Apparent Slowness Research Articles

Array analyses of volcanic earthquakes and tremor recorded at Las Cañadas caldera (Tenerife Island, Spain) during the 2004 seismic activation of Teide volcano

We analyze data from three seismic antennas deployed in Las Cañadas caldera (Tenerife) during May–July 2004. The period selected for the analysis (May 12–31, 2004) constitutes one of the most active seismic episodes reported in the area, except for the precursory seismicity accompanying historical eruptions. Most seismic signals recorded by the antennas were volcano-tectonic (VT) earthquakes. They usually exhibited low magnitudes, although some of them were large enough to be felt at nearby villages. A few long-period (LP) events, generally associated with the presence of volcanic fluids in the medium, were also detected. Furthermore, we detected the appearance of a continuous tremor that started on May 18 and lasted for several weeks, at least until the end of the recording period. It is the first time that volcanic tremor has been reported at Teide volcano. This tremor was a small-amplitude, narrow-band signal with central frequency in the range 1–6 Hz. It was detected at the three antennas located in Las Cañadas caldera. We applied the zero-lag cross-correlation (ZLCC) method to estimate the propagation parameters (back-azimuth and apparent slowness) of the recorded signals. For VT earthquakes, we also determined the S–P times and source locations. Our results indicate that at the beginning of the analyzed period most earthquakes clustered in a deep volume below the northwest flank of Teide volcano. The similarity of the propagation parameters obtained for LP events and these early VT earthquakes suggests that LP events might also originate within the source volume of the VT cluster. During the last two weeks of May, VT earthquakes were generally shallower, and spread all over Las Cañadas caldera. Finally, the analysis of the tremor wavefield points to the presence of multiple, low-energy sources acting simultaneously. We propose a model to explain the pattern of seismicity observed at Teide volcano. The process started in early April with a deep magma injection under the northwest flank of Teide volcano, related to a basaltic magma chamber inferred by geological and geophysical studies. The stress changes associated with the injection produced the deep VT cluster. In turn, the occurrence of earthquakes permitted an enhanced supply of fresh magmatic gases toward the surface. This gas flow induced the generation of LP events. The gases permeated the volcanic edifice, producing lubrication of pre-existing fractures and thus favoring the occurrence of VT earthquakes. On May 18, the flow front reached the shallow aquifer located under Las Cañadas caldera. The induced instability constituted the driving mechanism of the observed tremor.

Examination of systematic mislocation of South Sandwich Islands earthquakes using station pairs: Implications for inner core rotation

One of the most robust observations for the inner core rotation is that the differential PKP BC‐DF traveltimes from South Sandwich Islands (SSI) earthquakes to College, Alaska (and some other stations in Alaska), have increased systematically over the past 50 years. The time shift in the differential time residuals is some 0.3 s over 30 years. This temporal change is thought to result from a shift of the inner core structure from a superrotation of the inner core. However, the observation has been hotly debated and has been suggested to be an artifact of systematic earthquake mislocation. Here we examine this issue using three mutually independent approaches, with a goal to quantify the amount of the systematic mislocation, if any. All three approaches involve differencing data between north stations at a similar azimuth as the SSI‐Alaska azimuth and south stations at nearly the opposite azimuth. Mislocation along this azimuth has greatest impact on the PKP differential times. In approach 1, we examine how the double difference of P traveltimes between a north station and a south station changes with time. Because the differential apparent slowness between the P traveltimes of the pairs is 8 to 16 times that of the differential BC‐DF times, mislocation would cause a time shift in the double difference an order of magnitude larger than that of BC‐DF times. In approach 2, we compare directly difference in P arrival times between two SSI events that are close by and between a north and a south stations, which avoids using earthquake locations and Earth models entirely. In approach 3, we compare PKP traveltime residuals between a group of north and south stations. Because PKP traveltimes are generally not used for location in earthquake bulletins, they provide an independent data set to check earthquake location errors. The results from approaches 1 and 2, both of which use P data, are generally consistent. The mislocation is up to 3.6 ± 4.2 km (one standard deviation), explaining less than 10% of the observed COL temporal change. The results from approach 3 using PKP data show a larger mislocation but with a larger error (14.4 ± 10.8 km over 30 years). It can explain about 30% ± 22% of the observed COL temporal change. Therefore inner‐core superrotation is still the best explanation for the observed temporal change in the differential traveltimes.

