Global Digital Seismograph Network Research Articles

The 1980, 1997 and 1998 Azores earthquakes and some seismo-tectonic implications

We have studied the focal mechanisms of the 1980, 1997 and 1998 earthquakes in the Azores region from body-wave inversion of digital GDSN (Global Digital Seismograph Network) and broadband data. For the 1980 and 1998 shocks, we have obtained strike–slip faulting, with the rupture process made up of two sub-events in both shocks, with total scalar seismic moments of 1.9 × 10 19 Nm ( M w = 6.8) and 1.4 × 10 18 Nm ( M w = 6.0), respectively. For the 1997 shock, we have obtained a normal faulting mechanism, with the rupture process made up of three sub-events, with a total scalar seismic moment of 7.7 × 10 17 Nm ( M w = 5.9). A common characteristic of these three earthquakes was the shallow focal depth, less than 10 km, in agreement with the oceanic-type crust. From the directivity function of Rayleigh (LR) waves, we have identified the NW–SE plane as the rupture plane for the 1980 and 1998 earthquakes with the rupture propagating to the SE. Slow rupture velocity, about of 1.5 km/s, has been estimated from directivity function for the 1980 and 1998 earthquakes. From spectral analysis and body-wave inversion, fault dimensions, stress drop and average slip have been estimated. Focal mechanisms of the three earthquakes we have studied, together with focal mechanisms obtained by other authors, have been used in order to obtain a seismotectonic model for the Azores region. We have found different types of behaviour present along the region. It can be divided into two zones: Zone I, from 30°W to 27°W; Zone II, from 27°W to 23°W, with a change in the seismicity and stress direction from Zone I. In Zone I, the total seismic moment tensor obtained corresponded to left-lateral strike–slip faulting with horizontal pressure and tension axes in the E–W and N–S directions, respectively. In Zone II, the total seismic moment tensor corresponded to normal faulting, with a horizontal tension axis trending NE–SW, normal to the Terceira Ridge. The stress pattern for the whole region corresponds to horizontal extension with an average seismic slip rate of 4.4 mm/yr.

The November 14, 2001 west of Kunlun Mountain Pass earthquake: An earthquake with unsaturated surface wave magnitude

This paper presents an overview of the magnitude determination of the November 14, 2001 west of Kunlun Mountain Pass (KMP) earthquake at the juncture of Xinjiang and Qinghai, northwestern China. Comparisons are made among surface wave magnitudes determined by China National Digital Seismograph Network (CNDSN), National Earthquake Information Center (NEIC) of US Geological Survey (USGS) and moment magnitudes determined by different institutions in China and abroad. The result shows that different institutions yield different surface wave magnitudes, as different data and calculation formulae are used in magnitude determination. The magnitude of the earthquake in China’s Rapid Earthquake Information Release was given as MS=8.1; measurement given in the formally edited and published Observation Report of China Digital Seismograph Network is MS=8.2; and magnitude determined by USGS/NEIC is Ms=8.0. Soon after the occurrence of the KMP earthquake, Harvard University (Harvard), USGS/NEIC, Earthquake Research Institute, Tokyo University (ERI), Center for Analysis and Prediction, China Earthquake Administration (APCEA) and Institute of Geophysics, China Earthquake Administration (IGCEA) gave the moment magnitude MW as 7.8, 7.7, 7.7, 7.6 and 7.5, respectively, based on data from Global Seismograph Network (GSN), CNDSN and China Digital Seismograph Network (CDSN). These measurements, with an average value of MW=7.7, are close to each other. As moment magnitude is a physical quantity measuring the absolute size of an earthquake and has obvious advantages over conventional magnitude scale, and is the preferred magnitude of the international seismological community. It is concluded that the KMP earthquake is an earthquake with unsaturated surface wave magnitude with moment magnitude MW=7.7 and surface wave magnitude MS=8.0.

