Inversion Of Broadband Research Articles

Magmatic underplating, plumbing system, and carbon-enhanced electrical conductivity in the Paraná Magmatic Province

The origin of large igneous provinces (LIPs) is still an enigma but likely involves magma storage and pathways spread throughout the crust, requiring indirect methods for its study. Here, we present 3-D resistivity models derived from the inversion of broadband (∼0.0001–3000 s) magnetotelluric data with 9–13 km lateral spacing in the central Paraná Magmatic Province, an expressive Early Cretaceous LIP in South America. Our results map in greater detail the previously interpreted LIP magma conduit and support, in contrast with seismological models, significant magmatic underplating to explain the observed conductivity near the LIP central axis. The potential axial lava feeder appears as a pair of crustal conductors (5–15 km; >0.1 S/m) parallel to the region of maximum thickness of both pre-volcanic sedimentary rocks and erupted tholeiitic basalts along an extension of at least 800 km. We propose the high conductivity is due to graphite films of precipitated carbon during the ascension of carbon-bearing fluids released by crystallizing magmas underplated at the base of the crust. The association of high conductivity with underplating is supported by high Vp/Vs ratios close to the conductive lineament, by a lower-crustal zone of high P-wave velocities at the basin axis attributed to mafic intrusions, and by a residual gravity high interpreted as gabbros underplated/intruded in the lower crust. Moreover, the conductive lineament is spatially associated with intracrustal high densities inferred from geoid inversion and upper-crustal high P-wave velocities. Early CO2 release during crystallization of underplated magma before eruption could explain the time gap between the Weissert ocean anoxic event and the volcanism. Our study advances in the controversial topic of magmatic intrusive components in the Paraná LIP with implications for LIP generation and paleo-climate studies.

Geoacoustic inversion of wide-angle reflection-coefficient data to estimate sediment shear properties

This paper considers Bayesian geoacoustic inversion of broadband, wide-angle reflection-coefficient data including shear parameters in the seabed model to investigate the ability to estimate these parameters as well as effects on the estimation of other geoacoustic parameters, particularly compressional-wave attenuation. The seabed parameterization is based on the viscous grain-shearing (VGS) sediment-acoustic model, including the grain-to-grain shear modulus as an unknown parameter. VGS sediment parameters are transformed to density and frequency-dependent compressional- and shear-wave speeds and attenuations. Data prediction involves spherical-wave reflection-coefficient calculations. Trans-dimensional inversion, which samples probabilistically over the number of layers in the seabed model, is applied to combine quantitative model selection with parameter/uncertainty estimation. The inversion is applied to reflection-coefficient data sets collected on the New England Mud Patch, and inversion results are compared to those obtained under the common assumption of negligible shear effects (i.e., inverting for a fluid sediment model).

Magnetotelluric Evidence for Lithospheric Alteration Beneath the Wuyi‐Yunkai Orogen: Implications for Thermal Structure of South China

AbstractThe Wuyi‐Yunkai Orogen experienced a polyphase tectonomagmatism and is a key region for deciphering the alteration and thermal structure of the South China Block lithosphere. Herein, an electrical resistivity model of the lithosphere is presented via the three‐dimensional inversion of broadband (0.003–3600 s) magnetotelluric (MT) data collected along a 380‐km‐long profile comprising 62 MT sites across the Wuyi‐Yunkai Orogen, and the robustness of this model is critically evaluated through a series of sensitivity tests. The resistivity model reveals that the upper crust of the Cathaysia Block and the Wuyi‐Yunkai Orogen is dominated by high‐resistivity sedimentary cover interposed with low‐resistivity features, mainly along fault zones. High‐resistivity bodies and strong conductors in the upper crust are interpreted as magmatic rocks and tectonic mélanges, respectively. Another feature of this resistivity model is the presence of zones featuring enhanced electrical conductivity (<30 Ωm) extending from the lower crust to the upper mantle beneath the Wuyi‐Yunkai Orogen. The conductors in the lower crust are attributed to saline fluids from either the dehydration of the subducting Paleo‐Pacific slab or the regional metamorphism‐induced dehydration of sandy argillaceous rocks. In contrast, the conductors in the upper mantle are attributed to 4%–7% partial melt, which corresponds to the analyses of mantle xenoliths in South China. These conductors in the upper crust and upper mantle supply volatiles and heat to shallow geothermal systems. This work ultimately shows that the lithospheric thinning of South China is controlled mainly by mantle upwelling caused by the retreat of the subducting Paleo‐Pacific slab.

