The P-wavespeed structure in the lowermost 700km of the mantle below the central part of the Indian Ocean (original) (raw)
Related papers
Evidence for a shear velocity discontinuity in the lower mantle beneath India and the Indian Ocean
Physics of the Earth and Planetary Interiors, 1987
SH and sSH seismograms are modeled to determine the shear velocity structure in the D" region beneath India and the Indian Ocean. The signals show waveform complexities similar to those observed in data sampling the D" region beneath Alaska, the Caribbean, and Eurasia (Lay and Helmberger), which have been attributed to a 2.7% shear velocity discontinuity-280 km above the core-mantle boundary. The new data set consists of long-period tangential component recordings at WWSSN stations in Africa, the Middle East, and Europe for 11 intermediate and deep focus Indonesian earthquakes. In the distance range 70-82°the waveforms show an arrival between SH and ScSH with systematic moveout. From 89 to 940 there is a strong distortion of the SH waveforms, indicating the arrival of several phases closely spaced in time. The relative time shifts of similar complexity in the corresponding sSH phases requires a deep mantle origin. The depth dependence and moveout of the interference effects are well-predicted for both SH and sSH phases by a model with a lower mantle discontinuity. Alternative explanations of the interference as resulting from receiver reverberations, SKS contamination, multiple source complexity, or near source multipathing are ruled Out by systematic tests. While it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data, synthetic waveform modeling (using generalized ray theory and reflectivity) shows that the data can be well-fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions (Lay and Helmberger), but with a negative velocity gradient within the D" layer.
Depth variation of the mid-mantle seismic discontinuity
Geophysical Research Letters, 1997
Short-period array seismograms of deep events that occurred in the Indonesia, Japan and Izu-Bonin arcs are stacked and beam-formed to identify the near-source S-P converted waves that result from the mantle transition discontinuities. Most of the resulting images reveal the existence of a mid-mantle seismic discontinuity (“920 km discontinuity”) in these regions. Of the 15 events analyzed, three that occurred at the western end of the Indonesia arc show clear S-P arrivals observable even in individual seismograms. The mid-mantle discontinuity is characterized by large depth variation (900 ∼ 1080 km) and velocity contrast variation in different subduction zones. Especially, the depth variation of the mid-mantle discontinuity beneath the Indonesia arc, where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end, appears to be well correlated with the location of the high-velocity anomalies in recent tomographic models. However, the mid-mantle discontinuity cannot be simply coincided with the bottom of the high-velocity anomalies, because a velocity increase at the discontinuity is observed from the waveform analysis.
Geophysical Journal International, 2012
Current interpretations of seismic observations typically argue for significant chemical heterogeneity being present in the two large low shear velocity provinces under Africa and the Pacific. Recently, however, it has been suggested that large lateral temperature variations in the lowermost mantle resulting from a strong thermal gradient across D may provide an alternative explanation. In case of a high heat flux from the core into the mantle, the magnitude of shear wave velocity variations in tomographic models can be reconciled with isochemical whole mantle flow and a pyrolite composition. So far, the hypothesis of strong core heating has been tested in a consistent manner only against tomographic S-wave velocity models, but not against P-wave velocity models. Here, we explore a new approach to assess geodynamic models and test the assumption of isochemical whole mantle flow with strong core heating directly against the statistics of observed traveltime variations of both P and S waves. Using a spectral element method, we simulate 3-D global wave propagation for periods down to 10 s in synthetic 3-D elastic structures derived from a geodynamic model. Seismic heterogeneity is predicted by converting the temperature field of a high-resolution mantle circulation model (MCM) into seismic velocities using thermodynamic models of mantle mineralogy. Being based on forward modelling only, this approach avoids the problems of limited resolution and non-uniqueness inherent in tomographic inversions while taking all possible finite-frequency effects into account. Capturing the correct physics of wave propagation allows for a consistent test of the assumption of high core heat flow against seismic data.
Geophysical Journal of the Royal …, 1981
An 80-km seismic refraction line was recorded on an array of three ocean bottom seismometers located north-west of the northern tip of Explorer ridge and parallel to Revere-Dellwood fracture zone in the northeast Pacific Ocean. The explosion data were supplemented at shorter distances by seismograms obtained using a 16-litre airgun as source. The combined use of rotated SV component and polarization filtered record sections enabled identification and timing of the refracted S-wave on most sections. The travel time-distance relations for both Pand S-waves are interpreted in the intercept time (tau) and ray parameter domain using tau inversion to give extremal bounds for velocity-depth curves. A linearized inversion technique is also applied to give the smoothest velocity-depth profiles consistent with the travel time data. Amplitude analysis using disc ray theory synthetic seismograms further refine the P-wave velocity-depth models.
