Seismic Hazards in Northern New Mexico

SEISMIC HAZARDS IN NORTHERN NEW MEXICO? YES! Read on to find out more...

The Rio Grande rift is a major tectonic feature of western North America, and is expressed on the surface of the Earth as a series of elongate north-south trending basins that run from central Colorado, through the central parts of New Mexico, into northern Mexico where it blends with the greater Basin and Range tectonic province. The rift is the site of on-going east-west tectonic extension, which causes the Earth's crust to stretch, thin, and break. From the perspective of seismic hazards, the rift can be considered to be part of the Basin and Range tectonic province. The rift and the Basin and Range exhibit remarkably similar major tectonic and magmatic histories that span the last 30 million years. Historic seismicity in the Basin and Range province is characterized by widespread, abundant microseismic events, temporally punctuated by larger earthquakes that range from the limits of human perceptibility to potentially seriously damaging earthquakes of approximately magnitude 7. The Basin and Range province has experienced at least 11 earthquakes of magnitude 6.5 or larger in relatively recent historic times. Consistent with the historical record of the Basin and Range, paleoseismic evidence also indicates that large earthquakes of approximately magnitude 7 have occurred in the rift. Historical earthquakes in the rift include a magnitude 7.2 earthquake in northern Mexico in 1887, numerous magnitude 4 to 6 earthquakes in the Socorro area through the 20th Century, and magnitude 4 to 5+ events in Cerrillos and Dulce in 1918 and 1966, respectively. For more general information on earthquakes in New Mexico, visit the website of the New Mexico Bureau of Mines and Mineral Resources.

Los Alamos and the Jemez Mountains Satellite photo of the Jemez Mountains
Los Alamos lies near several major boundary faults of the Rio Grande rift in north-central New Mexico (map). The margin of the Rio Grande rift in the Los Alamos area is locally defined by the Pajarito fault system. Los Alamos also lies on the eastern flank of the Jemez Mountains, on a geographic feature known as the Pajarito Plateau (see satellite photo above). Volcanic activity in the Jemez Mountains spans the last 16 million years. Enormous explosive eruptions at 1.2 and 1.6 million years ago formed the Valles Caldera and the Bandelier Tuff, a series of ash-flows that form the Pajarito Plateau. The youngest eruptions from the Jemez Mountains volcanic field occurred as recently as about 50 thousand years ago, and there is the potential for renewed volcanism. Public perceptions are that seismicity in this region is rare. However, since establishment of the Los Alamos National Laboratory during World War II, there have been seven earthquakes felt by the residents of Los Alamos. Apparently the largest of these felt earthquakes were a magnitude 4 that occurred in 1952 and a magnitude 3.3 in 1971, both of which had reported Modified Mercalli Intensities of V in Los Alamos. More recently (in 1991 and 1998), Los Alamos has experienced very small magnitude earthquakes (M<2) with unusually high Modified Mercalli Intensities up to V. (explanations of various earthquake magnitude and intensity scales can be found at the website of the National Earthquake Information Center at http://wwwneic.cr.usgs.gov/neis/general/handouts/magnitude_intensity.html).
Recent paleoseismic studies on the Pajarito fault system indicate that large earthquakes of approximately magnitude 7 have occurred in geologically recent (Holocene), prehistoric times. This photograph shows one of the local faults. The exposure is about four meters in height, and shows Quaternary gravels (left) faulted against 7 million year old tuffs (right). Faults, such as this one, have been the focus of our detailed paleoseismic investigations which seek answers to the questions: how large can these earthquakes be? how often do they happen? and, when was the last one? Clearly the answers to these questions are fundamental to any probabilistic seismic hazard analyses.

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