The 2008 Wenchuan M S 8.0 earthquake occurred on Longmenshan fault zone (LMSF), which is at the eastern margin of the Tibetan plateau. The epicenter is near the Shuimogou earthquake swarm, which was thought to be triggered by the Zipingpu reservoir after its impounding in 2004. People have speculated that the large earthquake was triggered by the water filling of the reservoir. To figure out the role of the Zipingpu reservoir on the earthquake, the local seismicity recorded by the Zipingpu local seismic network during the period from 31 July 2004 to 11 May 2008 were analyzed in detail. The distribution of hypocenters showed that most earthquakes occurred on Yingxiu-Beichuan fault (YBF) in the reservoir area with hypocenters depth less than 10 km, which is a major source fault of the Wenchuan earthquake. Useful information on fault geometry in the depth was also obtained. The spatial-temporal distribution of hypocenters demonstrated clear migration pattern that indicated pore-pressure diffusion, it also showed a hydraulic diffusivity (D) of 0.7 m 2 /s. Previous experiments show the existence of the synergism process of the fault under a meta-instability state before fault sliding. It enhances the stress on the stronger portion of the fault and the synergism degree by reducing strength of the weak portions and by increasing the total length of weak portions. According to this view, the pore pressure diffusion by water filling of Zipingpu reservoir increased the total length of weak portions and enhanced the stress at the focal. GEODYNAMICS & TECTONOPHYSICS P U B L I S H E D B Y T H E I N S T I T U T E O F T H E E A R T H ' S C R U S T S I B E R I A N B R A N C H O F R U S S I A N A C A D E M Y O F S C I E N C E S T e c t o n o p h y s i c s Liu Yuanzheng et al.: Insights gained from the seismicity around the Zipingpu reservoir… 778ных местной сетью сейсмических наблюдений в районе Зипингпу в период с 31 июля 2004 г. по 11 мая 2008 г. Судя по распределению гипоцентров, большинство землетрясений произошли на разломе Инксю-Бейчуан в районе водо-хранилища, при этом глубина гипоцентров не превышала 10 км, и это основной разлом, инициировавший Венчуань-ское землетрясение. Кроме того, были получены полезные данные по глубинной геометрии разлома. По простран-ственно-временному распределению гипоцентров установлен характер миграции с рассеиванием порового давления, а также определен коэффициент гидравлической диффузии (D=0.7 м 2 /с). По результатам предыдущих экспериментов установлено наличие синергетического процесса на изучаемом разломе в метастабильном состоянии перед смеще-нием по разлому, что привело к усилению напряжений на прочном участке разлома и синергии при уменьшении прочности ослабленных участков, а также увеличении общей протяженности ослабленных участков разлома. По нашему мнению, рассеивание порового давления при заполнении водой водохранилища Зипингпу привело к увели-чению общей длины ослабленных участков разлома и увеличению напряжений в очаговой зоне.Ключевые слова: Венчуаньское землетрясение,...

The origins of tremors and their relation to fault slip and impending earthquakes remain unclear. Laboratory studies on tremor‐like acoustic emission events generated during preslip may shed some light on the above issues. We conducted stick‐slip experiments and observed tremor‐like events and preslip in the laboratory. The results show the following. (1) The dominant frequency of tremor‐like events increases with accelerating preslip. (2) The evolution of tremor‐like events is spatiotemporally consistent with that of preslip. (3) Interactions between preslip and high‐frequency events indicate the formation of a positive feedback effect between the expansion of preslip and failure of asperities, which leads to stick‐slip instability. Thus, we propose that tremors may be caused by stress variations derived from asperity interactions along fault surfaces at dry conditions. The increasing dominant frequency of tremors may imply the accelerating preslip of an upcoming earthquake.

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Field and experimental observations showed that preslip undergoes a transition from multiple to single preslip zones, which implies the existence of linkage of preslip zones before the fault instability. However, the observations of the linkage process, which is significant for understanding the mechanism of earthquake preparation, remains to be implemented due to the limitations of observation methods in previous studies. Detailed spatiotemporal evolutions of preslip were observed via a high-speed camera and a digital image correlation method in our experiments. The normalized length of preslip zones shows an increase trend while the normalized number of preslip zones (NN) shows an increase followed by a decrease trend, which indicate that the expansion of the preslip undergoes a transition from increase to linkage of the isolated preslip zones. The peak NN indicates the initiation of the linkage of preslip zones. Both the linkage of the preslip zones and the decrease in the normalized information entropy of fault displacement direction indicate the reduction of spatial complexity of preslip as the instability approaches. Furthermore, the influences of dynamic adjustment of stress along the fault and the interactions between the asperities and preslip on the spatial complexity of preslip were also observed and analyzed.

SUMMARY The effects of fault waviness on the fault slip modes are unclear. Laboratory study on the effects of the centimetre-scale fault contact distribution, which is mainly controlled by the fault waviness, on granodiorite stick-slip instabilities may help to unveil some aspects of the problem. The fast and slow stick-slip motions were separately generated in two granodiorite samples of the same roughness but different fault contact distributions in the centimetre scale in the laboratory. The experimental results show the following: (1) the fault with the small contact area and heterogeneous contact distribution generates fast stick-slip instabilities, while the fault with the large contact area and homogeneous contact distribution produces slow stick-slip events; (2) the nucleation processes of the fast stick-slip events are characterized by abrupt changes once the nucleation zones expand to the critical nucleation length that is observed to be shorter than the fault length, while the slow stick-slip events appear as a gradual evolution of the nucleation zones leading to total fault sliding. These indicate that, unlike the micron-scale fault contact distribution controlled by roughness, which depends mainly on the grain size of the abrasives used for lapping the fault surface, the centimetre-scale fault contact distribution, which depends mainly on the waviness of the fault surface profile, also plays an important role in the fault slip modes. In addition, the effects of the fault waviness on the fault friction properties are preliminarily analysed based on the rate- and state-dependent friction law.