Geophysical Journal International

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High-resolution palaeomagnetic record from Sea of Marmara sediments for the last 70 ka

Fri, 06/05/2020 - 00:00
SummaryMagnetostratigraphic and geochemical analyses were performed on two sediment cores recovered from the Sea of Marmara to investigate geomagnetic field variations over the last 70 ka. A chronology for each of the two cores was developed from eight AMS 14C datings, tephrochronology, and tuning of Ca concentrations with stadials and interstadials observed in Greenland ice core oxygen isotope data. Based on the age models, cores MD01–2430 and MRS-CS19 reach back to 70 ka and 32 ka, respectively. High average sedimentation rates of 43 cm/kyr for core MD01–2430 and 68 cm/kyr for core MRS-CS19 allow high-resolution reconstruction of geomagnetic field variations for the Sea of Marmara. Mineral magnetic properties are sensitive to glacioeustatic sea-level changes and palaeoclimate variations in this region, reflecting the variable palaeoenvironmental conditions of the Sea of Marmara during last 70 ka. Despite the impairment of the palaeomagnetic record in some stratigraphic intervals due to early diagenesis, relative palaeointensity variations in the Sea of Marmara sediments correlate well with similar records derived from other regions, such as the nearby Black Sea and the GLOPIS-75 stack. The directional record derived from the Sea of Marmara cores exhibits typical palaeosecular variation patterns, with directional anomalies at 41 ka and 18 ka, representing the Laschamps and postulated Hilina Pali excursions, respectively. Both directional anomalies are also associated with palaeointensity minima. A further palaeointensity minimum at 34.5 ka is likely related to the Mono Lake excursion, with no directional deviation documented in the Sea of Marmara palaeomagnetic record so far.

Application of rock physics modeling to investigate the differences between static and dynamic elastic moduli of carbonates

Thu, 06/04/2020 - 00:00
SummaryThe elastic moduli estimated through geophysical studies carried out in wells (logging data) differ from those obtained from the triaxial tests conducted in laboratory on the available core samples. Terminologically former and latter are referred to as dynamic and static elastic moduli respectively. Since the structural characteristics of rocks at the different scales, from micrometer to larger scales (tens of meter), are the controlling parameters of their dynamic and static moduli and their difference at the respective scale, in the present study we aim to investigate the influence of the measurable (or quantifiable) parameters of the pore space on these elastic moduli. To do so, 19 dry carbonate samples of different structural characteristics were collected. Their basic petrophysical properties such as porosity and permeability were measured in laboratory. The Ultra-Sonic Tomography was carried out to determine the heterogeneity degree, anisotropy system, and average acoustic wave velocities for each core sample. SEM images were analyzed to investigate the visual textural properties. The mineralogical composition of these samples was determined by the X-ray diffraction method. Based on the conducted experimental studies and using of the Effective Medium Theory, a unique rock physics model (‘petroelastic model’) was constructed for each core sample. The average (effective) microstructural parameters characterizing the pore space of the studied carbonate samples, along with their elastic moduli were estimated through solving the inverse problem and the measured acoustic wave velocities. A multistage statistical approach, including computation of correlation coefficients, optimized regression analysis, factor analysis and bootstrap resampling, was suggested to investigate the effect of each microstructural parameters on the static and dynamic Young's moduli, ratio of dynamic to static Young's moduli (k-value), dynamic Poisson's ratio, and mechanical properties (including unconfined compressive strength and internal friction angle). The obtained results show that the microstructural characteristics have different degrees of influence on the elastic moduli and can be successfully classified based on their physical nature. It was also concluded that the dynamic Poisson's ratio is independent of the studied, in this work, microstructural parameters.

