bluuebunny/crossref_metadata_2025 huggingface dataset:
Migration of wide‐aperture onshore‐offshore seismic data, central California: Seismic images of late stage subduction
Authors: Lafond Claude F. , Levander Alan
Published: February 2004

We conducted a high shot and receiver density, onshore‐offshore seismic experiment in 1986 across the central California transform margin as part of the Pacific Gas and Electric / EDGE Deep Crustal Geophysical Survey. The data set consists of land recordings of 406 marine air gun shots fired along a 70‐km track across the Santa Maria basin. The seismic vessel's track was collinear with 154 land seismic recorders deployed over 50 km across the Sur Obispo terrane and part of the Salinian block. The source‐receiver offsets range from 2.5 km to 120 km. The unique recording geometry, and high shot and receiver density, permit imaging the deeper crustal structure of the California transform margin but pose serious challenges for conventional seismic imaging methods. Consequently we developed a unified approach to wide‐angle imaging which incorporates a tomography technique for obtaining initial migration velocity models, and a prestack Kirchhoff depth migration with depth focusing of the main reflectors for velocity model refinement and imaging. This approach retains the velocity and dip information in the data and allows user control over the imaging process. We applied this processing to subsets of the onshore‐offshore data to obtain an upper crustal velocity model, and a 32‐km‐long, wide‐angle depth migrated image of the lower crust, in the coastal region under the Santa Maria basin, and the Sur‐Obispo terrane. The migration image shows a part of an underplated oceanic crustal layer, interpreted as the Monterey plate (Meltzer and Levander, 1991; Miller et al., 1992), which dips coastward at depths of 18 to 28 km. The plate appears to be kinked or imbricated beneath the coastline. The image correlates well with existing wide‐angle seismic interpretations based on ray tracing (Walter and Sharpless, 1987; Putzig, 1988; Howie et al., 1993). The migrated lower crustal image has more structural information than is apparent in conventional ray‐based interpretations along the same, crossing, and adjacent profiles. The crustal image demonstrates that automated imaging techniques applied to wide‐angle data can provide images of crustal structure comparable in fidelity to conventionally processed and migrated near‐vertical incidence data.
Hierarchical geometry of faulting
Authors: Ouillon G. , Castaing C. , Sornette D.
Published: February 2004

Several joint and fault patterns were mapped at different scales (from 1 cm to 100 km) on the sedimentary cover of Saudi Arabia. They were analyzed using a new multifractal algorithm improved to correct for irregular geometry of mapped domains and finite size effects. Moreover, a new technique (the Optimal Anisotropic Wavelet Coefficient method) was conceived. It allows transformation to large scales maps from smaller scales ones and quantifies the multiscale behavior of faulting anisotropy. It consists of finding at each point on a fault map an optimum filter that reveals the local structure. Fault length distributions are also computed. Our main conclusion is that different geometrical power laws and anisotropic textures hold separately in distinct limited ranges, separated by characteristic scales. We can observe a one‐to‐one correspondence between those characteristic scales and the depths of the main lithological or rheological interfaces in the Arabian crust. Fracturing and faulting can thus be better modeled as a hierarchical process, controlled in large part by the layering of the crust.
Magma ascent simulations of Etna's eruptions aimed at internal system definition
Authors: Dobran Flavio , Coniglio Silvia
Published: February 2004

The internal system of Etna was studied by employing a nonequilibrium two‐phase flow model of magma ascent which accounts for different gas‐magma flow regimes and gas loss to surrounding conduit fractures when the magma pressure exceeds the local lithostatic pressure. Simulations of different phases of the 1974 and 1989 eruptions of Etna with different flow regimes in conduits suggest that the pyroclastic, strombolian, lava fountaining, and effusive lava flow activity can be modeled by effectively accounting for different physical processes of magma ascent. Results from simulations and geophysical and geological data suggest that magma ascent at Etna may occur along a central conduit or inclined conduits from a magma storage region located from 8 to 9 km below the summit, depending on the regional tectonic stresses and characteristics of the magma supply system. The results also suggest the existence of a structurally weak zone from 1 to 4 km below the summit where magma may accumulate and from which fracture systems and magma ascent with gas loss to fractures may propagate to the eruptive vents at the surface. The predicted excess magmatic pressure over lithostatic in the upper regions of conduits without gas loss to fractures suggests fracturing of the volcanic edifice during initial stages of eruptions and subsequent magma ascent and gas loss along these fractures. Modeling of the changes of activity at Etna requires significant new efforts in geophysics aimed at high‐resolution internal system definition.
Terrestrial atmospheric effects on satellite eclipses with application to the acceleration of LAGEOS
Authors: Rubincam David Parry , Mallama Anthony
Published: February 2004