Torishima selects Glasgow for European base

Using a waveform inversion method, we studied the rupture processes of two deep earthquakes in the Izu-Bonin region. The earthquake source was modelled by space-time-dependent slip on a fault plane. The fault plane was divided into many subfaults, and the source was expressed in terms of the final dislocations on the subfaults, the rupture starting times at the subfault corners and the rise time of source function. From P-wave seismograms recorded by the Global Digital Seismograph Network, we first obtained the displacement waveforms by deconvolution techniques. Next, we inverted these waveform data to determine the model parameters. For the earthquake near Torishima on March 6, 1984, non-homogeneous stress distribution and variation in rupture velocity were revealed. The rupture initiated at a corner of a 54 × 36 km2 fault, and high-stress drop occurred in the middle region of the fault, about 30 km away from the initiation. The earthquake south off Honshu of April 24, 1984, did not have as complex a rupture process. The rupture propagated unilaterally with almost constant velocity.

Source time functions of the 1999, Jiji (Chi-Chi) earthquake from GDSN long period waveform data using aftershocks as empirical Green’s functions

A large earthquake (MW=7.6) occurred in Jiji (Chi-Chi), Taiwan, China on September 20, 1999, and was followed by many moderate-size shocks in the following days. Two of the largest aftershocks with the magnitudes of MW=6.1 and MW=6.2, respectively, were used as empirical Green’s functions (EGFs) to obtain the source time functions (STFs) of the main shock from long-period waveform data of the Global Digital Seismograph Network (GDSN) including IRIS, GEOSCOPE and CDSN. For the MW=6.1 aftershock of September 22, there were 97 pairs of phases clear enough from 78 recordings of 26 stations; for the MW=6.2 aftershock of September 25, there were 81 pairs of phases clear enough from 72 recordings of 24 stations. For each station, 2 types of STFs were retrieved, which are called P-STF and S-STF due to being from P and S phases, respectively. Totally, 178 STF individuals were obtained for source-process analysis of the main shock. It was noticed that, in general, STFs from most of the stations had similarities except that those in special azimuths looked different or odd due to the mechanism difference between the main shock and the aftershocks; and in detail, the shapes of the STFs varied with azimuth. Both of them reflected the stability and reliability of the retrieved STFs. The comprehensive analysis of those STFs suggested that this event consisted of two sub-events, the total duration time was about 26 s, and on the average, the second event was about 7 s later than the first one, and the moment-rate amplitude of the first event was about 15% larger than that of the second one.

Estimation of source parameters and depth of focus of the earthquake of Sikkim Himalaya by waveform modelling

The earthquake of 19 November 1980, which occurred in the Sikkim Himalaya was investigated by waveform modelling technique. The synthetic seismograms for both the P- and S-waves were generated using wave number integration method. The P- wave seismograms were generated using different velocity models for the source and receiver crusts. The S-wave seismograms were produced using only the half-space model for both the source and receiver crusts. Comparing the seismograms of these waves with the respective seismograms digitally recorded by the Global Digital Seismograph Network the source parameters have been estimated. The orientation parameters determined in this way are strike = 119 °, dip = 74° and dip direction = S29W for one modal plane and strike = 218 °, dip= 64° and dip direction = N52W for the other modal plane. The modal plane striking in NW direction has been preferred as representing the fault plane. The depth of the earthquake has been estimated to be 22 km. The total duration of the rupture process has been estimated to be 7 s.

Dawn of a New Era in Computational Global Seismology

Global seismology is a science driven by observations, and most major advances in our discipline can be linked to advances in the quality and quantity of seismic instrumentation. Normal-mode seismology was born in the early Sixties, when Caltech and UCLA seismologists identified fundamental toroidal and spheroidal modes in spectra obtained from time series recorded after the great 22 May 1960 Chilean earthquake. In the Eighties, the Harvard centroid-moment tensor project was initiated, and global tomographybecame feasible as a result of high-quality data recorded by the International Deployment of Accelerometers (IDA) and the Global Digital Seismographic Network (GDSN). Today, the Global Seismographic Network (GSN) contains more than 120 stations, the first permanent ocean-bottom seismometer (H20) has been installed, and in the near future USArray and the Advanced National Seismic System (ANSS) may blanket the contiguous United States with hundreds of broadband instruments. As in the past, these advances in instrumentation must be complemented by advances in theory and computation. Because of the success of simple spherically symmetric Earth models, first-order perturbation theory, and ray theory, global seismologists have been reluctant to embrace fully numerical approaches. Because the Earth is well approximated by a set of concentric spherical shells, synthetic seismograms based upon normal-mode summation or discrete-wavenumber/reflectivity …

