Spatial Distribution Of Aftershocks Research Articles

Source Models of the 2016 and 2022 Menyuan Earthquakes and Their Tectonic Implications Revealed by InSAR.

The Haiyuan fault system plays a crucial role in accommodating the eastward expansion of the Tibetan Plateau (TP) and is currently slipping at a rate of several centimeters per year. However, limited seismic activities have been observed using geodetic techniques in this area, impeding the comprehensive investigation into regional tectonics. In this study, the geometric structure and source models of the 2022 Mw 6.7 and the 2016 Mw 5.9 Menyuan earthquakes were investigated using Sentinel-1A SAR images. By implementing an atmospheric error correction method, the signal-to-noise ratio of the 2016 interferometric synthetic aperture radar (InSAR) coseismic deformation field was significantly improved, enabling InSAR observations with higher accuracy. The results showed that the reliability of the source models for those events was improved following the reduction in observation errors. The Coulomb stress resulting from the 2016 event may have promoted the strike-slip movement of the western segment of the Lenglongling fault zone, potentially expediting the occurrence of the 2022 earthquake. The coseismic slip distribution and the spatial distribution of aftershocks of the 2022 event suggested that the seismogenic fault may connect the western segment of the Lenglongling fault (LLLF) and the eastern segment of the Tuolaishan fault (TLSF). Additionally, the western segment of the surface rupture zone of the northern branch may terminate in the secondary branch close to the Sunan-Qilian fault (SN-QL) strike direction, and the earthquake may have triggered deep aftershocks and accelerated stress release within the deep seismogenic fault.

Stress triggering effect on the 2022 Honghe MS5.0 earthquake with historical strong earthquakes

The 2022 Honghe MS5.0 seismic event is intriguing due to its occurrence in the south of the Red River Fault, an area historically lacking seismic activities greater than MS5.0. To elucidate the seismogenic mechanism and scrutinize stress-triggered interactions, we calculated co-seismic and post-seismic Coulomb stress alterations induced by nine historical seismic events (M ≥ 6.0). The analysis reveals that these substantial seismic events provoked co-seismic stress augmentations of 1.409 bar and post-seismic stress increments of 0.159 bar. Noteworthy seismic events, such as the 1833 Songming, 1877 Shiping, 1913 Eshan, and 1970 Tonghai earthquakes, catalyzed the occurrence of the Honghe earthquake. Areas of heightened future seismic risk include the southern region of the Red River Fault and the eastern segments of the Shiping-Jianshui and Qujiang faults. Additionally, we assessed the correlation between the spatial distribution of aftershocks and the Coulomb stress shift triggered by the mainshock, taking into account the influence of calculation parameter settings.

Properties of the Petrinja (Croatia) earthquake sequence of 2020–2021 – Results of seismological research for the first six months of activity

We present locations of almost 14,000 events from the first six months of the Petrinja (Croatia) earthquakes sequence (mainshock 29 December 2020, Mw 6.4). The catalogue is estimated complete for ML ≥ 1.20. Initially sparse local seismograph network was densified a week after the mainshock, which reduced uncertainties of focal locations by about 75%. Source-specific station corrections were used in an iterative location scheme. The hypocentres were located with 30 different sets of velocity models and program control parameters in order to gain insight into the epistemic uncertainty of each earthquake location. The bulk of epicentres are located close to, and to the SE of the causative, dextral strike-slip Petrinja fault, but considerable activity has been triggered on smaller faults, up to 25 km away. About a half of the 78 first-motion polarity focal mechanism solutions (FMS) from the full first year of activity were found to have mechanisms similar to the foreshock, the mainshock and the largest of aftershocks, while the rest indicated almost pure reverse faulting. Strike-slip mechanisms occurred more often close to the main fault. The P-axes of FMS for aftershocks were found to be on the average rotated by 16° clockwise with respect to the P-axis of the mainshock, regardless of the style of faulting. The spatial distribution of aftershocks coincides very well with the areas of positive Coulomb stress change caused by the mainshock rupture. The inferred cut-off seismogenic depth in the area (7–10 km), estimated on the basis of published values of the geothermal gradient and the assumption of predominantly granitic upper crust, is considerably shallower than the depth of the deepest reliably located aftershocks (16–18 km). This fact may be explained by considering ophiolite-dominated crust, and by increased strain rate during the sequence, which raises the ability of crustal rocks for brittle failure.