Apparent migration of tremor source synchronized with the change in the tremor amplitude observed at Aso volcano, Japan

At Aso volcano, Kyushu, Japan, several different types of volcanic tremor have been observed for many years. One of them is the continuous tremor, the ground vibration which has dominant frequency between 3 and 10 Hz and has approximately constant amplitudes without any clear beginning and ending. We observed the continuous tremor at Aso using short period seismometer arrays for 3 days in 1999. We locate the source of the continuous tremor by seismic array data processing. We use the semblance coefficients in order to estimate the arrival azimuth and apparent slowness by grid search. The epicenters of the continuous tremor are located around the currently active crater, and the source depths are likely to be shallower than 600 m. We find that the estimated epicenter clearly migrates synchronized with the change in the tremor amplitude. The migration often occurs periodically with a period of about 80 s, but aperiodic occurrence of the migration is also often seen. In both cases, the epicenter is located southeastward (northwestward) when the amplitude is larger (smaller). We propose that there are two or more independent tremor sources with fixed locations, and that their amplitudes modulate either aperiodically or periodically with periods nearly 80 s. The tremor signals from those sources are mixed at the arrays, and the estimated epicenter parameters vary according to which signal dominates the seismograms. The simplest model is that there are two point sources, one at west and the other at south of the crater (we call the two sources as “NW source” and “SE source”, respectively), and the amplitude of the SE source changes with time. Consequently, the SE source dominates the seismogram when the observed amplitude is larger, whereas the NW source dominates when the amplitude is smaller. We generate synthetic seismograms, and apply the location technique to them to verify the validity of the two point source model and to search the locations of two sources which can explain the observed synchronization between the amplitude and the apparent epicenter location. We find that the distance between the two sources needs to be more than 400 m to agree with the observation. We also analyze the seismic array data observed in 2001, and infer that the NW source in 1999 may be identical to the tremor source of the 2001 data.

Acoustic Wave Propagation in a Borehole Penetrating an Inclined Layered Formation

AbstractUnderstanding acoustic wave propagation in an inclined fluid–filled borehole embedded in 3‐D media is essential for acoustic wave logging data processing and interpretation. A 3‐D staggered finite‐difference (FD) method is used to simulate borehole sonic waves in isotropic inclined layered formations. First, the FD results are compared with those obtained by a Real Axis Integration method for a monopole source in an open borehole vertically imbedded in homogeneous formation. A good agreement has been made for the two methods. Consequently, the FD solutions are confirmed by the analytical solutions. Then, the waveforms for a borehole with various deviations between the borehole axis and formation interface are calculated. Those for dipole sources are also simulated at a low source center frequency. The numerical results show that, when the source is under the inclined interface, equivalently going up in a fast (lower) formation, and also as the interface inclined angle increases, the determined slowness decreases from the slowness approximately correspondent to that of the slow (upper) formation, to the one in downward formation, and never reaches the real compressional slowness in the upper formation. The correspondent slowness curve, with respect to record points, and above the formation interface, becomes flatter with increasing distance from the source; while the short spacing detection makes the curve sharper. For all spacing with receivers in the upper formation, and the source under the interface in the lower formation, the apparent slowness becomes smaller than the real wave slowness in the upper formation. The larger the inclined angle is, the flatter are the curves away from the interface range for the long spacing. The shear wave slowness shows the similar features but more sensitive to the inclined angle than the compressional slowness. Also, this effect is controlled by the velocity contrast between the upper and lower formations. The above phenomenon is shown clearly in both snapshot visualization and slowness calculation with synthetic waveforms, and is well explained with the theory of ray acoustics.

