Hey fellow researchers, sorry for the gap in posting new info. My GPS base station, which was out for repairs came back and I've been catching up on
some field work that pays the bills for old AC. I wanted to follow closely the horizontal velocity vectors with the vertical velocity vectors and I
probably could have directly done that. I just wanted to add a few comments from my own personal knowledge regarding vertical accuracy pertaining to
GPS, DGPS, and CGPS.
Vertical accuracy, like horizontal accuracy can be determined various ways, however when speaking of verticle accuracy, we must be sure what datum we
are dealing with and if a Geoid Model is being used. When we refer to the "Z" aspect of an X.Y,Z coordinate, we can be refering to a few different
things depending on what our needs are. Elevation is usually referenced to mean sea level which isn't exactly the true sea level as that varies from
lunar cycle to lunar cycle...its just referenced to an average. HEA or height above elipsoid assumes the Earths shape to be constant and roughs in a
elipse using the average terrain. HEA is the difference between the sat reading and the elipsoid heght. A Geiod height, which I utilize for survey
GPS (Conus 99) allows a more accurate local terrain model and allows one to upload a reference into the data collector which uses that as its model.
With that explained a little, I'll go on with the data from UofU which is collected with carrier phase and thus has an error of 25% in some cases.
"While subsidence of the Snake River Plain over its 16 Ma history reflects the long-term deformation pattern of that region, the Yellowstone Plateau
has been the site of rapid crustal deformation in historic time. Pelton and Smith (1982) first documented the historical uplift of the Yellowstone
caldera of up to 76 cm based upon repeated 1st-order precision leveling in 1975-76-77 at benchmarks originally surveyed in 1923. From the mid-1970's
to 1984, leveling surveys revealed an additional 25 cm of uplift. However, by 1985 the deformation had reversed to subsidence, and the subsidence had
exceeded ~20 cm by 1995 (Dzurisin and Yama#a, 1987; Meertens and Smith, 1995).
Models of the uplift show that its depth extended from the surface to 9 km in depth. For the uplift data, a maximum volume change of 0.218 km³, in the
3 km to 6 km depth range was determined in the southern caldera and a second source of 0.185 km³ volume increase located in the northeast caldera. The
total volume change for the 1923 to 1977 data totaled ~0.73 km³ and was attributed to migration of magmas and/or hydrothermal fluids into the upper
crust. This corresponds to an inflation rate of 0.01 km³/yr to 0.03 km³/yr from magmatic fluids intruded into the upper crust.
Causative mechanisms for the caldera-wide uplift, summarized by Pelton and Smith (1982), included magmatic, tectonic, and glacial-isostatic sources,
but these authors suggested that the most likely source of the 1923-1977 uplift was by transport of magma. Dzurisin and others (1990) concluded that
basaltic intrusions into the mid- or upper crust or pressurization of a deep hydrothermal system by magmatic gas, or brine, released by
crystallization of a rhyolitic melt were also plausible sources for the uplift. "
Following the 50-year episode of historical uplift, deformation abruptly changed to subsidence of up to 6 cm between 1984 and 1985 (Dzurisin and
others, 1990), and subsidence has continued to 1991. Meertens and others (1992) reported subsidence of up to 7 cm for the period 1987 to 1991 over the
entire caldera using GPS (Global Positioning Satellites) corresponding to the same general area which experienced uplift from 1923 to 1977. The
spatial correlation between the area of uplift and the area of subsidence suggests a causative mechanism related to magmatic/hydrothermal intrusion
followed by fluid migration or degassing from the same source. The 1923-1977 uplift phase of the Yellowstone caldera is postulated to have resulted by
melts and/or hydrothermal fluid intrusion at depths of 3 to 6 km upper crust. These fluids may have in turn produced excess hydrothermal fluid/gases
in a shallow overlying layer which then degassed and/or experienced a reduced rate of hydrothermal fluid input, producing the observed subsidence from
Evidence of a widespread magmatic connection between the Yellowstone caldera and the Hebgen Lake fault was recently suggested by Savage and others
(1993) who modeled the strain field of the Hebgen Lake trilateration network for the period 1973-1987. The calculated uniaxial extension was 0.27
µstrains at an azimuth of 015°. Although these data can be explained by dislocation on a southward-dipping normal fault at Hebgen Lake, the data imply
significant deformation north of the surface projection of the Hebgen Lake fault. Savage and others (1993) suggested an alternate mechanism related to
magmatism, namely inflation of a vertical dike and accompanying rift that extends west-northwest ~100 km from the Yellowstone caldera to the Hebgen
Lake fault zone. This is a provocative interpretation of the crustal deformation, but it provides a tie between tectonic and magmatic mechanisms
across a large area of the Yellowstone Plateau.
Nonetheless, we believe that the of migration of melts and/or hydrothermal fluids into the mid- and upper-crust of the Yellowstone caldera is the most
plausible explanation for the modern crustal deformation. This mechanism must have been important over much of the 2 Ma volcanic history of
Yellowstone, and magmatism must have been the principal mechanism in the development of the resurgent domes. The modeled volume changes of
Yellowstone's historic crustal deformation are consistent with intrusion of hydrothermal fluids and rhyolitic/basaltic melts into the upper crust as
a likely mechanism. "
Just to add a little more fuel to kukla's fire about insufficient aftershocks, I found this statement regarding the Hedgon Lake network.
"Repeated GPS surveys of all or portions of the Hebgen Lake network between 1987-1993 revealed ongoing horizontal velocities at high rates of 3 to 5
mm/yr. extension in a N-NE direction. Remarkably, horizontal deformation has continued for several decades following the 1959 Hebgen Lake earthquake.
We note that there are insufficient aftershocks to match the equivalent seismic moment for this anomalously high rate. In contrast, no significant
vertical motion is apparent in the GPS measurements"
[Edited on 1-10-2003 by astrocreep]