ALBERT

Description: A better understanding of damage accumulation before dynamic failure events in geological material is essential to improve seismic hazard assessment. Previous laboratory tests revealed that such failure events are preceded by a phase of extending micro-cracks, leading to detectable changes in bulk seismic properties. We use seismic velocity estimates to measure changes in micro-crack populations and damage in intact actively and faulted Westerly granite samples. We use an array of 16 piezo-ceramic transducers to send and record ultrasonic pulses and to determine temporal seismic velocity changes. The coda-wave interferometry (CWI) and direct phase arrivals determine velocity changes.   The tests show that: 1) Higher confining pressures increase seismic velocities due to reduced pore space and no hysteresis effects during the unloading cycle. 2) During increasing differential stress, the crack growth before the onset of AE activity is aseismic and could be associated with a monitoring or deformation mechanism. However, the velocity measurement records the changes in a medium way before we observe AE or non-linearity in stress and strain data. 3) Direct waves exhibit strong anisotropy, increasing differential stress and accumulating damage before rock fracture. On the other hand, CWI produces velocity variations that are similar to P-wave velocity changes along the vertical axis. 4) Seismic interferometric measurements are thought to be highly sensitive to changes in the medium. However, we found that such measurements also depend on the type of rock damage (i.e., orientation and shape).

Description: A better understanding of damage accumulation before dynamic failure events in geological material is essential to improve seismic hazard assessment. Previous research has demonstrated the sensitivity of seismic velocities to variations in crack geometry, with established evidence indicating that initial crack closure induces rapid changes in velocity. Our study extends these findings by investigating velocity changes by applying coda wave interferometry (CWI). We use an array of 16 piezoceramic transducers to send and record ultrasonic pulses and to determine changes in seismic velocity on intact and faulted Westerly granite samples. Velocity changes are determined from CWI and direct phase arrivals. This study consists of three sets of experiments designed to characterize variations in seismic velocity under various initial and boundary conditions. The first set of experiments tracks velocity changes during hydrostatic compression from 2 and 191 MPa in intact Westerly granite samples. The second set of experiments focuses on saw-cut samples with different roughness and examines the effects of confining pressure increase from 2 to 120 MPa. The dynamic formation of a fracture and the preceding damage accumulation is the focus of the third type of experiment, during which we fractured an initially intact rock sample by increasing the differential stress up to 780 MPa while keeping the sample confined at 75 MPa. The tests show that: (i) The velocity change for rough saw cut samples suggests that the changes in bulk material properties have a more pronounced influence than fault surface apertures or roughness. (ii) Seismic velocities demonstrate higher sensitivity to damage accumulation under increasing differential stress than macroscopic measurements. Axial stress measured by an external load cell deviates from linearity around two-third through the experiment at a stress level of 290 MPa higher than during the initial drop in seismic velocities. (iii) Direct waves exhibit strong anisotropy with increasing differential stress and accumulating damage before rock fracture. Coda waves, on the other hand, effectively average over elastic wave propagation for both fast and slow directions, and the resulting velocity estimates show little evidence for anisotropy. The results demonstrate the sensitivity of seismic velocity to damage evolution at various boundary conditions and progressive microcrack generation with long lead times before dynamic fracture.

Description: The coda magnitude method of Mayeda and Walter (1996) provides stable source spectra and moment magnitudes (Mw) for local to regional events from as few as one station that are virtually insensitive to source and path heterogeneity. We applied the Coda Calibration Tool (CCT) to the 2019 Ridgecrest sequence and compare against results derived from the data driven, generalized inversion technique (GIT) as well as from results derived from finite-fault inversion and frequency-domain S-wave stress estimation (FSSE). We find excellent agreement over a broad range of event sizes and also confirm CCT provides stable Mw, eliminating the need for empirical magnitude relationships that tie ML to Mw. Coda has the advantage of requiring much fewer events and stations for calibration and routine measurement. Additionally, we use independent ground-truth source spectra constraints from coda spectral ratios to break the path and site trade-off, as well as not imposing a regional source scaling assumption or assuming a fixed stress drop for Green’s function events. The GIT and CCT approaches exhibit an increase in apparent stress with increasing magnitude to roughly Mw 5.5, then becomes constant. Furthermore, finite fault average apparent stress estimates and those derived from FSSE are also in good agreement with the larger, common events, further validating the GIT and CCT results. CCT stems from a multi-year collaboration between the US NDC and LLNL scientists, as well as collaboration with other institutions who are helping to evaluate and test the code with the goal of developing a fast and easy Java-based, platform independent coda envelope calibration and processing tool. CCT is freely available to the public on GitHub (https://github.com/LLNL/coda-calibration-tool). The tool can be used in routine processing to obtain stable source spectra for Mw, radiated seismic energy, apparent stress, corner frequency, and source discrimination on event type and/or depth.