Electronic Distribution of Hotel Rooms–An Exploratory Study of the European Hotel Industry

ABSTRACT Hotel groups have built up experience in distributing rooms electronically. More recently, the Internet has provided the opportunity to disrupt the traditional value chain, potentially disintermediating established providers, whilst presenting hotels with an unprecedented array of routes to market their products. The need for channel management expertise has become more acute than ever. Along with the need to understand consumers' online attitudinal preferences and concerns, we argue that there is a need to examine hotel providers' apparent slowness to take advantage of the new media. A research questionnaire, designed to establish the factors influencing hotels' choice of intermediaries, was developed from the extant literature. Principal component analysis uncovered an overriding factor, (referred to here as “risk preference”) which is shown to influence heavily channel strategy choice. This indicates the concern by hoteliers over the impact of negative direct or intermediary-related performance on hotel brand image and reputation. The results of this study further support the theory that the hotel industry has yet to define a strategic direction that will leverage the capabilities of the Internet.

Precise determination of the relative wave propagation parameters of similar events using a small‐aperture seismic array

We propose a method to determine accurately the relative wave propagation parameters (apparent slowness and propagation azimuth) of a cluster of seismic events with similar waveforms recorded on a seismic array. This relative slowness estimate (RelSE) method is based on precise measurements of the delays among arrivals of different earthquakes to each of the array receivers. Delays are determined using interpolations of the cross‐correlation functions of the earthquake waveforms. Accurate relative slowness vectors are estimated using a least squares fit of the observed delays to the delays corresponding to the arrivals of plane wave fronts. We tested the method using both synthetics and real data, in order to understand its resolution capabilities in presence of seismic noise and to assess the uncertainty regions associated with the slowness vector estimates. From these analyses, we establish a procedure to determine the 90% uncertainty regions associated with the estimates of relative slowness vectors. As an example of application of the RelSE method, we analyzed a multiplet composed of 16 similar earthquakes recorded during the 1999 seismic crisis at Deception Island volcano, Antarctica. Using a conventional slowness estimate method produces virtually the same result for every earthquake, because of the large uncertainties. Alternatively, using the RelSE method reduces the uncertainties of the estimates and allows to resolve the detailed distribution of (relative) apparent slowness vectors. Our results show that the slowness vectors are aligned within a narrow, north‐south trending band, which represents a clue toward the features of the source region and/or source distribution. We repeated the procedure using different earthquakes as master events. The estimated distribution of slowness vectors is similar in every case, which demonstrates that our results are independent of the choice of reference event.

Slowness‐weighted diffraction stack for migrating wide‐angle seismic data in laterally varying media

Wide‐angle prestack depth migration is an important tool for studying the nature of reflecting boundaries in the earth's crust. The slowness‐weighted diffraction stack (SWDS) method has been used to incorporate both two‐way traveltime constraints and slowness information in the migration. For this purpose, traveltimes and apparent slownesses of reflected arrivals must be calculated in the image space. Earlier applications of SWDS required a 1D or gently varying seismic velocity structure to obtain these quantities by ray tracing in the image space. I show that the apparent slownesses can also be derived directly from one‐way traveltime maps using Fermat's principle. The SDWS is applied to an existing onshore–offshore wide‐angle data set, and the example shows that the method can be used to image detailed reflectivity structure at great depths.

