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NASA NTRS1994

Controls on erosional retreat of the uplifted rift flanks at the Gulf of Suez and northern Red Sea

The Gulf of Suez and the Red Sea rigts are currently bordered by large asymmetric uplifts that are undergoing erosion. We find that the amount and timing of erosion vary systematically along the strike of the margin and have been controlled by variations in the perift stratigraphy. The perfit strata are compsoed of cliff-forming Eocene-Cretaceous carbonates overlaying the easily eroded Cretaceous-Cambrian 'Nubian' sandstone. This lithologic succession promotes scarp retreat of the sedimentary section, follwed by dissection of the underlying basement. The perift section thins from over 2000 m at the northern end of the rift to less htan 400 m at its junction with the Red Sea. Thus, at the northern part of the Gulf of Suez, the Nubian sandstone is minimally exposed, and the carbonates form a scarp at the rift border fault. Farther south, undercuttin of hte carbonates by erosion of the sandstion has resulted in scarp retreat. The escarpment cuts diagonally away from the border fault andis over 100 km inland from the border fault at the southernmost Gulf of Suez. The amount of retreat varies inversely with the sediment thickness. Exposure and erosion of basement are initiated by the retreate of the escarpment, and the depth of erosion, as indicated by fission track ages, increases with distance from the escarpment. These observations are explained by a model in which erosion along the Gulf of Suez is initiated as rift flank uplift becomes sufficiently large ot expose the friable sandstones. Undercutting the escarpment and exhumation of basement has been propagating northward and westward for at least 20 m.y. The average rate of scarp retreat has been 6 km/m.y. and the along-strike propagation of the erosion has been 12 km/m.y. The diachronous erosion of the rift flanks at the Gulf of Suez highlights the importance of distinguishing between the timing of uplift and of erosion. Both thermochronometric and stratigraphic data primarily indicate the timing of erosion, which may differ significantly form the timing of the uplift that initiates it. They must be interpreted carefully to avoid erroneous conclusions about rift tectonics.

Steckler, Michael S.↗
NASA NTRS1994

Implications of a global survey of venusian impact craters

We present a global survey of the areal distribution, size-frequency distribution, and morphometric properties of the venusian impact cratering record. We explore the resurfacing history of Venus, crater degradation, ejecta emplacement, and cratering mechanics. The number of volcanically embayed and tectonically deformed craters from 0.5 to 1.0 km above mean planetary radius is disproportionately high for an otherwise crater-deficient elevation range. More resurfacing occurred in this range, an elevation range dominated by volcanic rises, rifts, and coronae, than elsewhere on Venus. Although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets, some craters appear particularly `fresh' because thay have radar-bright floors, a radar-dark halo surrounding the ejecta blanket, and a west facing parabola of low radar return; 20, 35, and 8%, respectively, of craters with diameters greater than 22.6 km have these features. Characteristics of ejecta deposits for venusian craters change substantially with size, particularly at 20 km crater diameter, which marks the transition at which the boundaries of ejecta blankets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens. Secondary craters are a ubiquitous part of the ejecta blanket for craters over 50 km but occur infrequently as isolated rays about smaller craters. Comparison of complex craters found on Venus with those of other planets gave results that were consistent with the idea that interplanetary differences in complex crater shape are controlled by interplanetary differences in gravity and crustal strength. The interplanetary comparison indicates that Venus, the Moon, and Mercury appear to have stronger crusts than do Mars and Ganymede/Callisto.

Herrick, Robert R.↗
NASA NTRS1995

Quantitative characterization of the small-scale fracture patterns on the plains of Venus

The objectives of this research project were to (1) compile a comprehensive database of the occurrence of regularly spaced kilometer scale lineations on the volcanic plains of Venus in an effort to verify the effectiveness of the shear-lag model developed by Banerdt and Sammis (1992), and (2) develop a model for the formation of irregular kilometer scale lineations such as typified in the gridded plains region of Guinevere Planitia. Attached to this report is the paper 'A Tectonic Model for the Formation of the Gridded Plains on Guinevere Planitia, Venus, and Implications for the Elastic Thickness of the Lithosphere'.

