Moho Reflections Research Articles

DEEP REFLECTION SEISMIC DATA FOR IMPROVED IMAGING OF CRUST STRUCTURE: 2D CASE STUDY OF THE SOUTHWEST SUB-BASIN IN THE SOUTH CHINA SEA

Seismic imaging of crustal structures becomes difficult in the presence of rough basements or complex bathymetry. Here, we present a 900 km deep seismic reflection profile collected across the Southwest Sub-basin (SWSB) of the South China Sea. By analyzing the types and distinctions of noise and effective signals, we employed deep structure migration techniques to improve crustal structure imaging, wide-line processing to predict 3D space multiples, and F-K domain time-space variable adaptive de-ghosting and different offset stacking to enhance the weak signals reflected from deep strucutures. The imaged continental crustal structure in the Penxi Bank exhibits moderate thinning, down to 15 km, and is intersected by continental-ward low-angle normal faults. Within the limitations of the OBS P-wave velocity model, we detected sub-horizontal lower crustal reflections that may be indicative of a weak lower crust. Two small-scale rollover structures along detachment faults rooted and rafted to the top of these weak lower crust. Based on the presence of narrow continent-ocean transitions(COTs), continental-ward detachment faults, and high lithosphere heat flow, we deduced that the mantle lithosphere breakup occurred earlier than the crust in the SWSB. Moreover, the continent-ocean transitions and oceanic crust domains demonstrate rough basements with numerous faults and approximately 20% diffuse or weak Moho reflections. From the southern COT to the initial oceanic domain, the thickness of the crust gradually reduces to only 3-5 km. This suggests a relatively low magmatic budget and protracted tectonic extension from the continental breakup to the onset of seafloor spreading. Within the oceanic crust domain, the crust thickness ranges from approximately 4-6 km, indicating a thinner oceanic crust than normal crust. Lower crustal reflections with a ridge-ward dipping pattern terminate at the Moho reflections and are partly connected to syn-spreading faults, hinting at their possible generation through syn-spreading faulting.

The characteristics and distribution of the Moho reflections in the South China Sea Oceanic Basin

The distribution and characteristics of the Moho reflections (M-reflections) in oceanic basins record the seafloor spreading process and provide crucial insights into the formation and evolution of the oceanic basins. Approximately 20,000 km of multichannel seismic reflection sections are used to characterize the M-reflections and the crustal structures in the South China Sea oceanic subbasins. The identified M-reflections are mainly distributed in the northwest subbasin, the east subbasin, and the northern and southern flanks of the southwest subbasin (SWSB). They exhibit a banded distribution pattern in two-way traveltime along the north–south direction, with shallower depths observed in the central region and gradually deepening toward the northern and southern sides. In addition, the M-reflection amplitudes generally weaken, and the reflection events become less continuous as the igneous oceanic crust thins along with the seafloor spreading prior to the second ridge jump event. Ultimately, the M-reflection becomes indistinguishable in proximity to fossil spreading centers. These observations suggest a decreasing magma supply toward the cessation of the spreading. Despite the significance of Moho interface imaging at the crust-mantle boundary, the distinct M-reflections are absent in the SWSB and the vicinity of the Zhenbei-Huangyan Seamounts Chain, which are attributed to several factors, especially the vague nature of Moho interfaces.

The crustal structure of the western Amundsen Basin, Arctic Ocean, derived from seismic refraction/wide-angle reflection data

