Imperial Valley Region Research Articles

The Disappearing Lake: A Historical Analysis of Drought and the Salton Sea in the Context of the GeoHealth Framework.

The Imperial Valley region of Southeastern California has become one of the most productive agricultural regions in the state and has the highest rates of childhood asthma in California. Lack of precipitation in the Imperial Valley has caused the water level of the Salton Sea to recede to a record low since its formation in the early 1900s. Previous studies of wind and dust deposition conducted in other regions have shown how reduced precipitation, ground heating, and the diminishing water level in an arid climate pose a risk of exposing previously sequestered toxic chemicals to open air, adversely affecting lung health. The purpose of this study is to draw historical parallels between the Aral Sea and Salton Sea in the context of geomorphology, ecology, human health, economics, and human migration, to inform an assessment of environmentally related health impacts of those living in the Imperial Valley region. Future droughts and heatwaves are expected to rise in frequency and severity, disproportionately affecting those impacted by financial and health disparities. Future research must include the implications of population health in the context of GeoHealth as a result of the most recent drought and the receding water levels of the Salton Sea.

Gestión estratégica ambiental del aire en la frontera Mexicali–Imperial

El objetivo principal de este artículo es analizar el papel de la gestión local ambiental del aire, particularmente en el municipio de Mexicali, B.C., y su relación con el Valle Imperial, California en Estados Unidos. El documento plantea la premisa de que la contaminación ambiental del aire se ha agudizado (2010) y se mantendrá como un problema ambiental y social en la región de estudio porque no se ha conceptualizado desde la perspectiva de la gestión estratégica ambiental, especialmente de parte de las autoridades locales mexicanas.

Stress orientation and anisotropy based on shear-wave splitting observations in the Cerro Prieto fault area, Baja California, Mexico

Digital seismograms continuously recorded from 1988 to 1992 by two stations of the RESNOM seismic network in northern Baja California, Mexico, were used to search for probable shear-wave anisotropic characteristics in the region of the Cerro Prieto fault. Shear-wave splitting was identified in many of the three-component records analyzed. We measured the polarization direction of the leadingS wave inside theS-wave window as well as the delay times between fast and slow phases on those records displaying shear-wave splitting. For station CPX, which is nearest the Imperial Valley region to the north, the preferred polarization direction found in this study (azimuth 180°±10°) coincides with the direction of the regional maximum compressive stress determined for the region. This polarization direction can be interpreted in terms of the “Extensive Dilatancy Anisotropy” model as the effect of vertical parallel aligned cracks. The preferred polarization direction measured at LMX, however, gives an azimuth of 45°±5°. Thus, it appears that faults and fractures aligned oblique to the main tectonic trend have a greater influence on the anisotropic characteristics of the crust south of Cerro Prieto volcano than that of the regional stress field. Time delays between slow and fastS waves observed at CPX appear constant from 1988 to 1992 while delays measured at LMX for the same interval indicate a small increase with time which cannot be attributed to azimuthal variations of paths.

Historic earthquakes (1918 to 1923) and an assessment of source parameters along the San Jacinto fault system

Body waveform modeling is used to determine the source parameters of two earthquakes that occurred along the northern San Jacinto fault system in 1918 and 1923. The 1918 earthquake parameters are consistent with a 20- to 25-km rupture along the Claremont fault at a depth of 7 ± 5 km. Limited data for the 1923 event are more consistent with rupture on the San Jacinto fault than rupture along the San Andreas fault or a buried cross fault, although the greatest damage associated with the earthquake occurred closer to the San Andreas fault. Using the results of this study and previous studies of historic earthquakes along the southern San Jacinto fault, the relationships between rupture length and magnitude, focal depth and heat flow, and other parameters are examined in an effort to estimate the source properties of future events that are expected along currently quiescent portions of the fault zone. One important observation is that rupture lengths along the San Jacinto fault are shorter for an equivalent moment magnitude than those along the Imperial Valley-Mexicali fault system. The higher heat flow of the Imperial Valley region may be responsible for this difference.

