S U M M A R YNew empirical traveltime curves for the major seismic phases have been derived from the catalogues of the International Seismological Centre by relocating events by using P readings, depth phases and the iasp91 traveltimes, and then re-associating phase picks. A smoothed set of traveltime tables is extracted by a robust procedure which gives estimates of the variance of the traveltimes for each phase branch. This set of smoothed empirical times is then used to construct a range of radial velocity profiles, which are assessed against a number of different measures of the level of fit between the empirical times and the predictions of the models. These measures are constructed from weighted sums of L2 misfits for individual phases. The weights are chosen to provide a measure of the probable reliability of the picks for the different phases.A preferred model, ak1.?5, is proposed which gives a significantly better fit to a broad range of phases than is provided by the iusp9I and sp6 models. The differences in velocity between ak135 and these models are generally quite small except at the boundary of the inner core, where reduced velocity gradients are needed to achieve satisfactory performance for PKP differential time data.The potential resolution of velocity structure has been assessed with the aid of a non-linear search procedure in which 5000 models have been generated in bounds about ak135. Misfit calculations are performed for each of the phases in the empirical traveltime sets, and the models are then sorted using different overall measures of misfit. The best 100 models for each criterion are displayed in a model density plot which indicates the consistency of the different models. The interaction of information from different phases can be analysed by comparing the different misfit measures. Structure in the mantle is well resolved except at the base, and ak135 provides a good representation of core velocities.

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We relocate nearly 100,000 events that occurred during the period 1964 to 1995 and are well-constrained teleseismically by arrival-time data reported to the International Seismological Centre (ISC) and to the U.S. Geological Survey's National Earthquake Information Center (NEIC). Hypocenter determination is significantly improved by using, in addition to regional and teleseismic P and S phases, the arrival times of PKiKP, PKPdf, and the teleseismic depth phases pP, pwP, and sP in the relocation procedure. A global probability model developed for later-arriving phases is used to independently identify the depth phases. The relocations are compared to hypocenters reported in the ISC and NEIC catalogs and by other sources. Differences in our epicenters with respect to ISC and NEIC estimates are generally small and regionally systematic due to the combined effects of the observing station network and plate geometry regionally, differences in upper mantle travel times between the reference earth models used, and the use of later-arriving phases. Focal depths are improved substantially over most other independent estimates, demonstrating (for example) how regional structures such as downgoing slabs can severely bias depth estimation when only regional and teleseismic P arrivals are used to determine the hypocenter. The new data base, which is complete to about Mw 5.2 and includes all events for which moment-tensor solutions are available, has immediate application to high-resolution definition of Wadati-Benioff Zones (WBZs) worldwide, regional and global tomographic imaging, and other studies of earth structure.

The value to seismology of a global net of seismometers has long been recognized. Indeed, the first proposal for such a net came in 1895, only 6 years after the discovery that earthquake waves could be recorded at long distances, and the first net of standardized instruments was in existence by 1898 [Dewey and Byerly, 1969; Wood, 1942; Milne, 1899]. A true network, involving not only standardized high‐quality instruments but also the exchange and ready availability of data, did not come until the establishment of the World‐Wide Standard Seismograph Network (WWSSN) in the middle 1960's [Oliver and Murphy, 1971]. The importance of this to general seismology cannot be overstated; not only has it improved the quality of traditional research areas, but the availability of the data has suggested new types of investigations. In the decade since the WWSSN was built, two small networks have been set up: the High‐Gain Long‐Period instruments (HGLP) [Savino et al., 1972] and, very recently, the Seismic Research Observatories (SRO) [Peterson and Orsini, 1976]. Both of these networks use careful shaping of the instrument response to improve the detection of surface waves from very small events, which is important in lowering the magnitude threshold for discriminating earthquakes from explosions.

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The response of the Earth to an earthquake is a transient that is effectively zero several days after the event. A recording of the event, of finite duration in time, has a Fourier spectrum that is an entire, or integral, analytic function of frequency. We present a very simple procedure for computing the Fourier spectrum as a function of complex frequency; the analytically continued spectrum. By investigating the properties of the analytically continued spectrum we show how to extract high-Q modes, how t o estimate Q either from the amplitude or from the width of a resonance function, and how to improve the resolution of splitting to the theoretical maximum. Examples of these procedures, using observed data, are presented.Analytically continued seismic spectra Consider the displacement response of the Earth u(r, t ) to an earthquake (Gilbert & Dziewonski 1975,2.1 .I 1 ;hereafter M).where sk is the displacement of the kth free oscillation (normalized to a total energy of unity), .)k is the source excitation time function, cdk the frequency, and a k = uk/2Qk the damping parameter. The real part cdk and imaginary part (Yk of the kth complex normal mode eigenfrequency, (7k = cdk + i(Yk, are functionals of the mechanical structure of the Earth. There is considerable motivation to make accurate observations of numerous (Tk, and in particular to improve the observational accuracy of the (Yk's. In this paper we exploit the fact that u(r, t ) is available only for te [O, T] where T is the duration of the seismic

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By a process of matched filtering it is possible to deconvolve a small number of acceleration or strain records for the moment rate tensor of a seismic source. Theoretically, one horizontal accelerometer (or strain meter) or two vertical accelerometers is sufficient. Practically, five to ten records can be shown to suffice. Specifically, the mechanism of the Colombian event of July 31, 1970 can be retrieved from a sparse network of ten WWSSN vertical instruments.