Differential Time Residuals Research Articles

The inner core hemispheric boundary near 180 °W

The inner core (IC) east–west hemispheric dichotomy is widely recognized, but the reported position of the hemispheric boundary varies among studies due to uneven sampling coverage and the data analyzed. This study investigates the sharpness of the western hemispheric boundary (WHB) near 180°W by analyzing differential time residuals of PKiKP–PKPdf and PKP(bc–df) for PKPdf phases that sample 155°E–130°W in various azimuthal directions. Using PKiKP–PKPdf observations, the WHB is located at 175°E–180°W in the southern hemisphere, based mainly on the lateral isotropy–anisotropy transition. However, based on the lateral isotropic velocity contrast and this isotropy–anisotropy transition between the two hemispheres, its location is 170–160°W in the northern hemisphere. These findings indicate that the WHB is sharp and exhibits a latitudinal dependence with a 10°–20° kink, as well as 1.75% anisotropy in the uppermost IC across the 180–155°W range of the western hemisphere. As suggested by PKP(bc–df), the WHB could remain at 160°W at depth. The isotropic velocity contrast near the WHB (160°W) between the eastern and western hemispheres is lower than previous estimates using PKPdf phases sampling the bulk part of each hemisphere.

Fine Structure of P‐Wave Velocity Variations Underneath the Central Pacific from PKP Waves Recorded at the China Seismic Network (CSN)

AbstractSeismic PKP arrival times are sensitive to P velocity structure in the lowermost mantle. Earthquakes in South America recorded at China Seismographic Network (CSN) provide excellent samples of the lowermost mantle underneath the Central Pacific. Here we use differential travel times between the AB and DF branches of PKP waves to constrain the small‐scale variation of P velocity beneath the Central Pacific. The differential AB‐DF times reduce the biases/errors of earthquake locations, origin times and heterogeneity of the crust and upper mantle but are sensitive to the structure in the lowermost mantle. Our results show large positive AB‐DF differential time residuals across large areas of the lowermost mantle beneath the Central Pacific, which may indicate the source region of the “Pacific Super Plume”. The observed PKP residuals correlate with predictions from Grand's tomographic S model. Our inferred P velocity variation (which is about 2% in the bottommost 200 km D” layer) is about 36% of the S velocity variation. However, we observe significant variations in travel‐time residuals both inside the Central Pacific Plume and near its boundaries. The results have implications for further understanding the structure and nature of the Pacific Super Plume.

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.

A study of the uppermost inner core fromPKKPandP′P′ differential traveltimes

We have assembled a data set of PKKPdf−PKKPbc (PKKPdf−bc) and PKPPKPdf-PKKPbc differential traveltimes. This data set is used to study the Earth's uppermost inner core in the western hemisphere. Data from the eastern Pacific are recorded at the Canadian Yellowknife array and travel through the inner core twice, once perpendicular to the Earth's rotation axis and once parallel to the rotation axis. Our polar paths sample a similar region to paths from the South Sandwich Islands to Alaska, an important path geometry for inner core anisotropy studies. Our data set has the potential to be an independent source of information on the structure of the inner core, and to be a good supplement to past PKP studies. Differential time residuals (relative to isotropic inner core models) of up to several seconds are found. The differential traveltimes show significant scatter (up to 6 s) making it difficult to attribute them to inner core anisotropy alone. Alternatively, strong heterogeneity in the inner core and along the mantle paths may contribute to observations.

Evidence for a sharp lateral variation of velocity at the core–mantle boundary from multipathed PKPab

Rapid changes in differential travel times between the various PKP branches have been attributed to strong lateral velocity variations at the base of the mantle. Differential time PKPab–df residuals showing jumps of up to 2 s for paths only 100 km apart have been reported. Such extreme changes in travel times for signals with comparable wavelengths suggest ray bifurcation with multipaths containing slow and fast contributions to PKPab. Here we report on some well-documented normal and multipathed PKPab phases along with rapidly varying differential time residuals observed for Fiji events with paths sampling the northern edge of the mid-Pacific large slow structure beneath Hawaii. Systematic analyses of these data with two-dimensional models suggest the presence of a ridge-shaped structure at the core–mantle boundary containing abrupt reductions of P-wave velocity of up to 10%. This narrow ultralow velocity zone (ULVZ) is located at the boundary between a large scale low velocity and high velocity structure, corroborating recent dynamic models which propose that ULVZs preferentially occur at the boundary between hot and cold mantle domains and could be related to plume activity at Hawaii.

Scale lengths of shear velocity heterogeneity at the base of the mantle from S wave differential travel times

Resolving the seismic velocity heterogeneity in the lowermost mantle is essential for understanding the chemical and dynamical processes operating in the boundary layer between the core and the mantle. Several regions of the lowermost mantle appear to have abrupt increases in shear velocity several hundred kilometers above the core‐mantle boundary, at the top of the D” layer. This structure is revealed by seismic wave energy refracted by the velocity increase, resulting in an extra arrival (Scd) in teleseismic S wavetrains at distances from 65° to 95°. Anomalies in differential travel times of the extra arrival relative to direct S (which turns at shallower depths in the midmantle) or ScS (which reflects from the core‐mantle boundary below the velocity increase) provide probes of the velocity heterogeneity in the lowermost mantle with better vertical resolution than provided by conventional ScS‐S measurements. We explore the spatial patterns in Scd‐S, ScS‐Scd, and ScS‐S differential time residuals for paths through the lower mantle beneath Alaska, Eurasia, and India (all being regions with coherent Scd phases) to place constraints on the strength and scale lengths of shear velocity heterogeneity and/or variable topography of the lower mantle discontinuity in these regions. The observed patterns are poorly predicted by existing global tomographic models. Significant small‐scale heterogeneity, with lateral length scales of 200–500 km or less, exists even within regions that display a relatively uniform D” discontinuity structure over scale lengths of 1500–2000 km. The strongest travel time variations are associated with structure above the D” region, in contrast to common assumptions. Rapid lateral fluctuations in ScS‐Scd differential times suggest that the anomalies accumulate within a relatively thin zone, less than the overall thickness of D”. Correlations among the differential time anomalies indicate that Scd variations, ranging over about 4 s, are more pronounced than ScS variations and that the travel time fluctuations accumulate near the top of D” due to shear velocity heterogeneity of ±4% within a 50 km thick region and/or discontinuity topography of ±50 km, with scale lengths of 200–500 km. It appears that the upper portion of D” is much more heterogeneous than the deeper region. Heterogeneity within the layer appears to be decoupled from that at the top of the layer or in the overlying mantle.

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.