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Initial download of the metrics may take a while. Table of Contents. Previous article Next article. Article Abstract PDF Metrics Show article metrics. Services Same authors - Google Scholar. Bookmarking Mendeley. Fluid composition and degree of alteration of basalts had a negligible role in controlling the maximum fluid pressure that the experimental fault could sustain before failure i. In fact, more altered basalts i. However, this tiny discrepancy may be due to local heterogeneities in mineral composition and microstructure of basaltic samples.

The evidence of newly formed dolomite grains in the slipping zones of all the basalts i. We thank L. Tauro, E. Masiero, L. Peruzzo F. Zorzi, D. Pasqual, L.

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Kuo, F. Prando, and D. Cinti for technical support and analysis and P. McGrail for providing Columbia River basalt samples. We thank the Editor, N. Brantut, A.

Pluymakers, and an anonymous reviewer for their constructive comments that improved remarkably our study. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries other than missing content should be directed to the corresponding author for the article.

Volume 45 , Issue If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account. If the address matches an existing account you will receive an email with instructions to retrieve your username. Open access. Geophysical Research Letters Volume 45, Issue Research Letter Open Access. Piercarlo Giacomel Corresponding Author E-mail address: piercarlo.

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Please review our Terms and Conditions of Use and check box below to share full-text version of article. Abstract The safe application of geological carbon storage depends also on the seismic hazard associated with fluid injection. Figure 1 Open in figure viewer PowerPoint.

Frictional Instabilities

For each contact load, z plane scans were taken bottom up in 2 m steps for a total scan distance of m. The sc an starting point was determined from an initial zero -load position, with the upper vertical limit of the scan specified 25 m above the countersurface and the lower limit m below. Every scan was performed using the same initial datum Figure 3 3 fo r consistency in the following analysis Figure 3 -3 Illustration of scanning confocal microscopy of indentation contact.

For each indentation load, a scans were conducted along a series of delta z steps to provide a 3 dimensional red-green composite image. Using a standardized z reference frame allows for direct comparison of images from subsequent indentation experiments. Analysis of Contact As an initial survey of the indentation results, the confocal image stacks from three indents, 50 uN, uN, and uN, were rotated about the x -axis to provide z stacked planes.

An average intensity projection of the three indents was taken through the central um of contact Figure 3 -4 to eliminate the curvature effects of the contact while analyzing the contact regions PAGE 37 37 Figure 3 -4 Lateral average intensity projections of probe and countersurface. It can be clearly observed that there is no increase in intensity magnitude indicating that gel compression is not occurring and these hydrogel contacts behave as purely elastic materials.

All images were taken using the same microscope parameters and were not altered or thresholded beyond projecting the average values. From this projection, it can be clearly seen that the average intensity under the probe does not increase, and thus there is no compression or co mpaction of the structure occurring This corresponds to similar results produced by Schulze et al when using a glass probe Schulze et al.

Friction - Wikipedia

A Z -stack of images are combined in ImageJ. B The green channel representing the probe is subtracted from the red surface channel. C The resulting image stack is fit with a Gaussian to determine the center of contact and integrated around this axis. D the integrated image shows the half contact as revolved around the central probe axis. Because of overlap in excitation frequency bands for the green and red microspheres on the confocal microscope, a mild cross -fluorescence was observed in the TRITC channel.

Initial azimuthal averaging attempts were unable to threshold this cross-fluorescenc e out of the images resulting in an unsolvable fit attempt. To perform the azimuthal averaging of the contacts, ImageJ was used to subtract the probe image in green from the red, eliminating the cross fluorescence Figure 3 -5 a,b The resulting image s tack was then brought into MATLAB and each z -slice was thresholded to binary, such that the region excluded by probe subtraction would have an intensity value of zero and all surrounding values would be 1.

A gaussian fit between the slices was then perfor m ed to find the central x and y coordinates of contact Figure 3 -5 c. An azimuthal average was taken around the z axis through the contact centroid to generate PAGE 39 39 an image of the contact half -width Figure 3 -5d The results of azimuthal averaging can be se en in Figure 3 -6 After averaging, an attempt was made at fitting an arc with the probe radius to the edges of contact; however, due to extreme elastic flattening, no solutions were found to allow for calculating contact width using a deviation from in denter curvature.

Figure 3 -6 Results of azimuthal averaging of confocal z -stacks for all studied loads. The extents of elastic flattening in the contacts are evident, particularly at loads greater than N. To determine the contact widths, an alter native method was used.

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An approximately m wide section was targeted from around the center of contact for both channels and duplicated to a second image. This image was then rotated about its horizontal axis such that the z projected planes are now in the y direction. The maximum intensity values of each pixel were then projected to give a single plane at the PAGE 40 40 center of contact Figure 3 7 a,b.

Using ImageJ to overlay the two images, the overlapping region between the two slices could be identified Figure 3 7 c. Figure 3 -7 Projected image stacks of a probe, b counter surface, and c result of channel overlap calculations. The average gray value of each column was output for each load and used to produce intensity vs horizontal location plots Figure 3 -8 a. These values were smoothed using a 10 point floating average to mitigate the effects of artifacts and noise. From thes e plots a standard threshold value could be approximated for the limits of contact Figure 3 8 b.

The gray shaded area represents the specified threshold for determining the edges of contact. The corresponding contact widths and fit are reported in Figure 3 -9 Interestingly, even though the contact width approaches the radius of curvature of the probe, these contacts still a ppear to be purely elastic and obey classic Hertzian mechanics for nonadhesive contacts. The blue dashed line is a fit of contact width using non-adhesive Herztian contact mechanics.