Travel Time Residuals Research Articles

Recording earthquakes for tomographic imaging of the mantle beneath the South Pacific by autonomous MERMAID floats

SUMMARYWe present the first 16 months of data returned from a mobile array of 16 freely floating diving instruments, named mermaid for Mobile Earthquake Recording in Marine Areas by Independent Divers, launched in French Polynesia in late 2018. Our 16 are a subset of the 50 mermaid deployed over a number of cruises in this vast and understudied oceanic province as part of the collaborative South Pacific Plume Imaging and Modeling (SPPIM) project, under the aegis of the international EarthScope-Oceans consortium. Our objective is the hydroacoustic recording, from within the oceanic water column, of the seismic wavefield generated by earthquakes worldwide, and the nearly real-time transmission by satellite of these data, collected above and in the periphery of the South Pacific Superswell. This region, characterized by anomalously elevated oceanic crust and myriad seamounts, is believed to be the surface expression of deeply rooted mantle upwellings. Tomographically imaging Earth’s mantle under the South Pacific with data from these novel instruments requires a careful examination of the earthquake-to-mermaid traveltimes of the high-frequency P-wave detections within the windows selected for reporting by the discrimination algorithms on board. We discuss a workflow suitable for a fast-growing mobile sensor database to pick the relevant arrivals, match them to known earthquakes in global earthquake catalogues, calculate their traveltime residuals with respect to global seismic reference models, characterize their quality and estimate their uncertainty. We detail seismicity rates as recorded by mermaid over 16 months, quantify the completeness of our catalogue and discuss magnitude–distance relations of detectability for our network. The projected lifespan of an individual mermaid is 5 yr, allowing us to estimate the final size of the data set that will be available for future study. To prove their utility for seismic tomography we compare mermaid data quality against ‘traditional’ land seismometers and their low-cost Raspberry Shake counterparts, using waveforms recovered from instrumented island stations in the geographic neighbourhood of our floats. Finally, we provide the first analyses of traveltime anomalies for the new ray paths sampling the mantle under the South Pacific.

Insights from the P Wave Travel Time Tomography in the Upper Mantle Beneath the Central Philippines

In this paper, we present a high resolution 3-D tomographic model of the upper mantle obtained from a large number of teleseismic travel time data from the ISC in the central Philippines. There are 2921 teleseismic events and 32,224 useful relative travel time residuals picked to compute the velocity structure in the upper mantle, which was recorded by 87 receivers and satisfied the requirements of teleseismic tomography. Crustal correction was conducted to these data before inversion. The fast-marching method (FMM) and a subspace method were adopted in the forward step and inversion step, respectively. The present tomographic model clearly images steeply subducting high velocity anomalies along the Manila trench in the South China Sea (SCS), which reveals a gradual changing of the subduction angle and a gradual shallowing of the subduction depth from the north to the south. It is speculated that the change in its subduction depth and angle indicates the cessation of the SCS spreading from the north to the south, which also implies that the northern part of the SCS opened earlier than the southern part. Subduction of the Philippine Sea (PS) plate is exhibited between 14° N and 9° N, with its subduction direction changing from westward to eastward near 13° N. In the range of 11° N–9° N, the subduction of the Sulu Sea (SS) lies on the west side of PS plate. It is notable that obvious high velocity anomalies are imaged in the mantle transition zone (MTZ) between 14° N and 9° N, which are identified as the proto-SCS (PSCS) slabs and paleo-Pacific (PP) plate. It extends the location of the paleo-suture of PSCS-PP eastward from Borneo to the Philippines, which should be considered in studying the mechanism of the SCS and the tectonic evolution in SE Asia.