SUMMARY Two geophysical expeditions (LOMROG II and III) were carried out in 2009 and 2012 to acquire seismic data in the western Amundsen Basin in the Arctic Ocean, a basin formed by ultraslow seafloor spreading at the Gakkel Ridge. Previous studies show alternating magmatic and amagmatic segments at the ridge but it is unclear if such segmentation persisted throughout the entire opening history of the basin. The seismic refraction data were used to develop P-wave velocity models down to the uppermost mantle using forward modelling of traveltimes. The coincident seismic reflection data were used to constrain the geometry of the sedimentary layers and for characterizing the acoustic basement. 2-D gravity modelling was used to determine the Moho depth in areas when data quality was insufficient to resolve a Moho reflection. The models distinguish three different basement types: oceanic crust with layers 2 and 3, oceanic crust that is lacking a layer 3 and exhumed and serpentinized mantle. The maximum observed crustal thickness is 6 km. Areas with thin crust (<3 km) may be underlain by partially serpentinized mantle. Where exhumed mantle is observed, a serpentinization front separates highly serpentinized mantle at the top from partially serpentinized mantle below. The presence of oceanic crust off-axis of the presently amagmatic sector of the Gakkel Ridge indicates that there is both a spatial and temporal variation of crustal accretion processes at the ridge.

Crustal structure and its tectonic implications beneath the middle–lower Yangtze metallogenic belt in Anhui Province: 3D deep seismic sounding results from airgun source in inland waters

To evaluate the effectiveness of using airguns as seismic sources in inland waters to detect the regional crustal structure, a mobile large-capacity airgun excitation experiment was conducted in October 2015 in the Anhui section of the middle–lower Yangtze metallogenic belt. In this study, we extracted 1,957 first-arrival phases (Pg) and 2,179 Moho reflection phases (PmP) from the airgun seismic signals, and performed joint inversion of the traveltimes. The inversion results reveal the P-wave high-velocity anomalies above 7 km depth in the upper crust beneath the ore clustering areas, suggesting the source of mineralized materials. The crustal velocity structure characteristics substantially differed above and below a depth of 7 km, indicating the existence of a regional basement detachment surface. The velocity structure in the middle–lower crust, especially in the lower crust show lateral uniformity characteristic, which could be related to that the middle–lower Yangtze metallogenic belt had undergone a MASH metallization process. The Moho is 30–36 km deep, and its uplift zone extends from southwest to northeast in a “V” shape, which is consistent with the planar spreading characteristics of the metallogenic belt, indicating that the asthenosphere uplift and crustal thinning have had a controlling effect on the formation of the metallogenic belt. This study suggests the present-day crust in the region along the Yangtze River in Anhui retains the traces of lithosphere delamination-thinning and basaltic magma underplating during the Yanshan period. Our results indicates that airgun source detection in inland waters can effectively determine the continental crustal structure.

Enigmatic crustal and upper mantle structure in the NE Sino-Korean Craton based on nuclear explosion seismic data

We interpret the crustal and upper mantle structure along ∼2500 km long seismic profiles in the northeastern part of the Sino-Korean Craton (SKC). The seismic data with high signal-to-noise ratio were acquired with a nuclear explosion in North Korea as source. Seismic sections show several phases including Moho reflections (PmP) and their surface multiple (PmPPmP), upper mantle refractions (P), primary reflections (PxP, PL, P410), exceptionally strong multiple reflections from the Moho (PmPPxP), and upper mantle scattering phases, which we model by ray-tracing and synthetic seismograms for a 1-D fine-scale velocity model. The observations require a thin crust (30 km) with a very low average crustal velocity (ca. 6.15 km/s) and exceptionally strong velocity contrast at the Moho discontinuity, which can be explained by a thin Moho transition zone (< 5 km thick) with strong horizontal anisotropy. We speculate that this anisotropy was induced by lower crustal flow during delamination dripping. An intra-lithospheric discontinuity (ILD) at ∼75 km depth with positive velocity contrast is probably caused by the phase transformation from spinel to garnet. Delayed first arrivals followed by a long wave train of scattered phases of up to 4 s duration are observed in the 800–1300 km offset range, which are modelled by continuous stochastic velocity fluctuations in a low-velocity zone (LVZ) below the Mid-Lithospheric Discontinuity (MLD) between 120 and 190 km depth. The average velocity of this LVZ is about 8.05 km/s, which is much lower than the IASP91 standard model. This LVZ is most likely caused by rocks which are either partially molten or close to the solidus, which explains both low velocity and the heterogeneous structure.

