CRUSTAL UNLOADING BY QUARRYING AS A SOURCE OF INDUCED SEISMICITY IN PLAINFIELD, CONNECTICUT

Induced seismicity can cause millions of dollars of damage to infrastructure and lead to loss of human life. Many research efforts currently focus on earthquakes caused by fluid injection, considering the surge in seismic activity surrounding wastewater injection in Oklahoma, Texas, and other areas. However, other physical processes, including fluid removal, hydraulic fracturing, and mass loading/unloading must be considered for a complete understanding of seismic hazards caused by industrial activity. In this research, a 3D boundary element program (3D~Def) is used to simulate stress changes near Wauregan Quarry in Plainfield, Connecticut in order to analyze the possibility of inducing seismicity through crustal unloading. On January 12, 2015, a magnitude 3.1 mainshock near Wauregan Quarry caused modified Mercalli II-IV intensities. Shortly after the event, a team from Weston Observatory installed portable seismographs in the epicentral area. The portable array detected hundreds of small earthquakes from around the quarry, with 26 events that were accurately located. P-wave first motion directions obtained from the aftershock readings of these seismographs and surrounding networks suggest a thrusting focal mechanism. We collect 113 gravity measurements in the proximity of the quarry to compute a density model of the area and approximate fault orientations. Stable continental regions are commonly at near-critical stress conditions, such that small mechanical changes can trigger earthquakes. Quarry operations result in the removal of mass from the crust, which decreases lithostatic load. In a setting confined by compressional stress, decreasing the lithostatic load, or minimum principal stress (σ₃), shifts a Mohr-Coulomb diagram to failure, indicating rock fracture. A study published in 1998 by Seeber et al. investigates an earthquake that occurred near a quarry in the Cacoosing valley, Pennsylvania, which caused over $2 million of damage. In accordance with the seismic event near Wauregan Quarry, the study concluded that the Cacoosing valley event occurred as the result of a thrusting mechanism. Research exploring the relationship between earthquakes and quarry operations is necessary in order to reduce the risk of seismic hazards in stable continental regions for the future.