Search USGS⌕ Search

Geology topics

Jeanne L. Hardebeck

Publications and source records attributed to Jeanne L. Hardebeck.

At least 37 records · Page 2Linked to original sources
USGS2022

Physical properties of the crust influence aftershock locations

Aftershocks do not uniformly surround a mainshock, and instead occur in spatial clusters. Spatially variable physical properties of the crust may influence the spatial distribution of aftershocks. I study four aftershock sequences in Southern California (1992 Landers, 1999 Hector Mine, 2010 El Mayor—Cucapah, and 2019 Ridgecrest) to investigate which physical properties are spatially correlated with aftershock occurrence. I find that aftershocks correlate with several properties, including measures of stress and stress change from the mainshock, fault structure, kinematics, seismic velocity, and heat flow. Aftershock spatial density exhibits an order of magnitude or more variation as a function of these properties. I determine simple empirical relations between each of the properties and the aftershock spatial density, and use these relations to construct new spatial models that describe aftershock locations. The new spatial models are a significant improvement over a simple base model, but do not fully capture the dense spatial clustering of aftershocks. Numerous spatially varying physical properties exhibit no (or poor) correlation with aftershock spatial density, including temperature, rock composition, and rheological properties that might be expected to control aftershock occurrence. These results suggest that while spatially variable physical properties appear to influence aftershock locations, more work is necessary in order to establish the connections between aftershock occurrence and the causative physical properties.

Baja California, California, Nevada↗
USGS2022

Using machine learning techniques with incomplete polarity datasets to improve earthquake focal mechanism determination

Earthquake focal mechanisms are traditionally produced using P ‐wave first‐motion polarities and commonly require well‐recorded seismicity. A recent approach that is less dependent on high signal‐to‐noise exploits similar waveforms to produce relative polarity measurements between earthquake pairs. Utilizing these relative polarity measurements, it is possible to produce composite focal mechanisms for clusters within microseismic sequences using regional networks. However, missing or low‐confidence polarity measurements still limit our ability to calculate high‐quality composite focal mechanisms. Here, we replaced unreliable polarity measurements with estimates using iterative random forests, an unsupervised ensemble machine learning method. Using the imputed (“replaced”) polarity data, we then categorically clustered the events into families. As a case study, we applied this modified composite mechanism workflow to a multistation template matched catalog of an earthquake swarm that occurred during 2020 near the Maacama fault in northern California. We found that our modified methodology produced higher‐quality earthquake families and improved composite focal mechanisms, with fault‐plane uncertainties <35° for 94% of the families compared with 34% of families using the previous methodology.

Seismological Research Letters↗
USGS2022

Earthquakes in the shadows: Why aftershocks occur at surprising locations

For decades there has been a debate about the relative effects of dynamic versus static stress triggering of aftershocks. According to the static Coulomb stress change hypothesis, aftershocks should not occur in stress shadows—regions where static Coulomb stress has been reduced. We show that static stress shadows substantially influence aftershock occurrence following three M ≥ 7 California mainshocks. Within the modeled static Coulomb stress shadows, the aftershock rate is an order of magnitude lower than in the modeled increase regions. However, the earthquake rate in the stress shadows does not decrease below the background rate, as predicted by Coulomb stress change models. Aftershocks in the stress shadows exhibit different spatial–temporal characteristics from aftershocks in the stress increase regions. The aftershock rate in the stress shadows decays as a power law with distance from the mainshock, consistent with a simple model of dynamic stress triggering. These aftershocks begin with a burst of activity during the first few days after the mainshock, also consistent with dynamic stress triggering. Our interpretation is that aftershock sequences are the combined result of static and dynamic stress triggering, with an estimated ∼34% of aftershocks due to dynamic triggering and ∼66% due to static triggering.

