bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismicity and tectonic stress in the south‐central Pacific
Authors: Okal Emile A. , Talandier Jacques , Sverdrup Keith A. , Jordan Thomas H.
Published: February 2008

The 15‐station French Polynesian Seismic Network is used to study the intraplate seismicity of the South‐Central Pacific Ocean for the period January 1, 1965 to December 31, 1979. The overall pattern of seismicity shows a clustering of earthquakes at approximately 30 distinct localities, occurring both as discrete events and in swarms. Three localities are associated with known centers of active vulcanism (Moua Pihaa and Rocard seamounts in the Tahiti‐Méhétia area and Macdonald seamount in the Austral Islands). A set of eight localities is distributed along the tectonic axis of the Tuamotu Archipelago and its northwestward extention into the Line Islands, and a set of five is roughly aligned with the northeastern edge of the Tuamotu platform; both sets may be related to load inhomogeneities in the lithosphere. However, much of the seismicity is not correlated with major bathymetric features. With the exception of one isolated event, for example, none of the recorded activity occurs along the major fracture zones that cross the study area.

Only three localities have been the sites of two or more earthquakes with body‐wave magnitudes greater than 5.0; these we designate Regions A, B and C. Excluding Hawaii, Regions A, B and C are the most intense centers of seismicity within the Pacific plate interior, and together they account for more than 90% of the seismic energy release in the South‐Central Pacific. An analysis of water multiples from events in Regions A, B and C indicates hypocentral depths within the oceanic crust. Eight focal mechanisms have been obtained; these all have nearly horizontal, NW‐trending compressional axes, oriented approximately parallel to the direction of Pacific plate motion. The uniformity of this orientation over the large distances separating the epicenters (>2000 km) suggests that the mechanisms are indicative of a regional tectonic stress field, rather than locally disturbed stress patterns. These data thus provide additional constraints on the force balance models of plate tectonics.
Paleomagnetic test of the Early Tertiary Plate circuit between the Pacific Basin Plates and the Indian Plate
Authors: Gordon Richard G. , Cox Allan
Published: February 2008

The internal consistency of early Tertiary reconstructions of the Pacific, Antarctic, and Indian plates has been tested by combining seafloor spreading data and paleomagnetic data. These tests show that substantial motion has taken place during the Tertiary across a fossil plate boundary located within one of these plates. We consider two alternative models to reconcile the available paleomagnetic and plate tectonic information. In the first model, East and West Antarctica were separate plates, the early Tertiary position of West Antarctica being one in which the Antarctic Peninsula is adjacent to the southwest border of South America. This model is consistent with the paleomagnetic data from Australia and Antarctica. In the second model the northern and southern parts of the Pacific plate have undergone left‐lateral strike slip motion across a fossil plate boundary located northeast of Chatham Rise. This second model is supported by paleomagnetic data from the northern and southern segments of the Pacific plate. The joint assumptions (1) that the hot spots are fixed and (2) that during the early Tertiary the present Pacific plate was two separate plates (the Chatham Rise plate and the north Pacific plate) are consistent with all of the paleomagnetic data and all of the geometrical constraints of early Tertiary plate reconstructions. The joint assumptions (1) that the hot spots are fixed and (2) that the present Antarctic plate was two early Tertiary plates produce unacceptable overlap and are not consistent with Pacific plate paleomagnetic data unless the Chatham Rise has moved independently of adjacent seafloor.
The reversible Griffith crack: A viable model for dilatancy
Authors: Holcomb David J. , Stevens Jeffry L.
Published: February 2008

Previous work (paper 1) on the mechanism of dilatancy has outlined the deficiencies of the sliding crack model. We have developed a new model, based on reversible Griffith cracks, that is in accord with experimental results to data. These results can be summarized as follows: (1) dilatant rock exhibits volume increases at sufficiently high increasing differential stress, (2) there is substantial hysteresis in physical properties between increasing and decreasing stress, (3) examination of stressed and unstressed samples, using a scanning electron microscope, reveals stress‐induced cracks that are preferentially oriented with the crack plane close to the maximum compressive stress axis, (4) few shear cracks (required by the sliding crack model) are observed, and (5) dilatant rock exhibits ‘memory’ of the maximum differential stress since the last complete unloading. When returned to this stress state, the physical properties return to their previous values but at other stress states there is large hysteresis between loading and unloading. A classical Griffith crack exhibits all of these properties if it is postulated to be reversible. That is, below a critical stress, the crack closes in an unstable mode and healing occurs. This behavior requires close registration of the crack faces.
Investigation of two high stress drop earthquakes in the Shumagin Seismic Gap, Alaska
Authors: House Leigh , Boatwright John
Published: February 2008

