b-value of what? Complex behavior of the magnitude distribution during and within the 2016–2017 central Italy sequence

The Magnitude–Frequency-Distribution (MFD) of earthquakes is typically modeled with the (tapered) Gutenberg–Richter relation. The main parameter of this relation, the b-value, controls the relative rate of small and large earthquakes. Resolving spatiotemporal variations of the b-value is critical to understanding the earthquake occurrence process and improving earthquake forecasting. However, this variation is not well understood. Here we present unexpected MFD variability using a high-resolution earthquake catalog of the 2016–2017 central Italy sequence. Isolation of seismicity clusters reveals that the MFD differs in nearby clusters, varies or remains constant in time depending on the cluster, and features an unexpected b-value increase in the cluster where the largest event will occur. These findings suggest a strong influence of the heterogeneity and complexity of tectonic structures on the MFD. Our findings raise the question of the appropriate spatiotemporal scale for resolving the b-value, which poses a serious obstacle to interpreting and using the MFD in earthquake forecasting.

Spatiotemporal Variations of Focal Mechanism Solutions and Stress Field of the 2019 Changning Ms 6.0 Earthquake Sequence

The June 17, 2019, MS 6.0 Changning earthquake is the largest recorded event in the Sichuan basin, spatiotemporal variations of stress field may shed light on the seismogenic mechanism of the earthquake. We determined the focal mechanism solutions (FMSs) of 124 earthquakes with MS ≥ 3.0 occurring in the Changning area from April 1, 2007, to February 29, 2020, and analyzed changes of FMSs and stress field before and after Changning earthquake. The Changning aftershocks were predominantly thrust fault earthquakes, followed by strike slip. The P-axis azimuths of the aftershock FMSs were oriented predominantly in the NEE direction, notably differing from the NWW-oriented P-axis azimuths of pre-earthquake FMSs; it shows the rotation of local stress field before and after the Changning earthquake, it is speculated that the change of stress field in Changning area may be caused by long-term water injection and salt mining activities. From the southeast to the northwest of the aftershock zone, the azimuths of principal compressive stress (S1) change from NEE to near-EW in both horizontal and vertical planes. Significant changes occurred in the FMS types and stress field of the aftershock zone following the Changning earthquake, the FMSs became diverse, the S1 azimuth of the Changning area changed from NWW to NEE, and then EW, the plunge and stress tensor variances increased, it reflects that the stress field of the Changning area adjusts continually with time.

Hydrochemistry of the Lhasa River, Tibetan Plateau: Spatiotemporal Variations of Major Ions Compositions and Controlling Factors Using Multivariate Statistical Approaches

Spatiotemporal variations of the hydrochemical major ions compositions and their controlling factors are essential features of a river basin. However, similar studies in the southern Tibetan Plateau are relatively limited. This study focuses on the chemical compositions of the dissolved loads in the Lhasa River (LR) in the southern Tibetan Plateau. Two sampling campaigns were conducted during the rainy and dry seasons across the LR basin to systematically investigate the spatiotemporal variations of water chemistry and sources of the dissolved loads. The results show that the river water possesses slight alkalinity with an average pH of 8.05 ± 0.04. Total dissolved solids (TDS) and oxidation-reduction potential (ORP) range widely from 39.8 mg/L to 582.6 mg/L with an average value of 165.6 ± 7.7 mg/L and from −9.4 mV to 295 mV with a mean value of 153.7 ± 6.9 mV, respectively. The major cations follow the decreasing order of Ca2+, Mg2+, Na+, and K+ while HCO3−, SO42−, Cl−, and NO3− for anions. Ca2+ and Mg2+ account for 87.8% of the total cations, while HCO3− and SO42− accounts for 93.9% of the total anions. All the major ions show higher concentrations in the dry season. NO3−, HCO3−, and Mg2+ show significant spatial variations due to the influence of basin lithology and anthropogenic activity. Multi-variables statistical analysis reveals that the mechanisms controlling the LR hydrochemistry are mainly carbonate weathering followed by silicate weathering. Geothermal springs and anthropogenic activities also play crucial roles in altering river water ions composition in the middle stream and downstream. The relatively high NO3− value (3 ± 0.2 mg/L) suggests water quality will be under the threat of pollution with the increase of anthropogenic activities.