The first week of the internship is almost over, and I have already accumulated a dense pile of research papers thicker than my copy of Atlas Shrugged -- thank you Danielle for the light reading! Several of the papers served the purpose of providing a historical background on similar case studies of potentially induced earthquakes near wastewater injection sties, including a 200-some-page white paper on the topic released by the National Academy of Sciences; other papers served the purpose of expanding on the methods of measuring shear wave splitting (SWS) and structural anisotropy. Finally, I am working my way through the manual on how to implement the code -- called MFAST -- that I'll be using this summer to analyze shear wave splitting measurements. MFAST stands for Multiple Frequency Automatic Splitting Technique, which is essentially a hodge-podge of code using different shell scripts, Seismic Analysis Code (SAC), our dear old friend Generic Mapping Toolkit (GMT), SPLIT cluster analysis code, TauP toolkit, Fortran, and various UNIX tools, all of which come together and make processing these measurements really fast and mostly free of user bias. Sounds good to me!
Below is an example of some recorded wiggles from a practice data set that I have been playing around with using SAC.
The first and second rows show recorded horizontal ground motions in the E-W and N-S directions, respectively. The third row shows recorded vertical ground motions. As you may have already guessed, the P-wave arrival (marked by the IPUO line) appears first in the vertical component and the S-wave arrivals (marked by the T0 line) appear later and more clearly in the horizontal components. A careful eye might notice that the arrival times of the E-W and N-S S-waves do not exactly match up. In fact, the E-W shear wave arrives about a quarter of a second earlier than the N-S shear wave. Identifying and measuring this time lag is a significant component of my research this summer, for this time lag tells me something about the degree of anisotropy in the Earth. Additionally, by noting which horizontal component of the S-waves arrives first (in this example the E-W component), we can infer that the E-W shear wave must have been the fast wave, and therefore the fast (or preferred) direction of the anisotropic medium must also be E-W. Wherever the crust is fractured and faulted, the orientations of the cracks determine the fast direction. For a given preferred orientation, if the time lag between the two horizontal component shear waves changes over time, one can infer that the stresses acting along the faults must be changing; the cracks will dilate and contract as stresses increase and decrease, and therefore cause the speeds of the fast and slow waves to change. Fluid injection has the potential to induce such changes in stress along faults, and if enough fluid pressure is exerted, an earthquake can occur.
This image from Wikipedia demonstrates the basic idea of shear wave splitting.
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