Seismic Hazards in the Reno-Carson City Urban Corridor

Spring 1992

The Reno-Carson City urban corridor has one of the highest seismic hazards in the state of Nevada. Historical earthquakes are often the most convincing evidence of a local seismic hazard, and the Reno-Carson City urban corridor has had several damaging historical earthquakes. The largest earthquakes in the region are listed below. The earliest accounts of earthquakes are from the Pyramid Lake and Virginia Range areas, to the north and east of Reno. A large earthquake (estimated to be magnitude 7 to 7.3) is thought to have occurred near Pyramid Lake around 1845 or 1852, based on Paiute Indian recollections. On March 15, 1860, an earthquake with a magnitude of about 7 caused damage in Carson City but, based on aftershocks, its epicenter was probably closer to Pyramid Lake. On December 26 and 27, 1869, an interesting pair of earthquakes occurred, just eight hours apart. The first, with a magnitude perhaps as large as 6.7, occurred either in the Olinghouse area or the Virginia Range. The second, with a magnitude of around 6.1 apparently occurred closer to Carson City. An earthquake on June 3, 1887 was one of the most damaging to Carson City, bringing down chimneys and partly collapsing walls. The Reno area experienced two strong earthquakes of magnitude 6+ in 1914 that were separated by only two months. These brought down chimneys in a localized part of Reno. The most recent earthquake causing slight damage occurred just north of Carson City on January 2, 1991 and had a magnitude of 4.6. Smaller earthquakes are felt in the corridor on the average of a several times per year.

Significant Historical Earthquakes in the Reno-Carson City Urban Corridor

Date             Nearest Community   Intensity    Magnitude
1845(?) or 1852  Pyramid Lake?       VIII?        7-7.3 
1860, Mar. 15    Pyramid Lake?       VI           7 
1868, May 29     Virginia City       VII          6 
1869, Dec. 26    Virginia City       VI           6.7 
1869, Dec. 27    Steamboat Springs   VIII         6.1 
1887, June 3     Genoa               VIII         6+ 
1894, Nov. 18    Virginia City       VI           ? 
1896, Jan. 2     Carson City         VI           ? 
1897, May 15     Carson City         V            ? 
1914, Feb. 1     Reno                VI-VII       6 
1914, Apr. 24    Reno                VII-VIII     6.4 
1930, Apr. 9     SE Lake Tahoe       VI           4.3 
1933, June 2     Wabuska             VII          6.1 
1942, Dec. 3     Reno                VI           5.9 
1948, Dec. 2     Verdi               VII          6 
1952, May 9      Steamboat Springs   VII          5.1 
1966, Sep. 1     Truckee             VII          6 
1978, Sep. 4     Genoa               VI           5.2 
1991, Jan. 2     Carson City         V-Vl         4.6 

Note: Dates are based on Pacific Standard Time. Intensity is a measure of the damage and effects of an earthquake. The intensity scale used here is the Modified Mercalli (MM) intensity scale. MM V is a strongly felt earthquake, MM VI is an earthquake that cracks walls, MM VII is an earthquake that topples chimneys, and MM VIII is an earthquake that causes significant damage to weak structures.

Active faults capable of generating strong earthquakes are located throughout the Reno-Carson City urban corridor. Some of the larger active fault zones are shown on the map on the next page. These faults are normal, lateral, and oblique slip in nature, and are responsible for much of the present topography. These fault zones or systems can be generally placed into four groups: range-bounding normal faults, strike-slip faults, distributed systems, and seismogenic lineaments.

Range-bounding normal faults are present throughout the region, and generally have northerly trends. The most pronounced example in the region is the Carson Range fault system. This system bounds the eastern side of the Carson Range and includes the Genoa fault zone (GFZ on map) bounding the west side of Carson and Eagle Valleys, and the Mount Rose fault zone (MRFZ on map) including faults along western Washoe Valley and the eastern flank of Mount Rose.

The Genoa fault zone in particular is a classic range-bounding normal fault that has a vertical slip rate of around one millimeter per year. Earthquakes as large as magnitude 7.4 can occur along this fault zone. Taken with the slip rate, the estimated displacement from earthquakes of this size can be used to estimate a possible average earthquake recurrence interval. For the Genoa fault zone, this works out to be the occurrence of an earthquake about once every 3,000 years. The northern part of the Genoa fault zone is just to the west of Carson City.

The larger strike-slip faults in the region generally occur east and north of the urban areas, and generally trend northwest or northeast in a conjugate pattern. These include the Honey Lake, Warm Springs Valley, Pyramid Lake, and Olinghouse fault zones. The first three of these are part of a regional feature referred to as the Walker Lane. These faults appear to have remarkably high rates of activity and short recurrence intervals for this region. For example, the Pyramid Lake fault zone is thought to have an earthquake-recurrence interval of around 1,700 years and may have been involved in the 1845(?) or 1852 earthquake. Earthquakes as large as magnitude 7 to 7.5 may be associated with these larger, lateral fault systems.

Distributed fault zones, characterized by numerous short, densely spaced faults, are more difficult to interpret. The distribution can result from a variety of effects including surficial material response and inherited structural patterns. More work needs to be done with these systems before we can fully understand them, but important large earthquakes, such as the 1932 Cedar Mountain earthquake (magnitude 7.2) near Gabbs, Nevada indicate that active distributed systems can produce damaging earthquakes. Seismogenic lineaments are trends or alignments of faults, earthquakes, or low topography that are thought to have greater earthquake potentials than the background level, but do not necessarily rupture as through-going structural features. The principal evidence that these lineaments are seismogenic is the existence of intermittent fault scarps and recorded seismicity. An example is the Carson lineament. This is a northeast-trending zone of low topography formed by a crude synform in superjacent rocks, with discontinuous Quaternary fault scarps, and associated seismicity. The western part of the Carson lineament appears to interrupt the fault trends along the Carson Range fault system, and the eastern end approaches the southern end of the Pyramid Lake fault zone. Maximum earthquake magnitudes associated with these seismic zones are thought to be in the range of 6.6 to 7. Historical earthquakes of magnitude 6 have occurred along the Dog Valley lineament in 1948 and 1966.

The zoning map in the 1991 edition of the Uniform Building Code shows the eastern portion of the Reno-Carson City corridor to be in the most hazardous category (Zone 4) and the western portion to be in the next most hazardous (Zone 3). Unfortunately, the zone boundary running down the middle of the corridor divides both Reno and Carson City into two zones with differing building code requirements. This boundary should be more logically placed to the west along the Sierran front and both Reno and Carson City should be in Zone 4.

The maps in the National Earthquake Hazard Reduction Program Recommended Provisions for Earthquake Resistant Design, published by the Federal Emergency Management Agency, also show most of the urban corridor in their most hazardous categories, but curiously downgrade Storey and Douglas Counties into less hazardous categories. Since the Genoa fault zone is in Douglas County, the hazard rating in this area should be as high as the rest of the corridor.

The seismic hazard of the Reno-Carson City urban corridor is clearly great, and a return of the level of earthquake activity that occurred in the corridor in the latter half of the 19th century is a definite possibility. Because return of such earthquake activity would cause far more damage today due to the vastly greater population and number of man-made structures, it would be prudent to be prepared. Earthquake mitigation activities can take place on a personal level, in the home and at the office, and on a professional level (designing earthquake resistant buildings, for example). Further information, including a pamphlet on preparing for and surviving earthquakes, is available from the Nevada Bureau of Mines and Geology (784-6691) and the Seismological Laboratory (784-4975) at the University of Nevada, Reno.

---Craig dePolo, Research Geologist