We examine spatial variability of attenuation in the Earth's atmosphere as a cause of asymmetrical eclipses and consequent acceleration of LAGEOS, i.e., the solar radiation pressure on LAGEOS due to the Earth's penumbra. Measurements of atmospheric attenuation derived from the satellite‐borne Stratospheric Aerosol and Gas Experiment after the eruption of Mount Pinatubo were used to simulate the largest expected aerosol content of the atmosphere. In our experiment one hemisphere was loaded with volcanic aerosols, while the other was not. The difference between attenuation in the two hemispheres sets a maximum reasonable limit to the size of eclipse asymmetry. This condition would accelerate LAGEOS only about 0.2 picometers per second squared (pm s−2 or 10−12 m s−2) and indicates that eclipse asymmetry can only account for about 40–50% of the remaining unmodeled residuals. This is slightly less than the penumbral acceleration found by Vokrouhlicky et al. (1994).
Seismic attenuation structure of the East Pacific Rise near 9°30′N
Authors: Wilcock William S. D. , Solomon Sean C. , Purdy G. M. , Toomey Douglas R.
Published: February 2004

We describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9°32′N on the East Pacific Rise. Over 3500 estimates of t* are obtained from 0.4‐ to 0.7‐s‐long windows aligned with crustal P phases, including diffractions above and below a midcrustal magma chamber and Moho reflections which cross the rise axis at a range of lower crustal depths. We apply a smoothest model inversion algorithm to the t* measurements to derive images of apparent crustal Q−1 both 20 km off axis and within a 16×16 km area centered on the rise crest. The models resolve regions of high attenuation in the uppermost crust and in a low‐Q zone which extends from midcrustal to lower‐crustal depths beneath the rise axis. Off axis, Q values in the upper 1 km average 35–50, while at depths greater than 2–3 km Q is at least 500–1000. The high levels of attenuation in the uppermost crust probably result from the combined effect of factional, fluid flow, and scattering mechanisms. Within 1–3 km of the rise axis, Q increases markedly in the uppermost 1 km to about 65. If the increase in attenuation off axis is entirely due to the ∼300‐m increase in the thickness of layer 2A extrusives required by seismic velocity measurements, then Q in layer 2A must be about 10–20. No measurements of Q are obtained in the immediate vicinity of the 1.6‐km‐deep axial magma lens because no wave paths cross the rise axis through this region. The diffractions beneath the magma chamber and the Moho reflections require a low‐Q region, with minimum Q values of 20–50, which extends from no more than 2.5 km depth to the base of the crust. These values are similar to laboratory measurements of Q obtained at solidus temperatures and constrain the low‐Q region to contain no more than a few percent melt. The axial magma chamber, which comprises a melt lens and an underlying crystal mush zone, must be confined to a narrow, 1‐km‐thick region through which no rise‐crossing paths pass. Inversions for along‐axis structure in the low‐Q anomaly show a 20–25% increase in attenuation at 2–3 km depth north of 9°34′N but resolve no such trend at 4–6 km depth. The along‐axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along‐axis variations in magma supply.
Surface rupture earthquakes over the last ∼1000 years in the Wellington region, New Zealand, and implications for ground shaking hazard
Authors: Van Dissen Russ J. , Berryman Kelvin R.
Published: February 2004

The Wellington region is cut by five active right‐lateral strike‐slip faults: Wairarapa, Wellington, Ohariu, Shepherds Gully/Pukerua, and Wairau faults that have average recurrence intervals of meter‐scale surface rupture that range from ∼500 years to 5000 years, and lateral slip rates that range from 1 to 10 mm/yr. Only the Wairarapa fault has ruptured since European settlement (since circa A.D. 1840). Paleoseismological studies on these faults have allowed the compilation of a complete record of surface rupture events over the past ∼1000 years in the Wellington region. Within this time period, there does not appear to be any temporal clustering of surface rupture events on adjacent faults. TheM8 A.D. 1855 Wairarapa earthquake did not trigger rupture on any other fault in the region. The most recent surface‐faulting event on the Wellington fault (290–440 cal years B. P.) (cal years are calendar years before A.D. 1950) does not coincide with rupture of any other onland fault, and over 300 years separate the timing of the second most recent rupture on the Wellington fault (660–720 cal years B.P.) and the most recent rupture of the Ohariu fault (1060–1140 cal years B.P.). The most recent rupture of the Shepherds Gully/Pukerua fault is probably older than that of the Ohariu fault. The apparent nonclustering of surface rupture earthquakes in the Wellington region has been documented only for the on‐land strike‐slip faults. There are other possible seismogenic sources in the region, and thus important issues remain to be addressed regarding the history of large earthquakes in the Wellington region: (1) the seismogenic potential and earthquake recurrence interval of the subduction thrust beneath Wellington is not known; (2) the timing of rupture events on the offshore portion of the Wairau fault is not known; and (3) paleoseismic data are not available for the section of the Wellington fault north of the Wellington‐Hutt Valley segment. Estimates of earthquake hazard in the Wellington region, for all return times greater than 50 years, that incorporate paleoseismicity data are between one and two Modified Mercalli (MM) intensity units higher than the hazard based solely on the historical seismicity catalog, and the hazard is spatially more variable. Using a deterministic attenuation model, the level of shaking hazard approaches near maximum values within a return time of ∼500 years, largely reflecting the recurrence interval (500–770 years) of surface rupture earthquakes on the Wellington fault. Inclusion of a plausible model for magnitude 8 subduction zone earthquakes does not affect the level of MM intensity in Wellington region at return times greater than 500 years but does make a small contribution to the hazard at return times between 50 and 500 years.
Crustal structure and tectonics of the southeastern Canadian Cordillera
Authors: Zelt C. A. , White D. J.
Published: February 2004

