We estimate Brune‐type stress drops for over 70,000 southern California M 1.5–4 earthquakes from 1996 to 2019 using a P‐wave spectral decomposition approach. Based on our recent work documenting ...hard‐to‐resolve trade‐offs between absolute stress drop, stress drop scaling with moment, high‐frequency falloff rate, and empirical corrections for path and attenuation terms, we adopt a new approach in which the average corner frequency of the smallest earthquakes within a short distance from each target event is fixed to a constant value. This removes any true coherent spatial variations in stress drops among the smallest events but ensures that any spatial variations seen in larger event stress drops are real and not an artifact of inaccurate path corrections. Applying this approach across southern California, we document spatial variations in stress drop that agree with previous work, such as lower‐than‐average stress drops in the Salton Trough, as well as small‐scale stress drop variations along many faults and aftershock sequences. We observe an apparent increase in Brune‐type stress drop with moment for M 3–4 earthquakes, but their spectra can be fit equally well with self‐similar models with a high‐frequency falloff rate shallower than f−2.
Plain Language Summary
Earthquakes vary widely in their properties. One key parameter used by seismologists to describe earthquakes is their stress drop, a measure of the forces unleashed during fault rupture. High‐stress drop earthquakes are particularly dangerous because they generate stronger ground motions than low‐stress drop events of the same size. Previous studies have suggested that parts of southern California are more prone to high‐stress drop earthquakes than other regions. However, stress drop estimates have large uncertainties and studies do not always agree very well with each other, making it difficult to be sure that these observed spatial variations are real. Here, we introduce a new approach for earthquake stress drop estimation that is more stable than previous methods for assessing differences in average levels of stress drop across southern California. Applying our method to over 70,000 earthquakes from 1996 to 2019, we find that there are indeed substantial regional variations in stress drop with the Imperial Valley and Salton Sea region near the US‐Mexico border having particularly low earthquake stress drops.
Key Points
We estimate Brune‐type stress drops for over 70,000 southern California M 1.5–4 earthquakes using P‐wave spectral decomposition
We stabilize the estimation of empirical attenuation corrections by forcing small‐earthquake corner frequencies to a fixed value
We observe large‐scale variations in median stress drop across the region and deviations from a self‐similar Brune model for M > 3 events
The Mw 7.2 El Mayor‐Cucapah (EMC) earthquake ruptured a complex fault system in northern Baja California that was previously considered inactive. The Cerro Prieto Geothermal Field (CPGF), site of the ...world's second largest geothermal power plant, is located approximately 15 km to the northeast of the EMC hypocenter. We investigate whether anthropogenic fluid extraction at the CPGF caused a significant perturbation to the stress field in the EMC rupture zone. We use Advanced Land Observing Satellite interferometric synthetic aperture radar data to develop a laterally heterogeneous model of fluid extraction at the CPGF and estimate that this extraction generates positive Coulomb stressing rates of order 15 kPa/yr near the EMC hypocenter, a value which exceeds the local tectonic stressing rate. Although we cannot definitively conclude that production at the CPGF triggered the EMC earthquake, its influence on the local stress field is substantial and should not be neglected in local seismic hazard assessments.
Key Points
Geothermal energy production causes surface subsidence and crustal stressingProduction at CPGF generates positive Coulomb stresses in EMC rupture zoneAnthropogenic stresses exceed the tectonic loading rate at EMC hypocenter
The Permian Basin has a long history of induced earthquakes, but the seismicity rates have increased dramatically over the past two decades and included a MW 5.0 likely induced by wastewater disposal ...(WD) in March 2020. A detailed characterization of the proliferation of seismicity in the Permian Basin throughout this time period is needed for improving the scientific understanding of the mechanisms responsible and for mitigating future seismic hazard. Due to a sparse regional seismic network before the advent of Texas Seismological Network in 2017, we characterize seismicity using the 10‐station TXAR array that is 100s of km away from most of the seismicity, with the objective of improving upon the substantial contributions from previous work. By exploiting the nature of waveform similarity, we detect events with template matching, performing a quantitative analysis of spatially varying detection capabilities throughout the study area. From an initial catalog of 10,753 events, we identify 45,009 earthquakes and 10,208 quarry blasts. Using our catalog of earthquakes, we improve epicentral locations, compare relative magnitude techniques, and associate earthquakes to WD or hydraulic stimulations. We further use our earthquake catalog to investigate the relationship between seismicity and human activities near the city of Pecos, Texas. Through a comparison of our earthquake catalog with industrial records, we determine that the vast majority seismicity near Pecos, Texas, since 2000 is likely induced by an increase of WD at wells injecting at depths greater than 1.5 km.
