Seismic Tomography Studies Research Articles

The Magma Feeding System of the Klyuchevskaya Group of Volcanoes (Kamchatka)

Using observations from a large-scale seismic station network, we built a seismic tomography model of velocity anomalies for longitudinal (P-wave) and transverse (S-wave) seismic waves beneath the Klyuchevskaya group of volcanoes. The observational network (about 150 km) and the number of seismic stations (about 200) made it possible to determine the velocity structure to depths of 150 km. A low-velocity anomaly is observed almost beneath all volcanoes in the Klyuchevskaya group of volcanoes at a depth of 50 km. Beneath the Shiveluch volcano, in the region where the subduction plate discontinues, an anomaly can be detected to depths of 150 km. Most likely, in this zone, there is a channel for the penetration of magmatic melts to the surface along the boundary of the submerging Pacific Plate ending. This is indirectly confirmed by the presence of a mantle plume to a depth of 1000 km, as previously shown by the seismic tomography studies and an anomalous heat flow at the junction of the Kuril–Kamchatka and Aleutian island arcs.

The influence of fluid type on elastic wave velocity and attenuation in volcanic rocks

Volcano-seismic signatures are the convolution of source and path effects. The source signature provides important information about the dynamic rock deformation and fluid resonance processes within the volcano, but path effects are the result of the volcanic plumbing, the edifice physical conditions, and the complex geological structure. Complicating the inversion of seismic data further, is the influence of fluid properties. Here, we use ultrasonic experiments to understand the effect of variable fluid viscosity and bulk modulus on P-wave attenuation and velocity for volcanic rocks from White Island volcano (Whakaari), New Zealand.Our experimental conditions for high-porosity ash tuff and low-porosity lava resemble shallow and deep volcano facies respectively. P-wave velocity, Vp, and quality factor, Qp, increase with the viscosity and bulk modulus of the saturating fluid and decrease with porosity and fracture volume. Tuffs are highly sensitive to saturating fluid, with variations in Vp of up to 67% and Qp of 383%. Vp and Qp in fractured lavas are slightly less sensitive to fluids (24% and 296% respectively), but Vp can be used to identify lithology for these tuff vs. lavas. Comparison of the experimental data with rock physics models suggests that squirt flow is the dominant attenuation mechanism for viscosities greater than that of water, while scattering losses may be important in gas saturated rocks. Our results have significant implications for the interpretation of seismic tomography studies and for volcano hazard monitoring based on the characteristics of detected seismicity.

High density crustal intrusive bodies beneath Shillong plateau and Indo Burmese Range of northeast India revealed by gravity modeling and earthquake data

Modeling and analysis of gravity and earthquake data of a critically dynamic northeastern region of India assumes a special significance for understanding the seismicity and geodynamics. In this paper, the relation between lateral heterogeneity of gravity interfaces with subsurface structures, and seismic characteristics of northeast India are studied in detail to understand the seismicity and dynamics of the region using Bouguer gravity, earthquake and magnetotelluric data. The modern scaled power spectral analysis of gravity data has brought out crustal lateral heterogeneity at a depth of 8.33 km, 19.73 km and 37.14 km that correlate well with 10 km upper crust, 20 km lower crust and a Moho at 28–42 km revealed by 1D seismic crustal velocity and tomography studies. The inferred crustal depths support thinner crust over uplifted Shillong Plateau and gradual increase of depths over subduction interface of Indo Burmese Range. 2D gravity modeling constrained by seismic velocity model and 3D Euler deconvolution technique along two specific E-W and three N-S regional gravity transects delineated three significant high density anomalous heterogeneous intrusive bodies presumably due to underplating beneath Shillong Plateau, Mikir Massif and Indo Burmese Range. Based on the seismicity and stress pattern analysis through seismic ‘b’-value variation, we infer that the study region exhibits compressive stress concentrations in the form of brittle deformation at shallow depths and weakening of lower crust by high density intrusive (mafic/Ultramafic) bodies of ductile nature which may cause intense seismicity besides the ongoing collision of north drifting Indian plate with Eurasia to the north and the eastern subduction interface of Indo Burmese Range. We also infer that the NE to NW trending V-shaped areas outlined north of Shillong plateau, south of Mikir massif and NE-SW trending thrust Belt of Schuppen exhibit aseismic nature and may be the future impounding seismic sources in this region.

