Outer trench slope extension to frontal wedge compression in a subducting plate

Faulting in subducting plates is a critical process that changes the mechanical properties the subducting lithosphere and serves as a carrier of surface materials into mantle wedges. Two intraplate earthquake sequences located in the northern Manila subduction system were investigated in this study, which revealed distinct fault planes but a contrasting seismogeny over the northern Manila Trench. The seismic sequences analyzed in this study were of small-to-moderate events. The events were separately acquired by two ocean-bottom seismometer networks deployed on the frontal accretionary wedge in 2005 and the outer trench slope in 2006. The retrieved seismicity in the frontal wedge (in 2005) mainly included the overpressured sequence, whereas that in the approaching plate (in 2006) was aftershocks of an extensional faulting sequence. The obtained seismic velocity models and Vp/Vs ratios revealed that the overpressure was likely caused by dehydration within the shallow subduction zone. By using the near-field waveform inversion algorithm, we determined focal mechanism solutions for a few relatively large earthquakes. Data from global seismic observations were also used to conclude that stress transfer may be responsible for the seismic activity in the study area in 2005–2006. In late 2005, the plate interface in the frontal wedge area was unlocked by overpressure effect with the thrusting-dominant sequence. This event changed the stress regime across the Manila Trench and triggered the normal fault extension at the outer trench slope in mid-2006. However, the hybrid focal solution indicating reverse and strike-slip mechanisms provided in this study revealed that the plate interface had become locked again in late 2006.

Fluids Triggered the 2021 Mw 6.1 Yangbi Earthquake at an Unmapped Fault: Implications for the Tectonics at the Northern End of the Red River Fault

On 21 May 2021 a magnitude Mw 6.1 earthquake occurred in Yangbi region, Yunan, China, which was widely felt and caused heavy casualties. Imaging of the source region was conducted using our improved double-difference tomography method on the huge data set recorded by 107 temporary stations of ChinArray-I and 62 permanent stations. Pronounced structural heterogeneities across the rupture source region are discovered and locations of the hypocenters of the Yangbi earthquake sequence are significantly improved as the output of the inversion. The relocated Yangbi earthquake sequence is distributed at an unmapped fault that is almost parallel and adjacent (∼15 km distance) to the Tongdian–Weishan fault (TWF) at the northern end of the Red River fault zone. Our high-resolution 3D velocity models show significant high-velocity and low-VP/VS ratios in the upper crust of the rupture zone, suggesting the existence of an asperity for the event. More importantly, low-VS and high-VP/VS anomalies below 10 km depth are imaged underlying the source region, indicating the existence of fluids and potential melts at those depths. Upward migration of the fluids and potential melts into the rupture zone could have weakened the locked asperity and triggered the occurrence of the Yangbi earthquake. The triggering effect by upflow fluids could explain why the Yangbi earthquake did not occur at the adjacent TWF where a high-stress accumulation was expected. We speculate that the fluids and potential melts in the mid-to-lower crust might have originated either from crustal channel flow from the southeast Tibet or from local upwelling related to subduction of the Indian slab to the west.

Seismic Analysis of Moderate Size Earthquakes Recorded on Stations at Close Epicentral Distances

In this study, we analyzed the near-field seismic records of two moderate sized earthquakes in the Western Balkan region: the September 2016 Skopje earthquake, magnitude ML5.3 and the March 2020 Zagreb earthquake, magnitude ML5.5. Such recordings at close epicentral distances are rare and are thus very useful for testing some of the theoretical assumptions used in modeling earthquake risk. Firstly, response spectra were computed using the digital time histories for the three closest stations to the Skopje 2016 earthquake and the two closest stations to the Zagreb 2020 earthquake. Their characteristics were examined in terms of frequency and peak amplitude ranges. Secondly, the Nakamura method was applied to the records from the selected five stations coded SKO, FCE, IZIIS, QUHS, and QARH. The results of the spectral analysis were compared with interpretations from the geological and geotechnical maps at each location. Our findings support the idea that these combined methods can be used to categorize the underlying structural profile to a first approximation and can be used to derive velocity models.