An Aftershock Deletion Method Based on Fault Buffer Zone

The existence of aftershocks in an earthquake sequence can impact the analysis of the mainshock. In this study, we present a method for deleting an aftershock sequence based on the spatial relationship between earthquakes and faults. This method improves the performance of space window selection in the classical K-K method by eliminating aftershocks with an ideal fault buffer zone. The determination of fault buffer zones is based on a trial-and-error analysis of 69,714 earthquake records from the China Seismic Network Center (CENC) collected between 1980 and 2020. We selected 20 typical big earthquakes (ML7.0–8.0 or ~Ms6.6–8.0; for earthquakes above magnitude Ms7 or ML7.2, ML is approximately equal to Ms) as the mainshocks to establish the fault buffer zones. We also propose an empirical formula to determine the distance of the fault buffer zone by counting the aftershock deletion effect at different buffer distances. Compared with the classical K-K method, our method considers the correlation between the spatial distribution of aftershocks and faults, eliminates earthquake groups that are not related to the mainshock, greatly reduces the spatial range of aftershocks, improves the performance of deleting aftershocks of different magnitudes, and provides a new rule and reference for aftershock deletion.

Afterslip of the Mw 8.3 2015 Illapel Earthquake Imaged Through a Time‐Dependent Inversion of Continuous and Survey GNSS Data

AbstractWe use continuous and survey GNSS data to image the spatial and temporal evolution of afterslip during the 2 months following the Mw 8.3 2015 Illapel earthquake. Our approach solves for the incremental daily slip at the subduction interface using nonnegative least squares with spatial and temporal Laplacian regularization constraints. We find that afterslip developed at three specific areas at the megathrust, surrounding the coseismic rupture. In addition, well resolved afterslip also occurs within the coseismic rupture area that experienced ∼4 m of seismic slip. Our afterslip model shows striking correlations with the spatial distribution of aftershocks and repeating earthquakes. We capture the local afterslip triggered by a Mw 6.8 and two 6.9 aftershocks that ruptured downdip and north of the coseismic rupture, respectively. The latter ones were possibly triggered by the afterslip that developed north of the rupture. We also find a pulse of enhanced aseismic slip lasting a few days south of the rupture that spatially and temporally correlates with a seismicity burst. We finally find that areas of enhanced afterslip spatially correlates with areas having experienced regular seismic swarms observed during the years prior to the Illapel earthquake. This correlation supports the view of localized fluid high pore pressure areas behaving aseismically and surrounding a highly locked asperity, preventing the seismic rupture to propagate into them.

Rupture process and aftershock focal mechanisms of the 2022 M6.8 Luding earthquake in Sichuan

According to the China Earthquake Networks Center, a strong earthquake of M6.8 occurred in Luding County, Ganzi Tibetan Autonomous Prefecture, Sichuan Province, China (102.08°E, 29.59°N), on September 5, 2022, with a focal depth of 16 km. Rapid determination of the source parameters of the earthquake sequence is vital for post-earthquake rescue, disaster assessment, and scientific research. Near-field seismic observations play a key role in the fast and reliable determination of earthquake source parameters. The numerous broadband seismic stations and strong-motion stations recently deployed by the National Earthquake Intensity Rapid Report and Early Warning project have provided valuable real-time near-field observation data. Using these near-field observations and conventional mid- and far-field seismic waveform records, we obtained the focal mechanism solutions of the mainshock and M ≥ 3.0 aftershocks through the waveform fitting method. We were further able to rapidly invert the rupture process of the mainshock. Based on the evaluation of the focal mechanism solution of the mainshock and the regional tectonic setting, we speculate that the Xianshuihe fault formed the seismogenic structure of the M6.8 strong earthquake. The aftershocks formed three spatially separated clusters with distinctly different focal mechanisms, reflecting the segmented nature of the Xianshuihe fault. As more high-frequency information has been applied in this study, the absolute location of the fault rupture is better constrained by the near-field strong-motion data. The rupture process of the mainshock correlates well with the spatial distribution of aftershocks, i.e., aftershock activities were relatively weak in the maximum slip area, and strong aftershock activities were distributed in the peripheral regions.

