bluuebunny/crossref_metadata_2025_split huggingface dataset:
Seismological aspects of the Guatemala Earthquake of February 4, 1976
Authors: Kanamori Hiroo , Stewart Gordon S. 
Published: February 2008

Detailed analyses of teleseismic surface waves and body waves from the Guatemala earthquake of February 4, 1976, show the following: (1) Left lateral displacement along a vertical fault with a strike varying from N66°E to N98°E is consistent with the teleseismic data. (2) The seismic moment was 2.6 × 1027 dyn cm. The directivity of the surface wave radiation indicates an asymmetric (1:2.3) bilateral faulting with a total length of 250 km. In modeling the displacement a rupture velocity of 3 km/s was used, and the fault curvature was included. (3) If a fault width of 15 km is assumed, the average offset is estimated to be about 2 m. This value is about twice as large as the average surface offset. (4) Although the observed directivity suggests a uniform overall displacement along the fault, the body wave analysis suggests that the earthquake consists of as many as 10 independent events, each having a seismic moment of 1.3–5.3 × 1026 dyn cm and a fault length of about 10 km. The spatial separation of these events varies from 14 to 40 km. This multiple‐shock sequence suggests that the rupture propagation is jagged and partially incoherent with an average velocity of 2 km/s. (5) The average stress drop estimated from surface waves is about 30 bars, but the local stress drop for the individual events may be significantly higher than this. (6) The complex multiple event is a manifestation of a heterogeneous distribution of the mechanical properties along the fault, which may be caused by either asperities, differences in strength, differences in pore pressure, differences in slip characteristics (stable sliding versus stick slip), or combinations of these factors. (7) This complexity has important bearing on the state of stress along transform faults and is important in assessing the effect of large earthquakes along other transform faults like the San Andreas.Appendix Table Al is available with entire article on microfiche.Order from American Geophysical Union, 1909 K Street, N. W.,Washington, D. C. 20006. Document J78‐004; $1.00. Payment mustaccompany order.
Egress of hot water from midocean ridge hydrothermal systems: Some thermal constraints
Authors: Sleep Norman H. , Wolery Thomas J. 
Published: February 2008

The flow rates necessary for hot water to vent directly into the ocean at midocean ridge hydrothermal systems are obtained by balancing heat lost by conduction through the walls of the hydrothermal passages with the heat brought up in the water. Widespread porous flow is unlikely to vent hot water even at the ridge axis. More spatially restricted flow was modeled by planar and tubular crack zones. The planar zones can vent water only through young (age <1 m.y. B.P.) crust and then must be spaced on the order of kilometers. Tubular zones vent hot water on moderately old crust (1 m.y. B.P. <age <11 m.y. B.P.). A factor of 2 or 3 change in flow rate can change the temperature in a crack zone from slow cooling as the fluid rises to venting of hot water. Clogging will eliminate vents where extensive cooling or mixing occurs in the uppermost regions. The flow volumes predicted for cracks are large enough that minor and trace constituents may clog crack zones.Appendix is available with entire article on microfiche. Order fromAmerican Geophysical Union, 1909 K Street, N. W., Washington, D. C. 20006. Document J78‐008; $1.00. Payment must accompanyorder.
Stratigraphy of Oceanus Procellarum basalts: Sources and styles of emplacement
Authors: Whitford‐Stark James L. , Head James W. 
Published: February 2008

The appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D. . 20009. Document J80‐005; $1.00. Payment must accompany order.The basalts within Oceanus Procellarum have been named the Oceanus Procellarum Group and subdivided into four major geologic units on the basis of morphology, spectral reflectance, and other remote sensing information in order to develop a framework of regional stratigraphy for the largest lunar mare. The oldest mare basalt unit identified, the Repsold Formation (blue color, low albedo, high crater density, very weak 3.8‐cm radar backscatter, and a weak 1‐μm reflectance) crops out in NW Procellarum and may be correlative with Apollo 11 and 17 high‐titanium basalts. The next youngest unit, the Telemann Formation (red, higher albedo, high crater density, high DL values, and an average to strong 1‐μm reflectance) extends over wide areas of Procellarum and crops out as extensive surface units and as local patches surrounding earlier, topographically higher units. Numerous large sinuous rules in the Aristarchus Plateau were a major source for some of these deposits. Pyroclastic deposits forming the dark red mantle were also emplaced at the Aristarchus Plateau during these eruptions. The Hermann Formation (intermediate color, low albedo, intermediate crater density, and average to strong 1‐μm reflectance) occurs as extensive plains developed from less voluminous eruptions from sources at the Marius Hills complex, the Rümker Hills, and in Nubium, Cognitum, and near Delisle. The youngest unit, the Sharp Formation (blue color, very low albedo, low crater density, and a strong 1‐μm reflectance) is volumetrically small and is derived primarily from vents near the highland/mare boundary. Many Sharp Formation members have associated sinuous rules. Basalts of the Hermann Formation were sampled by Apollo 12; those of the Telemann Formation may be similar to the Luna 24 VLT basalts, while those of the Repsold Formation may be comparable with the Apollo 11, 17 and Luna 16 samples. The Sharp Formation, the youngest basalts, remains unsampled. The total volume of Procellarum basalts is estimated at 8.7×105 km3 (about 10% of the total mare volume). The relative volumes of the individual formations are not reflected by the surface area covered because the youngest flows tend to be thin and widespread. For example, the Sharp Formation has a surface area of 72,000 km2 but an estimated volume of only 1.8×104 km3, while the Repsold Formation has a present surface area of only 22,000 km2 but an estimated volume of 2.1×105 km3. The vast majority of lavas in Procellarum were extruded early in the phase of lava flooding, prior to about 3.5 b.y.
Seismicity parameters preceding moderate to major earthquakes
Authors: von Seggern David , Alexander Shelton S. , Baag Chang‐Eob 
Published: February 2008

Seismic events reported in the bulletins of the two large arrays, LASA and NORSAR, were merged with those from the NEIS bulletin for the period 1970–1977. Using a lower cutoff of mb= 5.8, 510 ‘main shocks’ within the P range of LASA or NORSAR were selected for this period; and various seismicity trends prior to them were investigated. A search for definite foreshocks, based on a significantly short time delay to the main shock, revealed that the true rate of foreshock occurrence was less than 20%. Foreshocks are almost exclusively associated with shallow (h < 100 km) main shocks. To establish common features, a method of averaging seismicity from many regions was used to suppress the randomness of the seismic behavior of each region. This averaging shows that the seismicity level around the main shock increases somewhat for 10 days before main shocks; this feature peaks in the last 3–4 hours prior to the main shocks. The averaging also reveals that the mean magnitude of events near the main shock increases prior to main shocks but only by a few hundredths of a magnitude unit. Again by averaging, the seismicity about main shocks is shown to tend with time toward the main shock as its origin time is approached, but the average effect is small (∼10% change). By expanding or contracting each region's time scale before averaging to relate to the magnitude of the main shock, these features are enhanced. Using a new variable to track the departures from both spatial and temporal randomness, the Poisson‐like behavior of deeper seismicity (>100 km) was demonstrated. For shallow events (<100 km) this variable reveals numerous instances of clustering and spatial‐temporal seismic gaps, with little tendency toward a uniformity of behavior prior to main shocks. A statistical test of the validity of seismic precursors was performed for approximately 90 main shock regions which had sufficient seismicity. Using a five‐variable vector (interevent time, interevent distance, magnitude, epicentral distance to main shock, and depth difference relative to main shock) for each event in a ‘precursory’ time window of 500 days before the main shock and for each event in a ‘normal’ time window of 500 days before that, the null hypothesis of equal vector means between the two groups was tested. At 90% confidence level, less than 30% of the main shock regions were thus found to exhibit precursory seismicity changes.Appendices are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐007; $1.00. Payment must accompany order.
Geodetic measurement of crustal deformation on the San Andreas, Hayward, and Calaveras Faults near San Francisco, California
Authors: Prescott W. H. , Lisowski M. , Savage J. C. 
Published: February 2008