Moho reflections within seismic noise autocorrelations

The noise recorded at a seismic station has information about some properties of the medium such as their discontinuities. The depth of these changes in velocity can be retrieved using auto-correlation and stacking methods applied to the seismic noise. We developed a new method to constrain Moho reflections hidden within the autocorrelation of seismic noise. This is done by filtering the data in different frequency bands (each one defined by a central frequency), applying the auto-correlation and stacking process per central frequency and finally stacking the frequencies of interest. Synthetic tests showed that phases associated with a discontinuity at depth arrive at the same lag time across different frequencies, while side lobes and constructive interactions of the random noise have time shifts in the lag time. We tested the method using data from the MASE array installed in Central Mexico and the results show a good agreement with previous works. We found that exploring the frequency content with different bandwidth filters and stacking only those with useful information helps to improve the SNR and to find the phase associated with the Moho discontinuity at depth.

The seismic reflection Moho across the mid-Norwegian continental rifted margin

The Moho – defined as the boundary that separates the Earth’s crust from the underlying upper mantle at depths of about 5 to 7 km beneath the oceans and 30 to 40 km below continents – has been difficult to pin down with observations. Different methods have yield a variety of results, suggesting that this simple definition may be misleading. Here, we present high-resolution and deep penetrating seismic reflection data that image the whole margin basement of the mid-Norwegian rift system, including the lower basement, Moho and upper mantle, from the proximal to the outer margin domains. We propose a determination of the seismic reflection Moho in this modern rifted margin context and describe its structural heterogeneity along the margin. Finally, the seismic facies, reflectivity, geometries and structural details of the seismic reflection Moho are described and discussed in terms of tectonic deformation pattern.

Feasibility of Estimating Parameters and Enhancing Reflections of Dipping Moho From Teleseismic P Wave Coda Autocorrelation

AbstractTeleseismic P wave coda autocorrelation can retrieve reflections from subsurface structural boundaries, which can be used in lithospheric imaging. However, dipping structures cause difficulty in extraction of reflections and errors in depth estimation. Based on a previously proposed velocity analysis method, we formulated a method for estimating the dipping parameters and correcting the effects of dipping structures. First, we derived an arrival time formula for the PP reflection of the dipping Moho, which includes the effects of the ray parameters, dipping structures, and azimuthal distribution of available events. Second, the formula was used for move out correction and velocity analysis. Finally, by scanning the peak of the Moho energy group in the velocity spectrum, which varies with the azimuth and dip angles, a method was proposed to estimate the dipping parameters of the Moho. We referred to the scanning map as a dipping spectrum. A field data test at station NE44 revealed the prospects of the practical applications of this method. The dip angle of the Moho below the station was 3°, and exhibited an azimuth angle of 25°NE. The stacked waveform of the Moho reflection was significantly enhanced. This method was successfully applied to more stations adjacent to station NE44. Our results provide new insight on the Moho in this area, which are important for imaging and interpretation in areas that contain a dipping Moho such as plate subduction zones.

The Moho reflectivity of the subduction beneath the Southwestern Alps from ambient seismic noise autocorrelations

SUMMARY The Western Alps shows a complex crustal organization due to the subduction of the European Plate beneath the Adriatic Plate and exhumation of the mantle wedge. The lithospheric structure of the Western Alps, that may hold significance for understanding orogenic processes and evolution, has been the subject of many geophysical studies, but the Moho profile remains unclear and this has led to controversies about the depth and extent of the European Plate beneath the Adriatic Plate. With the goal of retrieving detailed information on crustal constitution, we use autocorrelation of seismic ambient noise as a tool to map the body wave reflectivity structure at the subduction zone under the southwestern Alps. We use data recorded by the China–Italy–France Alps (CIFALPS) seismic transect, that includes 45 stations located approximately 5–10 km apart along a profile crossing the Alpine continental subduction in the Western Alps. We analyse the data set in four different frequency bands between 0.09 and 2 Hz. We automatically pick the arrival time of the Moho reflection in the second derivative of the envelope of the autocorrelation stack using prior Moho information. The 0.5–1 Hz frequency band mostly gives the best result due to the clear changes in reflectivity along the waveforms of the autocorrelation stacks after the picked arrival times of the Moho reflections. We find spatial coherence between 18 and 23 km depth in the western portion of the profile, indicating relatively homogeneous crustal rocks, and highly reflective structure under the central mountain range, due to the existence of a highly faulted zone. The very thin crust and the underlying mantle wedge known as the Ivrea body show instead high transparency to seismic waves and absence of reflections. The subduction profile of the European Plate shows a steep trend as compared to previous studies. We discuss autocorrelation stacks and Moho depths obtained from the arrival times of the picked reflectivity change in comparison with previous studies to validate the different reflection structures. Stacked ambient noise autocorrelations reliably image varied crustal properties and reflectivity structures in the highly heterogeneous region of the southwestern Alps.