Physical Properties of the Crust Influence Aftershock Locations

AbstractAftershocks 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.

Aftershock Probabilistic Seismic Hazard Analysis of Major Earthquakes in Western Zagros in Iran and Verification of Two Applied Methods

ABSTRACT Aftershock probabilistic seismic hazard analysis (APSHA) is essentially useful for various earthquake engineering problems. In this paper, a series of aftershock seismic hazard analyses were conducted in western Zagros region in Iran using different approaches. The results were obtained in terms of exceedance probabilities of peak ground accelerations within different time intervals following the mainshock. The results were generally compatible with the recorded data; however, the accuracy depended on the causative fault geometry and spatial distribution of aftershocks. The accuracy decreased in shorter time intervals after the mainshock and also in longer time spans especially when strong aftershocks occurred.

Effects of large aftershocks on spatial aftershock forecasts during the 2017–2019 western Iran sequence

SUMMARY On 12 November 2017, an earthquake with a moment magnitude of 7.3 struck the west of Iran near the Iraq border. This event was followed about 9 and 12 months later by two large aftershocks of magnitude 5.9 and 6.3, which together triggered intensive seismic activity known as the 2017–2019 Kermanshah sequence. In this study, we analyse this sequence regarding the potential to forecast the spatial aftershock distribution based on information about the main shock and its largest aftershocks. Recent studies showed that classical Coulomb failure stress (CFS) maps are outperformed by alternative scalar stress quantities, as well as a distance-slip probabilistic model (R) and deep neural networks (DNN). In particular, the R-model performed best. However, these test results were based on the receiver operating characteristic (ROC) metric, which is not well suited for imbalanced data sets such as aftershock distributions. Furthermore, the previous analyses also ignored the potential impact of large secondary earthquakes. For the complex Kermanshah sequence, we applied the same forecast models but used the more appropriate MCC-F1 metric for testing. Similar to previous studies, we also observe that the receiver independent stress scalars yield better forecasts than the classical CFS values relying on the specification of receiver mechanisms. However, detailed analysis based on the MCC-F1 metric revealed that the performance depends on the grid size, magnitude cut-off and test period. Increasing the magnitude cut-off and decreasing the grid size and period reduce the performance of all methods. Finally, we found that the performance of the best methods improves when the source information of large aftershocks is additionally considered, with stress-based models outperforming the R model. Our results highlight the importance of accounting for secondary stress changes in improving earthquake forecasts.

Source Characteristics of the Shallow 2019 Ms 6.0 Changning, China, Earthquake Sequence in the Salt Mining Area