Analysis of a geodetic network of 115 lines crossing the San Andreas, Hayward, and Calaveras faults in the vicinity of San Francisco Bay and measured repeatedly between 1970 and 1980 has revealed details about the accommodation of relative plate motion in this area. The most striking result is that the deformation is not uniformly distributed across the area. In the east bay, along the Hayward and Calaveras faults, all motion appears to take place as slip directly on the fault, with no accumulation of strain in the adjacent crust. On both the Calaveras and the Hayward faults the rate obtained for the 1970 to 1980 period agrees with geologic rates spanning a few million years and with creep rates spanning a few decades. The Hayward fault slip rate is 7 ± 1 mm/yr. The Calaveras fault slip rate is 7 ± 1 mm/yr, with perhaps half of this slip distributed across a zone a few kilometers wide, probably as inelastic deformation of weak near‐surface material. The absence of strain accumulation in the east bay is surprising since the Hayward and Calaveras faults have been the site of large earthquakes in the past. A block located east of the Calaveras fault and south of the Las Positas fault has been rotating clockwise at a rate of 0.3 ± 0.1 μrad/yr with very little internal deformation. Along the San Francisco peninsula no detectable slip occurs (less than 1.5 mm/yr) at the surface, but appreciable strain is accumulating. Near fault shear strain rates are 0.6 ± 0.1 μstrain/yr (engineering) with direction N47°W ± 9. The slip rate near the San Andreas fault is 12.2 ± 3.9 mm/yr distributed across a broad zone. The relative motion across the whole region during the period 1970–1980 is 32.1 ± 7.4 mm/yr.Supplement available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N. W., Washington, D. C. 20009. Document J81‐011; $1.00. Payment must accompany order.
Uncertainties in the relative positions of the Australia, Antarctica, Lord Howe, and Pacific Plates since the Late Cretaceous
Authors: Stock Joann , Molnar Peter 
Published: February 2008

We determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the Pacific and Antarctica plates, between Australia and Antarctica, and between the Lord Howe Rise and Australia. We combined these to yield a range of possible finite rotations describing the relative positions of the Pacific, Australia, Antarctica, and Lord Howe plates since the Late Cretaceous. If the Pacific‐Australia plate boundary has had its present trend since anomaly 18 time, reconstructions show 330±110 km of motion of the Pacific plate relative to the Lord Howe Rise since anomaly 5 time (9.8 m.y.), 420±110 km since anomaly 6 time (19.5 m.y.), 770±330 km since anomaly 13 time (35.6 m.y.), and 820±260 km since anomaly 18 time (43.0 m.y.). We examined two cases for times prior to anomaly 18, assuming a Late Cretaceous age of Australia‐Antarctica separation. If a plate boundary existed between the Lord Howe Rise and Pacific plates since the Late Cretaceous, with no plate boundary in Antarctica, reconstructions with the Lord Howe Rise fixed predict 610 ± 200 km of westward motion of the Pacific plate between the times of anomalies 31 and 22, followed by 260±100 km of northward motion between the times of anomalies 22 and 18. If the Lord Howe Rise was fixed to the Pacific plate until the Eocene, but a plate boundary existed between East and West Antarctica, reconstructions show very little motion across this boundary between the times of anomalies 31 and 22, followed by convergence between the times of anomalies 22 and 18. This second case also brings 70–80 m.y. paleomagnetic poles from the Pacific and East Antarctica plates into better agreement than the first case, but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated.Appendices are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82‐01; $1.00. Payment must accompany order.
Aftershocks of the Coyote Lake, California, earthquake of August 6, 1979: A detailed study
Authors: Reasenberg P. , Ellsworth W. L. 
Published: February 2008