Two moderate size (mb = 5.8, 6.0) earthquakes occurred within a local network of short‐period seismograph stations in the Shumagin Islands, Alaska, on April 6, 1974. The network recorded about 70 aftershocks over the next 2 weeks. Both main shocks triggered a strong motion accelerograph (SMA) at Sand Point, 50 km NNW of their epicenters. High‐quality relative locations obtained from local network arrivals for the main shocks and 25 aftershocks yield depths between 37 and 43 km and define a plane dipping about 30° to the NW. A nearly pure thrust focal mechanism for the larger (mb = 6.0) earthquake was obtained from long‐period data. The fault plane inferred from this mechanism dips 30° in the direction N16°W. The sequence was located in the upper portion of the dipping seismic zone beneath the eastern Aleutians and was presumably related to underthrusting of the Pacific plate beneath the North America plate. Estimates of the source parameters of these earthquakes were obtained from analysis of the SMA data and World‐Wide Standard Seismographic Network (WWSSN) short‐period data. The WWSSN data indicate that the earthquakes had approximately circular rupture areas. Modeling the SMA records with a quasi‐dynamic source model provides the following estimates of the source parameters for the mb = 5.8 and 6.0 earthquakes, respectively: moments, Mo = 3.6 and 6.6 × 1024 dyne cm and static stress drops, Δσ = 890 and 650 bars. A high frequency spectral falloff of ω−3 suggests that the ruptures stopped gradually. The b value of the sequence, 0.34, is quite low. Since the sequence occurred within an area that is likely to have a high potential for a future large earthquake, it is suggested that the low b value and the high stress drops of the main shocks represent precursory phenomena.
Tectonics and seismicity of the Toktogul Reservoir Region, Kirgizia, USSR
Authors: Simpson D. W. , Hamburger M. W. , Pavlov V. D. , Nersesov I. L.
Published: February 2008

The 214 m high Toktogul Dam will impound a 19.5‐km3 reservoir on the Naryn River in central Kirgizia. The reservoir lies along the Talas‐Fergana fault, a major tectonic feature of central Asia. The nature of Cenozoic movements along the fault is in debate; however, there is considerable evidence of present‐day right‐lateral movement. The region is one of moderate to high seismicity. A magnitude 7.6 earthquake occurred in 1946, approximately 65 km from the dam, near the intersection of the Talas‐Fergana fault and Chatkal ranges. Construction of the dam was completed in 1975 and by late 1979 the water level in the reservoir reached three fourths its final planned depth. Low‐magnitude, induced earthquakes began as the water level first exceeded 100 m in late 1977. A microearthquake survey using portable instruments was carried out in July‐August 1978 and a seven‐station telemetered network was installed in October 1978. The largest of the induced earthquakes has been of magnitude 2.5; the activity is in the immediate vicinity of the dam at very shallow depths (2–5 km) and is related to water level. Preliminary analysis of data gathered during 1978–1979 shows a zone of activity, extending to 5 km depth, supporting geological evidence for an active reverse fault beneath the dam and parallel to the canyon of the Naryn River. Where the reservoir crosses the Talas‐Fergana fault, 15 km upstream from the dam, the water level has not yet reached 100 m and seismicity along the Talas‐Fergana has not shown any significant change related to filling of the reservoir.
Transformation and analysis of record sections
Authors: Phinney Robert A. , Chowdhury K. Roy , Frazer L. Neil
Published: February 2008

Slant stacking is used to transform an observed record section Φ(x,t) into its plane wave decomposition Ψ(p,τ). The invertible transformation untangles travel time triplications and removes focusing and defocusing effects, yielding signals with the plane wave amplitudes and phases. The principal arrival, composed of refraction and wide angle reflections, appears in these coordinates as a display of the intercept time function τ(p), which may be inverted by any of several currently popular methods. The transformation into the (p,τ) domain; is an objective procedure which removes many of the interpreter's difficulties by casting the picking process into much simpler variables. Moreover, the decomposition into plane waves directly yields the transient reflectivity matrix of the (one dimensional) medium. Several lines of argument lead us to conclude that the comparison of data and synthetic data should be done in the (p,τ) domain. The use of these techniques requires that the wave field be sampled with sufficient spatial density of receivers to avoid wave number aliasing. We show examples of plane wave decomposition for shallow data from the COCORP deep crustal reflection program.
The accommodation of relative motion at depth on the San Andreas Fault System in California
Authors: Prescott William H. , Nur Amos
Published: February 2008

Plate motion below the seismogenic layer along the San Andreas fault system in California is generally assumed to occur by aseismic slip along a deeper extension of the fault. It is also possible that below the seismogenic layer, deformation is distributed laterally over a zone. Several observed features of the San Andreas fault in California have implications about the mode of accommodation of relative motion along the plate boundary beneath the seismogenic zone: the shallow depth of all earthquakes in California, the depth to which coseismic slip occurred during the 1906 San Francisco earthquake, the broad zone of strain accumulation, the broad heat flow anomaly, and the existence of widely separated parallel faults. The observations strongly imply that below the seismogenic zone, relative motion is distributed over a zone and occurs by inelastic flow rather than by aseismic slip on discrete fault planes. The existence of multiple faults further suggests that tractions at the base of the brittle layer are significant over time periods of years to hundreds of years.
Precise P and S wave velocity variations, crack density, and saturation changes
Authors: Buchbinder G. G. R.
Published: February 2008

