Dramatic Cross Section through a Caldera and its Inferred Subjacent Magma Chamber

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This diagram represents a cross-section of a caldera with vivid colorization. Note that it has no vertical scale exaggeration (except for the oil well!). A number of the details have been added by the artist to represent things whose actual details are unknown, yet there is confidence that they are present in some form, e.g., the various fissures.

FIGURE 7. Cross Section through Long Valley Caldera and Its Inferred Subjacent Magma Chamber. Long Valley Calcfera was produced by eruption of Bishop Tuff 730,000 years ago, which fills the caldera above the subsided Sierran basement block. Partial emptying of the upper part of the chamber during these eruptions paused collapse of the roof of the chamber and sinking of the Sienan block about 2000 m into the magma below. Subsequent renewed magma pressure caused arching of the caldera floor to form the resurgent dome about 700,00 years ago. Since then the magma chamber has been cooling and crystallizing from the walls inward (red), gradually reducing the size of the partially molten chamber (yellow-orange). Recent (1979-1985) renewed rise of the resurgent dome (0.5 m) and surrounding earthquake activity indicates renewed filling of the chamber with magma from depth and possibly injection of a dike along the caldera ring fracture below the south moat. The shallower cupola (hump) of magma on the right, based on Sanders's seismic attenuation studies, appears to be the center of the current uplift. The heavier lines at the upper left and lower right margins of the contemporary magma chamber mark the location of seismic phase transitions detected by Hill, Leutgert, and colleagues. (Cross section prepared by Roy Bailey and Dave Hill, U.S. Geological Survey, Apr. 30, 1985. Publication by permission of the American Geophysical Union.)

Early in 1980 Alan Ryall, taking note of the 1978 magnitude-5.7 quake and subsequent swarms, predicted that a large quake could be expected in the region within the foreseeable future. In May of 1980 four quakes of magnitude 6 occurred within hours, followed by a magnitude 5.9 in September 1981, and three magnitude-5.0-5.2 quakes in January 1983 within a swarm of thousands of smaller quakes. Repeated geodetic surveys of the region have shown that the resurgent dome has been stretching laterally and has been lifted vertically approximately 50 cm since 1979. Jim Savage and Malcolm Clark of the Geological Survey have mathematically modeled the swelling as due to expansion of a spherical chamber at a depth of l0-11 km beneath the resurgent dome by the injection of 0.15 cubic km of magma (see Figure 7).

Seismology has burgeoned into a modern science -- force-fed by federal funding to advance technology for detecting underground nuclear explosions and predicting earthquakes, and by industry to improve tools for gas and oil exploration. Computers, seismic instrument systems, telemetry, and data reduction have played key roles in this growth.

M. R. Raugh, "Modeling California Earthquakes and Earth Structures", Communications of the ACM, Volume 28, Number 11, Pages 1130-1150 (November 1985).

Full paper (PDF)

Document contributed by Xiang Rong Mei and assembled by Colin Rego, Northeastern University, 3/2004. The text in this article had been obtained by OCR and had errors which we have attempted to correct in the excerpts here (RPF).

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