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

Abstract

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 (TLS). We demonstrate that significant magnitude discrepancies (up to a unit higher) occur between seismic events recorded on nearby stations (<5 km) compared with those at greater distances. This is due to the influence of sedimentary layers, which are generally lower in velocity and more attenuating than the underlying crystalline basement rocks, and requires a change in the attenuation term of the M L scale. This has a significant impact on the United Kingdom’s (U.K.) hydraulic fracturing TLS, whose red light is set at M L 0.5. Because the nominal detectability of the U.K. network is M L 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 that proximity has on travel path velocities and attenuation, then perform a damped least‐squares inversion to determine appropriate constants within the M L 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 ray path within a slower more attenuating sedimentary layer compared with the continental crust. We therefore recommend that the magnitude scale M L=log( A )+1.17log( r )+0.0514 r −3.0 should be used when local monitoring networks are within 5 km of the event epicenters.