Travel Time Residuals Research Articles

Seismic evidence for slab graveyards atop the Core Mantle Boundary beneath the Indian Ocean Geoid Low

The Indian Ocean Geoid Low (IOGL) that spans a vast areal extent south of the Indian subcontinent is a spectacular feature on the Earth, whose origin still remains ambiguous. In this study, we investigate the seismic character of the lower mantle below this geoid low utilizing the travel time and amplitude residuals of high quality S and ScS phases from 207 earthquakes recorded at 276 stations in the epicentral distance range of 36°–90°. For comparison, we also perform a similar exercise for a region of geoid high in the vicinity. Results reveal large variations in the ScS travel times indicating that the lowermost mantle beneath the IOGL region is heterogeneous. The ScS-S differential travel times are ∼3s slower than those predicted by the IASP91 model, primarily due to velocity increase in the lowermost mantle beneath the IOGL region and ∼2s higher than the IASP91 beneath the geoid high region, due to velocity decrease in the lowermost mantle. The largest negative residuals from manual method (−7.72s) are concentrated below the IOGL. Iterative matching of differential travel time residuals reveals that the maximum positive and negative residuals can be explained in terms of a reduction in shear velocity of 0.9% and an increase of 1.6% respectively in a ∼1000km thick layer above the Core Mantle Boundary. Further, the ScS/S amplitude residuals beneath the IOGL are positive, implying high impedance contrast at the Core Mantle Boundary, owing to the presence of high velocity material. We attribute these high velocities to the presence of dehydrated high density slab graveyards atop the Core Mantle Boundary beneath the Indian Ocean. Release of water at the mid-to-upper mantle depths due to the dehydration of subducted slabs causing a reduction in density and velocity of the ambient mantle, could be responsible for the geoid low.

Imaging complexity in the earth — Case studies with optimized ray tomography

When building velocity models for seismic depth imaging, a key tool used in the industry is ray-based tomography. In the past 10 years, the resolution of tomographic solutions has seen a continuous increase because of evolving sophistication in methodologies and technology. A vital issue in the data domain is accuracy and density of residual-moveout picks that are used to derive tomographic velocity-model updates. A new automated method allows for precise tracking of accurate residual moveout on prestack depth-migrated gathers and consequently the fast determination of dense, high-quality traveltime residuals for seismic tomography. Synthetic and real data examples from this method demonstrate the value that accurate information concerning local wave paths inherent in these picks brings to the problem of resolving small-scale velocity anomalies. The determination of such small-scale anomalies ultimately leads to flatter prestack depth-migrated gathers and consequently better-focused structural images.

Detection and Delineation of a Fracture Zone with Observation of Seismic Shear Wave Anisotropy in the Upper Rhine Graben, SW Germany

Local seismic shear wave anisotropy is studied with recordings of microearthquakes near Landau and Insheim in the central Upper Rhine Graben, SW Germany. Although the recordings have a low signal-to-noise level and there is a complex heterogeneous 3-D tectonic structure, a time separation δt between horizontally polarised SH-waves and vertically polarised SV-waves can be observed in seismograms and particle motion diagrams. The observations can be explained by azimuthal anisotropy in the upper crust with a direction φ0 = 155° east of north for the fast polarisation direction of SV-waves. A gradient of time separation with distance x of δt/x ~ 10 ms/km can explain the data. This model can be interpreted with a classical scenario of fluid-filled (sub-)vertical cracks with a preferred NNW-SSE orientation. Known faults strike NNW-SSE around Landau and Insheim and the seismicity pattern is also oriented in this direction. This direction coincides with the regional orientation of the maximum horizontal stress (σH), and fluids apparently exist at depth as known from geothermal water extraction. Furthermore, we find that 3-D seismic velocity heterogeneities have a much larger influence on the precision of the microearthquake location than the anisotropy effect in this complex tectonic region. This is obvious from the up to five times larger travel time residuals (maximum −0.5 s and +0.35 s), which are used as station corrections during the location procedure, compared to the anisotropic δt observations (maximum 0.1 s).

