Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations

Seismological Society of America (SSA) | August 1st, 2017

The San Andreas fault (SAF) is one of the most studied strike‐slip faults in the world; yet its subsurface geometry is still uncertain in most locations. The Salton S

The San Andreas fault (SAF) is one of the most studied strike‐slip faults in the world; yet its subsurface geometry is still uncertain in most locations. The Salton Seismic Imaging Project (SSIP) was undertaken to image the structure surrounding the SAF and also its subsurface geometry. We present SSIP studies at two locations in the Coachella Valley of the northern Salton trough. On our line 4, a fault‐crossing profile just north of the Salton Sea, sedimentary basin depth reaches 4 km southwest of the SAF. On our line 6, a fault‐crossing profile at the north end of the Coachella Valley, sedimentary basin depth is ∼2–3km∼2–3  km and centered on the central, most active trace of the SAF. Subsurface geometry of the SAF and nearby faults along these two lines is determined using a new method of seismic‐reflection imaging, combined with potential‐field studies and earthquakes. Below a 6–9 km depth range, the SAF dips ∼50°–60°∼50°–60° NE, and above this depth range it dips more steeply. Nearby faults are also imaged in the upper 10 km, many of which dip steeply and project to mapped surface fault traces. These secondary faults may join the SAF at depths below about 10 km to form a flower‐like structure. In Appendix D, we show that rupture on a northeast‐dipping SAF, using a single plane that approximates the two dips seen in our study, produces shaking that differs from shaking calculated for the Great California ShakeOut, for which the southern SAF was modeled as vertical in most places: shorter‐period (⁠T<1sT<1  s⁠) shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall, compared to shaking calculated for a vertical fault.

Tectonic evolution of the Salton Sea inferred from seismic reflection data

Nature Portfolio (Springer Nature) | July 26th, 2009

Oblique extension across strike-slip faults causes subsidence and leads to the formation of pull-apart basins such as the Salton Sea in southern California. The f

Oblique extension across strike-slip faults causes subsidence and leads to the formation of pull-apart basins such as the Salton Sea in southern California. The formation of these basins has generally been studied using laboratory experiments or numerical models. Here we combine seismic reflection data and geological observations from the Salton Sea to understand the evolution of this nascent pull-apart basin. Our data reveal the presence of a northeast-trending hinge zone that separates the sea into northern and southern sub-basins. Differential subsidence (>10 mm yr−1) in the southern sub-basin suggests the existence of northwest-dipping basin-bounding faults near the southern shoreline, which may control the spatial distribution of young volcanism. Rotated and truncated strata north of the hinge zone suggest that the onset of extension associated with this pull-apart basin began after ∼0.5 million years ago. We suggest that slip is partitioned spatially and temporally into vertical and horizontal domains in the Salton Sea. In contrast to previous models based on historical seismicity patterns, the rapid subsidence and fault architecture that we document in the southern part of the sea are consistent with experimental models for pull-apart basins.

Ten Rivers in America's Future, No. 2. The Central Valley of California

U.S. Water Resources Policy Commission | July 1st, 1950

The Alkaline and Boracic Lakes of California

Scientific American | July 7th, 1877

Immediately east of the range of the Sierra Nevada is an extensive region of alkaline lakes and hot springs, of which very large areas are almost totally barren, the onl

Immediately east of the range of the Sierra Nevada is an extensive region of alkaline lakes and hot springs, of which very large areas are almost totally barren, the only vegetation consisting of wild sage, yucca, a few cacti, and scanty tufts of bunch-grass. This district affords, in its many extensive craters and in its lavas, basalts, and obsidians, the most conclusive evidence of its volcanic origin, while its solfataras and boiling springs may be regarded as the last representatives of active volcanicity. Although this region is one of great scientific interest, and may eventually become industrially important, it appears to be but little known in this country, and it has therefore been thought that a brief description of the district, as well as that of the borax lakes, lying on the western side of the Sierra, might not be without interest to English readers. The most remarkable of the alkaline lakes of this portion of California are Mono and Owens Lakes.