Migration velocity analysis based on focusing diffractions generated using the CRS wavefront attributes: Application to real land data

In land seismic data processing, the prestack time migration (PSTM) image remains the standard imaging output, but a reliable migrated image of the subsurface depends on the accuracy of the migration velocity model. We have adopted two new algorithms for time-domain migration velocity analysis based on wavefield attributes of the common-reflection-surface (CRS) stack method. These attributes, extracted from multicoverage data, were successfully applied to build the velocity model in the depth domain through tomographic inversion of the normal-incidence-point (NIP) wave. However, there is no practical and reliable method for determining an accurate and geologically consistent time-migration velocity model from these CRS attributes. We introduce an interactive method to determine the migration velocity model in the time domain based on the application of NIP wave attributes and the CRS stacking operator for diffractions, to generate synthetic diffractions on the reflection events of the zero-offset (ZO) CRS stacked section. In the ZO data with diffractions, the poststack time migration (post-STM) is applied with a set of constant velocities, and the migration velocities are then selected through a focusing analysis of the simulated diffractions. We also introduce an algorithm to automatically calculate the migration velocity model from the CRS attributes picked for the main reflection events in the ZO data. We determine the precision of our diffraction focusing velocity analysis and the automatic velocity calculation algorithms using two synthetic models. We also applied them to real 2D land data with low quality and low fold to estimate the time-domain migration velocity model. The velocity models obtained through our methods were validated by applying them in the Kirchhoff PSTM of real data, in which the velocity model from the diffraction focusing analysis provided significant improvements in the quality of the migrated image compared to the legacy image and to the migrated image obtained using the automatically calculated velocity model.

SOURCE PARAMETERS of CRIMEAN-BLACK SEA EARTHQUAKES in 2015

The spectral and dynamic source parameters (М0, r0, , , ησ, , u, Eu and Mw) of 16 Crimean earthquakes with КП=6.5–10.8, restored by amplitude spectra of compression and shear seismic waves recorded by digital regional seismic stations are analyzed. Approximation of the spectra and source parameters calculation is performed in the framework of the Brune dislocation model. The highest values of dynamic parameters (М0, r0, , , ησ, u, EU и Mw) are obtained for the earthquakes on June 13 and August 16 with h=11 km, h=7 km respectively and КП=10.8, which occurred in the Azov-Kuban and Kerch-Anapa areas. The radiation friction r for all earthquakes had a negative value, pointing to a complex slide of the rupture in the source. Within the whole energy range, the average value of the released stress did not exceed Δσ=8∙105 PA (8 bar) and apparent stress ησ <11∙105 PA (11 bar). For most 2015 earthquakes, the average M0 и r0 values were within the confidence intervals of long-term dependencies M0(КП), r0(КП). The values of r0 were evenly distributed concerning the regression r0(КП) and М0 is mostly located below the average according to М0 (КП). The maximum deviations of M0 from the long-term М0(КП) dependence were obtained for the most strong earthquakes on June 13 and August 16 with КП=10.8. These deviations can be associated with participation in average M0 of the "Sevastopol" station data which give low values of М0 and possible errors in determining the focal depths influencing the choice of environment velocity models to calculate М0. For the most strong earthquake of August 16 with Мw=3.8, which occurred in the Kerch-Anapa region, a solution of focal mechanism was obtained. The earthquake occurred under the action of horizontal latitudinal tensile forces. The type of movement in the source is an oblique normal fault. Both nodal planes have near-meridional (STKNP1=167°) and near-diagonal (STKNP2=336°) strike.

Saliency-Map Guided Salt Prediction by a Multi-Channel Convolutional Neural Network

One of the major challenges in seismic imaging is accurately delineating subsurface salt. Since a salt boundary has strong impedance compared with other sediments, we build a saliency map with intensity and orientation to create a pixel-level model for salt interpretation. In this abstract, we train a saliency-map as an additional attribute to combine with the original seismic to predict salt bodies. We also train a saliency-map to classify multiple geological facies in a multi-channel convolutional neural network with residual net architecture to help build subsurface velocity models. Two examples are shown which demonstrate that a saliency-map-plus-seismic model successfully improves the accuracy of salt prediction and reduces artifacts.

Accelerating Subsurface Data Processing and Interpretation with Cloud-Based Full Waveform Inversion Systems