Slip distribution and rupture history of the August 11, 2012, double earthquakes in Ahar – Varzaghan, Iran, using joint inversion of teleseismic broadband and local strong motion data

We use combined teleseismic and strong motion data sets to investigate finite-fault slip models for a double of earthquakes that occurred on August 11, 2012, in northwestern Iran near the cities of Ahar and Varzaghan. The data include teleseismic P-waveforms retrieved from broadband seismic stations located between 30°–94° from the earthquakes and local strong motion data recorded by the Iran Strong Motion Network, installed and operated by the Building and Housing Research Centre. We first invert teleseismic P-waveforms and local strong motion data separately. For the first event (12:23 UTC), the teleseismic broadband inversion yields a somewhat deeper and simpler distribution of slip than the local strong motion inversion. The strong motion inversion results in a more complex distribution because of higher frequency content but can also be influenced by complexities in the propagation path. For the second event (12:34 UTC), the slip distribution from strong motion data is more similar to the teleseismic result and shows a simple slip area with a small relative movement to the west. To resolve the differences between the results of these two data sets and obtain a better constrained slip model, we perform a joint inversion of teleseismic broadband and local strong motion data.The joint inversion for the first event shows two asperities with a maximum slip of 3.9 m up- dip from the hypocenter and extending to the west between depths of 1 and 5 km. A second narrower high-slip area is seen just above the hypocenter from 6 to 10 km depth. The total moment for this earthquake is calculated to be Mo = 3.8 × 1025 dyn-cm (3.8 × 1018 N.m) (Mw 6.4). For the second event, the results of the joint inversion show a simple slip distribution that is mainly confined in a single patch around the hypocenter with a depth range from about 10 to 13 km and maximum slip of 1.9 m. We compute a total seismic moment of Mo = 1.6 × 1025 dyn-cm (1.6 × 1018 N.m) (Mw 6.1) for the second event. The largest stress drops for the first event occur above the hypocenter with an average stress drop over the rupture area of 120 bar (12 Mpa). For the second event, the maximum stress drop occurs at the reported focal depth with an average stress drop over the rupture area of 80 bar (8 Mpa).

Seismotectonics of the Trans-Himalaya, Eastern Ladakh, India: Constraints from moment tensor solutions of local earthquake data

The seismotectonic scenario of the northwest part of the India-Asia collision zone is studied by analyzing the local earthquake data (M~1.4–4.3) recorded by a broadband seismological network consisting of 14 stations. Focal Mechanism Solutions (FMSs) of 13 selected earthquakes were computed through waveform inversion of three-component broadband records. Depth distribution of the earthquakes and FMSs of local earthquakes obtained by waveform inversion reveal kinematics of the major fault zones present in eastern Ladakh. A most pronounced cluster of seismicity is observed in the Karakoram Fault (KF) zone down to a depth of ~65km. The FMSs reveal a transpressive environment with an inferred strike slip fault plane parallel to the KF. It is argued that the KF penetrates down to the lower crust and is a manifestation of active underthrusting of Indian lower crust beneath Tibet. Two clusters of microseismicity are observed at a depth range 5–20km at the northwestern and southeastern fringes of the Tso Morari gneiss dome, which can be correlated to the activities along the Zildat fault and Karzok fault, respectively. The FMSs obtained for representative earthquakes show thrust fault solutions for the Karzok fault, and normal fault solutions for the Zildat fault. It is suggested that the Zildat fault is acting as a detachment, facilitating the exhumation of the Tso Morari dome. On the other hand, the Tso Morari dome is underthrusting the Karzok ophiolite at its southern margin along the Karzok fault due to gravity collapse.

