Is Coulomb Stress the Best Choice for Aftershock Forecasting?

<p>The Coulomb failure stress (CFS) criterion is the most commonly used method for predicting spatial distributions of aftershocks following large earthquakes. However, large uncertainties are always associated with the calculation of Coulomb stress change. The uncertainties mainly arise due to nonunique slip inversions and unknown receiver faults; especially for the latter, results are highly dependent on the choice of the assumed receiver mechanism. Based on binary tests (aftershocks yes/no), recent studies suggest that alternative stress quantities, a distance‐slip probabilistic model as well as deep neural network (DNN) approaches, all are superior to CFS with predefined receiver mechanism. To challenge this conclusion, which might have large implications, we use 289 slip inversions from SRCMOD database to calculate more realistic CFS values for a layered half‐space and variable receiver mechanisms. We also analyze the effect of the magnitude cutoff, grid size variation, and aftershock duration to verify the use of receiver operating characteristic (ROC) analysis for the ranking of stress metrics. The observations suggest that introducing a layered half‐space does not improve the stress maps and ROC curves. However, results significantly improve for larger aftershocks and shorter time periods but without changing the ranking. We also go beyond binary testing and apply alternative statistics to test the ability to estimate aftershock numbers, which confirm that simple stress metrics perform better than the classic Coulomb failure stress calculations and are also better than the distance‐slip probabilistic model.</p>

Shallow slip of blind fault associated with the 2019 Ms 6.0 Changning earthquake in fold-and-thrust belt in salt mines of Southeast Sichuan, China

SUMMARY An earthquake with a magnitude of Ms 6.0 and shallow focal depth of ∼4 km struck the Changning county, Sichuan province, China on 2019 June 17. The hypocentre is located in the fold-and-thrust belt with plentiful shale gas and salt mine resources. One hypothesis is that the shallow fault could be affected by the artificial pressure water injection including the disposal of wastewater, fracking shale gas extraction and salt mining in Changning area. In this study, SAR (Synthetic Aperture Radar) images, historical earthquakes, aftershocks and seismic reflection data were collected to jointly investigate the characteristics of the 2019 Changning earthquake. The source model inferred from the InSAR coseismic deformation observation reveals that the 2019 Changning earthquake is attributed to a blind fault dipping to southwest with dominant thrust and sinistral strike slip. Moreover, a small shallow fault developing within the Changning anticline was triggered by the main shock, which contributed to the surface displacements as observed in the north of the epicentre. The estimated maximum slip of 0.49 m is located at the depth of ∼1.9 km, ∼9 km northwest of the epicentre. The Coulomb failure stress change caused by the previous two large earthquakes, which occurred in the hydraulic fracturing area, suggesting that they have little effect on the initial rupture of the 2019 Changning earthquake. Despite this, they have a positive triggering effect on the fault rupture in the northwest of the seismogenic fault. In addition, the analysis on the relation between the positive Coulomb failure stress change and the aftershocks suggests that the aftershocks may have different motion patterns from the main shock. The analysis also shows the earthquakes occurrence in the seismogenic zone may be affected by the high pore pressure due to the long-term injection of salt mining for more than three decades.

Significance of Nonplanar Rupture of the Mainshock and Optimal Faulting in Forecasting Aftershocks of the 2015 Mw 7.8 Gorkha Earthquake

We computed the stress-change tensor around the 2015 Mw 7.8 Gorkha earthquake with two different rupture models: a simple uniformly dipping model and a complex ramp-flat-ramp-flat fault-slip model. In general, the Coulomb failure stress changes (ΔCFS) computed on the optimally orientated faults based on a ramp-flat-ramp-flat fault-slip model showed the best spatial correlation with the aftershock seismicity. This close relationship was further verified by the focal mechanism solutions of 17 intermediate-size aftershocks. The ΔCFS calculated from the known focal mechanisms of most events were close to the values computed from the optimal fault planes and slip directions using the complex slip model with a nonplanar rupture along the Main Himalaya thrust. We further computed the stress accumulation in the seismic gap regions located around the Gorkha earthquake and between the 1505 and 1934 Mw 8+ historical earthquakes. We found a significant increase of the Coulomb failure stress by 0.2–0.5 MPa caused by the three earthquakes, especially at the shallow ramp of the seismic gap, which indicates an enhanced seismic risk around the Kathmandu area.

Is Coulomb stress the best choice for aftershock forecasting?

