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We investigate the source characteristics of picoseismicity (Mw < −2) recorded during a hydraulic fracturing in situ experiment performed in the underground Äspö Hard Rock Laboratory, Sweden. The experiment consisted of six stimulations driven by three different water injection schemes and was performed inside a 28‐m‐long, horizontal borehole located at 410‐m depth. The fracturing processes were monitored with a variety of seismic networks including broadband seismometers, geophones, high‐frequency accelerometers, and acoustic emission sensors thereby covering a wide frequency band between 0.01 and 100,000 Hz. Here we study the high‐frequency signals with dominant frequencies exceeding 1000 Hz. The combined seismic network allowed for detection and detailed analysis of 196 small‐scale seismic events with moment magnitudes MW < −3.5 (source sizes of decimeter scale) that occurred solely during the stimulations and shortly after. The double‐difference relocated hypocenter catalog as well as source parameters were used to study the physical characteristics of the induced seismicity and then compared to the stimulation parameters. We observe a spatiotemporal migration of the picoseismic events away and toward the injection intervals in direct correlation with changes in the hydraulic energy (product of fluid injection pressure and injection rate). We find that the total radiated seismic energy is extremely low with respect to the product of injected fluid volume and pressure (hydraulic energy). The radiated seismic energy correlates well with the hydraulic energy rate. The obtained fault plane solutions for particularly well‐characterized events signify the reactivation of preexisting rock defects under influence of increased pore fluid pressure on fault plane orientations in good correspondence with the local stress field orientation.

This article aims to investigate the frequency-magnitude characteristics and lower magnitude limits of the microseismic catalog recorded with a seismic network sensitive to high frequencies at Mponeng mine, South Africa. The network, composed of one three-component accelerometer and eight acoustic emission sensors, is located at a depth of 3.5 km below the surface and covers the limited volume of approximately 300 × 300 × 300 m. The three-component accelerometer was used to estimate the moment magnitude for the limited number of 135 events (M w ranged from 4:1 to 0:3) well recorded by the network. We use the relation between the moment magnitude estimated from accelerometer data and radiated energy/moment magnitude estimated from acoustic emission sensors to extend the catalog to lower magnitudes. The magnitude of completeness of selected spatiotemporal subsets of the catalog was estimated for: (1) an aftershock sequence of an M w 1.9 event that occurred approximately 30 m from the network, and (2) postblasting activity during working days, located more than 80 m from the network. The data follow the Gutenberg-Richter (GR) frequency-magnitude relationship with no visible deviation from selfsimilar behavior of seismicity between M w 4:4 and 1:9 for the aftershock sequence and between 3:5 and 1:5 for the postblasting dataset. We estimated the magnitude of completeness of selected subset as low as 4:3 (b 1:26) for the aftershock sequence and 3:4 (b 1:17) for the postblasting activity. Differences in magnitude of completeness are attributed to location of recorded activity and site effects.

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