A geophysical study of the Castle Mountain Fault, southcentral Alaska

The Castle Mountain Fault (CMF) lies less than 50 km from Alaska's most populous city of Anchorage and represents an important seismic hazard to the Anchorage-Matanuska-Susitna Valley region in southcentral Alaska. The character of seismicity varies along the CMF. Although the western CMF (between 149.5° and 151° W) shows evidence for Late Pleistocene or Holocene reverse and dextral movement, it has been seismically quiet over the past ~50 years of local seismic network operation. In contrast, the CMF east of 149.5° W is associated with considerable background seismicity, including the Mw 5.81984 strike-slip Sutton earthquake. We have used gravity and magnetic data, constrained with results of previous seismic reflection and seismic tomography studies, to model subsurface structure of the CMF in an effort to determine what may control the variation in its seismic behavior. Our models are consistent with a component of high angle reverse faulting (northwest side up, dip~85°to 87°) that appears to have vertically offset Late Mesozoic rocks 2000 to 2500 m. Lack of data coverage precludes an estimate of its total strike-slip motion. Holocene movement on the CMF appears to be associated with a region where both sides of the fault consist of sedimentary rocks as defined by a gravity and magnetic low. If most of the vertical offset has occurred since the Miocene in response to collision of the Yakutat microplate with North America, then our results are comparable to recent thermochronological and paleoseismic estimates of vertical slip rates. Subduction of the edge of the Yakutat microplate appears to spatially influence seismic behavior along the CMF while the occurrence of the 1964 Great Alaska earthquake has also temporally affected local seismicity.

Mantle flow below the central and greater Alpine region: insights from SKS anisotropy analysis at AlpArray and permanent stations

Abstract. The Alpine chain in western and central Europe is a complex orogen developed as a result of the African–Adriatic plate convergence towards the European continent and the closure of several Tethys oceanic branches. Seismic tomography studies detected high-wave-speed slabs plunging beneath the orogen to variable depths and a potential change in subduction polarity beneath the Central Alps. Alpine subduction is expected to leave a significant imprint on the surrounding mantle fabrics, although deformation associated with the Hercynian Orogeny, which affected Europe prior to the collision with Adria, may have also been preserved in the European lithosphere. Here we estimate SKS anisotropy beneath the central and greater Alpine region at 113 broadband seismic stations from the AlpArray experiment as well as permanent networks from Italy, Switzerland, Austria, Germany, and France. We compare the new improved dataset with previous studies of anisotropy, mantle tomography, lithospheric thickness, and absolute plate motion, and we carry out Fresnel analysis to place constraints on the depth and origin of anisotropy. Most SKS directions parallel the orogen strike and the orientation of the Alpine slabs, rotating clockwise from west to east along the chain, from −45 to 90∘ over a ∼700 km distance. No significant changes are recorded in Central Alps at the location of the putative switch in subduction polarity, although a change in direction variability suggests simple asthenospheric flow or coupled deformation in the Swiss Central Alps transitions into more complex structures beneath the Eastern Alps. SKS fast axes follow the trend of high seismic anomalies across the Alpine Front, far from the present-day boundary, suggesting slabs act as flow barriers to the ambient mantle surrounding them for hundreds of km. Further north across the foreland, SKS fast axes parallel Hercynian geological structures and are orthogonal to the Rhine Graben and crustal extension. However, large splitting delay times (>1.4 s) are incompatible with a purely lithospheric contribution but rather represent asthenospheric flow not related to past deformational events. West of the Rhine Graben, in northeastern France, anisotropy directions are spatially variable in the proximity of a strong positive seismic anomaly in the upper mantle, perhaps perturbing the flow field guided by the nearby Alpine slabs.

Study of passive seismic tomography with various grid by using Matlab

Tomography technique is one method in geosciences adapted from medical field like CT Scan and MRI. In medical field, tomography is used to describe human’s body condition but in geoscience it is used to describe subsurface of the earth, like passive seismic tomography. Subsurface description like velocity data that expressed in tomogram is a result of inversion from travel time and ray tracing data. Tomography is done in various grid model (regular and irregular grid). Irregular grid give a better tomogram result, it is caused by it does not need big data of hypocenter or station. The distribution of grid also showed the good propagation model of ray tracing that influenced the tomogram result.

