Wave Anomalies Research Articles

Heterogeneous mantle effects on the behaviour of SmKS waves and outermost core imaging

SUMMARYSeismic traveltime anomalies of waves that traverse the uppermost 100–200 km of the outer core have been interpreted as evidence of reduced seismic velocities (relative to radial reference models) just below the core–mantle boundary (CMB). These studies typically investigate differential traveltimes of SmKS waves, which propagate as P waves through the shallowest outer core and reflect from the underside of the CMB m times. The use of SmKS and S(m-1)KS differential traveltimes for core imaging are often assumed to suppress contributions from earthquake location errors and unknown and unmodelled seismic velocity heterogeneity in the mantle. The goal of this study is to understand the extent to which differential SmKS traveltimes are, in fact, affected by anomalous mantle structure, potentially including both velocity heterogeneity and anisotropy. Velocity variations affect not only a wave's traveltime, but also the path of a wave, which can be observed in deviations of the wave's incoming direction. Since radial velocity variations in the outer core will only minimally affect the wave path, in contrast to other potential effects, measuring the incoming direction of SmKS waves provides an additional diagnostic as to the origin of traveltime anomalies. Here we use arrays of seismometers to measure traveltime and direction anomalies of SmKS waves that sample the uppermost outer core. We form subarrays of EarthScope's regional Transportable Array stations, thus measuring local variations in traveltime and direction. We observe systematic lateral variations in both traveltime and incoming wave direction, which cannot be explained by changes to the radial seismic velocity profile of the outer core. Moreover, we find a correlation between incoming wave direction and traveltime anomaly, suggesting that observed traveltime anomalies may be caused, at least in part, by changes to the wave path and not solely by perturbations in outer core velocity. Modelling of 1-D ray and 3-D wave propagation in global 3-D tomographic models of mantle velocity anomalies match the trend of the observed traveltime anomalies. Overall, we demonstrate that observed SmKS traveltime anomalies may have a significant contribution from 3-D mantle structure, and not solely from outer core structure.

SKS Polarization Anomalies Due to the Coriolis Force

ABSTRACT The Earth’s Coriolis force has been well-known to impact surface waves and normal modes, which is essential to accurately interpret these waves. However, the Coriolis force on body waves has been assumed to be negligible and mostly ignored. It has been previously shown that the Coriolis force impacts polarizations of shear waves, whereas the wavefronts remain unaffected. We expand on the potential influences of Earth’s Coriolis force on shear-wave polarization measurements by conducting 3D numerical simulations for elastic waves generated by earthquake and explosive sources in a radially symmetric, and 3D mantle and crustal models. The Coriolis force can produce polarization anomalies of mantle shear waves up to 7° and core phases, such as SKS and SKKS, up to 4°. Uncorrected shear-wave polarizations due to the Coriolis force can cause an additional source of error (5°–10° in fast direction, and 0.2–0.3 s delay time depending on the method and seismic phase), inaccurate interpretation of station misalignments, and imprecise estimates of the core–mantle boundary topography. We show how to correct for the Coriolis force on teleseismic shear waves using 1D ray tracing for well-isolated phases. We recommend the use of full waveform simulations to accurately account for earthquake sources parameters, poorly isolated phases that could include interfering phase arrivals within the measurement time window, and the effect of the Coriolis force on the polarizations of shear waves.

ENSO Modoki Impacts on the Interannual Variations of Spring Antarctic Stratospheric Ozone