The 1998–1999 seismic series at Deception Island volcano, Antarctica

During the 1998–1999 Antarctic summer the pattern of seismic activity at Deception Island volcano changed significantly. The change was characterized by the occurrence of an intense swarm of volcano–tectonic (VT) earthquakes. More than 2000 VT earthquakes with S–P times smaller than 4 s were recorded in the period January–February 1999. Pure volcanic events were also detected; especially long-period (LP) events, volcanic tremor and some hybrid events. Seismic monitoring was performed using two short-period small-aperture arrays, among other instruments. Based on their signal-to-noise ratios we selected 863 VT earthquakes, 350 LP events and tremor episodes, and 9 hybrid events for analysis. We estimated apparent slowness and back-azimuth for all events using the Zero Lag Cross-Correlation array technique. Combining this information with S–P times and other indirect evidence, we identified two different source regions. LP seismicity is located less than 1–1.5 km southwest of the Fumarole array site. These events are likely to have a hydrothermal origin. VT earthquakes and hybrid events are located at depths of 0.3–10 km in an area under the bay of Deception Island. The area extends from the Fumarole array to the northeast with epicentral distances that range from 0.5 to 12 km. Most hypocenters are clustered in a small volume of around 8 km 3. The sources of the LP seismicity and the VT earthquakes are spatially distinct, which indicates that they are not produced by the same mechanisms. Moment magnitude analyses of the VT earthquakes provide an average magnitude of 0.5 and very low average stress drop, around 1 bar. A study of first motion of the P-waves suggests that the events in this small source region should have a variety of source mechanisms. This is supported by the existence of families of events with the same waveforms. The occurrence of repeating fracture processes with low stress drop and small fault dimensions can be explained by the lubrication of pre-existing zones of weakness by pressurized fluids. The most probable hypothesis that explains the generation of this seismic series at Deception Island is: a seismic series caused by the stress generated by the uplift of the source area due to a magmatic injection in depth. We favor this hypothesis since it is compatible with the majority of the characteristics of the seismicity and explains the spatial and temporal behavior of the series.

Anisotropic inversion of refracted waves in vertical cable data in the presence of dip

Refracted arrivals are analysed to estimate the near-surface anisotropy of marine sediments using a vertical-cable (VC) configuration. In the presence of dip, the horizontal and vertical ray-slownesses are obtained from the observed apparent slownesses in the up- and downdip directions using a sum or difference at each azimuth. The multiple azimuths generated by a VC geometry permit the ray-slowness distribution of the marine sediments to be determined. An inversion procedure is developed to provide dip and anisotropy parameters for refractive layers from the measured refraction traveltimes in multilayered azimuthally isotropic and anisotropic media. Two sets of transversely isotropic models are used to analyse the azimuthal variations of apparent and ray slownesses. In the first set, we fix the anisotropic parameters of the models but vary the dip (0°, 5° and 10°) to test the effects of the presence of dip. In the second set, we vary the P-wave anisotropy strength (5.2%, 10.3%, 15.8% and 22.0%) to examine the sensitivity and accuracy of ray-slowness approximations which are independent of dip. We test this inversion procedure on synthetic P-wave VC data calculated for six different models by a finite-difference method. The results of applications to real VC data acquired from the North Sea are also presented.

Wide-angle Reflection Profiling with Quarry Blasts in the Eastern Tennessee Seismic Zone

This study evaluates the feasibility of using relatively small, delay-fired quarry blasts as energy sources for wide-angle reflection profiling of the middle to lower crust beneath the southern half of the eastern Tennessee seismic zone. The test recordings targeted structure on both sides of the New York–Alabama lineament, a prominent northeast-trending zone of steep magnetic gradients. The lineament marks a near-vertical boundary between zones of high and low seismic activity and also correlates with a linear boundary between blocks of high- and low-velocity crust imaged by joint hypocenter-velocity inversion of regional P and S arrival time data. Timed blasts at four crushed-stone quarries were recorded with a portable array of 19 PRS-4 recorders and three-component, 4.5-Hz geophones with stations at roughly 200 m intervals. Source-receiver offsets ranged from 16 to 67 km. Blasts were timed with a PRS-4 deployed at the quarry sites. The blasts ranged from 75 to 542 ms in duration and from 259 to 942 lbs. explosive per delay. The effects of ripple firing are clearly evident in trace autocorrelations and as coherent peaks and nodes in the amplitude spectra for each shot; the latter show signal energy at frequencies up to 45 Hz. Source wavelets estimated by minimum entropy deconvolution have durations that are consistent with the reported durations of the blasts. Unprocessed field records recorded over distances between 16 and 29 km show strong reflections from the basal decollement (depth 4.5 km) and deeper interfaces within the basement. Shot gathers and slant stacks for blasts recorded at offsets between 62 and 67 km show coherent energy with apparent slownesses and travel times consistent with precritical P and S reflections from the lower crust and Moho. Although sparsely sampled, preliminary migrated sections suggest the presence of several highly reflective structures, including a concentration of reflectors at a depth of about 25 km, near the maximum reported depth for earthquakes in the region, a zone of south-dipping reflectors at depths between 27 and 40 km, northwest of the New York–Alabama lineament, and a clustering of reflectors at depths between 40 and 50 km, possibly marking the crustmantle transition. They also show north-dipping reflectors that correlate with the top of a prominent wedge of reflective crust imaged in nearby vertical-incidence profiles. The results demonstrate that signals from relatively small delayfired blasts can be quite useful for imaging the crust.

