The purpose of the two-color EDM measurements depend upon the network: Northern California: Two radial networks were set-up to measure the strain accumulation from the Gorda plate being subducted under the North American plate. These measurements were made four times over the interval between 1991 and 1996. The networks were centered at 41.9861 -123.9374 and 41.5996 -122.9337 Hollister: A single radial network of 9 baselines spanned the Calaveras fault. The goal was to document transient deformation associated with aseismic slip (creep) on that fault. The measurements were made roughly once per day. These measurements span the interval between 1975 and 1980. And the network was centered at 36.8557, -121.4069 ‘ Parkfield, CA: The goal of this network was to document the strain accumulation leading to an anticipated, M6 earthquake that historically occurs at 11-to-38-year intervals. This network has two parts, one being a radial network that was measured once per day, and two other radial networks that were measured once per year. Measurements were initiated in 1983 and ceased in 2005 following the M6 Parkfield earthquake that occurred in 2004. Currently, deformation monitoring uses a dense set of GNSS receivers that record continuously. The center of the EDM network was located 35.8881, -120.4299 Long Valley caldera: This network was located near Mammoth Lakes, eastern California. Following the combination of detection of uplift of the caldera, four M6 earthquakes in 1980, and a vigorous swarm of earthquakes in the caldera’s south moat in 1983, USGS increased the temporal and spatial monitoring of deformation of the caldera in 1983. This included using the two-color EDM to measure changes in line-length of 8 baselines that span both the south moat and the resurgent dome of the caldera. From early 1984 through the early 2000s, measurements were made nearly daily. Over time, the network was expanded to five additional, radial networks to capture the spatial distribution of deformation. Typically, these auxiliary networks were measured once per year. The EDM network was phased out in favor of a dense distribution of continuously recording GNSS receivers. Several publications document a few episodes of inflation of the caldera as well as faulting in the caldera’s south moat. Pearblossom: Following the apparent “discovery” of the so-called “Palmdale bulge”, and its ensuing controversy (https://www.nature.com/articles/283247a0.pdf), the two-color EDM was moved from the Hollister network in 1980. Initially, measurements were made daily to detect transient deformation. This radial network included several baselines that spanned the San Andreas fault, which last ruptured in 1857. Early measurements appeared to show a transient deformation, but subsequent observations over the next 20 years strongly suggested that the deformation rate was constant and that the ‘anomaly’ was likely due to monument instabilities. The center of this network was located at 34.4582, -117.8444 Buttes: This network is located 30 KM north of the Pearblossom network in the Mojave desert. The goal was to quantify off-fault strain accumulation. Measurements were done annually for 7 years. The center of this network was located at 34.6815, -117.7218 Pinon: This network spanned the Pinon Flat Observation, a location where several different geodetic techniques were compared. The primary comparison for the 2-color EDM is a three component, long-baseline, (800 meter) laser strainmeter operated by the Institute of Geophysics and Planetary Physics of UC, San Diego. Due to topographic constraints, this network could only measure baselines within a 150-degree sector rather than the typical radial distribution achieved with the other networks. Measurements of the Pinon network spanned a 14-year interval starting in 1986. Measurements were made several times each year. The network was centered at 33.5743, -116.4455 Anza: To the west of the Pinon network is the San Jacinto fault. This network spans that fault using two central sites with each consisting of eight baselines. This network was measured annually for 10 years starting in 1988. The network was centered at 33.5678, -116.5939
