Ground Motion Prediction Equations in the San Jacinto Fault Zone: Significant Effects of Rupture Directivity and Fault Zone Amplification

We present a new set of Ground Motion Prediction Equations (GMPEs) for horizontal Peak Ground Acceleration, Peak Ground Velocity, and 5 % damped pseudo-spectral acceleration (PSA), developed for the San Jacinto Fault Zone (SJFZ) area. Besides using these equations to quantify seismic shaking in the area, the results allow us to examine the physics and local properties controlling the observed ground motions. The analyzed dataset includes ~30,000 observations from ~800 events spanning a magnitude range of 1.5 < M < 6.0 and recorded by up to 140 stations at epicentral distances ranging from essentially zero to 150 km. The local GMPE is developed for the SJFZ by applying classical regression techniques with predictive variables that include first distance and magnitude, and then site characteristics, rupture directivity, and fault zone amplification. The significance of these effects is determined by measuring the uncertainty-reduction of the GMPE due to each factor. The results show that, in contrast to many regional studies, traditional site characteristic has a relatively minor effect on peak amplitudes in our study area. However, rupture directivity is a significant factor controlling the amplitudes of ground motion even for small events. The dense seismic network and newly developed directivity tool enable us to extract efficiently directivity effects with statistical significance, using the ground-motion dataset during the regression analysis process. The obtained rupture directivities are consistent with the main focal mechanism orientations and surface trace orientations, known from other studies, and predictions for bimaterial ruptures in the trifurcation area of the SJFZ. Fault zone amplification is a second important factor, showing strong impact on the peak ground motion values, with increasing role for the lower frequency range (<10 Hz) examined in the 5 % damped PSA values. We also observe signatures of large amplitude-variances, which indicate additional source-related control on the distribution of amplitudes (besides rupture directivity) for aftershocks close in time and location to the M _L 5.1 earthquake of March 2013. Using the full set of records we present the most complete set of GMPEs for the SJFZ area, including a higher-amplitude prediction for regions in the direction of rupture.     

Surveying Infrasonic Noise on Oceanic Islands

v--vAn essential step in the establishment of an International Monitoring System (IMS) infrasound station is the site survey. The survey seeks a location with relatively low infrasonic noise and the necessary logistical support. This paper reports results from our surveys of two of the oceanic sites in the IMS - the Azores and Cape Verde. Each survey sampled infrasonic noise, wind velocity, air temperature and humidity for ~3 weeks at 4 sites near the nominal IMS locations. The surveys were conducted on Sao Miguel (the main island in the Azores) and Maio (Cape Verde). Infrasonic noise was measured using the French MB2000 microbarometer.¶During our 3-week experiment in January the trade winds at Cape Verde varied little from an azimuth of 63°. Because of the unvarying wind azimuth, the experiment gave us an opportunity to examine the effectiveness of a forest at reducing both wind speed and infrasonic noise. We find that the thick Acacia forest on Maio reduces wind speeds at a 2 m elevation by more than 50% but does not reduce infrasonic noise at frequencies below 0.25 Hz. This forest serves as a high-frequency filter and clearly does not reduce long-period noise levels which are due to large-scale turbulence in the atmospheric boundary layer above the forest. This is consistent with our observations in the Azores where the relationship between infrasonic noise and wind speed is more complex due to frequent changes in wind azimuth.¶In Cape Verde, wind speed and infrasonic noise are relatively constant. The diurnal variations are clearly seen however the microbarom is only rarely sensed. In the Azores, during our 3-week experiment in November and December of 1998, wind speed and infrasonic noise change rapidly. At this location, daily noise level swings of 40 to 50 dB at 0.1 Hz are not uncommon in the early winter and are due to changes in wind speed and atmospheric turbulence. The effectiveness of an infrasound station in the Azores will be strongly dependent on time during the winter season.¶The two surveys illustrate some of the difficulties inherent in the selection of sites for 1 to 3 km aperture arrays on oceanic islands. Due to elevated noise levels at these sites, 8 element, 2 km aperture arrays are strongly preferred.     

