Searching for hidden earthquakes in Southern California Ross, Zachary E; Trugman, Daniel T; Hauksson, Egill ...
Science (American Association for the Advancement of Science),
05/2019, Letnik:
364, Številka:
6442
Journal Article
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Earthquakes follow a well-known power-law size relation, with smaller events occurring much more often than larger events. Earthquake catalogs are thus dominated by small earthquakes yet are still ...missing a much larger number of even smaller events because of signal fidelity issues. To overcome these limitations, we applied a template-matching detection technique to the entire waveform archive of the regional seismic network in Southern California. This effort resulted in a catalog with 1.81 million earthquakes, a 10-fold increase, which provides important insights into the geometry of fault zones at depth, foreshock behavior and nucleation processes, and earthquake-triggering mechanisms. The rich detail resolved in this type of catalog will facilitate the next generation of analyses of earthquakes and faults.
Foreshocks have been documented as preceding less than half of all mainshock earthquakes. These observations are difficult to reconcile with laboratory earthquake experiments and theoretical models ...of earthquake nucleation, which both suggest that foreshock activity should be nearly ubiquitous. Here we use a state‐of‐the‐art, high‐resolution earthquake catalog to study foreshock sequences of magnitude M4 and greater mainshocks in southern California from 2008–2017. This highly complete catalog provides a new opportunity to examine smaller magnitude precursory seismicity. Seventy‐two percent of mainshocks within this catalog are preceded by foreshock activity that is significantly elevated compared to the local background seismicity rate. Foreshock sequences vary in duration from several days to weeks, with a median of 16.6 days. The results suggest that foreshock occurrence in nature is more prevalent than previously thought and that our understanding of earthquake nucleation may improve in tandem with advances in our ability to detect small earthquakes.
Plain Language Summary
Earthquakes often occur without warning or detectable precursors. Here we use a new, highly complete earthquake catalog to show that most mainshock earthquakes in southern California are preceded by elevated seismicity rates—foreshocks—in the days and weeks leading up to the event. Many of these foreshock earthquakes are small in magnitude and hence were previously undetected by the seismic network. These observations help bridge the gap between observations of real earth fault systems and laboratory earthquake experiments, where foreshock occurrence is commonly observed.
Key Points
We analyze foreshock activity in a catalog of more than 1.8 million earthquakes in southern California
Foreshock occurrence significantly exceeds the background seismicity rate for 72% of candidate mainshocks
Durations of elevated foreshock activity range from days to weeks for these sequences
Seismic swarms show the structure
Faults responsible for earthquakes are idealized into two dimensions, despite fault zones being complicated, three-dimensional structures. Ross
et al.
used machine ...learning to find 22,000 seismic events near Cahuilla, California, during a seismic swarm. They used the locations and sizes of these events to show how the complex structure of the fault interacted with natural fluid injections from below. The authors' methods highlight the complexities of one fault and suggest a way to characterize other faults around the world.
Science
, this issue p.
1357
Locating 22,000 events from a seismic swarm shows the complex interplay between earthquakes, fluids, and fault geometry.
The vibrant evolutionary patterns made by earthquake swarms are incompatible with standard, effectively two-dimensional (2D) models for general fault architecture. We leverage advances in earthquake monitoring with a deep-learning algorithm to image a fault zone hosting a 4-year-long swarm in southern California. We infer that fluids are naturally injected into the fault zone from below and diffuse through strike-parallel channels while triggering earthquakes. A permeability barrier initially limits up-dip swarm migration but ultimately is circumvented. This enables fluid migration within a shallower section of the fault with fundamentally different mechanical properties. Our observations provide high-resolution constraints on the processes by which swarms initiate, grow, and arrest. These findings illustrate how swarm evolution is strongly controlled by 3D variations in fault architecture.
