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21.
Seismicity and volcanism of Jan Mayen Island   总被引:1,自引:0,他引:1  
The small, arctic Jan Mayen Island, site of the World's northernmost active volcano, Beerenberg, is part of the mid-Atlantic ridge system and located along the Jan Mayen Fracture Zone (JMFZ). Recent data from the local seismic network, and fault plane solutions from the global network, indicate that the island is located at a ridge-crest intersection, which might explain the origin of the island and the associated volcanism. Moreover, the new data suggest a series of offset segments of the Mohn's Ridge, overlapping in an en echelon pattern. In January 1985, a flank eruption was for the first time observed with the local seismic network. Volcanic tremors and low-frequency events were observed on 5 January at 2230 h and 10 hours later the first large earthquake occurred. No visual confirmation of the eruption was made until 6 January at 1630 h. The seismic observations rule out the possibility that the large earthquake caused the eruption; it seems more likely that the changes in local stress conditions triggered the earthquake and that the eruption started before the first large earthquake. Recent observations show that the local network provides an efficient tool for monitoring and warning of volcanic activity. However, since there was no change in the local seismicity in the days or months before the 1985 eruption, it seems to be difficult to make long-term predictions of Beerenberg flank eruptions without using other techniques such as observations of tilt.  相似文献   
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The M w = 8.0 Wenchuan earthquake of May 12, 2008, caused destruction over a wide area. The earthquake cost more than 69,000 lives and the damage is reported to have left more than 5 million people homeless. It is estimated that 5.36 million buildings were destroyed and 21 million buildings were damaged in Sichuan and the nearby provinces. Economic losses due to the event are estimated to be 124 billion USD. From a field reconnaissance trip conducted in October 2008, it is evident that the combination of several factors, including mountainous landscape, strong ground shaking, extensive landslides and rock-falls, has exacerbated the human and economic consequences of this earthquake. Extensive damage occurred over a wide area due to the shear size of the earthquake rupture combined with poor quality building construction. In order to investigate the ground shaking during the earthquake, we have conducted a strong ground motion simulation study, applying a hybrid broadband frequency technique. The preliminary results show large spatial variation in the ground shaking, with the strongest ground motions along the fault plane. The simulation results have been calibrated against the recorded ground motion from several near-field stations in the area, and acceleration values of the order of 1 g are obtained, similar to what was recorded during the event. Comparison with the damage distribution observed in the field confirms that the effect of fault rupture complexity on the resulting ground motion distribution also controls to a large extent the damage distribution. The applied simulation technique provides a promising platform for predictive studies.  相似文献   
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Site response measurements provide information on the amplification of ground motions generated by local conditions. Recent studies of large destructive earthquakes have shown that damage during the earthquakes are often caused by the amplification of seismic waves in near-surface sedimentary layers. The estimation of site response is therefore critical, in order to evaluate the true seismic hazard potential of a given area. We investigated local site amplifications in the South Iceland Seismic Zone (SISZ). Nine digital seismographs were deployed, temporarily, in an area of approximately 400 km2, in the westernmost part of the SISZ. Among the 90 events recorded, 15 were used in this study, including a magnitude 3.1 (ML) event and selected aftershocks, which occurred in the northern outskirts of the village Hveragerdi. Single Station Spectral Ratios (SSSR) of the recorded earthquakes revealed some of the effects of local site conditions. Spectral amplification factors of 2–5 on average, can be expected in the SIL area, depending on the sediment type and thickness. Higher site amplifications occur in the southern part of the study region, where the thickest sedimentary cover is found. Spectral amplification, related to topographical effects, is observed at the bedrock reference station, Bjarnastadir. Standard Spectral Ratios (SSR), with respect to the bedrock reference station, Bjarnastadir, were also calculated for some stations, in order to compare the two spectral ratio results. The two methods show a good correlation at some stations, whereas at others they vary considerably. The comparison between the earthquake and ambient noise data, on the other hand, gave better correlation when the SSSR method is used.  相似文献   
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Areas of low strain rate are typically characterized by low to moderate seismicity. The earthquake catalogs for these regions do not usually include large earthquakes because of their long recurrence periods. In cases where the recurrence period of large earthquakes is much longer than the catalog time span, probabilistic seismic hazard is underestimated. The information provided by geological and paleo-seismological studies can potentially improve seismic hazard estimation through renewal models, which assume characteristic earthquakes. In this work, we compare the differences produced when active faults in the northwestern margin of the València trough are introduced in hazard analysis. The differences between the models demonstrate that the introduction of faults in zones characterized by low seismic activity can give rise to significant changes in the hazard values and location. The earthquake and fault seismic parameters (recurrence interval, segmentation or fault length that controls the maximum magnitude earthquake and time elapsed since the last event or Te) were studied to ascertain their effect on the final hazard results. The most critical parameter is the recurrence interval, where shorter recurrences produce higher hazard values. The next most important parameter is the fault segmentation. Higher hazard values are obtained when the fault has segments capable of producing big earthquakes. Finally, the least critical parameter is the time elapsed since the last event (Te), when longer Te produces higher hazard values.  相似文献   
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