Emergence of Low-Frequency Aftershocks of the 2019 Ridgecrest Earthquake Sequence

ABSTRACT Low-frequency earthquakes (LFEs) generally have relatively stronger spectral components in the lower frequency range compared with what is expected for regular earthquakes based on their magnitude. LFEs generally occur in volcanic systems or deep (>∼15 km) in plate boundary fault zones; however, LFEs have also been observed in nonvolcanic, upper crustal settings. Because there are few studies that explore the spatiotemporal behaviors of LFEs in the shallow crust, it remains unclear whether the shallow-crustal LFEs reflect local attenuation in their immediate vicinity or differences in their source mechanism. Therefore, it is important to identify shallow-crustal LFEs and to characterize their spatiotemporal activity, which may also improve our understanding of LFEs. In this study, we focus on detecting shallow-crustal LFEs and explore the possible generation mechanisms. We analyze 29,646 aftershocks in the 2019 Ridgecrest, California, earthquake sequence, by measuring the frequency index (FI) to identify candidate low-frequency aftershocks (LFAs), while accounting for the magnitude dependency of the FI. Using small earthquakes (ML 1–3) recorded in the borehole stations to minimize the attenuation effects in near-surface layers, we identify 68 clear LFAs in total. Based on their distribution and comparisons with other seismic parameters measured by Trugman (2020), the LFAs possess distinct features from regular events in the same depths range, including low corner frequencies and low stress drops. Events in the close vicinity of LFAs exhibit lower average FI values than regular aftershocks, particularly if the hypocentral distance between an LFA and its neighbors is less than 1 km. Our results suggest that LFAs are related to local heterogeneity or a highly fractured fault zone correlated with an abundance of cross faults induced by the aftershock sequence at shallow depths. Zones of high pore-fluid pressure in intensely fractured fault zones could cause the bandlimited nature of LFAs and LFEs in general.

Nanotwin orientation on history-dependent stress decay in Cu nanopillar under constant strain

Cu with nanotwin (NT) possesses great electrical, mechanical, and thermal properties and has potential for electronic applications. Various studies have reported the effect of NT orientation on Cu mechanical properties. However, its effect on Cu stress-relaxation behavior has not been clarified, particularly in nano-scale. In this study, Cu nanopillars with various orientations were examined by a picoindenter under constant strain and observed by in-situ TEM. The angles between the twin plane and the loading direction in the examined nanopillars were 0°, 60°, to 90°, and a benchmark pillar of single-crystal Cu without NT was examined. The stress drops were respectively 10%, 80%, 4%, and 50%. Owing to the interaction by NT, the dislocation behavior in nanopillars was different from that in bulk or in thin film samples. Especially, the rapid slip path of dislocations to go to the free surface of the nanopillar induced a dislocation-free zone in the 0° nanopillar, which led to work-softening. On the contrary, a high dislocation density was observed in the 90° nanopillar, which was generated by dislocation interaction and obstruction of dislocation slip by twin planes, and it led to work-hardening. The findings reveal the NT orientation in Cu nanopillars affected stress relaxation significantly.

Dynamic rupture initiation and propagation in a fluid-injection laboratory setup with diagnostics across multiple temporal scales

Fluids are known to trigger a broad range of slip events, from slow, creeping transients to dynamic earthquake ruptures. Yet, the detailed mechanics underlying these processes and the conditions leading to different rupture behaviors are not well understood. Here, we use a laboratory earthquake setup, capable of injecting pressurized fluids, to compare the rupture behavior for different rates of fluid injection, slow (megapascals per hour) versus fast (megapascals per second). We find that for the fast injection rates, dynamic ruptures are triggered at lower pressure levels and over spatial scales much smaller than the quasistatic theoretical estimates of nucleation sizes, suggesting that such fast injection rates constitute dynamic loading. In contrast, the relatively slow injection rates result in gradual nucleation processes, with the fluid spreading along the interface and causing stress changes consistent with gradually accelerating slow slip. The resulting dynamic ruptures propagating over wetted interfaces exhibit dynamic stress drops almost twice as large as those over the dry interfaces. These results suggest the need to take into account the rate of the pore-pressure increase when considering nucleation processes and motivate further investigation on how friction properties depend on the presence of fluids.

