David B. Slemmons

Geometric pattern, rupture termination and fault segmentation of the Dixie Valley—Pleasant Valley active normal fault system, Nevada, U.S.A.

Abstract Active fault systems of both interplate and intraplate settings clearly show patterns of segmentation. Fault segmentation, therefore, is very important for understanding fault behavior and assessing seismic hazard. The complexity of geometric patterns along fault systems has a strong influence on the propagation of earthquake ruptures, because it can form structural heterogeneities that tend to inhibit rupture propagation. This implies that there should be relations between fault segmentation and the complexity of fault geometry. The active normal faults of the Dixie Valley—Pleasant Valley system provide an excellent example to study such relations and to establish the geometric method of normal fault segmentation in the Basin and Range province. Studies of fault segmentation in the Dixie Valley—Pleasant Valley fault system indicate that there are changes of both geometric pattern and geomorphic character in the area of the segment boundary. The characteristics of segment boundaries on normal faults of the Dixie Valley—Pleasant Valley system can be explained in terms of the nature of rupture propagation and termination. These changes may be helpful criteria for distinguishing normal fault segmentation at least in the Basin and Range province.

Rupture terminations and size of segment boundaries from historical earthquake ruptures in the Basin and Range Province

Abstract The fault-segmentation method is commonly used to estimate the potential earthquake size. Segment boundaries play an important role in arresting earthquake ruptures from event to event. In the Basin and Range Province, earthquake rupture terminations are commonly associated with structural discontinuities, but not all-structural discontinuities have the capability to terminate an earthquake rupture. The size of structural discontinuities with respect to the rupture length and displacement may play an important role in controlling rupture termination. Studies of the geometric pattern of seven well-documented historical earthquake ruptures in the Basin and Range Province reveal three important relationships between rupture termination and the size of structural discontinuities. Firstly, terminations of normal faulting earthquakes are often associated with structural discontinuities, at least for the five historical ruptures studied in this paper. Secondly, the sizes of structural discontinuities at the end of earthquake rupture zones are generally the largest among the structural discontinuities within the rupture zone. Thirdly, there appears to be a trend that larger earthquake ruptures are stopped by larger structural discontinuities. The size of structural discontinuities that stopped the earthquake seems to scale with the length and displacement of earthquake surface rupture.

Active faults of Alaska

Abstract Geologically young displacements have been observed along 24 faults in an area of Alaska of approximately 624,000 km 2 . Active faults of southern Alaska include the Patton Bay and Hanning Bay reverse-slip faults, both reactivated in 1964. The Fairweather strike-slip fault experienced surface faulting in 1958 and possibly in 1899. Three normal-slip faults trending northerly were not reactivated in 1964; these are the Johnstone Bay fault, Ragged Mountain fault, and the Long Glacier fault. The Castle Mountain fault trends northeasterly and is marked by a low, south-facing escarpment. Active faults in the Alaska Range and adjacent areas include those of the Denali fault system, including the Totschunda fault and the McGinnis Glacier fault; inactive faults include the Hines Creek fault. Other active faults include the Donnelly Dome fault, the Granite Mountain fault, and the Healy Creek fault. The north front of the Alaska Range west of Donnelly Dome is a monocline with few associated surface faults. The Clearwater Lake fault, located along the Tanana River east of Delta Junction, appears to be active, although an origin of landforms due to faulting cannot be proven. Active faults in the Tanana—Kuskokwim Lowland and Yukon—Tanana Upland include the Minto, Shaw Creek, Champion Creek, and Tintina faults. The active Kaltag fault, located southwest of Tanana, does not appear to connect by means of an active trace with the Tintina fault, which is 300 km to the east. Active faults of the Kokrine—Hodzana Highlands include the Kanuti and Dall Mountain faults. Active faults west of the Kanuti include the Huslia and a fault along Shoestring Sand Dune, both in the Koyukuk Flats. The active Kobuk—Alatna Hills fault trends westerly south of the Brooks Range. No active faults were observed in the central and eastern Brooks Range, Arctic Foothills, or Arctic Coastal Plain in the area of study.

