Spatiotemporal Variations in Gutenberg–Richter b-value Depending on the Depthand Lateral Position in the Earth’s Crust of the Garm Region, Tajikistan
Detailed studies of the spatiotemporal variations in the Gutenberg–Richter (GR) b-value and in the minimum magnitude of the predicted earthquake (MPE) depending on the depth and lateral position of the sample of earthquakes in the Garm region, Tajikistan, are conducted. The time variations in the b-value obtained from the data on the earthquakes in the different depth ranges indicate that most of the “strong” events with M ≥ MPE were preceded by significant time anomalies concentrated in the vicinity of the source depths of these earthquakes. The maximum amplitudes of these anomalies are concentrated in the vicinity of the hypocenters of the strong earthquakes and subsequently attenuate with distance from a hypocenter. The observed time anomalies in b-value, which have a positive bay-like shape, are not accidental as suggested by their sufficient statistical representativity (18 events). It is concluded that the used approach will allow us to estimate the depth of the source of the future strong earthquake. The quality assessment of earthquake prediction based on 38 “strong” earthquakes with M ≥ MPE that occurred in seven local regions during a 23-year observation period shows that in 84% of cases, the presence of the anomalies in the b-value is accompanied by the successful forecasts. At the same time, the overall estimate for the probability of a medium-term forecast of “strong” earthquakes, with the allowance for the false alarms and missed events, is 71%. Meanwhile, the forecasting quality of strong earthquakes substantially increases when the time variations in the b-value are differentially monitored in the different depth intervals of the Earth''s crust. It is shown that the parameter of the minimum MPE determined from the right edge of the linear segment of the GR relationship is a characteristic of the structural blocks of the Earth''s crust and varies both in area and in depth. It is hypothesized that the front of the deformation waves arising on certain time intervals in a number of the local regions of the sample is probably detected. The propagation velocities of the supposed deformation waves are 40–50 km/yr with the front propagating from NE to SE. The results of the study can be used for medium-term forecasting the earthquakes with M ≥ MPE, for estimating the depth of the expected earthquake, and for the assessment the general seismic hazard in the seismically active regions.