Joint interpretation of seismic refraction tomography and electrical resistivity tomography by cluster analysis to detect buried cavities

In the last few years, the geophysical methods of seismic refraction tomography (SRT) and electrical resistivity tomography (ERT) are among the most used geophysical techniques for the reconstruction of subsoil geometries, for the investigation of underground cavities and also for the archaeological prospecting. However, the main disadvantage of each geophysical method is the difficulty of final interpretation of the data. In order to eliminate artifacts and generally improve the reliability and accuracy of geophysical interpretation, it is useful to perform a joint approach of different geophysical methods, also introducing the a priori information. In this work, it is shown the integrated study of seismic refraction tomography and electrical resistivity tomography techniques, the two geophysical methods are tested on both synthetic and real data and the integration of data is useful in detecting buried cavities and also evaluate their geometric characteristics. Likelihood parameters has been defined and tested, in order to help recognizing voids from other lithological structures. Finally, a statistical approach based on cluster analysis of the P-wave velocity, the density of the seismic rays and the electrical resistivity of the synthetic and experimental models was used. Multi-space cluster distribution maps were built, allowing to better define and interpret the anomalies of the subsoil.

Seismic Tomography Model for the Crust of Southern Crimea and Adjacent Northern Black Sea

This new seismic tomography study for the crust of southern Crimea and the northeastern Black Sea based on low-magnitude (M ≤ 3) earthquakes indicates significant heterogeneity of the crust at depths of 15–35 km. In the present paper we discuss the velocity distribution for P- and S-waves and give geological and tectonic interpretations in the context of collisional interaction between the Black Sea microplate and the Scythian plate. The joint use of data on both types of waves (P- and S-waves) increases the reliability of the interpretation and allows us to estimate the crustal composition of Mountainous Crimea (MC) and subcrustal mantle of the northeastern Black Sea. We have detected high velocity regions in the MC crust (Vp = 6.5‒6.8 km/s, Vs = 3.7‒3.9 km/s, Vp/Vs = 1.75‒1.9), which have complex configurations and can be deep sources of mid-Mesozoic intrusions known in the MC (Ayu-Dag and Castel). The high-velocity area is separated from eastern Crimea by a linear nearly north–south low-velocity zone located between the cities of Sudak and Feodosiya. The latter is interpreted as a weakened crustal zone associated with the Korsak–Feodosiya fault. The high-velocity region beneath the Black Sea (south of Kerch Peninsula) at depths of 25–40 km most likely belongs to the subcrustal mantle (Vp/Vs = 1.80–1.85) that underlies a thin suboceanic crust of the East Black Basin. The resulting crustal model for southern Crimea and adjacent part of the Black Sea is consistent with the parameters of other geophysical fields.

Integrating Reservoir Dynamics, Crustal Stress, and Geophysical Observations of the Laguna del Maule Magmatic System by FEM Models and Data Assimilation

AbstractThe Laguna del Maule volcanic field is a large rhyolitic magmatic system in the Chilean Andes, which has exhibited frequent eruptions during the past 20 ka. Rapid surface uplift (>20 cm/year) has been observed since 2007 accompanied by localized earthquake swarms and microgravity changes, indicating the inflating magma reservoir may interact with a preexisting weak zone (i.e., Troncoso fault). In this investigation, we model the magma reservoir by data assimilation with Interferometric Synthetic Aperture Radar data. The reservoir geometry is comparable to the magma body inferred by seismic tomography, magnetotelluric, and gravity studies. The models also suggest that a weak zone, which has little effect on surface displacement, is important as a fluid transport channel to promote earthquakes and microgravity changes. In particular, concentrated dilatancy within the weak zone facilitates the microfracture formation during reservoir inflation. High‐pressure fluid can inject into the weak zone from the magma reservoir to trigger earthquakes and further migrate upward to create positive gravity changes by occupying unsaturated storages. The pore pressure will then decrease, halting the seismicity swarm until the next cycle. This “hydrofracturing” process may release some accumulated stress along the magma reservoir delaying an eventual eruption in turn. Besides, the resultant models are propagated forward in time to evaluate potential stress trajectories for future unrest.