Understanding the connection between seismic activity and the earthquake nucleation process is a fundamental goal in earthquake seismology with important implications for earthquake early warning ...systems and forecasting. We use high-resolution acoustic emission (AE) waveform measurements from laboratory stick-slip experiments that span a spectrum of slow to fast slip rates to probe spatiotemporal properties of laboratory foreshocks and nucleation processes. We measure waveform similarity and pairwise differential travel-times (DTT) between AEs throughout the seismic cycle. AEs broadcasted prior to slow labquakes have small DTT and high waveform similarity relative to fast labquakes. We show that during slow stick-slip, the fault never fully locks, and waveform similarity and pairwise differential travel times do not evolve throughout the seismic cycle. In contrast, fast laboratory earthquakes are preceded by a rapid increase in waveform similarity late in the seismic cycle and a reduction in differential travel times, indicating that AEs begin to coalesce as the fault slip velocity increases leading up to failure. These observations point to key differences in the nucleation process of slow and fast labquakes and suggest that the spatiotemporal evolution of laboratory foreshocks is linked to fault slip velocity.
While the rupture processes of nearby earthquakes are often highly similar, characterizing the differences can provide insight into the complexity of the stress field and fault network in which the ...earthquakes occur. Here we perform a comprehensive analysis of earthquake waveform similarity to characterize rupture processes in the vicinity of Ridgecrest, California. We quantify how similar each earthquake is to neighboring events through cross correlation of full waveforms. The July 2019 Ridgecrest mainshocks impose a step reduction in earthquake similarity, which suggests variability in the residual stress field and activated fault structures on length scales of hundreds of meters or less. Among these aftershocks, we observe coherent spatial variations of earthquake similarity along the mainshock rupture trace, and document antisimilar aftershock pairs with waveforms that are nearly identical but with reversed polarity. These observations provide new, high‐resolution constraints on stress transfer and faulting complexity throughout the Ridgecrest earthquake sequence.
Plain Language Summary
Earthquakes that occur nearby to one another typically broadcast similar seismic signals. In this work, we show that the
M6.4 and
M7.1 earthquakes that occurred as part of July 2019 Ridgecrest, California, earthquake sequence triggered measurable changes in the similarity of earthquake waveforms throughout the nearby region. This implies high levels of complexity in the crustal stress field and active fault structures on the scale of tens to hundreds of meters. The Ridgecrest mainshocks caused earthquakes to become less similar on average, with systematic spatial variations along the rupture planes in correspondence to the level of mainshock fault slip. These observations form the basis for future work relating measurements of earthquake similarity to changes in stress and strength in Earth's crust.
Key Points
We use earthquake waveform similarity as a tool to study how stress and faulting evolve during the Ridgecrest sequence
Ridgecrest aftershocks have lower similarity than pre‐event seismicity, implying stress and fault heterogeneity at 100‐m length scales
Ridgecrest aftershocks show coherent spatial variations in similarity that correlate with along‐strike variations in mainshock fault slip
Understanding the generation of damaging, high‐frequency ground motions during earthquakes is essential both for fundamental science and for effective hazard preparation. Various theories exist ...regarding the origin of high‐frequency ground motions, including the standard paradigm linked to slip heterogeneity on the rupture plane, and alternative perspectives associated with fault complexity. To assess these competing hypotheses, we measure ground motion amplitudes in different frequency bands for 3 ≤ M ≤ 5.8 earthquakes in Southern California and compare them to empirical ground motion models. We utilize a Bayesian inference technique called the Integrated Nested Laplace Approximation (INLA) to identify earthquake source regions that produce higher or lower ground motions than expected. Our analysis reveals a strong correlation between fault complexity measurements and the high‐frequency ground motion event terms identified by INLA. These findings suggest that earthquakes on complex faults (or fault networks) lead to stronger‐than‐expected ground motions at high frequencies.
Plain Language Summary
An important and unresolved question in earthquake science is how damaging, rapid ground shaking is generated during an earthquake. Various ideas currently exist to explain the cause of such ground motions, with the standard view attributing strong ground motion to frictional variations on the fault plane that ruptures during an earthquake. However, recent studies have also indicated that geometric complexities within fault networks may likewise influence the strong ground shaking. To help resolve this conundrum, we analyzed the ground motions produced by earthquakes in Southern California to assess the dependence of these ground motions to the complex fault networks on which the earthquakes occur. Our findings indicate that complex fault network systems have a substantial influence on how damaging earthquake ground shaking could be. These results have broad implications for our understanding of the physics of earthquakes and have important implications for earthquake hazards.