The P-wave boundary of the Large-Low Shear Velocity Province beneath the Pacific

The Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle represent volumetrically significant thermal or chemical or thermo-chemical heterogeneities. Their structure and boundaries have been widely studied, mainly using S-waves, but much less is known about their signature in the P-wavefield. We use an extensive dataset recorded at USArray to create, for the first time, a high-resolution map of the location, shape, sharpness, and extent of the boundary of the Pacific LLSVP using P(Pdiff)-waves. We find that the northern edge of the Pacific LLSVP is shallow dipping (26° relative to the horizontal) and diffuse (∼120 km wide transition zone) whereas the eastern edge is steeper dipping (70°) and apparently sharp (∼40 km wide). We trace the LLSVP boundary up to ∼500 km above the CMB in most areas, and 700 km between 120° and 90°W at the eastern extent of the boundary. Apparent P-wave velocity drops are ∼1–3% relative to PREM, indicating a strong influence of LLSVPs on P-wave velocity, at least in the high-frequency wavefield, in contrast to previous studies. A localised patch with a greater velocity drop of ∼15–25% is detected, defined by large magnitude gradients of the travel-time residuals. We identify this as a likely location of an Ultra-Low Velocity Zone (ULVZ), matching the location of a previously detected ULVZ in this area. The boundary of a separate low velocity anomaly, of a similar height to the LLSVP, is detected in the north–west Pacific, matching tomographic images. This outlier appears to be connected to the main LLSVP through a narrow channel close to the CMB and may be in the process of joining or splitting from the main LLSVP. We also see strong velocity increases in the lower mantle to the east of the LLSVP, likely detecting subducted material beneath central America. The LLSVP P-wave boundary is similar to that determined in high-resolution S-wave studies and follows the −0.4% ΔVS iso-velocity contour in the S40RTS tomography model. Additionally, the LLSVP boundary roughly matches the shape of the −0.4% ΔVP iso-velocity contour of the P-wave model GyPSuM but defines an area more similar to that defined by the 0.0% VP iso-velocity contour. High resolution P-wave velocity determination allows for estimation of the ratio of P- and S-wave velocity anomalies (RS,P) which can be used to indicate dominantly thermal or chemical control of seismic velocities. Although the RS,P is found here to be approximately 2.4, which is indicative of a thermo-chemical anomaly. However, this result contains a large amount of uncertainty and the implications for the origin of LLSVPs likely remain inconclusive. Nonetheless, other observations of the Pacific LLSVP are consistent with a thermo-chemical anomaly whose shape and boundary sharpness are controlled by proximity to active and past subduction.

Seismic response and anisotropy of a model hcp iron inner core

We present a framework for simulating the measurement of seismic anisotropy in a model inner core by computing travel time residuals of synthetic seismic rays propagated through the model. The method is first tested on simple inner core structural models consisting of layers with distinct anisotropy, as often proposed in the literature. Those models are not consistent with geodynamics. Hence, we extend the method to a numerically grown inner core composed of ɛ-Fe with flow generated from an excess of crystallization in the equatorial belt, inducing polycrystalline textures. The global inner core anisotropy is seven times smaller than that of the single-crystal. Compositional stratification amplifies the global anisotropy by 15% while the addition of solidification textures reduces it by a factor of two. As such, and within the tested geodynamical models, no published elastic model of ɛ-Fe at inner core conditions allows one to reproduce the 3% cylindrical anisotropy reported in seismology publications. In addition, our models demonstrate that additional information, such as the depth dependence and the spread of the observed anisotropy is a key for revealing the dynamics and history of the inner core.

