On the nature of long-period long-duration seismic events detected during hydraulic fracturing

Abstract

Long-period long-duration (LPLD) seismic events are low-amplitude tremor-like seismic signals that have been observed in some microseismic monitoring data sets acquired during hydraulic fracturing operations. The LPLD events have been interpreted to be associated with slow slip along preexisting fractures presumed to either have high clay content or be misaligned with respect to the current-day principal stress directions. However, a recent study indicates that regional earthquakes, when recorded on vertical downhole monitoring arrays, have similar signal characteristics to LPLD events and that care must be taken when analyzing and interpreting such signals. Using data from a hydraulic fracturing microseismic data set in which LPLD events have previously been identified and well documented, together with data from the EarthScope Transportable USArray, we have investigated the hypothesis that the documented LPLD events were regional earthquakes. We have determined that the LPLD events corresponded with signals recorded on the USArray at distances of up to 350 km away from the injection well, although they were not listed in any regional earthquake catalog. The spatial coverage of the USArray allows the sources of many of the LPLD events to be relocated outside of the treatment well area and thus suggests that they are regional earthquakes of magnitude smaller than M2.5 rather than locally sourced events related to the hydraulic fracturing stimulation process.