Ground-Motion Prediction Equations for Shallow-Crustal and Upper-Mantle Earthquakes in Japan Using Site Class and Segmented Geometric Attenuation Functions for the Horizontal Component

ABSTRACT We present a set of ground-motion prediction equations for the horizontal components of the strong-motion records from shallow-crustal and upper-mantle earthquakes in Japan. We used a reasonably large dataset from earthquakes with a moment magnitude (Mw) over 4.9 and a reliable earthquake category determined by the tectonic location of earthquakes up to the end of 2012. The modeling of source and site effects in this study is similar to that by Zhao, Zhou, et al. (2016); a two-segmented model for the magnitude scaling function hinged at Mw 7.1 and site-period-based site classes were used as the site effect proxy that accounts for the effect of the nonlinear soil response. We used two types of multisegmented geometric attenuation functions with the distance break points based on the distributions of within-event residuals versus source distance. We found that the geometric attenuation functions with three distance break points (70, 130, and 220 km) for the shallow-crustal earthquakes and with two distance break points (90 and 150 km) for the upper-mantle events improved the model performance significantly. The first two break points for the shallow-crustal events are similar to those for the attenuation of the Fourier spectrum due to Moho reflection, but the corresponding coefficients between these spectra differ significantly. For the upper-mantle events, the geometric attenuation coefficient is somewhat surprisingly close to −1.0 for the first segment, 0.0 for the second segment, and −0.5 for the third segment at many periods, because these values are close to the classic attenuation rates of the Fourier spectrum for shallow-crustal events but having different distance break points. Moho reflection is not usually present in the ground motions from deep events, and we can only provide a plausible explanation; a future study is warranted for a theoretical explanation.

North-dipping relict subduction of the Paleo-Asian Ocean at the southeastern margin of the Mongolian Terrane: Study of two parallel deep seismic profiles

The final closure of the Paleo-Asian Ocean (PAO) and its tectonic characteristics have been debated for several decades owing to a lack of high-resolution information on the lithosphere structure. Scholars have been attempting to explain deep tectonic evolutionary processes while studying continental growth at the southern margin of the Mongolian Terrane. In a bid to provide a new interpretation of the deep structure with a higher resolution, we study two reprocessed deep seismic reflection profiles. We studied the northern part (210 km long) of the 630-km-long deep seismic reflection profile extending across the North China Craton (NCC) margin to the northern Sino-Mongolia border in the west; and a parallel profile (80 km long) in the east near the Sino-Mongolia border. Both profiles are characterized by consistently north-dipping layered reflections projecting from the lower crust to the upper mantle, with an estimated thickness of 3.6 to 6 km between adjacent reflections beneath the Uliastai and Hegenshan belts. Arched reflections are observed in the middle and lower crust; these may have been caused by later magmatic activities. In addition, the Moho reflection is observed to be fairly continuous and flat in most parts of these two profiles. The layered lower crust reflections and mantle reflections serve as important evidence that northward subduction occurred during the closure of the Paleo-Asian Ocean at the southeastern margin of the Mongolian Terrane. We propose a detailed model of the evolutionary processes from the early Paleozoic to early Mesozoic. The proposed model explains how these deep reflections were formed.