Crustal P-wave velocity structure and earthquake distribution in the Jiaodong Peninsula, China

The Jiaodong Peninsula of eastern China is densely populated and prone to frequent moderate-strong earthquakes that are mostly distributed along a group of northeastern oriented Mesozoic-Cenozoic faults. An improved understanding of the seismic velocity structure in the area is of great significance for the roles that the faults played in the formation and evolution of the various tectonic features and for the assessment of future earthquake risks. By utilizing earthquake records of the Shandong Provincial Seismic Network for the period from January 2013 to January 2020, this study simultaneously re-locates earthquakes and determines the three-dimensional P-wave velocity structure beneath the Jiaodong Peninsula using regional P-wave travel times by applying the double-difference tomography method. After the relocation, the travel time residuals are reduced by an order of magnitude, and the epicenters are more concentrated and demonstrate a closer spatial relationship with known active faults. The resulting P-wave velocity structure at different horizon depths indicates lateral heterogeneities in the study area. Clear differences in the characteristics of velocity anomalies are observed between the Jiaobei Uplift, Jiaolai Basin and Sulu ultra-high pressure metamorphic belt. The anomalies are mostly NE oriented, which is consistent with the strike of the regional faults and may suggest structural control of the faults to the geological configurations. Earthquakes generally occurred along the edges of high or low velocity regions, and areas accommodating the intersections of the Penglai-Weihai, Penglai-Qixia and Mouping-Jimo fault zones have the greatest potential for future damaging earthquakes in the area.

Earthquake location in tectonic structures of the Alpine Chain: the case of the Constance Lake (Central Europe) seismic sequence

A set of four magnitude Ml ≥ 3.0 earthquakes including the magnitude Ml = 3.7 mainshock of the seismic sequence hitting the Lake Constance, Southern Germany, area in July–August 2019 was studied by means of bulletin and waveform data collected from 86 seismic stations of the Central Europe-Alpine region. The first single-event locations obtained using a uniform 1-D velocity model, and both fixed and free depths, showed residuals of the order of up ± 2.0 s, systematically affecting stations located in different areas of the study region. Namely, German stations to the northeast of the epicenters and French stations to the west exhibit negative residuals, while Italian stations located to the southeast are characterized by similarly large positive residuals. As a consequence, the epicentral coordinates were affected by a significant bias of the order of 4–5 km to the NNE. The locations were repeated applying a method that uses different velocity models for three groups of stations situated in different geological environments, obtaining more accurate locations. Moreover, the application of two methods of relative locations and joint hypocentral determination, without improving the absolute location of the master event, has shown that the sources of the four considered events are separated by distances of the order of one km both in horizontal coordinates and in depths. A particular attention has been paid to the geographical positions of the seismic stations used in the locations and their relationship with the known crustal features, such as the Moho depth and velocity anomalies in the studied region. Significant correlations between the observed travel time residuals and the crustal structure were obtained.

P Wave Tomography Beneath Greenland and Surrounding Regions: 1. Crust and Upper Mantle

AbstractWe study the 3‐D P wave velocity (Vp) structure of the crust and upper mantle beneath Greenland and surrounding regions using the latest P wave arrival time data. The Greenland Ice Sheet Monitoring Network (GLISN), initiated in 2009, is an international project for seismic observation in these regions using 34 stations. We use a regional seismic tomography method to simultaneously invert P wave arrival times of local earthquakes and P wave relative traveltime residuals of teleseismic events. These data are extracted from the ISC‐EHB catalog; however, for the teleseismic data, we picked new arrival times from seismograms using a cross‐correlation method. Our tomographic results clearly reveal the Iceland plume, the Jan Mayen plume, and a newly discovered “Svalbard plume,” which merge together in the mantle transition zone. A high‐Vp body exists beneath the Greenland Sea, which might act as an obstacle against the rising Svalbard plume. Furthermore, our results reveal a remarkable low‐Vp anomaly elongated in the NW‐SE direction at depths ≤250 km beneath central Greenland, which is connected with the Iceland and Jan Mayen hotspots. Although previous studies have suggested a similar feature, our result is the first to show the low‐Vp zone existing at all depths in the Greenland lithosphere, and its spatial distribution coincides with a high heat flux region. These characteristics indicate that the low‐Vp zone reflects residual heat from the Iceland plume when the Greenland lithosphere passed over this plume at ~80–20 Ma. Our results also indicate the possible existence of residual heat from the Jan Mayen plume.