Modeling reflection and scattering by deep-ocean turbidites

Turbidites are commonly found in deep ocean sediments and are composed of multiple thin layers of fine and coarser sediments, the coarser sediments being deposited by turbidity currents. Although acoustics is a unique means to quantitatively probe the physical and geophysical properties of turbidites, research on this subject is still limited. The work of Gilbert and of Holland and Muncill has shown that this layering can cause acoustic reflection to be very strong, with bottom loss less than that expected for hard seafloors. We examine this problem further, looking at the effect of layering on penetration into the seafloor and also considering scattering by roughness of the inter-layer interfaces. It is found that penetration can be severely limited in some cases, and that this behavior can be understood as a form of Anderson localization. Prospects for inversion of broadband, vertical-incidence data are considered. [Work supported by ONR.]

Fast computation of seabed spherical-wave reflection coefficients in geoacoustic inversion.

This paper develops a fast numerical approach to computing spherical-wave reflection coefficients (SWRCs) for layered seabeds, which provides substantial savings in computation time when used as the forward model for geoacoustic inversion of broadband seabed reflectivity data. The approach exploits the Sommerfeld-integral representation of SWRCs as the Hankel transform of a function proportional to the plane-wave reflection coefficient (PWRC), and applies Levin integration to the rapidly oscillating integrand cast as the product of a (pre-computed) media-independent matrix and a vector involving PWRCs at a sparse sampling of integration angles. Compared to conventional Simpson's rule integration for computation of the SWRC, the Levin integration yields speed-up factors of an order of magnitude or more. Further, it results in reduced memory requirements for storage of pre-computed quantities, a desirable property when a graphics processing unit (GPU) is used for parallel computation of SWRCs. The paper applies trans-dimensional Bayesian inversion to investigate the impact of forward modeling in terms of PWRCs and SWRCs on the estimation of geoacoustic parameters and uncertainties. Model comparisons are quantified in simulated- and measured-data inversions by comparing the estimated geoacoustic parameters to the true parameters or core measurements, respectively, and by calculating the deviance information criterion for model selection.

Combined Inversion of Broadband and Short-Period Waveform Data for Regional Moment Tensors: A Case Study in the Alborz Mountains, Iran

Abstract In this study, we suggest a novel approach for the retrieval of regional moment tensors for earthquakes with small to moderate magnitudes. The first modification is the combined inversion of broadband and short‐period waveform data. The broadband waveforms are inverted in a frequency range suitable for surface waves, whereas for the short‐period data a frequency range suitable for body waves is applied. The second modification is the use of first‐motion body‐wave polarities to select the most probable solution out of all solutions from inversion. To combine three different criteria for selecting the most probable solution (i.e., residual from inversion, double‐couple content of solution, number of nonmatching first‐motion body‐wave polarities), the L2 norm is applied to the normalized parameters. We chose five earthquakes within the Alborz mountains, Iran, as a case study (3.1≤ M w ≤4.1). In this area, several factors exacerbate the difficulty of performing inversion for moment tensors, for example, a heterogeneous station network and large azimuthal gaps. We have demonstrated that our approach supplies reliable moment tensors when inversion from broadband data alone fails. In one case, we successfully retrieved a stable solution from short‐period waveform data alone. Thus, our approach enables successful determination of seismic moment tensors wherever a sparse network of broadband stations has thus far prevented it.

Data error covariance in matched-field geoacoustic inversion

Many approaches to geoacoustic inversion are based implicitly on the assumptions that data errors are Gaussian-distributed and spatially uncorrelated (i.e., have a diagonal covariance matrix). However, the latter assumption is often not valid due to theory errors, and can lead to reduced accuracy for geoacoustic parameter estimates and underestimation of parameter uncertainties. This paper examines the effects of data error (residual) covariance in matched-field geoacoustic inversion. An inversion approach is developed based on a nonparametric method of estimating the full covariance matrix (including off-diagonal terms) from the data residuals and explicitly including this covariance in the misfit function. Qualitative and quantitative statistical tests for Gaussianity and for correlations in complex residuals are considered to validate the inversion results. The approach is illustrated for Bayesian geoacoustic inversion of broadband, vertical-array acoustic data measured in the Mediterranean Sea.

