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Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA
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- PublicationOpen AccessSeismicity induced by massive wastewater injection near Puerto Gaitán, Colombia(2020-08-06)
; ; ; ; ; ; ; ; ; Seven years after the beginning of a massive wastewater injection project in eastern Colombia, local earthquake activity increased significantly. The field operator and the Colombian Geological Survey immediately reinforced the monitoring of the area. Our analysis of the temporal evolution of the seismic and injection data together with our knowledge of the geological parameters of the region indicate that the surge of seismicity is being induced by the re-injection of produced water into the same three producing reservoirs. Earthquake activity began on known faults once disposal rates had reached a threshold of ∼2 × 106 m3 of water per month. The average reservoir pressure had remained constant at 7.6 MPa after several years of production, sustained by a large, active aquifer. Surface injection pressures in the seismically active areas remain below 8.3 MPa, a value large enough to activate some of the faults. Since faults are mapped throughout the region and many do not have seismicity on them, we conclude that the existence of known faults is not the only control on whether earthquakes are generated. Stress conditions of these faults are open to future studies. Earthquakes are primarily found in four clusters, located near faults mapped by the operator. The hypocentres reveal vertical planes with orientations consistent with focal mechanisms of these events. Stress inversion of the focal mechanisms gives a maximum compression in the direction ENE-WSW, which is in agreement with borehole breakout measurements. Since the focal mechanisms of the earthquakes are consistent with the tectonic stress regime, we can conclude that the seismicity is resulting from the activation of critically stressed faults. Slip was progressive and seismic activity reached a peak before declining to few events per month. The decline in seismicity suggests that most of the stress has been relieved on the main faults. The magnitude of a large majority of the recorded earthquakes was lower than 4, as the pore pressure disturbance did not reach the mapped large faults whose activation might have resulted in larger magnitude earthquakes. Our study shows that a good knowledge of the local fault network and conditions of stress is of paramount importance when planning a massive water disposal program. These earthquakes indicate that while faults provide an opportunity to dispose produced water at an economically attractive volume–pressure ratio, the possibility of induced seismicity must also be considered.234 12 - PublicationRestrictedCascadia Tremor Located Near Plate Interface Constrained by S Minus P Wave Times(2009-01-30)
; ; ; ; ; ; ; ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Creager, K. C.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA. ;Galluzzo, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Malone, S.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA. ;Vidale, J. E.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA. ;Sweet, R.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA. ;Wech, G.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA.; ; ; ; ; ; Nonvolcanic tremor is difficult to locate because it does not produce impulsive phases identifiable across a seismic network. An alternative approach to identifying specific phases is to measure the lag between the S and P waves. We cross-correlate vertical and horizontal seismograms to reveal signals common to both, but with the horizontal delayed with respect to the vertical. This lagged correlation represents the time interval between vertical compressional waves and horizontal shear waves. Measurements of this interval, combined with location techniques, resolve the depth of tremor sources within T2 kilometers. For recent Cascadia tremor, the sources locate near or on the subducting slab interface. Strong correlations and steady S-P time differences imply that tremor consists of radiation from repeating sources.300 31 - PublicationRestrictedTidal Modulation of Nonvolcanic Tremor(2007-11-22)
; ; ; ; ; ;Rubinstein, J. L.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Vidale, J. E.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA ;Creager, K. C.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA ;Wech, A. G.; Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA; ; ; ; Episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of Japan and Cascadia. In Cascadia, such episodes typically last a few weeks, and differ from “normal” earthquakes in their source location and momentduration scaling. The three most recent episodes in the Puget Sound/Southern Vancouver Island portion of the Cascadia subduction zone have been exceptionally well recorded. In each episode, we see clear pulsing of tremor activity with periods of 12.4 and 24-25 hours, the same as the principal lunar and lunisolar tides. This indicates that the small stresses associated with the solid-earth and ocean tides influence the genesis of tremor much more effectively than they do “normal” earthquakes. Because the lithostatic stresses are 105 times larger than those associated with the tides, we argue that tremor occurs on very weak faults.284 27