Abstract

In 1976 and 1977, two unusually deep earthquakes of ML 4.6 and 4.2, respectively, occurred near the town of Crownpoint in northwestern New Mexico. Both events were felt extensively in the Four Corners region with maximum reported Modified Mercalli intensities of VI. These earthquakes are two of the larger earthquakes observed within the Colorado Plateau and are among the strongest earthquakes reported in New Mexico outside the Rio Grande rift. Relocations of the events placed them within the Chaco slope along the southern portion of the San Juan basin—an area that appears to have exhibited a low level of historical seismicity and tectonism possibly since Laramide times. The most significant aspect of the 1976 and 1977 Crownpoint earthquakes is their focal depths of 41 and 44 km, respectively. Such depths would locate the events in the lowermost crust in the vicinity of the Moho in sharp contrast to the upper crustal origin of most intraplate earthquakes. Although located within the Colorado Plateau, revised fault plane solutions exhibit normal faulting along northwest-trending planes and northeast-southwest-trending minimum compression, which are characteristic of Basin and Range-Rio Grande rift extension. This observation suggests that this portion of the Colorado Plateau is located within a tectonic transition zone between the Basin and Range-Rio Grande rift and Colorado Plateau stress provinces.

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