The 1994 Double Spring Flat Earthquake

Winter 1994

At 5:23 A.M. (PDT) on Monday, September 12, 1994, a moderately strong earthquake occurred in a remote part of the southern Pine Nut Mountains in western Nevada. The tremor, measuring 6.0 on the Richter magnitude scale, was the largest earthquake in Nevada in 28 years. The quake was centered about 16 km southeast of Gardnerville and 17 km northeast of Markleeville, just northwest of Double Spring Flat. In keeping with tradition, this seismic event was named the "Double Spring Flat" earthquake after the nearest significant placename.

The Double Spring Flat earthquake strongly shook much of western Nevada and eastern California and was felt from Sacramento to Elko. There was no loss of life and only minor property damage, primarily due to the quake's moderate size and occurrence in a sparsely populated area. Strong shaking from the earthquake lasted about ten seconds, knocking objects from shelves and rattling nerves throughout several communities near the epicenter, including the Double Spring Flat, Topaz Ranch Estates, Minden/Gardnerville, Markleeville, and Woodfords areas. The most significant damage included a toppled chimney at the Minden Inn and a damaged foundation in Double Spring Flat. The earthquake also triggered a few minor landslides on Kingsbury Grade (S.R. 207) and the Monitor Pass road (S.R. 89), and at least one on the Sonora Pass road (S.R. 108).

Several small foreshocks, with magnitudes of up to 2.6, occurred over a 12-day period leading up to the main earthquake. Large earthquakes are sometimes preceded by small earthquakes, which often occur in swarms or clusters, but usually such activity dies down without a large event occurring.

As of October 31, 1994, more than 900 aftershocks of the Double Spring Flat earthquake had been recorded. The largest aftershock (Richter magnitude 5.0) occurred about 11-1/2 hours after the main event (4:57 P.M. PDT). This large aftershock occurred in the Leviathan Canyon (Bryant Creek) area about 4 km southwest of the mainshock epicenter. Eight events of magnitude >4 followed the main earthquake; this is a typical number of aftershocks in this size range, although the sequence is somewhat unusual in that half of these occurred in an off-fault cluster on a single day more than a week after the main event. At the present time, aftershocks continue, but are decreasing in size and frequency.

The aftershock pattern forms a strong northeast alignment slightly more than 10 km in length. A secondary northwest aftershock alignment extends to the southeast from the mainshock; this alignment is dominated by the off-fault activity mentioned above and appears to represent triggered activity rather than the main-event fault plane. Clearly defined alignments are believed to typically reflect aftershocks occurring around the margins of, and therefore outlining, the fault area that slipped during the main earthquake. Based on this aftershock pattern, the Double Spring Flat earthquake appears to have occurred along a northeast- trending fault. No surface displacements or ground cracking were found that could be associated with such a northeast trend, however. It therefore appears that fault displacement died out before reaching the ground surface, as is typical for earthquakes with magnitudes of less than about 6.5. The earthquake, which was centered about 5 km below the ground surface, involved oblique slip motion, with fault displacement being mostly left-lateral strike-slip (horizontal) and partly normal (vertical).

The Double Spring Flat earthquake occurred within a stepover area between two principal Sierra Nevada frontal faults, the Genoa fault and the Antelope Valley fault. These two faults form the steep mountain fronts west of Minden/Gardnerville and Topaz Lake, respectively, and are principal structures of a transition zone between the Sierra Nevada to the west and Basin and Range to the east. The earthquake also occurred within the southern part of a complex north-trending fault zone which extends northward from this stepover area through the eastern part of Carson Valley (the East Carson Valley or Pine Nut Mountains fault zone); the causative fault can be considered part of that zone.

Many active Quaternary faults have been identified in the Double Spring Flat earthquake area, including a few that generally parallel the dominant northeast-trending aftershock alignment. Minor ground cracking was found along a few of the faults in the epicentral area, but these ground cracks occurred along northwest-trending faults and therefore appear to have been caused by secondary fault movements or shaking, rather than being directly related to the northeast-trending fault that was the source of the earthquake. The ground cracking formed zones generally trending northwest to north-northwest, with individual crack trends ranging from northwest to north-northeast. The cracks displayed extensional openings, typically from 1 to 10 mm wide and up to 20 mm wide in a few places (such cracks often appear somewhat wider due to sloughing or collapse of material into the crack). None of the cracks displayed definitive indications of lateral or vertical separations. All identified zones of cracking, with the exceptions of those in Fish Spring Flat and Smith Valley (see below), were located within 4 km of the epicenter of the main shock. The most pronounced cracks were located <3 km east of the epicenter, forming three short zones (0.5 to 1.0 km long) along north-northwest-trending, west-facing Quaternary scarps. Cracks to the west of the epicenter occurred along northwest-trending faults and generally had smaller openings; most occurred along a discontinuous zone possibly>3 km long and the others formed a short (about 130 m long), relatively minor, left-stepping zone. Additional zones of cracking similar to these may have escaped detection. Cracks were difficult to find in many areas, being masked by a duff layer (pine needles and twigs), heavy grass, or moderately steep rocky terrain, but no larger cracks or surface displacements appear to have occurred.

In addition to the ground cracking in the epicentral area, the Double Spring Flat earthquake apparently also triggered similar ground cracking along faults in Fish Spring Flat (about 15 km to the north) and in Smith Valley (about 25 km to the east). Both of these faults have histories of cracking, as reported by local residents. In 1988, a 1-km-long zone of cracking formed along a late Quaternary fault bounding the west side of Fish Spring Flat; minor cracking along a 1/2-km-long section of this same fault was found after the Double Spring Flat earthquake. In Smith Valley, a local resident (Mark Owens) noted ground cracking along a mapped fault in about 1980. Owens periodically checked the fault over subsequent years, including as recently as late August 1994. The crack did not reopen until the time of the Double Spring Flat earthquake.

There have been few moderate to large magnitude earthquakes in western Nevada over the past few decades, but the region had an extensive history of seismic activity prior to that, particularly during the mid to late 1800s. In fact, Nevada is the third most earthquake-prone state in the country, ranking behind only Alaska and California (albeit considerably lower than those two states). Most of the earthquake activity in Nevada has occurred in the west-central part of the state.

The 1994 Double Spring Flat earthquake served as a potent reminder that western Nevada is "earthquake country" and that residents should be prepared for future earthquakes. Several simple and inexpensive measures in your home or business can greatly reduce earthquake damage and prevent loss of life. For further information on earthquake hazards and preparedness, contact the Nevada Bureau of Mines and Geology (784-6691) or the University of Nevada Seismological Laboratory (784-4975).

--- Alan Ramelli, Research Geologist