Analysis the 1978–2008 crustal and sub-crustal earthquake catalog of Vrancea region

The crustal and sub-crustal seismicity that occurred in the Vrancea area from 1978 to 2008 is investigated. Due to quarry blast contamination, the analysis of the crustal seismicity was restricted to events that occurred between 20 km and 60 km, considering sub-crustal events as all those that occurred at depths larger than 60 km. The completeness magnitude was estimated between 2.3 and 2.5 for the crustal seismicity and between 2.9 and 3.1 for the sub-crustal one. The Gutenberg-Richter b-values show clear differences between crustal and sub-crustal seismicity, with values around 0.9 and 1.3–1.4, respectively. The analysis performed in this study represents a preliminary issue for the seismic hazard assessment of surrounding regions like Moldova, for which the earthquakes generated in Vrancea represent an important threat.

Wide-angle seismic imaging of the Elberton granite, Georgia: A pilot study using instantaneous blasts at dimension-stone quarries

Wide-angle recordings of small, multiple-hole blasts at dimension-stone quarries in the Elberton granite show coherent reflections at offsets between 7 and 15 km. In contrast with much larger blasts at crushed-stone quarries, this type of blast is fired without delays between holes. The data were recorded with an array of twenty portable seismographs using three-component, 4.5-Hz geophones. Migration of source gathers for two of the blasts shows a number of subhorizontal interfaces at depths between 2 and 4 km that may represent a layered complex near the base of the granite. Other reflectors include southeast-dipping interfaces at depths between 6 and 9.3 km and a more gently southeast-dipping complex at 9.5–11 km that may represent the master decollement. The results of the pilot study show that this type of quarry blast can be useful for imaging structure within the upper crust. Although preliminary, the wide-angle results support the interpretation that the Elberton granite is a tabular body less than 4 km thick.