The slow self-arresting nature of low-frequency earthquakes

Low-frequency earthquakes are a series of recurring small earthquakes that are thought to compose tectonic tremors. Compared with regular earthquakes of the same magnitude, low-frequency earthquakes have longer source durations and smaller stress drops and slip rates. The mechanism that drives their unusual type of stress accumulation and release processes is unknown. Here, we use phase diagrams of rupture dynamics to explore the connection between low-frequency earthquakes and regular earthquakes. By comparing the source parameters of low-frequency earthquakes from 2001 to 2016 in Parkfield, on the San Andreas Fault, with those from numerical simulations, we conclude that low-frequency earthquakes are earthquakes that self-arrest within the rupture patch without any introduced interference. We also explain the scaling property of low-frequency earthquakes. Our findings suggest a framework for fault deformation in which nucleation asperities can release stress through slow self-arrest processes.

Matrix permeability and flow-derived DFN constrain reactivated natural fracture rupture area and stress drop — Marcellus Shale microseismic example

Microseismic events associated with shale reservoir hydraulic fracturing stimulation (HFS) are interpreted to be reactivations of ubiquitous natural fractures (NFs). Despite adoption of discrete fracture network (DFN) models, accounting for NFs in fluid flow within shale reservoirs has remained a challenge. For an explicit account of NFs, this study introduced the use of seismology-based relations linking seismic moment, moment magnitude, fault rupture area, and stress drop. Microseismic data from HFS monitoring of Marcellus Shale horizontal wells had been used to derive planar hydraulic fracture geometry and source properties. The former was integrated with associated well production data found to exhibit transient linear flow. Analytical solutions led to linear flow parameters (LFPs) and system permeability for scenarios depicting flow through infinite and finite conductivity hydraulic fractures. Published core plug permeability was stress-corrected for in-situ conditions to estimate average matrix permeability. For comparison, the burial and thermal history for the study area was used in 1D Darcy-based modeling of steady and episodic expulsion of petroleum to account for geologic timescale persistence of abnormal pore pressure. Both evaluations resulted in matrix permeability in the same picodarcy (pD) range. Coupled with LFPs, reactivated NF surface area for stochastic DFNs was estimated. Subsequently, the aforementioned seismology-based relations were used for determining average stress drops needed to estimate NF rupture area matching flow-based DFN surface areas. Stress drops, comparable to values for tectonic events, were excluded. One of the determined values matched stress drops for HFS operations in past and recent seismological studies. In addition, calculated changes in pore pressure matched estimates in the aforementioned studies. This study unlocked the full potential of microseismic data beyond extraction of planar geometry attributes and stimulated reservoir volume (SRV). Here, microseismic events were explicitly used in the quantitative account of NFs in fluid flow within shale reservoirs.

Source Properties of Moderate-Magnitude Earthquakes Along the Alpine Fault, New Zealand

