The 1994 Cacoosing Valley earthquakes near Reading, Pennsylvania: A shallow rupture triggered by quarry unloading

Effects of crustal stresses on fluid transport in fractured rock: case studies from northeastern and southwestern USA

“…For convenience and ease of referencing, the maximum horizontal stress, S H ′/K i d m , and the minimum horizontal stress, S h ′/K i d m , are oriented E-W and N-S, respectively. These stress orientations differ somewhat from those determined by Seeber et al (1998), but are generally consistent with principal stress directions for the northeastern United States (Zoback and Zoback 1989). The values of 2.25, 1.00, and 1.15 are the stress ratios determined by Seeber et al (1998) from shallow (<5 km) earthquake focal mechanisms.…”

“…5A we see that for greater depths, where all the tectonic stress ratios given by Seeber et al (1998) might be expected to hold, E-W oriented subvertical fractures have the largest dimensionless apertures (ς/ς 0 ≈0.49), subhorizontal fractures (e.g., bedding-plane partings) have slightly reduced dimensionless apertures (ς/ς 0 ≈0.46), and N-S-oriented vertical fractures have the smallest dimensionless apertures (ς/ς 0 ≈0.30). A different situation is depicted for shallow depths in Fig.…”

“…5B. Here, where only the horizontal tectonic stress ratios given by Seeber et al (1998) might be expected to hold and where the vertical stress ratio would be reduced because of proximity to the ground surface, subhorizontal fractures (e.g., bedding-plane partings) have the largest dimensionless apertures (ς/ς 0 ≈0.90), and E-W-and N-Soriented vertical fractures have basically unchanged dimensionless apertures (ς/ς 0 ≈0.53 and ς/ς 0 ≈0.30).…”

“…Other nearby regional structures include broad, low amplitude folds (Lyttle and Epstein 1987) with mostly gentle topographic relief. Zoback and Zoback (1989) summarize contemporary principal stress orientations for the eastern United States, including the Newark Basin, and present the orientation of the greatest horizontal principal stress, S H , as NE-SW to E-W. Seeber et al (1998) argue that the least horizontal principal stress, S h , and the vertical principal stress, S v , have similar magnitudes in the Newark Basin and would be expected to change relative positions, thereby resulting in both thrust (S H >S h >S v ) and strike-slip (S H >S v >S h ) faulting. These investigators also apply source data from a number of recent earthquakes in the Newark Basin to a fault kinematic model proposed by Michael (1984) to establish the direction of S H and the principal stress ratios for the region.…”

Seismic Hazards Assessment for Radioactive Waste Disposal Sites in Regions of Low Seismic Activity

“…Of additional worry is that the 'natural' hazard, estimated for an unexcavated virgin site, might be significantly increased by the construction of the repository. This might occur by mass removal (sizable earthquakes have been triggered by surface quarrying, see Yerkes et al, 1983;Seeber et al, 1998), stress redistribution (many mines induce considerable seismic activity as the rockmass readjusts, and there is evidence that mining at Sudbury has triggered shear slip on faults cut by tunnels), and hydrostatic changes (during construction the hydrostatic effect of the repository would be zero to stabilizing, but after closure the slow flooding could trigger activity, as has been seen from recently flooded mines in Canada). Although they might not be quantifiable, any of these construction-related responses could trigger nearby earthquakes that otherwise might not have occurred for tens of thousands of years, and so render even the best prior hazard estimates moot.…”