Seismica

When are Non-Double-Couple Components of Seismic Moment Tensors Reliable?

Boris Rösler, Seth Stein, Bruce D. Spencer — Sat, 11 Mar 2023 00:00:00 -0500

There has been considerable discussion as to how to assess when non-double-couple (NDC) components of seismic moment tensors represent real source processes. We explore this question by comparing moment tensors (MTs) of earthquakes in three global catalogs, which use different inversion procedures. Their NDC components are only weakly correlated between catalogs, suggesting that they are largely artifacts of the inversion. A monotonic decrease in the NDC components' standard deviation with magnitude indicates increased reliability of the NDC components for larger earthquakes. The standard deviation begins to decrease for large NDC components exceeding 60%, suggesting that they represent real source processes. Randomly generated NDC components with the same mean and standard deviation as in the MT catalogs only reproduce some of this decrease. Thus NDC components of large earthquakes and NDC components that exceed 60% are likely to represent real source processes.

PyRaysum: Software for Modeling Ray-theoretical Plane Body-wave Propagation in Dipping Anisotropic Media

Wasja Bloch, Pascal Audet — Wed, 15 Feb 2023 00:00:00 -0500

This article introduces PyRaysum, a Python software for modeling ray-theoretical body-wave propagation in dipping and/or anisotropic layered media based on the popular Fortran code Raysum. We improve and expand upon Raysum in several ways: 1) we significantly reduce the overhead by avoiding I/O operations; 2) we implement automatic phase labeling to facilitate the interpretation of complex seismograms; 3) we provide the means to correct inaccuracies in the calculated amplitude of free surface reverberations. We take advantage of the modern, object-oriented Python environment to offer various classes and methods to perform receiver function calculation, filtering and plotting. PyRaysum also integrates well with NumPy and ObsPy, two standard libraries for numerical computing and seismology. PyRaysum is built in Python version 3 and requires a Fortran compiler, but otherwise runs on all platforms. The software offers a high-level, ease-of-use user interface and is equipped with complete documentation and testing as well as tutorials to reproduce published examples from the literature. Time-optimized post-processing functions allow for the straightforward and efficient incorporation of PyRaysum synthetic data into optimization or probabilistic parametric search approaches.

3D Paleoseismology from iOS Lidar and Structure from Motion Photogrammetry: a case study on the Dog Valley fault, California

Ian Pierce, Richard Koehler — Thu, 12 Jan 2023 00:00:00 -0500

Measuring displacements of strike-slip paleoearthquakes from trenching excavations requires detailed 3D trenching excavations. Here a new methodology utilizing an iOS based laser scanner and structure-from-motion is used to reconstruct stratigraphy and trace a displaced fluvial channel sequence across the Dog Valley fault in Northeastern California. The Dog Valley fault is a left-lateral strike slip fault in the northern Walker Lane. The northern Walker Lane accommodates ~5-7 mm/yr of dextral shear; however, the relative rates of deformation and earthquake history of the fault have not been previously assessed. Here, we present geomorphic mapping observations and preliminary paleoseismic trenching results from the Dog Valley fault. Lidar data reveal a clear east-northeast striking fault trace that extends about ~25 km from the Prosser Creek drainage west of the Polaris Fault near Highway 89 to the northwest flank of Peavine Mountain. The main trace of the fault appears to project through Stampede dam. Youthful fault scarps are visible along much of the fault, with alternating northwest- and southeast-facing scarps. Clear lateral displacements are largely absent along the fault, however right-stepping fault strands, sidehill benches, linear valleys and ridges, and alternating scarp facing directions are all consistent with left-lateral strike slip displacement. Stratigraphic and structural relations exposed in the Dog Valley fault trench show clear truncations and tilting of bedded fluvial and peat deposits and provide evidence for the occurrence of two Holocene earthquakes: the most recent earthquake postdates ~8 ka, and an earlier earthquake is inferred to have occurred between 8491-8345 cal. ybp. Based on 3D excavations of a prominent channel margin, the most recent earthquake was associated with ~ 115 cm of left-lateral displacement, corresponding to an M6.7 earthquake.

