bluuebunny/crossref_metadata_2025 huggingface dataset:
On the relationship between mechanical and hydraulic apertures in rough‐walled fractures
Authors: Renshaw Carl E.
Published: February 2004

Accurate analyses of the hydrogeology of fractured rock require an understanding the flow characteristics of single fractures. It is well known that these flow characteristics are strongly controlled by fracture apertures. Recent investigations on the distribution of apertures in natural fractures suggest that the cubic law can accurately predict the fluid flux through rough‐walled fractures as long as the appropriate average fracture aperture is used. Combining the stochastic cubic law with a simple deformation model results in a nonlinear relationship between fracture hydraulic and mechanical aperture. This relationship is shown to be consistent with published experimental and numerical data above a critical minimum aperture. Below this minimum aperture, the transmissivity of the fracture is approximately constant. Results have implications for the interpretation of laboratory fracture flow data and raise important questions pertaining to the mechanics of fracture deformation below the critical minimum aperture.
Diffuse impact of the Mid‐Atlantic Ridge with the Romanche transform: an ultracold ridge‐transform intersection
Authors: Bonatti E. , Ligi M. , Carrara G. , Gasperini L. , Turko N. , Perfiliev S. , Peyve A. , Sciuto P. F.
Published: February 2004

The Romanche is a long offset (∼950 km), slow slip (∼1.7 cm/yr) transform; thus a hot ridge axis should meet a ∼50‐m.y.‐old, thick and cold lithosphere at the ridge‐transform intersection (RTI). A strong thermal/topographic “transform cold edge effect” is therefore predicted. A morphobathymetric, seismic reflection and petrologic study of the eastern Romanche RTI shows that as the Mid‐Atlantic Ridge approaches the transform, a well‐formed axial rift valley disappears about 80 km from the RTI and is substituted by short en echelon, poorly developed axial ridge segments; they too disappear about 30 km from the edge of the transform valley. The predicted gradual deepening of the ridge axis toward the transform was not observed. An active nodal deep and an “inside corner high” are also absent. These observations, and the distribution of earthquake epicenters, suggest a poorly developed, diffuse RTI. An inactive rift valley ∼80 km west of the present RTI suggests ridge jumping within the last ∼4 m.y. The present poorly developed RTI may reflect the attempts of an embryonic spreading axis to become established and to propagate toward the transform. We infer from bottom rock sampling that the basaltic crust is patchy or absent and mantle‐derived serpentinized peridotites outcrop ubiquitously on the seafloor starting ∼30 km from the edge of the transform valley. The unusually deep (∼4 km below sea level) axial ridge segments, the lack of crust, and the chemistry of the peridotites suggest a prevalently amagmatic regime due to an ultracold upper mantle in this region. Absence of basaltic crust would favor massive serpentinization of a several kilometers thick peridotite column. Mass balance modeling suggests that the decrease of density and volume expansion resulting from serpentinization could explain the absence of the predicted deepening of the seafloor as it approaches the transform. These results suggest that the topographic effect of the transform edge thermal contrast may disappear at ultracold RTIs and that ultracold RTIs are magma starved, short lived, and unstable in time and space.
Morphology of pyroclastic cones and tectonics
Authors: Tibaldi Alessandro
Published: February 2004

The relationships between morphology and spatial distribution of 1315 Quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings: Tepic Rift (Mexico), Ethiopian Rift, Mexican Volcanic Belt, Canary Archipelago, and Mount Etna. Field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone. The strike of the feeding plane is directly related to: (1) the elongation of cone base and crater, (2) the location of depressions on the crater rim, and (3) the alignment of pyroclastic cones in relation to a given vent spacing. In addition, the strike and dip of faults affect the direction of cone breaching. These relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9° and are especially sensitive to fault escarpment and cone height, lava and cone density, and fault orientation with respect to the dip of the volcanic substrate topography. Relations 1 and 2 become more pronounced for regions undergoing extensional tectonics, where edifices also have a larger dimension. Whereas breaching in the direction of the fault dip is more widespread in regions under extension, breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics.
Style and rate of Holocene slip, Palos Verdes fault, southern California
Authors: McNeilan Thomas W. , Rockwell Thomas K. , Resnick Gregory S.
Published: February 2004

Recent studies of the Palos Verdes fault in the Los Angeles Outer Harbor characterize the fault's Holocene activity. The studies included areal and local geophysical surveys, as well as drilling, sampling, and dating of the subsurface materials. The studies identify the fault's location in the shallow subsurface, document evidence of Holocene strike‐slip, quantify dip‐slip versus sediment age, and quantify the net slip where an early Holocene paleochannel crosses the fault. The slip rate for the Palos Verdes fault is constrained to between 2.7 and 3 mm/yr for the past 7.8 to 8 ka. Because the new evidence shows that the Palos Verdes fault has one of the highest slip rates in the Los Angeles basin, the studies provide significant new insight into the regional tectonics and the seismic risk of the Los Angeles region. Using this rate of slip and segmentation models for the fault, the coastal Los Angeles region can expect an MW7 to 7.2 event about every 400 to 900 years from this potential seismic source.
Increased mantle convection during the mid‐Cretaceous: A comparative study of mantle potential temperature
Authors: Ricciardi Karen , Abbott Dallas
Published: February 2004