The crustal structure of the Omineca (OB) and Foreland (FB) belts of the southeastern Canadian Cordillera are interpreted from the inversion of seismic refraction/reflection travel times and amplitudes, and modeling of the Bouguer gravity data from a 350‐km east‐west wide‐angle profile. The main features of the resultant velocity and density models include (1) a low average crustal velocity of 6.2 km/s, (2) variable upper crustal velocities (5.6–6.3 km/s) within the FB and Purcell Anticlinorium as compared to farther west in the OB (6.1–6.2 km/s), (3) a midcrustal (∼20 km depth) 0.4–0.5 km/s velocity increase within the OB, (4) eastward crustal thickening from 35 to 42 km over 80 km distance beneath the OB‐FB boundary, (5) the Slocan Lake fault (SLF) dipping east at 15–20° to at least 35 km depth, (6) decreased lower crustal velocities (6.7–6.4 km/s) across the SLF from the OB into the FB, (7) increased uppermost mantle velocities (7.9–8.0 km/s) from the OB into the FB, and (8) an average crustal density increase of 40 kg/m3 from the OB into the FB. Lower velocities, higher conductances, and increased densities within the upper crust (0–10 km depth) of the Purcell Anticlinorium appear to be associated with Middle and Upper Proterozoic rift‐related metasediments and gabbroic dikes (35–55% by volume) in contrast to the mainly Mesozoic felsic intrusives and metamorphic rocks that dominate farther west in the OB. Elevated crustal temperatures within the OB (a high heat flow province) are responsible for low‐velocity gradients to 20 km depth. A more mafic lower crust beneath the OB (inferred from higher velocities) and the westward decrease in crustal thickness are interpreted as resulting primarily from Middle and/or Late Proterozoic extension and rifting of the craton. Penetration of the SLF into the lower crust may be controlled by lateral strength contrasts at the edge of the craton. Farther west, a midcrustal strength contrast (a velocity boundary at 20–25 km depth) may have acted as a regional detachment zone during compressional and extensional tectonic episodes.
The July 12, 1993, Hokkaido‐Nansei‐Oki, Japan, earthquake: Coseismic slip pattern from strong‐motion and teleseismic recordings
Authors: Mendoza Carlos , Fukuyama Eiichi
Published: February 2004

We employ a finite fault inversion scheme to infer the distribution of coseismic slip for the July 12, 1993 Hokkaido‐Nansei‐Oki earthquake using strong ground motions recorded by the Japan Meteorological Agency within 400 km of the epicenter and vertical P waveforms recorded by the Global Digital Seismograph Network at teleseismic distances. The assumed fault geometry is based on the location of the aftershock zone and comprises two fault segments with different orientations: a northern segment striking at N20°E with a 30° dip to the west and a southern segment with an N20°W strike. For the southern segment we use both westerly and easterly dip directions to test thrust orientations previously proposed for this portion of the fault. The variance reduction is greater using a shallow west dipping segment, suggesting that the direction of dip did not change as the rupture propagated south from the hypocenter. This indicates that the earthquake resulted from the shallow underthrusting of Hokkaido beneath the Sea of Japan. Static vertical movements predicted by the corresponding distribution of fault slip are consistent with the general pattern of surface deformation observed following the earthquake. Fault rupture in the northern segment accounts for about 60% of the total P wave seismic moment of 3.4 × 1020 N m and includes a large circular slip zone (4‐m peak) near the earthquake hypocenter at depths between 10 and 25 km. Slip in the southern segment is also predominantly shallower than 25 km, but the maximum coseismic displacements (2.0–2.5 m) are observed at a depth of about 5 km. This significant shallow slip in the southern portion of the rupture zone may have been responsible for the large tsunami that devastated the small offshore island of Okushiri. Localized shallow faulting near the island, however, may require a steep westerly dip to reconcile the measured values of ground subsidence.
Horizontal and vertical crustal movements from three‐dimensional very long baseline interferometry kinematic reference frame: Implication for the reversal timescale revision
Authors: Heki Kosuke
Published: February 2004

Three‐dimensional kinematic reference frame of geodetic very long baseline interferometry (VLBI) stations, tied to a geologic plate motion model, was established using the GLB907 solution by first selecting globally distributed stable plate interior stations and then applying a small translation and a rotation for the entire network in a three‐dimensional space so that the differences in the “horizontal” velocities between the VLBI observations and the model predictions are minimized. Since the VLBI network is global, we only assume that the horizontal movements of technically stable stations obey a plate motion model; we need not introduce any unwarranted constraints to the vertical velocities of specific stations to realize the frame. A suggestive correlation was found between the estimated vertical velocities of North American stations and those predicted by a postglacial rebound model. The revision of the magnetic polarity timescale (MPTS) causes a uniform increase or decrease of the predicted velocities, which could be detected as the small difference between the measured and the predicted relative plate velocities. Direct estimation of the correction suggests that the VLBI data fit best to the model when the NUVEL1 model is corrected by +3.4% (±1.2%), which differs significantly from the −4.5% deduced from the astronomical MPTS calibration. This was further confirmed by estimating the rotation rates for individual plate pairs.
Rifting history of the northern Mariana Trough: SeaMARC II and seismic reflection surveys
Authors: Baker Nancy , Fryer Patricia , Martinez Fernando , Yamazaki Toshitsugu
Published: February 2004