Precise observations of travel time changes of P and S waves from 11 explosions at the same shot point in the Charlevoix region, Quebec, have been determined by separate cross correlations of the P and S waveforms. The changes in P and S wave travel times are equal to within ±4 ms and lead to the relation that dα = 3dβ. From the theory of O'Connell and Budiansky (1977) the derivatives of the normalized P and S wave velocities with respect to the normalized S wave velocity define the changes in crack density parameter ε and the partial saturation parameter ζ. Increases in travel time observed in the Charlevoix region suggest that the crack density parameter ε increases, while the partial saturation parameter ζ decreases, in agreement with the dilatancy hypothesis (Nur, 1972).
Gravity and structure of the continental margins of southwestern Mexico and northwestern Guatemala
Authors: Couch Richard , Woodcock Stephen
Published: February 2008

Geophysical measurements over the eastern end of the Tehuantepec Ridge and adjacent continental margins of southern Mexico and northern Guatemala indicate that the ridge is a fracture zone and that it marks the boundary between two different subduction provinces. A positive free‐air gravity anomaly which extends northwestward along the outer continental shelf of Guatemala curves abruptly landward in the Gulf of Tehuantepec. The positive shelf anomaly is on trend with the positive anomaly of the Nicoya Peninsula, Costa Rica, and suggests that rocks genetically related to Cretaceous rocks of the Nicoya Complex extend northwestward along the continental shelf to the Gulf of Tehuantepec. A crustal and subcrustal cross section of the continental margin of Guatemala, constrained by gravity, magnetic, and seismic refraction data, indicates that the rock strata causing the outer shelf gravity high dip landward, consistent with imbricate thrusting of the oceanic crust beneath and into the continental margin. A model crustal cross section of the continental margin of southern Mexico, north of the Tehuantepec Ridge, shows a markedly different margin structure with a relatively small amount of continental accretion and a continental crustal block extending to within approximately 25 km of the trench axis.
Shumagin Seismic Gap, Alaska Peninsula: History of great earthquakes, tectonic setting, and evidence for high seismic potential
Authors: Davies J. , Sykes L. , House L. , Jacob K.
Published: February 2008

The Shumagin seismic gap, a segment of the plate boundary along the eastern Aleutian arc, has not ruptured during a great earthquake since at least 1899–1903. Because at least 77 years have elapsed since the Shumagin Gap last ruptured in a great earthquake and repeat times for the 1938 rupture zone and part of the Shumagin Gap are estimated to be 50 to 90 years, a high probability exists for a great earthquake to occur within the Shumagin Gap during the next one to two decades. Reconsideration of the rupture zones of the Aleutian earthquakes of 1938, 1946, and 1948 suggests that those events did not break the interplate boundary beneath the Shumagin Islands. Thus, the Shumagin seismic gap extends from the western end of the 1938 rupture zone to the eastern end of that of 1946. These boundaries also coincide with transverse structural features. At least the eastern half of the Shumagin Gap broke in great earthquakes in 1788 and 1847 and possibly in 1898–1903. The Shumagin Gap is probably not the result of aseismic slip; rather, plate motion is accommodated there seismically and episodically and can be expected to produce large earthquakes in the future. Although there is no definitive evidence of long‐term precursors of a possible future earthquake, several observations suggest that the Shumagin Gap is in an advanced stage of the earthquake cycle. Both teleseismic and local network data indicate a near absence of seismic activity (M ≥ 2) above a depth of 30 km along the main thrust zone within the gap; this is in strong contrast to adjacent portions of the arc where seismic activity is scattered across most of the main thrust zone. Two earthquakes with high stress drops (600–900 bars), which occurred at the base of the main thrust zone, may indicate the accumulation of a considerable amount of strain energy within the gap. A possible seismic gap at the eastern end of the aftershock zone of the Aleutian earthquake of 1957 has been identified near Unalaska Island. An earthquake that nucleates in the Shumagin Gap could also rupture the possible Unalaska Gap to the west, the 1938 aftershock zone to the east, or both, with resultant magnitude up to Mw = 9.0. Alternatively, the Shumagin Gap alone, or in one of the above combinations could rupture in a series of very large earthquakes instead of a single great shock. Past Alaska‐Aleutian earthquakes, including those of 1788, 1938, 1946, 1957, 1964, and 1965, have generated very large tsunamis. Future large earthquakes in the Shumagin Islands region could generate wave heights of several tens of meters along shorelines near the rupture areas. The Shumagin Gap is one of two major gaps along the United States portion of the Alaska‐Aleutian plate boundary and is one of the few areas in the United States where processes leading to a great earthquake are likely to be observed within a reasonable span of time.
Precursors to the Kalapana M = 7.2 earthquake
Authors: Wyss Max , Klein F. W. , Johnston Arch C.
Published: February 2008

