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The 2019 Mw 7.1 Ridgecrest earthquake was the largest event in California over the past 20 years. The earthquake was preceded by a sequence of foreshocks. However, the physical processes leading to the mainshock remain unclear. Here, we image the ratios of compressional (P)‐ to shear (S)‐wave velocity (Vp/Vs) in the fault zones and examine the spatial and temporal evolution of near‐source material properties during the Ridgecrest earthquake sequence. We find that the Vp/Vs ratios are spatially homogeneous in the rupture zones, indicating a lack of fault‐zone material difference along strike. We identify an anomalously low Vp/Vs ratio fault patch near the mainshock hypocenter before its occurrence, which returned to the background value after the earthquake. This low Vp/Vs ratio suggests fluid overpressure, which may have facilitated the nucleation of the 2019 Ridgecrest mainshock. Plain Language Summary: Understanding the earthquake nucleation process has direct implications for earthquake physics and seismic hazards. Specifically, identifying the geophysical processes within fault zones that precede and result in subsequent earthquakes has been of great interest to the earthquake science community. This study explores how the ratio of compressional (P) wave speed to shear (S) wave speed changes in both space and time and their correlations with the subsequent seismicity evolution, focusing on the 2019 Ridgecrest earthquake sequence in California. In most slipping areas of the earthquakes, the P‐wave to S‐wave speed ratios are relatively uniform. However, in places where faults end, cross each other, or change direction, we observe unusual values. We find high ratios near the three major earthquakes on a small scale. Additionally, the ratios change where the main earthquake (magnitude 7.1) initiated. The ratios are low between a moderate (magnitude 5.4) and the main (magnitude 7.1) earthquake and increase after the main earthquake, indicating the presence of over‐pressurized fluids near the earthquake source. The associated high pore pressure might have helped nucleate the Ridgecrest mainshock. Our findings show that these speed ratios can be highly sensitive to seismic activities and could help us better understand how earthquakes start. Key Points: We examine the spatial and temporal evolution of near‐source material properties during the 2019 Ridgecrest earthquake sequenceThe spatially homogeneous Vp/Vs ratios in the rupture zones indicate little structural resistance during the earthquake rupturesThe temporal changes in Vp/Vs ratios suggest fluid presence, which may have facilitated the nucleation of the 2019 Ridgecrest mainshock [ABSTRACT FROM AUTHOR]

A laser image system has been established to quantify the characteristics of growing bubbles in quiescent shear-thinning fluids. Bubble formation mechanism was investigated by comparing the evolutions of bubble instantaneous shape, volume and surface area in two shear-thinning liquids with those in Newtonian liquid. The effects of solution mass concentration, gas chamber volume and orifice diameter on bubble detachment volume are discussed. By dimensional analysis, a single bubble volume detached within a moderate gas flowrate range was developed as a function of Reynolds number ,Re, Weber number, We, and gas chamber number, Vc, based on the orifice diameter. The results reveal that the generated bubble presents a slim shape due to the shear-thinning effect of the fluid. Bubble detachment volume increases with the solution mass concentration, gas chamber volume and orifice diameter. The results predicted by the present correlation agree better with the experimental data than the previous ones within the range of this paper.

Defect engineering is accepted as an efficient strategy for tuning the electronic structure and enhancing photocatalytic performance of layered double hydroxides (LDHs), an emerging family of two-dimensional photocatalysts, in reducing CO2 to hydrocarbons. Herein, we systematically investigate how the different types of structural defects influence electronic behavior, band edge position and Gibbs free energy barrier of CoAl-LDHs by density functional theory plus U (DFT + U) method. Our calculations indicate that CoAl-LDHs with Al defects exhibits stronger CO2 absorption capacity and narrower band gap comparing perfect CoAl-LDHs, and CoAl-LDHs with hydroxyl (–OH) defects reduce band edge down to − 1.397 V by harvesting electrons around Co atom. Furthermore, CoAl-LDHs with mixed defects of –OH and Al presents a potential synergy in photocatalytic reduction of CO2, whose calculated Gibbs free energies imply the optimal pathway for reducing CO2 to CH4 to be CO 2 → ∗ COOH → ∗ CO → ∗ CHO → ∗ CH 2 O → ∗ CH 3 O → ∗ CH 3 OH → ∗ CH 3 → CH 4 through altering the potential-determining step and lowering the Gibbs free energy barrier from 0.452 to 0.203 eV. Our achievement valuably predicts catalytic performance of CoAl-LDHs with single and mixed defects in photocatalytic CO2 reduction. [ABSTRACT FROM AUTHOR]