The Seismic Record↗
USGS2022

S/P amplitude ratios derived from single-component seismograms and their potential use in constraining focal mechanisms for micro-earthquake sequences

Focal mechanisms, which reflect the sense of slip in earthquakes, provide important constraints for understanding crustal tectonics and earthquake source physics, including the interactions among earthquakes during mainshock–aftershock sequences or seismic swarms. Focal mechanisms of small (magnitude ≲3.5) earthquakes are usually determined by first‐motion P ‐wave polarities, sometimes supplemented by the ratio of S ‐wave to P ‐wave amplitudes ( S/P ). However, focal mechanisms of such events can be difficult to reliably constrain, particularly with sparse recording networks or very small magnitude events. Here, we describe a method for deriving S/P amplitude ratios from P/P and S/S amplitude ratios measured on single seismic components between pairs of nearby events, as is often performed during correlation‐based earthquake detection and relocation. These measurements can be transformed into relative S/P amplitude ratios, or they can be combined with a smaller number of traditional S/P amplitude ratios to provide a single‐channel estimation of full S/P ratios, even for low signal‐to‐noise‐ratio events not routinely cataloged and not amenable to traditional S/P ratio processing. This approach has the potential to greatly expand the applicability of S/P amplitude ratios, providing additional constraints for focal mechanisms of small earthquakes, particularly for spatially concentrated seismicity sequences.

California↗
USGS2022

Prospective and retrospective evaluation of the U.S. Geological Survey public aftershock forecast for the 2019-2021 Southwest Puerto Rico Earthquake and aftershocks

The M w "> M w 6.4 Southwest Puerto Rico Earthquake of 7 January 2020 was accompanied by a robust fore‐ and aftershock sequence. The U.S. Geological Survey (USGS) has issued regular aftershock forecasts for more than a year since the mainshock, available on a public webpage. Forecasts were accompanied by interpretive and informational material, published in English and Spanish. Informational products included narrative “scenarios” for how the aftershock sequence could play out, infographics, and a report on the potential duration of the aftershock sequence through the next decade. Forecasts are based on the epidemic‐type aftershock sequence (ETAS) model and generated using the USGS AftershockForecaster software—an interactive graphical user interface built on the OpenSHA platform ( Field et al. , 2003 ). The initial forecast is based on past sequences in similar tectonic environments; subsequent forecasts are tuned to the ongoing sequence via Bayesian model updating. Probabilistic aftershock forecasts for the next day, week, month, and year were publicly released and archived at a daily to monthly tempo, allowing for a truly prospective test of the forecast. Here, we evaluate the forecast over the first year of the recorded aftershocks. The ETAS‐based forecast performed well overall, successfully capturing both the chance of having at least one earthquake of a given magnitude in a forecast interval as well as the non‐Poissonian distribution of the total number of aftershocks within an interval. A retrospective analysis shows that the ETAS model is a substantial improvement over the existing Reasenberg and Jones (1989) forecast model. The exercise also reveals some limitations of the current model, in particular, with respect to nonstationarities in the aftershock magnitude distribution and model parameters throughout the evolving sequence.

Puerto Rico↗
USGS2021

Does earthquake stress drop increase with depth in the crust?

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.

Journal of Geophysical Research↗
USGS2021

A unified model of crustal stress heterogeneity from borehole breakouts and earthquake focal mechanisms

Observations of crustal stress orientation from the regional inversion of earthquake focal mechanisms often conflict with those from borehole breakouts, possibly indicating local stress heterogeneity, either laterally or with depth. To investigate this heterogeneity, we compiled SHmax estimates from previous studies for 57 near‐vertical boreholes with measured breakout azimuths across the Los Angeles region. We identified subsets of earthquake focal mechanisms from established earthquake catalogs centered around each borehole with various criteria for maximum depth and maximum lateral distance from the borehole. Each subset was independently inverted for 3‐D stress orientation and corresponding SHmax probability distributions, then compared with the corresponding borehole breakout‐derived estimate. We find good agreement when both methods sample the basement stress (breakouts are close to the sediment‐basement interface), or when both methods sample the mid‐basin stress (sufficient earthquakes are present within a sedimentary basin). Along sedimentary basin margins, in contrast, we find acceptable agreement only when focal mechanisms are limited to shallow and close earthquakes, implying short‐length‐scale heterogeneity of <20 km. While the region as a whole shows evidence of both lateral and vertical stress orientation heterogeneity, we find a more homogeneous stress state within basement rock, over length scales of 1–35 km. These results reconcile the apparently conflicting observations of short‐length‐scale heterogeneity observed in boreholes, which sample primarily the basins, with the relative homogeneity of stress inferred from focal mechanisms, which sample primarily the basement, and imply distinct regimes for the appropriate use of each type of stress indicator.

