Michelle Sneed

Water-level changes induced by local and distant earthquakes at Long Valley caldera, California

Distant as well as local earthquakes have induced groundwater-level changes persisting for days to weeks at Long Valley caldera, California. Four wells open to formations as deep as 300 m have responded to 16 earthquakes, and responses to two earthquakes in the 3-km-deep Long Valley Exploratory Well (LVEW) show that these changes are not limited to weathered or unconsolidated near-surface rocks. All five wells exhibit water-level variations in response to earth tides, indicating they can be used as low-resolution strainmeters. Earthquakes induce gradual water-level changes that increase in amplitude for as long as 30 days, then return more slowly to pre-earthquake levels. The gradual water-level changes are always drops at wells LKT, LVEW, and CH-10B, and always rises at well CW-3. At a dilatometer just outside the caldera, earthquake-induced strain responses consist of either a step followed by a contractional strain-rate increase, or a transient contractional signal that reaches a maximum in about seven days and then returns toward the pre-earthquake value. The sizes of the gradual water-level changes generally increase with earthquake magnitude and decrease with hypocentral distance. Local earthquakes in Long Valley produce coseismic water-level steps; otherwise the responses to local earthquakes and distant earthquakes are indistinguishable. In particular, water-level and strain changes in Long Valley following the 1992 M7.3 Landers earthquake, 450 km distant, closely resemble those initiated by a M4.9 local earthquake on November 22, 1997, during a seismic swarm with features indicative of fluid involvement. At the LKT well, many of the response time histories are identical for 20 days after each earthquake, and can be matched by a theoretical solution giving the pore pressure as a function of time due to diffusion of a nearby, instantaneous, pressure drop. Such pressure drops could be produced by accelerated inflation of the resurgent dome by amounts too small to be detected by the two-color electronic distance-measuring network. Opening-mode displacement in the south moat, inferred to have followed a M4.9 earthquake on November 22, 1997, could also create extensional strain on the dome and lead to water-level changes similar to those following dome inflation. Contractional strain that could account for earthquake-induced water-level rises at the CW-3 well is inconsistent with geodetic observations. We instead attribute these water-level rises to diffusion of elevated fluid pressure localized in the south moat thermal aquifer. For hydraulic diffusivities appropriate to the upper few hundred meters at Long Valley, an influx of material at temperatures of 300°C can thermally generate pressure of 6 m of water or more, an order of magnitude larger than needed to account for the CW-3 water-level rises. If magma or hot aqueous fluid rises to within 1 km of the surface in the eastern part of the south moat, then hydraulic diffusivities are high enough to allow fluid pressure to propagate to CW-3 on the time scale observed. The data indicate that seismic waves from large distant earthquakes can stimulate upward movement of fluid in the hydrothermal system at Long Valley.

Time-varying land subsidence detected by radar altimetry: California, Taiwan and north China

Contemporary applications of radar altimetry include sea-level rise, ocean circulation, marine gravity, and icesheet elevation change. Unlike InSAR and GNSS, which are widely used to map surface deformation, altimetry is neither reliant on highly temporally-correlated ground features nor as limited by the available spatial coverage, and can provide long-term temporal subsidence monitoring capability. Here we use multi-mission radar altimetry with an approximately 23 year data-span to quantify land subsidence in cropland areas. Subsidence rates from TOPEX/POSEIDON, JASON-1, ENVISAT, and JASON-2 during 1992–2015 show time-varying trends with respect to displacement over time in California’s San Joaquin Valley and central Taiwan, possibly related to changes in land use, climatic conditions (drought) and regulatory measures affecting groundwater use. Near Hanford, California, subsidence rates reach 18 cm yr−1 with a cumulative subsidence of 206 cm, which potentially could adversely affect operations of the planned California High-Speed Rail. The maximum subsidence rate in central Taiwan is 8 cm yr−1. Radar altimetry also reveals time-varying subsidence in the North China Plain consistent with the declines of groundwater storage and existing water infrastructure detected by the Gravity Recovery And Climate Experiment (GRACE) satellites, with rates reaching 20 cm yr−1 and cumulative subsidence as much as 155 cm.

Land subsidence in the southwestern Mojave Desert, California, 1992–2009

In cooperation with the Mojave Water Agency (MWA), the U.S. Geological Survey (USGS) has been monitoring land subsidence in the southwestern Mojave Desert of California using satellite Interferometric Synthetic Aperture Radar (InSAR) combined with ground-based techniques. Maps of land subsidence constructed from the InSAR data have proven to be an economical means to evaluate subsidence—with the goal of identifying small problems before they become large ones. The maps of subsidence over the considerably large (nearly 5,000 square miles [mi2]) MWA management area (fig. 1) enabled researchers to detect small magnitude, localized areas of subsidence near five lakebeds.

