Local Magnitude Discrepancies for Near-Event Receivers: Implications for the U.K. Traffic-Light Scheme

Local seismic magnitudes provide a practical and efficient scale for the implementation of regulation designed to manage the risk of induced seismicity, such as Traffic Light Schemes. We demonstrate that significant magnitude discrepancies (up to a unit higher) occur between seismic events recorded on nearby stations (<5km) compared to those at greater distance. This is due to the influence of sedimentary layers, as these shallow layers are generally lower in velocity and more attenuating than the underlying crystalline basement rocks, and require a change in the attenuation term of the ML scale. This has a significant impact on the UK’s hydraulic fracturing Traffic Light Scheme whose ‘red’ light is set at ML = 0.5. As the nominal detectability of the UK network is ML = 2, this scheme will require the deployment of monitoring stations in close proximity to well sites. Using data collected from mining events near New Ollerton, Nottinghamshire, we illustrate the effects proximity has on travel path velocities and attenuation, then perform a damped least squares inversion to determine appropriate constants within the ML scale. We show that the attenuation term needs to increase from 0.00183 to 0.0514, and demonstrate that this higher value is representative of a raypath within a slower, more attenuating sedimentary layer compared to the continental crust. We therefore recommend that the magnitude scale !! = !"# ! + 1.17!"# ! + 0.0514! − 3.0 should be used when local monitoring networks are within 5km of the event epicentres.