SeaMARC II sidescan sonar data and seismic reflection profiles of the northernmost portion of the Mariana Trough suggest a rifting rather than a seafloor‐spreading origin for lithosphere within this active backarc basin of the western Pacific. Variations in morphology along the strike of the backarc basin suggest both a spatial and a temporal variation in the style of extension of the basin. The northern Mariana Trough widens from 70 km near 23°N to 135 km near 21°30′N. Major active fault zones bound both sides of the backarc basin. Active rifting and volcanism are predominant in a roughly 20‐km‐wide active rift zone along the eastern (arc) side of the basin from 24°N to 22°N. The locus of active rifting has remained near the volcanic front of the arc during extension and is probably controlled by lithospheric weakness, which is greatest at the volcanic front. The zone of rifting intersects the active volcanic arc at Nikko Seamount. The distinction between the mechanism for formation of the Mariana Trough proposed here and that of slow mid‐ocean ridge spreading is that no true plate boundary has developed in the northernmost Mariana Trough. Rather, the backarc basin is composed of arc lithosphere intruded by arc and backarc basin magmas. Volcanism both in the rift zone and throughout the basin is fault‐controlled. South of 22°N, the zone of active rifting diverges westward from the arc as the transition to true seafloor spreading is approached. The transition to true mid‐ocean ridge spreading occurs to the south of the survey area described here.
Segmentation of the Aleutian plate boundary derived from stress direction estimates based on fault plane solutions
Authors: Lu Zhong , Wyss Max
Published: February 2004

We propose a new method to investigate stress homogeneity along plate boundaries based on the cumulative misfit of individual fault plane solutions, calculated using assumed stress tensors. Using this method, some segments of faults can be defined, without the time‐consuming inversions for stress directions from earthquake fault plane solutions. We assume that the misfits are relatively constant within segments of uniform stress orientation but that they change abruptly at boundaries of segments. This assumption is supported by the pattern of misfits observed for about 400 earthquakes located along the Aleutian subduction zone from 170°E to 145°W, which occurred between 1964 and 1993. The efficacy of the approach is confirmed by stress inversion analysis of moving windows. The cumulative misfit as a function of strike along the Aleutian arc shows there are five first‐order boundaries where the stress directions change significantly. The first one is near 177.5°E, which corresponds to the northward projection of the Rat fracture zone. The second one falls on the Adak fracture zone. The third is near the locus where the Amlia fracture zone intersects the trench. The fourth lies on the easternmost end of the 1957 aftershock zone. The last is located where the two major asperities of the 1964 rupture are separated. The segmentation boundaries found in our analysis may be controlled by the fracture zones in the subducted plate. This observation may be interpreted as due to decoupling within the underthrusting plate along the zones of weakness (the fracture zones) across which stress may not be transmitted fully. The boundaries defined by our method may also be related to the asperity and aftershock distributions of great earthquakes. The focal mechanism data in the segments defined as stress‐homogeneous by our method are then used to invert for the principal stress orientations by employing the Focal Mechanism Stress Inversion (FMSI) computer programs of Gephart. We find that the stress directions within these segments are different from each other, where they can be defined well, and the average misfits in these volumes are very small (2.8° and 5.5°). The method is not robust enough to detect all the stress segmentation boundaries based on only one reference stress tensor. Analyses with different reference stress models help to define most or all the stress segmentation boundaries.
Introduction to Special Section: Paleoseismology
Authors: Yeats Robert S. , Prentice Carol S.
Published: February 2004

A proverb of Confucius states “Study the past if you would divine the future.” If we could learn about the past history of earthquakes on a specific fault, then we could serve society well by better forecasting the future earthquake behavior of that fault.

For most of the world, the period of historical records is short: about 200 years in California and less than that in New Zealand, Oregon, and other parts of the world. And even where the historical record is thousands of years long, such as in north central China or the eastern Mediterranean region, it is commonly difficult to correlate a major historical earthquake with a specific active fault. Even if this correlation could be made without ambiguity, the recurrence intervals for many faults are longer than even a historical record of several thousand years. For these reasons, the history of large earthquakes on faults must, for the most part, be learned from the geological record.
Coral ages and island subsidence, Hilo drill hole
Authors: Moore James G. , Ingram B. Lynn , Ludwig Kenneth R. , Clague David A.
Published: February 2004