Lithostratigraphic, borehole-geophysical, hydrogeologic, and hydrochemical data from the East Bay Plain, Alameda County, California

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Land subsidence, groundwater levels, and geology in the Coachella Valley, California, 1993-2010

Land subsidence associated with groundwater-level declines has been investigated by the U.S. Geological Survey in the Coachella Valley, California, since 1996. Groundwater has been a major source of agricultural, municipal, and domestic supply in the valley since the early 1920s. Pumping of groundwater resulted in water-level declines as much as 15 meters (50 feet) through the late 1940s. In 1949, the importation of Colorado River water to the southern Coachella Valley began, resulting in a reduction in groundwater pumping and a recovery of water levels during the 1950s through the 1970s. Since the late 1970s, demand for water in the valley has exceeded deliveries of imported surface water, resulting in increased pumping and associated groundwater-level declines and, consequently, an increase in the potential for land subsidence caused by aquifer-system compaction. Global Positioning System (GPS) surveying and Interferometric Synthetic Aperture Radar (InSAR) methods were used to determine the location, extent, and magnitude of the vertical land-surface changes in the southern Coachella Valley during 1993–2010. The GPS measurements taken at 11 geodetic monuments in 1996 and in 2010 in the southern Coachella Valley indicated that the elevation of the land surface changed –136 to –23 millimeters (mm) ±54 mm (–0.45 to –0.08 feet (ft) ±0.18 ft) during the 14-year period. Changes at 6 of the 11 monuments exceeded the maximum expected uncertainty of ±54 mm (±0.18 ft) at the 95-percent confidence level, indicating that subsidence occurred at these monuments between June 1996 and August 2010. GPS measurements taken at 17 geodetic monuments in 2005 and 2010 indicated that the elevation of the land surface changed –256 to +16 mm ±28 mm (–0.84 to +0.05 ft ±0.09 ft) during the 5-year period. Changes at 5 of the 17 monuments exceeded the maximum expected uncertainty of ±28 mm (±0.09 ft) at the 95-percent confidence level, indicating that subsidence occurred at these monuments between August 2005 and August 2010. At each of these five monuments, subsidence rates were about the same between 2005 and 2010 as between 2000 and 2005. InSAR measurements taken between June 27, 1995, and September 19, 2010, indicated that the land surface subsided from about 220 to 600 mm (0.72 to 1.97 ft) in three areas of the Coachella Valley: near Palm Desert, Indian Wells, and La Quinta. In Palm Desert, the average subsidence rates increased from about 39 millimeters per year (mm/yr), or 0.13 foot per year (ft/yr), during 1995–2000 to about 45 mm/yr (0.15 ft/yr) during 2003–10. In Indian Wells, average subsidence rates for two subsidence maxima were fairly steady at about 34 and 26 mm/yr (0.11 and 0.09 ft/yr) during both periods; for the third maxima, average subsidence rates increased from about 14 to 19 mm/yr (0.05 to 0.06 ft/yr) from the first to the second period. In La Quinta, average subsidence rates for five selected locations ranged from about 17 to 37 mm/yr (0.06 to 0.12 ft/yr) during 1995–2000; three of the locations had similar rates during 2003–mid-2009, while the other two locations had increased subsidence rates. Decreased subsidence rates were calculated throughout the La Quinta subsidence area during mid-2009–10, however, and uplift was observed during 2010 near the southern extent of this area. Water-level measurements taken at wells near the subsiding monuments and in the three subsiding areas shown by InSAR generally indicated that the water levels fluctuated seasonally and declined annually from the early 1990s, or earlier, to 2010; some water levels in 2010 were at the lowest levels in their recorded histories. An exception to annually declining water levels in and near subsiding areas was observed beginning in mid-2009 in the La Quinta subsidence area, where recovering water levels coincided with increased recharge operations at the Thomas E. Levy Recharge Facility; decreased pumpage also could cause groundwater levels to recover. Subsidence concomitant with declining water levels and land-surface uplift concomitant with recovering water levels indicate that aquifer-system compaction could be causing subsidence. If the stresses imposed by the historically lowest water levels exceeded the preconsolidation stress, the aquifer-system compaction and associated land subsidence could be permanent. Land Subsidence, Groundwater Levels, and Geology in the Coachella Valley, California, 1993–2010 By Michelle Sneed, Justin T. Brandt, and Mike Solt 2 Land Subsidence, Groundwater Levels, and Geology in the Coachella Valley, California, 1993–2010 Introduction Groundwater has been a major source of