Abstract Using observation and reanalysis data, we investigated the effect of the sea surface temperature anomalies associated with ENSO Modoki from September to October on interannual variations in Antarctic stratospheric ozone from October to November. It was found that the planetary wave anomalies generated by ENSO Modoki in the tropical troposphere propagate to the southern mid- and then high-latitude stratosphere. The planetary wave anomalies have a profound impact on the polar vortex, subsequently affecting the interannual variations in Antarctic stratospheric ozone. Further analysis revealed that the responses of the polar vortex and ozone to ENSO Modoki are mainly modulated by the wave-1 and wave-3 components, and the effect of wave 2 is opposite and offset by those of wave 1 and wave 3. The contribution of the residual waves (after removing waves 1, 2, 3, and the remaining waves) are relatively small. Furthermore, we evaluated the performance of CMIP6 models in simulating the impacts of ENSO Modoki on the southern stratospheric polar vortex and ozone. We selected seven models that include stratospheric processes and stratospheric chemical ozone. We found that all are capable of distinguishing between eastern Pacific ENSO and ENSO Modoki events. However, only GISS-E2-1-G and MPI-ESM-1-2-HAM can simulate the patterns of ozone, circulation, and temperature in the Southern Hemisphere in a manner that closely resembles the reanalysis results. Further analysis indicated that these two models can better simulate the propagation of planetary wave activities in the troposphere forced by ENSO Modoki, whereas the other models produce significantly different results to those obtained from observations. Significance Statement This study found a significant connection between ENSO Modoki and the interannual variability of Antarctic stratospheric ozone in austral spring and investigated the underlying physical mechanisms in detail. In addition, the performances of CMIP6 models in simulating the impact of ENSO Modoki on the southern stratospheric polar vortex and ozone were evaluated. This study not only helps to further understand the characteristics of past Antarctic ozone changes but also helps developers improve the performance of models in simulating Antarctic stratospheric changes.

Identifying potential porphyry copper mineralization at the Zhu’nuo ore-cluster district in western Gangdese, southern Tibet: Insights from shortwave infrared (SWIR) spectrometry and geochemical anomalies

The Zhu’nuo ore-cluster district in the western Gangdese belt, southern Tibet, is characterized by a significant Cu geochemical anomaly, where the Zhu’nuo super-large porphyry Cu deposit (PCD) was discovered by stream sediment geochemical method. However, due to the extended coverage of volcanic rocks of the Linzizong group and the large area of ore-forming element anomalies, it is quite difficult to locate the concealed PCDs only by geochemical method. Therefore, the application of new technical methods is urgently needed to guide the exploration breakthroughs in the Zhu’nuo district. Recently, our studies have shown that the wavelength variation of diagnostic absorption peaks of alteration minerals measured by shortwave infrared (SWIR) spectrometry is closely related to the mineralization temperature in the Zhu’nuo PCD, which can be footprints to trace the hydrothermal and/or mineralization (H/M) centers of porphyry ore systems. Hence, we carried out SWIR studies in the Luobuzhen-Cimabanshuo areas with high Cu geochemical anomaly. In this study, sixteen alteration minerals were identified by SWIR, consisting mainly of white mica, chlorite, epidote, and kaolinite. Among them, the white mica and chlorite were widely distributed in the study area, and the Al-OH wavelength position of white mica (∼2200 nm; Pos2200) and Fe-OH wavelength position of chlorite (∼2250 nm; Pos2250) varied between 2191–2219 nm and 2239–2260 nm, respectively. Moreover, there is an increasing trend of Pos2200 from porphyry center to periphery, whereas Pos2250 shows the opposite tendency. When combined with geochemical anomalies of stream sediments and geological characteristics and SWIR features in the Zhu’nuo PCD in study area, three potential H/M centers, namely Cimabanshuo, South Luobuzhen and Ya’nuri, were determined, which are dominated by short wave anomalies (Pos2200, 2191–2206 nm; Pos2350, 2243–2250 nm). Significantly, we showed that the Dongshibu-Hongshan-Luobuzhen may belong to the same porphyry system by SWIR. And SWIR implies that the true center is located in South Luobuzhen (H/M II), which has great prospecting potential in its depth, rather than the previously recognized Dongshibu Cu prospect delineated by stream sediment data. We found that the Dongshibu Cu prospect does not show any alteration information associated with porphyry mineralization as reflected by SWIR, consistent with the barren Cu results from the late drilling validation. This decoupling may be caused by the long-term migration and weathering of stream sediment samples, during which the obtained geochemical anomaly do not represent their in-situ information. Therefore, caution must be taken when using stream sediment data to delineate prospecting targets. Instead, SWIR is a potentially powerful vectoring tool to aid exploration for porphyry systems, especially in the western regions where the level of prospecting work is extremely low.

Association of QTc Interval and V4-S Wave With Appropriate ICD Therapy in Hypertrophic Cardiomyopathy.

