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91.
R. G. Pearce 《Geophysical Journal International》1981,64(1):187-200
Summary. It is shown that complex teleseismic P waveforms from a shallow earthquake in a tectonic area can be interpreted using a simple source model embedded in a plane layer velocity structure (with sea layer) whose details are based upon independent evidence. This gives hope that structural complexity in tectonic regions may not always make distant P -wave seismograms impossible to interpret, and that, instead, source complexity may be responsible for some of the many complex waveforms observed, even for earthquakes below magnitude m b 5.5. 相似文献
92.
93.
Fault plane solutions using relative amplitudes of P and pP 总被引:2,自引:1,他引:1
R. G. Pearce 《Geophysical Journal International》1977,50(2):381-394
Summary. One way of finding the fault plane orientations of small shallow earthquakes is by the generation of theoretical P -wave seismograms to match those observed at several distant stations. Here, a technique for determining the uniqueness of fault plane solutions computed using the modelling method of Douglas et al . is described. Relative amplitudes of P and pP , and their polarities if unambiguous, are measured on the observed seismograms to be modelled, and appropriate confidence limits are assigned to each measurement. A systematic search is then made for all fault plane orientations which satisfy these observations.
Examples show that if P and pP are not severely contaminated by other arrivals, a well-defined and unique fault plane orientation can often be computed using as few as three stations well distributed in azimuth. Further, even if pP is not identifiable on a particular seismogram, then an upper bound on its amplitude – deduced from the observed coda – still places a significantly greater constraint on the fault plane orientation than would be provided by a P onset polarity alone. Modelling takes account of all such information, and is able to further eliminate incompatible solutions (e.g. by the correct simulation of sP ). It follows that if solutions can be found which satisfy many observed seismograms, this places high significance on the validity of the assumed double-couple source mechanism.
This relative amplitude technique is contrasted with the familiar first motion method of fault plane determination which requires many polarity readings, whose reliabilities are difficult to quantify. It is also shown that fault plane orientations can be determined for earthquakes below the magnitude at which first motion solutions become unreliable or impossible. 相似文献
Examples show that if P and pP are not severely contaminated by other arrivals, a well-defined and unique fault plane orientation can often be computed using as few as three stations well distributed in azimuth. Further, even if pP is not identifiable on a particular seismogram, then an upper bound on its amplitude – deduced from the observed coda – still places a significantly greater constraint on the fault plane orientation than would be provided by a P onset polarity alone. Modelling takes account of all such information, and is able to further eliminate incompatible solutions (e.g. by the correct simulation of sP ). It follows that if solutions can be found which satisfy many observed seismograms, this places high significance on the validity of the assumed double-couple source mechanism.
This relative amplitude technique is contrasted with the familiar first motion method of fault plane determination which requires many polarity readings, whose reliabilities are difficult to quantify. It is also shown that fault plane orientations can be determined for earthquakes below the magnitude at which first motion solutions become unreliable or impossible. 相似文献
94.
A programme of research is underway at Aberdeen University on the conflicts arising in the North Sea between the fishing and offshore oil and gas industries. This research note summarizes the work that has been undertaken to date and the main aspects on which future work will concentrate. 相似文献
95.