March 19, 1996 Artux Xinjiang Earthquake: A Simple Unilateral Rupture Event

AbstractUsing the Global Digital Seismograph Network (GDSN) broadband P wave records and the adaptive hybrid global search algorithm, the rupture process of the 19 March, 1996 Artux, Xinjiang earthquake(MS6.9) is examined. Aftershock data and results of geological and seismic field investigation are applied to constrain the real fault plane. This event occurred in the western part of the Keping Fault zone and is a unilateral‐rupture thrust event with a small strike slip component. The fault plane has a strike of 252° and a dip of 30°. The rupture duration was 15 s and the focal depth was 13 km. The inferred slip distribution contains two major slip sources. A sub‐event with a slip amplitude of 0.3m and a risetime of 0.8 s is located at the hypocenter. The maximum slip amplitude of 1.0m is associated with a 3.5 s risetime subevent located to the east of the first major slip source. The distance between the two major slip sources is about 25 km, and corresponds with the 30 km difference between the instrumental and macroseismic epicenters.

Global mapping of topography on transition zone velocity discontinuities by stacking SS precursors

We stack long‐period, transverse‐component seismograms recorded by the Global Digital Seismograph Network (GDSN) (1976–1996), Incorporated Research Institutions for Seismology‐International Deployment of Accelerometers (IRIS‐IDA) (1988–1996), and Geoscope (1988–1996) networks to map large‐scale topography on the 410‐ and 660‐km seismic velocity discontinuities. Underside reflections from these discontinuities arrive as precursors to the SS phase, and their timing can be used to obtain global variations of the depth to the reflectors. We analyze over 13,000 records from events mb>5.5, focal depth <75 km, and range 110° to 180° by picking and aligning on SS, then stacking the records along the theoretical travel time curves for the discontinuity reflections. Separate stacks are obtained for 416 equally spaced caps of 10° radius; clear 410‐ and 660‐km reflections are visible for almost all of the caps while 520‐km reflections are seen in about half of the caps. The differential travel times between the precursors and the SS arrival are measured on each stack, with uncertainty estimates obtained using a bootstrap resampling method. We then compute discontinuity depths relative to the isotropic Preliminary Reference Earth Model (PREM) at 40‐s period, correcting for surface topography and crustal thickness variations using the CRUST5.0 model of Mooney et al. [1995], and for upper mantle S velocity heterogeneity using model S16B30 of Masters et al. [1996]. The resulting maps of discontinuity topography have more complete coverage than previous studies; observed depths are highly correlated between adjacent caps and appear dominated by large‐scale topography variations. The 660‐km discontinuity exhibits peak‐to‐peak topography of about 38 km, with regional depressions that correlate with areas of current and past subduction around the Pacific Ocean. Large‐scale topography on the 410‐km discontinuity is lower in amplitude and largely uncorrelated with the topography on the 660‐km interface. The width of the transition zone, WTZ, as measured by the separation between the 410‐ and 660‐km discontinuities, appears thickest in areas of active subduction (e.g., Kurils, Philippines, and Tonga) and thins beneath Antarctica and much of the central Pacific Ocean. Spatial variations in WTZ appear unrelated to ocean‐continent differences but do roughly correlate with the S16B30 velocities in the transition zone, consistent with a common thermal origin for both patterns. The lower‐amplitude 520‐km reflector is more difficult to resolve but appears to be a global feature as it is observed preferentially for those bounce point caps with the most data.

Global Stacking of Broadband Seismograms

INTRODUCTION Stacking is a technique that combines large numbers of seismograms with different source-receiver geometries into a single composite image that typically has better signal-to-noise ratios than the individual records. This method has been routinely used by the oil industry for some time to image crustal structure from digital waveform data obtained using controlled source experiments. The increased availability of digital recordings of earthquakes from both local and global networks has made it possible to apply the stacking approach to studies of deep Earth structure. Walck and Clayton (1984) combined short-period vertical records from the Southern California Seismic Network to obtain upper-mantle velocity structure. Similarly, Vidale and Benz (1993) used vertical short-period records from seismic networks in the western United States to image upper-mantle discontinuities in the vicinity of deep earthquakes. Stacks of long-period Global Digital Seismograph Network seismograms have been used to image global body- and surface-wave phases (Shearer,...