Key Points
45,009 earthquakes and 10,208 quarry blasts are identified in the Permian Basin region during 2000–2017
Small magnitude (M < 1) and larger magnitude earthquakes missed from prior analyses can be characterized 100s of km from the TXAR array
Seismicity near Pecos, Texas, is primarily driven by wastewater disposal at wells deeper than 1.5 km
Fault geometry affects the initiation, propagation, and cessation of earthquake rupture, as well as, potentially, the statistical behavior of earthquake sequences. We analyze 18,250 (-0.27 < M < 4.4) ...earthquakes of the 2016-2019 Cahuila, California, swarm and, for the first time, use these high-resolution earthquake locations to map, in detail, the roughness across an active fault surface at depth. We find that the strike-slip fault is 50% rougher in the slip-perpendicular direction than parallel to slip. 3D mapping of fault roughness at seismogenic depths suggests that roughness varies by a factor of 8 for length scales of 1 km. We observe that the largest earthquake (M 4.4) occurred where there is significant fault complexity and the highest measured roughness. We also find that b-values are weakly positively correlated with fault roughness. Following the largest earthquake, we observe a distinct population of earthquakes with comparatively low b-values occurring in an area of high roughness within the rupture area of the M 4.4 earthquake. Finally, we measure roughness at multiple scales and find that the fault is self-affine with a Hurst exponent of 0.52, consistent with a Brownian surface.
Low Frequency Earthquakes (LFEs) are slip events that occur repeatedly at source locations within the lower crust. LFEs, and the associated seismic broadcast known as tremor, have been observed in a ...diverse array of tectonic environments. Here we develop a suite of statistical tools to conduct a systematic study of the spatial and temporal correlations of the event occurrence patterns of the 88 LFE sources beneath the greater Parkfield section of the San Andreas Fault. We first examine correlations in the occurrence patterns on long time scales to show that the regions to the north and south of Parkfield behave independently. We next use the cumulative event signatures of each source to characterize the individual occurrence patterns on shorter time scales. Through application of a statistical clustering algorithm, we demonstrate that individual LFE sources form spatially coherent clusters that may represent localized elastic structures or asperities on the deep fault interface. We conclude by examining the fine-scale features of the event rates within the LFE occurrence patterns. Through quantitative comparison to analogous laboratory shear experiments on granular, fault gouge-like materials, we infer that the distinctive features of LFE occurrence patterns reflect variations in the in-situ stress and frictional conditions at the individual LFE source locations. These observations provide a framework to understand the spatial and temporal diversity of fault slip that occurs within the lower crust beneath Parkfield and that may influence seismic hazard in the region.
•LFE seismicity reveals decoupling of the lower crust near Parkfield.•Adjacent LFE sources form clusters with synchronous occurrence patterns.•LFE clusters may identify localized asperities on fault interface.•Systematic differences in occurrence style of LFE sources are quantified.•Slip properties of LFE sources inferred through comparison to lab experiments.
The source spectral properties of injection‐induced earthquakes give insight into their nucleation, rupture processes, and influence on ground motion. Here we apply a spectral decomposition approach ...to analyze P wave spectra and estimate Brune‐type stress drop for more than 2,000 ML1.5–5.2 earthquakes occurring in southern Kansas from 2014 to 2016. We find that these earthquakes are characterized by low stress drop values (median ∼0.4 MPa) compared to natural seismicity in California. We observe a significant increase in stress drop as a function of depth, but the shallow depth distribution of these events is not by itself sufficient to explain their lower stress drop. Stress drop increases with magnitude from M1.5 to M3.5, but this scaling trend may weaken above M4 and also depends on the assumed source model. Although we observe a nonstationary, sequence‐specific temporal evolution in stress drop, we find no clear systematic relation with the activity of nearby injection wells.
Plain Language Summary
The rate of earthquake occurrence in regions of oil and gas production in the central and eastern United States has increased sharply over the last 8 years. In this study, we analyze the source spectra, or frequency content, of earthquakes occurring in one such prominent region of active oil and gas production: southern Kansas. This study is one of the first and the largest to date that provides a quantitative comparison between the spectral properties of these earthquakes, which are potentially induced by human activity, and those of earthquakes that occur in California due to natural tectonic processes. We find that earthquakes in southern Kansas are depleted in high‐frequency energy compared to natural earthquakes in California but that their relative frequency content increases significantly with depth and with magnitude. We also observe significant spatial and temporal variations in source spectral properties that may in part be driven by widespread wastewater disposal during oil and gas production. Characterizing the source spectral properties of these earthquakes is important because it lends insight into the physical processes causing these events and because the frequency content of the source has a strong influence on the intensity of shaking felt by the local population.
