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Fault scarps contain information about the timing and frequency of paleoseismic activity. This information, however, is difficult to obtain because existing methods to directly date prehistoric earthquakes are inadequate. We developed a new dating approach based on accumulation of cosmogenic chlorine-36 in bedrock fault-scarp faces exposed to cosmic radiation by seismic activity during the late Quaternary.
Materials exposed at or near the surface are bombarded by cosmic-ray particles. Nuclear reactions between 35Cl, 39K and 40Ca in rocks and cosmic ray neutrons and muons produce cosmogenic 36Cl. The cosmogenic production rates from 39K and 40Ca are highest at the surface and decrease exponentially with depth; that from 35Cl has a maximum at ca. 40-60 cm below the surface, then decreases exponentially. At depths greater than a few meters, abundances of these nuclides are low because of the low non-cosmogenic production rates. Therefore, exposure of a previously buried rock at the surface, for instance by faulting, starts the cosmogenic clock. Because production rates of cosmogenic 36Cl from its three target nuclides have been determined empirically, its concentrations in surficial rocks can be used to calculate how long these rocks have been exposed at the surface - their surface exposure ages can be calculated.
We studied a scarp on the Hebgen Lake fault, Montana, U.S.A. The scarp has recorded multiple surface ruptures as bands of similar weathering characteristics in limestone bedrock. The most recent event, the 17 August 1959 M7.5 Hebgen Lake earthquake, produced scarps up to 7 m high. At our study location, this normal faulting event produced over 2 meters of freshly exposed fault plane in bedrock. At least four other bands 2 to 5 m wide (measured vertically in the fault plane) are recognized above the 1959 band. We collected samples from a single, continuous bedrock fault face, approximately every 1 m in the vertical, from the soil level, up to 7.6 m above the soil, plus one sample 12.9 m above the soil level. Going up this rock face, we observed characteristic surficial features that indicate progressively older exposure ages: (1) increased depth of weathering pits, (2) greater diameter lichens, (3) more oxidation, (4) greater vein height, and (5) poorer preservation of slickensides, gouges, pits, and other tectonically produced features on the fault plane.
Cosmogenic 36Cl surface exposure ages increase with height above the present soil level. The increase in raw 36Cl ages is gradual, although not uniform, from ca. 2 ky at the bottom to ca. 40 ky at the top, and no grouping of similar ages can be observed. When the raw ages are corrected for cosmogenic 36Cl produced at shallow depths, distinct groups of similar ages appear at ca. 3 ky, 18 ky and 40 ky. These ages increase from the bottom of the face toward the top, and reflect individual faulting events in the past. The observed pattern is in conformity with the seismic stratigraphy, which is important because it demonstrates that the cosmogenic 36Cl signal reflects exposure time for individual sections of the face. It is very encouraging that we are able to calculate progressively older ages as we move up along the face and towards parts that have been exposed at the surface for longer times. This ability to obtain stratigraphically correct chronology demonstrates the feasibility of our new approach to dating paleoearthquakes using cosmogenic isotopes. The method is applicable to most types of rocks, under
most geological conditions and on fault faces that have
been exposed for up to one million years. Because of these characteristics, the approach has potential to become an important tool for quantitative paleoseismology.