Application of high-rate GPS for earthquake rapid response and modeling: A case in the 2019 Mw 7.1 Ridgecrest earthquake

Thu, 06/04/2020 - 00:00
SummaryThe 2019 Mw 7.1 Ridgecrest earthquake opens an opportunity to investigate how soon we can produce a reliable fault geometry and subsequently a robust source model based on high-rate Global Positioning System (GPS) data. In this study, we conduct peak ground displacement (PGD) magnitude scaling, real-time centroid moment tensor (CMT) calculation, and rapid kinematic slip inversion. We conclude that a four-station PGD warning with a magnitude of Mw 7.03 can be issued at 24 s after initiation of rupture. Fast CMT inversion can initially recover the correct nodal planes at 30 s. The kinematic slip model reveals the Mw 7.1 earthquake is a predominant dextral strike-slip event with both normal and thrust components resolved. The earthquake shows bilateral rupture with a low propagation speed of ∼2.1 km/s and a slip maxima of ∼4 m. The total moment is 5.18 × 1019 N·m (Mw 7.11). We further suggest that a reasonable source model will be available in a simulated real-time mode within 30 s after the earthquake occurring, without using full high-rate GPS waveforms. This research highlights the significance of high-rate GPS for rapid earthquake response and modeling of kinematic rupture, which is also generalized by the hypothetical real-time GPS analysis for the 2016 Mw 7.8 Kaikoura earthquake and the 2010 Mw 7.2 El Mayor-Cucapah earthquake.

Inverting magnetotelluric data with distortion correction – Stability, uniqueness and trade-off with model structure

Thu, 06/04/2020 - 00:00
SummaryGalvanic distortion of magnetotelluric data is a common effect that can impede the reliable imaging of subsurface structures. Recently, we presented an inversion approach that includes a mathematical description of the effect of galvanic distortion as inversion parameters and demonstrated its efficiency with real data. We now systematically investigate the stability of this inversion approach with respect to different inversion strategies, starting models and model parametrizations. We utilize a dataset of 310 magnetotelluric sites that were acquired for geothermal exploration. In addition to impedance tensor estimates over a broad frequency range, the dataset also comprises transient electromagnetic measurements to determine near surface conductivity and estimates of distortion at each site. We therefore can compare our inversion approach to these distortion estimates and the resulting inversion models. Our experiments show that inversion with distortion correction produces stable results for various inversion strategies and for different starting models. Compared to inversions without distortion correction, we can reproduce the observed data better and reduce subsurface artefacts. In contrast, shifting the impedance curves at high frequencies to match the transient electromagnetic measurements reduces the misfit of the starting model, but does not have a strong impact on the final results. Thus our results suggest that including a description of distortion in the inversion is more efficient and should become a standard approach for magnetotelluric inversion.

Field surveys and numerical modeling of the 26 December 2004 Indian Ocean tsunami in the area of Mumbai, west coast of India

Thu, 06/04/2020 - 00:00
SummaryIn the aftermath of the 2004 Indian Ocean (Sumatra) tsunami, numerous survey teams investigated its effects on various locations across the Indian Ocean (IO). However, these efforts were focused only on locations that experienced major destruction and a high death toll. As a consequence, some Indian Ocean coastal megacities were not examined. Among the cities not surveyed was Mumbai, a principal port and economical capital of India on the west coast of India with a population of more than 12 million. Mumbai is at risk of tsunamis from two major subduction zones in the Indian Ocean: the Sumatra-Andaman subduction zone (SASZ) and the Makran subduction zone (MSZ). As a part of the present study, we conducted a field survey of the 2004 IO tsunami effects in Mumbai, analysed the available tide gauge records and performed tsunami simulations. Our field survey in January 2018 found runup heights of 1.6 − 3.3 m in the Mumbai area. According to our analysis of tide gauge data, tsunami trough-to-crest heights in Okha (550 km to the north of Mumbai) and in Mormugao (410 km to the south of Mumbai) were 46 and 108 cm, respectively. Simulations of a hypothetical MSZ Mw 9.0 earthquake and tsunami, together with the Mw 9.1 Sumatra-Andaman earthquake and tsunami, show that the tsunami heights generated in Mumbai by a MSZ tsunami would be approximately 11 times larger than those generated by the 2004 Sumatra-Andaman tsunami. This result indicates that future tsunami hazard mitigation for Mumbai needs to be based on a potential large MSZ earthquake rather than a SASZ earthquake.