Key Points
We estimate moment, corner frequency, and stress drop for more than 2,000 ML1.5‐5.2 earthquakes in Kansas
These events exhibit relatively low median values of stress drop that increase with hypocentral depth
We observe an increase in stress drop with magnitude, but this trend may weaken above M4
Does Earthquake Stress Drop Increase With Depth in the Crust? Abercrombie, Rachel E.; Trugman, Daniel T.; Shearer, Peter M. ...
Journal of geophysical research. Solid earth,
October 2021, 2021-10-00, 20211001, Letnik:
126, Številka:
10
Journal Article
Recenzirano
We combine earthquake spectra from multiple studies to investigate whether the increase in stress drop with depth often observed in the crust is real, or an artifact of decreasing attenuation ...(increasing Q) with depth. In many studies, empirical path and attenuation corrections are assumed to be independent of the earthquake source depth. We test this assumption by investigating whether a realistic increase in Q with depth (as is widely observed) could remove some of the observed apparent increase in stress drop with depth. We combine event spectra, previously obtained using spectral decomposition methods, for over 50,000 earthquakes (M0 to M5) from 12 studies in California, Nevada, Kansas and Oklahoma. We find that the relative high‐frequency content of the spectra systematically increases with increasing earthquake depth, at all magnitudes. By analyzing spectral ratios between large and small events as a function of source depth, we explore the relative importance of source and attenuation contributions to this observed depth dependence. Without any correction for depth‐dependent attenuation, we find a systematic increase in stress drop, rupture velocity, or both, with depth, as previously observed. When we add an empirical, depth‐dependent attenuation correction, the depth dependence of stress drop systematically decreases, often becoming negligible. The largest corrections are observed in regions with the largest seismic velocity increase with depth. We conclude that source parameter analyses, whether in the frequency or time domains, should not assume path terms are independent of source depth, and should more explicitly consider the effects of depth‐dependent attenuation.
Plain Language Summary
The stress release (or stress drop) during an earthquake provides information about the energy budget, and the slip and area of rupture, which are needed to investigate earthquake triggering and rupture dynamics. Stress drop is also an important element of seismic hazard forecasting since high stress drop earthquakes radiate more high frequency energy, resulting in stronger ground shaking. As depth increases in the earth, the stress on faults increases because of the increased weight of the rocks above. Therefore, many models predict that deeper earthquakes should have higher stress drops. Deeper earthquakes radiate more high frequency energy than shallow ones, and some studies have interpreted this as an increase in stress drop with depth. However, attenuation of seismic energy as the waves travel through the earth is also depth‐dependent, and this is rarely explicitly included in analyses. We perform a combined analysis of frequency spectra from over 50,000 previously studied earthquakes. We compare ratios of large to small magnitude earthquakes, from different depth ranges, to separate the effects of depth‐dependent source radiation from depth‐dependent attenuation. We find that depth‐dependent attenuation can have a first‐order effect and account for much of the previously reported apparent increase in stress drop with depth.
Key Points
A stacked spectral ratio approach can separate depth dependence of source and path effects
Analyses of spectral decomposition inversions suggest that previous reports of increase in stress drop with depth may be overstated
Source parameter analyses should explicitly include depth‐dependent attenuation models or empirical corrections
Geothermal energy is an important source of renewable energy, yet its production is known to induce seismicity. Here we analyze seismicity at the three largest geothermal fields in California: The ...Geysers, Salton Sea, and Coso. We focus on resolving the temporal evolution of seismicity rates, which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading. We develop an iterative, regularized inversion procedure to partition the observed seismicity rate into two components: (1) the interaction rate due to earthquake‐earthquake triggering and (2) the smoothly varying background rate controlled by other time‐dependent stresses, including anthropogenic forcing. We apply our methodology to compare long‐term changes in seismicity to monthly records of fluid injection and withdrawal. At The Geysers, we find that the background seismicity rate is highly correlated with fluid injection, with the mean rate increasing by approximately 50% and exhibiting strong seasonal fluctuations following construction of the Santa Rosa pipeline in 2003. In contrast, at both Salton Sea and Coso, the background seismicity rate has remained relatively stable since 1990, though both experience short‐term rate fluctuations that are not obviously modulated by geothermal plant operation. We also observe significant temporal variations in Gutenberg‐Richter b value, earthquake magnitude distribution, and earthquake depth distribution, providing further evidence for the dynamic evolution of stresses within these fields. The differing field‐wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics, suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within California's geothermal fields.
