Spatiotemporal Variations in Gutenberg–Richter b-Value Depending on the Depth and Lateral Position in the Earth’s Crust of the Garm Region, Tajikistan

Abstract

Spatiotemporal variations in the Gutenberg–Richter (GR) b-value and in the minimum magnitude of a predicted earthquake (MPE) are studied in detail depending on the depth and lateral position of the selected sample of earthquakes in the Garm region, Tajikistan. The time variations in b-value estimated from the data on the earthquakes in the different depth intervals indicate that most of the “strong” events with MMPE were preceded by the significant time anomalies localized in the vicinity of the source depths of these earthquakes. The maximum amplitudes of these anomalies gravitate to the vicinity of the hypocenters of the strong earthquakes and decay with distance from the hypocenter. The observed time anomalies in the b-value, which have a shape of a positive bay, are not accidental, which is demonstrated by their sufficient statistical representativeness (18 events). It is concluded that based on the used approach it will be possible to estimate the source depth of a future strong earthquake. The estimate of earthquake prediction quality based on 38 “strong” earthquakes with MMPE that occurred in seven local regions during a 23-year observation period shows that in 84% of cases, the emergence of b-value anomalies is accompanied by the successful forecasts. At the same time, the overall probability estimate of a medium-term forecast of the “strong” earthquakes with false alarms and missed events taken into account is 71%. Meanwhile, the forecasting quality of the strong earthquakes substantially increases if the b-value time variations are monitored separately in the different depth intervals of the Earth’s crust. It is shown that the parameter of the minimum MPE estimated from the rightmost part of the linear segment of GR relationship is a characteristic of the structural blocks of the Earth’s crust and varies both across the area and along the depth. It is hypothesized that the front of the deformation waves emerging on certain time intervals in a number of the local regions of the sample has probably been detected. The supposed deformation waves propagate with the velocities of 40–50 km/yr with their front moving NE to SE.The results of the study can be used for medium-term forecasting of the earthquakes with MMPE, for estimating the depth of the expected earthquake, and for overall seismic hazard assessment in the seismically active regions.

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Correspondence to G. A. Popandopoulos.

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Popandopoulos, G.A. Spatiotemporal Variations in Gutenberg–Richter b-Value Depending on the Depth and Lateral Position in the Earth’s Crust of the Garm Region, Tajikistan. Izv., Phys. Solid Earth 56, 337–356 (2020). https://doi.org/10.1134/S1069351320020081

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Keywords:

  • Gutenberg–Richter law
  • positive bay-like anomalies
  • detection of a front of deformation waves