Sammis, Charles G.↗
NASA NTRS1996

Volcanism in Eastern Africa

In 1891, the Virunga Mountains of Eastern Zaire were first acknowledged as volcanoes, and since then, the Virunga Mountain chain has demonstrated its potentially violent volcanic nature. The Virunga Mountains lie across the Eastern African Rift in an E-W direction located north of Lake Kivu. Mt. Nyamuragira and Mt. Nyiragongo present the most hazard of the eight mountains making up Virunga volcanic field, with the most recent activity during the 1970-90's. In 1977, after almost eighty years of moderate activity and periods of quiescence, Mt. Nyamuragira became highly active with lava flows that extruded from fissures on flanks circumscribing the volcano. The flows destroyed vast areas of vegetation and Zairian National Park areas, but no casualties were reported. Mt. Nyiragongo exhibited the same type volcanic activity, in association with regional tectonics that effected Mt. Nyamuragira, with variations of lava lake levels, lava fountains, and lava flows that resided in Lake Kivu. Mt. Nyiragongo, recently named a Decade volcano, presents both a direct and an indirect hazard to the inhabitants and properties located near the volcano. The Virunga volcanoes pose four major threats: volcanic eruptions, lava flows, toxic gas emission (CH4 and CO2), and earthquakes. Thus, the volcanoes of the Eastern African volcanic field emanate harm to the surrounding area by the forecast of volcanic eruptions. During the JSC Summer Fellowship program, we will acquire and collate remote sensing, photographic (Space Shuttle images), topographic and field data. In addition, maps of the extent and morphology(ies) of the features will be constructed using digital image information. The database generated will serve to create a Geographic Information System for easy access of information of the Eastem African volcanic field. The analysis of volcanism in Eastern Africa will permit a comparison for those areas from which we have field data. Results from this summer's work will permit further study and monitoring of the volcanic activity in the area. This is of concern due to the large numbers of refugees fleeing into Zaire where they are being positioned at the base of Mt. Nyiragongo. The refugees located at the base of the volcano are in direct hazard of suffocation by gas emission and destruction by lava flow. The results from this summer study will be used to secure future funding to enable continuation of this project.

Cauthen, Clay↗
NASA NTRS1996

Planetary Interiors

This report identifies two main themes to guide planetary science in the next two decades: understanding planetary origins, and understanding the constitution and fundamental processes of the planets themselves. Within the latter theme, four specific goals related to interior measurements addressing the theme. These are: (1) Understanding the internal structure and dynamics of at least one solid body, other than the Earth or Moon, that is actively convecting, (2) Determine the characteristics of the magnetic fields of Mercury and the outer planets to provide insight into the generation of planetary magnetic fields, (3) Specify the nature and sources of stress that are responsible for the global tectonics of Mars, Venus, and several icy satellites of the outer planets, and (4) Advance significantly our understanding of crust-mantle structure for all the solid planets. These goals can be addressed almost exclusively by measurements made on the surfaces of planetary bodies.

Banerdt, W. Bruce↗
NASA NTRS1997

Galileo Science Update Europa Unveiled

A five person panel discuss newly imaged photographs of the surface of Jupiter's satellite Europa. In the discussion the topics that are covered are: surface features, ice and water formation, erosion, volcanism, thermal dissipation, crustal spreading, plate tectonics, impact sites, exobiology, and life.The run time on this video is 49:48 the air date is 1/17/97.