Key Points
We investigate the influence of fault network complexity on the high‐frequency ground motions of earthquakes in California
We observe a strong correlation between fault complexity and residual ground motions at high frequencies
The correlation is frequency‐dependent, with stronger correlations observed at frequencies above 2 Hz
The spectra of earthquake waveforms can provide important insight into rupture processes, but the analysis and interpretation of these spectra is rarely straightforward. Here we develop a Bayesian ...framework that embraces the inherent data and modeling uncertainties of spectral analysis to infer key source properties. The method uses a spectral ratio approach to correct the observed S‐wave spectra of nearby earthquakes for path and site attenuation. The objective then is to solve for a joint posterior probability distribution of three source parameters—seismic moment, corner frequency, and high‐frequency falloff rate—for each earthquake in the sequence, as well as a measure of rupture directivity for select target events with good azimuthal station coverage. While computationally intensive, this technique provides a quantitative understanding of parameter tradeoffs and uncertainties and allows one to impose physical constraints through prior distributions on all source parameters, which guide the inversion when data is limited. We demonstrate the method by analyzing in detail the source properties of 14 different target events of magnitude M5 in southern California that span a wide range of tectonic regimes and fault systems. These prominent earthquakes, while comparable in size, exhibit marked diversity in their source properties and directivity, with clear spatial patterns, depth‐dependent trends, and a preference for unilateral directivity. These coherent spatial variations source properties suggest that regional differences in tectonic setting, hypocentral depth or fault zone characteristics may drive variability in rupture processes, with important implications for our understanding of earthquake physics and its relation to hazard.
Plain Language Summary
The frequency content of seismic waveforms contains important information about the physics of earthquakes. However, measuring earthquake properties in this way is notoriously difficult and the resulting source parameter estimates are often highly uncertain. In this work we develop a technique based on Bayesian probability theory to reliably characterize earthquake source parameters and fully quantify their uncertainties. We apply this method to study 14 widely felt earthquakes in southern California. While these earthquakes are comparable in size, we show that the events can have large differences in their rupture behavior and in the frequency content of their waveforms. These findings have important implications both for our physical understanding of earthquakes and their influence on regional seismic hazard.
Key Points
We describe a Bayesian technique for analyzing earthquake spectra to infer key source parameters and rupture directivity
We apply the method to characterize 14 earthquake sequences in southern California, each with a M5 target event
We observe systematic regional and depth‐dependent variations in stress drop, with most events exhibiting unilateral rupture
The spatial patterns of earthquake ground motion amplitudes are commonly represented using a double‐couple model that corresponds to shear slip on a planar fault. While this framework has proven ...largely successful in explaining low‐frequency seismic recordings, at higher frequencies the wavefield becomes more azimuthally isotropic for reasons that are not yet well understood. Here, we use a dense array of nodal seismometers in Oklahoma to study the radiation patterns of earthquakes in the near‐source region where the effects of wavefield scattering are limited. At these close distances, the radiation pattern is predominantly double couple at low frequencies (<15 Hz). At higher frequencies, the recorded wavefield contains significant isotropic and residual components that cannot be explained as path or site effects, implying complexity in the rupture process or local fault zone structure. These findings demonstrate that earthquake source complexity can drive variability in the ground motions that control seismic hazard.
Plain Language Summary
The amplitude of the ground motions produced by an earthquake is not constant in all directions, but instead varies systematically in relation to the orientation of the fault on which the earthquake occurs. Here, we study these directional variations in ground motions, known as the seismic radiation pattern, using a large data set of thousands of closely spaced seismometers recording nearby earthquakes. Our focus is on understanding how the spatial pattern of ground motions depend on their frequency of vibration. We find that at low frequencies, a simplified and widely used four‐lobed model of earthquake ground motions does a good job describing the observed seismic wavefield. At higher frequencies, however, this four‐lobed radiation pattern becomes less clear, deteriorating due to complexity in earthquake source processes and fault zone structure. Understanding the physical mechanisms driving spatial variations in ground motion will help create more accurate earthquake hazard forecasts for communities living near active faults.
Key Points
We study the frequency dependence of earthquake radiation patterns in the near‐source region using a dense seismometer array
At low frequencies (<15 Hz), radiation patterns show excellent agreement with the double‐couple model
At high frequencies, the double‐couple radiation pattern deteriorates, likely due to source and fault zone complexity
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