Complex Source Process of the 17 August 1999 Izmit, Turkey, Earthquake

Broadband teleseismic P -wave data at epicentral distances of 30°-90° are inverted to retrieve the complex source process of the 17 August 1999 Izmit earthquake. A comparison of the waveforms of the mainshock with those of the aftershocks suggests a spatially complex rupture process having a duration of more than 45 sec. Inversion and modeling of broadband P waveforms are performed in this time window, incorporating a priori field observations of coseismic displacements. Both single-point and multiple-point source representations are determined. We found that the rupture process can be divided into at least three steps. In the first step, rupture initiated from the hypocenter and extended bilaterally along the E-W-striking fault with an almost pure right-lateral strike-slip motion. The eastern portion of the rupture front passed through the Sapanca Lake and stopped around Akyazi, whereas the western portion stopped near Hersek. This step lasted for 22-23 sec, releasing a total moment of 1.43 × 1020 N m. The dislocation was concentrated briefly at two areas: one near Golcuk and the other between the Sapanca Lake and Akyazi. Slip in these two areas was concentrated in the upper 10 km of the crust. After 1-2 sec, a second source started, possibly initiating on an underwater fault west of Hersek. Although its rupture lasted for almost 20 sec, its moment release was very low, just 0.17 × 1020 N m. Another 8-9 sec later, the easternmost fault segment between Akyazi and the area west of Duzce was triggered, with a rupture lasting for more than 15 sec, producing little energy in the short-period band, with slip concentrated in a zone extending vertically 25 km to the surface, and with a moment of 0.61 × 1020 N m. Manuscript received 22 September 2000.

Anomalous aftershocks of deep earthquakes in Mariana

Seismic moment of aftershocks during the August 23, 1995 deep earthquake sequence in the Mariana amounts up to 17% of that of the main shock (Mw 7.0). Such a high aftershock activity is extremely unusual. Based on the inversion of broadband P and SH waveforms, however, we found nothing anomalous in the source mechanisms. Among the 33 largest deep earthquakes (Mw≥7.0) since 1958, only events in the Mariana have high aftershock activity. We associate this observation with the unique configuration of the Mariana slab which seems to plunge sub‐vertically into the lower mantle with little distortion.

Broadband matched-field inversion for integral geoacoustic properties of the ocean bottom: YELLOW SHARK 94 experimental results

Bottom geoacoustic properties were determined from inversion of broadband (200–800 Hz), multitone pressure fields measured at a shallow-water site in the western Mediterranean (YELLOW SHARK 94). Extensive oceanographic and geophysical information was obtained in situ to support and validate the inversion. Matched-field processing was applied for each tone frequency. Optimization of the environmental parameters was performed simultaneously across all propagated frequencies by minimizing a maximum-likelihood objective function with genetic algorithms (SAGA). The following conclusions were drawn from this study. (a) For a fixed source-vertical array configuration broadband tomographic measurements were a sine qua non condition to obtain meaningful inversion results. (b) The broadband inversion provided considerable stability and robustness with respect to volume and bottom variabilities. (c) Corresponding single-frequency inversions performed under the exact same conditions produced erratic results. (d) Integral geoacoustic properties were effectively determined within the constraints imposed by the bottom parametrization. (e) More detailed and accurate properties were obtained by including the range dependence of ocean sound-speed profile (SSP) in the forward modeling. (f) The broadband inversion provided robust, unique, and most likely correct, geoacoustic parameters, i.e., SSP, attenuation, density, and thickness of the top clay layer, and sound speed of the underlying silt layer.