Active ocean–continent transform margins: seismic investigation of the Cayman Trough-Swan Island ridge-transform intersection

SUMMARY The southern boundary of the Cayman Trough in the Caribbean is marked by the Swan Islands transform fault (SITF), which also represents the ocean–continent transition of the Honduras continental margin. This is one of the few places globally where a transform continental margin is currently active. The CAYSEIS experiment acquired an ∼165-km-long seismic refraction and gravity profile (P01) running across this transform margin, and along the ridge-axis of the Mid-Cayman Spreading Centre (MCSC) to the north. This profile reveals not only the crustal structure of an actively evolving transform continental margin, that juxtaposes Mesozoic-age continental crust to the south against zero-age ultraslow spread oceanic crust to the north, but also the nature of the crust and uppermost mantle beneath the ridge-transform intersection (RTI). The traveltimes of arrivals recorded by ocean-bottom seismographs (OBSs) deployed along-profile have been inverse and forward modelled, in combination with gravity modelling, to reveal an ∼25-km-thick continental crust that has been continuously thinned over a distance of ∼65 km to ∼10 km adjacent to the SITF, where it is juxtaposed against ∼3–4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are only sparsely observed, and, even then, only by a few OBSs located on the continental margin, the 7.5 km s–1 velocity contour is used as a proxy to locate the crust–mantle boundary along-profile. Along the MCSC, the crust–mantle boundary appears to be a transition zone, at least at the seismic wavelengths used for CAYSEIS data acquisition. Although the traveltime inversion only directly constrains the upper crust at the SITF, gravity modelling suggests that it is underlain by a higher density (&amp;gt;3000 kg m–3) region spanning the width (∼15 km) of its bathymetric expression, that may reflect a broad region of metasomatism, mantle hydration or melt-depleted lithospheric mantle. At the MCSC ridge-axis to the north, the oceanic crust appears to be forming in zones, where each zone is defined by the volume of its magma supply. The ridge tip adjacent to the SITF is currently in a magma rich phase of accretion. However, there is no evidence for melt leakage into the transform zone. The width and crustal structure of the SITF suggests its motion is currently predominantly orthogonal to spreading. Comparison to CAYSEIS Profile P04, located to the west and running across-margin and through 10 Ma MCSC oceanic crust, suggests that, at about this time, motion along the SITF had a left-lateral transtensional component, that accounts for its apparently broad seabed appearance westwards.

The nature of the Moho beneath fast‐spreading centers: Evidence from the Pacific plate and Oman ophiolite

AbstractIt is common knowledge that the Moho is the boundary between the crust and the Earth's mantle. Here we show along several seismic profiles through the Pacific Plate that a correlation exists between the strength of Moho reflections, crustal thickness, and water depth. Where the Moho can be detected clearly, the overlying oceanic crust is systematically thicker and the water depths are shallower. We suggest that two end‐members of oceanic crust exist in fast spreading environments: one thick, underlain by a clear Moho; the other thinner, without a Moho; with all intermediate situations. In the Oman ophiolite, the best‐preserved on‐land analogue of fossil oceanic lithosphere created by fast‐spreading, the boundary between the mantle peridotites and the lower crustal gabbros mainly consists of a dunitic transition zone (DTZ) ranging from a few meters to a few hundred meters in thickness. A sudden influx of seawater down to the base of the crust at the mid‐ocean ridge (MOR) results in the hydrous (re‐)melting of mantle peridotites, producing a dunitic residue at the crust–mantle boundary that represents the most reflective Moho. At the same time, the hydrous melting, in addition to the normal decompression melting, beneath the MOR, increases the thickness of the oceanic crust by enhancing magma production. In the absence of hydrous melting, the DTZ is thin or absent at the crust–mantle boundary, and instead the uppermost mantle harzburgite is intruded by gabbros, and/or the overlying crustal gabbro is intruded by numerous wehrlite bodies, which will be seismically gradational.