agricultural, municipal, and domestic water supply in Coachella Valley, California (fig. 1), since the early 1920s. Pumping of groundwater resulted in water-level declines as much as 15 meters (m), or 50 feet (ft), between the early 1920s and late 1940s. In 1949, the importation of Colorado River water through the Coachella Canal, a branch of the All-American Canal, to the southern Coachella Valley began. As a result of the importation of surface water, pumping of groundwater decreased in the southern Coachella Valley during the 1950s through the 1970s, and water levels in some wells in the lower valley recovered as much as 15 m (50 ft). Since the late 1970s, however, the demand for water in the southern Coachella Valley has exceeded the deliveries of imported surface water, pumping has increased, and water levels have again declined. By 2010, water levels in many wells in the southern Coachella Valley had declined 15–30 m (50–100 ft), and water levels in some wells were at their lowest recorded levels. The Coachella Valley Water District (CVWD) is currently involved in several agreements and projects including the Quantification Settlement Agreement, tiered-rate structures, aquifer-recharge projects, and conversion from groundwater to surface water resources for (primarily) golf course irrigation through the Mid-Valley Pipeline Project, that could reduce reliance on the groundwater resource (Coachella Valley Water District, 2012). Continued monitoring could track the effect these agreements and projects have on groundwater levels. Declining water levels can contribute to or induce land subsidence in aquifer systems that consist of a substantial fraction of unconsolidated fine-grained sediments (silts and clays). Ikehara and others (1997) reported as much as 150 millimeters (mm) ±90 mm (0.5 ft ±0.3 ft) of subsidence in the southern parts of the Coachella Valley between 1930 and 1996. Land subsidence can disrupt surface drainage and watersupply or flood-control conveyances; cause earth fissures; and damage wells, buildings, roads, and utility infrastructure. A large earth fissure was discovered in 1948 about 3 kilometers (km), or 2 miles (mi), north of Lake Cahuilla in La Quinta (unpublished field notes, Coachella Valley Water District, 1948). Because subsidence had not been documented in the southern parts of the Coachella Valley prior to the report by Ikehara and others (1997), it is not known if this fissure formed in response to differential land subsidence during the earlier period (early 1920s–late 1940s) of groundwater-level declines. However, fissuring has recurred in this area (Clay Stevens, TerraPacific Consultants, Inc., written commun., 2006). Subsidence-related earth fissures and reactivated surface faults have been identified in many other groundwater basins in the western United States (Holzer, 1984). The CVWD works cooperatively with local stakeholders to manage the water supply for a large part of the Coachella Valley (fig. 1). Because of the potential for groundwater pumping to cause land subsidence, the CVWD entered into a cooperative agreement with the U.S. Geological Survey (USGS) to monitor vertical changes in land surface to determine if land was subsiding in the Coachella Valley. In 1996, the USGS established a geodetic network of monuments to monitor vertical changes in land surface in the southern Coachella Valley by using Global Positioning System (GPS) surveys and to establish baseline values for comparisons with results of future surveys. This geodetic network can be surveyed periodically to determine the distribution and amount of land subsidence. Interferometric Synthetic Aperture Radar (InSAR) data collected since 1993 were used to detect and quantify land subsidence in areas distant from the geodetic monuments. Purpose and Scope The objectives of this study were to detect and quantify land subsidence in the southern Coachella Valley from 1993 through 2010 by completing GPS surveys at the established geodetic network of monuments and by using InSAR data. For purposes of this report, the southern Coachella Valley represents the southern half of the Coachella Valley, which extends from the communities of Palm Desert, Indian Wells, Indio, and La Quinta on the north to the Salton Sea on the south (fig. 1). This report presents the results and interpretations of GPS data collected at the monuments in the monitoring network during surveys in 1996, 1998, 2000, 2005, and 2010 and also of spatially detailed maps of vertical land-surface changes generated by using InSAR data collected between 1993 and 2010. The InSAR-generated maps extend from near Palm Desert to near the Salton Sea (fig. 1). Data showing groundwater-level changes from the early to mid1990s to 2010 were examined and compared with the GPS measurements and the InSAR-generated maps to determine if the vertical changes in land surface could be related to the changes in groundwater levels.