Search

This paper discusses the spatiotemporal pattern of seismicity during the impoundment of the Xiaowan reservoir area, Yunnan Province, China, in the period from May 21, 2005 to December 31, 2012. The filling operations took place in five phases, starting on December 16, 2008. A notable increase in seismicity was only observed during the third filling phase, starting on July 15, 2010. Seismicity increase was mostly localized within two clusters, located to the northwest and west of the dam. Seismicity rates in these clusters showed a significant correlation with the water level increase, with the seismicity starting to increase when the water level reached the area covered by the two clusters, which additionally support they were induced by the reservoir impoundment. We further investigate source parameters for 44 pre-impoundment earthquakes and 164 post-impoundment earthquakes with M ≥ 2.0. Corner frequencies, seismic moments, and stress drops are obtained based on the spectral analysis of regional data, upon corrections for geometrical spreading, frequency-dependent Q, and site effects. Our results show that during the post-impoundment phase reservoir-induced seismicity (RIS) inside the two clusters have systematically lower stress drops with respect to those occurring at further distance, and surrounding natural tectonic earthquakes, by a factor of about two to three times, suggesting a possible source characteristic that differentiate reservoir-induced seismicity from natural tectonic earthquakes. However, temporal stress drops changes within the clusters cannot be resolved. To compare the difference in stress drop between RIS and natural tectonic earthquakes, not only the difference in stress drop due to different region, but also the temporal change of background stress level even in the same region should be taken into account. [ABSTRACT FROM AUTHOR]

The source rupture process of the M7.0 Lushan earthquake was here evaluated using 40 long-period P waveforms with even azimuth coverage of stations. Results reveal that the rupture process of the Lushan M7.0 event to be simpler than that of the Wenchuan earthquake and also showed significant differences between the two rupture processes. The whole rupture process lasted 36 s and most of the moment was released within the first 13 s. The total released moment is 1.9×10N m with M=6.8. Rupture propagated upwards and bilaterally to both sides from the initial point, resulting in a large slip region of 40 km×30 km, with the maximum slip of 1.8 m, located above the initial point. No surface displacement was estimated around the epicenter, but displacement was observed about 20 km NE and SW directions of the epicenter. Both showed slips of less than 40 cm. The rupture suddenly stopped at 20 km NE of the initial point. This was consistent with the aftershock activity. This phenomenon indicates the existence of significant variation of the medium or tectonic structure, which may prevent the propagation of the rupture and aftershock activity. The earthquake risk of the left segment of Qianshan fault is worthy of attention. [ABSTRACT FROM AUTHOR]

We investigate the relationship between the impoundment and seismicity in the Longtan reservoir, southwestern China and find evidence that the seismicity was reservoir induced. After the reservoir impoundment, a pronounced increase in seismicity was observed in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling. The observed induced seismicity shows a strong correlation with the filling cycles. The activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults. Source parameters for 1,616 earthquakes with M 0.1-4.2 recorded by 24 fixed and temporary stations deployed around the reservoir were calculated after applying corrections for geometrical spreading, frequency-dependent Q, and site effects. The static stress drop and apparent stress in this area both appear to increase with increasing seismic moment over the entire magnitude range. Our results show that reservoir induced earthquakes have ten times lower average stress drop than natural tectonic earthquakes. These results may indicate that the reservoir induced seismicity can occur with a lower tectonic stress due to the high pore pressures of the underground medium, and that the effect of the water decreases the coefficient of friction. [ABSTRACT FROM AUTHOR]

A pronounced increase in seismicity started in and around Longtan reservoir, southwestern China after October 1, 2006 when it began the impoundment, and by the end of May 14, 2010, about 3,233 earthquakes with −0.6 ≤ M ≤ 4.2 had been located. This seismicity which occurred in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling. There were four water filling periods in the Longtan reservoir, and the observed reservoir-induced seismicity (RIS) shows a strong correlation with the filling cycles. After the first filling, there appears to be an instant undrained response due to an elastic response to the reservoir load in the third and fourth cluster. Then, this seismicity is followed by a delayed, drained response due to pore pressure diffusion, with the seismicity migrating outwards in one or more directions in the second and third filling period. The seismic diffusivity ( α) we estimated is about 4.54 × 10 cm/s. The activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults. The delayed seismic response to the filling in the third cluster was due to the combined effects of the lack of local fault intersecting the reservoir and lower permeability of the rock. The b value we obtained for reservoir-induced events was significantly different and higher than that of pre-impoundment natural tectonic earthquakes in the Longtan reservoir. The results of relocated earthquakes based on double difference earthquake location algorithm showed that their focal depths were mainly shallower than about 10 km and the distribution of relocated RIS in four clusters had no relation with these intersecting faults in the Longtan reservoir except the fifth cluster. All these characteristics of RIS in the Longtan reservoir indicate that they may relate to the coupled poroelastic response that includes both pore pressure diffusion and an undrained response, but the pore pressure diffusion and the water permeation appear to play a more important role on inducing the earthquakes in Longtan reservoir. [ABSTRACT FROM AUTHOR]