Mantle convection patterns of the past are not well known, yet an understanding of changing mantle convection characteristics is fundamental to understanding the evolution of plate tectonics. There are very few ways to examine mantle characteristics of the past. Changes in spreading rate and volcanic activity with time have been used to draw conclusions about historic changes in mantle activity. Mantle temperature has been found to be related to crustal thickness. With this relationship, crustal thicknesses may now yield new conclusions about historic changes in mantle characteristics. We have inferred changes in mantle convection patterns throughout the last 180 m.y. by examining variations in assumed crustal thickness within the Pacific basin. Crustal thicknesses were calculated from residual depth anomalies by assuming that residual depth anomalies are the result of isostatic compensation of variations in crustal thickness. Crustal thickness is determined at the time of crustal formation and is dependent upon the temperature of the mantle source material. Intraplate hot spot volcanism effects on crustal thickness were not ignored. Examination of variations in crustal thickness of crust of different ages can reveal information about changing temperatures of the mantle at the ridge through time. We have learned that mantle temperatures at the ridge during the mid‐Cretaceous were more variable than those temperatures at the ridge after the mid‐Cretaceous. Furthermore, we have inferred from the data that mantle temperatures at hot spots were higher during the mid‐Cretaceous than those at hot spots existing after the mid‐Cretaceous. We suggest that mantle convection at the ridge was more rapid during the mid‐Cretaceous causing a higher variability of temperatures at the ridge. We also note that this period of increased mantle convection is concurrent with the increased mantle temperatures at hot spots within the Pacific basin.
No tectonic rotation of the Tuscan Tyrrhenian margin (Italy) since Late Messinian
Authors: Mattei M. , Kissel C. , Funiciello R.
Published: February 2004

Extensive paleomagnetic sampling of 38 late Miocene and Pliocene sites has been carried out in the western Tuscan basins in Italy. These basins are located along the Tyrrhenian margin, and the studied formations were deposited at the same time as the opening of the Tyrrhenian Sea. The laboratory analyses show that, in most of the sites, pseudo‐single‐domain magnetites are the main magnetic carrier. The paleomagnetic data obtained from 27 suitable sites indicate that the Tuscan Tyrrhenian margin has not been affected by significant regional rotation since at least the late Messinian‐early Pliocene (about 5.5 m.y.). These data demonstrate that the rotations previously measured in the Meso‐Cenozoic units of the Apennines and of the Apulia foreland cannot be directly related to the late Messinian‐Pliocene phases of rifting in the Tyrrhenian Sea. Moreover, the data presented here conflict with published results from coeval formations in the Apennines and Sicily, in areas subjected to strong thrusting and shortening. In these areas the rotations control the orientation of the main compressional structures and give rise to the typical arcuate shape of the Apenninic chain. We suggest that the tectonic evolution of the northern Tyrrhenian Sea‐Apenninic chain system involved thrusting and shortening of the external (eastern) domains, with rotations of the allochthonous units, followed by gravitational collapse of the western sector. This second phase involved no tectonic rotation of the newly formed extensional basins.
Thermal modeling of the southern Alaska subduction zone: Insight into the petrology of the subducting slab and overlying mantle wedge
Authors: Ponko Stefan C. , Peacock Simon M.
Published: February 2004

In order to predict metamorphic conditions near the slab/mantle‐wedge interface and possible melting relations beneath the volcanic arc in southern Alaska, we have constructed a two‐dimensional kinematic thermal model of the southern Alaska subduction zone that includes an abrupt change in slab dip at 40‐km depth, time‐dependent convergence rate, and variable shear heating. The numerical results indicate that the shallow‐dipping slab segment in the southern Alaska subduction zone increases slab‐surface temperatures up to 180 ° C at depths < 40 km and up to 90 °C at greater depths. Shear heating may increase slab‐surface temperatures an additional 140 °C at depths < 40 km and 70 °C at greater depths. Calculated pressure‐temperature (PT) paths indicate that oceanic crust subducting beneath the southern Alaska forearc is undergoing blueschist‐facies metamorphism. Calculated slab PT paths do not intersect the wet basaltic solidus for shear stresses ≤ 10% lithostatic pressure, suggesting that observed arc magmas are not derived from partial melting of subducting oceanic crust. The mantle wedge adjacent to the subducting slab lies at suosolidus conditions, and at depths < 75 km, numerous hydrous phases are stable including serpentine, talc, amphibole, and chlorite. Low P wave velocities observed in the mantle wedge adjacent to the subducting slab probably reflect extensive hydration of mantle‐wedge peridotite caused by fluids liberated from the subducting oceanic crust and sediments. Calculated PT conditions in the core of the mantle wedge permit partial melting if H2O is present either as a free fluid phase or fixed in amphibole.
Tholeiitic‐alkalic transition at subglacial volcanoes, Tuya region, British Columbia, Canada
Authors: Moore James G. , Hickson Catherine J. , Calk Lewis C.
Published: February 2004

Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volcanoes in the Stikine volcanic belt of northern British Columbia. The volcanoes rise 700, 500, and 400 m above their bases and are about 3.2, 1.6, and 2.6 km3 in volume, respectively. They began eruptive activity under several hundred meters of overlying glacial ice, or water in an ice‐impounded lake, and undegassed pillow lava was erupted and forms the bases of all three. Later, as the vents grew into shallow water, explosive phreatomagmatic activity erupted partly degassed glassy tuffs. Finally, when the volcano emerged through the surface of the ice or water (or the water was drained), degassed subaerial lava flows were erupted and were converted to assemblages of foreset‐bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes. The undegassed subglacial pillow base of Ash Mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes; at South Tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava; at Tuya Butte the pillow base is overlain by foreset‐bedded pillow lava, pillow breccias, and hyaloclastite tuffs, which in turn are overlain by subaerial lava flows composing a small shield volcano. The undegassed basal subglacial pillow lava of the three volcanoes contain 0.10 ± 0.01 wt % sulfur and ∼0.5 wt % H2O. The overlying partly degassed assemblages contain 0.06 ± 0.02% sulfur and ∼0.2% H2O at Ash Mountain, 0.07±0.01% sulfur at South Tuya, and 0.03±0.01% sulfur at Tuya Butte. The differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, as shown by change to more explosive activity and then to subaerial lava flows (and by a marked reduction of sulfur in volcanic glass), the magma shifted from tholeiitic to alkalic composition. This transition occurs at each of the three volcanoes. The tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source. Prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under subglacial conditions. As the volcano grew through the ice (or ice‐impounded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts, particularly of the more volatile‐rich alkalic melts, causing them to rise to the top of the chamber and erupt.
The 1992 M = 7 Cape Mendocino, California, earthquake: Coseismic deformation at the south end of the Cascadia megathrust
Authors: Murray Mark H. , Marshall Grant A. , Lisowski Michael , Stein Ross S.
Published: February 2004

We invert geodetic measurements of coseismic surface displacements to determine a dislocation model for the April 25, 1992, M = 7 Cape Mendocino, California, earthquake. The orientation of the model slip vector, which nearly parallels North America‐Juan de Fuca relative plate convergence, and the location and orientation of the model fault relative to the offshore Cascadia megathrust, suggest that the 1992 Cape Mendocino earthquake is the first well‐recorded event to relieve strain associated with the Cascadia subduction zone. We use data from three geodetic techniques: (1) the horizontal and vertical displacements of 13 monuments surveyed with the Global Positioning System, corrected for observed horizontal interseismic strain accumulation, (2) 88 section‐elevation differences between leveling monuments, and (3) the uplift of 12 coastal sites observed from the die‐off of intertidal marine organisms. Maximum observed displacements are 0.4 m of horizontal movement and 1.5 m of uplift along the coast. We use Monte Carlo techniques to estimate an optimal uniform slip rectangular fault geometry and its uncertainties. The optimal model using all the data resolves 4.9 m of slip on a 14 by 15 km fault that dips 28° SE. The fault extends from 1.5 to 8.7 km in depth and the main‐shock hypocenter is close to the downdip projection of the fault. The shallowly dipping fault plane is consistent with the observed aftershock locations, and the estimated geodetic moment is 3.1 × 1019 N m, 70% of the seismic moment. Other models that exclude leveling data collected in 1935 and 1942 are more consistent with seismological estimates of the fault geometry. If the earthquake is characteristic for this segment, the estimated horizontal slip vector compared with plate convergence rates suggests a recurrence interval of 140 years, with a 95% confidence range of 100–670 years. The coseismic uplift occurred in a region that also has high Quaternary uplift rates determined from marine terrace studies. If repeated ruptures of this southernmost segment of the Cascadia megathrust are responsible for the Quaternary uplift, a comparison of the coseismic uplift with coastal uplift rates suggests a recurrence interval of 200–400 years. Thus comparing horizontal and vertical coseismic to long‐term deformation suggests a recurrence interval of about 100–300 years for M = 7 events at the south end of the Cascadia megathrust.
Paleoseismology of the northern piedmont of Tianshan Mountains, northwestern China
Authors: Deng Qidong , Zhang Peizhen , Xu Xiwei , Yang Xiaoping , Peng Sizhen , Feng Xianyue
Published: February 2004