Inversion of broad-band multitone acoustic data from the YELLOW SHARK summer experiments

Integral geoacoustic properties of the sea bottom were determined from full-field inversion of broad-band, water-borne, acoustic propagation data. The data were obtained during the YELLOW SHARK 94 experiment along a 15-km mildly range-dependent transect at a shallow water site in the western Mediterranean. Seven tones from 200 Hz to 800 Hz were transmitted simultaneously by a mid-depth acoustic projector, bottom-moored at different ranges from a vertical array that spanned the water column below the thermocline. Extensive oceanographic and geophysical information were obtained in situ to support and validate the inversion. Matched-field processing was applied to the received pressure fields for each tone frequency. Optimization of the environmental parameters was performed simultaneously across all propagated frequencies. A maximum-likelihood objective function included the linear (Bartlett) cross correlator at the individual frequencies. Genetic algorithms searched for the global minimum of this objective function. The convergence and accuracy of the inversion were determined from statistical analysis of the a posteriori distribution of the candidate environmental models produced by the search algorithm. The following conclusions were drawn from this study. 1) For a fixed source-vertical array configuration broad-band tomographic measurements were a sine qua non to obtain meaningful inversion results. 2) The broad-band inversion provided considerable stability and robustness with respect to volume and bottom variabilities. 3) Corresponding single-frequency inversions performed under the exact same conditions produced erratic results. 4) Integral geoacoustic properties of the bottom were effectively determined within the constraints imposed by the bottom parameterization. 5) More detailed and accurate properties were obtained by including the range dependence of ocean sound-speed profile in the forward modeling. 6) The broad-band-inverted sound-speed profile, attenuation, density, and thickness of the top clay layer, and sound speed of the underlying silt layer, were in close agreement with the independent geophysical measurements.

The March 25, 1990 (Mw = 7.0, ML = 6.8), earthquake at the entrance of the Nicoya Gulf, Costa Rica: Its prior activity, foreshocks, aftershocks, and triggered seismicity

On March 25, 1990 a large earthquake (Mw = 7.0, ML = 6.8) occurred at the entrance of the Nicoya Gulf, Costa Rica, at 1322:55.6 UTC, producing considerable damage in central Costa Rica and generating much interest about whether or not the Nicoya seismic gap (Nishenko, 1989) had broken. The local country‐wide seismographic network recorded 6 years of activity prior to this large earthquake, 16 hours of foreshocks, the mainshock, and its aftershocks. This network is operated jointly by the Costa Rica Volcanological and Seismological Observatory at the National University (OVSICORI‐UNA), and the Charles F. Richter Seismological Laboratory at the University of California, Santa Cruz (CFRSL‐UCSC). We obtained high resolution locations from this network and located the mainshock at 9°38.5′N, 84°55.6′W (depth is 20.0 km) and the largest foreshock (Mw = 6.0, March 25, 1990, at 1316:05.8 UTC) at 9°36.4′N, 84°57.1′W (depth is 22.4 km). We find that the aftershock zone abuts the southeast boundary of the Nicoya seismic gap, suggesting that the seismic gap did not rupture. Since the installation of the local network in April 1984 to March 24, 1990, nearly 1900 earthquakes with magnitudes from 1.7 to 4.8 (318 with magnitude 3.0 or larger) have been located at the entrance of the Nicoya Gulf, one of the most active regions in Costa Rica. The March 25 earthquake occurred at the northwest edge of this region, where a sequence of foreshocks began 16 hours prior to the mainshock. The spatial‐temporal distribution of aftershocks and directivity analysis of the mainshock rupture process using teleseismic records both indicate a southeast propagating rupture. The mainshock ruptured an asperity of approximately 600 km2 of area, with this area expanding to 4000 km2 after 7 days. We present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount. Inversion of teleseismic broadband and long‐period P and SH waves yields a thrust faulting mechanism with the shallow plane striking 292°, dipping 26°, and with a rake of 88°, in agreement with the subduction of the Cocos plate under the Caribbean plate. Local first motions for the largest foreshock and the mainshock agree with this solution. We also present evidence suggesting that the March 25, 1990, earthquake triggered and reactivated several seismic swarms in central Costa Rica and temporally decreased the activity in the epicentral area of the July 3, 1983 (Ms = 6.2), Pérez Zeledón earthquake.