Transition zone velocity gradients and the 520‐km discontinuity

Stacks of long‐period Global Digital Seismograph Network (GDSN) seismograms at 110° to 180° epicentral distance reveal precursors to SS that result from underside reflections off upper mantle seismic discontinuities. The 410‐ and 660‐km discontinuities are obvious in these stacks, but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410‐ and 660‐km reflections. These sidelobes result from the limited bandwidth of the GDSN instrument responses and the effect of crustal reverberations on the SS reference phase. The crustal effects can be minimized by restricting the records to oceanic bounce points where the ∼6‐km‐thick crust has little effect on the long‐period waveforms. Over 2000 long‐period, transverse‐component seismograms with oceanic SS bounce points recorded by the GDSN from 1976 to 1991 are manually edited, aligned on SS, and then stacked using a new procedure that weights the records by data quality. The resulting image shows a clear reflection from a 520‐km discontinuity that cannot be explained as a sidelobe artifact, confirming earlier results of Shearer [1990, 1991] and Revenaugh and Jordan [1991]. By stacking along the expected travel time curves for discontinuity phases, the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time. The features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces. Using geometrical ray theory and assuming a constant velocity versus density scaling relationship, I fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion. The inverse problem is addressed by performing a deconvolution of the profile with the SS reference phase (after a correction for attenuation), followed by a direct mapping of reflectivity versus time into velocity versus depth. Velocity‐depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models, except that the SS precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660‐km discontinuity. Estimated discontinuity shear impedance changes are 6.7 ± 1.1% at 420 km, 2.9 ± 0.7% at 519 km, and 9.9 ± 1.5% at 663 km. The impedance change near 520 km is consistent with current mineral physics results for the olivine β to γ phase change and places constraints on the fraction of olivine in the transition zone.

The July 12, 1993, Hokkaido‐Nansei‐Oki, Japan, earthquake: Coseismic slip pattern from strong‐motion and teleseismic recordings

We employ a finite fault inversion scheme to infer the distribution of coseismic slip for the July 12, 1993 Hokkaido‐Nansei‐Oki earthquake using strong ground motions recorded by the Japan Meteorological Agency within 400 km of the epicenter and vertical P waveforms recorded by the Global Digital Seismograph Network at teleseismic distances. The assumed fault geometry is based on the location of the aftershock zone and comprises two fault segments with different orientations: a northern segment striking at N20°E with a 30° dip to the west and a southern segment with an N20°W strike. For the southern segment we use both westerly and easterly dip directions to test thrust orientations previously proposed for this portion of the fault. The variance reduction is greater using a shallow west dipping segment, suggesting that the direction of dip did not change as the rupture propagated south from the hypocenter. This indicates that the earthquake resulted from the shallow underthrusting of Hokkaido beneath the Sea of Japan. Static vertical movements predicted by the corresponding distribution of fault slip are consistent with the general pattern of surface deformation observed following the earthquake. Fault rupture in the northern segment accounts for about 60% of the total P wave seismic moment of 3.4 × 1020 N m and includes a large circular slip zone (4‐m peak) near the earthquake hypocenter at depths between 10 and 25 km. Slip in the southern segment is also predominantly shallower than 25 km, but the maximum coseismic displacements (2.0–2.5 m) are observed at a depth of about 5 km. This significant shallow slip in the southern portion of the rupture zone may have been responsible for the large tsunami that devastated the small offshore island of Okushiri. Localized shallow faulting near the island, however, may require a steep westerly dip to reconcile the measured values of ground subsidence.

Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS, and SS — SdS travel time residuals