Validation of Peak Ground Velocities Recorded on Very-high rate GNSS Against NGA-West2 Ground Motion Models

Brendan Crowell, Jensen DeGrande, Timothy Dittmann, Jessica Ghent — Wed, 01 Mar 2023 00:00:00 -0500

Observations of strong ground motion during large earthquakes are generally made with strong-motion accelerometers. These observations have a critical role in early warning systems, seismic engineering, source physics studies, basin and site amplification, and macroseismic intensity estimation. In this manuscript, we present a new observation of strong ground motion made with very high rate (>= 5 Hz) Global Navigation Satellite System (GNSS) derived velocities. We demonstrate that velocity observations recorded on GNSS instruments are consistent with existing ground motion models and macroseismic intensity observations. We find that the ground motion predictions using existing NGA-West2 models match our observed peak ground velocities with a median log total residual of 0.03-0.33 and standard deviation of 0.72-0.79, and are statistically significant following normality testing. We finish by deriving a Ground Motion Model for peak ground velocity from GNSS and find a total residual standard deviation 0.58, which can be improved by ~2% when considering a simple correction for Vs30.

Ocean Surface Gravity Wave Excitation of Flexural Gravity and Extensional Lamb Waves in Ice Shelves

Lauren Abrahams, Jose Mierzejewski , Eric Dunham, Peter D. Bromirski — Thu, 16 Mar 2023 00:00:00 -0400

Flexure and extension of ice shelves in response to incident ocean surface gravity waves have been linked to iceberg calving, rift growth, and even disintegration of ice shelves. Most modeling studies utilize a plate bending model for the ice, focusing exclusively on flexural gravity waves. Ross Ice shelf seismic data shows not only flexural gravity waves, with dominantly vertical displacements, but also extensional Lamb waves, which propagate much faster with dominantly horizontal displacements. Our objective is to model the full-wave response of ice shelves, including ocean compressibility, ice elasticity, and gravity. Our model is a 2D vertical cross-section of the ice shelf and sub-shelf ocean cavity. We quantify the frequency-dependent excitation of flexural gravity and extensional Lamb waves and provide a quantitative theory for extensional Lamb wave generation by the horizontal force imparted by pressure changes on the vertical ice shelf edge exerted by gravity waves. Our model predicts a horizontal to vertical displacement ratio that increases with decreasing frequency, with ratio equal to unity at ~0.001 Hz. Furthermore, in the very long period band (<0.003 Hz), tilt from flexural gravity waves provides an order of magnitude larger contribution to seismometer horizontal components than horizontal displacements from extensional Lamb waves.

Self-sufficient seismic boxes for monitoring glacier seismology

Ana Nap, Fabian Walter, Martin Lüthi, Adrien Wehrlé — Thu, 19 Jan 2023 00:00:00 -0500

Glacier seismology is a valuable tool for investigating ice flow dynamics, but sufficient data acquisition in remote and exposed glaciated terrain remains challenging. For data acquisition on a highly crevassed and remote outlet glacier in Greenland we developed self-sufficient and easily deployable seismic stations, "SG-boxes". The SG-boxes contain their own power supply via solar panel, a three-component omni-directional geophone and a GNSS receiver. The SG-boxes can be deployed and retrieved from a hovering helicopter, allowing for deployment in difficult terrain. To assess their performance we conducted a field test comparing the SG-boxes to established on-ice geophone installations at Gornergletscher in Switzerland. Moreover, data from a first SG-box deployment in Greenland were analyzed. The SG-boxes exhibit consistently higher noise levels relative to colocated conventional geophones and a correlation between noise levels, wind and air temperature is found. Despite their noise susceptibility, the SG-boxes detected a total of 13,114 Gornergletscher icequakes over 10 days, which is 30% of the total number of icequakes detected by conventional geophone stations. Hence, even in sub-optimal weather conditions and without additional noise reduction measures, the SG-boxes can provide unique and valuable data from challenging glaciated terrain where no conventional seismic installations are possible.