Mountain Building in Taiwan: Insights From 3‐D Geodynamic Models

AbstractTaiwan is widely considered to be a typical example of an arc‐continent collision surrounded by two opposite dipping subduction zones. The manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood. Various hypotheses have been proposed, but the geodynamic feasibility of those remains to be tested. Here we present 3‐D thermomechanical models to study the geodynamic evolution process of a Taiwan‐like setting after an initial transform fault was consumed. In our model setup, the boundary between the Eurasian plate and the South China Sea is northeast trending. The results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small‐scale mountain belt with high topography and crustal exhumation occurs in most cases. The Eurasian continental crust is exhumed in a dome‐like manner exposing higher‐grade metamorphic rocks, facilitated by high erosion rates and a weak continental lower crust rheology, but inhibited by the presence of a weak arc. A high topography within the orogen, as well as continental slab detachment, can develop for the convergence direction of N307° and large convergence rates. Our modeling results are thus generally consistent with the Eurasian slab‐tearing model proposed for Taiwan based on seismic tomographic studies, and we suggest that the main characteristic features in Taiwan can be explained by the combined effects of fast erosion, a weak lower crust, fast convergence, and a small convergence azimuth.

Noble Gas and Carbon Isotope Systematics at the Seemingly Inactive Ciomadul Volcano (Eastern‐Central Europe, Romania): Evidence for Volcanic Degassing

AbstractCiomadul is the youngest volcano in the Carpathian‐Pannonian Region, Eastern‐Central Europe, which last erupted 30 ka. This volcano is considered to be inactive, however, combined evidence from petrologic and magnetotelluric data, as well as seismic tomography studies, suggests the existence of a subvolcanic crystal mush with variable melt content. The volcanic area is characterized by high CO2 gas output rate, with a minimum of 8.7 × 103 t/year. We investigated 31 gas emissions at Ciomadul to constrain the origin of the volatiles. The δ13C–CO2 and 3He/4He compositions suggest the outgassing of a significant component of mantle‐derived fluids. The He isotope signature in the outgassing fluids (up to 3.10 Ra) is lower than the values in the peridotite xenoliths of the nearby alkaline basalt volcanic field (R/Ra 5.95 Ra ± 0.01), which are representative of a continental lithospheric mantle and significantly lower than MORB values. Considering the chemical characteristics of the Ciomadul dacite, including trace element and Sr–Nd and O isotope compositions, an upper crustal contamination is less probable, whereas the primary magmas could have been derived from an enriched mantle source. The low He isotopic ratios could indicate a strongly metasomatized mantle lithosphere. This could be due to infiltration of subduction‐related fluids and postmetasomatic ingrowth of radiogenic He. The metasomatic fluids are inferred to have contained subducted carbonate material resulting in a heavier carbon isotope composition (δ13C is in the range of −1.4‰ to −4.6‰) and an increase of CO2/3He ratio. Our study shows the magmatic contribution to the emitted gases.

Detailed seismic imaging of Merapi volcano, Indonesia, from local earthquake travel-time tomography

Mt. Merapi, located in central Java, Indonesia, is one of the most active volcanoes in the world. It has been subjected to numerous studies using a variety of methods, including tomographic imaging, in an attempt to understand the structure and dynamics of its magmatic plumbing system. Results of previous seismic tomographic studies that include Mt. Merapi poorly constrain the location of its underlying magma source due to limited data coverage. In order to comprehensively understand the internal structure and magmatism of Mt. Merapi, a project called DOMERAPI was conducted, in which 53 broadband seismic stations were deployed around Mt. Merapi and its neighbourhood for approximately 18 months, from October 2013 to April 2015. In this study, we compare Vp, Vs, and Vp/Vs tomograms constructed using data obtained from local (DOMERAPI) and regional seismic networks with those obtained without DOMERAPI data. We demonstrate that the data from the DOMERAPI seismic network are crucial for resolving key features beneath the volcano, such as high Vp/Vs ratios beneath the Merapi summit at ∼5 km and ∼15 km depths, which we interpret as shallow and intermediate magma bodies, respectively. Furthermore, west-east vertical sections across Mt. Merapi, and a “dormant” (less active) volcano, Mt. Merbabu, exhibit high Vp/Vs and low Vp/Vs ratios, respectively, directly beneath their summits. This observation likely reflects the presence (for Mt. Merapi) and absence (for Mt. Merbabu) of shallow magma bodies near the surface.