BackgroundVentricular arrhythmias in patients with hypertrophic cardiomyopathy (HCM) may lead to sudden cardiac death (SCD). We aimed to investigate the relationship between electrocardiogram (ECG) indicators and the risk of appropriate implantable cardioverter-defibrillator (ICD) therapy in HCM.MethodsThe HCM patients receiving ICD implantation were enrolled consecutively. QT interval correction (QTc) was calculated using Bazett's formula. Long or deep S wave in V4 lead was defined as duration time >50 ms and/or voltage amplitude >0.6 mV. The endpoint in our study was at least one ICD appropriate therapy triggered by ventricular tachyarrhythmia (VT) or ventricular fibrillation (VF), including anti-tachyarrhythmia pacing (ATP) and electrical shock.ResultsA total of 149 patients with HCM (mean age 53 ± 14 years, male 69.8%) were studied. Appropriate ICD therapies occurred in 47 patients (31.5%) during a median follow-up of 2.9 years. Cox regression analysis showed that long or deep S wave in V4 lead [hazard ratio (HR) 1.955, 95% confidence interval (CI) 1.017–3.759, P = 0.045] and QTc interval (HR 1.014, 95% CI 1.008–1.021, P < 0.001) were independent risk factors for appropriate ICD therapy. The ROC showed that the optimal cut-off point value for the QTc interval to predict the appropriate ICD therapy was 464 ms, and the AUC was 0.658 (95% CI 0.544–0.762, P = 0.002). The AUC for S wave anomalies in V4 lead was 0.608 (95% CI 0.511–0.706, P = 0.034). We developed a new model that combined the QTc interval and S wave anomalies in V4 lead based on four patient groups. Patients with QTc ≥464 ms and long or deep V4-S wave had the highest risk of developing appropriate ICD therapy (log-rank P < 0.0001). After adding QTc interval and V4-S wave anomalies into the HCM-risk-SCD model, the prediction effect of the new model was significantly improved, and the NRI was 0.302.ConclusionsIn this HCM cohort, QTc and S wave anomalies in V4 lead were found to be significant and strong predictors of the risk of appropriate ICD therapy. Patients with QTc ≥464 ms and long or deep S wave had the highest risk. After QTc interval and V4-S wave anomalies adding to the HCM-risk-SCD model, the prediction effect is significantly improved.

BUILDING A TOMOGRAPHIC VELOCITY MODEL FOR SAMOYLOV ISLAND AREA (LENA DELTA) FROM LOCAL SEISMOLOGICAL DATA FOR THE PERIOD OF 2019–2021

In our paper we present the results of seismotomographic inversion for the local seismological monitoring data obtained in the area of the Samoylov Island (Lena River delta) in 2019–2021. Tomographic velocity model was used for refining hypocenter locations of local earthquakes and for geological interpretation. The results are shown as maps of anomalies of seismic waves and Vp /Vs ratios for the 5 and 10 km depths. The velocity anomalies structure made it possible to interpret low Vp /Vs ratio as rocks related to the Siberian platform, and to compare between the boundary of the low Vp /Vs area and the trace of the known geological fault running along the Olenekskaya Channel.

Lower crustal attenuation in northeastern Tibetan Plateau from ML amplitude

This study investigated the crustal attenuation structures of Sg and Lg waves of the northeastern Tibetan Plateau. We collected ML amplitude data recorded at 168 permanent stations between 1985 and 2016 and 11 temporary broadband stations between 2014 and 2016. Detailed Q0 variation maps of Sg and Lg waves were obtained by applying ML amplitude tomography. The average Q0 values of the Sg and Lg wave were 440 and 220, respectively. Relatively high attenuation anomalies of both waves appeared in the central and eastern regions of the Bayan Har Block and the east edge of the Qiangtang Block, which may be related to partial melting, high geotemperature, and strong tectonic processes. High attenuation anomalies were also found in the Qilian Orogenic Belt and Hetao Graben, which may be related to their active tectonic behavior and densely distributed faults. The relatively low attenuation anomalies of both waves were revealed in the Alax and Ordos blocks, Qaidam, Tarim, Qinghai Lake, and Gonghe basins, which can be explained by the tectonically stable properties and ancient composition of geological elements. These results indicate that the path between the highly attenuated lower crust of the Bayan Har Block and the Qilian Orogenic Belt is obstructed by three adjacent low attenuated areas (i.e., the Qilian, Qinghai Lake, and Gonghe basins); thus, it appears unlikely that a crustal flow channel from the interior of the Tibetan Plateau to the Qilian Orogenic Belt will form.