Travel-time, time-term, and basement depth maps for the Imperial Valley region, California, from explosions

In this study, we have produced maps of reduced travel time for various shot points in the Imperial Valley, a time-term map, and a basement depth map. The time-term map effectively integrates all the features seen on individual travel-time maps, and the basement depth map is derived from the time-term map by a simple linear relation. The basement depth map represents the depth to the top of metasedimentary basement ( Vp ≈ 5.65 km/sec) in the central Imperial Valley and the top of crystalline rocks on West Mesa and in the Chocolate Mountains. We estimate the standard error in this map to be 0.6 km. The basement depth map indicates depths of up to 6 km in the Imperial Valley and depths generally less than 3 km on West Mesa. Major zones of high gradient in depth separate the Imperial Valley from West Mesa on the west and from the Chocolate Mountains on the east. Because West Mesa and the Chocolate Mountains are underlain by different types of basement than is the Imperial Valley, these major zones of high gradient apparently mark the locations of major crustal boundaries. Most active faults have basement scarps along at least parts of their lengths. Basement highs with relief as great as 3 km occur under major geothermal areas in the Imperial Valley, presumably indicating the presence of igneous intrusions and (or) metamorphism of sedimentary rocks to a relatively higher level in the crust.

Spatial and temporal variations in seismicity in the Imperial Valley (1902-1984)

abstractEarthquakes with ML ≧ 4.5 that occurred between 1932 and 1973 in the Imperial Valley region and their related foreshocks and aftershocks are relocated using station delays derived from calibration events that were well recorded at regional distances between 1979 and 1981. The relocated seismicity shows that there was an increase in seismicity in regions northeast of the Imperial fault 5 to 6 yr prior to the 1940 (Ms = 7.1) Imperial Valley earthquake. A study of intensity reports for felt earthquakes prior to 1932 also suggests an increase in seismicity in regions surrounding the Imperial fault as early as 1917. Isoseismal patterns, description of the mainshock sequence, and patterns of seismicity before and after the 1915 Imperial Valley earthquakes suggest that the earthquakes occurred off the Imperial fault. In this case, seismicity between 1906 and 1940 would nearly encircle the portion of the Imperial fault that ruptured during 1940. In both the 1940 and 1979 Imperial Valley earthquake sequences, aftershocks of ML ≧ 4.5 occurred only for 2 months following the mainshock. The aftershock sequences were followed by periods of quiescence lasting up to 10 yr in the region within 30 km of the mainshock. This is in contrast to ML > 6.0 earthquakes occurring near the edges of the Imperial Valley that had aftershocks of ML ≧ 4.5 occurring for up to 4 yr after the mainshock. These differences may be related to the high heat flow within the Imperial Valley.

Depth of seismicity in the Imperial Valley Region (1977–1983) and its relationship to heat flow, crustal structure and the October 15, 1979, earthquake

Focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the Imperial Valley‐southern Peninsular Ranges are used to study relationships between the depth of seismicity, heat flow, and crustal structure. This study used relocated A and B quality events from the California Institute of Technology catalog that were carefully selected to insure focal depth precision of ±2 km. Regional variations in focal depth appear to be related to regional heat flow variation, whereas local variations in focal depth, especially in the central Imperial Valley, may be related to crustal structure. These variations are studied by rheologie modeling. A comparison of focal depths of earthquakes occurring before and after the October 15, 1979 (M=6.6), earthquake indicates that aftershocks during the first 2 months of the sequence were 2–3 km deeper than earthquakes occurring in other time periods. The deepest earthquakes in Imperial Valley are spatially associated with a subbasement dome near the northern end of the Imperial fault. This dome coincides with the region where the Imperial fault undergoes a transition from stick‐slip behavior to aseismic fault creep. Models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity. A relocation of the 1940 (M=7.1) mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock.