Source record↗
NASA NTRS1996

The Surfaces of Pluto and Charon

Much of the surface of Pluto consists of high-albedo regions covered to an unknown depth by Beta-N2, contaminated with CH4, CO, and other molecules. A portion of the exposed surface appears to consist of solid H2O. The remainder is covered by lower albedo material of unknown composition. The N2 ice may occur as polar caps of large extent, leaving ices and other solids of lower volatility in the equatorial regions. The low-albedo material found primarily in the equatorial regions may consist in part of solid hydrocarbons and nitriles produced from N2 and CH4 in the atmosphere or in the surface ices. Alternatively, it may arise from deposition from impacting bodies and/or the chemistry of the impact process itself. Charon's surface is probably more compositionally uniform than that of Pluto, and is covered by H2O ice with possible contaminants or exposures of other materials that are as yet unidentified. The molecular ices discovered on Pluto and Charon have been identified from near-infrared spectra obtained with Earth-based telescopes. The quantitative interpretation of those data has been achieved through the computation of synthetic spectra using the Hapke scattering theory and the optical constants of various ices observed in the laboratory. Despite limitations imposed by the availability of laboratory data on ices in various mixtures, certain specific results have been obtained. It appears that CH4 and CO are trace constituents, and that some fraction of the CH4 (and probably the CO) on Pluto is dissolved in the matrix of solid N2. Pure CH4 probably also occurs on Pluto's surface, allowing direct access to the atmosphere. Study of the nitrogen absorption band at 2.148 micrometers shows that the temperature of the N2 in the present epoch is 40 +/-2 K. The global temperature regime of Pluto can be modeled from observations of the thermal flux at far-infrared and millimeter wavelengths. The low-albedo equatorial regions must be significantly warmer than the polar regions covered by N2 (at T = 40 K) to account for the total thermal flux measured. At the present season, the diurnal skin depth of the insolation-driven thermal wave is small, and the observed mm-wave fluxes may arise from a greater depth. Alternatively, the mm-wave flux may arise from the cool, sublimation source region. The surface microstructure in the regions covered by N2 ice is likely governed by the sintering properties of this highly volatile material. The observed nitrogen infrared band strength requires that expanses of the surface be covered with cm-sized crystals of N2. Grains of H2O ice on Charon, in contrast, are probably of order 50 micrometers in size, and do not metamorphose into larger grains at a significant rate. Because of the similarities in size, density, atmosphere and surface composition between Pluto and Neptune's satellite Triton, the surface structures observed by Voyager on Triton serve as a plausible paradigm for what might be expected on Pluto. Such crater forms, tectonic structures, aeolian features, cryovolcanic structures, and sublimation-degraded topography as are eventually observed on Pluto and Charon by spacecraft will give information on their interior compositions and structures, as well as on the temperature and wind regimes over the planet's extreme seasonal cycle.

Cruikshank, Dale P.↗
NASA NTRS1995

The Geology of Callisto

The geology of Callisto is not boring. Although cratered terrain dominates Callisto (a key end-member of the Jovian satellite system), a number of more interesting features are apparent. Cratered terrain is broken into irregular map-able bright and dark subunits that vary in albedo by a factor of 2, and several relatively smooth units are depleted of small craters. Some of these areas may have been volcanically resurfaced. Lineaments, including parallel and radial sets, may be evidence for early global tectonism. Frost deposition occurs in cold traps, and impact scars have formed from tidally disrupted comets. Geologic evidence suggests that Callisto does have a chemically differentiated crust. Central pit and central dome craters and palimpsests are common. The preferred interpretation is that a relatively ice-rich material, at depths of 5 km or more, has been mobilized during impact and exposed as domes or palimpsests. The close similarity in crater morphologies and dimensions indicates that the outermost 10 km or so of Callisto may be as differentiated as on Ganymede. The geology of cratered terrain on Callisto is simpler than that of cratered terrain on Ganymede, however. Orbital evolution and tidal heating may provide the answer to the riddle of why Callisto and Ganymede are so different (Malhotra, 1991). We should expect a few surprises and begins to answer some fundamental questions when Callisto is observed by Galileo in late 1996.

Schenk, Paul M.↗
NASA NTRS1995

Comment on 'The Global Resurfacing of Venus' by R. G. Strom, G.G. Schaber, and D.D. Dawson