AbstractThe 17 June 2019 Ms 6.0 Changning earthquake occurred on the southern margin of the Sichuan basin in China, which breaks the historical record for the largest earthquake in the Sichuan basin. Based on the abundant local seismic, strong motion, and Interferometric Synthetic Aperture Radar line of sight displacement data, we investigate the detailed source characteristics of this earthquake sequence. We determine focal mechanisms of 68 ML≥2.0 aftershocks with P wave first-motion polarities and S/P amplitude ratios. The triangle diagram of focal mechanisms shows that 82% of the aftershocks have thrust faulting mechanisms. The spatial distribution of aftershocks together with the determined focal mechanisms indicates that this earthquake sequence was mainly controlled by the southeast–northwest-extended faults. In addition, the revealed diversity of aftershock focal mechanisms implies that some small subsidiary faults with different geometries and motion features have likely been ruptured. The kinematic finite-fault joint inversion results reveal a complex rupture process of the mainshock on two fault segments with different geometries. The rupture initiated on the southeastern fault segment with a gentle dip angle and then jumped to the vertical northwestern fault segment. The main rupture length and duration are approximately 11 km and 8 s, respectively. The released total scalar seismic moment during the rupture process is 4.6×1017 N·m, corresponding to a moment magnitude of Mw 5.7. Our results suggest that the nucleation and the rupture initiation and propagation of the 2019 Ms 6.0 Changning earthquake sequence were likely controlled by the intrinsic structure and stress heterogeneities of the involved seismogenic faults, as well as the variation in pore-fluid pressure caused by the long-term water injection in the Changning salt mining area and adjacent areas.

Multi-Segment Rupture Model of the 2016 Kumamoto Earthquake Revealed by InSAR and GPS Data

The 2016 Kumamoto earthquake, including two large (Mw ≥ 6.0) foreshocks and an Mw 7.0 mainshock, occurred in the Hinagu and Futagawa fault zones in the middle of Kyushu island, Japan. Here, we obtain the complex coseismic deformation field associated with this earthquake from Advanced Land Observation Satellite-2 (ALOS-2) and Sentinel-1A Interferometric Synthetic Aperture Radar (InSAR) data. These InSAR data, in combination with available Global Positioning System (GPS) data, are then used to determine an optimal four-segment fault geometry with the jRi method, which considers both data misfit and the perturbation error from data noise. Our preferred slip distribution model indicates that the rupture is dominated by right-lateral strike-slip, with a significant normal slip component. The largest asperity is located on the northern segment of the Futagawa fault, with a maximum slip of 5.6 m at a 5–6 km depth. The estimated shallow slips along the Futagawa fault and northern Hinagu fault are consistent with the displacements of surface ruptures from the field investigation, suggesting a shallow slip deficit. The total geodetic moment release is estimated to be 4.89 × 1019 Nm (Mw 7.09), which is slightly larger than seismological estimates. The calculated static Coulomb stress changes induced by the preferred slip distribution model cannot completely explain the spatial distribution of aftershocks. Sensitivity analysis of Coulomb stress change implies that aftershocks in the stress shadow area may be driven by aseismic creep or triggered by dynamic stress transfer, requiring further investigation.

Is the Aftershock Zone Area a Good Proxy for the Mainshock Rupture Area?

ABSTRACTThe locations of aftershocks are often observed to be on the same fault plane as the mainshock and used as proxies for its rupture area. Recent developments in earthquake relocation techniques have led to great improvements in the accuracy of earthquake locations, offering an unprecedented opportunity to quantify both the aftershock distribution and the mainshock rupture area. In this study, we design a consistent approach to calculate the area enclosed by aftershocks of 12 Mw≥5.4 mainshocks in California, normalized by the mainshock rupture area derived from slip contours. We also investigate the Coulomb stress change from mainshock slip and compare it with the aftershock zone. We find that overall, the ratios of aftershock zone area to mainshock rupture area, hereinafter referred to as “aftershock ratio”, lie within a range of 0.5–5.4, with most values being larger than 1. Using different slip-inversion models for the same mainshock can have a large impact on the results, but the ratios estimated from both the relocated catalogs and Advanced National Seismic System catalog have similar patterns. The aftershock ratios based on relocated catalogs of southern California fall between 0.5 and 4.3, whereas they exhibit a wider range from 1 to 5.4 for northern California. Aftershock ratios for the early aftershock window (within one-day) show a similar range but of smaller values than using the entire aftershock duration, and we propose that continuing afterslip could contribute to the expanding aftershock zone area following several mainshocks. Our results show that areas with positive Coulomb stress change scale with aftershock zone areas, and spatial distribution of aftershocks represents stress release from mainshock rupture and continuing postseismic slip.