Simulations of strong ground motion for earthquakes in the Mexicali--Imperial Valley region

Summary. In this paper computer modelling is used to test simple approximations for simulating strong ground motions for moderate and large earthquakes in the Mexicali–Imperial Valley region. Initially, we represent an earthquake rupture process as a series of many independent small earthquakes distributed in a somewhat random manner in both space and time along the rupture surface. By summing real seismograms for small earthquakes (used as empirical Green's functions), strong ground motions at specific sites near a fault are calculated. Alternatively, theoretical Green's functions that include frequencies up to 20 Hz are used in essentially similar simulations. The model uses random numbers to emulate some of the non-deterministic irregularities associated with real earthquakes, due either to complexities in the rupture process itself and/or strong variations in the material properties of the medium. Simulations of the 1980 June 9 Victoria, Baja California earthquake (ML= 6.1) approximately agree with the duration of shaking, the maximum ground acceleration, and the frequency content of strong ground motion records obtained at distances of up to 35 km for this moderate earthquake. In the initial stages of modelling we do not introduce any scaling of spectral shape with magnitude, in order to see at what stage the data require it. Surprisingly, such scaling is not critical in going from M= 4–5 events to the M= 6.1 Victoria earthquake. However, it is clearly required by the El Centro accelerogram for the Imperial Valley 1940 earthquake, which had a much higher moment (Ms∼ 7). We derive the spectral modification function for this event. The resulting model for this magnitude ∼ 7 earthquake is then used to predict the ground motions at short distances from the fault. Predicted peak horizontal accelerations for the M ∼ 7 event are about 25–50 per cent higher than those observed for the M= 6.1 Victoria event.

Background seismicity rates and precursory seismic quiescence: Imperial Valley, California

The problem of recognizing precursory seismicity rate changes consists of two steps. First, a background or normal rate must be determined and, second, anomalous rates must be recognized. In this paper, we address the first of these steps, determining a background seismicity rate. Two problems must be dealt with if a meaningful background rate is to be determined. First dependent events must be recognized and removed from consideration and, second, detection or reporting changes must be accounted for. A number of techniques are available for recognizing dependent events. We used the one developed by McNally (1976) in this work. The technique we used for recgonizing detection and reporting changes was developed by Habermann (1983). We demonstrate these techniques on seismicity from the Imperial Valley region of California. After a reasonable background rate is determined, we examine seismicity rates in this region prior to the Imperial Valley earthquake of 15 October 1979. The source area of the earthquake shows a possible precursory quiescence which started during July 1979. The time of this rate decrease is close to the time of a change in the seismic instrumentation in the Imperial Valley which makes it difficult to determine whether the decrease is real or related to detection. Several other seismicity rate changes observed in the Imperial Valley appear to be related to changes in the operation of the network there. These include quiescent periods which occur only if magnitude cutoffs are applied to the data and periods of low reporting of events with M ≧ 2.5. These observations suggest that magnitudes were not reported for some events during several time periods. All of these observations indicate that determination of detection histories is crucial for meaningful seismicity studies.

A seismic refraction survey of the Imperial Valley Region, California

The U.S. Geological Survey conducted an extensive seismic refraction survey in the Imperial Valley region of California in 1979. The Imperial Valley is located in the Salton Trough, an active rift between the Pacific and North American plates. Forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0.5–1 km. More than 1300 recording locations were occupied, and more than 3000 usable seismograms were obtained. We analyzed five profiles using a standard ray‐tracing program, constructed a contour map of reduced travel times from our most widely recorded shot point, and modeled an existing gravity profile across the Salton Trough. Results are itemized: (1) All models have in common a sedimentary layer (Vp = 1.8–5.0 km/s), a “transition zone” (Vp = 5.0–5.65 km/s), a basement (Vp = 5.65 km/s in the Imperial Valley, 5.9 km/s on the bordering mesas), and subbasement (Vp = 7.2 km/s). (2) The sedimentary layer ranges in thickness along the axis of the Salton Trough from 3.7 km (Salton Sea) to 4.8 km (U.S.‐Mexican border). On the bordering mesas it is quite variable in thickness. (3) The “transition” zone is about 1 km thick in most places. In the Imperial Valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement. On the bordering mesas, however, there is a discontinuity at the top of this zone. (4) There are apparently two types of basement. On the bordering mesas, basement is crystalline igneous and metamorphic rocks. In the Imperial Valley, basement is mostly lower‐greenshist‐facies sedimentary rocks, based primarily on the smooth transition in character from sediment to basement arrivals, the low value of basement velocity, and the fact that deep (4 km) wells in the valley penetrate only the upper part of the known Cenozoic stratigraphic column for the Salton Trough. (5) The subbasement, or intermediate crustal layer, ranges in depth along the axis of the Salton Trough from 16 km (Salton Sea) to 10 km (U.S.‐Mexican border). Gravity modeling requires that this layer deepen and/or pinch out beneath the bordering mesas and mountain ranges. Based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the Imperial Valley, the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust. (6) Several structures are seen that affect basement, transition zone, and deeper parts of the sedimentary layer. They include a scarp along the Imperial fault, as much as 1 km down to the northeast, and a scarp passing roughly along the topographic boundary between the Imperial Valley and the bordering mesa to the west, as much as 3½ km down to the east. We interpret the latter scarp to be the suture, or rift boundary, between the older crystalline basement on the mesa and the younger metasedimentary basement in the Imperial Valley. (7) On a contour map of reduced travel time from our most widely recorded shot point, subtle patches of early arrivals among otherwise late arrivals in the central Imperial Valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150°C. Apparently the Salton Trough is a location where new crust is being generated. As the rift opens, mafic intrusive rocks fill it from below as sedimentary rocks fill it from above. Rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth (metasedimentary basement in the Imperial Valley) and thus consolidates the new crust.