Seismic imaging of Santorini: Subsurface constraints on caldera collapse and present-day magma recharge

Volcanic calderas are surface depressions formed by roof collapse following evacuation of magma from an underlying reservoir. The mechanisms of caldera formation are debated and predict differences in the evolution of the caldera floor and distinct styles of magma recharge. Here we use a dense, active source, seismic tomography study to reveal the sub-surface physical properties of the Santorini caldera in order to understand caldera formation. We find a ∼3-km-wide, cylindrical low-velocity anomaly in the upper 3 km beneath the north-central portion of the caldera, that lies directly above the pressure source of the 2011-2012 inflation. We interpret this anomaly as a low-density volume caused by excess porosities of between 4% and 28%, with pore spaces filled with hot seawater. Vents that were formed during the first three phases of the 3.6 ka Late Bronze Age (LBA) eruption are located close to the edge of the imaged structure. The correlation between older volcanic vents and the low-velocity anomaly suggests that this feature may be long-lived. We infer that collapse of a limited area of the caldera floor resulted in a high-porosity, low-density cylindrical volume, which formed by either chaotic collapse along reverse faults, wholesale subsidence and infilling with tuffs and ignimbrites, phreatomagmatic fracturing, or a combination of these processes. Phase 4 eruptive vents are located along the margins of the topographic caldera and the velocity structure indicates that coherent down-drop of the wider topographic caldera followed the more limited collapse in the northern caldera. This progressive collapse sequence is consistent with models for multi-stage formation of nested calderas along conjugate reverse and normal faults. The upper crustal density differences inferred from the seismic velocity model predict differences in subsurface gravitational loading that correlate with the location of 2011-2012 edifice inflation. This result supports the hypothesis that sub-surface density anomalies may influence present-day magma recharge events. We postulate that past collapses and the resulting topographical and density variations at Santorini influence magma focusing between eruptive cycles, a feedback process that may be important in other volcanoes.

3-D Seismic Tomographic study of Sinabung Volcano, Northern Sumatra, Indonesia, during the inter-eruptive period October 2010–July 2013

We estimated 3D P-wave velocity (Vp), S-wave velocity (Vs), and Vp/Vs tomographic structures in and around the Sinabung Volcano area, Northern Sumatra, Indonesia during the intereruptive period between October 2010 and July 2013 using high-quality P- and S-wave phases from >600 local volcano tectonic (VT) earthquakes and >5000 phase picks. We used the iterative damped-least-squares method of SIMULPS12 to simultaneously invert for the velocity structure and hypocenter relocations. We find that the majority of the VT earthquakes locate beneath Sinabung volcano to depths of about 8 km, with distal hypocenters dominantly to the north and northwest of the summit. We find six anomalous regions with the most notable including an area of low seismicity, high Vp/Vs and low Vp at depths of around 5 km slightly south of the summit. We believe this anomaly most likely represents hot material with some partial melt that may be the source for the 2010 phreatic eruptions and continued unrest throughout the study period. We also find a region below the summit with high seismicity, high Vp, high Vs and low Vp/Vs that we believe likely represents old intrusive material that has been fractured by the processes of the 2010 phreatic eruptions. Other shallow anomalous values of Vp, Vs and Vp/Vs are consistent with the known geology of Sinabung: anomalies of high Vp and Vs likely represent old, well-consolidated rock, and are observed to the SW and NE of Sinabung; while low Vp, low Vs and high Vp/Vs likely represent increased fracture densities, temperatures, or gas contents in geothermal features to the west of the summit. Finally we compare our results to those of the tomographic study of the October to November 2013 eruptive period in Nugraha et al. (2019) and find evidence that suggests magma rose from the lower region imaged in our study to within a few kilometers of the surface.

