Lesley Perg

Elevated shear zone loading rate during an earthquake cluster in eastern California

We compare geodetic velocity to geologic fault slip rates to show that tectonic loading was doubled across the eastern California shear zone (ECSZ) during a cluster of major earthquake activity. New slip rates are presented for six dextral faults that compose the ECSZ in the central Mojave Desert. These rates were determined from displaced alluvial fans dated with cosmogenic 10 Be and from a displaced lava flow dated with 40 Ar/ 39 Ar. We find that the sum geologic Mojave ECSZ slip rate, ≤6.2 ± 1.9 mm/yr, is only half the present-day geodetically measured velocity of 12 ± 2 mm/yr. These rates account for cumulative fault slip and geodetic observations that span the 60-km-wide shear zone; therefore this difference cannot be attributed to postseismic relaxation. Redistribution of tectonic loading over the earthquake cycle at a regional scale suggests that earthquake clustering may be enhanced via feedback with weakening of ductile shear zones.

Slip rate of the Calico fault: Implications for geologic versus geodetic rate discrepancy in the Eastern California Shear Zone

[1] Long-term (10 5 years) fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the Eastern California Shear Zone (ECSZ). The Calico fault is one of a family of dextral faults that traverse the Mojave Desert portion of the ECSZ. Its slip rate is determined from matching and dating incised Pleistocene alluvial fan deposits and surfaces displaced by fault slip. A high-resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features. The oldest geomorphically preserved alluvial fan, unit B, is displaced 900 ± 200 m from its source at Sheep Springs Wash in the northern Rodman Mountains. This fan deposit contains the first preserved occurrence of basalt clasts derived from the Pipkin lava field and overlies Quaternary conglomerate deposits lacking these clasts. The 40 Ar/ 39 Ar dating of two flows from this field yields consistent ages of 770 ± 40 ka and 735 ± 9 ka. An age of 650 ± 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic 3 He exposure date of 653 ± 20 ka on a basalt boulder from the surface of unit B. This assigned age and offset together yield a mid-Pleistocene to present average slip rate of 1.4 ± 0.4 mm/yr. Ayounger fan surface, unit K, records 100 ± 10 m of dextral displacement and preserves original depositional morphology of its surface. Granitic boulders and pavement samples from this surface yield an average age of 56.4 ± 7.7 ka after taking into account minimal cosmogenic inheritance of granitic clasts. The displaced and dated K fans yield a slip rate of 1.8 ± 0.3 mm/yr. Distributed deformation of the region surrounding the fault trace, if active, could increase the overall displacement rate to 2.1 ± 0.5 mm/yr. Acceleration of slip rate from an average of 1.4 mm/yr prior to � 50 ka to 1.8 mm/yr since � 50 ka is possible, though a single time-averaged slip rate of 1.6 ± 0.2 mm/yr satisfies the data. These rates are faster than any other paleoseismic or long-term slip rate yet determined for other dextral faults in the Mojave Desert and imply that fault slip rates and earthquake productivity are heterogeneous across this portion of the ECSZ. Total displacement across the Calico fault diminishes northward as shear is distributed into folding and sinistral faults in the Calico Mountains. This pattern is consistent with an approximately threefold drop in geologic slip rate as the Calico fault steps over onto the Blackwater fault and demonstrates the significance of fault interaction for understanding the pattern of present-day strain accumulation in the ECSZ.

Continental‐scale relationship between bankfull width and drainage area for single‐thread alluvial channels

We explore the bankfull width (Wbf) versus drainage area (Ada) relationship across a range of climatic and geologic environments and ask (1) is the relationship between ln(Wbf) and ln(Ada) best described by a linear function and (2) can a reliable relationship be developed for predicting Wbf with Ada as the only independent variable. The principal data set for this study was compiled from regional curve studies and other reports that represent 1018 sites (1 m ≤ Wbf ≤ 110 m and 0.50 km2 ≤ Ada ≤ 22,000 km2) in the continental United States. Two additional data sets were used for validation. After dividing the data into small, medium, and large-size basins which, respectfully, correspond to Ada < 4.95 km2, 4.95 km2 ≤ Ada < 337 km2, and Ada ≥ 337 km2, regression lines from each data set were compared using one-way analysis of covariance (ANCOVA). A second ANCOVA was performed to determine if mean annual precipitation (P) is an extraneous factor in the Wbf versus Ada relationship. The ANCOVA results reveal that using Ada alone does not yield a reliable Wbf versus Ada relationship that is applicable across a wide range of environments and that P is a significant extraneous factor in the relationship. Considering data for very small basins (Ada ≤ 0.49 km2) and very large basins (Ada ≥ 1.0 × 105 km2) we conclude that a two-segment linear model is the most probable form of the ln(Wbf) versus ln(Ada) relationship. This study provides useful information for building complex multivariate models for predicting Wbf.