The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption

Wed, 06/03/2020 - 00:00
SummaryOn May 10th, 2018, an unprecedented long and intense seismic crisis started offshore, east of Mayotte, the easternmost of the Comoros volcanic islands. The population felt hundreds of events. Over the course of one year, 32 earthquakes with magnitude greater than 5 occurred, including the largest event ever recorded in the Comoros (Mw = 5.9 on May 15th, 2018). Earthquakes are clustered in space and time. Unusual intense long lasting monochromatic very long period events were also registered. From early July 2018, Global Navigation Satellite System stations and Interferometric Synthetic Aperture Radar registered a large drift, testimony of a large offshore deflation. We describe the onset and the evolution of a large magmatic event thanks to the analysis of the seismicity from the initiation of the crisis through its first year, compared to the ground deformation observation (GNSS and InSAR) and modelling. We discriminate and characterise the initial fracturing phase, the phase of magma intrusion and dike propagation from depth to the sub-surface, and the eruptive phase that starts on July 3rd, 2018, around fifty days after the first seismic events. The eruption is not terminated two years after its initiation, with the persistence of an unusual seismicity, whose pattern has been similar since summer 2018, including episodic very low frequency events presenting a harmonic oscillation with a period of ∼16 s. From July 2018, the whole Mayotte Island drifted eastward and downward at a slightly increasing rate until reaching a peak in late 2018. At the apex, the mean deformation rate was 224 mm yr−1 eastward and 186 mm yr−1 downward. During 2019, the deformation smoothly decreased and in January 2020, it was less than 20 per cent of its peak value. A deflation model of a magma reservoir buried in a homogenous half space fits well the data. The modelled reservoir is located 45 ± 5 km east of Mayotte, at a depth of 28 ± 3 km and the inferred magma extraction at the apex was ∼94 m3 s−1. The introduction of a small secondary source located beneath Mayotte Island at the same depth as the main one improves the fit by 20 per cent. While the rate of the main source drops by a factor of 5 during 2019, the rate of the secondary source remains stable. This might be a clue of the occurrence of relaxation at depth that may continue for some time after the end of the eruption. According to our model, the total volume extracted from the deep reservoir was ∼2.65 km3 in January 2020. This is the largest offshore volcanic event ever quantitatively documented. This seismo-volcanic crisis is consistent with the trans-tensional regime along Comoros archipelago.

Digital rock physics in four dimensions: Simulating cementation and its effect on seismic velocity

Wed, 06/03/2020 - 00:00
SummaryPorosity exerts a strong control on the mechanical and hydraulic properties of rocks, but can often only be imaged indirectly from the surface using geophysical measurements, such as seismic velocity. Understanding and quantifying the relationship between seismic velocity and porosity is therefore a fundamental goal of many rock physics models. Simulating the geological processes that control porosity to generate digital rocks, and numerically modelling wave propagation to estimate their elastic properties, allows for flexible and rapid calibration of velocity-porosity trends. Here, the initial deposition of two digital carbonate sediments are simulated: grainstone (near spherical grains) and coquina (anisotropic shell fragments). The gradual precipitation of cement is then simulated, resulting in a suite of 3D volumes of varying porosity with otherwise constant and known mineral and grain phases. These models are then used as input to a 3D acoustic staggered-grid finite difference simulation of wavefield propagation, from which we estimate bulk seismic velocity and calculate the estimated bulk modulus. The resulting bulk modulus varies systematically with respect to porosity within the physical limits imposed by the Hashin-Shtrikman bounds. The samples exhibit anisotropy in the measured velocity consistent with structural anisotropy due to the settling of elongate grains under gravity. We use the resulting bulk velocity-porosity trends to test competing rock physics models, including one that accounts for varying effective pore-aspect ratio with porosity. The results validate the hypothesis that there is a power-law relationship between effective pore aspect ratio and porosity. This relationship is consistent with similar results obtained from a suite of natural carbonate grainstones examined in the laboratory. The results show the optimal rock physics model to be relatively insensitive to the degree of anisotropy in the fabric of the starting material, and may now be used with more confidence to link observed changes in effective pore aspect ratio to changes in porosity due to a range of geological processes, for example fracturing, dissolution and compaction, where other process-based models are available.