Johan Camitz Freysteinn Sigmundsson Gillian Foulger Cord-Hinrich Jahn Christof Völksen Pall Einarsson 《Bulletin of Volcanology》1995,57(2):136-145
GPS geodetic measurements were conducted around the Askja central volcano located at the divergent plate boundary in north Iceland in 1987, 1990, 1992 and 1993. The accuracy of the 1987 and 1990 measurements is in the range of 10 mm for horizontal components; the accuracy of the 1992 and 1993 measurements is about 4 mm in the horizontal plane. Regional deformation in the Askja region is dominated by extension. Points located outside a 30–45 km wide plate boundary deformation zone indicate a displacement of 2.4±0.5 cm/a in the direction N 99°E±12° of the Eurasian plate relative to the North American plate in the period 1987–1990. Within the plate boundary deformation zone extensional strain accumulates at a rate of 0.8 strain/a. Displacement of control points next to Askja (>7 km from the caldera center) in the periods 1990–1993 and 1992–1993 show deflation and contraction towards the caldera. These results are in accordance with the results obtained by other geodetic methods in the area, which indicate that the deflation at Askja occurs in response to a pressure decrease at about 2.8 km depth, located close to the center of the main Askja caldera. A Mogi point source was fixed at this location and the GPS data used to solve for the source strength. A central subsidence of 11±2.5 cm in the period 1990–1993 is indicated, and 5.5±1.5 cm in the period 1992–1993. The maximum tensional strain rate, according to the point source model, occurs at a horizontal distance of 2.5–6 km from the source, at the same location as the main caldera boundary. Discrepancies between the observed displacements and predicted displacements from the Mogi model near the Askja caldera can be attributed to the regional eastwest extension that occurs at Askja. 相似文献
96.
Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust 总被引:61,自引:0,他引:61
Two geochemical proxies are particularly important for the identification and classification of oceanic basalts: the Th–Nb proxy for crustal input and hence for demonstrating an oceanic, non-subduction setting; and the Ti–Yb proxy for melting depth and hence for indicating mantle temperature and thickness of the conductive lithosphere. For the Th–Nb proxy, a Th/Yb–Nb/Yb projection demonstrates that almost all oceanic basalts lie within a diagonal MORB–OIB array with a principal axis of dispersion along the array. However, basalts erupted at continental margins and in subduction zones are commonly displaced above the MORB–OIB array and/or belong to suites with principal dispersion axes which are oblique to the array. Modelling of magma–crust interaction quantifies the sensitivity of the Th–Nb proxy to process and to magma and crustal compositions. For the Ti–Yb proxy, the equivalent Ti/Yb–Nb/Yb projection features a discriminant boundary between low Ti/Yb MORB and high Ti/Yb OIB that runs almost parallel to the Nb/Yb axis, reflecting the fact that OIB originate by melting beneath thicker lithosphere and hence by less melting and with residual garnet. In the case of volcanic-rifted margins and oceanic plume–ridge interactions (PRI), where hot mantle flows toward progressively thinner lithosphere (often becoming more depleted in the process), basalts follow diagonal trends from the OIB to the MORB field. Modelling of mantle melting quantifies the sensitivity of the Ti–Nb proxy to mantle potential temperature and lithospheric thickness and hence defines the petrogenetic basis by which magmas plot in the OIB or MORB fields. Oceanic plateau basalts lie mostly in the centre of the MORB part of that field, reflecting a high degree of melting of fertile mantle. Application of the proxies to some examples of MORB ophiolites helps them to be further classified as C (contaminated)-MORB, N (normal)-MORB, E (enriched)-MORB and P (plume)-MORB ophiolites, which may add a useful dimension to ophiolite classification. In the Archean, the hotter magmas, higher crustal geotherms and higher Th contents of contaminants all result in widespread crustal input that is easy to detect geochemically with the Th–Nb proxy. Application of this proxy to Archean greenstones demonstrates that almost all exhibit a crustal component even when reputedly oceanic. This indicates, either that some interpretations need to be re-examined or that intra-oceanic crustal input is important in the Archean making the proxy less effective in distinguishing oceanic from continental settings. The Ti–Yb proxy is not effective for fingerprinting Archean settings because higher mantle potential temperatures mean that lithospheric thickness is no longer the critical variable in determining the presence or absence of residual garnet. 相似文献
97.
98.