Retrograde Schists of the Amphibolite Facies at broken hill,New South Wales

Aluminous, mafic, felsic, calcareous, and sulphide‐rich rocks have been involved in localized deformation and retrograde metamorphism at Broken Hill, western New South Wales, where retrograde schist‐zones intersect high‐grade, regional metamorphic rocks of the lower granulite facies (or the amphibolite‐granulite facies transition). Although technically retrograde, the schists contain mineral assemblages indicative of the lower amphibolite facies. The schist‐zones were formed by local folding, apparently as part of the third stage of deformation in the Broken Hill area.     

CLIMATIC CONTROL OF SOIL ACIDITY

A curve-fit model of the relationship between soil pH (A horizon) and climate forms the basis of a map of predicted pH for the conterminous United States. This map was produced with the aid of a large climatic data base, a computer mapping program and subroutines to map modeled values of soil pH. Patterns of pH follow well-recognized patterns of climate, although the pH of 7 line deviates greatly from Marbut's line dividing pedalfers from pedocals. The overall accuracy of the map and model was checked using a validation data set. Geographic patterns of pH are realistic, although land use, topography, and parent material may cause local variations about the predicted mean for a given region. Several apparently anamolous map patterns were also checked but found to be accurate. Prediction of the detailed pattern of pH will require variables in addition to climate.     

Review of Microstructural Evidence of Magmatic and Solid-State Flow

Evidence of magmatic flow includes: (a) parallel to sub-parallel alignment of elongate euhedral crystals (e.g., of feldspar or hornblende) that are not internally deformed, (b) imbrication (‘tiling’) of elongate euhedral crystals that are not internally deformed, (c) insufficient solid-state strain in regions between aligned or imbricated crystals to accommodate phenocryst rotation, (d) elongation of microgranitoid enclaves without plastic deformation of the minerals, (e) magmatic flow foliations and elongate microgranitoid enclaves deflected around xenoliths, and (f) schlieren layering (if due to flow sorting) in the absence of plastic deformation of the minerals involved. These features are consistent with rotation of crystals in a much weaker medium, namely a melt phase, at a stage when the magma has become viscous enough to preserve the alignment.Evidence of solid-state flow includes: (a) internal deformation and recrystallization of grains, (b) recrystallized “tails,” (c) elongation of recrystallized aggregates (e.g. of quartz and mica), (d) grainsize reduction, (e) fine-grained folia anastomosing around less deformed relics, (f) microcline twinning, (g) myrmekite, (h) flame perthite, (i) boudinage of strong minerals, typically with recrystallized aggregates of weaker minerals (e.g. quartz and mica) between the boudins, (j) foliation passing through, rather than around enclaves, and (k) heterogeneous strain with local mylonitic zones.Several criteria suggest “submagmatic flow,” including recrystallized feldspar, inferred transitions from magmatic imbrication to solid-state S/C arrangements, evidence of c-slip in quartz, and especially evidence of migration of residual melt into lower-pressure sites.Recent experimental studies indicate that a change from grain-supported flow to suspension flow typically occurs in deforming magmas at melt contents of between 20% to 40%, and that large amounts of strain may accumulate in magmas without being recorded by the final fabric. At lower melt percentages, perhaps as low as a few percent, depending on the minerals and their shapes, strain may be accommodated by: (a) melt-assisted grain-boundary sliding, (b) contact-melting assisted grain-boundary migration, (c) strain partitioning into melt-rich zones, (d) intracrystalline plastic deformation (c-slip in quartz indicating plastic deformation at temperatures near the granite solidus), and (f) transfer of melt to sites of low mean stress. The only indication of strain in the absence of crystal plasticity may be an alignment of crystals. Moreover, magmatic flow microstructures may be destroyed by fracturing, crystal plasticity and recrystallization before the magma reaches its solidus.Many rocks show evidence of solid-state flow superimposed on magmatic flow. Evidence of magmatic flow is commonly preserved in deformed felsic metamorphic rocks: for example the alignment of rectangular K-feldspar megacrysts and of microgranitoid enclaves. However, absence of alignment does not preclude a magmatic origin for K-feldspar megacrysts in felsic gneisses, as magmatic flow may cease before the magma becomes viscous enough to preserve an alignment.     

