The 'Carina Flare' supershell: probing the atomic and molecular ISM in a Galactic chimney     

J. R. Dawson  N. Mizuno  T. Onishi  N. M. McClure-Griffiths  Y. Fukui 《Monthly notices of the Royal Astronomical Society》2008,387(1):31-44

The 'Carina Flare' supershell, GSH 287+04−17, is a molecular supershell originally discovered in  ¹²CO( J = 1–0)  with the NANTEN 4 m telescope. We present the first study of the shell's atomic ISM, using H  i 21-cm line data from the Parkes 64-m telescope Southern Galactic Plane Survey. The data reveal a gently expanding,  ∼230 × 360  pc H  i supershell that shows strong evidence of Galactic Plane blowout, with a break in its main body at   z ∼ 280  pc and a capped high-latitude extension reaching   z ∼ 450  pc. The molecular clouds form comoving parts of the atomic shell, and the morphology of the two phases reflects the supershell's influence on the structure of the ISM. We also report the first discovery of an ionized component of the supershell, in the form of delicate, streamer-like filaments aligned with the proposed direction of blowout. The distance estimate to the shell is re-examined, and we find strong evidence to support the original suggestion that it is located in the Carina Arm at a distance of  2.6 ± 0.4 kpc  . Associated H  i and H₂ masses are estimated as   M _H I≈ 7 ± 3 × 10⁵ M_⊙  and     , and the kinetic energy of the expanding shell as   E _K ∼ 1 × 10⁵¹  erg. We examine the results of analytical and numerical models to estimate a required formation energy of several 10⁵¹ to  ∼10⁵²  erg, and an age of  ∼10⁷ yr  . This age is compatible with molecular cloud formation time-scales, and we briefly consider the viability of a supershell-triggered origin for the molecular component.  相似文献   

Fidelity of the integrated kinetic energy factor as an indicator of storm surge impacts     

Bass  Benjamin  Irza  John N.  Proft  Jennifer  Bedient  Philip  Dawson  Clint 《Natural Hazards》2017,85(1):575-595

Natural Hazards - After the devastating hurricane season of 2005, shortcomings with the Saffir–Simpson Hurricane Scale’s (SSHS) ability to characterize a tropical cyclone’s...  相似文献   

Discovery of a Color-selected Quasar at z = 5.50     

Stern D  Spinrad H  Eisenhardt P  Bunker AJ  Dawson S  Stanford SA  Elston R 《The Astrophysical journal》2000,535(2):L75-L78

I show how the existing observational data on Local Group dwarf galaxies can be used to estimate the average star formation law during the first 3 Gyr of the history of the universe. I find that the observational data are consistent with the orthodox Schmidt law with a star formation efficiency of about 4% if the star formation is continuous (during the first 3 Gyr). The efficiency is proportionally higher if most of the gas in the dwarfs was consumed (and never replenished) in a short time interval well before the universe turned 3 Gyr.  相似文献   

Sampling-based flood risk analysis for fluvial dike systems     

Richard Dawson  Jim Hall  Paul Sayers  Paul Bates  Corina Rosu 《Stochastic Environmental Research and Risk Assessment (SERRA)》2005,19(6):388-402

A dike system of moderate size has a large number of potential system states, and the loading imposed on the system is inherently random. If the system should fail, in one of its many potential failure modes, the topography of UK floodplains is usually such that hydrodynamic modelling of flood inundation is required to generate realistic estimates of flood depth and hence damage. To do so for all possible failure states may require 1,000s of computationally expensive inundation simulations. A risk-based sampling technique is proposed in order to reduce the computational resources required to estimate flood risk. The approach is novel in that the loading and dike system states (obtained using a simplified reliability analysis) are sampled according to the contribution that a given region of the space of basic variables makes to risk. The methodology is demonstrated in a strategic flood risk assessment for the city of Burton-upon-Trent in the UK. 5,000 inundation model simulations were run although it was shown that the flood risk estimate converged adequately after approximately half this number. The case study demonstrates that, amongst other factors, risk is a complex function of loadings, dike resistance, floodplain topography and the spatial distribution of floodplain assets. The application of this approach allows flood risk managers to obtain an improved understanding of the flooding system, its vulnerabilities and the most efficient means of allocating resource to improve performance. It may also be used to test how the system may respond to future external perturbations.  相似文献   

