Identifying fault heterogeneity through mapping spatial anomalies in acoustic emission statistics

Abstract

[1] Seismicity clusters within fault zones can be connected to the structure, geometric complexity and size of asperities which perturb and intensify the stress field in their periphery. To gain further insight into fault mechanical processes, we study stick-slip sequences in an analog, laboratory setting. Analysis of small scale fracture processes expressed by acoustic emissions (AEs) provide the possibility to investigate how microseismicity is linked to fault heterogeneities and the occurrence of dynamic slip events. The present work connects X-ray computer tomography (CT) scans of faulted rock samples with spatial maps of b values (slope of the frequency-magnitude distribution), seismic moments and event densities. Our current experimental setup facilitates the creation of a series of stick-slips on one fault plane thus allowing us to document how individual stick-slips can change the characteristics of AE event populations in connection to the evolution of the fault structure. We found that geometric asperities identified in CT scan images were connected to regions of low b values, increased event densities and moment release over multiple stick-slip cycles. Our experiments underline several parallels between laboratory findings and studies of crustal seismicity, for example, that asperity regions in lab and field are connected to spatial b value anomalies. These regions appear to play an important role in controlling the nucleation spots of dynamic slip events and crustal earthquakes.

Preslip and cascade processes initiating laboratory stick slip

“…We find a linear dependence between A 0 and M 0 (i.e., a slope of 1 on the log-log plot) such that M 0 = αA 0 C , where C = 1 and α = 0.22 Nm/V. Some AE studies have assumed C = 3/2, which is consistent with earthquakes if it is assumed that magnitude M = log 10 (A 0 ) [e.g., Goebel et al, 2012]. Our results indicate that the relationship between f 0 and the frequency of peak sensor sensitivity can affect the relationship between M and A 0 .…”

“…In previous studies, the recording system saturated for at least 5 ms during each DSE [Thompson et al, 2009;Goebel et al, 2012]. Our split-gain recording system did not saturate, and this allows us to study the amplitude and spectra of DSEs (section 3.3) and to search for immediate aftershocks.…”

“…• Readme • Figure S1 • Figure S2 • Figure S3 • Figure S4 • Figure S5 • Figure S6 • Figure S7 • Figure S8 • Figure S9 Correspondence to: G. C. McLaskey, gmclaskey@usgs.gov obey both Gutenberg-Richter frequency-magnitude statistics and Omori-type aftershock decay [Goebel et al, 2012;Lei, 2012;Weeks et al, 1978]. It has been assumed that both AEs and DSEs are in some sense small-scale versions of earthquakes and can provide insights into earthquake mechanics [e.g., Lei et al, 2003;Thompson et al, 2009;Johnson et al, 2013;Goebel et al, 2014aGoebel et al, , 2014b.…”

Triggering Processes in Rock Fracture

“…A natural high density inclusion (max size of 1 × 0.9 cm) was located near the notch [33], as revealed by x-ray computed tomography imaging. Largescale imperfections in the Wgrn07 sample were introduced by two 15 mm deep saw-cut notches inclined at 30°to the loading axis [34,35]). …”

“…The high compressive loading stresses and specific rock properties favored the formation of compaction bands in Bleuerswiller sandstone (Vo3, porosity 24%, 80 MPa confining pressure, wet, 10 MPa pore pressure [32]) and Bentheim sandstone (Be7a, porosity 22%, 160 MPa confining pressure, dry [33]) and the formation of localized fracture zones in two Westerly granite samples (Wgrn07, Wgp01, porosity ≈ 0.15% − 0.7%, 75 MPa confining pressure, dry [34,35]). No large-scale imperfections were identified in samples Vo3 and Wgp01.…”