Dip moveout of converted waves and parameter estimation in transversely isotropic media

For transverse isotropy with a vertical symmetry axis (VTI media), P‐wave reflection data alone are insufficient for building velocity models in depth. Here, we show that all parameters of VTI media responsible for propagation of P‐ and SV‐waves (the P‐wave and S‐wave vertical velocities VP0 and VS0 and the anisotropic parameters ε and δ) can be obtained by combining P‐wave traveltimes with the moveout of PS‐waves converted at a horizontal and dipping interface. Using converted modes, rather than pure S‐waves, avoids the need for expensive shear‐wave excitation on land and makes the method suitable for offshore exploration.The inversion algorithm is based on a new analytic description of the dip moveout of PS‐waves developed for symmetry planes of anisotropic media (and for any vertical plane in models with weak azimuthal anisotropy). The common‐midpoint (CMP) traveltime–offset relationship, derived in a parametric form and represented through the components of the slowness vector of the P‐ and S‐waves, makes it possible to compute the moveout curve of the PS‐wave without two‐point ray tracing. This formalism also leads to closed‐form solutions for moveout attributes, such as the coordinates (xmin, tmin) of the traveltime minimum, the normal‐moveout (NMO) velocity defined at x = xmin and the slope of the moveout curve (apparent slowness) at zero offset.The parameter‐estimation algorithm operates with reflection moveout of P‐ and PS‐waves from a horizontal and dipping reflector. The NMO velocities of P‐ and PS‐waves from horizontal events and the ratio of the corresponding zero‐offset traveltimes yield three equations for the four unknown medium parameters. The remaining parameter is found from an overdetermined system of equations that includes the P‐wave NMO velocity and moveout attributes of the PS‐wave for a dipping event. Numerical analysis shows that the PS‐wave dip‐moveout signature plays a crucial role in obtaining accurate estimates of the anisotropic parameters. The joint inversion of P and PS data provides the necessary information not only for P‐wave depth imaging in VTI media, but also for the processing of PS‐waves, including re‐sorting of PS traces into common‐reflection‐point gathers and transformation to zero offset (TZO).

The Acquisition of Japanese Numeral Classifiers: Linkage between Grammatical Forms and Conceptual Categories.

This study examines the acquisition of Japanese numeral classifiers in Japanese preschool children, ages 3 to 6, with a primary emphasis on developing comprehension ability. Numeral classifiers, widely distributed in languages of East and Southeast Asia and the New World, are a group of morphemes that usually occur adjacent to quantity expressions. The selection of numeral classifiers is determined by the inherent semantic properties of the noun whose quantity is being specified, suggesting that developing patterns of comprehension should be linked to underlying patterns of semantic and conceptual development. Previous research claims that children acquire certain distributional patterns very early but that the acquisition of the semantic system is a very slow process. We argue instead that different techniques and stimulus contrast sets reveal a much greater sensitivity to semantic relations in young children than was previously considered possible. Reasons for the apparent slowness in classifier acquisition are also discussed.