Key Points
We quantitatively compare changes in seismicity at The Geysers, Salton Sea, and Coso geothermal fields
Fluid injection drives seasonal seismicity rate transients at The Geysers but not Salton Sea or Coso
Each field exhibits significant changes in b value, magnitude distribution, and depth distribution
Fault location and geometry are critical considerations in the reactivation of preexisting faults. Here, we combine relocated earthquake catalogs and focal mechanisms to delineate seismogenic faults ...in Oklahoma and southern Kansas and analyze their stress state. We first identify and map seismogenic faults based on earthquake clustering. We then obtain an improved stress map using 2,047 high‐quality focal mechanisms. The regional stress map shows a gradual transition from oblique normal faulting in western Oklahoma to strike‐slip faulting in central and eastern Oklahoma. Stress amplitude ratio shows a strong correlation with pore pressure from hydrogeologic models, suggesting that pore pressure exhibits a measurable influence on stress patterns. Finally, we assess fault stress state via 3‐D Mohr circles; a parameter understress is used to quantify the level of fault criticality (with 0 meaning critically stressed faults and 1 meaning faults with no applied shear stress). Our results indicate that most active faults have near vertical planes (planarity >0.8 and dip >70°), and there is a strong correlation between fault length and maximum magnitude on each fault. The fault trends show prominent conjugate sets that strike 55–75° and 105–125°. A comparison with mapped sedimentary faults and basement fractures reveals common tectonic control. Based on 3‐D Mohr circles, we find that 78% of the faults are critically stressed (understress ≤0.2), while several seismogenic faults are misoriented with high understress (>0.4). Fault geometry and local stress fields may be used to evaluate potential seismic hazard, as the largest earthquakes tend to occur on long, critically stressed faults.
Key Points
Stress inversion reveals stress heterogeneity and a gradual transition from strike-slip to oblique normal faulting in Oklahoma
Most but not all reactivated faults are optimally oriented in the present‐day stress field in Oklahoma
Comparison of reactivated faults, basement fractures, and mapped sedimentary faults suggests common tectonic control
Tectonic faults fail through a spectrum of slip modes, ranging from slow aseismic creep to rapid slip during earthquakes. Understanding the seismic radiation emitted during these slip modes is key ...for advancing earthquake science and earthquake hazard assessment. In this work, we use laboratory friction experiments instrumented with ultrasonic sensors to document the seismic radiation properties of slow and fast laboratory earthquakes. Stick‐slip experiments were conducted at a constant loading rate of 8 μm/s and the normal stress was systematically increased from 7 to 15 MPa. We produced a full spectrum of slip modes by modulating the loading stiffness in tandem with the fault zone normal stress. Acoustic emission data were recorded continuously at 5 MHz. We demonstrate that the full continuum of slip modes radiate measurable high‐frequency energy between 100 and 500 kHz, including the slowest events that have peak fault slip rates <100 μm/s. The peak amplitude of the high‐frequency time‐domain signals scales systematically with fault slip velocity. Stable sliding experiments further support the connection between fault slip rate and high‐frequency radiation. Experiments demonstrate that the origin of the high‐frequency energy is fundamentally linked to changes in fault slip rate, shear strain, and breaking of contact junctions within the fault gouge. Our results suggest that having measurements close to the fault zone may be key for documenting seismic radiation properties and fully understanding the connection between different slip modes.
Plain Language Summary
Tectonic faults can slip rapidly within a few seconds producing intense ground shaking and radiating high‐frequency seismic energy, or they can slip slowly over much longer time scales and emanate weak seismic signals. Understanding the seismic properties of slow and fast earthquakes is a key goal in earthquake science and could have important implications for earthquake hazard. Here, we use laboratory friction experiments instrumented with ultrasonic transducers and document systematic variations in seismic properties for slow and fast laboratory earthquakes. Our data show that both slow and fast laboratory earthquakes radiate measurable high‐frequency energy. Fault slip rate plays a key role in modulating high‐frequency energy and we propose that the origin of this high‐frequency energy originates from the breaking of grain contacts. The high‐frequency characteristics of slow and fast lab earthquakes seem to follow different scaling relationships, which could have important implications for illuminating the connections between slow and fast earthquakes in natural fault systems.
Key Points
We document seismic radiation properties of laboratory earthquakes for a spectrum of failure modes from stable sliding to fast stick‐slip
The full spectrum of labquake failure modes radiate measurable high‐frequency energy between 100 and 500 kHz
High‐frequency energy in slow and fast laboratory earthquakes scales systematically with fault slip rate
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