On the basis of P, PP, S, SS arrival times and SS ‐ S410S, SS ‐ S660S differential times, we construct models of mantle P and S velocity structure and boundary topography of the 410‐km and 660‐km discontinuities. Events from the catalog of the International Seismological Centre (ISC) are relocated relative to the International Association of Seismology and Physics of the Earth's Interior 1991 (IASP91) velocity model using both P and S arrival times. The arrival times are corrected for ellipticity and the PP and SS residuals are corrected for the topography at the bounce point. The cap‐averaged PP ‐ P and SS ‐ S differential time residuals, plotted at the PP and SS surface reflection points, form broad coherent patterns. The geographic distribution of the cap averaged residuals agrees quite well with PP ‐ P and SS‐S differential time residuals derived from long period Global Digital Seismograph Network (GDSN) data. A robust lp inversion scheme is used to infer global mantle structure. Synthetic tests indicate that for regions well sampled by SS ‐ S410S and SS ‐ S660S differential times, the velocity estimates are not seriously contaminated by the topography of the 410‐ and 660‐km discontinuities. However, estimates of boundary deflections may be influenced by extensive P and S velocity variations of 3% or greater. We find the 410‐km discontinuity to be depressed by as much as 24 km beneath North America. Conversely, the discontinuity is deflected upward underneath Eurasia. In some regions the topography of the 660‐km discontinuity is quite distinct from that of the 410‐km discontinuity, but the two appear to be positively correlated. A series of depressions are found at several intersections of the 660‐km discontinuity with known subduction zones. The elevated topography in the 410‐km discontinuity beneath Europe is underlain by a trough in the 660‐km discontinuity. A number of subduction zones are characterized by a thinning of the transition zone. Negative P and S velocity anomalies, underlying back‐arc basins and tectonically active continental regions, encircle the Pacific. Where they are resolved, the stable continental cratons are systematically positive velocity features that extend below 200 km. With the inclusion of PP and SS travel time residuals we are better able to constrain midmantle structure. Most notably, in the depth range 35–660 km beneath the Northwest Pacific we observe high P velocity. Where they are resolved, mid‐ocean ridges are most clearly imaged as low velocity features in the S model. The northern portion of the Mid‐Atlantic Ridge is underlain by negative S velocity anomalies. In the Pacific, the East Pacific Rise is an extensive low S velocity anomaly.

On the structure of the lowermost mantle beneath the southwest Pacific, southeast Asia and Australasia

The region of the lowermost mantle beneath the southwest Pacific, Australasia and southeast Asia (50°S–50°N and 80–190°E) has been studied using a wide variety of seismic techniques. We complement these studies with results obtained from long-period Global Digital Seismograph Network (GDSN) data using a recently developed phase-stripping technique that permits the isolation of D″ reflections from the stronger neighbouring S and ScS signals. We identify patches with D″ reflections and areas where we cannot confidently determine the presence or absence of a D″ reflection. A synthesis of our results with other studies suggests that D″ varies dramatically through the region, generally thickening from 100–150 km in a central zone to about 300 km at the eastern and western margins. However, there are irregularities in this overall pattern, including areas where there seems to be little evidence for a D″ discontinuity. Inspection of waveform amplitudes shows considerable scatter in not only the D″ reflected phases, but also core-mantle boundary reflected phases. Experiments with synthetic seismograms for a variety of D″ models and the observed lateral variability through the region suggest that this is to be expected. Furthermore, ray-theory calculations for large-scale 3D Earth models predict significant variations (±30%) in S-wave amplitudes of lowermost mantle turning rays. Finally, we investigate possible correlations between lower-mantle velocity, flow and D″ thickness. We find some correlation between D″ thickness and lower-mantle velocities obtained from tomographic inversions, with a thin D″ layer in high-velocity regions and a thickening of the layer toward slower regions. The relationship between predicted lower-mantle flow and D″ thickness is less clear. These results are qualitatively consistent with thermal boundary layer predictions for D″, but do not preclude the possibility of compositionally distinct material in the layer.

A P wave velocity model of Earth's core

Present Earth core models derived from the retrieval of global Earth structure are based on absolute travel times, mostly from the International Seismological Centre (ISC), and/or free‐oscillation eigenfrequencies. Many core phase data are left out of these constructions, e.g., PKP differential travel times, amplitude ratios, and waveforms. This study is an attempt to utilize this additional information to construct a model of core P wave velocity which is consistent with the different types of core phase data available. In conjunction with our waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements, which were the highest‐quality observations chosen from a large population of Global Digital Seismograph Network (GDSN) records. As a result of fitting these various data sets, a one‐dimensional P wave velocity model of the core, PREM2, is proposed. This model, modified from the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981), shows a better fit to the combined data set than any of the existing core models. Major features of the model include a sharp velocity discontinuity at the inner core boundary (ICB), with a large jump (0.78 km/s), and a low velocity gradient at the base of the fluid core. The velocity is nearly constant over the lower 100 km of the outer core. The model features a depth‐dependent Qα structure in the inner core such that a constant t* for the inner core fits the amplitude ratios and waveforms of short‐period waves moderately well. This means the top of the inner core is more attenuating than the deeper part of the inner core. In addition, the P velocity in the lowermost mantle is reduced from that of PREM as a baseline adjustment for the observed separations of the DF and AB branches of PKP at large distances.