Darrell S. Kaufman Britta J. L. Jensen Alberto V. Reyes Caleb J. Schiff Duane G. Froese Nicholas J. G. Pearce 《第四纪科学杂志》2012,27(4):344-359
Radiocarbon‐dated sediment cores from six lakes in the Ahklun Mountains, south‐western Alaska, were used to interpolate the ages of late Quaternary tephra beds ranging in age from 25.4 to 0.4 ka. The lakes are located downwind of the Aleutian Arc and Alaska Peninsula volcanoes in the northern Bristol Bay area between 159° and 161°W at around 60°N. Sedimentation‐rate age models for each lake were based on a published spline‐fit procedure that uses Monte Carlo simulation to determine age model uncertainty. In all, 62 14C ages were used to construct the six age models, including 23 ages presented here for the first time. The age model from Lone Spruce Pond is based on 18 ages, and is currently the best‐resolved Holocene age model available from the region, with an average 2σ age uncertainty of about ± 109 years over the past 14.5 ka. The sedimentary sequence from Lone Spruce Pond contains seven tephra beds, more than previously found in any other lake in the area. Of the 26 radiocarbon‐dated tephra beds at the six lakes and from a soil pit, seven are correlated between two or more sites based on their ages. The major‐element geochemistry of glass shards from most of these tephra beds supports the age‐based correlations. The remaining tephra beds appear to be present at only one site based on their unique geochemistry or age. The 5.8 ka tephra is similar to the widespread Aniakchak tephra [3.7 ± 0.2 (1σ) ka], but can be distinguished conclusively based on its trace‐element geochemistry. The 3.1 and 0.4 ka tephras have glass major‐ and trace‐element geochemical compositions indistinguishable from prominent Aniakchak tephra, and might represent redeposited beds. Only two tephra beds are found in all lakes: the Aniakchak tephra (3.7 ± 0.2 ka) and Tephra B (6.1 ± 0.3 ka). The tephra beds can be used as chronostratigraphic markers for other sedimentary sequences in the region, including cores from Cascade and Sunday lakes, which were previously undated and were analyzed in this study to correlate with the new regional tephrostratigraphy. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
99.
The creation of ‘usable science’ is widely promoted by many environmental change focused research programs. Few studies however, have examined the relationship between research conducted as part of such programs and the decision-making outcomes that the work is supposed to advance, and is constrained by limited methodological development on how to empirically assess the ‘usability’ of science. Herein, this paper develops a conceptual model and assessment rubric to quantitatively and systematically evaluate the usability of climate change research for informing decision-making. We focus on the process through which data is collected, analyzed and reported and examine the extent to which key principles of usable science are integrated into project design, using grant proposals as our data source. The approach is applied to analyze climate change research conducted as part of the International Polar Year in Canada, with 23 projects identified as having explicit goals to inform decision-making.While the creation of usable science was promoted by funded projects in the International Polar Year, this was not generally reflected in research design: fewer than half determined objectives with input of decision makers, decision context was not widely considered, and knowledge users were not widely reported to be engaged in assessing the quality of data or in resolving conflict in evidence. The importance of science communication was widely emphasized, although only 8/23 projects discussed tailoring specific results for end user needs. Thus while International Polar Year research has made significant advances in understanding the human dimensions of Arctic climate change, key attributes necessary for determining success in linking science to decision-making (pertinence, quality, timeliness) were not captured by many projects. Integrating these attributes into research design from the outset is essential for creating usable science, and needs to be at the forefront of future research programs which aim to advance societal outcomes. The framework for assessing usability here, while developed and tested in an Arctic climate change context, has broader applicability in the general environmental change field. 相似文献
100.
In 2001, legislative measures were introduced in the UK to restrict usage of antifouling agents in small (<25 m) vessel paints to dichlofluanid, zinc pyrithione and zineb. This removed the previously popular booster biocides diuron and Irgarol 1051 from the market. To investigate the impact of this legislation, water samples were taken from locations where previous biocide levels were well documented. Results from analyses demonstrate a clear reduction in water concentrations of Irgarol 1051 (between 10% and 55% of that found during pre-restriction studies), indicating that legislation appears to have been effective. Although other booster biocides were screened for (chlorothalonil, dichlofluanid and Sea-Nine 211), they were below the limits of detection (<1 ng/l) in all samples. A survey of chandlers and discussions with legislative authorities supports these results and concurs the removal of Irgarol 1051 based paints from the market using simple regulations at a manufacturer level with little regulation at a retailer level. 相似文献