A 25.8‐m‐thick sedimentary section containing coral fragments occurs directly below a surface lava flow (the ∼1340 year old Panaewa lava flow) at the Hilo drill hole. Ten coral samples from this section dated by accelerator mass spectrometry (AMS) radiocarbon and five by thermal infrared multispectral scanner (TIMS) 230Th/U methods show good agreement. The calcareous unit is 9790 years old at the bottom and 1690 years old at the top and was deposited in a shallow lagoon behind an actively growing reef. This sedimentary unit is underlain by a 34‐m‐thick lava flow which in turn overlies a thin volcaniclastic silt with coral fragments that yield a single 14C date of 10,340 years. The age‐depth relations of the dated samples can be compared with proposed eustatic sea level curves after allowance for island subsidence is taken. Island subsidence averages 2.2 mm/yr for the last 47 years based on measurements from a tide gage near the drill hole or 2.5–2.6 mm/yr for the last 500,000 years based on the ages and depths of a series of drowned coral reefs offshore from west Hawaii. The age‐depth measurements of coral fragments are more consistent with eustatic sea levels as determined by coral dating at Barbados and Albrolhos Islands than those based on oxygen isotopic data from deep sea cores. The Panaewa lava flow entered a lagoon underlain by coral debris and covered the drill site with 30.9 m of lava of which 11 m was above sea level. This surface has now subsided to 4.2 m above sea level, but it demonstrates how a modern lava flow entering Hilo Bay would not only change the coastline but could extensively modify the offshore shelf.
Surface wave propagation in central Asia: Observations of scattering and multipathing with the Kyrgyzstan broadband array
Authors: Pavlis Gary L. , Mahdi Hanan
Published: February 2004

We studied the propagation of Rayleigh waves at regional distances in central Asia using a combination of array processing techniques and surface wave analysis. We present results from the detailed analysis of three representative events recorded by a 10‐station, broadband network that has been running in the central Asian country of Kyrgyzstan since 1991: anMs= 5.1 event near Ashkhabad, Turkmenistan; anMs= 5.8 event in south central Tibet; and the October 7, 1994, nuclear explosion at Lop Nor. We find there is a remarkable difference in the propagation characteristics of surface waves along these three paths. The path from the event in Turkmenistan is simple and is well approximated by propagation through a laterally homogeneous medium. Array processing shows the entire Rayleigh wave train stacks coherently and arrives from an azimuth close to that predicted by a great circle path. Furthermore, estimates of dispersion curves and fundamental mode signals determined for individual stations show little variation across the array. The Tibet and Lop Nor paths are completely different. We find strong evidence for complicated multipathing and scattering effects along both of these paths. We observe a three‐stage pattern in the Tibet case: (1) the early, lowest‐frequency part of the Rayleigh wave packet arrives as a coherent signal from close to the great circle path azimuth; (2) this is overpowered in the period range around 20 s by a strong multipath signal that propagates across the array from a much more southerly azimuth; and (3) periods below 20 s rapidly become incoherent, and the signal does not have a well‐defined direction of propagation. The Lop Nor path shows similar complexity. On this path there is little dispersion for measurable periods greater than 10 s, so the low‐frequency energy arrives in an Airy phase. The Airy phase stacks somewhat coherently (it stacks, but significant power is lost in the best beam), and slowness analysis shows it arrives from an azimuth 14° north of the great circle path azimuth. Like the Tibet event, the direction of propagation of the shorter periods cannot be resolved. These results are consistent with crustal structure in central Asia. The multipathing and scattering from the Tibet and Lop Nor events are not surprising given these waves propagate across highly variable crust and high topography resulting from the collision of India with Eurasia. The relative simplicity of the Turkmenistan event path reinforces this as almost all of that path traverses the relatively undeformed Turan platform of Uzbekistan.
Earthquake hazards of active blind‐thrust faults under the central Los Angeles basin, California
Authors: Shaw John H. , Suppe John
Published: February 2004

We document several blind‐thrust faults under the Los Angeles basin that, if active and seismogenic, are capable of generating large earthquakes (M= 6.3 to 7.3). Pliocene to Quaternary growth folds imaged in seismic reflection profiles record the existence, size, and slip rates of these blind faults. The growth structures have shapes characteristic of fault‐bend folds above blind thrusts, as demonstrated by balanced kinematic models, geologic cross sections, and axial‐surface maps. We interpret the Compton‐Los Alamitos trend as a growth fold above the Compton ramp, which extends along strike from west Los Angeles to at least the Santa Ana River. The Compton thrust is part of a larger fault system, including a decollement and ramps beneath the Elysian Park and Palos Verdes trends. The Cienegas and Coyote Hills growth folds overlie additional blind thrusts in the Elysian Park trend that are not closely linked to the Compton ramp. Analysis of folded Pliocene to Quaternary strata yields slip rates of 1.4 ± 0.4 mm/yr on the Compton thrust and 1.7 ± 0.4 mm/yr on a ramp beneath the Elysian Park trend. Assuming that slip is released in large earthquakes, we estimate magnitudes of 6.3 to 6.8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps. Multiple‐segment ruptures could yield larger earthquakes (M= 6.9 to 7.3). Relations among magnitude, coseismic displacement, and slip rate yield an average recurrence interval of 380 years for single‐segment earthquakes and a range of 400 to 1300 years for multiple‐segment events. If these newly documented blind thrust faults are active, they will contribute substantially to the seismic hazards in Los Angeles because of their locations directly beneath the metropolitan area.
Modeling earthquake cycles in the Shumagin subduction segment, Alaska, with seismic and geodetic constraints
Authors: Zheng Gutuan , Dmowska Renata , Rice James R.
Published: February 2004