New Sources of Gold in the West: ABSTRACT

Records show discovery of, perhaps, 40,000 gold and silver deposits in western United States. Mine descriptions give dates of operation and allow correlations with history. These mine histories commonly indicate that the properties have further potential. Many of the prospects left 50, 75, and even 100 years ago are of interest today because of new geological concepts. Simple veins of yesterday may be the exposed parts of larger entities. The old ore controls, structures, or zones may be tied to larger lineaments, elements, or zones. In younger deposits, corollaries are often made with geothermal fields. Lineations and cross lineations require further consideration. Connections with plate tectonics come easily. As an example of the increase in size of useful parameters, the line of segmentation portrayed in coast ranges of northern California and Oregon might be looked upon as lines marking transverse zones of extension. Better known zones of extension are parts of the very large fault network centering around the San Andreas and Garlock faults, and parts of the Imperial Valley region south of the Salton Sea, a well known geothermal area. Corollaries with such areas are sought, for example, along lines marked by the Antler and Sevier tectonic belts in Nevada. In other western areas, pillow lavas occur in close proximity with gold occurrences which stimulates thinking along lines of spreading centers and submarine springs. The old gold and silver deposits appear to fit readily into a framework, in which treatment of prospects as segments within a geothermal model is helpful. Conversely, with the growth of geothermal development, the view of activity will be incomplete without consideration of the potential by-products, precious metals. End_of_Article - Last_Page 1354------------

Notes on a Map of the Laguna Salada Basin, Baja California, Mexico

N the northeastern part of the Mexican state of Baja California, almost adjoining the Imperial Valley, is the practically unknown Laguna Salada Basin.' While the two basins present close similarities of natural features, the dissimilaritv between their economic development is notable. The Laguna Salada Basin is uninhabited; the Salton Sink has a population measured in tens of thousands. The one, occupying nearly a million acres, is practically unused; the other, slightly larger in acreage, has been the scene of one of the most rapid and spectacular developments in the whole history of irrigation agriculture. That important development has resulted in the creation of nearly $I50,000,000 of property values with an annual production of agricultural commodities having a farm value of more than $25,000,000. Yet only thirty years ago the Imperial Valley region was a barren and uninhabited area, as the Laguna Salada Basin is today. Of recent years Mexican officials interested in the economic advancement of their country have given much attention to reclamation possibilities. They recognize the need for thorough preliminary investigations covering not only the engineering aspects of the project under consideration but also other features that affect the economic success of such developments. The first investigation of this type was made in the Laguna Salada Basin during the winter of 1923-1924. Since that time a score or more similar studies have been conducted in connection with reclamation projects in other parts of Mexico. One important phase of the investigation concerned the soils; the character of the various classes of soil was to be determined and the distribution of these classes plotted upon a suitable map. However, no such map, sufficiently accurate and detailed, was available; and the first need was for a survey of the basin and the preparation of a relatively detailed base map. In this work a closed transit and level traverse was made; beginning and ending at a known station in the city of Mexicali, this primary line extended longitudinally through