Seismic waveform inversion for core–mantle boundary topography

The topography of the core–mantle boundary (CMB) is directly linked to the dynamics of both the mantle and the outer core, although it is poorly constrained and understood. Recent studies have produced topography models with mutual agreement up to degree 2. A broad-band waveform inversion strategy is introduced and applied here, with relatively low computational cost and based on a first-order Born approximation. Its performance is validated using synthetic waveforms calculated in theoretical earth models that include different topography patterns with varying lateral wavelengths, from 600 to 2500 km, and magnitudes (∼10 km peak-to-peak). The source–receiver geometry focuses mainly on the Pdiff, PKP, PcP and ScS phases. The results show that PKP branches, PcP and ScS generally perform well and in a similar fashion, while Pdiff yields unsatisfactory results. We investigate also how 3-D mantle correction influences the output models, and find that despite the disturbance introduced, the models recovered do not appear to be biased, provided that the 3-D model is correct. Using cross-correlated traveltimes, we derive new topography models from both P and S waves. The static corrections used to remove the mantle effect are likely to affect the inversion, compromising the agreement between models derived from P and S data. By modelling traveltime residuals starting from sensitivity kernels, we show how the simultaneous use of volumetric and boundary kernels can reduce the bias coming from mantle structures. The joint inversion approach should be the only reliable method to invert for CMB topography using absolute cross-correlation traveltimes.

Crust and upper mantle structure of the New Madrid Seismic Zone: Insight into intraplate earthquakes

We determine a 3-D P-wave velocity model of the crust and upper mantle down to 400km depth to investigate structural heterogeneity and its influences on the generation of intraplate earthquakes in the New Madrid Seismic Zone. We used 4871 high-quality arrival times from 187 local earthquakes and 30,846 precise travel-time residuals from 1041 teleseismic events recorded by the EarthScope/USArray Transportable Array. Our results show that, beneath the Reelfoot rift, a significant low-velocity (low-V) zone exists in the upper mantle down to 200km depth, with a large volume of 200×200×150km3. The origin of the low-V zone may be related to the passage of the Bermuda hotspot and the stalled ancient Farallon slab materials foundering in the mantle transition zone. This low-V zone may have relatively low shear strength and act as a viscously weak zone embedded in the lithosphere, being apt to concentrate tectonic stress and transfer stress to the seismogenic faults in the upper crust, leading to the large intraplate earthquakes in the New Madrid Seismic Zone.

Crustal and uppermost mantle structure beneath western Tibet using seismic traveltime tomography

AbstractWe study the elastic wave speed structure of the crust and the uppermost mantle in western Tibet using P wave and S wave arrival times from regional earthquakes recorded by a temporary seismic network. We relocate the earthquakes, and subsequently invert travel time residuals for 3‐D distributions of wave speed. Resolution tests with a variety of input structures are used to verify the reliability of our results. The crust beneath western Tibet has low P wave speed (5.9–6.3 km/s) throughout its nearly 80 km thickness, with lower values in this range concentrated within the Lhasa block. Beneath the Himalaya wave speeds are higher. Southern and western limits of the slow material beneath the Tibetan Plateau correlate with the Karakoram fault, and dip beneath the plateau at ∼40° angle. We find no evidence of a subhorizontal low velocity zone in the crust. In the uppermost mantle, we find a long and narrow region of fast (up to 8.4 km/s) P wave speed extending from the Karakoram fault in NE direction, and crossing the Bangong‐Nujiang suture. In a north‐south cross section, the distribution of relatively fast P wave speed suggests a ramp‐flat geometry consistent with India underthrusting the Tibetan Plateau at least as far as 32.5°N. A plausible interpretation of the upper mantle fast feature is the formation of eclogite from the mafic lower‐crustal material of India after it is underthrust beneath Tibet. Notably, in western Tibet this process only takes place in a narrow region.

Slow wavespeeds and fluid overpressure in a region of shallow geodetic locking and slow slip, Hikurangi subduction margin, New Zealand