Deformation associated with the earthquake cycle in the Shumagin Islands segment of the Alaska‐Aleutian subduction zone is analyzed with the use of a two‐dimensional finite element model. The model consists of an oceanic plate dipping under an upper plate, both of which respond elastically to stress fluctuations in the earthquake cycle, and these are underlain by asthenospheric mantle and mantle wedge regions which respond viscoelastically. It is tailored to the geometry of the Shumagin Islands region, by using seismicity to define the position of the interplate interface and (partially) coupled region along it. The model is preconditioned by forcing this interface to undergo periodically repeated slips up to (and including) the time of the May 31, 1917, event (Ms = 7.4) in that region, with each chosen to be consistent with the moment and estimated rupture area of that event. We investigated the dependence of model results for geodetic signals on the strength of seismic coupling between the plates and viscoelastic relaxation of deviatoric stresses in the mantle, including in the mantle wedge close to the plate junction and along the aseismic downdip continuation of the thrust interface. In models with significant relaxation in the wedge or downdip thrust zone, results show that as the intraseismic stage matures, there is a region of diminished compressional strain rates, and even of locally extensional rates, on the Earth's surface above the downdip end of the seismically coupled zone. Based on the seismic estimates of the location of the coupled zone, this region is in the area of the Shumagin Islands. We find that if approximately 20% of the convergence takes place seismically (compatible with the previous seismic history), and if an extensive region of relaxed deviatoric stress is assumed to be present in the wedge and/or along the downdip interface, then deformations predicted by the model can be made consistent with the measured strain data from the Shumagin Islands geodetic network [Lisowski et al., 1988; Larson and Lisowski, 1994], as well as uplift and tilt data [Savage and Plafker, 1991; Beavan, 1992]. Our model simulations here suggest that the Shumagin segment is capable of large earthquakes. The hypothesis of totally aseismic subduction is not similarly consistent with all geodetic constraints.
Ridge segmentation and the magnetic structure of the Southern Mid‐Atlantic Ridge 26°S and 31°–35°S: Implications for magmatic processes at slow spreading centers
Authors: Weiland Charles M. , Macdonald Ken C. , Grindlay Nancy R.
Published: February 2004

Along‐axis profiles of three‐dimensional magnetic inversions for the Mid‐Atlantic Ridge (MAR) 31°–35°S show low magnetization near the middle of ridge segments and high magnetization at the segment tips for three adjacent spreading segments; thus there is an inverse relation between axial magnetization and axial topography. The ridge segment at 26°S on the MAR has the same inverse relationship between magnetization and topography. The common occurrence of this relationship suggests that it reflects a fundamental process of crustal accretion at the MAR. We analyze the rock magnetic properties from 42 locations within the four ridge segments in the South Atlantic to constrain the inherent trade‐off between source intensity and source thickness in the magnetization model. The natural remanent magnetization (NRM) intensities from the four ridge segments, averaged together, correlate with the magnetic inversion profiles. This finding implies that changes in the magnetization of the extrusives may account for much of the observed magnetic anomaly amplitude variation. A direct correlation of FeO content and magnetization suggests that magnetic anomaly amplitudes may be an indicator of FeTi‐rich basalts at the slow spreading MAR, even though the iron content of the basalts from high magnetization areas is not as high as observed at Pacific spreading centers. Despite the different magma plumbing systematics of the Pacific spreading centers and the MAR, it appears that the segment‐scale magma system of the MAR also results in segment‐scale crustal magnetization variations. Further evidence that the axial magnetic variations result from source intensity variations is that older isochrons have higher intensities near the ridge‐discontinuities, similar to the behavior on‐axis. Between 0 and 5 Ma the decay in magnetization is ∼50% independent of location within a spreading segment.
Interpolation of principal stress directions by nonparametric statistics: Global maps with confidence limits
Authors: Bird Peter , Li Yao
Published: February 2004

We suggest a simple method for interpolation of data on the horizontal most compressive principal stress direction (
) which makes no assumptions about their distribution, except that the probability of a given discrepancy is primarily a function of distance. Using the 6000 data of the World Stress Map, we obtain empirical two‐point distributions of direction differences at a number of distances. Then, to find the probability distribution at an interpolation point, we form the product of the distributions implied by all the available data within the correlation horizon (with or without preaveraging of clustered data). It is then simple to select the most likely direction and 90% (or other) confidence bounds. The resulting global maps of
directions have 90% confidence limits of less than ±45° over 20–65% of the globe, and small uncertainties of less than ±10° over 5–25% of the globe (depending on the method). Both maps illustrate how radial
directions encircle high topography (or unusually shallow bathymetry), suggesting a generally low level of deviatoric stress in the lithosphere.
Geomagnetic changes across the last reversal recorded in lava flows from La Palma, Canary Islands
Authors: Quidelleur Xavier , Valet Jean‐Pierre
Published: February 2004

