Lateral structure of the subducting pacific plate beneath the Hokkaido corner from intermediate and deep earthquakes

We present a study of the lateral structure and mode of deformation in the transition between the Kuril and Honshu subduction zones. We begin by examining the source characteristics of the January 19, 1969, intermediate depth earthquake north of Hokkaido in the framework of slab-tearing, which for the December 6, 1978 event has been well documented by previous studies. We use a least-squares body wave inversion technique, and find that its focal mechanism is comparable to the 1978 event. To understand the cause of these earthquakes, which in the case of the 1978 event occurred on a vertical tear fault but does not represent hinge faulting, we examine the available International Seismological Centre [ISC] hypocenters and Harvard centroid-moment tensor [CMT] solutions to determine the state of stress, and lateral structure and segmentation in the Kuril and northern Honshu slabs. These data are evaluated in the framework of two models. Model (A) requires the subducting slab at the Hokkaido corner to maintain surface area. Model (B) requires slab subduction to be dominated by gravity, with material subducting in the down-dip direction. The distribution of ICS hypocenters shows a gap in deep seismicity down-dip of the Hokkaido corner, supporting model (B). From the CMT data set we find that three types of earthquake focal mechanisms occur. The first (type A) represents dip-slip mechanisms consistent with down-dip tension or compression in the slab in a direction normal to the strike of the trench. These events occur throughout the Honshu and Kuril slabs with focal mechanisms beneath Hokkaido showing NNW plungingP andT axes consistent with the local slab geometry. The second (type B) occurs primarily at depths over 300 km in the southern part of the Kuril slab with a few events in the northern end of the Honshu deep seismicity. These earthquakes have focal mechanisms with P axes oriented roughly E-W, highly oblique to the direction of compression found in the type A events, with which they are spatially interspersed. The third (type C) group of earthquakes are those events which do not fit in either of the first two groups and consist of either strike-slip focal mechanisms, such as the tearing events, or oddly oriented focal mechanisms. Examination of the stress axes orientations for these three types reveals that the compressional axes of the type C events are consistent with those of type B. The slab tearing events are just differential motion reflecting the E-W compressive states of stress which is responsible for the type B family of events. There is no need to invoke down-dip extension which does not fit the slab geometry. We conclude that these two states of stress can be explained as follows: 1) The type A events and the seismicity distribution support model (B). 2) The type B and C events upport model (A). The solution is that the slab subducts according to model (B), but the flow in the mantle maintains a different trajectory, possibly induced by the plate motions, which produces the second state of E-W compressive stress.     

Stony meteorite porosities and densities: A review of the data through 2001

Abstract— In this review, we summarize the data published up to December 2001 on the porosity and density of stony meteorites. These data were taken from 925 samples of 454 different meteorites by a variety of techniques. Most meteorites have densities on the order of 3 to 4 g/cm³, with lower densities only for some volatile‐rich carbonaceous meteorites and higher densities for stony irons. For the vast majority of stones, porosity data alone cannot distinguish between different meteorite compositions. Average porosities for most meteorite classes are around 10%, though individual samples can range as high as 30% porosity. Unbrecciated basaltic achondrites appear to be systematically less porous unless vesicles are present. The measured density of ordinary chondrites is strongly controlled by the amount of terrestrial weathering the sample has undergone with porosities steadily dropping with exposure to the terrestrial environment. A theoretical grain density based on composition can model “pre‐weathered” porosities. The average model porosity for H and LL chondrites is 10%, while L chondrite model porosities average only 6%, a statistically significant difference.     

Impact of erosion and accretion on the distribution of enterococci in beach sands

Bacterial pathogens in coastal sediments may pose a health risk to users of beaches. Although recent work shows that beach sands harbor both indicator bacteria and potential pathogens, it is neither known how deep within beach sands the organisms may persist nor if they may be exposed during natural physical processes. In this study, sand cores of approximately 100 cm depth were collected at three sites across the beach face in Kitty Hawk, North Carolina, before, during, and after large waves from an offshore hurricane. The presence of DNA from the fecal indicator bacterium Enterococci was detected in subsamples at different depths within the cores by PCR amplification. Erosion and accretion of beach sand at the three sites were also determined for each sampling day. The results indicate that ocean beach sands with persisting enterococci signals could be exposed and redistributed when wind, waves, and currents cause beach erosion or accretion.     