Three-dimensional liquefaction potential analysis using geostatistical interpolation   总被引：1，自引：0，他引：1  

Kevin M. Dawson  Laurie G. Baise   《Soil Dynamics and Earthquake Engineering》2005,25(5):369-381

We applied three-dimensional geostatistical interpolation to evaluate the extent of liquefiable materials at two sites that liquefied during the 1994 Northridge Earthquake. The sites were the Balboa Blvd site and the Wynne Ave. site located in the alluvial San Fernando Valley. The estimated peak ground accelerations at the sites are 0.84 g (Balboa Blvd) and 0.51 g (Wynne Ave.). These sites were chosen because surface effects due to liquefaction were not predicted using available techniques based on thickness and depth of liquefiable layers (Ishihara [Ishihara K. Stability of natural deposits during earthquakes. Proceedings of the 11th international conference on soil mechanics and foundation engineering, vol. 1. Rotterdam, The Netherlands: A.A. Balkema; 1985. p. 321–76.]) and the Liquefaction Potential Index (Iwasaki et al. [Iwasaki T, Tatsuoka F, Tokida K, Yasuda S. A practical method for assessing soil liquefaction potential based on case studies at various sites in Japan. In: Proceedings of the second international conference on microzonation, San Francisco; 1978. p. 885–96.]). During the earthquake, both sites experienced surface effects including ground cracking and extension as a result of liquefaction. Foundations and buried utilities were damaged at both sites. The sites were investigated after the event by researchers with the United States Geologic Survey using standard penetration tests (SPT) and cone penetration tests. In this paper, liquefaction potential was estimated for each soil sample using results from SPTs according to the updated Seed and Idriss simplified procedure. The probability of liquefaction was estimated by applying an indicator transform to the results of the liquefaction potential calculation. We compared our results to detailed geologic mapping of the sites performed by other researchers. Using geostatistical interpolation to estimate the probability of liquefaction is a useful supplement to geologic evaluation of liquefaction potential. The geostatistical analysis provides an estimate of the continuous volume of liquefiable soil along with an assessment of confidence in an interpolation. The probability of liquefaction volumes compare well with those predicted using geologic interpretations.  相似文献   

Distinguishing tsunami and storm deposits: An example from Martinhal, SW Portugal   总被引：2，自引：0，他引：2  

S. Kortekaas  A.G. Dawson 《Sedimentary Geology》2007,200(3-4):208-221

Geological identification of past tsunamis is important for risk assessment studies, especially in areas where the historical record is limited or absent. The main problem when using the geological evidence is to distinguish between tsunami and storm deposits. Both are high-energy events that may leave marine traces in coastal stratigraphic sequences. At Martinhal, SW Portugal both storm surge and tsunami deposits are present at the same site within a single stratigraphic sequence, which makes it suitable to study the differences between them, excluding variations caused by local factors.

The tsunami associated with the Lisbon earthquake of November 1st 1755 AD, had a major impact on the geomorphology and sedimentology of Martinhal. It breached the barrier and laid down an extensive sheet of sand, as described in eyewitness reports. Besides the tsunami deposit the stratigraphy of Martinhal also displays evidence for storm surges that have breached and overtopped the barrier, flooding the lowland and leaving sand layers. Both marine-derived flood deposits show similar grain size characteristics and distinctive marine foraminifera. The most important differences are the rip-up clasts and boulders exclusively found in the tsunami deposit and the landward extent of the tsunami deposit that everywhere exceeds that of the storm deposits. Identification of both depositional units was only possible using a collection of different data and extensive stratigraphical information from cores as well as trenches.  相似文献   

Paleoseismology of the Gazikoy-Saros segment of the North Anatolia fault, northwestern Turkey: Comparison of the historical and paleoseismic records, implications of regional seismic hazard, and models of earthquake recurrence   总被引：1，自引：0，他引：1  

Thomas Rockwell  Aykut Barka  Timothy Dawson  Serdar Akyuz  Kim Thorup 《Journal of Seismology》2001,5(3):433-448