Response of ocean surface waves to the co‐occurrence of Boreal Summer Intra-Seasonal Oscillation and El Niño Southern Oscillation

Studies related to the combined effect of different atmospheric oscillations on the ocean surface waves are limited. The present study focuses on the change in wave climatology due to the combined effect of Boreal Summer Intra-Seasonal Oscillation (BSISO) and El Nino Southern Oscillation (ENSO) using 40 years of reanalysis data on the Asian Summer Monsoon Region (ASMR). Composite analysis of surface wind, significant wave height, wind-sea, swell and mean wave period are analysed corresponding to different ENSO–BSISO phase combinations. The study showed noteworthy differences in wave parameters when ENSO–BSISO phases are analysed together. In El Nino–BSISO combined phase, enhancement of north-easterly wind causes the advancement in the reversal of wind direction (north-easterly to monsoon enhanced south-westerly) and disrupted propagation of positive wind and wave anomalies. Conversely, during La Nina–BSISO co-occurrence, south-westerlies are enhanced and as a result, the negative north-easterly anomalies are disrupted. In all the ENSO–BSISO combined phases, maximum wave height variability occurs at the South China Sea owing to the combined action of the north Indian Ocean (NIO) and Western North Pacific (WNP) surface wind forcing. High fluctuations in Tm over the NIO and WNP are observed during different ENSO–BSISO phase combinations. The phase relationship of Hs with Tm and the significant height of wind-sea (Hsw) and swell (Hss) are assessed to understand the propagation of swells. Due to the effect of multiple atmospheric perturbations, significant changes in Hs occur over the coastal regions of the NIO.

Wave Anomaly Detection in Wave Measurements

AbstractQuality control measures for ocean waves observations are necessary to give confidence of their accuracy. It is common practice to detect anomalies or outliers in surface displacement observations by applying a standard deviation threshold. Besides being a purely statistical method, this quality control procedure is likely to flag extreme wave events erroneously, thereby impacting higher-order descriptions of the wave field. In this paper we extend the use of the statistical phase-space threshold, an established outlier detection method in the field of turbulence, to detect anomalies in a wave record. We show that a wave record in phase space (here defined as a diagram of displacement against acceleration) can be enclosed by a predictable ellipse where the major and minor axes are defined by the spectral properties of the wave field. By using the parameterized ellipse in phase space as a threshold to identify wave anomalies, this is a semiphysical filtering method. Wave buoy data obtained from a mooring deployed near King George Island, Antarctica [as part of the Antarctic Modeling Observation System (ATMOS)], and laser altimeter data obtained at the Northwest Shelf of Australia were used to demonstrate the functioning of the filtering methodology in identifying wave anomalies. Synthetic data obtained using a high-order spectral model are used to identify how extreme waves are positioned in phase space.

Anomalous Shubnikov-de Haas quantum oscillations in rare-earth tritelluride NdTe3

Materials with the structural motif of a square net have in recent years attracted attention for their often unexpected electronic properties. Here, we present low-temperature magnetotransport experiments on high-quality ${\mathrm{NdTe}}_{3}$ single crystals with a carrier mobility of $6\ifmmode\times\else\texttimes\fi{}{10}^{4}$ ${\mathrm{cm}}^{2}$/V s at 2.25 K; an increase of 58% is observed below the N\'eel temperature. Shubnikov-de Haas oscillations up to 9 T exhibit anomalies in their amplitude which deviates from the conventional Lifshitz-Kosevich behavior below the magnetic ordering temperature and around 20 K. The latter, which has so far not been observed in any tritelluride, suggests a field- and temperature-induced electronic structure transition. We provide a thorough analysis of three different samples, highlighting the importance of sample quality for elucidating details in the transport behavior. Our results on ${\mathrm{NdTe}}_{3}$ suggest that rare-earth tritellurides may display so-far obscured exotic electronic properties in addition to the well-studied high-temperature charge density wave anomalies.

Influence of the North Pacific Victoria Mode on the Madden–Julian Oscillation

Using the observational data and the Coupled Model Intercomparison Project phase 5 (CMIP5) models this study examined the influence of the North Pacific Victoria mode (VM) on the Madden–Julian Oscillation (MJO). The results show that the February–April VM had a significant influence on the development and propagation of the MJO over the equatorial central-western Pacific (ECWP) during spring (March-May) between 1979 and 2017. Specifically, MJO development was favored more by positive VM events than negative VM events. These complicated connections could have been caused by the SST gradient anomalies associated with positive VM events, enhancing the convergence of low-level over the ECWP. When this is combined with warm SST anomalies in the equatorial central Pacific it could have led to a boost in the Kelvin wave anomalies, resulting in enhanced MJO activity over the ECWP. These conclusions indicate that the VM is an important factor in MJO diversity.