The Geysers of California

Scientific American | September 28th, 1867

The Geysers of California are situated in lateral ravines of Pluton River, a tributary of Russian River. In gaining a clear idea of the California Geysers, it will be n

The Geysers of California are situated in lateral ravines of Pluton River, a tributary of Russian River. In gaining a clear idea of the California Geysers, it will be necessary to forget the geysers of Iceland, with their columns of water and capitals of cloud. Upon approaching those upon Pluton River, your first impression is that there has been a great conflagration, and that the fire engines are blowing off steam preparatory to going home. The gorge is lined with masses of smouldering ashes, from which hot steam is being drifted, by the wind, and, in some places, you can imagine that the embers are ready to relight. In the bottom of the canon, turbid and blackened water, from which vapor slowly lifts, is running among the discolored rocks. Here and there, escaping steam hisses, and, in some places, roars like the "exhaust " of an engine.

The Impact of a Severe Drought on Dust Lifting in California’s Owens Lake Area

Nature Portfolio (Springer Nature) | May 11th, 2017

Mineral dust aerosols are responsible for some of the largest sources of uncertainties in our current understanding of climate change. Here we show that a severe drought

Mineral dust aerosols are responsible for some of the largest sources of uncertainties in our current understanding of climate change. Here we show that a severe drought is having a significant impact in one of largest sources of mineral dust aerosols of the U.S., the Owens Lake area in California’s southwest. Measurements of aerosol concentration (PM2.5 particle matter) in the Owens Lake salty playa show that the annual mean concentration of PM2.5 aerosol has been increasing steadily since the beginning of the current drought, with periods of high aerosol concentration increasing from 4 months in 2013 to 9 months in 2015. Interestingly, the PM2.5 aerosol concentration usually increases abruptly from less than ~0.05 mg/m3 to ~0.25 mg/m3. This occurs when saltation events break salt crusts produced by the efflorescence of brine in the salty playa. The brine is produced by either rainfall or runoff water. Based on this observation, we hypothesize that there is an upper limit of ~0.25 mg/m3 in the annual mean PM2.5 aerosols concentration in the Owens Lake basin that might limit the impact of mineral dust aerosols on climate. Indeed, the upper annual limit of ~0.25 mg/m3 has been nearly reached during the current drought.

The Inundation of the Salton Basin by the Colorado River and How It Was Caused

Scientific American | April 14th, 1906

If the Colorado River continues to flow through the channel which it has been occupying during the last six months, the geography of the Southwest must be radically ch

If the Colorado River continues to flow through the channel which it has been occupying during the last six months, the geography of the Southwest must be radically changed, for at the present time but little water from the river reaches the Gulf of California, which until recently has formed its main estuary. Except when the river is in flood, the bulk of the water flows into what is known as the Salton Sink in southern California-a distance of fully 160 miles from the gulf.  The new channel of the Colorado takes a northwesterly course, while the channeI it formerly occupied is nearly south. It is perhaps needless to say that this watercourse is not only one of the most important in the Southwest, but is notable for the immense volume of water which it carries, especially during the flood seasons. But the quantity of detritus which it holds in solution is enormous, and is nearly equal to that carried down by the Mississippi at certain seasons of the year, owing to the topography of the region which drains into it. An idea of the quantity of water carried by the Colorado at different times of the year can be gained, when it is stated that actual measurements indicate a flow ranging as high as 30,000 cubic feet per second. It is estimated that if the silt and other material brought down the river in the course of a year were spread evenly over a given 'surface, it would cover no less than 35,000 acres to a minimum depth of one foot. Consequently, the Colorado is almost as much of a "land maker" as the Mississippi, acting like a gigantic suction dredge in carrying away the material in its vicinity and depositing it upon the lowlands near its mouth and in the Gulf of California.

