthquakes using a trade-off procedure. Calculations using synthetic travel-time data suggest that abrupt changes in slowness of ～ 0.4 s deg^?1 due to the presence of triplications are generally resolvable, provided that the effects of lateral variations are small, even with random epicentre mislocations in the range ± 0.5°. Slowness measurements at a network of temporary stations deployed across Australia do not show any discontinuities in slowness greater than 0.2 s deg^?1 in the distance range 45–54°. Similar measurements at the Warramunga array from the same source regions, however, suggest the presence of complexity in the slowness curve at distances close to 50°. Relative arrival times at the temporary network generally have standard deviations less than 0.25 s, thus suggesting that details of structure finer than those derived from conventional travel-time studies can be resolved.