Velocity structure of the earthquake zone of the M6.3 Yogyakarta earthquake 2006 from a seismic tomography study

Velocity structure of the earthquake zone of the M6.3 Yogyakarta earthquake 2006 from a seismic tomography study

Seismic velocity structure of the Jakarta Basin, Indonesia, using trans-dimensional Bayesian inversion of horizontal-to-vertical spectral ratios

Characterizing the interior structure of the Jakarta Basin, Indonesia, is important for the improvement of seismic hazard assessment there. A dense-portable seismic broad-band network, comprising 96 stations, has been operated between October 2013 and February 2014 covering the city of Jakarta. The seismic network sampled broad-band seismic noise mostly originating from ocean waves and anthropogenic activity. We used horizontal-to-vertical spectral ratio (HVSR) measurements of the ambient seismic noise to estimate fundamental-mode Rayleigh wave ellipticity curves, which were used to infer the seismic velocity structure of the Jakarta Basin. By mapping and modelling the spatial variation of low-frequency (0.124–0.249 Hz) HVSR peaks, this study reveals variations in the depth to the Miocene basement. These variations include a sudden change of basement depth from 500 to 1000 m along N–S profile through the centre of the city, with an otherwise gentle increase in basin depth from south to north. Higher frequency (2–4 Hz) HVSR peaks appear to reflect complicated structure in the top 100 m of the soil profile, possibly related to the sediment compaction and transitions among different sedimentary sequences. In order to map these velocity profiles of unknown complexity, we employ a trans-dimensional Bayesian framework for the inversion of HVSR curves for 1-D profiles of velocity and density beneath each station. Results show that very low-velocity sediments (<240 m s−1) up to 100 m in depth cover the city in the northern to central part, where alluvial fan material is deposited. These low seismic velocities and the very thick sediments in the Jakarta Basin will potentially contribute to seismic amplification and basin resonance, especially during giant megathrust earthquakes or large earthquakes with epicentres close to Jakarta. Results have shown good correlation with previous ambient seismic noise tomography and microtremor studies. We use the 1-D profiles to create a pseudo-3-D model of the basin structure which can be used for earthquake hazard analyses of Jakarta, a megacity in which highly variable construction practices may give rise to high vulnerability. The methodology discussed can be applied to any other populated city situated in a thick sedimentary basin.

Electrical resistivity cross-section across northern part of Saurashtra region: An insight to crystallized magma and fluids

Saurashtra region forms the Western part of the Deccan volcanic province (DVP) in India and is bounded by the Cambay rift basin (CRB) in the east, Narmada rift basin (NRB) in the South and Kachchh rift basin (KRB) in the north. This region is uplifted due to different stages of inter-continental breakup (Madagascar-India & Seychelles-India), different stages of rifting, reactivation of fault zones and Deccan volcanism. The major portion of Saurashtra region is covered by Deccan basalts underlain by Mesozoic sediments. Detailed magnetotelluric (MT) and long period magnetotelluric (LMT) soundings have been carried out in the northern part of Saurashtra with an aim to bring out the geoelectrical configuration of different basins (Jamnagar, Jasdan, and Cambay) along the profile. 2D inversion carried out for different modes: transverse electric (TE), transverse magnetic mode (TM) and TE + TM independently. The inversion of different modes shows distinct electrical signatures. Due to the 3D nature of the data, we have considered the TM mode of interpretation because it is less prone to 3D distortions. The final model represents large-scale heterogeneities in the crust due to the presence of different resistive and conductive blocks. High conductive lithosphere denotes the thermal influx of the mantle material to the base of the crust. Exchange of heat with the lower crust during basaltic magmatic underplating released carbonate fluids and/or dehydration of minerals resulting in high conductivity anomalies. A prominent conductive feature on the east side of the profile at upper crustal depths may be due to partial melts associated with mafic/ultra-mafic intrusions related to the Reunion plume activity. Different resistive blocks in conjunction with seismic tomography studies represent recrystallized older Precambrian crust that forms the uplifted blocks of Jamnagar, Jasdan and western part of Cambay basins.