Electrochemical and catalytic behaviour of Dawson-type complexes derived from [(1),2,3-P2Mo2W15O61]10− and first transition metal ions

The acid-base and electrochemical behaviour of the lacunary Dawson-type heteropolyanion α-[(1)-2,3-P2Mo2W15O61]10− and of its derivatives with VIV, MnII, FeIII, CoII, NiII, ZnII, and CuII has been studied as a function of pH. Essentially two groups can be distinguished. In the first group, constituted by the VIV, MnII, CoII, NiII, ZnII-substituted complexes, the redox activity, if any, of the heterometallic cations occurs far from the potential location of that of the molybdenum moieties. At pH 3, the molybdenum waves are very close to each other and to that of the precursor lacunary complex. For higher pH values the apparent slowness observed for the first wave of the lacunary compound becomes less pronounced in the substituted complexes, but is modulated differently by each substituent. For the FeIII and CuII substituted complexes, the redox activity of these cations can mix with that of the molybdenum centres, depending on the pH of the solution. Spectroelectrochemical experiments have helped to clarify the conditions of the separation of redox activities in the case of the FeIII-substituted complex. It has been possible to obtain an overall three electron process on the first wave of this compound. In contrast, copper deposition is observed with the CuII substituted complex in appropriate conditions. All the complexes show electrocatalytic properties on their respective first wave for the reduction of NO. Owing to the very high stability of the VIV-substituted complex throughout the whole pH domain, it has been possible to study the electrocatalytic oxidation of NADH to NAD+ at pH 8.

Nonlinear refraction traveltime tomography

A few important issues for performing nonlinear refraction traveltime tomography have been identified. They include the accuracy of the traveltime and raypath calculations for refraction, the physical information in the refraction traveltime curves, and the characteristics of the refraction traveltime errors. Consequently, we develop a shortest path ray‐tracing method with an optimized node distribution that can calculate refraction traveltimes and raypaths accurately in any velocity model. We find that structure ambiguity caused by short and long rays in the seismic refraction method may influence the inversion solution significantly. Therefore, we pose a nonlinear inverse problem that explicitly minimizes the misfits of the average slownesses (ratios of traveltimes to the corresponding ray lengths) and the apparent slownesses (derivatives of traveltimes with respect to distance). As a result, we enhance the resolution as well as the convergence speed. To regularize our inverse problem, we use the Tikhonov method to avoid solving an ill‐posed inverse problem. Errors in refraction traveltimes are characterized in terms of a common‐shot error, a constant deviation for recordings from the same shot, and a relative traveltime‐gradient error with zero mean with respect to the true gradient of the traveltime curve. Therefore, we measure the uncertainty of our tomography solution using a nonlinear Monte Carlo approach and compute the posterior model covariance associated with two different types of random data vectors and one random model vector. The nonlinear uncertainty analysis indicates that the resolution of a tomography solution may not correspond to the ray coverage. We apply this tomography technique to image the shallow velocity structure at a coastal site near Boston, Massachusetts. The results are consistent with a subsequent drilling survey.

Observations of high—frequency scattered energy associated with the core PhasePKKP

Stacks of global seismic network data reveal emergent high—frequency (∼1 Hz) energy extending back from the PKKPc caustic at 72° to a distance of about 60°. The arrival's emergent onset, long duration (∼150 s), apparent slowness, and proximity to the core phase PKKP indicate a deep scattering origin. However, the amplitude and timing of the observations are difficult to explain with simple models of PKKP scattering.

Array tracking of the volcanic tremor source at Deception Island, Antarctica

We have found experimental evidence which shows that the volcanic tremor recorded at Deception Island (South Shetland Islands, Antarctica) is a superposition in time of overlapping hybrid events. We studied data from a small aperture seismic array. Data analysis for tremor and hybrids included: (1) spectral analysis; (2) apparent slowness and back‐azimuth determination by using the zero‐lag cross‐correlation method; and (3) polarization analysis. Both types of events share these common features: (a) two dominant spectral bands at frequencies 1–3 Hz (the most energetic) and 4–8 Hz; (b) several coherent phases with the same back‐azimuth to the source and apparent slowness along the whole signal; (c) in the high frequency band, the apparent slowness is very low (around 0.17 s/km), indicating the propagation of body waves; (d) in the low frequency band, the apparent slowness is high (around 1.6 s/km), consistent with the presence of surface waves; and (e) clear P‐wave onset followed by a complex pattern of Rayleigh waves. Therefore, both types of events are strongly related because they share the same source region, the same wave‐propagation properties, and the same wave composition. Moreover, several arrivals, that resemble a single hybrid event, have been found along the tremor signals. Due to these reasons, we hypothesize that volcanic tremor of Deception Island is a superposition of hybrid type events. The source of both types could be the interaction between thaw water and hot materials in a shallow aquifer.