The distribution of impact craters on Venus has been the subject of a great deal of analysis since the return of Magellan data. Phillips el al. (1992) performed Monte Carlo two-dimensional (2-D) modeling of the areal distribution of craters, and the results of that exercise allowed a restricted, but still quite large, range of possible planetary resurfacing histories, including the possibility that the crater, were emplaced on a geologically inactive planet. However, the nonrandom distribution of embayed and deformed craters (Phillips el al., 1992), the hypsometric distribution of craters (Herrick and Phillips, 1994), the varied degradation states of craters (Izenberg et al., 1994), their nonrandom distribution with different geologic terrain types (Namiki and Solomon, 1994; Price et al, 1994), and three-dimensional resurfacing modeling (Bullock el al., 1993) all seem to argue against that particular possibility. In contrast, Strom el al. (1994) have collected a refined and more comprehensive data set of impact features, and they input these data into more sophisticated 2-D Monte Carlo modeling and statistical analyses of the areal distribution of craters, the hypsometric distribution of craters, and the number of embayed craters. They concluded that 'Venus experienced a global resurfacing event about 300 m.y. ago followed by a dramatic reduction of volcanism and tectonism. This global resurfacing event ended abruptly (less than 10 m.y.). The present crater population has accumulated since then and remains largely intact . . . only about 4%-6% of the planet has been volcanically resurfaced since the global event . . .' If these conclusions are well-founded, this work certainly represents a significant advancement in restricting tile number of plausible resurfacing histories for the planet. If Strom et al. (1994) are correct, it would also mean that all of the other aforementioned works are in error to various degrees, or at least represent overzealous interpretation of the data. However, we have identified apparent flaws in the observations, modeling, and interpretations presented by Strom el al. (1994) that lead us to question whether their conclusions are warranted. We limit our comments to three areas of their analysis: (1) observations pertaining to the number and area of disrupted and pristine craters and crater-related features, (2) modeling of the areal and elevation distribution of craters, and (3) interpretations of resurfacing models.

Herrick, Robert R.↗
NASA NTRS1997

Global Plate Velocities from the Global Positioning System

We have analyzed 204 days of Global Positioning System (GPS) data from the global GPS network spanning January 1991 through March 1996. On the basis of these GPS coordinate solutions, we have estimated velocities for 38 sites, mostly located on the interiors of the Africa, Antarctica, Australia, Eurasia, Nazca, North America, Pacific, and South America plates. The uncertainties of the horizontal velocity components range from 1.2 to 5.0 mm/yr. With the exception of sites on the Pacific and Nazca plates, the GPS velocities agree with absolute plate model predictions within 95% confidence. For most of the sites in North America, Antarctica, and Eurasia, the agreement is better than 2 mm/yr. We find no persuasive evidence for significant vertical motions (less than 3 standard deviations), except at four sites. Three of these four were sites constrained to geodetic reference frame velocities. The GPS velocities were then used to estimate angular velocities for eight tectonic plates. Absolute angular velocities derived from the GPS data agree with the no net rotation (NNR) NUVEL-1A model within 95% confidence except for the Pacific plate. Our pole of rotation for the Pacific plate lies 11.5 deg west of the NNR NUVEL-1A pole, with an angular speed 10% faster. Our relative angular velocities agree with NUVEL-1A except for some involving the Pacific plate. While our Pacific-North America angular velocity differs significantly from NUVEL-1A, our model and NUVEL-1A predict very small differences in relative motion along the Pacific-North America plate boundary itself. Our Pacific-Australia and Pacific- Eurasia angular velocities are significantly faster than NUVEL-1A, predicting more rapid convergence at these two plate boundaries. Along the East Pacific Pise, our Pacific-Nazca angular velocity agrees in both rate and azimuth with NUVFL-1A.

Larson, Kristine M.↗
NASA NTRS1995

Global Stratigraphy of Venus: Analysis of a Random Sample of Thirty-Six Test Areas

