Geologists looking at recent sediments — Part 3: 

The Rio Grande in Valencia County flows in the Rio Grande Rift, a trough built more than 25 million years with sediments as much as 20,000 feet deep, about 40 miles wide, bordered to the east by the Manzano Mountains.

Near the foot of these mountains, the Hubbel Spring Fault, also called the Los Ojuelos Fault is a rift-related 50-mile long fault system. This fault zone includes about a dozen parallel north-south strands scattered broadly between three miles west of the base of the Manzano Mountains and the western edge of the Llano de Manzano near Tomé Hill.

A major strand of the fault can be seen as a ridge (called a “fault scarp”) south of the eastern end of Meadow Lake, marked also by a line of increased vegetation and occasional large brush or small trees, and year-round springs where the fault intersects groundwater flow from the Manzanos.

A compilation of drops in topography associated with the fault strands across the Llano de Manzano shows a total of about 100 feet of surface drop to the west in the past 100,000 years.

To further investigate the past movements along the fault, geologists excavated and mapped east-west trenches, including a 45-feet long, 23-feet deep trench excavated in 2004 at Carrizozo Springs, about a mile south of Meadow Lake. The geologists climbed in the trench to observe and document the layers visible on the sidewalls of the trench.

They mapped in detail 14 horizontal sediments layers, and any disturbances of these layers (see cross-section). The stratigraphy indicated multiple past events showing the layers dropping to the west. These represent four to five large earthquakes in the past 80,000 years that each caused one to 15 feet of vertical movement. They also collected eight samples for approximate dating to evaluate the time lapses between apparent earthquakes.

The data indicate an average recent topography sink rate of possibly one inch every 300 years and confirm that this fault system is active.

The recurrence of large earthquakes can be estimated at every 12,000 to 25,000 years, and the last event was about 12,000 years ago. However, the fact that there are multiple strands with broad spacing between the fault strands makes any reliable estimate of future earthquake frequency, magnitude, and location very uncertain. (Detailed internet information can be found by searching for USGS Quaternary Faults, Hubbel Spring Fault).

(Paul Parmentier is a retired geologist from California now living in Los Lunas. He shares the rich geology and nature features in Valencia County in a monthly column.)