Two adjacent sequences of 41 and 21 individual lava flows have been sampled in the island of La Palma. Although no detailed correlation can be established between the two sections, they both encompassed the Matuyama‐Brunhes (M‐B) reversal. Stepwise alternating field (AF) and thermal demagnetization in air and in vacuum accompanied by investigations of the rock magnetic properties allowed us to identify several lava flows with the same signature and thus most likely associated with very short episodes of intense volcanic activity. The intermediate virtual geomagnetic pole (VGP) positions do not cluster within any preferred geographic areas. The polarity change is followed by a rebound (or an excursion) with a cluster of VGPs over northeastern India. Whether or not this feature is truly associated with the transition or correlated with the Delta event (0.69 Ma) will hopefully be resolved by further dating. Paleointensity experiments were performed on 60 samples using the modified Thellier method [Coe, 1967]. Corrections for magnetomineralogical changes induced by heating were applied using the technique of partial thermoremanent magnetization (pTRM) checks [Valet et al., 1994b] with a significant increase in the success rate (from 38% to 58%). Twenty‐four independent estimates of the paleofield range between 3.9 and 46.0 μT. The field intensity was reduced to at least one tenth of the present‐day value during the transition. Low (transitional) field intensities are also observed during the rebound (or excursion). The lava flows preceding the polarity change show evidence for a significant drop in the dipole field intensity, while the posttransitional flows exhibit high field intensities. These features are compatible with the asymmetrical saw‐tooth pattern recently observed in sedimentary records [Meynadier et al., 1994]. The characteristics of the M‐B transition emerging from a compilation of the four available volcanic records are not inconsistent with the hypothesis of rapidly time‐varying nondipole transitional components favored by the decrease of the dipole and its subsequent recovery in the opposite polarity state.
Ps conversions from the upper mantle transition zone beneath the Canadian landmass
Authors: Bostock M. G.
Published: February 2004

Broadband teleseismic P waveforms recorded on the Canadian National Seismograph Network have been processed to isolate near‐receiver Ps conversions originating from upper mantle discontinuities at approximately 410‐ and 660‐km depth. A simultaneous deconvolution procedure which removes source effects and exploits the move‐out characteristics of the weak Ps phases is used to raise signal above the noise levels in the P coda. Large‐amplitude arrivals exhibiting times and slownesses consistent with an interpretation as Ps converted phases from the 410‐ and 660‐km discontinuities are observed at 18 and 20 of the 24 stations examined, respectively. The arrival times, corrected for crustal heterogeneity, show strong correlation with upper mantle structure above 410‐km depth as determined in previous studies. Stations in the Cordillera above low‐velocity mantle are characterized by residuals up to 3.3 s slow relative to the IASP91 model, whereas sites overlying the high‐velocity root below the Canadian Shield are advanced by as much as 1.9 s. The difference between the P660s and P410s times provides information on the structure of the transition zone beneath Canadian stations. The trend of the data indicates that topography on the discontinuities is at or below resolvable levels (±5 km) over most of the region, in general accord with a previous study of the transition zone beneath continental stations but in contrast with results for the Pacific basin.
Winter water masses and nutrients in the northern Gulf of California
Authors: Lavín M. F. , Gaxiola‐Castro G. , Robles J. M. , Richter K.
Published: February 2004

Hydrographic data (salinity, temperature, O2, PO4, NO3 and SiO2) collected in the northern Gulf of California between February 27 and March 3, 1988, reveal that bottom water formation took place that winter. North of 30.5°N, salinity increased with depth from ∼35.30 practical salinity units (psu) at the surface to 35.57 psu at the bottom of the 200‐m deep Wagner Basin; below ∼25 m, temperature was almost homogeneous, at ∼15°C (±0.4°C), with some inversions. The TS diagrams and the distribution of dissolved oxygen and nutrients suggest that the most likely origin of this bottom water is the shallow coastal region (<40 m deep) of the northern Gulf. Elsewhere, the characteristic Gulf of California decrease of salinity with depth was found, from over 35.25 in the surface to 34.90 at 200 m. In the upper ∼120 m, intermediate salinity (35.0 to 35.3) and nutrient concentrations (phosphate, 1.5–2.0 μM; nitrate, 12–16 μM; and silicate, 25–32 μM) identify the Gulf of California Water this winter. In the deeper layers, low salinity (<35.0) and high nutrients (phosphate, >2.2 μM; nitrate, >22 μM; and silicate, >35 μM) indicate the presence of oceanic water from Guaymas Basin, probably Subsurface Subtropical Water. The boundary between the two regimes was ∼18 km wide, with clearly defined bottom fronts and intrusions at all depths. Of the several late‐winter hydrographic data sets available, only that from March 1973 presents a similar distribution of high‐salinity bottom water. Therefore interannual variability (not necessarily El Niño‐Southern Oscillation (ENSO) related) can have a profound effect on vertical convection, which can occur both in ENSO and non‐ENSO years. An important and as yet unexplained difference between the two data sets is that there was more Gulf of California Water in the northern Gulf of California in 1973 than in 1988.
Tower‐based measurements of normalized radar cross section from Lake Ontario: Evidence of wind stress dependence
Authors: Colton Marie C. , Plant William J. , Keller William C. , Geernaert Gerald L.
Published: February 2004

