Geology topics

G. King

Publications and source records attributed to G. King.

4 recordsLinked to original sources

USGS2007

Coseismic and post-seismic signatures of the Sumatra 2004 December and 2005 March earthquakes in GRACE satellite gravity

The GRACE satellite mission has been measuring the Earth's gravity field and its temporal variations since 2002 April. Although these variations are mainly due to mass transfer within the geofluid envelops, they also result from mass displacements associated with phenomena including glacial isostatic adjustment and earthquakes. However, these last contributions are difficult to isolate because of the presence of noise and of geofluid signals, and because of GRACE's coarse spatial resolution (>400 km half-wavelength). In this paper, we show that a wavelet analysis on the sphere helps to retrieve earthquake signatures from GRACE geoid products. Using a wavelet analysis of GRACE geoids products, we show that the geoid variations caused by the 2004 December ( M w = 9.2) and 2005 March ( M w = 8.7) Sumatra earthquakes can be detected. At GRACE resolution, the 2004 December earthquake produced a strong coseismic decrease of the gravity field in the Andaman Sea, followed by relaxation in the area affected by both the Andaman 2004 and the Nias 2005 earthquakes. We find two characteristic timescales for the relaxation, with a fast variation occurring in the vicinity of the Central Andaman ridge. We discuss our coseismic observations in terms of density changes of crustal and upper-mantle rocks, and of the vertical displacements in the Andaman Sea. We interpret the post-seismic signal in terms of the viscoelastic response of the Earth's mantle. The transient component of the relaxation may indicate the presence of hot, viscous material beneath the active Central Andaman Basin.

Geophysical Journal International↗

USGS1994

Block versus continuum deformation in the Western United States

The relative role of block versus continuum deformation of continental lithosphere is a current subject of debate. Continuous deformation is suggested by distributed seismicity at continental plate margins and by cumulative seismic moment sums which yield slip estimates that are less than estimates from plate motion studies. In contrast, block models are favored by geologic studies of displacement in places like Asia. A problem in this debate is a lack of data from which unequivocal conclusions may be reached. In this paper we apply the techniques of study used in regions such as the Alpine-Himalayan belt to an area with a wealth of instrumental data-the Western United States. By comparing plate rates to seismic moment release rates and assuming a typical seismogenic layer thickness of 15 km it appears that since 1850 about 60% of the Pacific-North America motion across the plate boundary in California and Nevada has occurred seismically and 40% aseismically. The San Francisco Bay area shows similar partitioning between seismic and aseismic deformation, and it can be shown that within the seismogenic depth range aseismic deformation is concentrated near the surface and at depth. In some cases this deformation can be located on creeping surface faults, but elsewhere it is spread over a several kilometer wide zone adjacent to the fault. These superficial creeping deformation zones may be responsible for the palaeomagnetic rotations that have been ascribed elsewhere to the surface expression of continuum deformation in the lithosphere. Our results support the dominant role of non-continuum deformation processes with the implication that deformation localization by strain softening must occur in the lower crust and probably the upper mantle. Our conclusions apply only to the regions where the data are good, and even within the Western United States (i.e., the Basin and Range) deformation styles remain poorly resolved. Nonetheless, we maintain that block motion is the deformation style of choice for those continental regions where the data are best.

Earth and Planetary Science Letters↗

USGS1993

Aspects of the biogeochemistry of methane in Mono Lake and the Mono Basin of California

Above-ambient levels of methane and higher hydrocarbons were detected in the atmosphere of the Mono Basin. These gases emanated from several different sources, including natural gas seeps (thermogenic and biogenic), and methanogenic activity in sediments. Seeps were distributed over nearly 33% of the lake bottom and were also present in the exposed former lakebed. They originated from one or more natural gas deposits that underlie the basin. Seeps associated with hot springs had a thermogenic character, whereas the others had the characteristics of bacterially formed gases. The radiocarbon content of methane in all seep gases was low (5-11% modern carbon), indicating an age of greater than about 20,000 years. Dissolved methane increased with depth in the sediments, ultimately reaching saturation levels (1-3 mM). The outward diffusive flux of methane into the anoxic bottom waters elevated concentrations to 60 µM during the 1984-1988 meromictic interval. The radiocarbon content of hypolimnion methane was equivalent to that of the dissolved inorganic carbon (~80% modern carbon), indicating derivation from current methanogenic activity. Oxidation by anaerobic bacteria was the major sink for this methane. Methane that escaped oxidation transited to the epilimnion where it attained supersaturation (0.1-3.0 µM) relative to the atmosphere. Measured outward fluxes over a 2-year period were highly variable, ranging from 0.01 to 4.82 mmoles CH 4 m -2 d -1 . Bacterial activity was detected in the sediments of the hypolimnion High rates of sulfate reduction (~200 µmoles L -1 d -1 ) were measured in the upper 5-cm interval but decreased > 10-fold with depth. The highest rate of methanogenic activity was ~1 µmole L -1 d -1 , as measured by reduction of 14 CO 2 . Bacteria metabolized dimethylsulfide, trimethylamine, acetate, and glucose to primarily CO 2 and some CH 4 . Turnovers of glucose (k = 0.16 d -1 ) and trimethylamine (k = 0.09 d -1 ) were highest in the upper 2.5-cm interval and decreased as much as 10-fold with depth. These results confirm that a low rate of methanogenic activity occurs in this extreme environment and is the source of methane to the hypolimnion.

California↗

USGS1990

The origin of large local uplift in extensional regions

Large localized uplift is commonly observed in continental regions undergoing extension. These observations can be modelled by planar, high-angle normal faulting of an elastic upper crust overlying an inviscid lower crust. Isostasy provides the necessary driving force. The model quantifies the role of flexural rigidity, density variations in the crust, and erosion and deposition of sediment.

Nature↗