Mantle dynamics of the Andean Subduction Zone from continent-scale teleseismic S-wave tomography

SUMMARY The Andean Subduction Zone is one of the longest continuous subduction zones on Earth. The relative simplicity of the two-plate system has makes it an ideal natural laboratory to study the dynamics in subduction zones. We measure teleseismic S and SKS traveltime residuals at >1000 seismic stations that have been deployed across South America over the last 30 yr to produce a finite-frequency teleseismic S-wave tomography model of the mantle beneath the Andean Subduction Zone related to the Nazca Plate, spanning from ∼5°N to 45°S and from depths of ∼130 to 1200 km. Within our model, the subducted Nazca slab is imaged as a fast velocity seismic anomaly. The geometry and amplitude of the Nazca slab anomaly varies along the margin while the slab anomaly continues into the lower mantle along the entirety of the subduction margin. Beneath northern Brazil, the Nazca slab appears to stagnate at ∼1000 km depth and extend eastward subhorizontally for >2000 km. South of 25°S the slab anomaly in the lower mantle extends offshore of eastern Argentina, hence we do not image if a similar stagnation occurs. We image several distinct features surrounding the slab including two vertically oriented slow seismic velocity anomalies: one beneath the Peruvian flat slab and the other beneath the Paraná Basin of Brazil. The presence of the latter anomaly directly adjacent to the stagnant Nazca slab suggests that the plume, known as the Paraná Plume, may be a focused upwelling formed in response to slab stagnation in the lower mantle. Additionally, we image a high amplitude fast seismic velocity anomaly beneath the Chile trench at the latitude of the Sierras Pampeanas which extends from ∼400 to ∼1000 km depth. This anomaly may be the remnants of an older, detached slab, however its relationship with the Nazca–South America subduction zone remains enigmatic.

Combining asynchronous data sets in regional body-wave tomography

SUMMARY Regional body-wave tomography is a very popular tomographic method consisting in inverting relative traveltime residuals of teleseismic body waves measured at regional networks. It is well known that the resulting inverse seismic model is relative to an unknown vertically varying reference model. If jointly inverting data obtained with networks in the vicinity of each other but operating at different times, the relative velocity anomalies in different areas of the model may have different reference levels, possibly introducing large-scale biases in the model that may compromise the interpretation. This is very unfortunate as we have numerous examples of asynchronous network deployments which would benefit from a joint analysis. We show here how a simple improvement in the formulation of the sensitivity kernels allows us to mitigate this problem. Using sensitivity kernels that take into account that data processing implies a zero mean residual for each event, the large-scale biases that otherwise arise in the inverse model using data from asynchronous station deployment are largely removed. We illustrate this first with a very simple 3-station example, and then compare the results obtained using the usual and the relative kernels in synthetic tests with more realistic station coverage, simulating data acquisition at two neighbouring asynchronous networks.

Imaging of the Upper Mantle Beneath Southeast Asia: Constrained by Teleseismic P-Wave Tomography

It is of great significance to construct a three-dimensional underground velocity model for the study of geodynamics and tectonic evolution. Southeast Asia has attracted much attention due to its complex structural features. In this paper, we collected relative travel time residuals data for 394 stations distributed in Southeast Asia from 2006 to 2019, and 14,011 seismic events were obtained. Then, teleseismic tomography was applied by using relative travel time residuals data to invert the velocity where the fast marching method (FMM) and subspace method were used for every iteration. A novel 3D P-wave velocity model beneath Southeast Asia down to 720 km was obtained using this approach. The tomographic results suggest that the southeastern Tibetan Plateau, the Philippines, Sumatra, and Java, and the deep part of Borneo exhibit high velocity anomalies, while low velocity anomalies were found in the deep part of the South China Sea (SCS) basin and in the shallow part of Borneo and areas near the subduction zone. High velocity anomalies can be correlated to subduction plates and stable land masses, while low velocity anomalies can be correlated to island arcs and upwelling of mantle material caused by subduction plates. We found a southward subducting high velocity body in the Nansha Trough, which was presumed to be a remnant of the subduction of the Dangerous Grounds into Borneo. It is further inferred that the Nansha Trough and the Dangerous Grounds belong to the same tectonic unit. According to the tomographic images, a high velocity body is located in the deep underground of Indochina–Natuna Island–Borneo–Palawan, depth range from 240 km to 660 km. The location of the high velocity body is consistent with the distribution range of the ophiolite belt, so we speculate that the high velocity body is the remnant of thee Proto-South China Sea (PSCS) and Paleo-Tethys. This paper conjectures that the PSCS was the southern branch of Paleo-Tethys and the gateway between Paleo-Tethys and the Paleo-Pacific Ocean. Due to the squeeze of the Australian plate, PSCS closed from west to east in a scissor style, and was eventually extinct under Borneo.

