Philippe Guéguen

In August 2019, a multistage hydraulic fracturing (HF) operation was carried out at Preston New Road, United Kingdom. HF caused abundant seismic activity that culminated with an ML 2.9 event. The seismic activity was recorded by a downhole array of 12 sensors located in a nearby monitoring well. About 55,556 events were detected and located in real time during the operation by a service company. In this study, we first improve the number of detections by applying template matching and later calculate the moment magnitude of the associated earthquakes. Then we show that by separately analyzing the periods during and immediately after injection, distinct patterns can be identified. We observe an increase in the delay and decrease in amplitude of peak seismicity during subsequent phases of injection. After injection, the seismicity decay can be described by the Omori–Utsu law. The decay rate tends to slow with each successive injection, in particular during the later injection stages. In addition, the frequency–magnitude distribution evolves from a tapered distribution (lack of large events) to a bilinear distribution (excess of large events). This evolution is gradual, with the corner magnitude increasing with each injection. We interpret these patterns as the result of the combined effect of two factors: (1) the stimulated volume becoming increasingly aseismic and (2) the gradual increase in its size, which increases the probability of triggered events on preexisting faults. More generally, these patterns suggest that seismic activity during injection is strongly influenced by the injection history and is modulated by local conditions such as stress state, fault structure, and permeability.

Herein, we discuss amplitude–frequency noise models for high-quality accelerometric monitoring of a civil engineering building and the benefits of seismic building monitoring policies in weak-to-moderate seismic regions. Since 2004, the city hall building in Grenoble (French Alps) has been monitored continuously. First, accelerometric data from one continuous year are used to derive broadband noise models for the bottom and top of the building. The noise models are compared with (1) the noise sensitivity of the high-gain accelerometer installed in the building and low-cost sensor sensitivity models; (2) the typical earthquake response curves given by Clinton and Heaton (2002); and (3) the earthquakes recorded in the Northern Alps. Then, using earthquakes data, this study highlights threshold values for signal-to-noise ratio (≥3 or 9 dB) recordings of earthquake as a function of magnitude and distance for weak-to-moderate earthquakes. We present a preliminary cost-benefit analysis of instrumentation for such regions according to seismic hazard and instrumentation quality. For weak-to-moderate seismic regions like Grenoble area, the capability of high-dynamic accelerometers to record low-amplitude ground motions and building responses is confirmed and encouraged to enable high-quality observation of building response over a broad range of frequencies. Bearing in mind that full-scale building test data are of greater interest for improving our understanding of building response than even the most sophisticated models, the recording of weak-to-moderate earthquakes in building must be broadened using high dynamic instruments to obtain more comprehensive and advanced results.