PAHRUMP REGIONAL PLANNING DISTRICT MASTER PLAN 2010

Transcription

1 CHAPTER TWENTY SEISMIC SAFETY PLAN (This Plan is from the 1999 Pahrump Regional Planning District Master Plan) Pursuant to NRS , this topic consists of an identification and appraisal of seismic hazards such as susceptibility to surface ruptures from faulting, to ground shaking or to ground failures. The RPC may develop a seismic safety element after implementation of the Master Plan. The purpose of this section "consists of an identification and appraisal of seismic hazards such as Susceptibility to surface ruptures from faulting, to ground shaking or to ground failures." This section will present the Goals and policy of Nye County as it relates to the geologic hazards of earthquakes and land subsidence. EARTHQUAKES Seismic activity in Nye County has been and is related to man-made and natural causes. Manmade seismic activity has resulted from underground nuclear testing. It was generally of short duration with the only effect being minor inconvenience to those that experienced the tremor. There is no evidence that any structural damage to local buildings has resulted from nuclear testing. Natural causes of seismic activity are due to shifts in the earth s crust or tectonic faulting. Tectonic faulting results from the separation or movement of part of the earth s crust in relation to another. These faults resulted from earth movement that occurred in the middle to late Pleistocene era (1.5 million years ago) and traverse the county in a north-south trending series that created the basin/ valley arrangement of the state (Figure 19.1). A good example of a major active fault is the San Andreas Fault running up the coast of California from San Diego to San Francisco. Movement along this fault has resulted in numerous costly earthquakes such as Loma Prieta and Huntridge. Major earthquake activity in Nevada is concentrated along a series of faults extending in a northerly direction from the Owen s Valley in California to Winnemucca, with the greatest activity in the Reno-Winnemucca-Tonopah triangle (Figures 19.2 and 19.3). July 21, 2010 Version Page 140

2 There is also potential danger due to liquefaction, an earthquake hazard where the support capabilities of the ground give way during intense shaking. Liquefaction occurs when seismic waves pass through saturated granular soil, distorting its granular structure and causing some of the empty spaces between granules to collapse. Liquefaction causes lateral spreads (horizontal movements of commonly 10 to 15 feet, but up to 100 feet), flow failures (massive flows of soil or slides, typically hundreds of feet, but up to 12 miles), and loss of bearing strength (soil deformations causing structures to settle or tip), all causing severe damage to property. The effects of earthquake waves at the surface can be measured using the Modified Mercalli Intensity (MMI) Scale, which consists of arbitrary rankings based on observed effects, or the more common Richter Magnitude Scale, a mathematical basis that expresses the effects of an event in magnitude (M). The first recorded earthquake in Nye County occurred on December 20, 1932, at Cedar Mountain. This earthquake was recorded as a severe MMI XII event. Over 20 years later, a moderate MMI VI earthquake was recorded on July 6, 1954, in Gabbs. Only six months later, on December 16, 1954, an MMI IX earthquake was felt in Beatty. In recent years, the Little Skull Mountain earthquake (M 5.6 on the Richter Scale) occurred at the Nevada Test Site on June 29, This earthquake, the largest ever recorded at the site, is thought to have been triggered by an M 7.0 earthquake that occurred in Landers, California, 24 hours earlier. On August 1, 1999, an M 5.7 earthquake occurred near Scotty s Junction, 34 miles northwest of Beatty. Although it was reported that the area at the epicenter shook quite hard, no reports of significant damage or injuries were reported in this relatively unpopulated area. Repeated, clustered, low-magnitude (Magnitude less than 4.0) earthquakes are often recorded along the Rock Valley fault zone in the Nevada Test Site. Nye County is characterized by parallel mountain ranges and valleys, bounded by normal-slip faults. There are 270 known normal-slip faults within Nevada, with several relatively small (12- to 24-mile-long) faults within and around Nye County. Figure 19.4 shows the location of the faults in Nye County. Although relatively small in size, these fault zones are capable of delivering M earthquakes. Documented faults in this area include Rocky Valley, Pahranagat, Cane Spring, Timpahute, Frenchman Mountain, Whitney Mesa, Cashman, Decatur, Eglington, and West Charleston. In addition, larger faults, such as the 60-mile Pahrump Valley fault (potential M ) located in southern Pahrump Valley and the 71-mile Death Valley Fault (potential M ) and 11 l-mile Furnace Greek Fault (potential M ) in Death Valley pose great seismic hazards to Nye County. Additionally, Nye County is susceptible to background earthquakes, which are not linked to any known fault and do not rupture at the surface, as well as earthquake sequences and earthquakes caused by subsurface faults. July 21, 2010 Version Page 141

