A mine blast is typically composed of many individual explosions detonated in a temporal and spatial grid, a process called delay firing. A recent experiment at a granite quarry in Massachusetts included detonation of a delay‐fired (DF) mining blast within 250 m of a single‐fired (SF) explosion. The DF blast consisted of 48 holes detonated over 0.495 s. It produced longer duration Rayleigh waves with envelope functions that peaked later than Rayleigh waves generated by the SF explosion. Measured group velocity dispersion curves for recordings of the DF blast were slower than the estimates for the SF explosion, and with larger differences noted at near‐source stations. Using a formula derived initially by Ben‐Menahem (1961) for dynamically rupturing fault lines, we correct the dispersion curves using an initial group delay of 0.32 s. We demonstrate that for delay‐firing patterns of ∼0.5 s, the effect of the initial group delay is not significant beyond 25 km; however, for longer‐duration blasts, the effect could be observed at distances beyond 100 km. Mine blast firing patterns should be considered prior to using the resulting dispersion curves for crustal structure determination.
Online Material: MPEG movies of the DF and SF shots.
