Crustal deformation patterns are affected by multiscale granulation and healing processes associated with phase transitions between continuum and discrete states of rocks. The ongoing ...continuum‐discrete transitions are accompanied by progressive evolution of disordered fault networks to dominant localized fault zones, development of bimaterial interfaces, and increasing dynamic weakening of fault surfaces. Results on individual fault zones point to three general dynamic regimes. The first is associated with broad range of heterogeneities, little dynamic weakening, power law frequency‐size statistics, temporal clustering of intermediate and large events, and accelerated seismic release before large earthquakes. The second is associated with relatively uniform localized structures, significant dynamic weakening, characteristic earthquake statistics, and quasi‐periodic temporal occurrence of large events without precursory accelerated release. For a range of conditions, the fault zone response can switch back and forth between the foregoing two dynamic regimes. Higher temperature, fluid content, and thickness of sedimentary cover reduce the seismic coupling in a region and change the properties of local earthquake sequences. Brittle regions with high seismic coupling have few foreshocks and long‐duration aftershock sequences with high event productivity, whereas more viscous regions with low seismic coupling have increased foreshocks activity and low‐productivity aftershock sequences or swarms. The results provide criteria for organizing data in classes associated with different evolutionary stages and different regional conditions. An ability to recognize the dynamic regime of a given fault zone or a region can increase the information content of the data and lead to improved strategies for seismic hazard assessment.
We attempt to clarify processes associated with the 2019 Ridgecrest earthquake sequence by analyzing space‐time variations of seismicity, potency values, and focal mechanisms of earthquakes leading ...to and during the sequence. Over the 20 years before the Mw7.1 mainshock, the percentage of normal faulting events decreased gradually from 25% to below 10%, indicating a long‐term increase of shear stress. The Mw6.4 and Mw7.1 ruptures terminated at areas with strong changes of seismic velocity or intersections with other faults producing arresting barriers. The aftershocks are characterized by highly diverse focal mechanisms and produced volumetric brittle deformation concentrated in a 5–10 km wide zone around the main ruptures. Early aftershocks of the Mw7.1 event extended over a wide area below typical seismogenic depth, consistent with a transient deepening of the brittle‐ductile transition. The Ridgecrest earthquake sequence produced considerable rock damage in the surrounding crust including below the nominal seismogenic zone.
Plain Language Summary
The Eastern California Shear Zone is one of the seismically most active regions in Southern California and hosted in the last few decades several large earthquakes. The most recent of these is the 5 July 2019, Ridgecrest earthquake with magnitude 7.1, which was followed by a vigorous aftershock sequence. To clarify processes associated with the Ridgecrest earthquake sequence, we analyze properties of earthquakes before, during and after the 2019 Ridgecrest mainshock. The fraction of normal faulting events was reduced gradually in the 20 years before the mainshock, indicating a long‐term increase of shear stress. The Ridgecrest earthquake and a moderate event with magnitude 6.4 about 34 hr earlier terminated in areas with high seismic velocity or intersections with other faults, which might act as barriers that arrested the ruptures. The aftershocks following the Ridgecrest mainshock have highly diverse mechanisms and are widely distributed within 5–10 km wide zone around the main rupture. Many of the early aftershocks are deeper than the regular seismogenic zone. The results highlight the strongly heterogeneous volumetric nature of crustal deformation during large earthquakes off main plate‐boundary faults.
Key Points
The fraction of normal faulting events in the area dropped in the past 20 years from >25% to <10%, implying increasing shear stress
The Mw6.4 and Mw7.1 events terminated at areas with strong changes of seismic velocity or junctions with other faults acting as barriers
Aftershocks with diverse mechanisms produced significant potency in a 5–10 km wide zone, including deeper‐than‐usual early events
Proper classification of nontectonic seismic signals is critical for detecting microearthquakes and developing an improved understanding of ongoing weak ground motions. We use unsupervised machine ...learning to label five classes of nonstationary seismic noise common in continuous waveforms. Temporal and spectral features describing the data are clustered to identify separable types of emergent and impulsive waveforms. The trained clustering model is used to classify every 1 s of continuous seismic records from a dense seismic array with 10–30 m station spacing. We show that dominate noise signals can be highly localized and vary on length scales of hundreds of meters. The methodology demonstrates the complexity of weak ground motions and improves the standard of analyzing seismic waveforms with a low signal‐to‐noise ratio. Application of this technique will improve the ability to detect genuine microseismic events in noisy environments where seismic sensors record earthquake‐like signals originating from nontectonic sources.
