SCEC Award Number 16159 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Comparison of Source Inversions and Stress Drops with In-Situ Observations of Faulting
Investigator(s)
Name Organization
William Ellsworth Stanford University Margaret Boettcher University of New Hampshire
Other Participants Pamela Moyer (UNH PhD student)
SCEC Priorities 3c, 4a, 6b SCEC Groups FARM, Seismology, SIV
Report Due Date 03/15/2017 Date Report Submitted 07/07/2017
Project Abstract
 The primary goal of our project was to obtain surface and underground seismic data from a ML 5.5 earthquake to use as a test case for the Source Inversion Validation Project (SIV). The rupture zone of the 2014 ML 5.5 Orkney, South Africa earthquake is also the target of an International Continental Scientific Drilling Program (ICDP) drilling in a deep gold mine that began in June 2017. We obtained the data and performed a kinematic source inversion to obtain a spatially and temporally varying image of the mainshock using surface station data. Our results show a slow initiation that began about 1 second prior to the mainshock. We calculated stress drop for an initial set of aftershocks (0.9 ≤ Mw ≤ 2.4) in the high slip region of the fault plane. Using an empirical Green’s function spectral ratio method we determined stress drops in the range of 5-95 MPa. A two-layer velocity model for the region was obtained using both in-mine and surface station data with the Fortran code VELEST. Finally, in our recent publication [Moyer et al., 2017], we encourage other seismologists to use this dataset as an SIV test case.
Intellectual Merit The construction of the open data set for the 2014 ML 5.5 Orkney, South Africa earthquake together with the open invitation for its analysis moves the SCEC-led Source Inversion Validation (SIV) project from the realm of synthetic data into the world of real earthquakes. Our initial models of the earthquake, although intentionally simple, and associated measurements of the source parameters of its aftershocks, demonstrate the value of more thorough investigations through a SIV community-based effort. Through coordination with the International Continental Scientific Drilling Program drilling project now in progress, the opportunities for bringing seismological inferences and physical samples of faulting together are potentially revolutionary for our ability to interpret earthquake source processes from afar.
Broader Impacts A fundamental goal of SCEC is to understand the physical processes that occur within the Earth during an earthquake. Until now we have only been able to piece together a patchwork of data from earthquakes whose rupture surface we never see and surface outcrops of ancient exhumed faults we take to be analogs. We have put in place the infrastructure needed to bring remote and direct measures of faulting together in a collaborative and open research environment.
Exemplary Figure Figure 3. Stress drop of some aftershocks of the August 5, 2014 ML 5.5 Orkney earthquake. (a) Three-dimensional map showing the location of the surface stations (black triangles), in-mine station (colored triangles), hypocenter of the mainshock (red star), rupture plane of the mainshock (green zone), and aftershocks (gray dots). (b) Stress drop results from spectral analysis for 23 aftershocks of the ML 5.5 Orkney earthquake sized by magnitude (0.9 ≤ Mw ≤ 2.4). Stress drop values range from 4.5 MPa (blue) to 94 MPa (red). All stress drop results are from the high slip region of the fault plane. Example spectral ratio method analysis is given in panels (c)-(f), with velocity waveforms from an in-mine station show for a target event (c) and an EGF (d), the spectrum of the target event (blue) and EGF (red) along with their respective noise spectra (e), and the spectral ratio (green) of the target event and EGF at each component that passes our quality tests along with the best-fit Brune’s model (black) to all the spectral ratios and the identified corner frequency (black inverted triangle).