%0 Journal Article %1 niu_etal:2008 %A Niu, Fenglin %A Silver, Paul G. %A Daley, Thomas M. %A Cheng, Xin %A Majer, Ernest L. %D 2008 %I Nature Publishing Group %J Nature %K geophysics seismics seismology %N 7201 %P 204--208 %R 10.1038/nature07111 %T Preseismic velocity changes observed from active source monitoring at the Parkfield SAFOD drill site %U http://dx.doi.org/10.1038/nature07111 %V 454 %X Measuring stress changes within seismically active fault zones has been a long-sought goal of seismology. One approach is to exploit the stress dependence of seismic wave velocity, and we have investigated this in an active source cross-well experiment at the San Andreas Fault Observatory at Depth (SAFOD) drill site. Here we show that stress changes are indeed measurable using this technique. Over a two-month period, we observed an excellent anti-correlation between changes in the time required for a shear wave to travel through the rock along a fixed pathway (a few microseconds) and variations in barometric pressure. We also observed two large excursions in the travel-time data that are coincident with two earthquakes that are among those predicted to produce the largest coseismic stress changes at SAFOD. The two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture stress induced changes in crack properties, as observed in early laboratory studies.

@article{niu_etal:2008, abstract = {Measuring stress changes within seismically active fault zones has been a long-sought goal of seismology. One approach is to exploit the stress dependence of seismic wave velocity, and we have investigated this in an active source cross-well experiment at the San Andreas Fault Observatory at Depth (SAFOD) drill site. Here we show that stress changes are indeed measurable using this technique. Over a two-month period, we observed an excellent anti-correlation between changes in the time required for a shear wave to travel through the rock along a fixed pathway (a few microseconds) and variations in barometric pressure. We also observed two large excursions in the travel-time data that are coincident with two earthquakes that are among those predicted to produce the largest coseismic stress changes at SAFOD. The two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture stress induced changes in crack properties, as observed in early laboratory studies.}, added-at = {2012-09-01T13:08:21.000+0200}, author = {Niu, Fenglin and Silver, Paul G. and Daley, Thomas M. and Cheng, Xin and Majer, Ernest L.}, biburl = {https://www.bibsonomy.org/bibtex/254dff1a783388a1b423b33fe7053acdb/nilsma}, day = 10, doi = {10.1038/nature07111}, interhash = {09c2b6d26b83215b78baf247321f0953}, intrahash = {54dff1a783388a1b423b33fe7053acdb}, issn = {0028-0836}, journal = {Nature}, keywords = {geophysics seismics seismology}, month = jul, number = 7201, pages = {204--208}, publisher = {Nature Publishing Group}, timestamp = {2021-02-09T13:26:58.000+0100}, title = {Preseismic velocity changes observed from active source monitoring at the Parkfield SAFOD drill site}, url = {http://dx.doi.org/10.1038/nature07111}, volume = 454, year = 2008 }