Plain Language Summary
Improvements in microseismic detection will advance observations of failure processes on faults subjected to slowly accumulating tectonic stress. Continuous seismic waveforms contain copious variations of nontectonic signals that inhibit the detection of genuine microearthquakes. Developing a framework to identify emergent and impulsive signals originating from natural and anthropogenic activity will advance seismic network monitoring capabilities. We apply unsupervised machine learning techniques to classify multiple types of weak ground motions that occur ubiquitously in continuous seismic records. A trained model is used to label every 1 s of a dense geophone array to provide a high‐resolution description of the anatomy of continuous seismic records. Further methodology developments and application in various environments have high potential for improving the ability to monitor earthquakes and other sources of ground motion.
Key Points
Emergent and impulsive signals in continuous seismic waveforms are identified using cluster analysis on a dense array data
An unsupervised learning model is trained to identify multiple classes of noise using temporal and spectral data features
A more complete understanding of seismic noise signals will improve the ability to detect genuine microseismic events
We analyze space‐time variations in the depth distribution of seismicity in Southern and Baja California, focusing on transients following four M ≥ 6.7 mainshocks. The regular brittle‐ductile ...transition depth is estimated at different locations as the local bottom of 99,636 background events and is compared with the bottom of events within earthquake clusters. The four M ≥ 6.7 mainshock‐aftershock sequences exhibit early aftershocks with depths up to 5 km below the regular brittle‐ductile transition depth and epicentral distances up to 15 km from the mainshock ruptures. The maximum aftershock depth increases abruptly following the mainshocks and recovers to the background level after several years. The wide‐spread deeper‐than‐usual early aftershocks favor classical brittle‐ductile transition over change from unstable to stable frictional response as the mechanism governing the base of the seismogenic zone. Episodic transient deepening of the brittle‐ductile transition following major earthquakes can have important long‐term effects on the lower crust.
Plain Language Summary
Rock deformation in the Earth's crust changes from brittle failure involving localized fracturing, frictional sliding and seismicity in the upper crust to ductile deformation involving distributed flow at greater depth. Analysis of four well‐located M ≥ 6.7 mainshock‐aftershock sequences in Southern and Baja California indicates that many deeper‐than‐usual aftershocks occur after moderate to large mainshocks and are widely distributed around the mainshock rupture zones. The maximum aftershock depth extends up to 5 km below the regular bottom of the seismogenic zone following the mainshocks and decreases back to the background level after several years. The observed transient deepening of seismicity is consistent with geological observations of localized brittle failures below the regular seismogenic zone. The results have important implications for the mechanics governing the base of seismicity and the long‐term properties and dynamics of the lower crust.
Key Points
Four M ≥ 6.7 events have aftershocks 5 km below the regular seismogenic zone around the mainshock ruptures
The maximum aftershock depth increases abruptly after the mainshocks and decreases gradually to the background level after a few years
The results are consistent with the transient deepening of the brittle‐ductile transition depth generated by the mainshocks
We image the shallow seismic structure across the Southern San Andreas Fault (SSAF) using signals from freight trains and trucks recorded by a dense nodal array, with a linear component perpendicular ...to SSAF and two 2D subarrays centered on the Banning Fault and Mission Creek Fault (MCF). Particle motion analysis in the frequency band 2–5 Hz shows that the examined traffic sources can be approximated as moving single‐ or multi‐point sources that primarily induce Rayleigh waves. Using several techniques, we resolve strong lateral variations of Rayleigh wave velocities and Q‐values across the SSAF, including 35% velocity reduction across MCF toward the northeast and strong attenuation around the two fault strands. We further resolve 10% mass density reduction and 45% shear modulus decrease across the MCF. These findings suggest that the MCF is currently the main strand of the SSAF in the area with important implications for seismic hazard assessments.
Plain Language Summary
Imaging the internal structure of fault zones is essential for understanding earthquake properties and processes. Here we utilize seismic data generated by trains and trucks in the Coachella valley and recorded by a dense seismic array to image the subsurface structure of two main strands of the Southern San Andreas Fault (SSAF). Several types of analyses allow us to resolve seismic velocities, attenuation coefficients, and mass density across the entire San Andreas Fault zone. The results show a clear contrast in physical properties across the Mission Creek strand of the SSAF, highlighting the presence of a bimaterial fault interface and suggesting that it is the main active strand of SSAF. The research opens up possibilities for using common rail and road traffic signals to derive high resolution imaging results of subsurface seismic properties at other locations.