Aftershock hypocenters and focal mechanism solutions for the Coyote Lake, California, earthquake reveal a geometrically complex fault structure, consisting of multiple slip surfaces. The faulting surface principally consists of two right stepping en echelon, northwest trending, partially overlapping, nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 Parkfield, California, earthquake. The overlap occurs near a prominent bend in the surface trace of the Calaveras fault at San Felipe Lake. Slip during the main rupture, as inferred from the distribution of early aftershocks, appears to have been confined to a 14‐km portion of the northeastern sheet between 4‐ and 10‐km depth. Focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex, with left stepping imbricate structure. Seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock. Aftershocks in this zone define a single vertical plane 8 km long between 3‐ and 7‐km depth. Within the overlap zone between the two main slip surfaces, the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone, in close agreement with the stress perturbations predicted by crack interaction models. Aftershock activity in the overlap zone is not associated with a simple dislocation surface. Space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone. This alternation is consistent with local stress perturbations predicted by crack interaction models. We conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which, in turn, control the displacements.Table 5 is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82‐006; $1.00. Payment must accompany order.
The Manicouagan Impact Structure: An analysis of its original dimensions and form
Authors: Grieve Richard A. F. , Head James W. 
Published: February 2008

Manicouagan, Quebec (51°23'N; 68°42'W) is the most intensively studied large complex terrestrial impact structure in a predominantly crystalline target. The ground truth data available from Manicouagan have considerable potential for interpreting the subsurface characteristics of lunar impact structures of comparable morphology and size. Two contrasting hypotheses, however, exist for the preerosional form of Manicouagan: (1) a multi‐ring basin with a final diameter of 75 km and a transient cavity diameter of 30–45 km [Floran and Dence, 1976], and (2) an endogenically modified peak‐ring basin with a final diameter of 100 km and a transient cavity diameter of 80 km [Orphal and Schultz, 1978]. Both hypotheses are based largely on topographic data and interpret the prominent 65 km diameter annular moat as a graben‐like feature formed either (1) by collapse of the outer slope of the transient cavity rim or (2) by uplift of the final crater floor through post‐impact intrusive activity.The present analysis suggests that this annular moat is primarily an erosional feature and that the original form of the impact structure cannot be determined unequivocally on the basis of present topography. If all the available topographic, geologic, and geophysical data are considered, however, then an internally consistent interpretation of original dimensions is possible. The distribution of shock deformation effects in the basement rocks of the present crater floor suggests an original transient cavity diameter of 60 km. The 55 km‐diameter, impact melt‐covered inner plateau provides a minimum estimate for the diameter of the final floor of the crater and the annular moat, with its outliers of downdropped Ordovician limestone, is interpreted as marking the contact between the floor and the innermost slump blocks of the final rim. There is no compelling evidence to regard the annular moat as a graben. By analogy with fresh lunar and mercurian structures, a floor width of 55 km suggests a final rim diameter of 86–95 km. This is consistent with the topographic data from outside the annular moat and with the residual peripheral gravity low, which suggests a final rim diameter of 85–95 km. Due to erosion, it is difficult to assign Manicouagan to a particular morphological class of impact structure, but on the basis of the available data it is most likely that the preerosional form was that of a central peak crater or possibly a peak‐ring basin. At the time of its formation, Manicouagan may have been dimensionally and possibly morphologically similar to the 96 km diameter lunar crater Copernicus.
Relative motions between oceanic plates of the Pacific Basin
Authors: Engebretson David C. , Cox Allan , Gordon Richard G. 
Published: February 2008

