The Mono Lake Case | ScienceGate

abstract Microseismicity, composite focal-mechanism solutions, and previously-published focal parameter data are used to determine the current tectonic activity of the prominent zone of seismicity in western Nevada and eastern California, termed the Nevada Seismic Zone. The microseismicity substantially agrees with the historic seismicity and delineates a narrow, major zone of activity that extends from Owens Valley, California, north past Dixie Valley, Nevada. Focal parameters indicate that a regional pattern of NW-SE tension exists for the western Basin and Range and is now producing crustal extension within the Nevada Seismic Zone. An eastward shift of the seismic zone along the Excelsior Mountains and left-lateral strike-slip faulting determined from a composite focal mechanism indicate transform-type faulting between Mono Lake and Pilot Mountain. Based on these results and other data, it is suggested that the Nevada Seismic Zone is caused by the interaction of a westward flow of mantle material beneath the Basin and Range Province with the boundary of the Sierra Nevada batholith.

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Sierra Nevada-Great Basin boundary zone: Earthquake hazard related to structure, active tectonic processes, and anomalous patterns of earthquake occurrence

Bulletin of the Seismological Society of America ◽

10.1785/bssa0700051557 ◽

1980 ◽

Vol 70 (5) ◽

pp. 1557-1572

Author(s):

J. D. VanWormer ◽

Alan S. Ryall

Keyword(s):

Sierra Nevada ◽

Great Basin ◽

Temporal Variations ◽

Mono Lake ◽

Local Network ◽

Low Velocity ◽

Owens Valley ◽

Walker Lake ◽

Fault Plane Solutions ◽

The North

abstract Precise epicentral determinations based on local network recordings are compared with mapped faults and volcanic features in the western Great Basin. This region is structurally and seismically complex, and seismogenic processes vary within it. In the area north of the rupture zone of the 1872 Owens Valley earthquake, dispersed clusters of epicenters agree with a shatter zone of faults that extend the 1872 breaks to the north and northwest. An area of frequent earthquake swarms east of Mono Lake is characterized by northeast-striking faults and a crustal low-velocity zone; seismicity in this area appears to be related to volcanic processes that produced thick Pliocene basalt flows in the Adobe Hills and minor historic activity in Mono Lake. In the Garfield Hills between Walker Lake and the Excelsior Mountains, there is some clustering of epicenters along a north-trending zone that does not correlate with major Cenozoic structures. In an area west of Walker Lake, low seismicity supports a previous suggestion by Gilbert and Reynolds (1973) that deformation in that area has been primarily by folding and not by faulting. To the north, clusters of earthquakes are observed at both ends of a 70-km-long fault zone that forms the eastern boundary of the Sierra Nevada from Markleeville to Reno. Clusters of events also appear at both ends of the Dog Valley Fault in the Sierra west of Reno, and at Virginia City to the east. Fault-plane solutions for the belt in which major earthquakes have occurred in Nevada during the historic period (from Pleasant Valley in the north to the Excelsior Mountains on the California-Nevada Border) correspond to normaloblique slip and are similar to that found by Romney (1957) for the 1954 Fairview Peak shock. However, mechanisms of recent moderate earthquakes within the SNGBZ are related to right- or left-lateral slip, respectively, on nearly vertical, northwest-, or northeast-striking planes. These mechanisms are explained by a block faulting model of the SNGBZ in which the main fault segments trend north, have normal-oblique slip, and are offset or terminated by northwest-trending strike-slip faults. This is supported by the observation that seismicity during the period of observation has been concentrated at places where major faults terminate or intersect. Anomalous temporal variations, consisting of a general decrease in seismicity in the southern part of the SNGBZ from October 1977 to September 1978, followed by a burst of moderate earthquakes that has continued for more than 18 months, is suggestive of a pattern that several authors have identified as precursory to large earthquakes. The 1977 to 1979 variations are particularly noteworthy because they occurred over the entire SNGBZ, indicating a regional rather than local cause for the observed changes.

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