Key Points
We detect frequent seismic signals from rail and road traffic in a dense array across the southern San Andreas fault zone
We use the traffic signals to image shallow structural properties across the Banning and Mission Creek fault strands
The resolved velocity and density contrasts across the Mission Creek fault suggest it is the main active strand of the Southern San Andreas Fault in the area
We use recent results on statistical analysis of seismicity to present a robust method for comprehensive detection and analysis of earthquake clusters. The method is based on nearest‐neighbor ...distances of events in space‐time‐energy domain. The method is applied to a 1981–2011 relocated seismicity catalog of southern California having 111,981 events with magnitudes m ≥ 2 and corresponding synthetic catalogs produced by the Epidemic Type Aftershock Sequence (ETAS) model. Analysis of the ETAS model demonstrates that the cluster detection results are accurate and stable with respect to (1) three numerical parameters of the method, (2) variations of the minimal reported magnitude, (3) catalog incompleteness, and (4) location errors. Application of the method to the observed catalog separates the 111,981 examined earthquakes into 41,393 statistically significant clusters comprised of foreshocks, mainshocks, and aftershocks. The results reproduce the essential known statistical properties of earthquake clusters, which provide overall support for the proposed technique. In addition, systematic analysis with our method allows us to detect several new features of seismicity that include (1) existence of a significant population of single‐event clusters, (2) existence of foreshock activity in natural seismicity that exceeds expectation based on the ETAS model, and (3) dependence of all cluster properties, except area, on the magnitude difference of events from mainshocks but not on their absolute values. The classification of detected clusters into several major types, generally corresponding to singles, burst‐like and swarm‐like sequences, and correlations between different cluster types and geographic locations is addressed in a companion paper.
Key Points
Earthquake clusters are identified in southern California
Accuracy and stability of detection is tested using ETAS model
Several new cluster features are reported
We investigate the non‐double‐couple components of 224 M ≥ 3.0 earthquakes in the 2019 Mw7.1 Ridgecrest sequence, which occurred on a complex fault system in the Eastern California Shear Zone. Full ...moment tensors are derived using waveform data from near‐fault and regional stations with a generalized cut‐and‐paste inversion and 3‐D velocity and attenuation models. The results show limited Compensated Linear Vector Dipole components, but considerable explosive isotropic components (5%–15% of the total moments) for approximately 50 earthquakes. Most of these events occur between the Mw6.4 foreshock and 1 day after the Mw7.1 mainshock and are mainly distributed around the rupture ends and fault intersections. The percentage of isotropic components is reduced considerably when data recorded by near‐fault stations are not included in the inversions, highlighting the importance of near‐fault data. The results suggest that high‐frequency damage‐related radiation and other local dilatational processes are responsible for the observed isotropic source terms.
Plain Language Summary
Earthquakes occur when rocks below the surface break and move rapidly. Deriving earthquake source mechanisms provides information on the involved physical processes. We examine source mechanisms of M ≥ 3.0 earthquakes in the 2019 Mw7.1 Ridgecrest sequence, using waveforms from 39 near‐fault and regional stations. Many earthquakes are found to have considerable isotropic radiation that is not expected for pure slip motion along faults. The isotropic radiation reflects motions normal to the faults that may be caused by complex fault geometry, transient fluid pressure effects, and generation of microcracks in the rupture zones. We systematically investigate the possibility of each mechanism by analyzing the spatiotemporal variations of events with considerable isotropic components. The results suggest that rock damage in the rupture zone likely provides a major contribution to the isotropic radiation.
Key Points
Fifty out of 224 M ≥ 3.0 earthquakes show considerable isotropic components not resolved without near‐fault data
Events with large isotropic components occurred early in the sequence near rupture ends and fault intersections
Rock fracturing in earthquake source volumes likely contributes significantly to the isotropic components
We provide high‐resolution seismic imaging of the central Garlock fault using data recorded by two dense seismic arrays that cross the Ridgecrest rupture zone (B4) and the Garlock fault (A5). ...Analyses of fault zone head waves and P‐wave delay times at array A5 show that the Garlock fault is a sharp bimaterial interface with P waves traveling ∼5% faster in the northern crustal block. The across‐fault velocity contrast agrees with regional tomography models and generates clear P‐wave reflections in waveforms recorded by array B4. Kirchhoff migration of the reflected waves indicates a near‐vertical fault between 2 and 6 km depth. The P‐wave delay times imply a ∼300‐m‐wide transition zone near the Garlock fault surface trace beneath array A5, offset to the side with faster velocities. The results provide important constraints for derivations of earthquake properties, simulations of ruptures and ground motion, and future imaging studies associated with the Garlock fault.