The very low-frequency transmitter radio wave anomalies related to the 2010 Ms 7.1 Yushu earthquake observed by the DEMETER satellite and the possible mechanism

Abstract. Earthquakes may disturb the lower ionosphere through various coupling mechanisms during the seismogenic and coseismic periods. The VLF (very low-frequency) signal radiated from ground-based transmitters will be affected when it penetrates the disturbed ionosphere above the epicenter area, and this anomaly can be recorded by low-Earth orbit satellites under certain conditions. In this paper, the temporal and spatial variation of the signal-to-noise ratio (SNR) of the VLF transmitter signal in the ionosphere over the epicenter of 2010 Yushu Ms 7.1 earthquake in China is analyzed using DEMETER (Detection of Electro-Magnetic Emission Transmitted from Earthquake Regions) satellite observation. The results show that SNR over the epicenter of the Yushu earthquake especially in the southwestern region decreased (or dropped) before the main shock, and a GPS–TEC (Global Positioning System; total electron content) anomaly accompanied, which implies that the decrease in SNR might be caused by the enhancement of TEC. A full-wave method is used to study the mechanism of the change in SNR before the earthquake. The simulated results show SNR does not always decrease before an earthquake. When the electron density in the lower ionosphere increases by 3 times, the electric field will decrease about 2 dB, indicating that the disturbed-electric-field decrease of 20 % compared with the original electric field and vice versa. It can be concluded that the variation of electron density before earthquakes may be one of the important factors influencing the variation of SNR.

Evaluation on the CFSv2 forecasts of three cold waves during 2015–2016 in China

In this paper, by using forecasts of the highest and lowest daily surface air temperatures from the Coupled Forecast System version 2 (CFSv2) over a period of 9 months, NCEP/DOE reanalysis data from the National Centers for Environmental Prediction (NCEP), and data from the U.S. NOAA Climate Prediction Center (CPC), three cold waves are identified during 2015–2016 in China, and the CFSv2 forecasts of these cold waves are evaluated. The results show that the high-resolution CPC data and the low-resolution NCEP/DOE data both very clearly show the temperature drops and temperature anomalies of these cold waves. The earlier the forecast lead time is, the greater the errors in the beginning and ending dates of the cold waves between the CFSv2 forecasts and the observations; there are errors of 1–2 days in the durations of the cold waves between the CFSv2 forecasts and the observations. CFSv2 exhibits certain abilities to forecast the overall temperature drops and temperature anomalies of cold waves at forecast lead times of 0, 5, 10, and 15 days, but the CFSv2 prediction ability is poor at forecast lead times exceeding 20 days. CFSv2 can predict the spatial distribution of temperature drops better than it can predict that of temperature anomalies, but the absolute errors in the observed temperature drops are greater than those in the observed temperature anomalies. The temperature drops from the CFSv2 forecasts are obviously underestimated in North China, South China, and their nearby areas but overestimated in Mongolia and Northeast China. The temperature anomalies from the CFSv2 forecasts are obviously underestimated in the inland areas of China but overestimated in the surrounding oceans and adjacent coastal areas. The difference fields at forecast lead times of 0 and 30 days are basically the same, but the absolute values of the differences increase with increases in the forecast lead time (with a few exceptions).

Aftershocks localization and shallow crustal velocity structure following the &amp;lt;italic&amp;gt;M&amp;lt;/italic&amp;gt;s6.9 Mainling earthquake in Tibet, China