Origin of pyroxene-plagioclase aggregates in a rhyodacite

Bodies of pyroxene-bearing rhyodacite in New England, New South Wales, previously considered to be intrusions, are reinterpreted as ignimbritic flows. Small aggregates of pyroxene and plagioclase are inferred to be either compositionally modified crystalline residuals from partial melting or crystal cumulates. An earlier hypothesis involving mixing of solid biotite-diorite with a rhyolitic liquid is questionable, and the origin of the rhyodacite can be explained in terms of recent experimental work on crystal-liquid equilibria under inferred crustal conditions.     

The relationship between winter minimum temperatures and spring phenology in a Southern Appalachian Cove

Summary Winter minimum temperatures were recorded at four sites in Station Cove, a forested flat-bottomed ravine in the southern Appalachians of South Carolina, U.S.A. Unlike minimum temperatures found in frost hollows, cove minima were warmer than those found outside of the cove. Within the cove, minimum temperatures at study sites in the narrow, gorge-like western portion near a 20 m high cascade were warmer than those in the broader, eastern portion. Additionally, minimum temperatures near the ground were warmer than those at 1.4 m height above the surface. Selected spring-flowering plant species bloomed earlier in the western portion of the cove, while for the same plant species, within cove flowering dates were 2 to 3 weeks earlier than those outside of the cove. Station Cove, therefore, appears to harbor a microclimate that allows for much earlier flowering and leafing out of plant species in spring than does the surrounding region.

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Der Zusammenhang zwischen Winter-Minimumtemperaturen und Frühlingsphänologie in einem Tal der südlichen Appalachen

Zusammenfassung Winter-Minimumtemperaturen wurden an vier Meßstellen in Station Cove, einer bewaldeten Schlucht mit flachem Boden in den südlichen Appalachen von South Carolina, U.S.A., registriert. Im Gegensatz zu den Minimumtemperaturen, die in Frostlöchern auftreten, waren die Talschluchtminima wärmer als die außerhalb der Schlucht gemessenen. Innerhalb des Tales waren die Minimumtemperaturen im schluchtartigen, westlichen Ende in der Nähe eines 20 m hohen Wasserfalls wärmer als die im breiteren, östlichen Teil des Tales. Außerdem waren die Minimumtemperaturen in Bodennähe wärmer als die in 1,4 m Höhe gemessenen. Einige Arten von Frühlingsblumen blühten im westlichen Teil der Talschlucht früher als im östlichen Teil. Dieselben Arten blühten in der Schlucht 2 bis 3 Wochen früher als außerhalb der Schlucht. Station Cove ist daher durch ein Mikroklima charakterisiert, das im Vergleich zum umliegenden Terrain ein früheres Austreiben und Blühen der Pflanzen im Frühling erlaubt.

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Efficiency of Lg Propagation from SmS Dynamic Ray Tracing in Three-dimensionally Varying Crustal Waveguides

The effects of variations in crustal and basin thicknesses on the efficiency of Lg propagation in Central Asia are investigated by comparing the predictions of ray theoretical modeling of Lg as multiple, critically reflected, SmS rays with observations at the KNET, CNET, and ILPA arrays. Ray modeling predicts that strong spatial gradients in crustal thickness within 10 to 50 km of either the source or receiver can affect Lg efficiency over paths exceeding 500 km. Variations in the efficiency of Lg propagation to KNET, including instances of strong variations across the aperture of the array, are consistent with predicted blockage of SmS by high slopes of Moho topography associated with the Hindu Kush and Pamir mountains. At CNET and ILPA the blocking effects of Moho slopes associated with the Caucausus and Zagros mountain belts and Black and Caspian Seas are accentuated by the effects of thick basins. Thick basins (greater than 2000 m) are found to have large effects on the efficiency of Lg propagation from shallow (1 km) crustal sources, blocking either a portion or all of the Lg phase along paths crossing these basins. The basin effects are strongly sensitive to source depth, nearly vanishing for sources 15 km and deeper.