Is High Topography Around the North Atlantic Supported From the Upper Mantle?

AbstractThe traveltime residuals of 393 receiver sites with more than 21,000 P wave and 9,700 S wave arrivals are used to investigate the upper‐mantle P and S wave velocity structure down to the depth of 500 km beneath the regions of high topography of the North Atlantic Caledonian crust. In east Greenland, we observe high‐velocity anomalies beneath high topography. In northern Norway and Sweden, a stable thick lithosphere beneath both the high topography and the Baltic shield exists. Only southern Norway exhibits a weak correlation between high topography and low‐velocity anomalies of P and S waves. Consequently, the correlation of topography versus average relative traveltime residuals in four regions of proposed Cenozoic epeirogenic uplift reveals positive correlation only in southern Norway and negative or absence of correlation in other regions. These results show that high topography does not correlate with the inferred velocity structure of the upper mantle in the North Atlantic realm. Consequently, our findings weaken the hypothesis of dynamic topography and epeirogenic uplift of the Atlantic margins during the Late Cenozoic.

Lateral Velocity Gradients in the African Lower Mantle Inferred From Slowness Space Observations of Multipathing

AbstractLarge low‐velocity provinces (LLVPs) are hypothesized to be purely thermal features or possess some chemical heterogeneity but which exactly remains ambiguous. Regional seismology studies typically use travel time residuals and multipathing identification in the waveforms to infer properties of LLVPs. These studies have not fully analyzed all available information such as measuring the direction and inclination of the arrivals. These measurements would provide more constraints of LLVP properties such as the boundary velocity gradient and help determine their nature. Here, we use array seismology to measure backazimuth (direction) and horizontal slowness (inclination) of arriving waves to identify structures causing multipathing and wavefield perturbation. Following this, we use full‐wavefield forward modeling to estimate the gradients required to produce the observed multipathing. We use SKS and SKKS data from 83 events sampling the African LLVP, which has been extensively studied providing a good comparison to our observations. We find evidence for structures at heights of up to 600 km above the core‐mantle boundary causing multipathing and wavefield perturbation. Forward modeling shows gradients of up to 0.7% δVs per 100 km (0.0005 km s−1 km−1) can produce multipathing with similar backazimuth and horizontal slowness to our observations. This is an order of magnitude lower than the previous strongest estimates of −3% δVs per 50 km (0.0044 km s−1 km−1). As this is lower than that predicted for both thermal and thermochemical structures, lateral velocity gradients capable of producing multipathing are not necessarily evidence for a thermochemical nature.

Teleseismic P-wave crustal tomography of the Weifang segment on the Tanlu fault zone: A case study based on short-period dense seismic array experiment