Geophysical investigation of dewatering in Lolo Creek, Southwest Missoula, Montana, USA

Interest in finding the causes of reoccurring late summer dewatering of Lolo Creek, Montana, led to geophysical and hydrogeologic studies to understand the cause and effect relationship between surface water uses, groundwater uses and stream flow, critical to water management decisions. Electrical Resistivity Tomography (ERT), Spontaneous Potential (SP), Multichannel Analysis of Surface Waves (MASW) and Seismic Refraction Tomography methods were used to provide information on potential seepage pathways leading to dewatering of Lolo Creek and the subsurface lithology of the area. Results from the MASW and seismic refraction tomography studies showed the bedrock at depths of about 35 m close to Highway 93 and 27 m at the end of the 288 m East-West seismic line running through Lewis and Clark Drive. ERT results showed a shallow, high- resistive layer underlain by low resistive layers. The Eocene bedrock (mylonite) was found to have a resistivity between 180 and 400 Ωm. A geologic fault was inferred in a 360 m East-West ERT profile. SP measurements around the inferred fault indicate a zone of infiltration, with SP values between −18 mV to −2 mV. A 1D-depth velocity profile obtained from MASW survey located close to East-West ERT profile, indicates a low velocity subsurface between 0 and 39 m, which is interpreted to be fractured bedrock. This fractured, shallow bedrock and associated fault work as a seepage path from the shallow to deep aquifer and could possibly account for dewatering of the Creek during low-flow periods in the late summer.

Mantle Transition Zone Thickness Beneath the Middle East: Evidence for Segmented Tethyan Slabs, Delaminated Lithosphere, and Lower Mantle Upwelling

AbstractWe use a very large seismic data set to provide a comprehensive image of the mantle transition zone (MTZ) beneath the Middle East. We utilized the technique of Common Conversion Point stacking of P wave receiver functions to investigate the topography on the 410‐ and 660‐km discontinuities defining the upper and lower boundaries of the MTZ. Our results show significant topography on the 410‐ and 660‐km discontinuities and corresponding variations in the MTZ thickness. The MTZ topography is broadly consistent with the results of seismic tomography studies, implying the presence of both cold thermal anomalies imparted by detached Tethyan slabs and lithospheric segments and hot thermal anomalies induced by upwelling of lower mantle material. The MTZ topography in the northern Middle East is dominated by the presence of patches of cold material that are intermittently separated by regions of hot to normal MTZ. Our results suggest that instead of a continuous slab, the Tethyan slab in the Middle East is strongly segmented along the strike of the subduction boundary. Furthermore, we find evidence for a significant gap in subduction extending from the eastern edge of the Cyprean arc to NW Iran. The southern Middle East is dominated by the processes related to the mantle upwelling beneath the Afar depression. Our results imply that buoyant lower mantle material enters the MTZ beneath the Afar depression and then spreads laterally to the northeast beneath the western Arabia, flowing within the MTZ and in the upper mantle.

Lithospheric layering beneath the contiguous United States constrained by S-to-P receiver functions

To image upper mantle seismic discontinuities beneath the contiguous United States, a total of 284,121 S-to-P receiver functions (SRFs) recorded by 3,594 broadband seismic stations in the EarthScope Transportable Array and other permanent and temporary deployments are stacked in circular bins of 2° in radius. A robust negative arrival, representing a sharp discontinuity of wave speed reduction with depth, is visible in virtually all the stacked traces in the depth range of 30–110 km. Beneath the western U.S., the mean depth of this discontinuity is 69±17 km, and beneath the eastern U.S., it is 76±5 km, both are comparable to the depth of the tomographically-determined lithosphere–asthenosphere boundary (LAB). In contrast, the depth of the discontinuity beneath the stable cratonic region of the central U.S. is 87±6 km, which is significantly shallower than the ∼250 km LAB depth determined by seismic tomography. Based on the amplitude of the corresponding arrival in the SRFs and findings from previous seismic tomography and mantle xenolith studies, this discontinuity beneath the central U.S. is interpreted as the top of an intra-lithospheric low wave speed, probably phlogopite-rich layer. The observations provide new constraints on a number of regional scale tectonic processes, such as lithospheric stretching in the Texas–Louisiana Gulf Coastal Plain and the Basin and Range Province, and possible lithospheric basal erosion beneath the northeastern U.S.