JGR Solid Earth↗
USGS2021

Spatial clustering of aftershocks impacts the performance of physics‐based earthquake forecasting models

I explore why physics‐based models of earthquake triggering rarely outperform statistical models in prospective testing, outside of limited spatial‐temporal windows. Pseudo‐prospective tests on suites of synthetic aftershock sequences show that a major factor is the level of unmodeled spatial clustering of the direct aftershocks triggered by the mainshock. The synthetic sequences are generated from generalized “physical” triggering models, optionally superimposed on background heterogeneity that controls the level of clustering. The statistical Epidemic Type Aftershock Sequence (ETAS) model performs relatively better the more clustered the direct aftershocks, while the true generalized “physical” model performs relatively worse. Real aftershocks appear to be sufficiently clustered to allow ETAS to perform as well as or better than physical models such as Coulomb stress triggering. A likely cause of the spatial clustering of direct aftershocks is heterogeneity of the background physical conditions, which typically is not modeled in physics‐based forecasts. This implies that the forecast performance of physical models could be substantially improved through a better understanding of the interaction between earthquake stress changes and variable background physical conditions such as stress state, fault strength, and fluid pressure.

JGR Solid Earth↗
USGS2020

Localized fluid discharge by tensile cracking during the post-seismic period in subduction zones

It is thought that extensional structures (extensional cracks and normal faults) generated during the post-seismic period create fluid pathways that enhance the drainage of the subducting plate interface, thus reducing the pore pressure and increasing fault strength. However, it remains to be elucidated how much pore fluid pressure decreases by the extension crack formation. Here we examined i) the pore fluid pressure decrease, and ii) the degree fault strength recovery by the extension crack formation during the post-seismic period by analyzing extension quartz veins exposed around the Nobeoka Thrust, southwestern Japan. The Nobeoka Trust is an on-land analog of the modern splay fault at shallow depths (~8 km) in the Nankai Trough. The poro-elastic model of extensional quartz vein formation indicates that the formation of extensional cracks only releases up to ~7–8% of the total pore fluid pressure at ~8 km depth. The pore pressure around the Nobeoka Thrust was close to lithostatic pressure during the entire seismic cycle. The estimated effective frictional coefficient along the Nobeoka Thrust after this small fluid-loss by the extensional crack formation does not exceed 0.15. Hence, the pore fluid pressure reduction due to the post-seismic extensional cracks contributes little to increase the fault strength of the megasplay fault.

Scientific Reports↗
USGS2020

A stress-similarity triggering model for aftershocks of the MW6.4 and MW7.1 Ridgecrest earthquakes

The July 2019 M w "> M w 6.4 and 7.1 Ridgecrest earthquakes triggered numerous aftershocks, including clusters of off‐fault aftershocks in an extensional stepover of the Garlock fault, near the town of Olancha, and near Panamint Valley. The locations of the off‐fault aftershocks are consistent with the stress‐similarity model of triggering, which hypothesizes that aftershocks preferentially occur in areas where the mainshock static stress change tensor is similar in orientation to the background stress tensor. The background stress field is determined from the inversion of earthquake focal mechanisms, with the spatial resolution adapted to the local density of earthquakes. The mainshock static stress change is computed using finite‐source models for the M w "> M w 6.4 foreshock and M w "> M w 7.1 mainshock. I quantify the similarity between these two stress fields using the tensor dot product of the normalized deviatoric stress tensors. The off‐fault aftershocks in the Garlock stepover and the Olancha area fall within lobes of positive stress similarity, whereas the aftershocks near Panamint Valley are partially within a lobe. The cluster in the Garlock fault stepover and the smaller of two clusters near Olancha occur in regions of locally anomalous background stress that results in higher stress similarity. I compute the spatial density of M &#x2265; 2.0 "> M≥2.0 aftershocks and find that the aftershock density increases as a function of stress similarity, with a factor of &#x223C; 15 "> ∼15 difference between high stress‐similarity and low stress‐similarity areas. This result is robust with respect to the choice of mainshock model and the uncertainty of the background stress field. The aftershock density varies substantially inside the high stress‐similarity lobes, however, indicating that other variable background conditions, such as material properties, temperature, and fluid pressure, may also be playing a role. Specifically, temperature and fluid pressure conditions might help explain the low rate of aftershocks in the Coso geothermal field.

California↗
USGS2020

Are the stress drops of small earthquakes good predictors of the stress drops of moderate-to-large earthquakes?