The Trinidad and Tobago Microzonation Project: Port of Spain

Wed, 06/03/2020 - 00:00
SummaryIn the present study, we present the results from the microzonation study conducted in Port of Spain, capital of the Republic of Trinidad & Tobago. A dense grid of single-site recordings was used to determine the fundamental frequency of soil above bedrock, while a grid of 26 array recordings comprised the database for finding the 1-D shear wave velocity, with depth. The resonant frequency was found to range from < 1.0 Hz, for the deeper sediments to the south, near the coast, to above 4.0 Hz, on the northern outskirts of the city, closer to the rock formations. The array data processing revealed a shear wave velocity less than 360 m/s, for the alluvial deposits, whilst for the harder formations, the velocity was at least 1000 m/s. To validate the results, a parametric investigation, using synthetic seismograms of ambient noise for simplified 1-D models of the Port of Spain basin sediments, was conducted. A 3-D geological model of the basin was developed, by integrating the experimental results with the simulated data. The model suggests a gradual increase, from north to south, in sediment depth down to ∼160 m. In order to understand and explain the variation of the resonance frequency, a review of the historical development of the area, for the past 250 years, revealed large-scale, non-engineered land reclamation in the 19th and 20th Centuries, resulting in areas with anomalously high amplification of seismic motion.

Analysis of the June 2017 Maoxian Landslide Processes with Force Histories from Seismological Inversion and Terrain Features

Tue, 06/02/2020 - 00:00
SummaryA devastating landslide occurred in Maoxian (China) on June 24, 2017, which generated strong signals that were recorded by a regional seismic network. We determined the landslide force history from long–period seismic waves and identified eight subevents. For each subevent, we obtained an independent force history and calculated its sliding path. The shape of the terrain before and after the landslide was found to play a critical role in the motion of the sliding mass. A combination of seismic and terrain data was used to discriminate between or relate the subevents to each other, and to locate the initiation point of each sliding path. We explain the Maoxian landslide dynamics as the combination of the rock collapse, centripetal acceleration of the sliding body, deceleration and acceleration once again after overcoming obstacles along the sliding path.

3D joint geodetic and strong motion finite fault inversion of the May 12th, 2008 Wenchuan, China Earthquake

Tue, 06/02/2020 - 00:00
SummaryWe present a source inversion of the 2008 Wenchuan, China earthquake, using strong-motion waveforms and geodetic offsets together with three-dimensional (3D) synthetic ground motions. We applied the linear multiple time window technique considering geodetic and dynamic Green's functions computed with the finite element method and the reciprocity and Strain Green's Tensor formalism. All ground motion estimates, valid up to 1 Hz, accounted for three-dimensional effects, including the topography and the geometry of the Beichuan and Pengguan faults. Our joint inversion has a higher moment (M0) than a purely geodetic inversion and the slip distribution presents differences when compared to one-dimensional model source inversions. The moment is estimated to be M0 = 1.2 × 1021 Nm, slightly larger than other works. Our results show that considering a complex 3D structure reduces the size of large areas of 10 m slip or greater by distributing it in wider zones, with reduced slips, in the central portion of the Beichuan and the Pengguan faults. Finally, we compare our source with a relocated aftershock catalog and conclude that the 4–5 m slip contours approximately bound the absence or presence of aftershocks.