Enhanced Acoustic Backscatter Due to High Abundance of Sand Dollars,Dendraster excentricus

Abstract

Detailed acoustic surveys of benthic sediments were conducted in July 1995 and September 1998 in the vicinity of Humboldt Bay, California. During these surveys, a band of enhanced acoustic backscatter was observed offshore from the bay entrance, approximately parallel to the isobaths, in water depths ranging from 16–24 m. In order to assess the cause of the increase in backscatter levels, a more comprehensive study was conducted in August and September 1999 using 100 kHz side-scan sonar, bottom grab sampling and underwater video recording. New observations indicated that a dense population of sand dollars (Dendraster excentricus) coincided with the enhanced backscatter band. Compared to the two previous acoustic studies, the central section of the band expanded westward by 180 m and the southern section of the band shifted eastward by 160 m, possibly resulting from a change in the biological or physical factors which influence the location and breadth of sand dollars.

The relationship between high sand dollar abundance and enhanced acoustic backscatter was further verified in the near shore region off Samoa Beach California, where a dense, banded population of sand dollars was previously observed. Video footage confirmed the presence of a band of sand dollars, also nominally parallel to the isobaths, in water depths of 8–15 m. A band of enhanced backscatter coincided with the dense sand dollar population. The identification of dense aggregations of sand dollars through enhanced acoustic backscatter could lead to the use of acoustic techniques to study sand dollar distributions and abundance.     

Improving the time resolution of surfzone bathymetry using in situ altimeters

Surfzone bathymetry often is resolved poorly in time because watercraft surveys cannot be performed when waves are large, and remote sensing techniques have limited vertical accuracy. However, accurate high-frequency bathymetric information at fixed locations can be obtained from altimeters that sample nearly continuously, even during storms. A method is developed to generate temporally and spatially dense maps of evolving surfzone bathymetry by updating infrequent spatially dense watercraft surveys with the bathymetric change measured by a spatially sparse array of nearly continuously sampling altimeters. The update method is applied to observations of the evolution of shore-perpendicular rip current channels (dredged in Duck, NC, 2012) and shore-parallel sandbars (observed in Duck, NC, 1994). The updated maps are compared with maps made by temporally interpolating the watercraft surveys, and with maps made by spatially interpolating the altimeter measurements at any given time. Updated maps of the surfzone rip channels and sandbars are more accurate than maps obtained by using either only watercraft surveys or only the altimeter measurements. Hourly altimeter-updated bathymetric estimates of five rip channels show rapid migration and infill events not resolved by watercraft surveys alone. For a 2-month observational record of sandbars, altimeter-updated maps every 6 h between nearly daily surveys improve the time resolution of rapid bar-migration events.     