The northern piedmont of the Tianshan Mountains consists of three rows of Cenozoic EW‐striking fold and reverse fault zones, with en échelon right‐lateral steps. The southernmost row involves sediments as young as lower Pleistocene, but there is no evidence of activity along this row during the last 30,000 years. The central row is composed of three linear anticlines (Houerguos, Manas, and Tugulu) and associated reverse faults. The northernmost row includes the Dushanzi, Halaande, and Anjihai anticlines and their associated reverse faults. Abundant fault scarps and folds of late Pleistocene to Holocene river terraces across the anticlines within the central and the northernmost rows indicate recent folding and reverse faulting. We divide the northern piedmont into the Dushanzi and Manas fold and reverse fault zone. In the Dushanzi zone, we excavated 15 trenches across scarps controlled by reverse faults and back thrusts. By comparing 11 trench logs among the 15 trenches, we identify three paleoearthquakes since 13,000 years B.P. The first event occurred between 11,300 and 13,300 years B.P., and the second and the third events occurred 6300–8400 and 3000–5000 years B.P., respectively. Considering the uncertainties of the data, the average recurrence interval for large earthquakes in the Dushanzi zone is about 4000 years. A large earthquake along this zone is expected in the near future because the elapsed time since the last surface‐rupturing event is already 3000–4000 years. Three large trenches and several small trenches excavated across the fault scarps along the Manas fold and reverse fault zone reveal four events. The first, second, and third events occurred at 18,000–13,000 years B.P., 11,300–10,500 years B.P., and 6900–3600 years B.P., respectively. The fourth, the latest one, is the 1906 M = 7.7 Manas earthquake. Field investigation suggests that the 1906 Manas earthquake occurred along a blind thrust fault. This earthquake formed three discontinuous zones of fresh surface ruptures, the longest of which is only 8 km long along the eastern segment of the Tugulu reverse fault, and was associated with a zone of uplift 130 km long related to the Manas earthquake. The average recurrence interval along this zone is probably 5000–6000 years. Therefore it is unlikely that a large earthquake will occur along this zone in the near future because the Manas earthquake occurred only 89 years ago.
Velocity structure and seismicity of southeastern Tennessee
Authors: Kaufmann Ronald Douglas , Long Leland Timothy
Published: February 2004

The seismic zone in southeastern Tennessee is at the confluence of major crustal features, which have been interpreted largely from potential data, and their relation to seismicity could help us understand why major earthquakes sometimes occur in the eastern United States. In this paper we solve for the previously unknown velocity structure of the upper crust by an inversion of travel time residuals from relocated earthquakes. The gravity anomalies are included by using a linear relation between average anomalous density and average anomalous velocity. The velocity model demonstrates that the seismicity is concentrated in areas of average to below average velocity and does not appear to be associated with one of the previously identified major crustal features. The high‐velocity zones mark areas that are generally lacking in seismicity. The association of earthquake hypocenters with regions of low‐velocity crustal rocks is consistent with other intraplate seismic zones, and this association supports the conjecture that intraplate earthquakes occur in crust that may have been weakened. The velocity anomalies at midcrustal depths do not support the New York‐Alabama (NY‐AL) lineament as a linear feature extending through southeastern Tennessee and parallel to contours in gravity anomalies as originally proposed. A continuation of the (NY‐AL) lineament to the southwest requires either a 15 degree southwestward change in direction or a displacement to be consistent with the velocity anomalies. The seismically active areas in southeastern Tennessee do not appear to be constrained by the major crustal features, but instead, the seismicity is characterized by the distribution of hypocenters and their association with low‐velocity regions at midcrustal depths.
Geomagnetic polarity bias patterns through the Phanerozoic
Authors: Algeo Thomas J.
Published: February 2004

Phanerozoic geomagnetic polarity bias patterns have been reconstructed using polarity data from 278 stratigraphic formations of Cambrian‐Jurassic age combined with data from an established geomagnetic polarity timescale for the Cretaceous–Recent. In addition to the well‐known Cretaceous Normal Polarity Superchron and Kiaman Reversed Polarity Superchron, other first‐order polarity features are recognized: (1) a Middle Cambrian‐Middle Ordovician Burskan Reversed Polarity Bias Interval, (2) a Late Ordovician‐Late Silurian Nepan Normal Polarity Bias Interval, (3) an Early Jurassic Normal Polarity Bias Interval, and, possibly, (4) a Middle Jurassic Normal Polarity Bias Interval. A combination of strong polarity bias and low reversal rates during the Ordovician may indicate the existence of a “dual‐polarity superchron” containing a single major polarity transition, the Middle Ordovician Polarity Shift. Reconstruction of an accurate Phanerozoic polarity trend permits application of a “polarity bias test” to evaluate the primary character of magnetic remanences. A polarity bias test of British Siluro‐Devonian remanences reveals that group “A” remanences (0°–20° paleopole latitude; 30–100% normal polarity) exhibit polarity concordance with coeval non‐British remanences, whereas group “B” remanences (25°–50° paleopole latitude; 0–20% normal polarity) are strongly discordant, suggesting that the latter are largely of secondary origin. Analysis of groups of magnetic remanences also permits estimation of (1) characteristic timescales for formation polarity data and (2) evaluation of sources of age‐dependent polarity‐ratio variance. For the Cambrian‐Jurassic polarity data set, formations exhibit a mean characteristic timescale of 1.0–1.5 m.y., and circa 50% of polarity‐ratio variance is attributable to paleomagnetic sampling of a binomial variable (i.e., geomagnetic field polarity) and 50% to other factors (i.e., stochastic and systematic depositional and sampling biases, incorrect age estimates for stratigraphic formations and characteristic remanences, complex magnetizations, and low epochal reversal frequencies).
Fracture interface waves
Authors: Gu Boliang , Nihei Kurt T. , Myer Larry R. , Pyrak‐Nolte Laura J.
Published: February 2004