Travel times recorded onshore from active-source marine seismic surveys were used to determine Hikurangi forearc wavespeeds. Ray-path midpoints sample forearc crust above the shallow (<10–12 km) subduction thrust. The southern region is locked to c. 30 km depth, exhibits slow slip at 30-45 km depth, and is similar to other subduction zones. Our 1D southern model has a rapid increase in seismic velocity (Vp) from the seabed to 4 km depth, consistent with rapidly-deposited clastic sediments, and near-constant Vp=5.0±0.2 km/s at depths of 4–10 km within the forearc. The northern region has slow-slip events at shallow depths of c. 5–15 km, beneath the volume sampled by our analysis. Average travel times at offsets of 20–80 km are >1 s more than at equivalent offsets in the south, and Vp increases with depth within the forearc from 3.5±0.1 km/s at 4 km depth to 4.5±0.2 km/s at 10 km depth. We interpret the low wavespeeds in terms of compaction disequilibrium and compare seismic inferences of anomalously-high porosity (>10%) with hydrostatic reference compaction models to show effective stress is low in both the north (27±10 Mpa) and south (36±14 MPa). In the south, pore space and conduits have a high chance of being localized on faults or sand layers, the wedge is clearly compressionally critical, and we suggest the higher seismic velocities recorded primarily reflect higher effective stress levels transmitted through the rock framework. The observed wedge geometry and our estimate of fluid pressure within the wedge (λ=0.87±0.05) suggest a weak overpressured subduction thrust. In the north, we suggest high fluid pressure is maintained by a large fluid inventory from subducting sediment, a pore-space geometry characterized by pervasively-fractured rock and mudstone, and a lower mean stress due to its stable and non-critical wedge geometry. We consider both end-member assumptions of convergent and tensile failure, the latter of which reveals the possibility of near-lithostatic fluid pressure on the north Hikurangi subduction interface, although this remains unproven. Our observations are consistent with high fluid pressures encountered in petroleum wells at depths <3 km and we observe a clear spatial correlation between the distribution of residual travel-times and the maximum depth of geodetic coupling and slow-slip. Our observations provide evidence for a causative link between high fluid pressure and anomalously-shallow slow slip events and depth of geodetic locking.

P-wave velocity and density structure beneath Mt. Vesuvius: a magma body in the upper edifice?

&lt;p&gt;A high-resolution image of the compressional wave velocity and density structure in the shallow edifice of Mount Vesuvius has been derived from simultaneous inversion of travel times and hypocentral parameters of local earthquakes and from gravity inversion. The robustness of the tomography solution has been improved by adding to the earthquake data a set of land based shots, used for constraining the travel time residuals. The results give a high resolution image of the P-wave velocity structure with details down to 300-500 m. The relocated local seismicity appears to extend down to 5 km depth below the central crater, distributed into two clusters, and separated by an anomalously high Vp region positioned at around 1 km depth. A zone with high Vp/Vs ratio in the upper layers is interpreted as produced by the presence of intense fluid circulation alternatively to the interpretation in terms of a small magma chamber inferred by petrologic studies. In this shallower zone the seismicity has the minimum energy, whilst most of the high-energy quakes (up to Magnitude 3.6) occur in the cluster located at greater depth. The seismicity appears to be located along almost vertical cracks, delimited by a high velocity body located along past intrusive body, corresponding to remnants of Mt. Somma. In this framework a gravity data inversion has been performed to study the shallower part of the volcano. Gravity data have been inverted using a method suitable for the application to scattered data in presence of relevant topography based on a discretization of the investigated medium performed by establishing an approximation of the topography by a triangular mesh. The tomography results, the retrieved density distribution, and the pattern of relocated seismicity exclude the presence of significant shallow magma reservoirs close to the central conduit. These should be located at depth higher than that of the base of the hypocenter volume, as evidenced by previous studies.&lt;/p&gt;

Seismic images of magmatic rifting beneath the western branch of the East African rift

We have performed a tomographic study using a joint data set that includes local and teleseismic events, recorded by a temporary network in the western branch of the East African rift system. From the travel time residuals, we derive a three‐dimensional model of seismic P‐wave velocity anomalies for the crust and upper mantle down to a depth of 80 km. Particular attention is paid to the verification of the inversion results by various resolution tests. The results show that the eastern rift shoulder is characterized by relatively high seismic velocities. Lower velocities are obtained beneath the entire length of the rift valley and the Rwenzori Mountains. A prominent feature is observed north‐east of the mountain range: here we detected a vertically oriented, cylindrical low‐velocity anomaly with maximum amplitudes in the middle crust and the upper mantle lithosphere. We suggest that this anomaly indicates reservoirs of molten material related to the ongoing rifting process within this segment of the rift. Just above this anomaly, at depths between 5 and 16 km, earthquake swarms exist. The observed reduction in P‐wave velocity is used to provide constraints on the possible melt content and temperature anomaly in the uppermost mantle. The observed 3–5% P‐velocity decrease can be explained by melt fraction up to 2%–3.3% or alternatively by a temperature increase of at least 248 to 376 K and even higher‐temperature anomalies are possible if lower ambient temperatures in the reference mantle are assumed. Probably, the two effects act in combination.