The age relations between 36 impact craters with dark paraboloids and other geologic units and structures at these localities have been studied through photogeologic analysis of Magellan SAR images of the surface of Venus. Geologic settings in all 36 sites, about 1000 x 1000 km each, could be characterized using only 10 different terrain units and six types of structures. These units and structures form a major stratigraphic and geologic sequence (from oldest to youngest): (1) tessera terrain; (2) densely fractured terrains associated with coronae and in the form of remnants among plains; (3) fractured and ridged plains and ridge belts; (4) plains with wrinkle ridges; (5) ridges associated with coronae annulae and ridges of arachnoid annulae which are contemporary with wrinkle ridges of the ridged plains; (6) smooth and lobate plains; (7) fractures of coronae annulae, and fractures not related to coronae annulae, which disrupt ridged and smooth plains; (8) rift-associated fractures; and (9) craters with associated dark paraboloids, which represent the youngest 1O% of the Venus impact crater population (Campbell et al.), and are on top of all volcanic and tectonic units except the youngest episodes of rift-associated fracturing and volcanism; surficial streaks and patches are approximately contemporary with dark-paraboloid craters. Mapping of such units and structures in 36 randomly distributed large regions (each approximately 10(exp 6) sq km) shows evidence for a distinctive regional and global stratigraphic and geologic sequence. On the basis of this sequence we have developed a model that illustrates several major themes in the history of Venus. Most of the history of Venus (that of its first 80% or so) is not preserved in the surface geomorphological record. The major deformation associated with tessera formation in the period sometime between 0.5-1.0 b.y. ago (Ivanov and Basilevsky) is the earliest event detected. In the terminal stages of tessera fon-nation, extensive parallel linear graben swarms representing a change in the style of deformation from shortening to extension were formed on the tessera and on some volcanic plains that were emplaced just after, and perhaps also during the latter stages of the major compressional phase of tessera emplacement. Our stratigraphic analyses suggest that following tessera formation, extensive volcanic flooding resurfaced at least 85% of the planet in the form of the presently-ridged and fractured plains. Several lines of evidence favor a high flux in the post-tessera period but we have no independent evidence for the absolute duration of ridged plains emplacement. During this time, the net state of stress in the lithosphere apparently changed from extensional to compressional, first in the form of extensive ridge belt development, followed by the formation of extensive wrinkle ridges on the flow units. Subsequently, there occurred local emplacement of smooth and lobate plains units which are presently essentially undefortned. The major events in the latest 10% of the presently preserved history of Venus (less than 50 m.y. ago) are continued rifting and some associated volcanism, and the redistribution of eolian material largely derived from impact crater deposits.

Basilevsky, Alexander T.↗
NASA NTRS1993

Preliminary Stratigraphic Basis for Geologic Mapping of Venus

The age relations between geologic formations have been studied at 36 1000x1000 km areas centered at the dark paraboloid craters. The geologic setting in all these sites could be characterized using only 16 types of features and terrains (units). These units form a basic stratigraphic sequence (from older to younger: (1) Tessera (Tt); (2-3) Densely fractured terrains associated with coronae (COdf) and in the form of remnants among plains (Pdf); (4) Fractured and ridged plains (Pfr); (5) Plains with wrinkle ridges (Pwr); (6-7) Smooth and lobate plains (Ps/Pl); and (8) Rift-associated fractures (Fra). The stratigraphic position of the other units is determined by their relation with the units of the basic sequence: (9) Ridge bells (RB), contemporary with Pfr; (10-11) Ridges of coronae and arachnoids annuli (COar/Aar), contemporary with wrinkle ridges of Pwr; (12) Fractures of coronae annuli (COaf) disrupt Pwr and Ps/Pl; (13) Fractures (F) disrupt Pwr or younger units; (14) Craters with associated dark paraboloids (Cdp), which are on top of all volcanic and tectonic units except the youngest episodes of rift-associated fracturing and volcanism; (15-16) Surficial streaks (Ss) and surficial patches (Sp) are approximately contemporary with Cdp. These units may be used as a tentative basis for the geologic mapping of Venus including VMAP. This mapping should test the stratigraphy and answer the question of whether this stratigraphic sequence corresponds to geologic events which were generally synchronous all around the planet or whether the sequence is simply a typical sequence of events which occurred in different places at diffferent times.

Basilevsky, A. T.↗
NASA NTRS1996

Detecting Amino Acids on Mars

Understanding the events that led to the origin of life on Earth is complicated by the lack of geological evidence from the period around four billion years (4 Gyr) ago when the transition from prebiotic chemistry to biochemistry is believed to be occurred. Although erosion and plate tectonics have since erased the terrestrial geological record from the time of the origin of life, there is possibility that information about this period of Earth history may still be preserved on Mars. A major goal of the NASA Space Exploration Program is to search for evidence of abiotic chemistry and extinct or extant life on Mars. During the next decade, spacecraft will orbit Mars, land on the surface, and return with surface samples for analysis. The question is what compounds should we search for, either directly on the planet or in samples returned to Earth, that will answer unambiguously whether abiotic and/or biotic organic molecules are present.