3 Although Nye County has not been a priority for seismic monitoring, studies prepared by the Federal Emergency Management Agency (FEMA) suggest that a Northridge-sized M 6.5 or greater earthquake will occur in the Las Vegas metro area (including Nye County) once every 300 years. Pahrump-area historical earthquake activity is below Nevada state average. It is 222% greater than the overall U.S. average. Figure 19.5 shows the location of the faults in the valley. Table 19.a describes the recorded seismic activity in the area since Seismic Activity Since 1972 Time Date Magnitude Distance from Pahrump 10:00 6/29/ mi. 15:15 2/28/ :00 6/13/ :30 10/12/ :06 8/1/ :00 4/4/ TABLE 19.A LAND SUBSIDENCE In the southwestern United States, agricultural and urban areas that depend on groundwater pumping are prone to land subsidence. Land subsidence, or the lowering of the earth s surface, can be due to man-made processes such as ground water pumping or natural causes. Land subsidence occurs when declining water levels lead to compaction of subsurface soils and bedrock as water is extracted. A lesser amount of subsidence occurs with the recoverable compression of course-grained sands and gravel deposits. A common resultant feature that accompanies and is evidence of subsidence is earth fissures, which are tension cracks in the sediment above the water table. The two causes of subsidence are grouped into two categories: endogenic and exogenic subsidence. Endogenic subsidence occurs within the earth and is due to tectonism, volcanism, and continental drift. Exogenic subsidence occurs mainly at the earth s surface and can result from natural causes as well as induced by the activities of man. Basically, Exogenic subsidence is basically the result of a loss of support. There are three processes that could result in a loss of support. First, loss of support can be caused by fluid extraction as in the case of groundwater withdrawal. Second, loss of support on a regional in scale can be caused by an increase of loading from the weight July 21, 2010 Version Page 142

4 of a body of water such as a lake. A third process (hydro-compaction) that could cause a loss of support is the adding water to, or saturating, of a collapsible soil that has a loose grain structure. Groundwater withdrawal is thought to be the most common reason for localized ground subsidence as found in the San Joaquin Valley, California; Central Arizona; Denver, Colorado; London, England and Osaka, Japan. Groundwater withdrawal is also the primary factor in localized subsidence in Pahrump Valley. It is important to understand the distinction between fault movement and fissure movement. Fault movement is associated with the release of natural forces, while fissure movement is associated with hydraulically driven forces associated with groundwater withdrawal. Fissures tend to occur near faults for very good reasons, but what causes fissure movement is very different from what causes fault movement. Thus, one can understand why exploring the causes of groundwater withdrawal related fissures and possibly discovering a method of making accurate predictions about when and where they should occur is very important to Nye County. The results of this study should provide a significant management tool for government agencies, public utilities and private industry in order to avoid or mitigate the potential hazards of subsidence. Land subsidence has been documented in Nye County since the early 1980s, when fissures in the Town of Pahrump were first observed and mapped. However, land subsidence most likely began to occur in the mid to late 1950s, when the rate of pumped water for irrigated land began to exceed the perennial yield of the aquifer. Land subsidence has been documented at 14 USGS and Nevada Department of Transportation Pahrump Valley monument stations from 1981 to During this period, many homes in the Pahrump Valley have displayed subsidence failure including cracked and uneven foundations and cement pads. Four large fissures (approximately one mile long) are present to the south and southeast of the Town of Pahrump. The greatest degree of subsidence has occurred in three major areas of Pahrump: the southern portion of town, along SR 160, and in the area of the western fissure. 14 monument stations placed around the valley have shown subsidence to range from 1 inch to 18 inches over the past 24 years. Because there is no source of artificial recharge for the Pahrump Valley aquifer, this problem may become more severe if overdrafting as a practice continues or grows. Mitigation measures include the review of building plans for geologic hazards, the requirement of a soils engineering report for non-residential development plans, and a geo-technical investigation report on any housing development within 500 feet of a documented fault or fissure. These measures can be incorporated in the current plot / site plan review process currently being conducted by county staff. Nye County could develop policy that would include, but not be limited to, discouraging development where seismic problems cannot be mitigated and amendments to the Land Use Plan to properly reclassify those areas unsuitable for development because of July 21, 2010 Version Page 143