Regional landscape ecological security is an important issue for ecological security, and has a great influence on national security and social sustainable development. The Pearl River Delta (PRD) in southern China has experienced rapid economic development and intensive human activities in recent years. This study, based on landscape analysis, provides a method to discover the alteration of character among different landscape types and to understand the landscape ecological security status. Based on remotely sensed products of the Landsat 5 TM images in 1990 and the Landsat 7 ETM+ images in 2005, landscape classification maps of nine cities in the PRD were compiled by implementing Remote Sensing and Geographic Information System technology. Several indices, including aggregation, crush index, landscape shape index, Shannon's diversity index, landscape fragile index, and landscape security adjacent index, were applied to analyze spatial-temporal characteristics of landscape patterns in the PRD. A landscape ecological security index based on these outcomes was calculated by projection pursuit using genetic algorithm. The landscape ecological security of nine cities in the PRD was thus evaluated. The main results of this research are listed as follows: (1) from 1990 to 2005, the aggregation index, crush index, landscape shape index, and Shannon's diversity index of nine cities changed little in the PRD, while the landscape fragile index and landscape security adjacent index changed obviously. The landscape fragile index of nine cities showed a decreasing trend; however, the landscape security adjacent index has been increasing; (2) from 1990 to 2005, landscape ecology of the cities of Zhuhai and Huizhou maintained a good security situation. However, there was a relatively low value of ecological security in the cities of Dongguan and Foshan. Except for Foshan and Guangzhou, whose landscape ecological security situation were slightly improved, the cities had reduced values in landscape ecological security, with the most decreased number 0.52 in Zhaoqing. Results of this study offer important information for regional eco-construction and natural resource exploitation. [ABSTRACT FROM AUTHOR]

High-resolution three-dimensional V and V/ V images in the Longtan reservoir area were obtained from local earthquake data by using 3,178 events with total 24,153 P-wave and 23,987 S-wave arrivals collected from 23 seismic stations. The tomographic images show that significant V heterogeneities can be seen at layers of different depth in the Longtan reservoir area. Low- V anomalies both beneath and around the main rivers in the reservoir area may be related to the composition of rocks which are mainly deposit carbonate and arenaceous shale, which contributes to water saturation. We deduced that the high porosity rocks beneath the main rivers may be fully saturated with water. The phenomenon that V is relatively high in the area which is 10-20 km away from the rivers indicates that horizontal saturation of water is limited within a small range of area that is about 10-20 km from the main rivers. The characteristic is significant that seismicity in the Longtan reservoir area is coincident with the distribution of the low- V area. V/ V tomographic images show that V/ V ranges from 1.8 to 2.05 in shallow layers above 4 km depth beneath the Longtan reservoir, suggesting the properties of the rocks are limestone and shale. At the depth of 7 km, the distribution of V/ V image varies quite remarkably, especially in the dam area. This demonstrates that the range of influence by the saturation of water in the media below the reservoir surface can reach 4-7 km depth in the dam area. [ABSTRACT FROM AUTHOR]

Moment tensor solution, rupture process and rupture characteristics of the great Wenchuan M8.0 earthquake are studied by using 39 long-period P and SH waveforms with evenly azimuth coverage of stations. Our results reveal that the Wenchuan M8.0 event consisted of 5 sub-events of Mw⩾7.3 occurring succesively in time and space. Rupture started with a Mw7.3 introductory strike-slip faulting in the first 12 s, then within 12-40 s, two sub-events with Mw7.6 and Mw7.4 occurred within 80 km northeast from the initial point with the dominant rupture type of thrust moving. From 40 to 62 s, a Mw7.5 and Mw7.4 right-lateral strike-slip type of sub-events occurred on the two sides of Beichuan, 120 km away northeast from the initial point. The whole rupture process lasted 105 s and unilaterally propagated from the initial point on the WS section of the Yingxiu-Beichuan fault to the NE direction, resulting in a 230-km-long surface rupture zone and the average surface dislocation is up to 4 m. Two asperities are identified and the whole rupture process is formed by WS and NE parts. In the WS part named Dujiangyang-Wenchuan, where the initial point is located, the rupture process showed reverse faulting with the maximum slip of 8.2 m. Around Mianzhu, rupture changed to right-lateral strike slip faulting and formed a Beichuan-Qingchuan large slip area. The rupture area on this part is about 10 km in depth, shallower than on the WS part. The maximum slip is 6.53 m. Consequently, there formed 2 segments with the surface dislocation larger than 6 m. One is the Dujiangyan-Wenchuan segment with the maximum surface displacement of 6.44 m, the other is the Beichuan-Qingchuan segment with the maximum surface displacement of 6.53 m. This segmentation may have its geological and tectonic background. [ABSTRACT FROM AUTHOR]