Surface Faulting Events Research Articles

Recurrent Holocene strike-slip faulting, Pyramid Lake fault zone, western Nevada

Trenches excavated at two sites on scarps of the Pyramid Lake fault zone revealed evidence of recurrent Holocene strike-slip faulting. Control for estimating the ages of faulting events is provided by soil stratigraphy, C-14 ages on organic sediments, and the Mazama ash (7000–6700 yr), which is present at both sites. The first site displayed evidence of at least three, probably four, and possibly five displacements within the past 7500 yr. Evidence of at least three and probably four post–Mazama-ash faulting events was found at the second trench site. The juxtaposition of lithologically contrasting deposits and units with different thicknesses on opposite sides of the faults indicates strike-slip faulting. The estimated average recurrence of surface faulting events (M s ⩾ 6.5) is about 1800 yr assuming four post–Mazama-ash events and about 2400 yr for three events. The most recent event occurred within the past few hundred years. The rate of recurrence of surface faulting events on the Pyramid Lake fault zone is three to five times higher than for most faults in the western Basin and Range province.

Recurrence of moderate to large magnitude earthquakes produced by surface faulting on the Wasatch fault zone, Utah

No historical earthquake associated with surface faulting have occurred along the Wasatch fault zone during at least the past 133 yr. Exploratory trenching, analysis of fault scarp morphology and scarp-derived colluvial deposits, and detailed mapping at two locations along the fault zone indicated there have been repeated moderate to large magnitude earthquakes ( M 6 1 2 to 7 1 2 ) associated with surface faulting during the late Pleistocene and Holocene. These detailed geological studies provide new data on the style of deformation associated with normal-slip faulting, amount of cumulative displacement, amount of displacement per event, and the recurrence intervals of surface faulting earthquakes. At the Kaysville site, 30 km north of Salt Lake City, at least three surface faulting events have produced 10 to 11 m of cumulative net vertical tectonic displacement since the middle Holocene; estimates of the net tectonic displacement for individual faulting events range from 1.7 to 3.7 m. The two most recent events occurred within the past 1,580 ± 150 yr and interval between these events was from 500 to 1,000 yr. Along this segment of the fault, the average recurrence interval is probably close to 1,000 yr. The middle to late Holocene slip rate is 1.8 (+1.0; −0.6) nm/yr. At the Hobble Creek site, 46 km south of Salt Lake City, six or seven surface faulting events have produced 11.5 to 13.5 m of cumulative net vertical tectonic displacement during the past 12,000 to 13,000 yr; the average net tectonic displacement per event is 0.8 to 2.8 m. The average Holocene recurrence interval at this location is 1,500 to 2,600 yr and the slip rate is 1.0 ± 0.1 mm/yr. If the recurrence intervals on these segments of the Wasatch fault zone are typical of the other segments of the zone, the recurrence interval of moderate to large magnitude earthquakes for the entire Wasatch fault zone may be 50 to 430 yr.

Seismic moments of major earthquakes and the average rate of slip in central Asia

Seismic moments for 12 major earthquakes (M ≥ 7.6) in central Asia from 1911 to 1967 were calculated from long-period Rayleigh and Love wave spectral densities. With fault lengths estimated from geological field observations of surface faulting, intensity distributions, or master event relocations of aftershocks, the calculated moments place bounds on the average slip and fault widths. The following table summarizes the calculated moments, estimated fault lengths, and inferred possible average displacements.