The thermal conductivity of meteorites: New measurements and analysis

We have measured the thermal conductivity at low temperatures (5-300 K) of six meteorites representing a range of compositions, including the ordinary chondrites Cronstad (H5) and Lumpkin (L6), the enstatite chondrite Abee (E4), the carbonaceous chondrites NWA 5515 (CK4 find) and Cold Bokkeveld (CM2), and the iron meteorite Campo del Cielo (IAB find). All measurements were made using a Quantum Design Physical Properties Measurement System, Thermal Transport Option (TTO) on samples cut into regular parallelepipeds of ∼2-6 mm dimension. The iron meteorite conductivity increases roughly linearly from 15 W m⁻¹ K⁻¹ at 100 K to 27 W m⁻¹ K⁻¹ at 300 K, comparable to typical values for metallic iron. By contrast, the conductivities of all the stony samples except Abee appear to be controlled by the inhomogeneous nature of the meteorite fabric, resulting in values that are much lower than those of pure minerals and which vary only slightly with temperature above 100 K. The L and CK sample conductivities above 100 K are both about 1.5 W m⁻¹ K⁻¹, that of the H is 1.9 W m⁻¹ K⁻¹, and that of the CM sample is 0.5 W m⁻¹ K⁻¹; by contrast the literature value at 300 K for serpentine is 2.5 W m⁻¹ K⁻¹ and those of enstatite and olivine range from 4.5 to 5 W m⁻¹ K⁻¹ (which is comparable to the Abee value). These measurements are among the first direct measurements of thermal conductivity for meteorites. The results compare well with previous estimates for meteorites, where conductivity was derived from diffusivity measurements and modeled heat capacities; our new values are of a higher precision and cover a wider range of temperatures and meteorite types. If the rocky material that makes up asteroids and provides the dust to comets, Kuiper Belt objects, and icy satellites has the same low thermal conductivities as the ordinary and carbonaceous chondrites measured here, this would significantly change models of their thermal evolution. These values would also lower their thermal inertia, thus affecting the Yarkovsky and YORP evolution of orbits and spin for solid objects; however, in this case the effect would not be as great, as thermal inertia only varies as the square root of the conductivity and, for most asteroids, is controlled by the dusty nature of asteroidal surfaces rather than the conductivity of the material itself.     

Characterization of Mason Gully (H5): The second recovered fall from the Desert Fireball Network

Mason Gully, the second meteorite recovered using the Desert Fireball Network (DFN), is characterized using petrography, mineralogy, oxygen isotopes, bulk chemistry, and physical properties. Geochemical data are consistent with its classification as an H5 ordinary chondrite. Several properties distinguish it from most other H chondrites. Its 10.7% porosity is predominantly macroscopic, present as intergranular void spaces rather than microscopic cracks. Modal mineralogy (determined via PS‐XRD, element mapping via energy dispersive spectroscopy [EDS], and X‐ray tomography [for sulfide, metal, and porosity volume fractions]) consistently gives an unusually low olivine/orthopyroxene ratio (0.67?0.76 for Mason Gully versus ~1.3 for typical H5 ordinary chondrites). Widespread “silicate darkening” is observed. In addition, it contains a bright green crystalline object at the surface of the recovered stone (diameter ≈ 1.5 mm), which has a tridymite core with minor α‐quartz and a rim of both low‐ and high‐Ca pyroxene. The mineralogy allows the calculation of the temperatures and ?(O₂) characterizing thermal metamorphism on the parent body using both the two‐pyroxene and the olivine‐chromite geo‐oxybarometers. These indicate that MG experienced a peak metamorphic temperature of ~900 °C and had a similar ?(O₂) to Kernouvé (H6) that was buffered by the reaction between olivine, metal, and pyroxene. There is no evidence for shock, consistent with the observed porosity structure. Thus, while Mason Gully has some unique properties, its geochemistry indicates a similar thermal evolution to other H chondrites. The presence of tridymite, while rare, is seen in other OCs and likely exogenous; however, the green object itself may result from metamorphism.     

Finite Fault Modeling in a Tsunami Warning Center Context

The US NOAA/NWS tsunami warning centers have relied on earthquake location and depth, and scalar measures of earthquake size and slowness to assess the potential for the generation of a destructive tsunami by an earthquake. Recent earthquakes, such as Peru 2001, Sumatra 2004 and the Java 2006, manifest some of the difficulties the warning centers face as they try to cope with unusual earthquakes. We have undertaken a study of a simple teleseismic waveform inverse model and applied it to the earthquakes of June 23, 2001 in Peru and of July 17, 2006 in Java. Synthetic numerical experiments suggest that the most salient features of the rupture history of an earthquake can be recovered. Furthermore the calculations can be conducted quickly enough to be useful in a warning center context. We have applied our technique to the Peru 2001 and recent Java 2006 earthquakes. Our overall results are consistent with those obtained from other studies. The results show why the Peru event initially looked slow to the US tsunami warning centers and that the Java event is a truly slow or tsunami earthquake. Clearly, the warning centers stand to benefit from an increased understanding of the earthquakes they monitor. Furthermore, better knowledge of the slip distribution along a fault will improve tsunami wave-height forecasts.