Interface waves on a single fracture in an elastic solid are investigated theoretically and numerically using plane wave analysis and a boundary element method. The finite mechanical stiffness of a fracture is modeled as a displacement discontinuity. Analysis for inhomogeneous plane wave propagation along a fracture yields two dispersive equations for symmetric and antisymmetric interface waves. The basic form of these equations are similar to the classic Rayleigh equation for a surface wave on a half‐space, except that the displacements and velocities of the symmetric and antisymmetric fracture interface waves are each controlled by a normalized fracture stiffness. For low values of the normalized fracture stiffness, the symmetric and antisymmetric interface waves degenerate to the classic Rayleigh wave on a traction‐free surface. For large values of the normalized fracture stiffness, the antisymmetric and symmetric interface waves become a body S wave and a body P wave, respectively, which propagate parallel to the fracture. For intermediate values of the normalized fracture stiffness, both interface waves are dispersive. Numerical modeling performed using a boundary element method demonstrates that a line source generates a P‐type interface wave, in addition to the two Rayleigh‐type interface waves. The magnitude of the normalized fracture stiffness is observed to control the velocities of the interface waves and the partitioning of seismic energy among the various waves near the fracture.
Equatorial origin for Lower Jurassic radiolarian chert in the Franciscan Complex, San Rafael Mountains, southern California
Authors: Hagstrum Jonathan T. , Murchey Benita L. , Bogar Robert S.
Published: February 2004

Lower Jurassic radiolarian chert sampled at two localities in the San Rafael Mountains of southern California (∼20 km north of Santa Barbara) contains four components of remanent magnetization. ComponentsA,B′, andBare inferred to represent uplift, Miocene volcanism, and subduction/accretion overprint magnetizations, respectively. The fourth component (C), isolated between 580° and 680°C, shows a magnetic polarity stratigraphy and is interpreted as a primary magnetization acquired by the chert during, or soon after, deposition. Both sequences are late Pliensbachian to middle Toarcian in age, and an average paleolatitude calculated from all tilt‐correctedCcomponents is 1° ± 3° north or south. This result is consistent with deposition of the cherts beneath the equatorial zone of high biologic productivity and is similar to initial paleolatitudes determined for chert blocks in northern California and Mexico. This result supports our model, in which deep‐water Franciscan‐type cherts were deposited on the Farallon plate as it moved eastward beneath the equatorial productivity high, were accreted to the continental margin at low paleolatitudes, and were subsequently distributed northward by strike‐slip faulting associated with movements of the Kula, Farallon, and Pacific plates. Upper Cretaceous turbidites of the Cachuma Formation were sampled at Agua Caliente Canyon to determine a constraining paleolatitude for accretion of the Jurassic chert sequences. These apparently unaltered rocks, however, were found to be completely overprinted by theAcomponent of magnetization. Similar in situ directions and demagnetization behaviors observed in samples of other Upper Cretaceous turbidite sequences in southern and Baja California imply that these rocks might also give unreliable results.
Distinct S wave reflector in the midcrust beneath Nikko‐Shirane volcano in the northeastern Japan arc
Authors: Matsumoto Satoshi , Hasegawa Akira
Published: February 2004

Distinct S waves reflected from a midcrustal seismic velocity discontinuity are detected beneath Nikko‐Shirane volcano in the southernmost part of the northeastern Japan arc. A detailed travel time analysis of the reflected S waves by using data acquired through a dense seismic network temporarily set up in this region shows that this unusual S wave reflector is distributed over an area of 15 × 15 km2 at depths ranging from 8 to 15 km. The reflector has a conical shape becoming shallow toward the summit of Nikko‐Shirane volcano. Observed amplitude spectral ratios of reflected S waves to direct S waves show that the reflector body has a strong velocity contrast to the surrounding medium and its thickness is of the order of 100 m at most. The reflector body is approximated by two thin layers probably filled with partially molten materials. Cutoff depth for shallow seismicity in this area is 3–5 km above the reflector and becomes shallow toward Nikko‐Shirane volcano, nearly parallel to the reflector. The depth to brittle‐ductile transition zone seems to be prescribed by the existence of the reflector body, which is perhaps a thin magma body.
Coseismic crustal deformations of 1994 Northridge, California, earthquake detected by interferometric JERS 1 synthetic aperture radar
Authors: Murakami Makoto , Tobita Mikio , Fujiwara Satoshi , Saito Takashi , Masaharu Hiroshi
Published: February 2004

Coseismic deformations of the January 17, 1994, Northridge, California earthquake are mapped using interferometric combination of synthetic aperture radar images acquired by the Japanese Earth Resource Satellite (JERS 1) on April 30, 1993, and June 14, 1994. We make an interferogram in a rectangular area with a size of 18.6 km (N‐S) by 29.9 km (E‐W). Although coherence is not perfect in some areas, fringes in flat and inhabited areas clearly show the coseismic movements. To remove ambiguity of the fringes due to inaccurate orbital information, we use a least squares adjustment to minimize the difference between the radar and the Global Positioning System (GPS) measurements. The difference between the radar and GPS measurements is less than 3.5 cm. The observed fringe patterns agree with a GPS‐based theoretical fault model within 10 cm. We also find a lobed pattern that suggests up to 20 cm of subsidence within a small area. The results suggest that the L band (23.5 cm) radar used by JERS 1 is more robust to spatial and temporal decorrelation than the C band radar used by the ERS 1 and RADARSAT satellites.
Extensional processes in continental lithosphere
Authors: Ruppel Carolyn
Published: February 2004

