Seismic Velocity Structure of the Irish Crust from Quarry Blasts

Abstract

Travel-time tomography uses the travel times of seismic waves between pairs of sources and receivers to constrain the elastic properties of the subsurface. However, the low rate of natural seismicity in Ireland limits the application of standard local earthquake tomography. This study uses seismic wave arrival times from controlled explosions generated during quarry and mining activities to refine the constraints on the velocity structure of the Irish crust. Previous seismic studies have utilised (i) surface wave dispersion from teleseismic earthquakes, providing broad insights into the lithospheric structure, and (ii) spatially sparse seismic reflection and refraction profiles. While these studies have delineated major tectonic features, such as the late-Caledonian Leinster Granite and a crustal boundary linked to the closure of the Iapetus Ocean, the precise boundaries of these features remain unresolved. Subašić (2021) employed the FMTOMO package to compute a preliminary 3D travel-time tomography model of the Irish crust based on quarry blast data. FMTOMO (Rawlinson et al., 2006) uses a gradient-based subspace inversion scheme to derive a seismic velocity model from observed travel times. In this study, we re-evaluate and expand the input dataset used by Subašić (2021) and focus on optimising the regularisation parameters of the tomographic inversion. Event classification into natural earthquakes and quarry explosions is performed using the spectral ratio method applied to S-wave trains, a procedure developed and routinely applied by the Irish National Seismic Network (INSN). The updated dataset includes 1,411 quarry blast events with P- and S-wave travel-time measurements from 2013–2014, a period of increased station density due to temporary seismic deployments. Quarry blasts, being surface explosions, are assumed to have well-constrained surface locations. A catalogue of 234 quarry sites in Ireland was initially compiled from satellite imagery by the INSN. Hypocentre locations for each event are first calculated from phase arrival times and subsequently relocated to the nearest quarry. Given that quarry mines in Ireland typically range from hundreds of metres to a maximum of ~1.5 km in length, most events fall within the error margin of the initial locations. For events located beyond a 3 km radius of known quarries, additional searches for unrecorded sites were conducted. Satellite imagery inspections of these unclassified events identified 25 additional quarries. The operational status of these quarries during the study period was confirmed using historical imagery from Google Earth by comparing quarry areas before and after the analysed time frame