Using multi-component ambient seismic noise cross-correlations to identify higher mode rayleigh waves and improve dispersion measurements

Mon, 06/01/2020 - 00:00
SummaryAmbient seismic noise cross-correlation with three-component sensors yields a nine-component empirical Green's tensor, in which four components of the radial-vertical plane contain Rayleigh waves. We exploit the retrograde elliptical nature of particle motion of the fundamental mode Rayleigh wave to correct the phase of the four radial-vertical components and stack them to obtain an average fundamental mode Rayleigh-wave time-series. This technique can suppress incoherent noise and wave packets that do not follow the targeted elliptical particle motion. The same technique can be used to isolate the first higher mode Rayleigh wave that follows prograde elliptical particle motion. We first demonstrate the effectiveness of the method on synthetic waveforms and then apply it on noise cross-correlations computed in Central California. Using this method, we isolate 1st higher mode Rayleigh waves on noise cross-correlations in the Great Valley, California, which provides new phase velocity constraints for estimating velocity structure in the sedimentary basin. We also obtain improved estimates of fundamental mode Rayleigh wave dispersion for surface-wave tomography. The waveforms stacked assuming retrograde particle motion return at least ∼20 per cent more group velocity dispersion measurements satisfying a minimum signal-to-noise ratio criteria than the individual components for periods ∼4–18 s. For equivalent group velocity measurements, signal-to-noise ratio for the stacked estimate of the fundamental mode Rayleigh wave is on average 40 per cent greater than that measured on the individual components at periods less than 10 s. The technique also provides an easy way to detect large errors in sensor orientation.

Deformation of a spherical, viscoelastic, and incompressible Earth for a point load with periodic time change

Mon, 06/01/2020 - 00:00
SummaryPlanetary-scale mass redistributions occur on Earth for certain spatiotemporal periods, and these surface mass changes excite the global periodic loading deformations of a viscoelastic Earth. However, the characteristics of periodic viscoelastic deformations have not been well investigated even in a simple Earth model. In this study, we derive the semi-analytical Green's functions (fully-analytical Love numbers) for long-standing point sources with given periods using a modified asymptotic scheme in a homogeneous Maxwell spherical Earth model. Here, the asymptotic scheme is needed in order to obtain accurate semi-analytical time-dependent Green's functions. The amplitudes and phases of the Green's functions may be biased if only the series summations of the Love numbers are used because the influence of viscoelasticity is degree-dependent. We compare the viscoelastic and elastic periodic Green's functions with different material viscosities and loading periods and investigate the amplitude increase percentage and phase delay of the periodic displacement and geoid change. For example, our analysis revealed that the viscosity increases the amplitude by 40 per cent–120 per cent and delays the phase approximately -100°–60° for the displacement and geoid change when bearing a 10-year loading period, assuming a viscosity of 1018 Pa·s and a shear modulus 4 × 1010 Pa.

Magnetotelluric imaging of the Mérida Andes and surrounding areas in Venezuela

Mon, 06/01/2020 - 00:00
SummaryThe Caribbean and South American tectonic plates bound the north-eastwards expulsion of the North Andean Block in western Venezuela. This complex geodynamic setting resulted in the formation of major strike-slip fault systems and sizeable mountain chains. The 100 km wide Mérida Andes extend from the Colombian/Venezuelan border to the Caribbean coast. To the north and south, the Mérida Andes are bound by hydrocarbon-rich sedimentary basins. Knowledge of lithospheric structures, related to the formation of the Mérida Andes, is limited though, due to a lack of deep geophysical data. In this study, we present results of the first broadband magnetotelluric profile crossing the Mérida Andes and the Maracaibo and Barinas - Apure foreland basins on a length of 240 km. Geoelectrical strike and dimensionality analysis are consistent with one- or two- dimensional subsurface structures for the sedimentary basins but also indicate a strong three- dimensional setting for the Mérida Andes. Using a combination of 2D and 3D modelling we systematically examined the influence of 3D structures on 2D inversions. Synthetic data sets derived from 3D modelling allow identification and quantification of spurious off-profile features as well as smoothing artefact due to limited areal station coverage of data collected along a profile. The 2D inversion models show electrically conductive basins with depths of 2 to 5 km for the Barinas-Apure and 2 to 7 km for the Maracaibo basins. A number of resistive bodies within the Maracaibo basin could be related to active deformation causing juxtaposition of older geological formations and younger basin sediments. The most important fault systems of the area, the Boconó and Valera Faults, cross-cut the Mérida Andes in NE-SW direction along its strike on a length 400 km and N-S direction at its centre on a length 60 km, respectively. Both faults are associated with sub-vertical zones of high electrical conductivity and sensitivity tests suggest that they reach depths of up to 12 km. A sizeable conductor at 50 km depth, which appears consistently in the 2D sections, could be identified as an inversion artefact caused by a conductor east of the profile. We speculate the high conductivity associated with the off-profile conductor may be related to the detachment of the Trujillo Block. Our results partially support the ”floating orogen hypothesis” developed to explain the geodynamic evolution of western Venezuela and they highlight the relevance of the Trujillo Block in this process.