Wave equation tomographic velocity inversion method based on the Born/Rytov approximation

This paper discusses Born/Rytov approximation tomographic velocity inversion methods constrained by the Fresnel zone. Calculations of the sensitivity kernel function and traveltime residuals are critical in tomographic velocity inversion. Based on the Born/Rytov approximation of the frequency-domain wave equation, we derive the traveltime sensitivity kernels of the wave equation on the band-limited wave field and simultaneously obtain the traveltime residuals based on the Rytov approximation. In contrast to single-ray tomography, the modified velocity inversion method improves the inversion stability. Tests of the near-surface velocity model and field data prove that the proposed method has higher accuracy and Computational efficiency than ray theory tomography and full waveform inversion methods.

Parameterization of 18th January 2011 earthquake in Dalbadin Region, Southwest Pakistan

An earthquake of magnitude 7.3Mw occurred on 18th January 2011 in Southwestern Pakistan, Baluchistan province (Dalbadin Region). The area has complex tectonics due to interaction of Indian, Eurasian and Arabian plates. Both thrust and strike slip earthquakes are dominant in this region with minor, localized normal faulting events. This earthquake under consideration (Dalbadin Earthquake) posed constraints in depth and focal parameters due to lack of data for evaluation of parameters from Pakistan, Iran or Afghanistan region. Normal faulting mechanism has been proposed by many researchers for this earthquake.In the present study the earthquake was relocated using the technique of travel time residuals. Relocated coordinates and depth were utilized to calculate the focal mechanism solution with outcome of a dominant strike slip mechanism, which is contrary to normal faulting. Relocated coordinates and resulting mechanism are more reliable than many reporting agencies as evaluation in this study is augmented by data from local seismic monitoring network of Pakistan. The tectonics in the area is governed by active subduction along the Makran Subduction Zone. This particular earthquake has strike slip mechanism due to breaking of subducting oceanic plate. This earthquake is located where oceanic lithosphere is subducting along with relative movements between Lut and Helmand blocks. Magnitude of this event i.e. Mw=7.3, re evaluated depth and a previous study of mechanism of earthquake in same region (Shafiq et al., 2011) also supports the strike slip movement.

Hypocentral parameter inversion for regions with poorly known velocity structures

Abstract The determination of accurate hypocentral parameters is crucial in seismic monitoring, and is highly dependent on the accuracy of the implemented velocity model in conventional methods. A method to determine accurate hypocentral parameters based on an approximate velocity model is desirable. We introduce an iterative velocity updating scheme that can be readily combined with conventional hypocentral inversion methods. The algorithm searches for an optimum velocity model in a prescribed velocity range that minimizes the traveltime residuals. The hypocentral parameters are determined using the optimum velocity model. The proposed scheme reduces the dependence on a given velocity model in hypocentral inversion, providing reasonable hypocentral parameters based on an approximate velocity model. The feasibility and accuracy of the algorithm are tested with synthetic and field data. The scheme yields hypocentral parameters that are as accurate as those from full inversion methods but with approximately 70 times lower cost in terms of computational time. The proposed scheme can be readily implemented in any conventional method that is based on a fixed velocity model.

