The 20th Anniversary of the Installation of the Small-Aperture Mikhnevo Array for Monitoring Induced Seismicity

Abstract—Over the 20 years of continuous operation of the Mikhnevo small-aperture seismic array (SASA), vast experience in recording ultraweak signals generated by regional and global seismicity has been accumulated. High-resolution data processing methods have been developed and applied, including beam-forming and waveform cross correlation. In this review of the results of instrumental observations and processing, two approaches to reducing the detection threshold for seismic events when monitoring induced seismicity are considered: the use of array stations and the waveform cross-correlation method (WCCM). The efficiency of the approaches with respect to the detection, location, and identification of weak seismic sources is illustrated by the aftershock sequence of the earthquake near Mariupol that occurred on August 7, 2016, as well as the aftershocks of the fifth and sixth announced explosions in the DPRK, detected during the period from September 9, 2016, to September 11, 2021. The coordinates of the earthquake were estimated using the data of the Mikhnevo array and the temporary Rostov-on-Don SASA of IDG RAS. The location accuracy is comparable to the accuracy provided by 49 three-component (3-C) stations of FRC GS RAS. In the five days after the earthquake, 12 aftershocks were detected and located with respect to the mainshock using the WCC method. The group stations of the International Monitoring System (IMC) AKASG and BRTR and the 3-C station KBZ also participated in the detection and estimation of the parameters. The network of stations of the FRC GS RAS detected five aftershocks, and the IMC did not detect a single one. The location of explosions in the DPRK using the WCCM made it possible to determine their relative location with an accuracy of 100–200 m. The sixth explosion could not be accurately located with respect to the others due to the finite size of its source, which introduced significant changes in the differential travel time, depending on the direction to the station. The WCCM was also used to detect and identify weak seismic events within the DPRK Punggye-ri test site using template waveforms from explosions and aftershocks of the fifth and sixth tests, recorded at the IMC array stations KSRS and USRK. Over a 5-year observation period, 89 events were detected. Based on estimates of the cross-correlation characteristics of signals at both stations, it was possible to divide the general aftershock sequence into two separate ones associated with processes in the zones of influence of the fifth and sixth explosions.