Using gravity gradients to estimate fault parameters in the wichita uplift region

Mon, 06/01/2020 - 00:00
SummaryThe geologic setting of southwestern Oklahoma and northeastern Texas is an ideal example of an aulacogen, the result of the tectonic evolution of a failed rift of the North American continent during the Paleozoic era (540–360 Ma). The Wichita Province forms the uplifted basement portion of this Southern Oklahoma Aulacogen (SOA). The major fault zones to its north and south are clearly evident in gravity gradient maps produced by the recently constructed Earth Gravitational Model 2008 (EGM2008). Fault parameters, such as the dip angle, location, and density contrasts have been estimated from profiles of seismic data and local gravimetry in the 1990s. On the other hand, gravitational gradients that are derived from EGM2008 and then combined to form the differential field curvature are particularly indicative of linear structures such as dip-slip faults. They are used here exclusively, that is, without additional geophysical constraints, in an optimal, least-squares estimation based on the Monte Carlo technique of simulated annealing to determine dip angle and location parameters of the major faults that border the Wichita Uplift region. Results show that these faults have small dip angles, in basic agreement with the low-angle faults inferred from seismic studies. The EGM2008 gradients also appear in some cases to provide an improved map of the major faults in the region, thus offering a strong constraint on their location.

Separation of tectonic and local components of horizontal GPS station velocities: a case study for glacial isostatic adjustment in East Antarctica

Sat, 05/30/2020 - 00:00
SummaryAccurate measurement of the local component of geodetic motion at GPS stations presents a challenge due to the need to separate this signal from the tectonic plate rotation. A pressing example is the observation of glacial isostatic adjustment (GIA) which constrains the Earth’s response to ice unloading, and hence, contributions of ice-covered regions such as Antarctica to global sea level rise following ice mass loss. While both vertical and horizontal motions are of interest in general, we focus on horizontal GPS velocities which typically contain a large component of plate rotation and a smaller local component primarily relating to GIA. Incomplete separation of these components introduces significant bias into estimates of GIA motion vectors. We present the results of a series of tests based on the motions of GPS stations from East Antarctica: 1) signal separation for sets of synthetic data that replicate the geometric character of non-separable, and separable, GIA-like horizontal velocities; and 2) signal separation for real GPS station data with an appraisal of uncertainties. For both synthetic and real motions, we compare results where the stations are unweighted, and where each station is areal-weighted using a metric representing the inverse of the spatial density of neighbouring stations. From the synthetic tests, we show that a GIA-like signal is recoverable from the plate rotation signal providing it has geometric variability across East Antarctica. We also show that areal-weighting has a very significant effect on the ability to recover a GIA-like signal with geometric variability, and hence on separating the plate rotation and local components. For the real data, assuming a rigid Antarctic plate, fitted plate rotation parameters compare well with other studies in the literature. We find that 25 out of 36 GPS stations examined in East Antarctica have non-zero local horizontal velocities, at the 2σ level, after signal separation. We make the code for weighted signal separation available to assist in the consistent appraisal of separated signals, and the comparison of likely uncertainty bounds, for future studies.