Variations ofPwave speeds in the mantle transition zone beneath the northern Philippine Sea
Journal of Geophysical Research, 1997
Using waveforms and travel times from deep earthquakes, we constructed 16 seismic profiles, each of which constrains the radial variation in Vp over a small area beneath the northern Philippine Sea. Taken together, the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high Vp (up to 3% faster than average Earth models) originally suggested by travel time tomography. Unlike travel time tomography, which relies heavily on arrival times of the direct P phase, we utilize the waveforms and move-out of later arrivals that mainly sample the mantle transition zone of interest. Our results identify three important characteristics of the northern Philippine Sea anomaly that are distinct from previous results. First, being approximately a subhorizontal, laterally uniform feature, the anomaly is localized beneath the northwestern comer of the Philippine Sea, within a region of approximately 500 x 500 km 2 immediately east of the Ryukyu arc. Second, the anomaly is well constrained to occur in the lower portion of the transition zone, extending all the way down to the 660-km discontinuity. Third, the presence of such a distinct anomaly reduces the contrast in V, across the 660-km discontinuity from approximately 6% to 3%. Such a configuration •s consistent wath the interpretanon that the anomaly is caused by a remnant of subducted slab, as negative buoyancy should rest the slab just above the 660-km discontinuity where resistance to subduction is expected from a negative Clapeyron slope during the spinel-Mg-Fe-perovskite transition. between the upper and the lower mantle [e.g., Anderson, 1989; Lay, 1994; Silver et at., 1988]. Several recent studies showed evidence that the extent of slab penetration into the lower mantle seems to vary laterally along subduction zones such that at least some subducted slabs remain stagnant in the upper mantle [e.g., Glennon and Chen, 1995a; Ding and Grand, 1994], while others plunge steeply into the lower mantle [e.g., Jordan, 1977; Creager and Jordan, 1984, 1986]. For instance, on the basis of long-period (-20 s) converted and reflected body waves, Shearer and Masters [ 1992] and Shearer [ 1991 ] contended that the topography of the 660-km discontinuity shows
Mapping the lowermost mantle using core-reflected shear waves
Journal of Geophysical Research, 1994
A map of laterally varying D-double prime velocities is obtained for the region from 50 deg S to 50 deg N in latitude and 70 deg E to 190 deg 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-double prime paths, and the remaining residuals are interpreted as the result of D-double prime 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-double prime 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-double prime 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.
Analysis of earthquakes in the distance range 40–70° and inferred lower mantle structure
Physics of the Earth and Planetary Interiors, 1982
. Analysis of earthquakes in the distance range 40-70°and inferred lower mantle structure. Phys. Earth Planet. Inter., 28: 242-250. About fifty earthquakes in the distance range 40-70°and azimuthal range 45-120°from the Celebes, Philippines, Mariana and Kurile Island regions, and recorded at Gauribidanur seismic array in southern India, were used in the present study. Measurements on slowness and apparent azimuths were made on the first 30 s of the short period P-wave trains using an adaptive processing technique. Analysis of this data set has revealed no strong evidence for any triplications in the travel-time curve over the ranges in question. The P-wave velocity increases continuously with an almost uniform gradient below 1000 km depth range and is in very close agreement with the JB model. Almost all the observed slowness values of the events were anomalously low and consistent suggesting that they are caused by some azimuthal dependent structure near the array.
Geophysical Research Letters, 1983
Observations of high-frequency (HF) Pn phases by an oceanbottom seismometer (OBS) array were made at distances between 6 ø and 18 ø in the northwestern Pacific. At this distance range, mantle-refracted P phases arrive earlier than HF-Pn phases. These two phases are successfully separated for near surface earthquakes by applying appropriate filtering, since the dominant frequencies of HF-Pn phases are higher than 6 Hz and the frequencies of mantle-refracted P phases are rather low, about 3 Hz. The apparent velocities of HF-Pn phases in this distance range are estimated to be from 8.3 to 8.1 km/sec or even lower, while those of mantle-refracted P waves are from 8.4 to 8.6 km/sec. The reported value of 8.33 km/sec for Hf-Pn phases appears to be an average of both HF-Pn and mantle-refracted P phases. Therefore, these observations suggest that a proposed additional HF-Pn wave guide for the higher velocity of 8.33 km/sec is questionable. western Pacific at distances between 9 ø and 40 ø Bull Seismol Soc Amer , 61, 65-78, , ß ß ß ß __ 1971o Walker, DoA., High-frequency Pn and S n phases recorded in the Western Pacific, J. Geo•hys.
Search for seismic discontinuities in the lower mantle
Geophysical Journal International, 2001
Indications of lower mantle discontinuities have been debated for decades, but still little is known about their properties, and their origins are enigmatic. In our study broad-band recordings of deep events are examined for the presence of signals from the lower-mantle discontinuities with a novel technique. We deconvolve vertical component of the P-wave coda in the period range around 10 s by the S waveform and stack many deconvolved traces with moveout time corrections. In synthetic seismograms for an earth model without lower mantle discontinuities, the strongest signal thus detected in the time window of interest is often s'410'P phase (generated as S and reflected as P from the '410 km' discontinuity above the source). In actual seismograms there are other phases that can be interpreted as converted from S to P at discontinuities in the lower mantle beneath the seismic source. We summarize the results of processing the seismograms (1) of deep events in Sunda arc at seismograph stations in east Asia, (2) deep Kermadec-Fiji-Tonga events at the J-array and FREESIA networks in Japan and stations in east Asia, and (3) deep events in the northwest Pacific region (Mariana, Izu-Bonin and the Japan arc) recorded at stations in north America. In our data there are indications of discontinuities near 860-880, 1010-1120, 1170-1250 and 1670-1800 km depths. The clearest signals are obtained from the discontinuity at a depth of 1200 km. We argue that the '900', '1200' and '1700 km' discontinuities are global, but laterally variable in both depth and strength. Seismic stratification of the lower mantle may have bearings on the patterns of subduction, as revealed by tomographic models.