Detecting aftershocks using a dense N-array: The case of the 2019 M4.1 Black Rock, Utah sequence

On April 14, 2019 a moderate earthquake (M 4.1) occurred 20 km ENE of Black Rock, Utah and ~66 km NNE of Milford, Utah. It was preceded by an M2.07 foreshock and the largest aftershock (M2.68) occurred ten minutes later. For the first three days after the mainshock the University of Utah Seismograph Stations (UUSS) cataloged 19 events recorded by the regional network. Coincident in time with the sequence a dense circular array consisting of 151 5-Hz three component nodal geophones over a 5 km aperture was in operation. The array was located at the proposed Utah FORGE geothermal research facility footprint in southwestern Utah, 30-35 km SSW from the mainshock's epicenter. The nodals were distributed in five concentric rings centered around well 58-32, and were recording continuously for 23 days at 1 kHz, beginning April 7, 2019. To take advantage of the dense array we explore different array-based detection algorithms for identifying aftershocks during the first three days of the sequence, namely, (i) the frequency-domain array-based detection algorithm (Linville et al., 2018), (ii) the local similarity-based detection algorithm (Li et al., 2018), and (iii) the algorithm based on the envelope of stacked subarrays (Meng and Ben-Zion, 2017). Using as ground truth the events detected and located by the UUSS, we aim to increase the number of true detections, particularly for the first hours after the mainshock. Preliminary results show that the implemented algorithms surpass the permanent network's detection capability. For the first two hours following the mainshock we are able to at least double the number of detections compared to routine process.