Fault Instability and Its Relation to Static Coulomb Failure Stress Change in the 2016 Mw 6.5 Pidie Jaya Earthquake, Aceh, Indonesia

Herein, we applied the fault instability criterion and integrated it with the static Coulomb stress change (ΔCFS) to infer the mechanism of the 2016 Mw 6.5 Pidie Jaya earthquake and its aftershock distribution. Several possible causative faults have been proposed; however, the existence of a nearby occurrence, the 1967 mb 6.1 event, created obscurity. Hence, we applied the fault instability analysis to the Pidie Jaya earthquake 1) to corroborate the Pidie Jaya causative fault analysis and 2) to analyze the correlation between ΔCFS distribution imparted by the mainshock and the fault instability of the reactivated fault planes derived from the focal solution of the Pidie Jaya aftershocks. We performed the fault instability analysis for two possible source faults: the Samalanga-Sipopok Fault and the newly inferred Panteraja Fault. Although the maximum instability value of the Samalanga-Sipopok Fault is higher, the dip value of the Panteraja Fault coincides with its optimum instability. Therefore, we concluded that Panteraja was the causative fault plane. Furthermore, a link between the 1967 mb 6.1 event and the 2016 Mw 6.5 earthquake is discussed. To analyze the correlation between the fault instability and the ΔCFS, we resolved the ΔCFS of the Pidie Jaya mainshock on its aftershock planes and compared the ΔCFS results with the fault instability calculation on each aftershock plane. We discussed the possibility of conjugate failure as shown by the aftershock fault instability. Related to the ΔCFS and fault instability comparison, we found that not all the aftershocks have positive ΔCFSs, but their instability value is high. Thus, we suggest that the fault plane instability plays a role in events that do not occur in positive ΔCFS areas. Apart from these, we also showed that the off-Great Sumatran Fault (Panteraja and Samalanga-Sipopok Faults) are unstable in the Sumatra regional stress setting, thereby making it more susceptible to slip movement.

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.

Source Model of the 2014 Mw 6.9 Yutian Earthquake at the Southwestern End of the Altyn Tagh Fault in Tibet Estimated from Satellite Images

Multiple fault segments ruptured during the 2014 Yutian earthquake, but the detailed source parameters and the mechanism of rupture complexity remain poorly understood. Here, we use high-resolution TanDEM-X satellite data and Satellite Pour l’Observation de la Terre-6/7 images to map the coseismic ground deformation field of the event. We find that the majority of coseismic slip occurred in the upper 10 km with the maximum left-lateral fault slip of ∼2.5 m at ∼6 km depth. The fault ruptured across a large 4.5 km extensional stepover from one left-lateral fault segment to another, with some right-lateral relay faulting in between. We find that the earthquake was followed by shallow afterslip concentrating at the southwestern end of coseismic rupture, in an area of many aftershocks and positive Coulomb failure stress change. Our findings demonstrate the power of satellite remote sensing technology in constraining source geometry and slip model of complex earthquakes when ground measurements are limited.

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>

Automatic Inversion of Rupture Processes of the Foreshock and Mainshock and Correlation of the Seismicity during the 2019 Ridgecrest Earthquake Sequence

The 2019 Ridgecrest, California, earthquake sequence included an Mw 6.4 foreshock on 4 July, followed by an Mw 7.1 mainshock about 32 hr later. We determined the rupture patterns of the foreshock and mainshock by applying the automatic iterative deconvolution and stacking method to strong-motion records. The foreshock was characterized by a unilateral rupture toward the southwest, and the shallow portion had a relatively large slip with the maximum value of ∼1.4 m. The mainshock presents an asymmetrical bilateral rupture with an average rupture velocity of 2.0 km/s. More than 80% of the seismic moment was released on the northwest segment of the fault, producing a maximum slip of ∼5.2 m. With the two inferred slip models, we calculated the Coulomb failure stress change (ΔCFS) to analyze the spatial–temporal correlation of the seismicity activity in this sequence. The result shows that the epicenter of the Mw 7.1 mainshock was brought 0.4 bars closer to failure by the Mw 6.4 foreshock, and the stress-increased zone has a good spatial consistence with the coseismic slip distribution of the mainshock and the aftershock distribution of the foreshock. Besides, the positive ΔCFS induced by the mainshock also enhanced its aftershock activity, especially at depths of 4–10 km where the major rupture occurred, inferring that the mainshock-induced ΔCFS may be responsible for the occurrence of aftershocks. In addition, we test the effects of different cutoff frequencies and crust velocity structures on the inversion results. The result reveals that the main source rupture characteristics are almost independent of these factors, implying a high reliability of automation inversion of strong-motion data. Overall, this work indicates that automatic inversion of strong-motion data can provide reliable and rapid rupture model, which is essential for earthquake emergency responses and tsunami early warnings.

Sensitivities of Geodetic Source Analyses to Elastic Crust Heterogeneity Constrained by Seismic Tomography for the 2017 Mw 6.5 Jiuzhaigou, China, Earthquake

ABSTRACT The 2017 Mw 6.5 Jiuzhaigou earthquake (JE) struck a rugged area of the Jiuzhaigou Valley in eastern Tibet that has experienced frequent seismic activity over the last few decades. We use finite‐element models (FEMs) and Sentinel‐1 Interferometric Synthetic Aperture Radar observations to characterize the earthquake source. The FEM domain accommodates a heterogeneous (HET) distribution of realistic crustal materials inferred by regional seismic tomography data. The HET‐derived source configurations yield a significantly smaller misfit, at the 95% confidence level, than that estimated for a homogeneous (HOM) half‐space. The former generally requires a lower degree of smoothing constraint, highlighting that the HET solutions are systematically more compatible with the surface observations than the HOM solutions. The magnitudes of induced Coulomb failure stress change (ΔCFS) estimated by the HET solution drastically differ (by >0.1 MPa) from those calculated by the HOM solution. The postearthquake stability of near‐field faults is generally overestimated by the HOM estimations, whereas some localities of negative ΔCFSHOM are predicted with positive ΔCFSHET. These results highlight the sensitivities of both slip and stress estimations to the complexity of the adopted elastic modeling domain, leading to more accurate aftershock hazard assessments. The HET‐resolved seismic rupture reveals two major slip asperities of magnitude up to 0.83 m distributed along the fault strike, which is coherent with the aftershock distribution. Two aftershock clusters are consistently found near or below these two peak‐slip zones, which are imaged by the HET model but absent in the HOM solution. The JE hypocenter and aftershocks are bounded below by a negative velocity anomaly (ΔVP, ΔVS down to −4%) at ∼18 km depth. Such low‐velocity layers of reduced strength may be relevant to the vertical distribution of seismicity and earthquake slip, which provide insights into assessing the seismic hazards and aftershock‐prone areas of the eastern Tibetan margin.