Long- and Short-Term Stress Interaction of the 2019 Ridgecrest Sequence and Coulomb-Based Earthquake Forecasts

AbstractWe first explore a series of retrospective earthquake interactions in southern California. We find that the four Mw≥7 shocks in the past 150 yr brought the Ridgecrest fault ∼1 bar closer to failure. Examining the 34 hr time span between the Mw 6.4 and Mw 7.1 events, we calculate that the Mw 6.4 event brought the hypocentral region of the Mw 7.1 earthquake 0.7 bars closer to failure, with the Mw 7.1 event relieving most of the surrounding stress that was imparted by the first. We also find that the Mw 6.4 cross-fault aftershocks shut down when they fell under the stress shadow of the Mw 7.1. Together, the Ridgecrest mainshocks brought a 120 km long portion of the Garlock fault from 0.2 to 10 bars closer to failure. These results motivate our introduction of forecasts of future seismicity. Most attempts to forecast aftershocks use statistical decay models or Coulomb stress transfer. Statistical approaches require simplifying assumptions about the spatial distribution of aftershocks and their decay; Coulomb models make simplifying assumptions about the geometry of the surrounding faults, which we seek here to remove. We perform a rate–state implementation of the Coulomb stress change on focal mechanisms to capture fault complexity. After tuning the model through a learning period to improve its forecast ability, we make retrospective forecasts to assess model’s predictive ability. Our forecast for the next 12 months yields a 2.3% chance of an Mw≥7.5 Garlock fault rupture. If such a rupture occurred and reached within 45 km of the San Andreas, we calculate it would raise the probability of a San Andreas rupture on the Mojave section by a factor of 150. We therefore estimate the net chance of large San Andreas earthquake in the next 12 months to be 1.15%, or about three to five times its background probability.

Effects of imparted Coulomb stress changes in the seismicity and cluster of the December 2017 Hojedk (SE Iran) triplet

The Hojedk area in the southeastern Iran experienced three earthquakes during 12 days in December 2017. These events occurred in proximity to a region that has experienced several moderate and large events in the past. In this paper, the role of Coulomb stress changes in occurrence of the Hojedk cluster was investigated, and also the produced stress changes due to these events on the surrounding and optimally oriented faults was calculated. Previous earthquakes (events in the Golbaf–Sirch region, the Shahdad slip, and the Dahuiyeh–Zarand earthquake) imparted positive stress changes on the fault planes of the Hojedk earthquakes, especially on the ruptured plane of the first main shock. The Hojedk first main shock triggered the second one by imparting maximum stress changes of about 11.7 MPa on the fault plane of this event, and these two events brought the third main shock to failure by producing more than 6.0 MPa stress changes on its ruptured plane. Among the major active faults in the surrounding area, the middle part of the Nayband fault, northern part of the Golbaf–Sirch fault, and southern part of the Kuhbanan fault received positive stress changes and southen parts of the Ravar and all parts of the Chatrud faults received negative stress changes due to the Hojedk earthquakes. Furthermore, our results for correlation between Coulomb stress changes and seismicity distribution showed that following the Dahuiyeh–Zarand earthquake, the majority of the seismicity located on the positive stress area and Coulomb stress changes have a controlling role on the spatial distribution of seismicity even after a decade. The occurrence of the Hojedk first main shock affects the spatial distribution of aftershocks until the occurrence of the next main hocks. By triggering the next one, the stress patterns change in the area, and seismicity follows from the stress patterns of the last event.

Elastic strain energy and pore-fluid pressure control of aftershocks

Aftershocks are well-known, however their triggering mechanisms remain unclear. The Coulomb failure stress change (ΔCFS) has been widely implemented to understand the spatial distribution of aftershocks. Here we propose a new metric for evaluating aftershock generation based on the energetics of shear faulting in a prestressed state. Unlike ΔCFS, the new metric depends not only on coseismic stress changes but also on background crustal stresses. The energetics-based formulation expands the ΔCFS defined on a specific plane into a generalized failure stress defined in a three-dimensional space. We examined the diagnostic ability of the new metric by applying receiver operating characteristic analysis to 12,175 aftershocks (M≥0) that followed the 1992 Landers earthquake. With a realistic background stress field inferred from thousands of earthquake focal mechanisms, the new metric was able to discriminate the triggering mechanisms of the aftershocks: two-thirds were direct responses to coseismic stress changes and one-fifth resulted from strength decreases owing to pore-fluid pressure increases.