Understanding the earth's subsurface is critical to the needs of the exploration and production (E&P) industry for minimizing risk and maximizing recovery. Until recently, the industry's service sector has not made many advances in data-driven automated earth model building from raw exploration seismic data. But thankfully, that has now changed. The industry's leading technique to gain an unprecedented increase in resolution and accuracy when establishing a view of the interior of the earth is known as the Full Waveform Inversion (FWI). Advanced formulations of FWI are capable of automating subsurface model building using only raw unprocessed data. Cloud-based FWI is helping to accelerate this journey by encompassing the most sophisticated waveform inversion techniques with the largest compute facility on the planet. This combines to give verifiable accuracy, more automation and more efficiency. In this paper, we describe the transformation of enabling cloud-based FWI to natively take advantage of the public cloud platform's main strength in terms of flexibility and on-demand scalability. We start from lift-and-shift of a legacy MPI-based application designed to be run by a traditional on-prem job scheduler. Our specific goals are to (1) utilize a heterogeneous set of compute hardware throughout the lifecycle of a production FWI run without having to provision them for the entire duration, (2) take advantage of cost-efficient spare-capacity compute instances without uptime guarantees, and (3) maintain a single codebase that can be run both on on-prem HPC systems and on the cloud. To achieve these goals meant transitioning the job-scheduling and "embarrassingly parallel" aspects of the communication code away from using MPI, and onto various cloud-based orchestration systems, as well as finding cloud-based solutions that worked and scaled well for the broadcast/reduction operation. Placing these systems behind a customized TCP-based stub for MPI library calls allows us to run the code as-is in an on-prem HPC environment, while on the cloud we can asynchronously provision and suspend worker instances (potentially with very different hardware configurations) as needed without the burden of maintaining a static MPI world communicator. With this dynamic cloud-native architecture, we 1) utilize advanced formulations of FWI capable of automating subsurface model building using only raw unprocessed data, 2) extract velocity models from the full recorded wavefield (refractions, reflections and multiples), and 3) introduce explicit sensitivity to reflection moveout, invisible to conventional FWI, for macro-model updates below the diving wave zone. This makes it viable to go back to older legacy datasets acquired in complex environments and unlock considerable value where FWI until now has been impossible to apply successfully from a poor starting model.

The Role of Legacy Seismic in Enhancing the Image in the Current Seismic Data – Abu Dhabi: Case Study

Acquiring surface seismic data can be challenging in areas of intense human activities, due to presence of infrastructures (roads, houses, rigs), often leaving large gaps in the fold of coverage that can span over several kilometers. Modern interpolation algorithms can interpolate up to a certain extent, but quality of reconstructed seismic data diminishes as the acquisition gap increases. This is where vintage seismic acquisition can aid processing and imaging, especially if previous acquisition did not face the same surface obstacles. In this paper we will present how the legacy seismic survey has helped to fill in the data gaps of the new acquisition and produced improved seismic image. The new acquisition survey is part of the Mega 3D onshore effort undertaken by ADNOC, characterized by dense shot and receiver spacing with focus on full azimuth and broadband. Due to surface infrastructures, data could not be completely acquired leaving sizable gap in the target area. However, a legacy seismic acquisition undertaken in 2014 had access to such gap zones, as infrastructures were not present at the time. Legacy seismic data has been previously processed and imaged, however simple post-imaging merge would not be adequate as two datasets were processed using different workflows and imaging was done using different velocity models. In order to synchronize the two datasets, we have processed them in parallel. Data matching and merging were done before regularization. It has been regularized to radial geometry using 5D Matching Pursuit with Fourier Interpolation (MPFI). This has provided 12 well sampled azimuth sectors that went through surface consistent processing, multiple attenuation, and residual noise attenuation. Near surface model was built using data-driven image-based static (DIBS) while reflection tomography was used to build the anisotropic velocity model. Imaging was done using Pre-Stack Kirchhoff Depth Migration. Processing legacy survey from the beginning has helped to improve signal to noise ratio which assisted with data merging to not degrade the quality of the end image. Building one near surface model allowed both datasets to match well in time domain. Bringing datasets to the same level was an important condition before matching and merging. Amplitude and phase analysis have shown that both surveys are aligned quite well with minimal difference. Only the portion of the legacy survey that covers the gap was used in the regularization, allowing MPFI to reconstruct missing data. Regularized data went through surface multiple attenuation and further noise attenuation as preconditioning for migration. Final image that is created using both datasets has allowed target to be imaged better.

Velocity Models for the Crust Hosting the Main Aftershock Cluster of the 2011 Mineral, Virginia, Earthquake

Three-dimensional P- and S-wave velocity (VP and VS) models are determined for the crust containing the main aftershock cluster of the 2011 Mineral, Virginia, earthquake using local earthquake tomography. The inversion uses a total of 5125 arrivals (2465 P- and 2660 S-wave arrivals) for 324 aftershocks recorded by 12 stations. The inversion volume (22 × 20 × 16 km) is completely contained within the Piedmont Chopawamsic metavolcanic terrane. The models are well resolved in the central portion of the inversion volume in the depth range 1–5 km; good resolution does not extend to the hypocenter depth of the mainshock. Most aftershocks are located within a northeast-trending, southeast-dipping region containing negative VP anomalies, positive VS anomalies, and VP/VS ratios as low as 1.53. These velocity results strongly argue for the presence of quartz-rich rocks, which we attribute to either the presence of a giant quartz vein system or metamorphosed orthoquarzite sandstones originally deposited on the Laurentian passive margin and subsequently incorporated into the Chopawamsic thrust sheets during island arc collision in the Taconic orogeny.