Ocean transform faults often generate characteristic earthquakes that repeatedly rupture the same fault patches. The westernmost Gofar transform fault quasi‐periodically hosts ∼M6 earthquakes every ∼5 years, and microseismicity suggests that the fault is segmented into five distinct zones, including a rupture barrier zone that may have modulated the rupture of adjacent M6 earthquakes. However, the relationship between the systematic slip behavior of the Gofar fault and the fault material properties is still poorly known. Specifically, the role of pore fluids in regulating the slip of the Gofar fault is unclear. Here, we use differential travel times between nearby earthquakes to estimate the in‐situ Vp/Vs of the fault‐zone materials. We apply this technique to the dataset collected by an ocean‐bottom‐seismometer network deployed around the Gofar fault in 2008, which recorded abundant microearthquakes, and find a moderate Vp/Vs of 1.75–1.80 in the rupture barrier zone and a low Vp/Vs of 1.61–1.69 in the down‐dip edge of the 2008 M6 rupture zone. This lateral variation in Vp/Vs may be caused by both pore fluids and chemical alteration. We also find a 5%–10% increase in Vp/Vs in the barrier zone during the 9 months before the mainshock. This increase may have been caused by fluid migrations or slip transients in the barrier zone. Plain Language Summary: Oceanic transform faults are ideal natural laboratories for studying earthquake physics due to their regular behaviors. The westernmost Gofar transform fault near the East Pacific Rise generates magnitude six earthquakes every 5–6 years on two rupture zones, respectively, which are separated by a barrier zone repeatedly stopping ruptures on the adjacent segments. The distinct behavior of the barrier zone might be due to differences in material properties from the rupture zones. To explore this hypothesis, we analyze thousands of small earthquakes in 2008 and find that the barrier zone has a higher ratio between P and S velocities than the rupture zone. This difference indicates that the barrier and rupture zones differ in their fluid content and chemical composition, which may have regulated their distinct slip behaviors. We also find an increase in the ratio between P and S velocities in the barrier zone during the 9 months before the magnitude six earthquake in 2008, which may reflect fluid flows or aseismic fault slips. Our findings suggest that pore fluids and the chemical composition of fault‐zone materials likely play a crucial role in regulating the slip behaviors of oceanic transform faults. Key Points: Rupture barrier zone has a moderate Vp/Vs of 1.75–1.80Down‐dip edge of the 2008 M6 mainshock has a low Vp/Vs of 1.61–1.69Vp/Vs in the rupture barrier zone increased in the 9 months before the mainshock [ABSTRACT FROM AUTHOR]

We use a dense seismic array of 1,108 vertical‐component geophones within a 600‐m footprint to detect thousands of small earthquakes near an active strand of the San Jacinto fault zone in southern California during a 26‐day period. We first correct site effects using multichannel cross‐correlations of the P‐waves of 256 cataloged earthquakes, and then perform beamforming analysis on the continuous waveforms in a slowness range from −0.4 to 0.4 s/km in both the east and north directions. At each time step, we identify the beam slowness with maximum amplitude and apply a picking algorithm to identify 13,408 events. These detections include over 55.6% of the events in the Quake Template Matching (QTM) catalog for all of southern California during the same time period and 70% of those within 100 km of the array. In addition, we detect over 10,000 new events, not in the QTM catalog. Many of these events can also be seen in records from nearby borehole seismic stations. Measured slownesses for the catalog and newly‐discovered events group into clusters that can be associated with QTM earthquake locations, but with slowness values considerably distorted from predictions based on a 1‐D velocity model, presumably owing to strong velocity heterogeneity near the San Jacinto Fault. Amplitudes of the detected events obey a Gutenberg‐Richter distribution with a b‐value close to one. Foreshocks are common among these detected events, increasing in rate before mainshocks following an inverse Omori's law. Plain Language Summary: We show that an array of over 1,000 seismometers located on a small patch of the San Jacinto Fault in southern California can detect many times more earthquakes than those in standard earthquake catalogs. Although accurate locations cannot be obtained for most of these newly identified events, their amplitudes indicate they obey similar scaling with size as larger magnitude earthquakes. Many of the larger events are preceded by foreshocks and the foreshock rate increases in the time leading up to the mainshocks. Our results suggest that future deployments of small‐aperture seismometer arrays could be used to complement existing seismic networks to probe earthquake activity in great detail. Key Points: We detect over 13,000 seismic events during 26 days in 2014 using a dense nodal array on the San Jacinto fault zone in southern CaliforniaMost of the detections are newly discovered earthquakes not in existing catalogsForeshocks are common among these earthquakes, increasing in the rate before their mainshocks following an inverse Omori's law [ABSTRACT FROM AUTHOR]