Estimating Rupture Dimensions of Three Major Earthquakes in Sichuan, China, for Early Warning and Rapid Loss Estimates

ABSTRACTLarge earthquakes, such as Wenchuan in 2008, Mw 7.9, Sichuan, China, provide an opportunity for earthquake early warning (EEW), as many heavily shaken areas are far (∼50 km) from the epicenter and warning times could be sufficient (≥5 s) to take preventive action. On the other hand, earthquakes with magnitudes larger than ∼M 6.5 are challenging for EEW because source dimensions need to be defined to adequately estimate shaking. Finite-fault rupture detector (FinDer) is an approach to identify fault rupture extents from real-time seismic records. In this study, we playback local and regional onscale strong-motion waveforms of the 2008 Mw 7.9 Wenchuan, 2013 Mw 6.6 Lushan, and 2017 Mw 6.5 Jiuzhaigou earthquakes to study the performance of FinDer for the current layout of the China Strong Motion Network. Overall, the FinDer line-source models agree well with the observed spatial distribution of aftershocks and models determined from waveform inversion. However, because FinDer models are constructed to characterize seismic ground motions (as needed for EEW) instead of source parameters, the rupture length can be overestimated for events radiating high levels of high-frequency motions. If the strong-motion data used had been available in real time, 50%–80% of sites experiencing intensity modified Mercalli intensity IV–VII (light to very strong) and 30% experiencing VIII–IX (severe to violent) could have been issued a warning with 10 and 5 s, respectively, before the arrival of the S wave. We also show that loss estimates based on the FinDer line source are more accurate compared to point-source models. For the Wenchuan earthquake, for example, they predict a four to six times larger number of fatalities and injured, which is consistent with official reports. These losses could be provided 1/2∼3 hr faster than if they were based on more complex inversion rupture models.

Variability of ETAS Parameters in Global Subduction Zones and Applications to Mainshock–Aftershock Hazard Assessment

ABSTRACTMegathrust earthquake sequences can impact buildings and infrastructure due to not only the mainshock but also the triggered aftershocks along the subduction interface and in the overriding crust. To give realistic ranges of aftershock simulations in regions with limited data and to provide time-dependent seismic hazard information right after a future giant shock, we assess the variability of the epidemic-type aftershock sequence (ETAS) model parameters in subduction zones that have experienced M≥7.5 earthquakes, comparing estimates from long time windows with those from individual sequences. Our results show that the ETAS parameters are more robust if estimated from a long catalog than from individual sequences, given individual sequences have fewer data including missing early aftershocks. Considering known biases of the parameters (due to model formulation, the isotropic spatial aftershock distribution, and finite size effects of catalogs), we conclude that the variability of the ETAS parameters that we observe from robust estimates is not significant, neither across different subduction-zone regions nor as a function of maximum observed magnitudes. We also find that ETAS parameters do not change when multiple M 8.0–9.0 events are included in a region, mainly because an M 9.0 sequence dominates the number of events in the catalog. Based on the ETAS parameter estimates in the long time period window, we propose a set of ETAS parameters for future M 9.0 sequences for aftershock hazard assessment (K0=0.04±0.02, α=2.3, c=0.03±0.01, p=1.21±0.08, γ=1.61±0.29, d=23.48±18.17, and q=1.68±0.55). Synthetic catalogs created with the suggested ETAS parameters show good agreement with three observed M 9.0 sequences since 1965 (the 2004 M 9.1 Aceh–Andaman earthquake, the 2010 M 8.8 Maule earthquake, and the 2011 M 9.0 Tohoku earthquake).