New gravity data and 3D density model constraints on the Ivrea Geophysical Body (Western Alps)

Sat, 05/30/2020 - 00:00
SummaryWe provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and three-dimensional (3D) density modeling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4 to 9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg/m3 value commonly used in such computations. We then develop a 3D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg/m3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea-level. The model sensitivity tests suggest that the ∼300-500 kg/m3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea-level, for the lowest and the highest density contrast respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometers. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼ 20 km in West-East direction, and steeply E-SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.

Spatial relationship between shallow very low frequency earthquakes and the subducted Kyushu-Palau Ridge in the Hyuga-nada region of the Nankai subduction zone

Sat, 05/30/2020 - 00:00
SummaryShallow very low frequency earthquakes (sVLFEs) have occurred recurrently at the shallow plate interface of the Hyuga-nada region of the western Nankai subduction zone, Japan. Although the locations of sVLFE epicentres have been determined using land-based seismic records with moderate accuracy, it is necessary to determine their locations more precisely to explore the relationship between sVLFEs and other shallow slow earthquakes and examine the structural factors that may control sVLFE activity. Here, we identified sVLFE epicentres using seismic records obtained from temporarily deployed ocean bottom seismometers (OBSs) in the Hyuga-nada region. Seismic observations involved the deployment of 5–13 OBSs for approximately 1 year, with deployments conducted three times during 2014–2016 each time with changing OBS numbers and array distribution. As a result, one sVLFE episode, containing successive Rayleigh wave pulses with slow velocities due to marine sediments, could be detected at a frequency band of 0.1–0.15 Hz per observation, resulting in a total of three episodes. Rayleigh wave amplitudes of ordinary earthquakes in the continuous records were suppressed using earthquake catalogues. We estimated the dispersion curve for the Rayleigh wave group velocity for each array, which represented the averaged group velocity within the array, using coda interferometry, and applied an envelope correlation method (ECM) using the group velocities to continuous records. These processing provided sVLFE epicentres with horizontal distance errors of < 5 km. Our results showed that sVLFEs depths, which were inferred from the contour line of the top of the Phillipine Sea Plate, had increased from < 10 km to 10–15 km in the region of the subducted Kyusyu-Palau Ridge (KPR). It was also apparent that migration of sVLFE epicentres occurred in 2015 from a depth of 15 km to shallower depths along the northern margin of the subducted KPR. These results identified the subducted KPR as a structural factor controlling the excitation conditions of sVLFE activities.

3D thermochemical structure of lithospheric mantle beneath the Iranian plateau and surrounding areas from geophysical-petrological modeling

Thu, 05/28/2020 - 00:00
SummaryWhile the crustal structure across the Iranian plateau is fairly well constrained from controlled source and passive seismic data, the lithospheric mantle structure remains relatively poorly known, in particular in terms of lithology. Geodynamics rely on a robust image of the present day thermochemical structure interpretations of the area. In this study, the 3D crustal and upper mantle structure of the Iranian plateau is investigated, for the first time, through integrated geophysical-petrological modeling combining elevation, gravity and gravity gradient fields, seismic and petrological data. Our modeling approach allows us to simultaneously match complementary data sets with key mantle physical parameters (density and seismic velocities) being determined within a self-consistent thermodynamic framework. We first elaborate a new 3D isostatically balanced crustal model constrained by available controlled source and passive seismic data, as well as complementary by gravity data. Next, we follow a progressively complex modeling strategy, starting from a laterally quasi chemically homogeneous model and then including structural, petrological and seismic tomography constraints. Distinct mantle compositions are tested in each of the tectonothermal terranes in our study region based on available local xenolith suites and global petrological data sets. Our preferred model matches the input geophysical observables (gravity field and elevation), includes local xenolith data, and qualitatively matches velocity anomalies from state of the art seismic tomography models. Beneath the Caspian and Oman seas (offshore areas) our model is defined by an average Phanerozoic fertile composition. The Arabian plate and the Turan platform are characterized by a Proterozoic composition based on xenolith samples from eastern Arabia. In agreement with previous studies, our results also suggest a moderately refractory Proterozoic type composition in Zagros-Makran belt, extending to Alborz, Turan and Kopeh-Dagh terranes. In contrast, the mantle in our preferred model in Central Iran is defined by a fertile composition derived from a xenolith suite in northeast Iran. Our results indicate that the deepest Moho boundary is located beneath the high Zagros Mountains (∼65 km). The thinnest crust is found in the Oman Sea, Central Iran (Lut Block) and Talesh Mountains. A relatively deep Moho boundary is modeled in the Kopeh-Dagh Mountains, where Moho depth reaches to ∼55 km. The lithosphere is ∼280 km thick beneath the Persian Gulf (Arabian-Eurasian plate boundary) and the Caspian Sea, thinning towards the Turan platform and the high Zagros. Beneath the Oman Sea, the base of the lithosphere is at ∼150 km depth, rising to ∼120 km beneath Central Iran, with the thinnest lithosphere (< 100 km) being located beneath the northwest part of the Iranian plateau. We propose that the present day lithosphere-asthenosphere topography is the result of the superposition of different geodynamic processes: i) Arabia-Eurasia convergence lasting from mid Jurassic to recent and closure of Neo-Tethys ocean, ii) reunification of Gondwanian fragments to form the Central Iran block and Iranian micro-continent, iii) impingement of a small-scale convection and slab break-off beneath Central Iran commencing in the mid Eocene and iv) refertilization of the lithospheric mantle beneath the Iranian micro-continent.