Since Vening‐Meinesz's realization that the East African Rift represented an extensional, not compressional, feature and since the widespread acceptance of plate tectonics two decades later, research on the nature and causes of extensional tectonism within continental lithosphere has intensified. Among the manifestations of extensional processes affecting continental lithosphere are passive margins (Atlantic margin), discrete intracontinental rift zones (East African Rift), diffuse rifts (Basin and Range Province), strike‐slip dominated rifts (Dead Sea Rift), and rifts in zones of regional compression (Tibetan grabens). Although no two rift zones are alike, continental rifts can generally be characterized by normal faulting with subsidiary strike‐slip faulting, lithospheric thinning which outpaces crustal stretching, varying amounts of alkaline magmatism, heat flow that is locally elevated near faults and magmatic centers, and crust that has experienced magmatic underplating and some amount of magmatic intrusion. Most aspects of rift related deformation can be explained in terms of three parameters: (1) lithospheric and (sometimes) asthenospheric thermal structure, (2) lithospheric (particularly crustal) rheology, and (3) temporal factors such as the absolute age, timing, and rate of extension. The interaction of these physical parameters determines the eventual outcome of rifting (failure or progression to complete continental breakup), the patterns of subsidence and uplift, and the mode of extensional deformation. Modes of rifting (the lithosphere's mechanical response to extensional stress) can be broadly divided into pure shear, simple shear, and lower crustal flow mechanisms. In a general sense, these categories of rift mechanisms can account for observations at rifted margins, in the Basin and Range Province, and at metamorphic core complexes respectively. The mechanisms of continental lithosphere rifting (the effects) are here distinguished from the processes which actually drive extension (the causes). Following the terminology of previous authors, the causal processes are categorized as either passive or active: Passive processes originate at plate boundaries or in response to convective drag on the base of the lithosphere, while active processes are seated in the sublithospheric mantle and typically involve the interaction of mantle plumes and the lithospheric plates. A peculiar set of factors, including the local stress state, the retreat of a subducting plate, and convective instability of thickened crustal roots, may drive extension in compressional settings like Tibet. In the future, rifting studies are likely to focus on further clarification of the active and passive rifting terminology, better constraints on deformation rates and lithospheric rheology and thermal structure, along‐axis segmentation of rifts, and the analogies and differences between rifting on Earth and nearby planets.
Global mapping functions for the atmosphere delay at radio wavelengths
Authors: Niell A. E.
Published: February 2004

I have developed expressions for calculating the ratios (mapping functions) of the “line of sight” hydrostatic and wet atmospheric path delays to their corresponding zenith delays at radio wavelengths for elevation angles down to 3°. The coefficients of the continued fraction representation of the hydrostatic mapping function depend on the latitude and height above sea level of the observing site and on the day of the year; the dependence of the wet mapping function is only on the site latitude. By comparing with mapping functions calculated from radiosonde profiles for sites at latitudes between 43°S and 75°N, the hydrostatic mapping function is seen to be more accurate than, and of comparable precision to, mapping functions currently in use, which are parameterized in terms of local surface meteorology. When the new mapping functions are used in the analysis of geodetic very long baseline interferometry (VLBI) data, the estimated lengths of baselines up to 10,400 km long change by less than 5 mm as the minimum elevation of included data is reduced from 12° to 3°. The independence of the new mapping functions from surface meteorology, while having comparable accuracy and precision to those that require such input, makes them particularly valuable for those situations where surface meteorology data are not available.
Oxygen isotope ratios in olivine from the Hawaii Scientific Drilling Project
Authors: Eiler John M. , Valley John W. , Stolper Edward M.
Published: February 2004

Oxygen isotope ratios of olivine in 23 tholeiites from the Hawaii Scientific Drilling Project (HSDP) core (15 from Mauna Kea, 8 from Mauna Loa) and three samples of outcropping subaerial or dredged submarine Mauna Kea lavas have been measured by laser fluorination. The δ18O values are 4.6–5.4‰, confirming previous observations that some Hawaiian lavas are derived from sources with δ18O values lower than typical upper mantle (δ18O olivine ≈ 5.2 ± 0.2‰). The Mauna Kea‐Mauna Loa transition marks a shift from δ18O values lower than the mantle average in Mauna Kea olivines (∼4.8) to more typical mantle values in Mauna Loa olivines. Lavas containing olivines with δ18O values similar to the typical upper mantle are associated with more “primitive” or less depleted radiogenic isotope characteristics; i.e., with higher 3He/4He (>13 Ra), higher 87Sr/86Sr (>0.7036) and lower ϵNd (<6.5), and with 206Pb/204Pb ratios less than ∼18.3. These relationships indicate that the δ18O values of the relatively enriched source components of the Hawaiian plume sampled by Mauna Loa lavas are comparable to (or greater than) the mantle average. This conclusion is supported by δ18O values of olivine from other high 3He/4He islands, which are also comparable to the upper mantle average. The low δ18O values in Hawaiian lavas are derived from a source having more MORB‐like, or depleted, He, Nd, and Sr isotope ratios, but more radiogenic Pb than is seen in the Mauna Loa lavas. Assimilation of 18O‐depleted lower oceanic crust from the underlying Pacific crust by hot, MgO‐rich parental magmas or melting of older, recycled oceanic crust entrained in the Hawaiian plume are both possible sources of this 18O‐depleted, MORB‐like component in Hawaiian magmas.
Stress/strain changes and triggered seismicity following the Mw7.3 Landers, California earthquake
Authors: Gomberg Joan
Published: February 2004