The 1896 high-quality relative travel-time residuals are collected by applying the Multi-Channel Cross-Correlation technique to the waveform data of teleseismic events recorded by a short-period dense seismic array consisting of 302 stations in the Weifang segment of the Tanlu fault zone. Using the Fast Marching Teleseismic Tomography technique, these residuals are inverted for a fine P-wave crustal velocity model under the study region down to 30 km depth with the resolution of ~20 km in horizontal directions and ~30 km in depth. We image obvious high-velocity anomalies to the west of the fault zone from the surface down to a depth of 30 km, and striking low-velocity anomalies appear within the fault zone around 36.5°N where the Changle volcano exists on the surface, whilst to the east velocity anomalies exhibit a complicated pattern. Meanwhile, our tomographic images roughly show a boundary between the fault zone and its surrounding areas. We infer that the fault zone provides a plausible channel for continuous upwelling of hot and wet materials from the mantle to the surface when it intruded into the crust. The process of upwelling of hot and wet materials from the mantle could be associated with mantle dynamics of the big mantle wedge due to the stagnation and dehydration of the subducted Pacific slab in the mantle transition zone under the region. Our present study demonstrates that it is possible that a fine crustal velocity structure can be obtained by teleseismic tomography with a short-period dense seismic array.

Detailed Structure of the Subducted Nazca Slab into the Lower Mantle Derived From Continent‐Scale Teleseismic P Wave Tomography

AbstractNazca subduction beneath South America is one of our best modern examples of long‐lived ocean‐continent subduction on the planet, serving as a foundation for our understanding of subduction processes. Within that framework, persistent heterogeneities at a range of scales in both the South America and Nazca plates is difficult to reconcile without detailed knowledge of the subducted Nazca slab structure. Here we use teleseismic travel time residuals from >1,000 broadband and short‐period seismic stations across South America in a single tomographic inversion to produce the highest‐resolution contiguous P wave tomography model of the subducting slab and surrounding mantle beneath South America to date. Our model reveals a continuous trench‐parallel fast seismic velocity anomaly across the majority of South America that is consistent with the subducting Nazca slab. The imaged anomaly indicates a number of robust features of the subducted slab, including variable slab dip, extensive lower mantle penetration, slab stagnation in the lower mantle, and variable slab amplitude, that are incorporated into a new, comprehensive model of the geometry of the Nazca slab surface to ~1,100 km depth. Lower mantle slab penetration along the entire margin suggests that lower mantle slab anchoring is insufficient to explain along strike upper plate variability while slab stagnation in the lower mantle indicates that the 1,000 km discontinuity is dominant beneath South America.

Lateral Variations of Shear‐Wave Velocity in the D″ Layer Beneath the Indian‐Eurasian Plate Collision Zone

AbstractSeismic tomography has demonstrated that the shear‐wave velocity is relatively high over a 3,000‐km wide region in the lowermost mantle beneath southern and eastern Asia. This seismic anomaly demarcates the current position of slab remnants that may have subducted in the Cretaceous. To further characterize the seismic structure at smaller scales, we measure 929 residual travel time differences (δt) between the phases ScS and S using recordings of eight earthquakes beneath the Indian Ocean at stations from the Chinese Digital Seismic Network. We interpret variations of δt up to 10 s as due to horizontal shear‐velocity variations in D″ beneath northern India, Nepal, and southwestern China. The shear velocity can vary by as much as 7% over distances shorter than 300 km. Our observations provide additional observational evidence that compositional heterogeneity and possibly melt contribute to the seismic structure of the lower mantle characterized by long‐term subduction and mantle downwelling.

Reassessment of source parameters of ‘major’ southern African earthquakes

AbstractModerate to large earthquakes within an earthquake catalogue contribute significantly to the seismic hazard and risk assessment results of any region. Thus it is prudent to ensure these events have reliable source parameters (epicentres and magnitude). The dataset of events compiled in this study contains a total of 117 instrumentally recorded events of magnitude M ≥5.0, whose parameters were obtained from the Council for Geoscience (CGS) and International Seismological Centre (ISC) databases. The events are mostly located in South Africa with a few in neighbouring countries. Parametric data made up of all available phase data and amplitudes associated with each of the earthquakes were compiled. The availability of these data enabled the earthquake epicentres and magnitude values to be recalculated using the velocity model and the local magnitude relation that are currently being used by the CGS in its analysis of national seismic data. The accuracy of the relocations was determined by producing and analysing three parameters, the azimuthal distribution of seismograph stations (GAP), root-mean-square of travel time residuals (RMS) and epicenter location error data. The analysis of these parameters showed that there was an improvement in the accuracy of the relocated events. Using the ISC location algorithm, iLOC, eight preselected events were further analysed. From this analysis, two earthquakes were found to satisfy the conditions for Ground Truth (GT595%) candidacy whilst four events satisfied the criteria for GT2090% candidacy.