The New Inland Sea

Scientific American | April 13th, 1907

The deep channel in the Alamo River, which passed Holtville in August, was gradually approaching Sharp's Heading, and it was recognized that when this cataract reached

The deep channel in the Alamo River, which passed Holtville in August, was gradually approaching Sharp's Heading, and it was recognized that when this cataract reached the heading it would be very difficult and expensive, and perhaps impossible, to maintain that heading. This, however, was not the only peril to the water supply of the valley. The channel of New River had eroded to such an extent that where the two streams separated it was estimated that four-fifths of the water was running down New River and only one fifth down the Alamo. While this proportion was favorable to the regimen of the Alamo and the safety of Sharp's Heading, it was very threatening in an other respect. It accelerated the cutting of the New River channel, in which was a great cataract four or five miles below the separation of the two streams, and this was, of course, advancing upstream. It was well recognized that when this cataract reached the Alamo the channel would be so deep that all of the water would run down New River and leave Sharp's Heading on dry land, without any water for the irrigation of the Imperial Valley. Threatened first with inundation, and next with the destruction of their entire water supply, the inhabitants of the Imperial Valley have naturally been almost in a state of panic for several months.

The Seasonal Cycle of Surface Soil Moisture

American Meteorological Society (AMS) | June 29th, 2022

The seasonal cycle contributes substantially to soil moisture temporal variability in many parts of the world, with important implications for seasonal forecasting releva

The seasonal cycle contributes substantially to soil moisture temporal variability in many parts of the world, with important implications for seasonal forecasting relevant to agriculture and the health of humans and ecosystems. There is considerable spatial variability in the seasonal cycle of soil moisture, yet a lack of global observations has hindered the development of parsimonious theories explaining that variability. Here, we use 6 years of global satellite observations to describe and explain the seasonal cycle of surface soil moisture globally. An unsupervised clustering algorithm is used to identify five distinct seasonal cycle regimes. Each seasonal cycle regime typically arises in both hemispheres, on multiple continents, and across substantially different local climates. To explain this spatial variability, we then show that the observed seasonal cycle regimes are reproduced very well by a simple but physically based water balance model, which only uses precipitation and downwelling surface shortwave radiation as inputs, and includes no free parameters. Surprisingly, no information on vegetation or land cover is required. To our knowledge, this is the first characterization of the seasonal cycle of surface soil moisture based on global observations.

Three-dimensional geologic mapping of the Cenozoic basin fill, Amargosa Desert basin, Nevada and California

U.S. Geological Survey (USGS) | July 1st, 2014

Understanding the subsurface geologic framework of the Cenozoic basin fill that underlies the Amargosa Desert in southern Nevada and southeastern California has been impr

Understanding the subsurface geologic framework of the Cenozoic basin fill that underlies the Amargosa Desert in southern Nevada and southeastern California has been improved by using borehole data to construct three-dimensional lithologic and interpreted facies models. Lithologic data from 210 boreholes from a 20-kilometer (km) by 90-km area were reduced to a limited suite of descriptors based on geologic knowledge of the basin and distributed in three-dimensional space using interpolation methods. The resulting lithologic model of the Amargosa Desert basin portrays a complex system of interfingered coarse- to fine-grained alluvium, playa and palustrine deposits, eolian sands, and interbedded volcanic units. Lithologic units could not be represented in the model as a stacked stratigraphic sequence due to the complex interfingering of lithologic units and the absence of available time-stratigraphic markers. Instead, lithologic units were grouped into interpreted genetic classes, such as playa or alluvial fan, to create a three-dimensional model of the interpreted facies data. Three-dimensional facies models computed from these data portray the alluvial infilling of a tectonically formed basin with intermittent internal drainage and localized regional groundwater discharge. The lithologic and interpreted facies models compare favorably to resistivity, aeromagnetic, and geologic map data, lending confidence to the interpretation.