Calculations of dynamic stresses and strains, constrained by broadband seismograms, are used to investigate their role in generating the remotely triggered seismicity that followed the June 28, 1992 Mw7.3 Landers, California earthquake. I compare straingrams and dynamic Coulomb failure functions calculated for the Landers earthquake at sites that did experience triggered seismicity with those at sites that did not. Bounds on triggering thresholds are obtained from analysis of dynamic strain spectra calculated for the Landers and Mw6.1 Joshua Tree, California earthquakes at various sites, combined with results of static strain investigations by others. I interpret three principal results of this study with those of a companion study by Gomberg and Davis [this issue]. First, the dynamic elastic stress changes themselves cannot explain the spatial distribution of triggered seismicity, particularly the lack of triggered activity along the San Andreas fault system. In addition to the requirement to exceed a Coulomb failure stress level, this result implies the need to invoke and satisfy the requirements of appropriate slip instability theory. Second, results of this study are consistent with the existence of frequency‐ or rate‐dependent stress/strain triggering thresholds, inferred from the companion study and interpreted in terms of earthquake initiation involving a competition of processes, one promoting failure and the other inhibiting it. Such competition is also part of relevant instability theories. Third, the triggering threshold must vary from site to site, suggesting that the potential for triggering strongly depends on site characteristics and response. The lack of triggering along the San Andreas fault system may be correlated with the advanced maturity of its fault gouge zone; the strains from the Landers earthquake were either insufficient to exceed its larger critical slip distance or some other critical failure parameter; or the faults failed stably as aseismic creep events. Variations in the triggering threshold at sites of triggered seismicity may be attributed to variations in gouge zone development and properties. Finally, these interpretations provide ready explanations for the time delays between the Landers earthquake and the triggered events.
Melt inclusions from chemically zoned ash flow sheets from the Southwest Nevada Volcanic Field
Authors: Vogel Thomas A. , Aines Roger
Published: February 2004

The purpose of this paper is to determine whether melt inclusions from three well‐studied, compositionally zoned ash flow sheets from the Southwest Nevada Volcanic Field can be used to determine the evolution of the parent magma bodies. All samples are from the quenched, glassy pumice fragments that occur in the upper and lower portions of the Topopah Spring Tuff (68.7–78.7% SiO2), the Rainier Mesa Tuff (55.0–76.3% SiO2), and the Ammonia Tanks Tuff (57.5–76.3% SiO2) (all SiO2 values anhydrous). Most of the melt inclusions in this study are in feldspar; a few are in quartz. There are large variations of volatile concentrations in melt inclusions within an individual phenocryst and large variations in melt inclusions in different phenocrysts within a pumice fragment. This finding demonstrates that the volatiles did not reequilibrate after the formation of the ash flow sheet. The volatile contents of most of these melt inclusions represent decompression and loss of some volatiles and therefore do not represent conditions at the time of entrapment. The presence of CO2 in many inclusions demonstrates that the magma evolved a gas phase in its preeruption evolution. The nonvolatile chemical variations of the melt inclusions from the Rainier Mesa and Ammonia Tanks Tuffs are similar. Melt inclusions from the lower‐silica pumice fragments show a wide range of chemical variation and monitor the liquid as it evolved. In both tuffs some melt inclusions are significantly more silica‐rich than the matrix glass, and indicate mixing with a more evolved magma. Most melt inclusions from the high‐silica pumice fragments from each of these tuffs show little chemical variation and are similar to the matrix glass. The fact that no lower‐silica melt inclusions occur in the high‐silica pumice fragments supports the model that the high‐silica magma did not evolve from the low‐silica magma. The maximum H2O concentrations of inclusions in each pumice fragment from the Rainier Mesa and Ammonia Tanks Tuffs show a general positive correlation with SiO2 concentrations of the pumice fragment. No such relationship is observed with the Topopah Spring Tuff.
Geometry and slip distribution in normal fault systems: Implications for mechanics and fault‐related hazards
Authors: Bruhn Ronald L. , Schultz Richard A.
Published: February 2004