At 06:34 CST on November 18, 2017, an M s6.9 earthquake struck Mainling prefecture, Nyingchi City, Tibet. The earthquake occurred at the frontier zone of collision and subduction in the eastern Himalayan syntaxis, a region of persistently strong compression between India and Eurasia. To monitor aftershock activity, the Institute of Tibetan Plateau Research, Chinese Academy of Sciences, installed 28 broadband seismic stations around the epicentral area and obtained 180 days of seismic observation data. The average spacing between stations was approximately 5 km, which yielded good coverage of the source area and enriched the seismic data for the Eastern Himalayan syntaxis. In this paper, the hypodense events are refined by the hypoDD double-difference seismic location method and the local tomography software (LOTOS-12), an algorithm for seismic location and velocity structure imaging joint inversion. The hypoDD seismic localization method uses waveform cross-correlation technology to achieve positioning using the minimum residuals of the actual and theoretical travel times of two earthquakes. It is widely used for close-range earthquakes and is suitable for regional earthquake research; the LOTOS algorithm is more appropriate for velocity structure inversion in regions with significant differences in physical properties. The results show that the aftershock zone is approximately 50 km long and 30 km wide and trends NW-SE. According to the aftershock distribution, three active faults are basically consistent with the strike of the Xixingla fault zone. These active fractures are similar in shape and have a NE dip direction, and the dip angles of the active faults gradually become steeper from NW to SE. Under the continuous NE thrusting action of the Namche Barwa syntaxis, the three secondary fractures have undergone bending deformation. This deformation is associated with the adjustment mode of the eastern Himalayan syntaxis where the Indian plate continues to push Eurasia northward. At the same time, combining these results with those of the LOTOS-12 algorithm, the distribution of the Pg wave and Sg wave velocity disturbance in the shallow crust (20 km) and the wave speed ratio ( V p/ V s) are obtained. The main shock was located in an abnormal transition zone of body wave velocity, and the aftershocks mostly occurred in the negative anomaly zone for P wave and S wave velocities, which reflects the characteristics of stratigraphic fractures and faults in the area with concentrated earthquakes. We suggest that this area should be avoided in the future when construction of engineering facilities is required north of Namche Barwa Peak. Overall, with Namche Barwa Peak as the boundary, the P wave and the S wave are generally characterized by high-speed anomalies in the north and low-speed anomalies in the south. This phenomenon may be related to the geological structure and petrophysical properties in this area, as the lithologies of the Lhasa terrane are old and cold; meanwhile, the metamorphic body in the south and the Namche Barwa syntaxis are relatively broken, the ages of rock formation are relatively young, and the lithologies are relatively hot and soft. In addition, high-speed P wave anomalies in the northwestern part of Namche Barwa Peak may be related to enrichment in garnet minerals in this area. The higher wave velocity ratio on the southwest side of the epicentral area, the lower P wave velocity on the right side of profile 1–1′ and the P wave on the left side of profile 2–2′ may be related to the fragmentation of the terrain in this area and the development of valley glaciers. The area where the wave velocity ratio is relatively large may also be related to the upwelling of asthenospheric and lithospheric material in the deep mantle.

Upper mantle slab under Alaska: contribution to anomalous core-phase observations on south-Sandwich to Alaska paths

Observations of travel time anomalies of inner core-sensitive PKPdf seismic body waves, as a function of path orientation with respect to the earth's rotation axis, have been interpreted as evidence of anisotropy in the inner core. Paths from earthquakes in the South Sandwich Islands to stations in Alaska show strongly anomalous travel times, with a large spread that is not compatible with simple models of anisotropy. Here we assess the impact of strong velocity heterogeneity under Alaska on the travel times, directions of arrival and amplitudes of PKPdf. We use 3D ray-tracing and 2.5D waveform modelling through a new, high-resolution tomography model of the upper mantle beneath Alaska. We find that the structure beneath Alaska, notably the subducting slab, is reflected in the patterns of these PKPdf observations, and this can be replicated by our model. We also find similar patterns in observed teleseismic P waves that can likewise be explained by our slab model. We conclude that at least 2 s of the travel time anomaly often attributed to inner core anisotropy is due to slab effects in the upper mantle beneath Alaska.