Recalibration of the Local Magnitude (ML) Scale for Earthquakes in the Yellowstone Volcanic Region

ABSTRACT The Yellowstone volcanic region is one of the most seismically active areas in the western United States. Assigning magnitudes (M) to Yellowstone earthquakes is a critical component of monitoring this geologically dynamic zone. The University of Utah Seismograph Stations (UUSS) has assigned M to 46,767 earthquakes in Yellowstone that occurred between 1 January 1984 and 31 December 2020. Here, we recalibrate the local magnitude (ML) distance and station corrections for the Yellowstone volcanic region. This revision takes advantage of the large catalog of earthquakes and an increase in broadband stations installed by the UUSS since the last ML update in 2007. Using a nonparametric method, we invert 7728 high-quality, analyst-reviewed amplitude measurements from 1383 spatially distributed earthquakes for 39 distance corrections and 20 station corrections. The inversion is constrained with four moment magnitude (Mw) values determined from time-domain inversion of regional-distance broadband waveforms by the UUSS. Overall, the new distance corrections indicate relatively high attenuation of amplitudes with distance. The distance corrections decrease with hypocentral distance from 3 km to a local minimum at 80 km, rise to a broad peak at 110 km, and then decrease again out to 180 km. The broad peak may result from superposition of direct arrivals with near-critical Moho reflections. Our ML inversion doubles the number of stations with ML corrections in and near the Yellowstone volcanic region. We estimate that the additional station corrections will nearly triple the number of Yellowstone earthquakes that can be assigned an ML. The new ML distance and station corrections will also reduce uncertainties in the mean MLs for Yellowstone earthquakes. The new MLs are ∼0.07 (±0.18) magnitude units smaller than the previous MLs and have better agreement with 12 Mws (3.15–4.49) determined by the UUSS and Saint Louis University.

Insights from synthetic seismogram modelling study of oceanic lower crust and Moho Transition Zone

The Moho discontinuity is one of the fundamental boundaries on Earth, separating the crust from the upper mantle. Arrival times of seismic reflection and refraction data are used to estimate the depth of the Moho, but the nature of the Moho remains illusive. We present a synthetic seismogram modelling study for a suite of possible velocity models of the oceanic lower crust and Moho transition zone (MTZ). For modelling, we have used a 45-km long multichannel streamer (MCS) geometry, to get insight about waveforms and amplitudes of seismic phases like the crustal refraction (Pg), Moho reflection (PmP) and mantle refraction (Pn), and the interactions between these phases. We find that the nature of PmP reflection not only depends on the velocity structure of the MTZ but also on the velocity structure of the lower crust and upper mantle. The triplication resulting from these three seismic arrivals and its lateral extent mainly depend on the velocity structure in the lower crust. Apart from the velocity in the crust, the critical distance for the PmP arrival depends on the velocity gradient in the MTZ. The amplitude versus offset analysis of the wide-angle PmP arrivals indicates that there are two broad high amplitude peaks for the MTZ thickness less than or equal to the dominant seismic wavelength and only one large narrow peak for a thicker MTZ. Consequently, strong PmP arrivals for a thick MTZ lie in a narrow offset range around the critical offset whereas they lie over a large offset range for a sharp Moho. We also have modelled the synthetic seismograms for velocity structures from the Oman and the Bay of Islands ophiolites. Our modelling results indicate that the quantitative analysis of amplitudes and waveforms of ultra-long offset MCS data could provide detailed information about the lower crust and MTZ.

Changes in elastic wave velocity during brittle deformation of gabbro and peridotite: Implications for oceanic Moho reflectivity