Eureka Valley Tuff, East-Central California and Adjacent Nevada

The Eureka Valley Tuff consists of two major ash-flow sheets and a local over-lying sequence of ash-flow tuff erupted from vents within the Little Walker caldera 11 mi west-northwest of Bridgeport, in east-central California. The lower of the two major ash-flow sheets, here named the Tollhouse Flat Member, is the “biotite-augite-latite” of Ransome (1898). The overlying By-Day Member can readily be identified by the absence of phenocrystic biotite and by paleomagnetic and other petrographic criteria. The recognition of the distinctive By-Day Member above the Tollhouse Flat Member both in the Bridgeport area and west of the Sierra crest unequivocally demonstrates the generally accepted correlation of the latitic ash-flow tuffs of the two areas. K-Ar age determinations indicate that the three members of the Eureka Valley Tuff were erupted within a very short interval of time about 9.5 m.y. ago.

Recent Crustal Movements in the Central Sierra Nevada–Walker Lane Region of California–Nevada: Part Iii, the Olinghouse Fault Zone

Abstract The Olinghouse fault zone is one of several NE—ENE-trending fault zones and lineaments, including the Midas Trench and the Carson—Carson Sink Lineament, which exhibit left-lateral transcurrent movement conjugate to the Walker Lane in western Nevada. The active portion of this fault zone extends for approximately 23 km, from 16 km east of Reno, Nevada, to the southern extent of Pyramid Lake. The fault can be traced for most of its length from its geomorphic expression in the hilly terrain, and it is hidden only where overlain by recent alluvial sediments. Numerous features characteristic of strike-slip faulting can be observed along the fault, including: scarps, vegetation lines, sidehill and shutter ridges, sag ponds, offset stream channels and stone stripes, enclosed rhombohedral and wedge-shaped depressions, and en-echelon fractures. A shear zone having a maximum observable width of 1.3 km is defined principally by Riedel shears and their symmetrical P-shears, with secondary definition by deformed conjugate Riedel shears. Several continuous horizontal shears, or principal displacement shears, occupy the axial portion of the shear zone. The existence of P-shears and principal displacement shears suggests evolution of movement along the fault zone analogous to the “Post-Peak” or “Pre-Residual Structure” stage. Historic activity (1869) has established the seismic potential of this zone. Maximum intensities and plots of the isoseismals indicate the 1869 Olinghouse earthquake had a magnitude of 6.7. Field study indicates the active length of the fault zone is at least 23 km and the maximum 1869 displacement was 3.65 m of left-slip. From maximum fault length and maximum fault displacement to earthquake magnitude relations, this corresponds to an earthquake of about magnitude 7.

Recent Crustal Movements in the Central Sierra Nevada–Walker Lane Region of California-Nevada: Part II, the Pyramid Lake Right-Slip Fault Zone Segment of the Walker Lane

Abstract The Pyramid Lake fault zone is within the Honey Lake—Walker Lake segment of the Walker Lane, a NW-trending zone of right-slip transcurrent faulting, which extends for more than 600 km from Las Vegas, Nevada, to beyond Honey Lake, California. Multiscale, multiformat analysis of Landsat imagery and large-scale (1: 12,000) lowsun angle aerial photography, delineated both regional and site-specific evidence for faults in Late Cenozoic sedimentary deposits southwest of Pyramid Lake. The fault zone is coincident with a portion of a distinct NW-trending topographic discontinuity on the Landsat mosaic of Nevada. The zone exhibits numerous geomorphic features characteristic of strike-slip fault zones, including: recent scarps, offset stream channels, linear gullies, elongate troughs and depressions, sag ponds, vegetation alignments, transcurrent buckles, and rhombohedral and wedge-shaped enclosed depressions. These features are conspicuously developed in Late Pleistocene and Holocene sedimentary deposits and landforms. The Pyramid Lake shear zone has a maximum observable width of 5 km, defined by Riedel and conjugate Riedel shears with maximum observable lenghts of 10 and 3 km, respectively. P-shears have formed symmetrical to the Riedel shears and the principal displacement shears, or continuous horizontal shears, isolate elongate lenses of essentially passive material; most of the shears are inclined at an angle of approximately 4° to the principal direction of displacement. This suggests that the shear zone is in an early “PreResidual Structure” stage of evolution, with the principal deformation mechanism of direct shear replacing the kinematic restraints inherent in the strain field. Historic seismic activity includes microseismic events and may include the earthquake of about 1850 reported for the Pyramid Lake area with an estimated Richter magnitude of 7.0. Based on worldwide relations of earthquake magnitude to length of the zone of surface rupture, the Pyramid Lake fault zone is inferred to be capable of generating a 7.0–7.5-magnitude event for a maximum observable length of approximately 6 km and a 6.75–7.25-magnitude event for a half length of approximately 30 km.