Paper in JGR on improved stress tensor inversion from focal mechanism

Another article related to stress tensor inversion has been published in JGR! This one discusses the improved methodology for stress tensor inversion.

Reference:

Martínez-Garzón, P., Y. Ben-Zion, N. Abofalthian, G. Kwiatek, and M. Bohnhoff (2016). A refined methodology for stress inversions of earthquake focal mechanisms. J. Geophys. Res., 121, 8666-8687, DOI: 10.1002/2016JB013493. [ Article Page ]

Abstract:

We develop an improved methodology for reliable high-resolution inversions of focal mechanisms to background stress field orientation and stress ratio R in two or three dimensions. The earthquake catalog is declustered to remove events likely affected strongly by local stress interactions rather than reflecting the large-scale background stress field. The declustered data are discretized with the k-means algorithm into groups containing a number of focal mechanisms between a minimum number Nmin and 2Nmin. Synthetic tests indicate that Nmin ≈ 30 provides stable inversion results under different stress regimes and noise conditions when R ≈ 0.5, while Nmin ≈ 45 is needed for R near 0 or 1. Additional synthetic tests compare the performance of selecting the fault plane of each focal mechanism using (a) the plane with lowest misfit angle between the slip vector from the focal mechanism and shear traction from the stress tensor and (b) the plane with highest instability coefficient representing proximity to the optimally oriented fault for given stress field and friction coefficient. The instability criterion is found to provide more accurate inversion results under all tested stress regimes, stress ratios, and noise conditions. The refined inversion methodology combines selecting fault planes using the instability criterion iteratively with a damped simultaneous inversion of different focal mechanism groups. Results characterizing neighborhoods of discretized domains merged during the damped inversion provide high-resolution information independent of the discretization. Some aspects of the methodology are illustrated with focal mechanism data from the San Jacinto Fault Zone in Southern California.

Paper in GRL on non-double-couple components at The Geysers geothermal field

Finally we published a paper related to investigation of seismic moment tensors of microseismic events at The Geysers geothermal field.

Reference

Martínez-Garzón, P., Kwiatek, G., Bohnhoff, M., and G. Dresen (2017). Volumetric components in the earthquake source related to fluid injection and stress state. Geophys. Res. Lett. 44, DOI: 10.1002/2016GL071963. [ Article Page ]

Abstract

We investigate source processes of fluid-induced seismicity from The Geysers geothermal
reservoir in California to determine their relation with hydraulic operations and improve the corresponding seismic hazard estimates. Analysis of 869 well-constrained full moment tensors (Mw 0.8–3.5) reveals significant non-double-couple components (>25%) for about 65% of the events. Volumetric deformation is governed by cumulative injection rates with larger non-double-couple components observed near the wells and during high injection periods. Source mechanisms are magnitude dependent and vary significantly between faulting regimes. Normal faulting events (Mw<2) reveal substantial volumetric components indicating dilatancy in contrast to strike-slip events that have a dominant double-couple source. Volumetric components indicating closure of cracks in the source region are mostly found for reverse faulting events with Mw>2.5. Our results imply that source processes and magnitudes of fluid-induced seismic events are strongly affected by the hydraulic operations, the reservoir stress state, and the faulting regime.

Paper on the influence of pore pressure changes on stress tensor inversion parameters

Another paper! This time we discuss the influence of temporal changes in pore pressure on the results of stress tensor inversion from focal mechanisms, with the special focus on stress shape ratio coefficient. There is also a neat presentation of the effects using the data from The Geysers geothermal field. The paper has been published in Geophysical Research Letters. Have fun!

Reference:

Martínez-Garzón, P., Vavryčuk, V., Kwiatek, G., and M. Bohnhoff. Sensitivity of stress inversion of focal mechanisms to pore pressure changes (2016). Geophys. Res. Lett., 43, 8441-8450, DOI: 10.1002/2016GL070145. [ Article Page ]

Paper on theoretical limits on detection and analysis of small earthquakes

Finally, the paper entitled “Theoretical limits on detection and analysis
of small earthquakes” has been published in Journal of Geophysical Research: Solid Earth. In this paper we investigate theoretical limits on detection and reliable estimates of source characteristics of small earthquakes using synthetic seismograms for shear/tensile dislocations on kinematic circular ruptures and observed seismic noise and properties of several acquisition systems (instrument response, sampling rate). The results presented in the manuscript provide guidelines on detection of small events in various geological environments, along with information relevant to reliable analyses of earthquake source properties.

Reference:

Kwiatek, G., and Y. Ben-Zion (2016), Theoretical limits on detection and analysis of small earthquakes, Journal of Geophysical Research-Solid Earth, 121, doi:10.1002/2016JB012908.