Normal faults are important sources of large, destructive earthquakes in continental rift provinces. We investigate the mechanical characteristics of multiple, linked master and subsidiary faults using two‐dimensional boundary element models. The master fault system is a Coulomb‐frictional fault that dips 50° from the surface to a depth of 15 km, where it is linked to a subjacent shear zone that creeps under low resolved shear stress. We consider two basic configurations, one in which the shear zone and master fault are coplanar, and the other in which the shear zone dips 20°, forming a bent or crudely listric fault system. Antithetic and splay faults are added to the master fault system in each case to model failure of multiple fault systems that are typical of half‐grabens. We focus on the mechanical interactions between the various faults in order to identify points of initial failure, spatial variations in the amount of fault slip, and the patterns of surface deformation that may precede catastrophic failure of the master fault system. In all cases, failure on the master frictional fault initiates at the base, where it joins the creeping shear zone, and at the Earth's surface during the same stress increment. The listric fault system fails at lower regional differential stress than the planar fault system. Failure of a large antithetic fault that dips at 70° from the surface to the base of the frictional master fault initiates at lower regional differential stress for the listric than for the planar case. Several repeated failure episodes may be required to cause surface offset on the antithetic fault if the master fault system is planar, but antithetic surface faulting may occur during each master fault failure episode in the listric case. Modeled splay faults dip from the surface at either greater (70°) or lower (50°) angles than the frictional master fault to a depth of 5 km, where they intersect the master fault. In both cases, surface slip is greater on the master fault and lesser on the splay fault. Surface deformation prior to catastrophic failure of the frictional master fault is generated by creep on the deep‐seated shear zone and during the initial stages of master fault rupture. Footwall uplift is predicted above a steeply dipping, locked Coulomb fault and creeping shear zone, but the footwall uplift is absent in the listric fault ‐ shear zone case. A localized zone of increased extensional strain is centered on the surface trace of the master fault as the result of near‐surface extensional and shear fracturing during the initial stages of fault slip. These geodetic anomalies are of interest because they are a near‐surface phenomenon which may also alter fluid flow and gas flux and be detectable by hydrologic and geochemical instrumentation as well as geodetic measurements.
Buoyancy sources for a large, unrooted mountain range, the Sierra Nevada, California: Evidence from xenolith thermobarometry
Authors: Ducea Mihai N. , Saleeby Jason B.
Published: February 2004

Xenoliths hosted by Cenozoic volcanic flows and plugs from the Central Sierra Nevada and Eastern Sierra Nevada, Owens Valley, and Inyo Mountains were studied for petrography and thermobarometry. The Central Sierra Nevada suite consists of abundant lower crustal feldspathic granulites, garnet clinopyroxenites, and mantle‐derived peridotites and garnet websterites. Mafic crustal assemblages occur down to ∼65–70 km, although below 35–40 km, they are mainly in the eclogite facies. In contrast, the Eastern Sierra Region suites show peridotitic, pyroxenitic, and harzburgitic assemblages at depths of ≥35–40 km. They define an adiabat in PT space (T ∼ 1180–1250°C), suggesting the presence of the asthenospheric upper mantle close to the base of the crust. The ultramafic mantle rocks from the Central Sierra Nevada also define an adiabatic slope in PT space, possibly an artifact of side heating from the east. There is xenolith evidence that the Sierra Nevada lost about half of its original crust on the eastern side of the range. Regardless of the actual mechanism of crustal thinning, the loss of the eclogitic lowermost crust and replacement by peridotite in the eastern Sierra Nevada is a process accompanied by a substantial density decrease (>100 kg/m3). Overall, if the mechanism of eclogitic lowermost crust removal is viable, there are enough buoyancy sources to explain topographic differences between the Sierra Nevada and the adjacent Basin and Range, assuming isostatic equilibrium.
On the inverse problem for earthquake rupture: The Haskell‐type source model
Authors: Das S. , Suhadolc P.
Published: February 2004

In order to gain insight into how to invert seismograms correctly to estimate the details of the earthquake rupturing process, we perform numerical experiments using artificial data, generated for an idealized faulting model with a very simple rupture and moment release history, and solve the inverse problem using standard widely used inversion methods. We construct synthetic accelerograms in the vicinity of an earthquake for a discrete analog of the Haskell‐type rupture model with a prescribed rupture velocity in a layered medium. A constant level of moment is released as the rupture front passes by. We show that using physically based constraints, such as not permitting back slip on the fault, allows us to reproduce many aspects of the solution correctly, whereas the minimum norm solution or the solution with the smallest first differences of moment rates in space and time do not reproduce many aspects for the cases studied here. With the positivity of moment rate constraint, as long as the rupturing area is allowed to be larger than that in the forward problem, it is correctly found for the simple faulting model considered in this paper, provided that the rupture velocity and the Earth structure are known. If, however, the rupture front is constrained either to propagate more slowly or the rupturing area is taken smaller than that in the forward problem, we find that we are unable even to fit the accelerograms well. Use of incorrect crustal structure in the source region also leads to poor fitting of the data. In this case, the proper rupture front is not obtained, but instead a “ghost front” is found behind the correct rupture front and demonstrates how the incorrect crustal structure is transformed into an artifact in the solution. The positions of the centroids of the moment release in time and space are generally correctly obtained.