Bada, Jeffrey L.↗
NASA NTRS1995

Geology and landscape evolution of the Hellas region of Mars

Hellas basin on Mars has been the site of volcanism, tectonism, and modification by fluvial, mass-wasting, and eolian processes over its more than 4-b.y. existence. Our detailed geologic mapping and related studies have resulted in the following new interpretations. The asymmetric distribution of highland massifs and other structures that define the uplifted basin rim suggest a formation of the basin by the impact of a low-angle bolide having a trajectory heading S 60 deg E. During the Late Noachian, the basin was infilled, perhaps by lava flows, that were sufficiently thick (>1 km) to produce wrinkle ridges on the fill material and extensional faulting along the west rim of the basin. At about the same time, deposits buried northern Malea Planum, which are interpreted to be pyroclastic flows from Amphitrites and Peneus Paterae on the basis of their degraded morphology, topography, and the application of a previous model for pyroclastic volcanism on Mars. Peneus forms a distinctive caldera structure that indicates eruption of massive volumes of magma, whereas Amphitrites is a less distinct circular feature surrounded by a broad, low, dissected shield that suggests generally smaller volume eruptions. During the Early Hesperian, an approx. 1- to 2-km-thick sequence of primarily fined-grained, eolian material was deposited on the floor of Hellas basin. Subsequently, the deposit was deeply eroded, except where armored by crater ejecta, and it retreated as much as 200-300 km along its western margin, leaving behind pedestal craters and knobby outliers of the deposit. Local debris flows within the deposit attest to concentrations of groundwater, perhaps in part brought in by outflow floods along the east rim of the basin. These floods may have deposited approx. 100-200 m of sediment, subduing wrinkle ridges in the eastern part of the basin floor. During the Late Hesperian and Amazonian, eolian mantles were emplaced on the basin rim and floor and surrounding highlands. Their subsequent erosion resulted in pitted and etched plains and crater fill, irregular mesas, and pedestal craters. Local evidence occurs for the possible former presence of ground ice or ice sheets approx. 100 km across; however, we disagree with a hypothesis that suggests that the entire south rim and much of the floor of Hellas have been glaciated. Orientations of dune fields and yardangs in lower parts of Hellas basin follow directions of the strongest winds predicted by a recently published general circulation model (GCM). Transient frost and dust splotches in the region are, by contrast, related to the GCM prediction for the season in which the images they appear in were taken.

Tanaka, Kenneth L.↗
NASA NTRS1997

Crustal deformation in southern California using SAR interferometry

The rate of slow deformation processes along faults activated during the Landers 1992 earthquake was investigated. The analysis was performed by combining pairs of ERS-1/2 synthetic aperture radar (SAR) of Southern California (U.S.). The interferograms revealed several centimeters of post-seismic rebound in step-overs of the 1992 break. The southern branches of the 1992 break experienced surface creep, producing sharp phase cuts in the interferometric maps. The same approach was used in the Los Angeles (CA) basin. The tectonic signal in the interferograms of the Los Angeles basin was intermingled with signals due to ground subsidence caused by oil and water withdrawal.