<p><b>The Alpine Fault is a major active continental transform fault that is late in its typical cycle of large earthquakes. Extensive paleoseismic research has revealed that the central segment of the Alpine Fault ruptures in M7+ earthquakes every 291±23 years and last ruptured in 1717 AD. The paleoseismic results also reveal that some places along the fault, which coincide with pronounced along-strike changes in fault characteristics, act as conditional barriers to rupture. The geometry, seismicity rates and geology of the Alpine Fault change along three principal segments (North Westland, Central and South Westland segments) but it is unclear whether source properties (e.g. stress drop) of near-fault seismicity also vary between those fault segments, and whether these properties have some influence on conditional segmentation of the Alpine Faultduring large earthquake rupture.</b></p> <p>To examine whether source properties of earthquakes can influence or elucidate the conditional segmentation of Alpine Fault earthquakes, we have computed stress drops of moderate-magnitude earthquakes occurring on and close to the Alpine Fault. We use an empirical Green’s function (EGF) approach and require each EGF earthquake to be highly correlated (cross-correlation ≥0.8) with its respective mainshock. We use data from dense, temporary seismometer networks, including DWARFS (Dense WestlandArrays Researching Fault Segmentation), a new two-part network designed to constrain seismogenic behaviour near key transitional boundaries. Our results investigate the spatial variability of these source properties along the length of the Alpine Fault, focusing on whether earthquakes at the rupture segment boundaries behave differently to those in the middle of previously identified rupture segments.</p> <p>We analyse individual P- and S-wave measurements of corner frequency and stress drop for 95 earthquakes close to (within 5 km) and on the Alpine Fault. Overall, the calculated stress drops range between 1–352 MPa and show good agreement with other studies both within New Zealand and worldwide. The stress drop values obtained for the three Alpine segment are: 1–143 MPa (median values of 8 and 9 MPa for P- and S-waves, respectively) for the South Westland/Central segment boundary zone, 2–309 MPa (median values of 17 and 39 MPa for P- and S-waves, respectively) for the Central segment and 1–352 MPa (median values of 15 and 19 MPa for P- and S-waves, respectively) for the North Westland/Central segment boundary zone. There are no marked differences in stress drop values along the North Westland and Central segments, but those values are slightly higher than along the South Westland segment.</p> <p>This may indicate a bigger difference in fault geometry, slip and seismicity rate compare with other segments, or that the South Westland segment is weaker than the other segments. We see no clear dependence of stress drop values on depth, magnitude or focal mechanism.</p>

Source Properties of Moderate-Magnitude Earthquakes Along the Alpine Fault, New Zealand

<p><b>The Alpine Fault is a major active continental transform fault that is late in its typical cycle of large earthquakes. Extensive paleoseismic research has revealed that the central segment of the Alpine Fault ruptures in M7+ earthquakes every 291±23 years and last ruptured in 1717 AD. The paleoseismic results also reveal that some places along the fault, which coincide with pronounced along-strike changes in fault characteristics, act as conditional barriers to rupture. The geometry, seismicity rates and geology of the Alpine Fault change along three principal segments (North Westland, Central and South Westland segments) but it is unclear whether source properties (e.g. stress drop) of near-fault seismicity also vary between those fault segments, and whether these properties have some influence on conditional segmentation of the Alpine Faultduring large earthquake rupture.</b></p> <p>To examine whether source properties of earthquakes can influence or elucidate the conditional segmentation of Alpine Fault earthquakes, we have computed stress drops of moderate-magnitude earthquakes occurring on and close to the Alpine Fault. We use an empirical Green’s function (EGF) approach and require each EGF earthquake to be highly correlated (cross-correlation ≥0.8) with its respective mainshock. We use data from dense, temporary seismometer networks, including DWARFS (Dense WestlandArrays Researching Fault Segmentation), a new two-part network designed to constrain seismogenic behaviour near key transitional boundaries. Our results investigate the spatial variability of these source properties along the length of the Alpine Fault, focusing on whether earthquakes at the rupture segment boundaries behave differently to those in the middle of previously identified rupture segments.</p> <p>We analyse individual P- and S-wave measurements of corner frequency and stress drop for 95 earthquakes close to (within 5 km) and on the Alpine Fault. Overall, the calculated stress drops range between 1–352 MPa and show good agreement with other studies both within New Zealand and worldwide. The stress drop values obtained for the three Alpine segment are: 1–143 MPa (median values of 8 and 9 MPa for P- and S-waves, respectively) for the South Westland/Central segment boundary zone, 2–309 MPa (median values of 17 and 39 MPa for P- and S-waves, respectively) for the Central segment and 1–352 MPa (median values of 15 and 19 MPa for P- and S-waves, respectively) for the North Westland/Central segment boundary zone. There are no marked differences in stress drop values along the North Westland and Central segments, but those values are slightly higher than along the South Westland segment.</p> <p>This may indicate a bigger difference in fault geometry, slip and seismicity rate compare with other segments, or that the South Westland segment is weaker than the other segments. We see no clear dependence of stress drop values on depth, magnitude or focal mechanism.</p>

