The Denver Earthquake Sequence of March–April 1981

Abstract

The Denver earthquake sequence of March–April 1981 was monitored by a network of four permanent and eight portable seismographs. In addition to the main shock (mb = 4.3) on 2 April, six microaftershocks (M < 2) during the subsequent two-week period were recorded and located. Five of those six events had epicenters within the most active area of the 1967–1968 Rocky Mountain Arsenal (RMA) sequence. A composite focal mechanism solution for the main shock and the six aftershocks showed a combination of reverse and strike-slip faulting (14% inconsistency in the 29 P-wave polarities) that is different from the predominantly normal faulting reported for the 1967–1968 RMA sequence. These different focal mechanisms, plus variable water-level response at the RMA well during the earthquake sequence in the 1960’s, may suggest the presence of a multiple fracture system in the source volume.

Raton Basin Induced Seismicity Is Hosted by Networks of Short Basement Faults and Mimics Tectonic Earthquake Statistics

“…Coal‐bed methane production and wastewater injection began in 1994 and its increase from ∼2000 to 2002 was accompanied by a rapid increase in seismicity (Figure 1b; Advanced National Seismic System Comprehensive Earthquake Catalog (ANSS ComCat)). Thus, the Raton Basin's rise of induced seismicity was ∼8 years ahead of the central U.S. average (Figure 1), and it is a valuable example of how induced seismicity can persist as injection decreases or potentially cease on local scales (e.g., Healy et al., 1968; Hsieh & Bredehoeft, 1981; Bollinger, et al., 1983). Historically, seismicity in the basin was rare but not absent.…”