Lake Van (Southeastern Turkey) Experiment: Receiver Function Analyses of Lithospheric Structure from Teleseismic Observations

The P and S wave receiver functions and their joint inversions are used to study the lithosphere beneath the Lake Van region using approximately 600 teleseismic earthquake data (Mw ≥ 5.8) at different azimuths collected from 10 permanent broadband stations operated by Kandilli Observatory and Earthquake Research Institute and Disaster and Emergency Management Authority in the region. The dataset is taken from the European Integrated Data Archive. The simulated annealing method is used for the joint inversion of P-wave and S-wave receiver functions, since the P410s phase cannot be detected reliably in the depth stacks. This may be due to the low olivine content and high basalt content at this depth. The inversion process is therefore performed without travel time residuals. The crustal thickness is observed at nearly 45 km in all the velocity models obtained from the inversion. The relatively low-velocity layer (Vs = 3.4 km/s) at depths of the middle crust may be associated with volcanic centers near Nemrut, in the west of the region. Additionally, another low-velocity layer (Vs = ~ 3.0 km/s) is observed in the upper crust around the Suphan Volcano. Also, Vp/Vs and Poisson’s ratios are calculated for the study area. Their high values (Vp/Vs ≥ 1.85, and σ ≥ 0.285) correspond to the partial melting of the lower crust in the region. According to velocity models obtained from the inversion results, the most important outcome is that the average variance of the Moho discontinuity (~ 45 km) and lithosphere–asthenosphere boundary (~ 90 km) under the seismograph stations can be interpreted as a likely part of the Arabian oceanic plate in relation to the collision zone.

Earthquake detection in the Jiangsu region, China using graphics-processing-unit-based Match & Locate and rapid earthquake association and location*

Earthquake detection and location are essential in earthquake studies, which generally consists of two main classes: waveform-based and pick-based methods. To evaluate the ability of two different methods, a graphics-processing-unit-based Match & Locate (GPU-M&L) method and a rapid earthquake association and location (REAL) method are applied to continuous seismic data recorded by 24 digital seismic stations from Jiangsu Seismic Network during 2013 for comparison. GPU-M&L is one of waveform-based methods by waveform cross-correlations while REAL is one of pick-based method to associate arrivals of different seismic phases and locate events through counting the number of P and S picks and travel time residuals. Twenty-six templates are selected from the Jiangsu Seismic Network local catalog by using the GPU-M&L. The number of newly detected and located events is about 2.8 times more than those listed in the local catalog. We both utilize a deep-neural-network-based arrival-time picking method called PhaseNet and a short-term/long-term average (STA/LTA) trigger algorithm for seismic phase detection and picking by applying the REAL. We then refine seismic locations using a least-squares location method (VELEST) and a high-precision relative location method (hypoDD). By applying STA/LTA and PhaseNet, 1006 and 1893 events are associated and located, respectively. The newly detected events are mainly clustered and show steeply dipping fault planes. By analyzing the performance of these methods based on long-term continuous seismic data, the detected catalogs by the GPU-M&L and REAL show that the magnitudes of completeness are 1.4 and 0.8, respectively, which are smaller than 2.6 given by the local catalog. Although REAL provides improvement compared with GPU-M&L, REAL is highly dependent on phase detection and picking which is strongly affected by signal-noise ratio (SNR). Stations at southeast of the study region with low SNR may lead to few detections in the same area.

ISC‐EHB 1964–2016, an Improved Data Set for Studies of Earth Structure and Global Seismicity

AbstractA data set of earthquake hypocenters and associated traveltime residuals for seismic phases recorded by seismograph stations globally is an essential starting point for most studies of global seismicity and Earth structure. Such data sets have been produced in various forms by national and international agencies since the beginning of instrumental seismology at the turn of the twentieth century. We have reprocessed the comprehensive data used to produce the routinely distributed bulletins of the International Seismological Centre (ISC) since 1964 to construct a new refined data set of hypocenters with improved focal depths and phase residuals. This data set, called ISC‐EHB, is used to reveal features of the seismotectonic zones in downgoing slabs in greater detail than previously routinely available.