Peltzer, G.↗
NASA NTRS1998

Permanent GPS Geodetic Array in Southern California

The southern California Permanent GPS Geodetic Array (PGGA) was established in the spring of 1990 to evaluate continuous Global Positioning System (GPS) measurements as a new too] for monitoring crustal deformation. Southern California is an ideal location because of the relatively high rate of tectonic deformation, the high probability of intense seismicity, the long history of conventional and space geodetic measurements, and the availability of a well developed infrastructure to support continuous operations. Within several months of the start of regular operations, the PGGA recorded far-field coseismic displacements induced by the June 28, 1992 (M(sub w)=7.3), Landers earthquake, the largest magnitude earthquake in California in the past 40 years and the first one to be recorded by a continuous GPS array. Only nineteen months later, on 17 January 1994, the PGGA recorded coseismic displacements for the strongest earthquake to strike the Los Angeles basin in two decades, the (M(sub e)=6.7) Northridge earthquake. At the time of the Landers earthquake, only seven continuous GPS sites were operating in southern California; by the beginning of 1994, three more sites had been added to the array. However, only a pair of sites were situated in the Los Angeles basin. The destruction caused by the Northridge earthquake spurred a fourfold increase in the number of continuous GPS sites in southern California within 2 years of this event. The PGGA is now the regional component of the Southern California Integrated GPS Network (SCIGN), a major ongoing densification of continuous GPS sites, with a concentration in the Los Angeles metropolitan region. Continuous GPS provides temporally dense measurements of surface displacements induced by crustal deformation processes including interseismic, coseismic, postseismic, and aseismic deformation and the potential for detecting anomalous events such as preseismic deformation and interseismic strain variations. Although strain meters yield much higher short-term resolution to a period of about 1 year, a single continuous GPS site is significantly less expensive than a single strain meter and probably has better long-term stability beyond a 1-year period. Compared to less frequent field measurements, continuous GPS provides the means to better characterize the errors in GPS position measurements and thereby obtain more realistic estimates of derived parameters such as site velocities.

Green, Cecil H.↗
NASA NTRS1996

Transient Creep of a Composite Lower Crust: A Polymineralic Basis for Rapidly Evolving Postseismic Deformation Modes - 2

Postseismic horizontal strain and displacement following the June 28, 1992, Landers, California, earthquake (M(sub W) 7.3) is broad scale and cannot be explained solely by delayed afterslip located at the rupturing fault trace. Both the observed strain at Pifion Flat Observatory (PFO) and observed Global Positioning System receiver velocities evolve rapidly after the Landers-Big Bear earthquake sequence. The observed exponential decay of these motions, with timescales of 4-34 days, may reflect a soft creep rheology in the lower crust and brittle-ductile transition zone or even within the seismogenic crust itself. Here a simple model of a two-dimensional screw dislocation in a layered Maxwell viscoelastic Earth is employed in conjunction with a composite rheology to demonstrate that the short timescale transient response modes (approx. = 4-34 days) are consistent with the behavior of a biviscous lower crust. The lowest viscosity of this system is derivable from laboratory experimental data on the long-term creep of natural quarztites, and the highest viscosity is consistent with isostasy-related lower crustal flow in a continental extensional tectonic environment. The model predicts significant stress relaxation at the base of the seismogenic crust. Near the base of the seismogenic zone, and about 4 km away from the mainshock, the rate of predicted relaxation is of the order of 0.01 MPa/ d during the first 20 days of postseismic flow. Oblate spheroidal inclusions at 5% concentration levels that are both aligned and fairly flat in shape and that have a viscosity of 3-4 x 10(exp 15) Pa s are consistent with both the amplitude and decay time of horizontal crustal strain observed at PFO after the Landers mainshock. It is speculated that the structures exposed in cross sections and in seismic reflection profiles of the lower crust that have mylonitic associations are, in part, the cause of such rapid postseismic evolution in southeastern California. Unmylonitized quartz-rich rock at sufficiently elevated temperatures could also contribute to the rapid decay modes.

Ivins, Erik R.↗
NASA NTRS1996

Global Seismicity: Three New Maps Compiled with Geographic Information Systems

This paper presents three new maps of global seismicity compiled from NOAA digital data, covering the interval 1963-1998, with three different magnitude ranges (mb): greater than 3.5, less than 3.5, and all detectable magnitudes. A commercially available geographic information system (GIS) was used as the database manager. Epicenter locations were acquired from a CD-ROM supplied by the National Geophysical Data Center. A methodology is presented that can be followed by general users. The implications of the maps are discussed, including the limitations of conventional plate models, and the different tectonic behavior of continental vs. oceanic lithosphere. Several little-known areas of intraplate or passive margin seismicity are also discussed, possibly expressing horizontal compression generated by ridge push.

Lowman, Paul D., Jr.↗