The Kalapana, Hawaii earthquake of November 1975 had a rupture length of 40 to 50 km and was located on the south flank of the active volcano Kilauea. The source mechanism was dip slip normal faulting on a plane dipping ∼20 degrees to the SE, with the greatest principal stress oriented in that direction and accumulated by volcanic intrusions into Kilauea's rifts. The source area of the 1975 earthquake was subject to intensive geological and geophysical research for many years before this earthquake because of its proximity to the volcano. We studied the distribution of epicenters for small earthquakes from 1962 to 1975. Seven seismographs located within 3 km of the aftershock area had been in operation for 3 to 10 years before the mainshock, and geodetic triangulations and trilaterations in the source area had been carried out repeatedly since 1914. We found that precursory changes occurred throughout most of the rupture area but in two distinctly different patterns. In the larger outer anomalous area the seismicity rate was decreased by 50% during the 3.8 years before the mainshock; in addition, several geodetic lines indicated anomalous strain release during this time. Within two inner areas the seismicity remained high, then increased shortly before the mainshock. In one of the inner areas a P wave travel time delay of 0.2 s could be detected, which began about 3.5 years before the mainshock. Within the other inner anomalous area geodetic strain was accumulating until the first half of 1975 when 3.5×10−4 strain (35 bars), was released aseismically. By contrast, the outer anomalous volume was experiencing strain softening from 1970/1971 on. We interpret these observations as indicating that strain softening by fault creep in the outer anomalous area transferred stress into two major asperities (locked portions of the fault). A velocity decrease and foreshocks were observed in one asperity and high stress accumulation in the other, implying that dilatancy of the crust probably occurred. Our model is qualitatively supported by the independent evidence of strong motion records which show that the Kalapana earthquake was a complex multiple rupture. We conclude that the Kalapana earthquake was preceded by a preparatory process which lasted 3.8 ± 0.3 years and which had dimensions of 45 × 10 km covering approximately the aftershock area.
Seismic and aseismic deformation associated with the 1952 Kern County, California, earthquake and relationship to the Quaternary history of the White Wolf Fault
Authors: Stein Ross S. , Thatcher Wayne
Published: February 2008

Synthesis of geodetic, geologic, and seismic data from the White Wolf fault, California, indicates that the fault separates an area of late Quaternary and continuing rapid uplift in the Tehachapi Mountains and Transverse Ranges from even more rapid subsidence in the southern San Joaquin Valley. On July 21, 1952, rupture of the White Wolf fault produced the ML = 7.2 Kern County earthquake. We used the aftershock zone to delimit the size of the faulted slip surface and applied constraints imposed by the known 1952–1953 horizontal shear strains to model the measured coseismic vertical displacements, with an elastic dislocation model. A curved fault trace with decreasing fault depth (27 to 10 km from the surface vertically to the base), slip (3 to 1 m), and dip (75° to 20°) from the 1952 epicenter at the southwest end of the fault toward the northeast provides the fit most consistent with the geodetic record, the measured seismic moment, the fault‐plane solution, and the pattern of surface rupture. Two short releveled lines near the 1952 epicenter tilted 4 and 17 μrad down to the north from 5–10 years before the earthquake; the preseismic tilts differ significantly from ten other surveys of these lines. Left‐lateral fault‐crossing shear strain from 0.2–20 years before the quake was two times greater than both preseismic off‐fault strains and the post‐seismic fault‐crossing strains. During the first seven years after the earthquake, aseismic deformation was negligible. From 1959 to 1972 uplift reached 160 mm over an area larger than the aftershock zone, rising first in the epicentral region and then at the northeast end of the fault. This was unaccompanied by any surface fault slip. Reconstruction of the vertical separation on the White Wolf fault from late Quaternary and late Miocene stratigraphic marker beds shows that the rate of reverse fault slip increased forty‐fold, from 0.1–0.2 mm/yr to 3–9 mm/yr, between the past 10–15 m.y. and the most recent 0.6–1.2 m.y. We estimate a 170‐ to 450‐yr average recurrence interval for earthquakes on the White Wolf fault with slip equivalent to that in 1952. The 1952 earthquake appears to be characteristic of the Quaternary record of fault displacement in the increase in White Wolf slip toward the San Andreas fault, the ratio of reverse to lateral slip (1.3:1), and the ratio of vertical fault slip to emergence of the hanging wall block (3:1). The >8500‐m‐deep sedimentary basin on the down‐thrown block cannot be explained by repeated slip of the White Wolf fault in an elastic medium.
Diminished tektite ablation in the wake of a swarm
Authors: Sepri Paavo , Chen Karl K. , O'Keefe John A.
Published: February 2008

Observations of ablation markings on tektite surfaces reveal that a large variation in aerodynamic heating must have occurred among the members of a swarm during atmospheric entry. In a few cases, the existence of jagged features indicates that these tektite surfaces may have barely reached the melting temperature. Such an observation seems to be incompatible with the necessarily large heating rates suffered by other tektites which exhibit the ring wave melt flow. A reconciliation is proposed in the form of a wake shielding model which is a natural consequence of swarm entry. Calculations indicate that the observed ablation variations are actually possible for swarm entry at greater than escape velocity. This aerodynamic conclusion provides support for the arguments favoring extraterrestrial origin of tektites.
Evidence for cold water circulation at DSDP site 395: Isotopes and chemistry of alteration products
Authors: Lawrence J. R. , Drever J. I.
Published: February 2008