We excavated five trenches across the North Anatolia fault zone (NAFZ)along the Ganos fault (Gazikoy-Saros segment), which last produced surfacerupture in 1912, near Kavakkoy where the fault enters the Gulf of Saros. The trenches exposed faulted sediments in a flood-plain environment withabundant detrital charcoal and scattered land-snail shells. Twenty-tworadiocarbon dates place constraints on the ages of the exposed sediments,which range from less than a few hundred years to about 6000 years inage. In two closely spaced trenches, we identified five discrete earthquakeevent horizons in the upper 2.5 m of stratigraphy based on abruptupward termination of shear zones, folding, fissuring, and abruptstratigraphic thickening, four of which may corresponded to historicallyrecorded large regional earthquakes. The earliest of the identified eventsoccurs below an unconformity and dates to about 4 ka B.P. The morerecent four events all occurred within the past 1000–1200 years and maycorrespond to large earthquakes in A.D. 824, ca 1354, 1509, 1766 and1912 (Ambraseys and Finkel, 1987, 1991, 1995). In another trench,we identified at least two events that have occurred during the past 500years and probably correspond to the large events of 1766 and 1912. These observations support an average return period of about 250–300years for the Gazikoy-Saros segment of the NAFZ. They also suggest thatthis segment, which is bound both to the east and west by large releasingstepovers, behaves in a quasi-periodic fashion, at least for the past severalsurface ruptures.Most of the 23 mm/yr of dextral shear between Anatolia and Europeobserved by GPS occurs on the North Anatolian fault. We use18 mm/yr and the 250–300 year recurrence rate, as determined fromour trenching and the historical record, to suggest that each of theearthquakes observed in our trenches produced several meters of slip,consistent with their inferred sizes from the extent of historical damage. Considering that Istanbul has not suffered a large nearby event in theMarmara Sea since 1766, we suggest that about 4 m of strain hasaccumulated across faults in the Marmara during these past centuries. Thisis similar to the average slip in many of the large earthquakes on the NorthAnatolian fault this century. If released seismically, this could result in anearthquake in the M 7.2–M 7.6 range, similar to the August and November,1999 earthquakes east of the Marmara Sea.  相似文献   

Simulation studies of electron acceleration by ion ring distributions in solar flares     

K. G. McClements  J. J. Su  R. Bingham  J. M. Dawson  D. S. Spicer 《Solar physics》1990,130(1-2):229-241

Using a 2 1/2-D fully relativistic electromagnetic particle-in-cell code (PIC) we have investigated a potential electron acceleration mechanism in solar flares. The free energy is provided by ions which have a ring velocity distribution about the magnetic field direction. Ion rings may be produced by perpendicular shocks, which could in turn be generated by the super-Alfvénic motion of magnetic flux tubes emerging from the photosphere or by coronal mass ejections (CMEs). Such ion distributions are known to be unstable to the generation of lower hybrid waves, which have phase velocities in excess of the electron thermal speed parallel to the field and can, therefore, resonantly accelerate electrons in that direction. The simulations show the transfer of perpendicular ion energy to energetic electrons via lower hybrid wave turbulence. With plausible ion ring velocities, the process can account for the observationally inferred fluxes and energies of non-thermal electrons during the impulsive phase of flares. Our results also show electrostatic wave generation close to the plasma frequency: we suggest that this is due to a bump-in-tail instability of the electron distribution.  相似文献   

A Holocene tsunami deposit in eastern Scotland     

David Long  David E. Smith  Alastair G. Dawson 《第四纪科学杂志》1989,4(1):61-66

A thin, regionally extensive, laterally persistent sand layer identified within the Holocene coastal sequences of eastern Scotland, dated to 7000 years BP, is suggested to be a tsunami deposit. The likely source of the tsunami wave is the earthquake induced second Storegga Slide on the Norwegian continental slope at least 750 km northeast of the deposit.  相似文献   

Identifying elements of the plumbing system beneath Kilauea Volcano, Hawaii, from the source locations of very-long-period signals     

Javier Almendros  Bernard Chouet  Phillip Dawson  Timothy Bond 《Geophysical Journal International》2002,148(2):303-312

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