We report here the dependence of the normalized radar cross‐section (NRCS) on incidence angle, azimuth angle, wind speed, wind stress, and atmospheric stratification for Ku band microwave backscatter from a lake. The measurements were made in autumn 1987 on Lake Ontario, using a rotating microwave system mounted on a research tower operated by the Canada Centre for Inland Waters. The results show that at intermediate incidence angles the NRCS on the lake generally increases faster with wind speed than it does on the ocean. We attribute this to the larger atmospheric drag coefficients which exist on the lake compared with the ocean, and we show that the results are more consistent with a dependence of the NRCS on wind stress than on wind speed near the surface. We find a stratification dependence of the NRCS similar to that previously reported at C band and show that at 40° and 60° incidence angles this dependence can be removed by parameterizing the NRCS in terms of either the friction velocity or neutral wind speed. At a 20° incidence angle the stratification dependence is not removed by this procedure.
Seasonal variability of light availability and utilization in the Sargasso Sea
Authors: Siegel David A. , Michaels Anthony F. , Sorensen Jens C. , O'Brien Margaret C. , Hammer Melodie A.
Published: February 2004

A 2 year time series of optical, biogeochemical, and physical parameters, taken near the island of Bermuda, is used to evaluate the sources of temporal variability in light availability and utilization in the Sargasso Sea. Integrated assessments of light availability are made by examining the depth of constant percent incident photosynthetically available radiation (%PAR) isolumes. To first order, changes in the depth of %PAR isolumes were caused by physical processes: deep convective mixing in the winter which led to the spring bloom and concurrent shallowing of %PAR depths and the occurrence of anomalous thermohaline water masses during the summer and fall seasons. Spectral light availability variations are assessed using determinations of diffuse attenuation coefficient spectra which illustrate a significant seasonal cycle in colored detrital paniculate and/or dissolved materials that is unrelated to changes in chlorophyll pigment concentrations. Temporal variations in the photosynthetic light utilization index Ψ are used to assess vertically integrated light utilization variations. Values of Ψ are highly variable and show no apparent seasonal pattern which indicates that Ψ is not simply a “biogeochemical constant” Determinations of in situ primary production rates and daily mean PAR fluxes are used to diagnose the relative role of light limitation in determining vertically integrated rates of primary production ∫PP. The mean depth of the light‐saturated zone (the vertical region where the daily mean PAR flux was greater than or equal to the saturation irradiance Ik) is only ∼40 m, although more than one half of ∫PP occurred within this zone. Production model results illustrate that accurate predictions of ∫PP are dependent upon rates of light‐saturated production rather than upon indices of light limitation. It seems unlikely that significant improvements in simple primary production models will come from the partitioning of the Earth's seas into biogeochemical provinces.
Toward closure of the inherent optical properties of natural waters
Authors: Pegau W. Scott , Zaneveld J. Ronald V. , Voss Kenneth J.
Published: February 2004

A fundamental relationship of inherent optical properties (IOP) is that the beam attenuation coefficient is the sum of the volume absorption and scattering coefficients (c = a + b). A relative calibration of a set of instruments can be provided using this IOP closure equation. Measurement of the true beam attenuation coefficient c is not practical as all attenuation instrumentation has some finite acceptance angle in which scattered light is collected. We provide a theoretical framework for measuring the attenuation and scattering coefficients in a consistent manner. Using this framework, we provide a practical version of the IOP closure equation. We apply the practical IOP closure equation to measurements made at Lake Pend Oreille, Idaho, in the spring of 1992. Results of this IOP closure indicate that the practical closure equation is a useful approach. Closure was achieved during some measurement sets but not at others. The intermittent lack of closure may be due to the method of determining the scattering coefficient from the general angle scattering meter or that the calibration of at least one of the instruments drifted during the time of the experiment.
Analysis of ocean color components within stratified and well‐mixed waters of the western English Channel
Authors: Hochman Herschel T. , Walsh John J. , Carder Kendall L. , Sournia A. , Muller‐Karger Frank E.
Published: February 2004

In situ pigment and dissolved organic carbon (DOC) data from two distinct hydrographic regions of the western English Channel are used to explore the possible marine DOC contamination of the past satellite estimates of phytoplankton biomass. To compare with field measurements, the individual spectral contributions of DOC, pigments, and water to the total diffuse attenuation coefficient,Kpar, are summed on a quantum basis within stratified waters near Plymouth, England; and for the spectrally averaged diffuse attenuation coefficient,Kd, on an energy basis within tidally mixed waters near Roscofif, France. In addition, coastal zone color scanner (CZCS) images from 1979 to 1986 were used to compute DOC concentrations for comparison with in situ values. Our analysis suggests that almost 50% of the color signal of satellite‐sensed pigments may be attributed to absorption by marine colored DOC (CDOC) within the English Channel. These results compare favorably to the in situ DOC measurements off Plymouth, but not to off‐Roscoff measurements, suggesting that there may be more CDOC in the stratified waters and more nonabsorbing DOC in the tidally mixed waters.