A variety of low temperature alteration products occurs in the 573‐m‐thick section of basaltic rock from Site 395. Carbonate veins, smectites (pre‐dominantly saponite), zeolites, goethite, opal, and gypsum, listed in order of decreasing abundance, are present. Values of δ18O (SMOW) of calcite veins range from +31.8 to +34.8‰, and δ13C (PDB) ranges from +2.3 to +4.4‰. Apparently the calcite veins were formed at 0–15°C in equilibrium with deep ocean water. Some veins may have formed during glacial periods when δ18O of the deep water was higher than it is presently. The deepest calcite veins were formed in the temperature range 8 – 15°C. Saponite veins associated with an intrusive dolerite sill (δ18O = +9.8 to +24.3‰, SMOW) are estimated to have formed in the temperature range 30 – 160°C. Very extensive circulation (i.e., large water/rock ratios) predominated during alteration of the basalts as evidenced by seawater carbon and sulfur isotope ratios in calcite and gypsum and carbon, sulfur and magnesium mass balance considerations.
Ground motion in the near field of a fluid‐driven crack and its interpretation in the study of shallow volcanic tremor
Authors: Chouet Bernard
Published: February 2008

We present a study of the motion of the ground in the near field of a fluid‐driven tensile crack embedded in a layered half space. The source that we consider is the jerky opening of a channel connecting two fluid‐filled cracks, and the cause of this opening is the excess pressure of fluid in one of the cracks. We make a complete representation of the three components of ground motion in the space, time, and frequency domains and analyze the effects of fluid compressibility, source depth, and medium structure on the ground response. The calculations show the presence of a dominant frequency of motion which depends not only on the source geometry and bulk modulus of the fluid but also on medium characteristics, receiver position, and the component of motion being considered. Using this source model, we view an episode of volcanic tremor as a continuous sequence produced by numerous jerky openings of channels occurring randomly in time along a chain of cracks. Our results are applied to the October 5–6, 1963, east rift eruption of Kilauea volcano, Hawaii, and found to be compatible with available seismic data, suggesting that magma is transported through an ensemble of cracks with the area of 1 × 1 km, each pair of cracks constituting an individual dike segment which opens in discrete increments at a rate of 1/s. We obtain the following parameters for each dike‐segment opening: increase in cavity volume, 40 m3; stress drop, 0.004 bar; seismic moment, 1019 dyn cm; and force applied by the fluid to open the dike, 1014 dyn. The total moment integrated over the entire duration of tremor for the eruption is 0.5 · 1024 dyn cm, roughly equivalent to a single magnitude M = 5 earthquake. The seismic source parameters and other field observations offer constraints on the process of mass transport and are compatible with the excess pressure and viscosity of magma of about 40 bars and 102 P, respectively.
Igneous evolution of Hess Rise: Petrologic evidence from DSDP leg 62
Authors: Windom Kenneth E. , Seifert Karl E. , Vallier Tracy L.
Published: February 2008

Igneous rock was recovered at three sites on Hess Rise, north‐central Pacific Ocean, during Deep Sea Drilling Project leg 62: tholeiitic basalt at site 464 (northern Hess Rise), trachyte at site 465, and rounded clasts of alkalic basalt at site 466 (both on southern Hess Rise). The depths at which the igneous rocks from sites 464 and 465 were recovered correspond to strong acoustic reflectors, whereas the clasts from site 466 were recovered from within calcareous sediment. Although all the rocks are moderately to highly altered, petrographic and chemical studies, especially rare earth element determinations, provide data on the nature of these rocks. Data from these studies, combined with geophysical models of the evolution of the Pacific plate during the Mesozoic, allow an interpretation of the formation of Hess Rise involving building of a volcanic platform by eruption of tholeiitic mid‐oceanic ridge basalt caused by the breakup of a R‐R‐R‐type triple junction during the middle Cretaceous. This period of platform building was accompanied by edifice building along the south edge of Hess Rise involving alkalic basalt and its differentiation products associated with a leaky transform fault. This interpretation agrees with proposed models for the origin of the tholeiitic parts of other oceanic plateaus but requires a different mechanism—namely, volcanism associated with leaky transform faults—to account for the alkalic rocks.
The Bermuda earthquake of March 24, 1978: A significant oceanic intraplate event
Authors: Stewart Gordon S. , Helmberger Donald V.
Published: February 2008

The Bermuda earthquake (M ∼ 6) occurred near the westerly extension of the Kane Fracture Zone roughly 370 km southwest of the island of Bermuda. It is one of the largest oceanic intraplate earthquakes to occur off the eastern coast of North America. Because of its size and location, it has provided an excellent set of WWSSN body waves. They can be used to infer its depth and faulting parameters by waveform modeling techniques. The results indicate a north‐northwest striking thrust mechanism (strike = N20°W, dip = 42°NE, rake = 90°) with the hypocenter located at a depth of 11 km, which for an oceanic crust places it predominantly in the mantle. The event had a seismic moment of 3.4 × 1025 dyne cm, and its time history was modeled with a symmetric trapezoidal time function 3 s in duration. The north‐northwest strike of the event is in good agreement with the bathymetry of the area, the epicenter being close to the southwestern edge of the Bermuda Rise. The strike of the event is also close to that of the inferred extensions of the present ridge fracture zones in the region. The presence of such fracture zones is indicative of local weak zones in the lithosphere. The Bermuda earthquake most likely is associated with one of these zones of weakness and is the result of the application of present day stress imposed on the region by the North American plate in the direction of its absolute motion. This is an important event in terms of understanding and estimating seismic hazard on the eastern seaboard of North America.
Inversion of slant stacks using finite‐length record sections
Authors: Brocher Thomas M. , Phinney Robert A.
Published: February 2008