Finite-fault analysis of the 1979 March 14 Petatlan, Mexico, earthquake using teleseismic P waveforms

SUMMARY Vertical, teleseismic P waves recorded for the 1979 March 14 Petatlan, Mexico, earthquake were used to derive the distribution of coseismic slip using a linear finite-fault inversion scheme that solves for the amount of slip in each of a series of consecutive time windows. Data recorded by six stations of the Global Digital Seismograph Network were inverted in addition to digitized analogue long-period recordings available from nine Worldwide Standardized Seismograph Network stations. The digital data include four broad-band and short-period velocity waveforms reconstructed from the short- and long-period components. The time-window approach allows for a variable rise time on the fault and accounts for the source multiplicity evident in the recorded P waveforms. Synthetic tests conducted using the inversion method on the limited data set, however, reveal that the data are insufficient to identify the exact dislocation duration on the fault. The method is thus implemented by prescribing the fault rise time using five consecutive 1 s time windows. The coseismic slip inferred from the P waves shows a small 70 cm peak near the earthquake hypocentre and a large zone of dislocation (1.2 m maximum) further south-east. The slip pattern covers depths from 3 to 25 km and is located south-east of other recent large interplate ruptures on the Michoacan segment of the Mexican subduction zone. This result indicates that the 1979 Petatlan earthquake broke an independent, adjacent portion of the Cocos-North America plate boundary. The seismic moment of 1.5 × 1027 dyn cm inferred from the P waves is approximately one-half the long-period moment estimated by other investigators from the observed surface waves. Although the discrepancy is within the uncertainty of the seismic-moment estimates, it may suggest the presence of a component of slow interplate motion that did not radiate significant P-wave energy.

Mapping the lowermost mantle using core‐reflected shear waves

A map of laterally varying D″ velocities is obtained for the region from 50°S to 50°N in latitude and 70°E to 190°E in longitude. Velocities are found using an analysis of the differential travel time residuals from 481 ScS‐S and 266 sScS‐sS phase pairs. The long‐period data are taken from the Global Digital Seismograph Network digital waveform catalog for the time period of January 1980 to March 1987. Each differential travel time is found by a cross correlation of the S phase ground displacement, corrected to simulate differential attenuation, with all following phases. Travel times are corrected for ellipticity and mantle heterogeneity outside of their D″ paths, and the remaining residuals are interpreted as the result of D″ heterogeneity. Ray‐tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies.The resulting map reveals significant long‐wavelength D″ structure including a 3% low‐velocity region beneath northeastern Indonesia, surrounded by three identified high‐velocity zones beneath northwestern Pacifica (+4%), Southeast Asia (+3%), and Australia (+3–5%). This structure is of continent/ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle. The low‐velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature D″ rock which has been heated by the core has become gravitationally unstable and begun to rise. A chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site, much the way continents are dragged toward subduction zones. The high‐velocity zones possibly result from the downward convection of cold lower mantle plumes, which pond at the core‐mantle boundary. These seismic anomalies may also contain a chemical signature from faster iron‐poor materials brought down through the lower mantle or the additional presence of SiO2 stishovite, perhaps in its higher‐pressure polymorph.

Lateral variations in D″ thickness from long‐period shear wave data

We explore global variations in D″ shear wave structure by examining 12 years of long‐period Global Digital Seismograph Network (GDSN) data to identify reflected phases from the D″ layer in the lowermost mantle. We restrict our search to epicentral distances between 63° and 74° where a precritical D″ reflection will lie between the S and ScS arrivals. With GDSN long‐period records a D″ signal at these ranges is generally obscured by the stronger neighboring phases, so to isolate the D″ reflections we develop a technique for stripping away the interfering S and ScS waveforms. We interpret travel time variations in the observed SdS phases in terms of variations in D″ thickness and generate maps of inferred D″ thickness for areas of good seismic coverage. The regions of observed D″ reflections are beneath Australasia, north central Asia, the Arctic, Alaska, and central America. We find considerable variation in D″ structure in these regions, a result consistent with previous studies which have found evidence for lateral heterogeneity in D″. On average, the estimated D″ thickness is 260 km, but there is a fairly uniform distribution of thicknesses between 150 km and 350 km. A possible correlation is observed between D″ thickness and regions of predicted strong horizontal mantle flow.