P and SH velocity structure in the upper mantle beneath Northeast China: Evidence for a stagnant slab in hydrous mantle transition zone

Using high-dense regional body waves for three deep earthquakes that occurred around Russia–China border, we investigate both S and P wave velocity structures in the mantle transition zone beneath Northeast China and northern part of North China Craton, where the northwestern Pacific plate is imaged to subhorizontally lie above the 660-km discontinuity. We observe an increasing trend of S–P travel time residuals along the epicentral distance within a distance range of 11–16.5°, indicating a velocity anomaly in MTZ. We seek the simplest model that explains the observed broadband waveforms and relative travel times of triplication for a confined azimuth sector. Both SH and P data suggest a ∼140±20km high velocity layer lying above a slightly depressed and broad 660-km discontinuity. Shear velocity reduction of ∼2.5% in the deeper part of the transition zone is required to compensate for the significantly large relative time between AB and CD triplicate branches and the increased trending of S–P travel time residuals as well. The MTZ, as a whole, is featured by low shear velocity and high Vp/Vs ratio. A water-rich mantle transition zone with 0.2–0.4wt% of H2O may account for the discrepancy between the observed Vp and Vs velocity structures. Our result supports the scenario of a viscosity-dominated stagnant slab with an increased thickness of ∼140km, which was caused by the large viscosity contrast between the lower and upper mantles. The addition of water and eastward trench retreat might facilitate stagnation of the subducting Pacific slab beneath Northeast China.

New constraints on the 3D shear wave velocity structure of the upper mantle underneath Southern Scandinavia revealed from non-linear tomography

We analyse travel times of shear waves, which were recorded at the MAGNUS network, to determine the 3D shear wave velocity ( v S ) structure underneath Southern Scandinavia. The travel time residuals are corrected for the known crustal structure of Southern Norway and weighted to account for data quality and pick uncertainties. The resulting residual pattern of subvertically incident waves is very uniform and simple. It shows delayed arrivals underneath Southern Norway compared to fast arrivals underneath the Oslo Graben and the Baltic Shield. The 3D upper mantle v S structure underneath the station network is determined by performing non-linear travel time tomography. As expected from the residual pattern the resulting tomographic model shows a simple and continuous v S perturbation pattern: a negative v S anomaly is visible underneath Southern Norway relative to the Baltic Shield in the east with a contrast of up to 4% v S and a sharp W–E dipping transition zone. Reconstruction tests reveal besides vertical smearing a good lateral reconstruction of the dipping v S transition zone and suggest that a deep-seated anomaly at 330–410 km depth is real and not an inversion artefact. The upper part of the reduced v S anomaly underneath Southern Norway (down to 250 km depth) might be due to an increase in lithospheric thickness from the Caledonian Southern Scandes in the west towards the Proterozoic Baltic Shield in Sweden in the east. The deeper-seated negative v S anomaly (330–410 km depth) could be caused by a temperature anomaly possibly combined with effects due to fluids or hydrous minerals. The determined simple 3D v S structure underneath Southern Scandinavia indicates that mantle processes might influence and contribute to a Neogene uplift of Southern Norway. ► New 3D upper mantle shear wave velocity ( v S ) model for Southern Scandinavia. ► A low v S anomaly underneath Southern Norway relative to the Baltic Shield. ► Lateral variations in lithospheric thickness as cause of the upper v S anomaly. ► Temperature anomaly beneath Southern Norway as possible cause of a deeper anomaly. ► Mantle processes might influence the Neogene uplift of the Scandes.

Crustal and uppermost mantle structure of Caucasus and surrounding regions

A 3-D P-wave velocity model is developed for the crust and uppermost mantle of Caucasus and the surrounding area by applying the tomographic method of Zhao et al. using 300 000 high-quality P-wave first arrivals from 43 000 events between 1964 and 2005. This tomographic method can accommodate velocity discontinuities such as the Moho in addition to smooth velocity variations. The spatial resolution is 1°×1° in the horizontal direction and 10 km in depth. The velocity images of the upper crust correspond well with the surface geology. Beneath the southern Caucasus high velocity anomalies are found in the middle crust and low velocity anomalies are found in the uppermost mantle. Relatively low Pn velocities are located under the Lesser Caucasus, eastern Turkey, and northern Iran. Higher Pn velocities occur under the eastern portion of the Black Sea and the southern Caspian Sea, and also extend into the eastern edge of Azerbaijan. Tomographic model significantly reduces the travel-time residuals.