Inversion of slant stacks for velocity‐depth structures using only the intercept time τ(p) is suboptimal. We propose inverting slant stacks using their integrated power P(p) as an additional constraint. First‐order discontinuities and changes in gradient of velocity‐depth functions, v(z), cause losses of P(p) only for record sections of finite length. In practice then, the observed P(p) may be used to constrain v(z). One advantage of the proposed method is that both τ(p) and P(p) are found directly from the slant stack. Another advantage is that velocity structures are rapidly and accurately inferred. Finally, the method is not subject to biases, as are methods which employ first arrivals only.
Energy extraction from fractured geothermal reservoirs in low‐permeability crystalline rock
Authors: Murphy Hugh D. , Tester Jefferson W. , Grigsby Charles O. , Potter Robert M.
Published: February 2008

Two hot dry rock geothermal energy reservoirs were created by hydraulic fracturing of Precambrian granitic rock on the west flank of the Valles Caldera, a dormant volcanic complex, in the Jemez Mountains of northern New Mexico. Heat was extracted in a closed‐loop mode of operation, injecting water into one well and extracting the heated water from a separate production well. The first reservoir was produced by fracturing the injection well at a depth of 2.75 km, where the indigenous rock temperature was 185°C. The relatively rapid decline in temperature of the water produced from the first reservoir, 100°C in 74 days, indicated an effective fracture radius of about 60 m with an average thermal power extracted of 4 MW. A second, larger reservoir was created by refracturing the injection well 180 m deeper. Downhole measurements of water temperature at the reservoir outlet as well as temperatures inferred from chemical geothermometry showed that the thermal decline of this reservoir was negligible; the effective heat transfer area of the new reservoir must be at least 45,000 m2, nearly 6 times larger than the first reservoir. In addition, reservoir residence time studies employing visible dye tracers indicated that the mean volume of the second reservoir is 9 times larger. Other measurements showed that flow impedances were low and that downhole water losses from these reservoirs should be manageable. The geochemistry of the produced water was essentially benign, with no scaling problems apparent. Moreover, the level of induced seismic activity was insignificantly small.
Solute partitioning under continuous cooling conditions as a cooling rate indicator
Authors: Onorato P. I. K. , Hopper R. W. , Yinnon H. , Uhlmann D. R. , Taylor L. A. , Garrison J. R. , Hunter R.
Published: February 2008

A mathematical model is developed to describe the concentration profiles as a function of time of a solute dissolved in two coexisting finite phases under continuous cooling conditions. A temperature‐dependent partitioning of the solute between the two phases is included. The model relates to the use of experimentally determined solute profiles to infer the rates at which various rocks cooled. The model employs finite difference equations and the Thomas tridiagonal method to solve the appropriate differential equations describing the solute partitioning. To illustrate the application of the model, the partitioning of zirconia between coexisting ilmenite and ulvöspinel is considered. It is shown that cooling rates can be calculated not only from solute concentration profiles within the grains but also from the ratio of the solute concentrations at a given distance on both sides of the interface or from the ratio of the average solute concentrations in the two phases. The cooling rates of a suite of Apollo 15 Elbow Crater gabbros are calculated from measured ratios of concentrations at 10 μm from the interfaces between the phases. A comparison is made of calculated concentration ratios 10 μm from the interface and cooling rates for equilibrium versus uniform initial concentrations. It is shown that the Elbow Crater gabbros could not have had a uniform concentration profiles at the previously suggested initial temperature of 1350 K. Also considered are the effects of the initial temperature, the grain sizes, and the diffusion coefficients of Zr in the phases on the calculated cooling rates. Finally, suggestions are made for other systems where the present model can be applied to estimate the thermal histories of rocks.
A stochastic fault model: 2. Time‐dependent case
Authors: Andrews D. J.
Published: February 2008

A random model of fault motion in an earthquake is formulated by assuming that the slip velocity is a random function of position and time truncated at zero, so that it does not have negative values. This random function is chosen to be self‐affine; that is, on change of length scale, the function is multiplied by a scale factor but is otherwise unchanged statistically. A snapshot of slip velocity at a given time resembles a cluster of islands with rough topography; the final slip function is a smoother island or cluster of islands. In the Fourier transform domain, shear traction on the fault equals the slip velocity times an impedance function. The fact that this impedance function has a pole at zero frequency implies that traction and slip velocity cannot have the same spectral dependence in space and time. To describe stress fluctuations of the order of 100 bars when smoothed over a length of kilometers and of the order of kilobars at the grain size, shear traction must have a one‐dimensional power spectrum is space proportional to the reciprocal wave number. Then the one‐dimensional power spectrum for the slip velocity is proportional to the reciprocal wave number squared and for slip to its cube. If slip velocity has the same power law spectrum in time as in space, then the spectrum of ground acceleration will be flat (white noise) both on the fault and in the far field.
Thermal regime of the Escalante Desert, Utah, with an analysis of the Newcastle Geothermal System
Authors: Chapman David S. , Clement Monica D. , Mase Charles W.
Published: February 2008