<p>The Coulomb failure stress (CFS) criterion is the most commonly used method for predicting spatial distributions of aftershocks following large earthquakes. However, large uncertainties are always associated with the calculation of Coulomb stress change. The uncertainties arise due to non-unique slip inversions and unknown receiver fault mechanism. Especially for the latter, uncertainties are highly dependent on the choice of the assumed receiver mechanism. There are two ways of defining the receiver faults, either by predefining fault kinematics by geological constraints, or by calculating optimally oriented planes, both ways are pretty unrealistic as real aftershocks show variable rupture mechanisms. Recent studies have proposed an alternative method based on deep learning to forecast aftershocks. Using a binary test (aftershocks yes/no), it has been shown that their method as well as alternative stress values, such as maximum shear or the von-Mises criteria, are more accurate and reliable than the classical CFS criterion with predefined receiver mechanism.</p><p>Here we use 351 slip inversions from SRCMOD database to calculate Coulomb failure stress on a layered-half space using variable receiver mechanisms as well as proposed alternative stress metrics. We also perform tests for different magnitude cut-offs, grid size variation, and aftershock duration to verify the use of ROC analysis for ranking of stress metrics. The observations suggest that introducing a layered-half space does not improve the stress maps and ROC curves. However, magnitude cut-off and aftershock duration does effect the efficiency of stress metric in a way that larger magnitudes and shorter aftershock durations are forecasted efficiently. Two alternative statistics based tests i.e. log-likelihood and information gain tests using rate-based forecasts (non-binary) are also performed to compare the ability of metrics to discriminate the regions with and without aftershocks. The results suggest that simple methods of stress calculations perform better than the classic Coulomb failure stress calculations.</p>

Induced or triggered? The deadly February 2019 Rongxian-Weiyuan ML 4.9 earthquake in the shale gas field in Sichuan, China

<p>Coinciding with the extensive hydraulic fracturing activities in the southern Sichuan basin, seismicity in the region has surged in the past a few years, including a number of earthquakes with magnitudes larger than 5. On 25 February 2019, an M<sub>L</sub>4.9 earthquake struck the Rongxian County, Sichuan, China and caused 2 fatalities and 12 injuries, the first deadly earthquake associated with shale gas production. The earthquake was preceded by two foreshocks with magnitudes of M<sub>L</sub>4.7 and M<sub>L</sub>4.3 within two days. We relocated the earthquake sequence using local and regional seismic network, and obtained the focal depths of the mainshock and two foreshocks at 1 and 3 km, respectively, much shallower than the report from catalogue. Most other smaller quakes were located at 2-6 km. The mainshock had also been well captured by InSAR images, which confirmed the shallow depth of ~1 km. Both seismic and geodetic data yielded thrust faulting mechanism for the mainshock, consistent with the mapped Molin fault in the region. The two foreshocks, however, occurred on an unmapped fault that has different orientation than the Molin fault. Injection wells are found in the vicinity of the two foreshocks and the fracking depth (~2.7 km) coincides with their focal depths, suggesting a possible causal relationship. The mainshock is located in the region with positive Coulomb failure stress caused the two foreshocks. The value of Coulomb failure stress change is 0.03 bar, smaller than the typical static triggering threshold. Therefore, the mainshock is likely caused by fracking by poroelastic stress transfer.</p>

The 2014 Mw 6.1 Ludian Earthquake: The Application of RADARSAT-2 SAR Interferometry and GPS for this Conjugated Ruptured Event

Although the Zhaotong–Ludian fault is a seismically active zone located in the boundary between the Sichuan–Yunnan block and the South China block, it has not experienced a large earthquake greater than Mw 7 since at least 1700. On 3 August, 2014, an Mw 6.1 earthquake (the Ludian earthquake) ruptured the Zhaotong active belt in Ludian County, Yunnan province, China. This earthquake was the largest earthquake recorded in the region since 2000, and it provides us with a unique opportunity to study the active tectonics in the region. The analysis of the aftershocks showed that two conjugate faults could have been involved in the event. We first used Global Positioning System (GPS) data and C-band RADARSAT-2 imagery to map the coseismic surface deformation. We then inverted the derived coseismic deformation for the slip distribution based on the constructed conjugate fault model. Finally, the coulomb failure stress due to the Ludian earthquake was estimated to investigate the potential seismic hazards in this region. Our investigations showed that the Ludian earthquake was mainly a bilateral rupture event. The major slip of the main shock was located at depths of 0–5 km, which is close but does not superpose with the aftershocks that are mostly located at depths of 5–20 km. Interestingly, the seismic moment released by the aftershocks (6.9 × 1018 N∙m) was greater than that of the main shock (2.6 × 1018 N∙m). This evidence suggests that the accumulated elastic strain at depths of 0–20 km could have been fully released by the Ludian earthquake and its subsequent aftershocks. Furthermore, our analysis of the coulomb failure stress changes due to the main shock showed that the aftershocks could be the result of dynamic triggering rather than static triggering.