Constraints on the crustal structure beneath the Sinai subplate, SE Mediterranean, from analysis of local and regional travel times

The Sinai Peninsula has been recognized as a subplate of the African Plate located at the triple junction of the Gulf of Suez rift, the Dead Sea Transform fault, and the Red Sea rift. The upper and lower crustal structures of this tectonically active, rapidly developing region are yet poorly understood because of many limitations. For this reason, a set of P- and S-wave travel times recorded at 14 seismic stations belonging to the Egyptian National Seismographic Network (ENSN) from 111 local and regional events are analyzed to investigate the crustal structures and the locations of the seismogenic zones beneath central and southern Sinai. Because the velocity model used for routine earthquake location by ENSN is one-dimensional, the travel-time residuals will show lateral heterogeneity of the velocity structures and unmodeled vertical structures. Seismic activity is strong along the eastern and southern borders of the study area but low to moderate along the northern boundary and the Gulf of Suez to the west. The crustal Vp/Vs ratio is 1.74 from shallow (depth ≤ 10 km) earthquakes and 1.76 from deeper (depth > 10 km) crustal events. The majority of the regional and local travel-time residuals are positive relative to the Preliminary Reference Earth Model (PREM), implying that the seismic stations are located above widely distributed, tectonically-induced low-velocity zones. These low-velocity zones are mostly related to the local crustal faults affecting the sedimentary section and the basement complex as well as the rifting processes prevailing in the northern Red Sea region and the ascending of hot mantle materials along crustal fractures. The delineation of these low-velocity zones and the locations of big crustal earthquakes enable the identification of areas prone to intense seismotectonic activities, which should be excluded from major future development projects and large constructions in central and southern Sinai.

Relocating the 2011 Tohoku-oki earthquakes (M 6.0–9.0)

The great Tohoku-oki earthquake (Mw 9.0) on 11 March 2011 was accompanied by nearly 100 large foreshocks and aftershocks (M≥6.0) in the Northeast Japan forearc. In this study we relocated 77 of these large events using three different velocity models. An updated three-dimensional velocity model significantly reduced the travel-time residuals in the earthquake relocation. We found that the large thrust earthquakes are generally located in or around high-velocity patches, suggesting that the generation of the large thrust earthquakes is controlled by structural heterogeneities in the megathrust zone. The high-velocity patches may represent asperities in the megathrust zone where the subducting Pacific plate and the overriding Okhotsk plate may be coupled strongly. Many non-thrust earthquakes occurred in the crust of the overriding Okhotsk plate, indicating that the stress field there has been changed by the Tohoku-oki mainshock. The present results may improve our understanding of the subduction dynamics and seismotectonics of the Northeast Japan arc.

P-wave velocity structure beneath the northern Antarctic Peninsula: evidence of a steeply subducting slab and a deep-rooted low-velocity anomaly beneath the central Bransfield Basin

Upper-mantle structure between 100 and 300 km depth below the northern Antarctic Peninsula is imaged by modelling P-wave traveltime residuals from teleseismic events recorded on the King Sejong Station (KSJ), the Argentinean/Italian stations (JUBA and ESPZ), an IRIS/GSN Station (PMSA) and the Seismic Experiment in Patagonia and Antarctica (SEPA) broad-band stations. For measuring traveltime residuals, we applied a multichannel cross-correlation method and inverted for upper-mantle structure using VanDecar's method. The new 3-D velocity model reveals a subducted slab with a ∼70° dip angle at 100–300 km depth and a strong low-velocity anomaly confined below the SE flank of the central Bransfield Basin. The low velocity is attributed to a thermal anomaly in the mantle that could be as large as 350–560 K and which is associated with high heat flow and volcanism in the central Bransfield Basin. The low-velocity zone imaged below the SE flank of the central Bransfield Basin does not extend under the northern Bransfield Basin, suggesting that the rifting process in that area likely involves different geodynamic processes.