United States Geological Survey

About: United States Geological Survey is a government organization based out in Reston, Virginia, United States. It is known for research contribution in the topics: Population & Groundwater. The organization has 17899 authors who have published 51097 publications receiving 2479125 citations. The organization is also known as: USGS & US Geological Survey.

Stress changes from the 2008 Wenchuan earthquake and increased hazard in the Sichuan basin

Abstract: On 12 May 2008, the devastating magnitude 7.9 (Wenchuan) earthquake struck the eastern edge of the Tibetan plateau, collapsing buildings and killing thousands in major cities aligned along the western Sichuan basin in China. After such a large-magnitude earthquake, rearrangement of stresses in the crust commonly leads to subsequent damaging earthquakes. The mainshock of the 12 May earthquake ruptured with as much as 9 m of slip along the boundary between the Longmen Shan and Sichuan basin, and demonstrated the complex strike-slip and thrust motion that characterizes the region. The Sichuan basin and surroundings are also crossed by other active strike-slip and thrust faults. Here we present calculations of the coseismic stress changes that resulted from the 12 May event using models of those faults, and show that many indicate significant stress increases. Rapid mapping of such stress changes can help to locate fault sections with relatively higher odds of producing large aftershocks.

Joint formation in granitic rock of the Sierra Nevada

Abstract: A single steeply dipping joint set in the Mount Givens Granodiorite, central Sierra Nevada, was studied to clarify the mechanics of fracture and joint formation in granitic rocks. The joints were filled with fluid during, or immediately following, formation; these fluids deposited epidote and chlorite within the joints. Examination of lithologic markers in outcrop and thin section demonstrates that relative displacements are normal to the joint surfaces. These observations rule out a shear origin for these joints. The measured extensional strain acommodated by joint dilation is on the order of 1 × 10 −4 to 5 × 10 −4 . A few joints in the area exhibit small strike-slip offsets. In these joints, the mineral fillings are sheared, indicating the strike-slip motion postdated the jointing. Individual joints consist of numerous subparallel, planar segments. The lengths of joints range from metres to nearly 100 metres. Shorter joints are more abundant than longer joints. The observed distribution of joint lengths is thought to result from the elastic interaction of adjacent joints. Shorter joints are prevented from further propagation by their long neighbors. Between mapped joints, small cracks that have lengths of several centimetres are found parallel to the longer joints. These cracks represent a growth stage between grain-scale microcracks and macroscopic joints. A method is developed for estimating the tensile stress responsible for initiating joint growth. The method requires knowledge of the extensional strain accommodated by joint dilation and the spatial density of joints, both of which can be determined by field observations. Calculations based on observations of joints in the Florence Lake area indicate relative tensile stresses (average remote stress plus internal fluid pressure) of approximately 1 MPa to 40 MPa. These values of stress and estimates of initial crack length are used to estimate the quasi-static fracture toughness of the granodiorite. The calculated fracture toughnesses range from 0.04 Mpa·m 1/2 to 4 Mpa·m 1/2 . The stress and fracture toughness estimates are compatible with existing data from laboratory fracture experiments.