Cracks that develop during brittle deformation easily modify rocks' physical properties; therefore, understanding of seismic properties of oceanic rocks during deformation is essential for interpretation of the seismic observations of the oceanic lithosphere. Here we measured elastic wave velocities of gabbro and peridotite during triaxial deformation experiments at room temperature, a confining pressure of 20 MPa, and a strain rate of ∼10−6 s−1. Gabbroic rocks showed significant decrease in elastic wave velocities approaching the maximum stress, which is comparable to that of typical crystalline rocks. On the other hand, peridotite showed relatively sudden and small decrease in elastic wave velocities close to the maximum stress. We inverted the observed velocities to crack density tensor based on an effective medium theory. The different changes in elastic wave velocities between gabbroic rocks and peridotites were interpreted by different characteristics of crack development, where limited number of cracks were opened in peridotite than in gabbroic rocks. The calculated reflection coefficient of the interface between the gabbro and peridotite increases from <0.1 to ∼0.25 as approaching the maximum stress. Synthetic seismograms computed from the crack-dependent velocity structures of the oceanic lithosphere indicate that the Moho reflectivity is highly sensitive to the contrast of crack-damage across the crust-mantle boundary. The existence of a heterogeneous crack-damaged zone in the oceanic lithosphere, which is likely related to intra-plate earthquakes, may result in the spatial variations of Moho reflection.

Serpentinized peridotite versus thick mafic crust at the Romanche oceanic transform fault

AbstractThe crust beneath transform faults at slow-spreading ridges has been considered to be thin, comprising a thin mafic layer overlying serpentinized peridotite. Using wide-angle seismic data, we report the presence of a Moho at ∼6 km depth and a low-velocity anomaly extending down to 9 km beneath the 20-km-wide Romanche transform valley floor in the equatorial Atlantic Ocean. The low crustal velocities above the Moho could be due to either highly serpentinized mantle peridotite or fractured mafic rocks. The existence of clear Moho reflections and the occurrence of a large crustal-depth rupture during the 2016 magnitude 7.1 earthquake suggest that the crust likely consists of fractured mafic material. Furthermore, the presence of low velocities below the Moho advocates for extensive serpentinization of the mantle, indicating that the Moho reflection is unlikely to be produced by a serpentinization front. The crust to the north of the transform fault likely consists of mafic material, but that in the south appears to be more amagmatic, possibly containing serpentinized peridotite. Our results imply that the transform fault structure is complex and highly heterogeneous, and thus would have significant influence on earthquake rupture and alteration processes.

Workshop report: Exploring deep oceanic crust off Hawai`i

Abstract. For more than half a century, exploring a complete sequence of the oceanic crust from the seafloor through the Mohorovičić discontinuity (Moho) and into the uppermost mantle has been one of the most challenging missions of scientific ocean drilling. Such a scientific and technological achievement would provide humankind with profound insights into the largest realm of our planet and expand our fundamental understanding of Earth's deep interior and its geodynamic behavior. The formation of new oceanic crust at mid-ocean ridges and its subsequent aging over millions of years, leading to subduction, arc volcanism, and recycling of some components into the mantle, comprise the dominant geological cycle of matter and energy on Earth. Although previous scientific ocean drilling has cored some drill holes into old (&gt; 110 Ma) and young (&lt; 20 Ma) ocean crust, our sampling remains relatively shallow (&lt; 2 km into intact crust) and unrepresentative of average oceanic crust. To date, no hole penetrates more than 100 m into intact average-aged oceanic crust that records the long-term history of seawater–basalt exchange (60 to 90 Myr). In addition, the nature, extent, and evolution of the deep subseafloor biosphere within oceanic crust remains poorly unknown. To address these fundamentally significant scientific issues, an international workshop “Exploring Deep Oceanic Crust off Hawai`i” brought together 106 scientists and engineers from 16 countries that represented the entire spectrum of disciplines, including petrologists, geophysicists, geochemists, microbiologists, geodynamic modelers, and drilling/logging engineers. The aim of the workshop was to develop a full International Ocean Discovery Program (IODP) proposal to drill a 2.5 km deep hole into oceanic crust on the North Arch off Hawai`i with the drilling research vessel Chikyu. This drill hole would provide samples down to cumulate gabbros of mature (∼ 80 Ma) oceanic crust formed at a half spreading rate of ∼ 3.5 cm a−1. A Moho reflection has been observed at ∼ 5.5 km below the seafloor at this site, and the workshop concluded that the proposed 2.5 km deep scientific drilling on the North Arch off Hawai`i would provide an essential “pilot hole” to inform the design of future mantle drilling.