The depth geodynamic state and its correlation with the surface geological and geophysical parameters along the sublatitudinal profile of Eurasia

A cross‐sections of longitudinal (P) and transverse (S) wave anomalies (attribute δ(VP/VS)) is constructed along the sublatitudinal profile from the Atlantic Ocean to the Pacific Ocean across the regions of the latest Eurasian volcanism. It is correlated with surface geophysical parameters interpretable in terms of geodynamics: heat flow, seismicity and integrated conductivity of the lithosphere. All the volcanic groups are related to the negative anomalies of S‐ and P‐wave velocity variations at depths, which are observed in the eastern part of the profile from Central Asia to the Pacific Ocean to depths of 1000 km. Such anomalies correlate with the heat flow anomalies and are thus indica‐ tive of a deep source. The absence of deep roots in the western part of the profile from the Caspian to the Western Mediterranean suggests lateral extension of the anomalously ‘hot’ mantle from the Afar branch of the African super‐ plume. The groups of volcanic formations in the Baikal region and the Far East are spatially associated with heat flow anomalies that are three times higher than the background values. A correlation between intraplate volcanism and the lithosphere conductivity suggests the presence of positive anomalies in all volcanic clusters, despite the fact that their background values are considerably different. In the continental part, velocity anomalies are typical of all volcanic groups with positive conductivity anomalies. It is evidenced by seismic tomography that all the volcanic groups (ex‐ cept the Alpine‐Caucasian) have ‘hot’ roots in the upper mantle to depths of 1200 km. The highest maximum conduc‐ tivity values are typical of the zones wherein high intraplate seismicity is absent. Along the profile, there are several zones of high intraplate seismicity, which are separated by aseismic zones or plate boundaries. This suggest the influ‐ ence of the heated state of the mantle and the occurrence of zones of increased conductivity in the lithosphere.

What happens to the ocean surface gravity waves when ENSO and MJO phases combine during the extended boreal winter?

The safety of vulnerable coastal and offshore infrastructures requires an in-depth understanding of wave variability and climate drivers. We investigate the association of significant wave height (Hs) and peak wave period (Tp) with the co-occurrence of El Nino–Southern Oscillation (ENSO) and the Madden–Julian Oscillation (MJO) at the global scale. We calculate composites of daily anomalies in modelled Hs, Tp, and surface wind for periods of ENSO–MJO phase combinations. Calculations spanned November–March seasons over the period 1979–2018. Wave anomalies are widespread across the world’s oceans, with remarkable strength during several ENSO–MJO phase combinations, demonstrating strong tropic–tropic and tropic-extratropic teleconnections. Positive Hs anomalies are strongest in the Pacific Ocean during El Nino–MJO phase 8, in the Atlantic Ocean during ENSO-neutral-MJO phase 3, and in the Indian Ocean during ENSO-neutral-MJO phase 4. Positive Tp anomalies are strongest in the Pacific Ocean during La Nina–MJO phase 8, in the Atlantic Ocean during El Nino–MJO phase 1, and in the Indian Ocean during El Nino–MJO phase 8. In the Southern Ocean, the strongest Hs anomalies occur during El Nino–MJO phase 8, whereas in the Maritime Continent, they appear during ENSO-neutral-MJO phases 5–6. Despite previous studies finding low correlations of ENSO indices with wave parameters in the North Atlantic, our results suggest that ENSO-related conditions play a significant role in the area when combined with certain MJO-related conditions. This study also reveals that the wave anomalies associated with ENSO–MJO phase combinations can be twice as strong as those found in previous work, related only to the MJO. Therefore, considering multiple concurrent climate patterns in the analysis of wave anomalies is essential to developing more reliable coastal management plans.

Hot, volatile‐poor, and oxidized magmatism above the stagnant Pacific plate in Eastern China in the Cenozoic

AbstractEastern China has experienced widespread and voluminous, basaltic to andesitic intraplate magmatism in the Cenozoic. Seismic tomography and kinematic reconstructions show that magmas have erupted above the stagnant Pacific plate, which currently extends within the mantle transition zone for &gt;1500 km beyond the active arc. Seismic studies also show in intriguing detail that low‐velocity wave anomalies reach from the mantle transition zone or below to major volcanic centers, suggesting an intimate relation between the magmatism and components derived from the mantle‐transition zone and possibly from deeper levels. Here, we provide new petrological, mineral chemical, and whole‐rock geochemical data showing that Cenozoic basaltic andesitic and andesitic magmas that have erupted above the present‐day edge of the subducted Pacific plate are largely unfractionated, ≥1130‐1160±15‐50 °C hot, mantle‐derived melts that were volatile‐poor (undegassed melt CO2 and H2O of ~2400‐3100±500 ppm and ~0.5‐0.6±0.4‐1.1 wt%), and relatively oxidized (~FMQ to ~FMQ+1.3±1.0). We further infer that the magmas were derived by partial melting of oxidized, H2O‐ and CO2‐poor eclogite‐rich sources (~FMQ‐1.0 to ~FMQ). We use this to suggest that the low‐velocity seismic anomalies below Eastern China largely reflect the presence of oxidized and enriched, hot and not necessarily particularly hydrous mantle domains as common interpretation has suggested. This is crucial information for quantifying material flow above and around the subducted Pacific plate and for constraining residence times of subducted components in Eastern Asia and globally.