The stress drops of small earthquakes often exhibit spatial patterns of variability. If moderate and large earthquakes follow the same spatial patterns, the stress drops of possible future damaging earthquakes could be better predicted by considering the stress drops of nearby small events. Better stress drop predictability could reduce ground-motion uncertainty in Probabilistic Seismic Hazard Assessment (PSHA) and Earthquake Early Warning (EEW). I find that for an internally consistent stress drop catalog of M1.8-3.1 events in southern California, the stress drops of the bigger earthquakes are predictable from the nearby smaller events. However, this catalog only weakly spatially correlates with another catalog of M3.0-5.8 earthquakes, and is spatially uncorrelated with five other stress drop catalogs of M≥3.4 earthquakes. For southern California events M5.5-7.5, stress drops compiled from the literature are weakly spatially correlated with the stress drops of the M1.8-3.1 events, although the correlations are not statistically significant. The lack of strong spatial correlation may be due to actual differences in the controlling factors of stress drop, for example dynamic weakening in moderate-to-large earthquakes. Alternatively, a stronger spatial correlation may exist that is obscured by methodological heterogeneity and large errors in the stress drop estimates. Either way, the stress drops of small earthquakes do not appear to be good predictors of the stress drops of nearby moderate-to-large earthquakes, at least for current techniques of stress drop estimation. If these results are representative, small-earthquake stress drops are not currently useful for substantially reducing uncertainty in PSHA and EEW.

California↗
USGS2020

Potential duration of aftershocks of the 2020 southwestern Puerto Rico earthquake

Abstract Aftershocks (earthquakes clustered spatially and chronologically near the occurrence of a causative earthquake) are ongoing in southwestern Puerto Rico after a series of earthquakes, which include a magnitude 6.4 earthquake that occurred near Barrio Indios, Guayanilla, on January 7, 2020, and affected the surrounding area. This report estimates the expected duration of these aftershocks by incorporating observations of aftershocks as of January 17, 2020, into a well-established statistical model of how earthquake sequences behave. Aftershocks will persist for years to decades, although with decreasing frequency, and earthquakes will likely be felt on a daily basis for up to several months. These estimates have significant uncertainty owing to different scenarios of how the earthquake sequence may evolve over time and could also change if a new large aftershock occurs. This report also estimates the amount of time remaining until the annual probability of magnitude 5, 6, and 7 or greater aftershocks—which could cause additional damage—decreases to 50, 25, 10, 5, and 1 percent. As of this writing, the chance of having a magnitude 6 or greater earthquake within a given year, going forward, will not fall below 25 percent for another 3 months to 3 years. The chance of having a magnitude 5 or greater earthquake will not fall below 25 percent for a decade or more. The aftershocks discussed in this report would be located in the same general area as the aftershocks that have already occurred. Our results do not imply a change in the risk of earthquakes in other parts of Puerto Rico. Resumen Las réplicas (terremotos agrupados espacial y cronológicamente cerca de la ocurrencia de un terremoto causante) están en curso en el suroeste de Puerto Rico después de una serie de terremotos, que incluyen un terremoto de magnitud 6.4, ocurrido cerca del Barrio Indios, Guayanilla, el 7 de enero de 2020 y que afectaron las áreas circundantes. Este informe estima la duración esperada de las réplicas incorporando observaciones de réplicas a partir del 17 de enero de 2020 en un modelo estadístico bien establecido de cómo se comportan las secuencias de terremotos. Las réplicas persistirán durante años o décadas, aunque con una frecuencia decreciente, y los terremotos probablemente se sentirán a diario durante varios meses. Estos estimados tienen una incertidumbre significativa debido a diferentes escenarios de cómo la secuencia del terremoto puede evolucionar con el tiempo y también podrían cambiar si ocurre una nueva réplica grande. Este informe también estima la cantidad de tiempo restante hasta que la probabilidad anual de réplicas de magnitud 5, 6 y 7 o más - que podría causar daños adicionales- disminuya a un 50, 25, 10, 5 y 1 por ciento. Al momento de escribir este artículo, la posibilidad de tener un terremoto de magnitud 6 en un año determinado, en el futuro, no caerá por debajo del 25 por ciento durante otros 3 meses a 3 años. La probabilidad de tener un terremoto de magnitud 5 o mayor no será inferior al 25 por ciento durante una década o más. Las réplicas discutidas en este informe se ubicarían en la misma área general que las réplicas que ya han ocurrido. Nuestros resultados no implican un cambio en el riesgo de terremotos en otras partes de Puerto Rico.