5 geologic conditions. A subsidence district could be designated so monitoring can be conducted and mitigation measures determined and carried out when necessary. Beginning with the data available and in cooperation with the other neighboring governments and agencies, the county should begin to maintain and periodically update maps of documented areas of collapsible soils, subsidence, faulting and fissuring. The County should make available to the public information concerning documented areas of seismic hazard, subsidence, and poor soil conditions. SUMMARY Seismic activity in Nye County has had significance in a geologic sense and in geologic time. Current building practices have been adequate to withstand seismic activity both man-induced through nuclear testing and natural from earthquakes. Research intending to update local seismic information may result in more stringent building standards. The pivotal issue in the valley is dealing with certain geologic deposits that are susceptible to horizontal movement and fissuring that may cause structural damage to buildings. The subsidence problem will continue to occur as long as groundwater withdrawal exceeds annual recharge, natural or injected. The most damaging result will be the spreading of existing fissures and the likely formation of new ones. These phenomena will make such things as the enforcement of adequate construction regulations necessary. It will also require consideration of land use density restrictions on susceptible geographic areas. Efforts to stabilize groundwater withdrawal practices should be prioritized locally and through State level legislation. GOALS, OBJECTIVES, AND POLICIES- SEISMIC SAFETY The following Goals, Objectives, and Policies are proposed to implement the purposes of this Element. Goal : Reduce the possibility of damage and losses due to earthquakes. Objective 1- Nye County protect existing assets, as well as any future development, from the effects of earthquakes. Policy 1- Continue to enforce the Uniform Building Code (UBC) provisions pertaining to grading and construction relative to seismic hazards. Policy 2- Continue to enforce UBC requirements for addressing liquefaction potential in the design of structures. Policy 3- Implement an Unreinforced Masonry (URM) building program that determines the structural safety of critical facilities, and retrofit buildings, if necessary. July 21, 2010 Version Page 144

6 Policy 4- Develop and provide managers of mobile home parks with information on how to improve the seismic performance of mobile homes. Policy 5- Encourage utility companies to evaluate the seismic risk to their highpressure transmission pipelines and implement mitigation measures, such as automatic shut-off valves The purpose of these policies is to protect life and property from the effects of earthquakes. Goal : Reduce the possibility of damage and losses due to land subsidence. Objective 1- Nye County protect existing assets, as well as new development, from land subsidence. Policy 1- Develop and adopt setbacks from mapped faults to help mitigate future fissure losses. Policy 2- Support an ordinance that will ensure effective withdrawal of groundwater that will not precede or exacerbate subsidence. The purpose of these polices is protect life and property from the effects of land subsidence. Implementation Actions The following actions will implement the above policies. By 2012, develop G1S maps of geologic hazards. By 2012, develop a Water withdrawal/ subsidence Ordinance. July 21, 2010 Version Page 145

7 Figure 19.1 July 21, 2010 Version Page 146

8 FIGURE 19.2 July 21, 2010 Version Page 147

9 FIGURE 19.3 July 21, 2010 Version Page 148

10 PAHRUMP REGIONAL PLANNING DISTRICT MASTER PLAN 2010 FIGURE 19.4 July 21, 2010 Version Page 149

11 FIGURE 19.5 July 21, 2010 Version Page 150