Plain Language Summary
Along the northern edge of the Mojave Desert, the Garlock fault intersects the San Andreas fault and is the second largest (∼300 km long) fault in Southern California. It can host M > 7 earthquakes that pose significant seismic hazard to densely populated communities. However, the subsurface structure of the Garlock fault is not well understood due to the sparse seismic network and lack of seismic activity nearby. The 2019 Ridgecrest earthquake sequence in the Eastern California Shear Zone led to a rapid deployment of several dense linear arrays with ∼100 m spacing and apertures of a few kilometers, which cross the Ridgecrest rupture zone and also the Garlock fault. The recorded seismic data is used here to illuminate the internal structure of the central Garlock fault. Analyses of P‐wave delay times and waves refracted along and reflected by the fault interface indicate a near‐vertical Garlock fault that separates two distinctive crustal blocks with different wave speeds. The resolved high‐resolution fault zone image can have important implications for multiple studies associated with the Garlock fault.
Key Points
We image the central Garlock fault using data of aftershocks of the 2019 Ridgecrest earthquake recorded by two dense linear arrays
A P‐wave velocity contrast across the fault (∼5% faster in the north) generates clear fault zone head and reflected waves
Kirchhoff migration of P waves reflected by the fault indicates a near‐vertical interface with a sharp impedance contrast
We perform seismic attenuation tomography of the shallow structure for the San Jacinto Fault in the Ramona Reservation of southern California. The study uses ambient seismic noise recorded by a ...linear array of 65 3‐C sensors across the fault. We extract amplitude decay information, with uncertainty, from noise interferometry functions. To account for strong heterogeneities in the complex shallow fault zone structure, we apply a frequency‐dependent amplitude correction for focusing/defocusing effects using inverted phase velocity maps obtained by solving the transport equation. We then estimate the attenuation structure based on the linear station‐triplet method for both Love and Rayleigh waves. The attenuation tomography indicates strong attenuation correlated with known San Jacinto Fault surface traces. The Love wave attenuation tomography reveals an asymmetric damage zone that exists primarily on the fault side with faster seismic velocity, consistent with earthquake ruptures on a fault bimaterial interface with preferred propagation direction to the northwest.
Plain Language Summary
The San Jacinto Fault Zone (SJFZ) is part of the boundary between the American and Pacific plates in Southern California. Earthquake ruptures produce rock damage that leads to attenuation of seismic waves during propagation. To study the attenuation structure of the fault zone, we extract amplitude information from background seismic noise recorded by a dense array of sensors across the fault. The results indicate an asymmetric attenuation and damage structure, with higher damage on the side where seismic waves travel faster. The rock damage asymmetry may have implications on the seismic shaking hazard from future large earthquakes on the SJFZ.
Key Points
We perform ambient noise attenuation tomography based on Rayleigh and Love waves on a linear array across the San Jacinto Fault northwest of Anza, California
We apply a frequency‐dependent amplitude correction for elastic focusing/defocusing effects due to strong lateral velocity variations
The attenuation tomography reveals an asymmetric damage zone concentrated on the stiffer side of fault
We introduce an algorithm for declustering earthquake catalogs based on the nearest‐neighbor analysis of seismicity. The algorithm discriminates between background and clustered events by random ...thinning that removes events according to a space‐varying threshold. The threshold is estimated using randomized‐reshuffled catalogs that are stationary, have independent space and time components, and preserve the space distribution of the original catalog. Analysis of catalog produced by the Epidemic Type Aftershock Sequence model demonstrates that the algorithm correctly classifies over 80% of background and clustered events, correctly reconstructs the stationary and space‐dependent background intensity, and shows high stability with respect to random realizations (over 75% of events have the same estimated type in over 90% of random realizations). The declustering algorithm is applied to the global Northern California Earthquake Data Center catalog with magnitudes m ≥ 4 during 2000–2015; a Southern California catalog with m ≥ 2.5, 3.5 during 1981–2017; an area around the 1992 Landers rupture zone with m ≥ 0.0 during 1981–2015; and the Parkfield segment of San Andreas fault with m ≥ 1.0 during 1984–2014. The null hypotheses of stationarity and space‐time independence are not rejected by several tests applied to the estimated background events of the global and Southern California catalogs with magnitude ranges Δm < 4. However, both hypotheses are rejected for catalogs with larger range of magnitudes Δm > 4. The deviations from the nulls are mainly due to local temporal fluctuations of seismicity and activity switching among subregions; they can be traced back to the original catalogs and represent genuine features of background seismicity.
Key Points
A new declustering method is proposed based on nearest‐neighbor analysis of earthquakes in time‐space‐magnitude domain
Declustering the examined catalogs with magnitude range Δm< 4 leads to a stationary field with independent space‐time components
Declustering data with Δm> 4 reveal nonstationary patterns attributed to the original catalog rather than the method
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