Northeastern California

We will begin our discussion of hazards in northeastern California with an overview of expected regional long-term rate of slip from global plate tectonics and compare that to geodetic, geological and seismological observations of the region and particular fault zones. Although abundant evidence of late Quaternary and Holocene faulting exists, several factors lead to considerable uncertainty in assigning the level of seismic activity expected from regional tectonics into discrete earthquake sources in this region. The most problematic factors are the great breadth and complexity of fault systems and the sparseness of Holocene deposits needed to judge recency of faulting and measure slip rates on particular faults. Hence, to account for seismic hazard we adopted four areal sources (Figure 2b, NE09 - NE12) in consultation with workers in adjoining states to develop a consistent tectonic model. We also delineated eight linear zones (NE01 - NE08) and discuss them in the context of the overall tectonic model.

Tectonic model for Northeastern California

Very Long Baseline Interferometry (VLBI) data show that considerable motion occurs between the generally rigid Sierra Nevada-Great Valley block and stable North America plate (11±1 mm/yr toward N50°W evaluated at Quincy (Figure 2b; Figure 10), Ward, 1990; Argus and Gordon, 1991). Geodetic networks and VLBI are beginning to explain how crustal strain is distributed in the eastern, central and southern Great Basin [Savage and others, 1992, 1995; Dixon, 1995]. Some east-west normal faulting is mostly concentrated on the Wasatch fault at the eastern boundary of the Great Basin (Table 2, Figure 10, Figure 11). A zone of north-northwesterly trending dextral shear branches from the San Andreas fault system in southern California and north of the Mojave Desert follows the western margin of the Great Basin. A major branch of this zone of dextral-normal faulting broadly distributes strain throughout the Central Nevada Seismic Zone. VLBI evidence and geologic evidence demonstrate that dextral zones re-enter California as two branches (HL, Honey Lake fault and L-T TZ, Lassen-Tahoe tectonic zone in Figure 10). VLBI stations at Quincy and Hat Creek (HTC) have maintained a consistent velocity differential that averaged 3.6 mm/yr at ~N54°W for a decade (D. F. Argus, writ. commun., 1995). We suggest that this dextral shear reflects permanent aseismic deformation near the Lassen volcanic center that lies between the two sites and reflects strain release principally of the L-T TZ, but also may release strain from the branch associated with the Honey Lake fault. Geologic slip rate evidence on the Honey Lake fault suggests a minimum of 2 mm/yr dextral slip rate on that branch of the shear zone [Wills and Borchardt, 1993].

Click for high-resolution image

Figure 10. Path across northern Great Basin and San Francisco Bay area for summation of long-term geologic slip vectors on active fault systems. Faults: SA, San Andreas; H, Hayward; C, Calaveras; GV, Great Valley thrusts; HL, Honey Lake; L, Likely; SV, Surprise Valley; BR, Black Rock; JM, Jackson Mountains. L-T TZ, Lassen-Tahoe tectonic zone. Stars, Holocene volcanic centers (MLV, Medicine Lake volcano). Triangles, VLBI sites (HTC, Hat Creek)

Figure 11. Slip vectors assumed for this study shown as green arrows and solid green lines.

To develop a defensible regional tectonic model for northeastern California we chose to close a circuit of slip rate vectors from the Pacific plate near the San Francisco Bay region where data are plentiful, through the northeastern California area near Quincy, across the northern Great Basin where active faulting is sparse and closing to the North American plate east of the Wasatch fault (Figure 10). Using the Nuvel-1A [DeMets and others, 1994] and the most recent VLBI model for the Pacific plate-Sierra Nevada block motion is parsimonious because Nuvel-1A may represent a minimum estimate [DeMets, 1995] and the Pacific-Sierran block model is well-corroborated for the SF Bay region as we discuss below. We considered a narrow interpretation of plate tectonic modeling prudent because northeastern California is something like a concealed triple junction partly analogous to the Mendocino triple junction that lies to the west, thus a different tectonic regime may apply. The shear zones of northeastern California resemble the San Andreas fault system of coastal California in that they are largely dextral and driven by interaction with the Pacific Plate, whereas the high Cascade volcanic structures from Lassen Peak northward are more closely associated with Cascadian subduction than with the San Andreas fault system. Some tectonic models argue for a continuation of substantial dextral shear northward from California and Nevada into Oregon [Wells, 1990; Pezzopane and Weldon, 1993], but these models are not yet corroborated because adequate geologic slip rate data and the necessary geodetic coverage are lacking for the region. The vector difference of the total Nuvel-1A motion and current VLBI Sierran-Pacific model is 9.1 mm/yr directed N51°W. Subtracting known slip rate vectors in the Basin and Range outside of the northeastern California shear zone yields 6 mm/yr directed N33°W available within the dextral shear zone (Table 2). Because it probably has the largest slip rate of any normal fault in the Great Basin, we gave much consideration to the rate on the Wasatch fault. Geologic data permit at most ~1-2 mm/yr for the Wasatch fault sensu strictu, whereas geodetic data suggest at least ~3 mm/yr. Hence, we assume 2.5 ± 1 mm/yr is approximately correct but may either integrate slip rate of other northeastern Great Basin faults or neglect some regional strain associated with the entire intermountain seismic belt.

We partition ~2 mm/yr of the 6 mm/yr of available dextral shear onto the larger northeastern faults where geologic slip rates are generally minimum estimates (e.g., NE01 and NE02) based on a single strand in a complex zone. Sources NE03 and NE04 are assumed to jointly continue the total slip rate of NE02 northward as they are distinctly active faults compatible in their geomorphic expression and fragmental slip rate data with this assumption (W. A. Bryant, writ. comm., 1996). The areal source from western Nevada (NE12) is assigned 4 mm/yr. Northward from this source, areally distributed slip rate on NE10 and NE09 is dropped to 2 mm/yr to reflect the contributions from specifically delineated fault zones (NE02, NE03, NE04) as discussed above. A small remainder, ~2 mm/yr of dextral shear directed N2°W, has been ignored in this analysis and could be presumed to be distributed in some highly uncertain way within the northern Great Basin between the Surprise Valley fault (NE05) and central Nevada.

Foothills fault zone (NE11)

We estimate that the slip rate of the Foothills fault system is well below the minimum of 0.1 mm/yr generally used as a filter in this study to eliminate low-activity faults that do not contribute to seismic hazard beyond the regional background level [Frankel and others, 1996]. However, because the fault zone has re-emerged as having continuing importance for engineering and major public policy reasons [Schwartz and others, 1996], we deemed it prudent to include this complex fault system as an areally distributed hazard (NE11).