Interactive three-dimensional visualization of seismic array data

We present a set of tools developed for the analysis of three-component seismic array data. The first tool involves the application of three-dimensional volume visualization techniques for displaying the results of single-component, array, slowness analysis calculations. This technique has proven useful for analysis of both body waves and surface waves recorded by broadband arrays with a wide range of apertures. The second tool presented is a tool for animation and interactive analysis of three-dimensional particle motions observed by an array of three-component instruments. This tool displays a three-dimensional track of particle motions over a specified time window as recorded by three-component array stations. In order to compare the amplitudes, waveforms, polarization and time delays of signals on different array stations the records are aligned by the time of arrival of a plane wave with the azimuth and the apparent slowness determined from two-dimensional semblance analysis. This technique provides an improved understanding of the spatial variability of seismic wave fields that is not possible with conventional, two-dimensional graphics.

Observations of high-frequency scattered waves using dense arrays at Teide Volcano

Abstract A study of the seismic coda of seven small earthquakes recorded on the Teide Volcano-Canary Islands (Spain) was carried out using a temporary, small-aperture, 12-station, seismic array. The purpose was to measure backazimuth, apparent velocity and the type of waves that compose the coda in the frequency range 4 to 6 Hz. We used the zero-lag cross-correlation (ZLC) method to obtain the components of the wave vector and three-component analysis techniques based on the covariance matrix of the signal in the time domain to investigate the polarization properties of the signals. The results show that a great part of the coda signals in the analyzed frequency range are almost uncorrelated, while a low number of isolated correlated arrivals show an apparent slowness between 2 × 10−3 and 2.5 × 10−4 sec/m and an almost random distribution of backazimuths. The correlated arrivals have been interpreted as generated by strong scatterers, probably related to the presence, in the study area, of surface topography irregularities (the volcanic cone and the caldera rim). The wave type varies from pure S-type waves to mixed surface waves with some Rayleigh components.

Seismic velocities at the core-mantle boundary inferred from P waves diffracted around the core

The very base of the mantle is investigated with core-diffracted P-wave (P diff) travel times published by the International Seismological Centre (ISC) for the period 1964–1987. Apparent slownesses are computed for two-station profiles using a difference method. As the short-period P diff mostly sample a very thin layer above the core-mantle boundary (CMB), a good approximation of the true velocity structure at the CMB can be derived from the apparent slownesses. More than 27000 profiles are built, and this provides an unprecedented P diff sampling of the CMB. The overall slowness distribution has an average value of 4.62 s/deg, which corresponds to a velocity more than 4% lower than that of most mean radial models. An analysis of the residuals of absolute ISC P and P diff travel times is independently carried out and confirms this result. It also shows that the degree of heterogeneities is significantly higher at the CMB than in the lower mantle. A search for lateral velocity variations is then undertaken; a first large-scale investigation reveals the presence of coherent slowness anomalies of very large dimensions of the order of 3000 km at the CMB. A tomographic inversion is then performed, which confirms the existence of pronounced (±8–10%) lateral velocity variations and provides a reliable map of the heterogeneities in the northern hemisphere. The influence of heterogeneity in the overlying mantle, of noise in the data and of CMB topography is evaluated; it seemingly proves minor compared with the contribution of heterogeneities at the CMB. Our results support the rising idea of a thin, low-velocity laterally varying boundary layer at the base of the D″ layer. The two principal candidate interpretations are the occurrence of partial melting, or the presence of a chemically distinct layer, featuring infiltrated core material.

Human gene therapy--an immature genie, but certainly out of the bottle.

The gene therapy community has swung from an excited and overoptimistic mood to one of disillusionment with the apparent slowness of progress and with the absence of truly therapeutic applications. Although gene therapy is yet to come of age, there can be no doubt that, in time, it will bear fruit.