Joint ambient noise autocorrelation and receiver function analysis of the Moho

SUMMARYIn the field of seismic interferometry, cross-correlations are used to extract Green’s function from ambient noise data. By applying a single station variation of the method, using autocorrelations, we are in principle able to retrieve zero-offset reflections in a stratified Earth. These reflections are valuable as they do not require an active seismic source and, being zero-offset, are better constrained in space than passive earthquake based measurements. However, studies that target Moho signals with ambient noise autocorrelations often give ambiguous results with unclear Moho reflections. Using a modified processing scheme and phase-weighted stacking, we determine the Moho P-wave reflection time from vertical autocorrelation traces for a test station with a known simple crustal structure (HYB in Hyderabad, India). However, in spite of the simplicity of the structure, the autocorrelation traces show several phases not related to direct reflections. Although we are able to match some of these additional phases in a qualitative way with synthetic modelling, their presence makes it hard to identify the reflection phases without prior knowledge. This prior knowledge can be provided by receiver functions. Receiver functions (arising from mode conversions) are sensitive to the same boundaries as autocorrelations, so should have a high degree of comparability and opportunity for combined analysis but in themselves are not able to independently resolve VP, VS and Moho depth. Using the timing suggested by the receiver functions as a guide, we observe the Moho S-wave reflection on the horizontal autocorrelation of the north component but not on the east component. The timing of the S reflection is consistent with the timing of the PpSs–PsPs receiver function multiple, which also depends only on the S velocity and Moho depth. Finally, we combine P receiver functions and autocorrelations from HYB in a depth–velocity stacking scheme that gives us independent estimates for VP, VS and Moho depth. These are found to be in good agreement with several studies that also supplement receiver functions to obtain unique crustal parameters. By applying the autocorrelation method to a portion of the EASI transect crossing the Bohemian Massif in central Europe, we find approximate consistency with Moho depths determined from receiver functions and spatial coherence between stations, thereby demonstrating that the method is also applicable for temporary deployments. Although application of the autocorrelation method requires great care in phase identification, it has the potential to resolve both average crustal P and S velocities alongside Moho depth in conjunction with receiver functions.

Investigation of crustal thickness and uppermost mantle velocity beneath Gujarat, western India, utilizing Moho reflected P phases

Crustal thickness and its variation can provide important insights into the tectonics of a region. The most popular methodology to determine crustal thickness – receiver function analysis utilizes converted phases from the Moho discontinuity. Although the reflection co-efficient is comparable to the transmission co-efficient, reflected phases are rarely used for determining crustal thickness. We tested and utilized such a rare methodology to determine the crustal thickness and uppermost mantle P-velocity beneath Gujarat, a seismically active region of the northwestern Deccan Volcanic Province. We used waveforms of ~2900 local and regional earthquakes (ML≥2.0) recorded by a network of broadband seismometers spanning the region. After manually marking the P arrivals from all the seismograms, computing and applying several signal-to-noise ratio criteria and aligning each seismogram to their P-onsets, a total of 9134 seismograms were stacked together. This stack enabled identification of the Moho reflected phases (PmP), based on the amplitude. Forward modelling of the observed PmP data yielded estimates of crustal thickness (H) and uppermost mantle velocities (v), given a crustal velocity structure or equivalent average velocity of the region. We divided the region into 0.5° × 0.5° grids and estimated the values of H and v at each grid point, using stacks of seismograms corresponding to Moho reflection points (within 50 km) of the grid. These values provided the lateral variation of the crustal thickness and uppermost mantle velocities across Gujarat. We found that the average uppermost mantle velocity is higher in the Kachchh region compared to that in Saurashtra, probably indicating past high-temperature events that transported and accumulated high velocity materials from greater depths beneath Kachchh. The crustal thickness also follows the same trend and maximum thickness of 43 km was observed within the center of the Kachchh rift with values gradually decreasing outwards from it. It is very likely that a thick crust beneath Kachchh includes a root. The average thickness and uppermost mantle velocity in Saurashtra were found to be 38.5 km and 7.9 km/s, respectively.