Twenty‐five new heat flow measurements are presented for the Escalante Desert region within the Great Basin of the western United States. Heat flow, excluding geothermal areas, ranges from 43 to 350 mW m−2, but much of the variability may be caused by deeply circulating groundwater redistributing the regional flux. A subset of 10 sites drilled specifically to characterize the heat flow of the region yielded a mean of 100 mW m−2 with a standard deviation of 22 mW m−2. A comparison of thermal conductivities of solid cylindrical discs and rock chips (rhyolite to andesite tuffs) confirmed the importance of porosity corrections to thermal conductivity measurements. A ‘blind’ geothermal system southwest of Newcastle, Utah, situated within the Escalante Desert, has also been studied. Temperatures of 110°C are observed only 75 m below the ground surface. Heat flow results from 11 drillholes in this region yield values between 163 and 3065 mW m−2. The 500 mW m−2 contour encloses an area of 9.4 km2. By integrating the excess heat flux (above background) over the thermal anomaly, we deduce a thermal power loss of 12.8 MW for this geothermal system, which corresponds to a subsurface water discharge of 32 kg s−1.
The Juan de Fuca Ridge—Hot Spot—Propagating Rift System: New tectonic, geochemical, and magnetic data
Authors: Delaney John R. , Johnson H. Paul , Karsten Jill L.
Published: February 2008

Underway geophysics, deep tow camera work, and preliminary geochemical and magnetic studies, from a 1980 cruise to the Juan de Fuca Ridge, allow the following interpretations: (1) the central third (between 45.5°N and 46.5°N) of the ridge is now actively spreading from a zone which is 10 to 20 km west of the axis bisecting the Brunhes‐Matuyama magnetic reversal boundary, (2) the apparent terminus of surficial igneous activity at the northern tip of the active ‘Cobb Propagator’ has been identified by dredging and camera work; the tip appears curved, or offset slightly, to the west, does not correspond to the northernmost tip of the central magnetic anomaly [Elvers et al., 1974], and spreading has been initiated beneath sediments farther north, and (3) iron and titanium in fresh glasses from the active volcanic zone exhibit a progressive enrichment northward, then a pronounced reversal to more ‘normal’ concentrations in the vicinity of the Cobb Offset.
Phase transition in SrO
Authors: Sato Yosiko , Jeanloz Raymond
Published: February 2008

Static‐compression experiments to 59 GPa (590 kbar), employing X ray diffraction through a diamond cell, demonstrate that strontium oxide (SrO) transforms from its initial B1 (NaCl‐type) to the B2 (CsCl‐type) structure at 36 (±4) GPa on the ruby fluorescence scale with a volume change at the transition ΔV/V1 ∼ −13%. These results confirm earlier predictions based upon systematics for oxides but are in lesser agreement with ab initio (modified electron gas) calculations. The new data for the B1 phase are consistent with previous ultrasonic and static‐compression measurements, which indicate that K0′ > 4. Data spanning the 25‐GPa range in pressure constrain the equation of state of the B2 phase. The change in bulk sound velocity at the transition is shown to be essentially zero, which is a significant deviation from the behavior predicted by velocity‐density systematics.
an interferometric technique for measuring velocity and attenuation in molten rocks
Authors: Katahara K. W. , Rai C. S. , Manghnani M. H. , Balogh J.
Published: February 2008

An interferometric technique has been developed for the purpose of measuring ultrasonic velocity and attenuation in rock melts. In essence, a long wave train is transmitted through two long buffer rods separated by a thin layer of melt. Multiple reflections in the melt layer interfere with each other and give rise to resonances for melt thicknesses equal to integral multiples of half the wavelength. The velocity and attenuation can be obtained by measuring the amplitude of the transmitted wave as a function of either the melt layer thickness or the frequency. The accuracy of the method was tested at room temperature by compressional wave measurements on water, mercury, and carbon disulfide. Velocities were found to be accurate to within 0.3%. Values of the specific dissipation factor Q−1 as low as 10−3 can be resolved for a liquid such as water with a low acoustical impedance, whereas Q−1 values as low as 10−4 can be measured for high‐impedance liquids such as mercury. Q−1 values obtained on carbon disulfide agree with previous work to within better than 0.001. Experiments on a viscous organic polymer show that shear measurements can be made and that Q−1 values up to at least 0.2 can be measured by this technique. The method has been successfully used to study a basalt melt to 1500°C. Sources of error and possible improvements are discussed, and the method is compared to previously used techniques. The method has the potential of measuring velocity and attenuation in partial melts and solids.
Application of wave field continuation to the inversion of refraction data
Authors: McMechan George A. , Clayton Robert W. , Mooney Walter D.
Published: February 2008

Three examples of the inversion of refraction data by downward continuation illustrate the applicability of the method to field data. The first example is a refraction profile from the Mojave Desert, California. These data are spatially aliased and contain clear evidence of lateral inhomogeneity. The inversion in this case produces a broken image in the slowness‐depth domain due to the lateral inhomogeneity, but a useful average velocity model is still obtained. The second example is a shallow marine reflection profile. Here, the truncation effects due to the finite horizontal aperture of the recording cable produce artifacts in the slowness‐depth domain. The velocity model is, however, distinct from these artifacts, and the presence of strong precritical reflections aids in the inversion. The third example is another shallow marine reflection profile. The inversion of these data illustrate the utilization of constraints provided by multiples as well as primary arrivals.
Injection and Thermal Breakthrough in Fractured Geothermal Reservoirs
Authors: Bödvarsson Gudmundur S. , Tsang Chin Fu
Published: February 2008