Appendix tables are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C., 20009. Document B84‐012; $2.50. Payment must accompany order.Relative motion poles describing the displacement histories between the Pacific plate and once adjacent oceanic plates (Farallon, Kula, Izanagi I, Izanagi II, and Phoenix) were derived for the late Mesozoic and Cenozoic eras. Because fracture zone and magnetic anomaly data are generally available from the Pacific plate but not from adjacent plates, a new method of analysis for onesided data was required. This analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions. The following are the main conclusions drawn from our analysis: (1) For time intervals of the order of 107 years, termed stages, relative motion poles for plate pairs remained nearly fixed. Between stages, shifts in poles were commonly both large and abrupt. Within stages, rates of plate motion were commonly observed to change markedly, indicating that plates changed speed more frequently than they changed direction. (2) The relative motions of all of the plates analyzed changed at about chron M11 (135 Ma), chron 34 (85 Ma), and chron 25 (56 Ma). (3) During the Early Cretaceous there were five oceanic plates in the Pacific basin rather than the four recognized by previous workers. (4) To determine the number of Farallon plates that existed to the east of the Pacific plate during the time interval from chron 34 (85 Ma) to chron 25 (56 Ma), fracture zones and magnetic anomalies that record Pacific‐Farallon spreading from the northern, central, and southern Pacific plate were analyzed separately and collectively. The analysis shows that a single Pacific‐Farallon relative motion pole and a single rate are consistent with all of the data. (5) Spreading rates along the Pacific‐Kula ridge decreased markedly between chrons 32b and 25 (72–56 Ma), probably in response to the arrival of buoyant Kula lithosphere at a subduction zone northwest of the Bering Sea. (6) Soon after chron 25 (56 Ma), a major reorganization is recorded along the Pacific‐Farallon boundary. The Kula‐Farallon boundary is also thought to have changed during this reorganization. (7) Kula‐Pacific anomalies north of anomaly 25 are modeled by assuming continued Kula‐Pacific spreading after chron 25 (56 Ma). (8) Probably by chron 18 (43 Ma), Pacific‐Kula spreading had ceased and the Kula‐Farallon ridge system had evolved into alignment with the Pacific‐Farallon spreading direction. However, the timing of these events is uncertain because Pacific‐Kula data younger than chron 25 (56 Ma) are sparse.
Paleodepth distribution of Neocene deep‐sea hiatuses
Authors: Keller Gerta , Barron John A. 
Published: February 2008

The depth of formation of Miocene to middle Pliocene deep‐sea hiatuses (NH1 to NH8)has been determined using 152 Deep Sea Drilling Project sites in the Atlantic, Pacific and Indian oceans. Megahiatuses representing maxima in deep‐sea erosion, occur at three water depths: below 3800 m, at intermediate depths between about 2000 and 3000 m, and in shallower waters above 1500 m paleodepth. Both the paleodepths and distribution of these hiatus maxima suggest that flow of Antarctic Bottom Water, Antarctic Intermediate Water, and North Atlantic Deep Water masses largely caused this erosion. Correlation of the hiatuses with oxygen isotope, carbonate and sea level records indicates that prior to 11 Ma, brief hiatuses including those resulting from nondeposition in high‐productivity equatorial regions largely correlate with global cooling episodes, high‐carbonate content, and lowstands of sea level. During the last 11 m.y., hiatuses also seem to correlate with cooling episodes, but carbonate dissolution is characteristic and sea levels may have been rising or at a lowstand. During the NH4, NH7, and NH8 intervals, it is possible that a rise in sea level corresponds with polar cooling, but there is uncertainty in correlation. Brief hiatuses during rising sea levels can be explained by basin‐shelf fractionation of carbonates. Hiatuses during sea level lowstands and cooling episodes may result from intensified bottom water circulation and increased corrosiveness of bottom water due to higher levels of CO2and increased productivity during increased upwelling.Supplement (Appendix 1) is availablewith entire article on microfiche. Orderfrom American Geophysical Union, 2000Florida Avenue, N.W., Washington, DC20009. Document P87001; $2.50. Paymentmust accompany order.