Anisotropy of Earth's inner core

In an effort to confirm inner core anisotropy, we conducted a systematic search for PKP ray paths with various angles from the Earth's spin axis. In particular, we studied paths nearly parallel to the spin axis (polar paths) and those nearly parallel to the equatorial plane (equatorial paths). Data for earthquakes and explosions were collected from Worldwide Standardized Seismograph Network (WWSSN), Long Range Seismic Measurements (LRSM), and Global Digital Seismograph Network (GDSN). Absolute times (DF, BC) and differential times (BC‐DF, AB‐DF) as well as waveform data were examined. For all polar paths, differential times of BC‐DF consistently yield residuals of 1.5 to 3.5s larger than equatorial paths. Absolute DF time residuals exhibit anomalies of the same magnitude (1 to 4s) with DF being early along polar paths while BC residuals have no obvious correlation with the differential time anomalies. DF phases appear multi‐pathed for polar paths and are relatively simple for equatorial paths. These results coupled with previous studies suggest an axisymmetric anisotropy at the top of the inner core.

Global mapping of upper mantle reflectors from long-period SS precursors

SUMMARY Long-period precursors to SS resulting from underside reflections off upper mantle discontinuities (SdS where d is the discontinuity depth) can be used to map the global distribution and depth of these reflectors. We analyse 5,884 long-period seismograms from the Global Digital Seismograph Network (1976-1987, shallow sources, transverse component) in order to identify SdS arrivals. Corrections for velocity dispersion, topography and crustal thickness at the SS bounce point, and lateral variation in mantle velocity are critical for obtaining accurate estimates of discontinuity depths. The 410 and 660 km discontinuities are observed at average depths of 413 and 653 km, and exhibit large-scale coherent patterns of topography with depth variations up to 40 km. These patterns are roughly correlated with recent tomographic models, with fast anomalies in the transition zone associated with highs in the 410 km discontinuity and lows in the 660 km discontinuity, a result consistent with laboratory measurements of Clapeyron slopes for the appropriate phase changes. The best resolved feature in these maps is a trough in the 660km discontinuity in the northwest Pacific, which appears to be associated with the subduction zones in this region. Amplitude variations in SdS arrivals are not correlated with discontinuity depths and probably result from focusing and defocusing effects along the ray paths. The SdS arrivals suggest the presence of regional reflectors in the upper mantle above 400 km. However, only the strongest of these features are above probable noise levels due to sampling inadequacies.

Inner Core Attenuation From Short-PeriodPkp(Bc)VersusPkp(Df)Waveforms

SUMMARY Differential waveform analysis provides an excellent tool for studying the attenuation properties of the top of the inner core. We analyse 108 PKP(BC) versus PKP(DF) waveforms from Global Digital Seismograph Network (GDSN) verticalcomponent seismograms to constrain the frequency and depth dependency of Qa in this region. We use both frequency- and time-domain techniques. In the timedomain method, the BC phase is mapped onto the DF phase using an attenuation band operator. The mapping operator is parameterized by the upper and lower cut-off frequencies of the absorption band, the time shift required to align these two phases, and t*, the integrated effect of Q;’ in the top of the inner core. In the frequency-domain analysis, multitaper spectral estimation is used to compute the complex spectrum of the two phases. The shape of the amplitude spectrum of the spectral ratio between these two phases gives an estimate of en. Similar results are obtained from frequency- and time-domain analysis but t he Qa obtained from frequency-domain analysis is approximately 20 per cent greater than the value obtained from time-domain analysis. We prefer the frequency-domain results since they are not affected by the presence of noise at higher frequencies. Apparent Qa values exhibit considerable scatter with no clear frequency or depth dependence. We find that the average value of Qa in the top of the inner core is about 360 which is consistent with previous body wave studies but differs by a factor of two from values obtained from studies of the decay of free oscillations.