Puerto Rico↗
USGS2018

Updated California aftershock parameters

Reasenberg and Jones (1989) introduced a statistical model for aftershock rate following a mainshock along with estimates of “generic” California parameter values based on past aftershock sequences. The Reasenberg and Jones (1989) model has been used for decades to issue aftershock forecasts following M &#x2265; 5 "> M ≥ 5 mainshocks in California. Here, we update the “generic” parameters for California through a fit to the aftershock sequences of M &#x2265; 5 "> M ≥ 5 mainshocks occurring since 1980. We find aftershock productivity values that are lower on average than the generic productivity reported by Reasenberg and Jones (1989) , likely because low‐productivity sequences were omitted from their analysis and possibly because of a trade‐off between productivity and b "> b ‐value. We confirm the observation of Llenos and Michael (2017) that southern California sequences are more productive on average than northern California sequences. The Mendocino area is much less productive; the hydrothermal areas in Long Valley, Coso, and the Salton Sea, in contrast, are much more productive. We also quantify the variability of the Reasenberg and Jones (1989) productivity parameter a "> a a between sequences with a normal distribution. This distribution of a "> a ‐values can be used to compute aftershock forecasts that include epistemic uncertainty and can be used as the prior for Bayesian updating of the a "> a a ‐value as a sequence progresses.

California↗
USGS2018

Temporal stress changes caused by earthquakes: A review

Earthquakes can change the stress field in the Earth’s lithosphere as they relieve and redistribute stress. Earthquake-induced stress changes have been observed as temporal rotations of the principal stress axes following major earthquakes in a variety of tectonic settings. The stress changes due to the 2011 Mw9.0 Tohoku-Oki, Japan, earthquake were particularly well documented. Earthquake stress rotations can inform our understanding of earthquake physics, most notably addressing the long-standing problem of whether the Earth’s crust at plate boundaries is “strong” or “weak.” Many of the observed stress rotations, including that due to the Tohoku-Oki earthquake, indicate near-complete stress drop in the mainshock. This implies low background differential stress, on the order of earthquake stress drop, supporting the weak crust model. Earthquake stress rotations can also be used to address other important geophysical questions, such as the level of crustal stress heterogeneity and the mechanisms of postseismic stress reloading. The quantitative interpretation of stress rotations is evolving from those based on simple analytical methods to those based on more sophisticated numerical modeling that can capture the spatial-temporal complexity of the earthquake stress changes.

Journal of Geophysical Research B: Solid Earth↗
USGS2018

Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia.

California↗
USGS2018

Testing for the ‘predictability’ of dynamically triggered earthquakes in Geysers Geothermal Field

The Geysers geothermal field is well known for being susceptible to dynamic triggering of earthquakes by large distant earthquakes, owing to the introduction of fluids for energy production. Yet, it is unknown if dynamic triggering of earthquakes is ‘predictable’ or whether dynamic triggering could lead to a potential hazard for energy production. In this paper, our goal is to investigate the characteristics of triggering and the physical conditions that promote triggering to determine whether or not triggering is in anyway foreseeable. We find that, at present, triggering in The Geysers is not easily ‘predictable’ in terms of when and where based on observable physical conditions. However, triggered earthquake magnitude positively correlates with peak imparted dynamic stress, and larger dynamic stresses tend to trigger sequences similar to mainshock–aftershock sequences. Thus, we may be able to ‘predict’ what size earthquakes to expect at The Geysers following a large distant earthquake.

California↗
USGS2017

Imaging shear strength along subduction faults

Subduction faults accumulate stress during long periods of time and release this stress suddenly, during earthquakes, when it reaches a threshold. This threshold, the shear strength, controls the occurrence and magnitude of earthquakes. We consider a 3-D model to derive an analytical expression for how the shear strength depends on the fault geometry, the convergence obliquity, frictional properties, and the stress field orientation. We then use estimates of these different parameters in Japan to infer the distribution of shear strength along a subduction fault. We show that the 2011 M w 9.0 Tohoku earthquake ruptured a fault portion characterized by unusually small variations in static shear strength. This observation is consistent with the hypothesis that large earthquakes preferentially rupture regions with relatively homogeneous shear strength. With increasing constraints on the different parameters at play, our approach could, in the future, help identify favorable locations for large earthquakes.

Geophysical Research Letters↗