Some Foreshocks and Aftershocks Associated with the Moiyabana Earthquake of 2017 in Botswana

On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associated with the Moiyabana earthquake. The foreshocks and aftershocks data used were from the Botswana Geoscience Institute (BGI) and the Seisan software was used to analyze the foreshock and aftershock events. The analyses revealed nine epicenter locations of foreshocks which are spread out across the country and most of them are located in the central and southern parts of Botswana, while the aftershocks are clustered around the mainshock. Although five of the nine foreshocks occurred far from the boundaries of major tectonic units, the other four occurred near key features such as the Zoetfontein, Lecha and Chobe faults. The spatial distribution of aftershocks indicates that the stress released by the mainshock, re-activated the planes of weakness in the vicinity of the mainshock and farther away from the mainshock. Hence, this affected the Zoetfontein fault, the boundary between Passarge basin and Magondi belt and the boundary between the Kaapvaal craton and Limpopo mobile belt. The aftershocks also show a northwest-southeast trend, which probably indicates the rupture plane; and mainly lie within the Limpopo mobile belt that is sandwiched between the Kaapvaal craton to the south and Zimbabwe craton to the north. Furthermore, the aftershocks concentration to the south reveals a close relation in demarcating the boundary of the Kaapvaal craton and the Limpopo mobile belt.

One neuron versus deep learning in aftershock prediction

29 August 2018: "Artificial intelligence nails predictions of earthquake aftershocks". This Nature News headline is based on the results of DeVries et al. (2018) who forecasted the spatial distribution of aftershocks using Deep Learning (DL) and static stress feature engineering. Using receiver operating characteristic (ROC) curves and the area under the curve (AUC) metric, the authors found that a deep neural network (DNN) yields AUC = 0.85 compared to AUC = 0.58 for classical Coulomb stress. They further showed that this result was physically interpretable, with various stress metrics (e.g. sum of absolute stress components, maximum shear stress, von Mises yield criterion) explaining most of the DNN result. We here clarify that AUC c. 0.85 had already been obtained using ROC curves for the same scalar metrics and by the same authors in 2017. This suggests that DL - in fact - does not improve prediction compared to simpler baseline models. We reformulate the 2017 results in probabilistic terms using logistic regression (i.e., one neural network node) and obtain AUC = 0.85 using 2 free parameters versus the 13,451 parameters used by DeVries et al. (2018). We further show that measured distance and mainshock average slip can be used instead of stress, yielding an improved AUC = 0.86, again with a simple logistic regression. This demonstrates that the proposed DNN so far does not provide any new insight (predictive or inferential) in this domain.

Effect of Fault Roughness on Aftershock Distribution: Plastic Off‐Fault Material Properties

AbstractWe perform spontaneous earthquake rupture simulations on rough strike‐slip faults with off‐fault plastic material properties. We examine the off‐fault stress change and coseismic damage pattern resulting from dynamic fault slip. We use the stress output from each simulation to calculate the Coulomb failure function (CFF). We calculate the CFF values on the extensional and compressional side of the fault using parallel receiver fault orientations. We also calculate the CFF values on the extensional side of the fault using variable receiver fault orientations determined using the angle at which plastic shear strain is maximum. We calculate the probability density function for the CFF values across the fault as a function of distance. We observe a similar overall trend of the CFF values with distance for the extensional and compressional sides—our simulations show a broad range of values in the near‐fault region, while a narrow range of values occurs further from the fault. In the near‐fault region, we observe many more positive CFF change zones using variable receiver fault orientations than using parallel orientations. We calculate the areas and amplitudes of these positive CFF zones as a function of distance away from the fault. Our comparison of CFF amplitudes as a function of rupture area suggests that the spatial aftershock distribution surrounding a fault is controlled by both stress heterogeneity and the coseismic damage zone complexity. The calculations of rupture areas using our model are consistent with ruptured areas of observed aftershocks in California.