Enhanced magnetic fields within a stratified layer

Thu, 05/28/2020 - 00:00
SummaryMounting evidence from both seismology and numerical experiments on core composition suggests the existence of a layer of stably stratified fluid at the top of Earth’s outer core. In such a layer, a magnetostrophic force balance and suppressed radial motion lead to stringent constraints on the magnetic field, named Malkus constraints, which are a much more restrictive extension of the well known Taylor constraints. Here, we explore the consequences of such constraints for the structure of the core’s internal magnetic field. We provide a new simple derivation of these Malkus constraints, and show solutions exist which can be matched to any external potential field with arbitrary depth of stratified layer. From considerations of these magnetostatic Malkus constraints alone, it is therefore not possible to uniquely infer the depth of the stratified layer from external geomagnetic observations. We examine two models of the geomagnetic field defined within a spherical core, which obey the Taylor constraints in an inner convective region and the Malkus constraints in an outer stratified layer. When matched to a single-epoch geomagnetic potential field model, both models show that the toroidal magnetic field within the outer layer is about 100 times stronger compared to that in the inner region, taking a maximum value of 8 mT at a depth of 70 km. The dynamical regime of such a layer, modulated by suppressed radial motion but also a locally enhanced magnetic field, may therefore be quite distinct from that of any interior dynamo.

Seismoelectromagnetic effects associated with the 15 February 2017 Veracruz earthquake (MW = 4.8)

Wed, 05/27/2020 - 00:00
SummaryIn this study, we investigated correlations between electromagnetic and seismic signals of the 15 February 2017 Veracruz, Mexico, earthquake (MW = 4.8). We carried out a time-frequency misfit analysis based on the continuous wavelet transform in order to compare electric, magnetic, and seismic records accurately. This analysis was performed for horizontal and vertical components separately. Our results from time-frequency misfit and goodness-of-fit criteria confirm the general similarity between seismic and electromagnetic signals both in frequency and time. Additionally, we studied the behavior of peak amplitudes of seismoelectromagenetic records as a function of magnitude and distance. Our observations are in good agreement with previous studies, confirming scaling with magnitude and attenuation with distance. Radiated seismic energy estimations were performed with two methods: Integration of velocity records, and Empirical Green Function, respectively. Estimated energy magnitudes (4.35 < Me < 4.98) are consistent with reported seismic magnitudes for this event. We propose a method for determining electric and magnetic coseismic energies based on the concept of energy flux as implemented in the frequency domain by the integration of electromagnetic records. The calculated energies showed that the radiated seismic energy is much higher than the electric and magnetic energies.

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