Accelerating full waveform inversion via source stacking and cross-correlations

SUMMARYAccurate synthetic seismic wavefields can now be computed in 3-D earth models using the spectral element method (SEM), which helps improve resolution in full waveform global tomography. However, computational costs are still a challenge. These costs can be reduced by implementing a source stacking method, in which multiple earthquake sources are simultaneously triggered in only one teleseismic SEM simulation. One drawback of this approach is the perceived loss of resolution at depth, in particular because high-amplitude fundamental mode surface waves dominate the summed waveforms, without the possibility of windowing and weighting as in conventional waveform tomography.This can be addressed by redefining the cost-function and computing the cross-correlation wavefield between pairs of stations before each inversion iteration. While the Green’s function between the two stations is not reconstructed as well as in the case of ambient noise tomography, where sources are distributed more uniformly around the globe, this is not a drawback, since the same processing is applied to the 3-D synthetics and to the data, and the source parameters are known to a good approximation. By doing so, we can separate time windows with large energy arrivals corresponding to fundamental mode surface waves. This opens the possibility of designing a weighting scheme to bring out the contribution of overtones and body waves. It also makes it possible to balance the contributions of frequently sampled paths versus rarely sampled ones, as in more conventional tomography.Here we present the results of proof of concept testing of such an approach for a synthetic 3-component long period waveform data set (periods longer than 60 s), computed for 273 globally distributed events in a simple toy 3-D radially anisotropic upper mantle model which contains shear wave anomalies at different scales. We compare the results of inversion of 10 000 s long stacked time-series, starting from a 1-D model, using source stacked waveforms and station-pair cross-correlations of these stacked waveforms in the definition of the cost function. We compute the gradient and the Hessian using normal mode perturbation theory, which avoids the problem of cross-talk encountered when forming the gradient using an adjoint approach. We perform inversions with and without realistic noise added and show that the model can be recovered equally well using one or the other cost function.The proposed approach is computationally very efficient. While application to more realistic synthetic data sets is beyond the scope of this paper, as well as to real data, since that requires additional steps to account for such issues as missing data, we illustrate how this methodology can help inform first order questions such as model resolution in the presence of noise, and trade-offs between different physical parameters (anisotropy, attenuation, crustal structure, etc.) that would be computationally very costly to address adequately, when using conventional full waveform tomography based on single-event wavefield computations.

Quantifying Asthenospheric and Lithospheric Controls on Mafic Magmatism Across North Africa

AbstractAfrican basin‐and‐swell morphology is often attributed to the planform of subplate mantle convection. Across North Africa, the coincidence of Neogene and Quaternary (i.e., &lt;23 Ma) magmatism, topographic swells, long wavelength gravity anomalies, and slow shear wave velocity anomalies within the asthenosphere provides observational constraints for this hypothesis. Admittance analysis of topographic and gravity fields corroborates the existence of subplate support. To investigate quantitative relationships between intraplate magmatism, shear wave velocity, and asthenospheric temperature, we collected and analyzed a suite of 224 lava samples from Tibesti, Jabal Eghei, Haruj, Sawda/Hasawinah, and Gharyan volcanic centers of Libya and Chad. Forward and inverse modeling of major, trace, and rare Earth elements was used for thermobarometric studies and to determine melt fraction as a function of depth. At each center, mafic magmatism is modeled by assuming adiabatic decompression of dry peridotite with asthenospheric potential temperatures of 1300‐1360 °C. Surprisingly, the highest temperatures are associated with the low‐lying Haruj volcanic center rather than with the more prominent Tibesti swell. Our results are consistent with earthquake tomographic models which show that the slowest shear wave anomalies within the upper mantle occur directly beneath the Haruj center. This inference is corroborated by converting observed velocities into potential temperatures, which are in good agreement with those determined by geochemical inverse modeling. Our results suggest that North African volcanic swells are primarily generated by thermal anomalies located beneath thinned lithosphere.