In this study, the problem of cold water injection into a fractured geothermal reservoir is considered. During injection, the cold water will advance along the fractures, gradually extract heat from the adjacent rock matrix, and eventually arrive at the production wells. If the injected water has not fully heated (up) by then, detrimental effects on energy production from decreasing fluid enthalpies may result. This indicates the need to establish criteria for designing an injection/production scheme for fractured geothermal reservoirs. The model considered in this work consists of an injection well fully penetrating a fractured geothermal reservoir containing equally spaced horizontal fractures. A constant‐temperature liquid water is injected into the fractures, and with the rock matrix assumed to be impermeable, the effect of heat conduction on the advancement of the ‘cold’ water along the fractures is observed. Key dimensionless parameters that describe the physical system are identified and type curves are generated. These are intended to be used in the design of injection/production systems, mainly for determining the appropriate locations and the flow rates of the injection wells. Also, temperature contour maps are given, and these can be used to estimate the amount of recoverable energy from the geothermal system, based on a given injection/production scheme. Numerical studies are also made to investigate the importance of the assumptions in the analytical work and to extend the work to cases where the rock matrix is permeable. The results are intended to be used as a constraint on results calculated using the type curves. Finally, two simple numerical examples using a hypothetical doublet and the Cerro Prieto geological model are given. In the doublet case, a reservoir with two to five fractures is considered, and breakthrough times of 13 and 65 years, respectively, are calculated. Using typical injection rate and the Cerro Prieto geological model, our calculations indicate that the thermal front will have advanced only 240 m away from the injection well after 30 years of injection. It is furthermore illustrated that this estimate is independent of fractures in the reservoirs.
The Bangui Magnetic Anomaly: Its geological origin
Authors: Regan Robert D. , Marsh Bruce D.
Published: February 2008

Satellite magnetometer data have revealed a long‐wave length magnetic anomaly in the vicinity of Bangui, Central African Republic. Substantiation of the anomaly has been made through analysis of air and ground magnetic surveys. A correlative gravity anomaly has also been observed in ground gravity data. The anomaly, in an area of highly metamorphosed Precambrian rocks, does not correlate with any surficial features but is definitely crustal in origin. An interpretation consistent with all available geophysical and geological data is that the anomaly could be the result of an early intrusion into the crustal rocks of this region, followed by subsidence and deformation.
Ignimbrites of Amazonis Planitia Region of Mars
Authors: Scott David H. , Tanaka Kenneth L.
Published: February 2008

A series of postulated ignimbrite units is mapped in the Amazonis, Memnonia, and Aeolis quadrangles of Mars. The units cover about 2.2×106 km2 within a broad but discontinuous and irregular belt trending east‐west along the highland‐lowland boundary. The ignimbrites overlie parts of the western and southern aureole materials of Olympus Mons but are embayed in places by the lava plains of the lowlands. Stratigraphic relations between the basalt flows from the Tharsis Montes region and the ignimbrites are not clearly defined; crater counts suggest that the younger ignimbrites postdate the lava flows. Crater counts per square kilometer for the ignimbrites range from 7.29±1.95×10−4 to 6.36±2.01×10−5 for craters larger than 1 km in diameter. The ignimbrite materials form thick (≥100 m), extensive, relatively flat sheets that are smooth to grooved or gently undulating. Grooved surfaces appear to be yardangs and, in most places, are not alined with prevailing wind directions. The seven mapped ignimbrite units are characterized by morphologic expression, stratigraphic position, and crater counts. Similarities to ignimbrites in the Pancake Range of central Nevada include (1) rounded patches of smooth, high‐albedo, nonwelded material superposed on jointed, low‐albedo, welded material, (2) local complementary joint sets in welded materials, and (3) thick flow sheets of great areal extent that follow but subdue underlying topography. Four major eruptive centers occur in areas where units are thickest and where a dominant, NNW‐SSE structural trend is expressed locally by unit margins, elongate collapse features, and normal faulting. A minimum volume of 3.85×106 km3 for the deposits has been calculated from thickness estimates based on shadow measurements and crater rim height relations.
A simple mechanical model of Valhalla Basin, Callisto
Authors: Melosh H. J.
Published: February 2008

The Valhalla basin on Callisto is a multiringed structu‐e that extends over much of the satellite's surface. Although its appearance differs in detail from lunar multiringed basins, its origin may also be attributed to lithospheric fragmentation accompanying collapse of the transient crater formed by an impact event. This paper explores the mechanics of the collapse process by treating the lithosphere as a thin elastic‐Von Mises plastic sheet (plane geometry) or shell (spherical geometry). Flow of the underlying asthenosphere inward toward the crater cavity induces plastic failure of the lithosphere and produces a characteristic pattern of faults in the disrupted lithosphere. The pattern and extent of faulting is a function of a single dimensionless parameter that involves the strength and thickness of the lithosphere, the crater depth and diameter, and the surface gravity of the planet. The tectonic structures of Valhalla correspond well with the failure pattern expected for a large crater produced in a thin (circa 30 km) weak (strength <100 bar) lithospheric shell.