Near-Source Ground Motions and Their Variability Derived from Dynamic Rupture Simulations Constrained by NGA-West2 GMPEs

ABSTRACT Empirical ground-motion prediction equations (GMPEs) lack a sufficient number of measurements at near-source distances. Seismologists strive to supplement the missing data by physics-based strong ground-motion modeling. Here, we build a database of ∼3000 dynamic rupture scenarios, assuming a vertical strike-slip fault of 36×20 km embedded in a 1D layered elastic medium and linear slip-weakening friction with heterogeneous parameters along the fault. The database is built by a Monte Carlo procedure to follow median and variability of Next Generation Attenuation-West2 Project GMPEs by Boore et al. (2014) at Joyner–Boore distances 10–80 km. The synthetic events span a magnitude range of 5.8–6.8 and have static stress drops between 5 and 40 MPa. These events are used to simulate ground motions at near-source stations within 5 km from the fault. The synthetic ground motions saturate at the near-source distances, and their variability increases at the near stations compared to the distant ones. In the synthetic database, the within-event and between-event variability are extracted for the near and distant stations employing a mixed-effect model. The within-event variability is lower than its empirical value, only weakly dependent on period, and generally larger for the near stations than for the distant ones. The between-event variability is by 1/4 lower than its empirical value at periods &gt;1 s. We show that this can be reconciled by considering epistemic error in Mw when determining GMPEs, which is not present in the synthetic data.

Enhanced Electroplasticity through Room-Temperature Dynamic Recrystallization in a Mg-3Al-1Sn-1Zn Alloy

It has been well known that electric pulse can be utilized to enhance the plasticity of metals, which is attributed to the change of dislocation dynamics, e.g., localized planar slip to homogeneous wavy slip. Here, we show another effect of pulse current, which facilitates texture weakening through room-temperature dynamic recrystallization and additionally improve the plasticity of a polycrystalline Mg-3Al-1Sn-1Zn alloy. By conducting a tensile test under electrical pulse, we found that the peak flow stress and fracture strain depend strongly on current density. As peak current densities increases, the flow stress drops and the fracture strain increases. Our Electron Backscatter Diffraction results suggest that dynamic recrystallization occurs at room temperature, which develops a weakened texture. Our work provides a new insight into electroplasticity mechanism in Mg alloys.

Bayesian Parameter Estimation for Space and Time Interacting Earthquake Rupture Model Using Historical and Physics-Based Simulated Earthquake Catalogs

ABSTRACT This article introduces a framework to supplement short historical catalogs with synthetic catalogs and determine large earthquakes’ recurrence. For this assessment, we developed a parameter estimation technique for a probabilistic earthquake occurrence model that captures time and space interactions between large mainshocks. The technique is based on a two-step Bayesian update that uses a synthetic catalog from physics-based simulations for initial parameter estimation and then the historical catalog for further calibration, fully characterizing parameter uncertainty. The article also provides a formulation to combine multiple synthetic catalogs according to their likelihood of representing empirical earthquake stress drops and Global Positioning System-inferred interseismic coupling. We applied this technique to analyze large-magnitude earthquakes’ recurrence along 650 km of the subduction fault’s interface located offshore Lima, Peru. We built nine 2000 yr long synthetic catalogs using quasi-dynamic earthquake cycle simulations based on the rate-and-state friction law to supplement the 450 yr long historical catalog. When the synthetic catalogs are combined with the historical catalog without propagating their uncertainty, we found average relative reductions larger than 90% in the recurrence parameters’ uncertainty. When we propagated the physics-based simulations’ uncertainty to the posterior, the reductions in uncertainty decreased to 60%–70%. In two Bayesian assessments, we then show that using synthetic catalogs results in higher parameter uncertainty reductions than using only the historical catalog (69% vs. 60% and 83% vs. 80%), demonstrating that synthetic catalogs can be effectively combined with historical data, especially in tectonic regions with short historical catalogs. Finally, we show the implications of these results for time-dependent seismic hazard.