Upper mantle tomography of the Western Junggar: Implications for its geodynamic evolution

A high-resolution 3-D P-wave velocity model of the upper mantle beneath the Western Junggar is determined by inverting a large number of relative travel-time residuals of teleseismic events. Our results reveal a prominent high-velocity (high-V) anomaly beneath the western Junggar basin and a low-velocity (low-V) anomaly beneath the Darbut belt. The NE-SW striking high-V anomaly at depths of 50–200 km beneath the western Junggar basin dips toward the northwest, which represents a fossil oceanic slab. The low-V anomaly may reflect an intraoceanic arc-related terrain that has experienced strong metasomatism by hot upwelling asthenosphere in the late Paleozoic, resulting in adakitic intrusions and concentration of porphyry CuAu deposits in the Darbut belt. These results shed new light on the geodynamic evolution of the Western Junggar and formation mechanisms of the adakites and porphyry CuAu deposits.

TX2019slab: A New P and S Tomography Model Incorporating Subducting Slabs

AbstractLarge numbers of earthquakes occur in subduction zones that are marked by dipping, narrow high seismic velocity slabs. The existence of these fast velocity slabs can cause serious earthquake mislocation problems that can bias estimates of seismic travel time residuals. This can affect the recovery of subducting slabs in tomography as well as introduce significant artifacts into lower mantle structure in tomography models. In order to better account for known subducting slabs, we performed a new P and S wave joint tomography inversion incorporating a three‐dimensional thermal model of subducting slabs in the starting model. In addition, velocity and source locations were inverted for simultaneously. Our new P and S models feature higher‐amplitude subducting slabs compared with previous global tomography results. The S to P heterogeneity ratio based on the new tomography model indicates that thermal elastic effects alone cannot explain all the heterogeneities in the lower mantle. Much of the observed abnormal S to P heterogeneity ratio can be explained by anelastic effects, the spin transition, and phase transitions of bridgmanite to post‐perovskite in the lower mantle.

Assessment of directional accuracy of GNSS-Acoustic measurement using a slackly moored buoy

We assessed the positioning accuracy of GNSS-Acoustic (GNSS-A) measurement using a slackly moored buoy. A key feature of real-time buoy-based GNSS-A measurement is that positioning must be performed via single ranging from an arbitrary observation position and encompassing the drifting range of the buoy. In this study, slack-line mooring was employed to resist strong Kuroshio current of up to 5.5 knots in the Nankai Trough. During a year-long sea trial, the buoy drifted within a circle of ~ 4000 m radius, which is much larger than the dimension of a seafloor transponder array. In the sea trial, more than 500 successful regular and on-demand ranging measurements were obtained at various observation positions. The results show that the horizontal positioning accuracy (2σ) of the array for single ranging was 46 cm when the buoy was inside the array and 97 cm when it was outside the array. These accuracies are comparable to those achieved through single ranging in traditional ship-based surveys at the same site. In addition to the distance dependency, we observed directivity in the accuracy, depending on the geometry between an observation point and seafloor transponders. To interpret the accuracy degradation and directivity as a function of observation position, we calculated error ellipses using a dilution-of-precision (DOP) analysis procedure for GNSS-A positioning. The error ellipses clearly illustrate that the variance in array positions is large in the line-of-sight direction from an observation point to the array center. We translated the error ellipses to positioning accuracies using the standard deviation of travel time residuals when an array position is estimated using all pings. The positioning accuracy resulting from the translation corresponded to that obtained using array positions in a buoy observation. The DOP analysis results and their translation into positioning accuracies enabled a detailed assessment of the directional accuracy of GNSS-A positioning at an arbitrary observation position, which is important for real-time measurement of seafloor movement in the event of a huge earthquake.