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1 ARMA Insights into dynamic asperity failure in the laboratory Selvadurai, P.A. University of California, Berkeley, CA, USA Glaser, S.D. University of California, Berkeley, CA, USA Copyright 2014 ARMA, American Rock Mechanics Association This paper was prepared for presentation at the 48 th US Rock Mechanics / Geomechanics Symposium held in Minneapolis, MN, USA, 1-4 June This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented. ABSTRACT: Our current laboratory investigations quantified the local stress states on a laboratory fault which control the transition of sliding from stable (quasi-static) to unstable (dynamic), commonly referred to as rupture nucleation. A fault was experimentally modeled using two Poly(methyl methacrylate) samples in a direct shear configuration. A pressure-sensitive film was employed to localize and map the contact junctions (asperities) throughout the interface A portion of the fault, characterized by a high density distribution of larger asperities, experienced considerably less slow (aseismic) slip than neighboring regions. Foreshocks detected acoustically were observed to coalesce in this region moments before the interface rapidly slipped. A high definition video camera was focused on this region and light passing through asperities appeared brighter than the non-contacting regions. Foreshocks in this slip deprived region caused, sudden changes in light intensity passing through these asperities and were concomitant with recorded acoustic emission. Source radius of failing asperities measured from the camera (0.56 mm) were similar to that estimated from the Brune corner frequency model (0.81 mm). Dynamic triggering of smaller strong asperities directly adjacent to larger asperities (measured by the camera) likely increased the high-frequency content measured in smaller foreshocks. Keywords: Acoustic emissions, earthquakes, scaling, frictional sliding, asperity failure. 1. INTRODUCITON Frictional faults have been studied in the laboratory with the goal of understanding the transition from immobility to sliding by many researchers [e.g. 1-3]. Mechanisms surrounding this transition to rapid sliding is not well understood and insight could potentially lead to better estimates of seismic hazard and earthquake prediction. The laboratory allows for the careful isolation of the complicated discrete asperity-level dislocation mechanisms believed to occur during rupture nucleation. In the past, laboratory experiments have been primarily used to develop empirical laws which define frictional stability [e.g. 4, 5], the most well-known being the rate and state friction laws. Parameters in these models are empirically derived and characterize a multitude of physical phenomena simultaneously. While this theory has been used quite effectively to understand the earthquake cycles [e.g. 6, 7], there are limitations from our lack of understanding the actual intrinsic mechanisms controlling friction. 2. EXPERIMETNAL FACILITIES 2.1. General In the experiments presented here, dry-friction conditions along the fault were carefully controlled during each experiment. The tests consisted of loading two samples of PMMA in a direct shear configuration shown in Figure 1a. The experimental facilities presented in this paper have been previously documented by Selvadurai and Glaser [8]. The surfaces of two samples of Poly(Methyl methacrylate) or PMMA are machined flat then sandblasted to create roughness profiles [9] more similar to those found in nature [10]. We employed a pressure sensitive film to detect and measure the contacting asperities formed between the interacting surfaces [11]. The film is polyethylene based and composed of microcapsules (10 µm resolution) that when compressed change color proportional to the applied pressure (±1.5 Pa). After loading in the fault the film was digitized using an image scanner, and algorithms in MATLAB were used to detect, size and

2 allowed us to detect from a near-field vantage point surface normal displacements over the frequency band ~ Fig. 1. a) General configuration of direct shear apparatus from the side view with general locations for the slip (NC1-NC7) and AE sensors (PZ1-PZ16). b) Detailed locations of the slip (red crosses) and acoustic (black triangles) sensor array with respect to the interface (white). catalogue all contacting asperities in the static state under normal load and not subjected to shear. During a test, shear/frictional stress along the fault was increased under constant normal stress until dynamic instability occurred, known as a stick-slip event (SSE). Prior to accelerated rupture, slow slip accumulated nonuniformly along the fault an observation occurring frequently in nature [e.g ]. Slow slip measurements were taken using slip sensors (NC1-NC7) placed near to the fault that measured slip in the direction of applied shear load (see Figure 1a for general location and Figure 1b for accurate locations). A region characterized by a lack of slow slip in relation to the neighboring sections of the fault was described as slipped deprived and may be similar to locked sections on faults in nature. Just prior to rapid slip, small highfrequency acoustic emission (AE) events were detected and located using an array of 16 piezoelectric acoustic emission sensors (PZ1-PZ16) placed along the underside of the base plate (locations are shown in Figure 1b). The AE sensors are absolutely calibrated [15, 16] which 8 khz 2.5 MHz, with an approximately 1 picometer noise floor [15]. These observed events are treated as foreshocks (FS) and they display pulse-like P and S wave components. The pulse-like characteristics also allow us to locate them along the interface. Sequences of foreshocks were consistently detected prior to rapid slip (seconds) and radiated from within the slip deprived section of the interface before each stick-slip event. Foreshock sequences varied in number, respective size, and spatio-temporal distribution. These characteristics and their relation to the external boundary conditions are not detailed in this discussion but have been formally discussed in a manuscript currently in preparation [17]. The goal of this discussion was to focus attention on the slip deprived region, specifically on the asperities which are visually observable moments before rapid slip due to the transparent properties of the PMMA samples Photography of interfacial asperities The unique setup and transparent properties of PMMA allowed us to directly observe the interface during the experiments. Figure 2a shows a schematic of the PMMA slider block pressed against the base plate where a Canon VIXIA HF M301 CMOS video camera was focused at a 35 o angle from the faulting plane. Figure 2b illustrates the theory which follows normal load experiments performed on PMMA-PMMA interfaces in the past [18]; (1) two nominally flat interfaces only touch on regions called asperities; (2) the light was transmitted more effectively through these contacting asperities; and (3) light was diffracted on open region in which no contact occurs. According to this methodology light passing through the asperities will appear brighter than the light diffracted through the interface. In Figure 2c, the raw image from one frame focused on the slip deprived region is shown. The CMOS video camera used in these experiments operated at a video frame rate of 30p; displaying 30 entire frames per second (Δt frame ~ 33.3 ms). The square CMOS array was 6.35 mm and images are obtained at a focal length of 4.1 mm. The field of view was approximately 41 mm and images are 640 x 1117 pixels per frame, making the resolution ~ 0.03 mm/pixel (~30 µm/pixel). No image correction was performed for the optical distortion as light passed through the slider sample and into the camera lens. Instead, a ruler was placed alongside of the interface in the x-direction and parallels in the y- directions were maintained by photo editing software. This corrects for the presence of orthoscopic distortions [19] and enables more accurate locations of the foreshocks with respect to the images from the camera. Future improvements of the imaging of failing asperities will investigate the variations in radiation patterns and rise time with respect to the changes in light intensity

3 along asperities at ~333 times the temporal resolution. This will be performed using a high speed (HS) camera operating at 10,000 frames per second (Δt frame = 0.25 ms). For this, the HS camera will trigger simultaneously with the AE data acquisition system in order to have well defined time stamps on each image. 3. EXPERIMENT Figure 3 shows a typical result for the direct shear experiment and the sensor measurements taken throughout each test. Phase I of the experiment consisted of loading the slider block against the base plate under a constant normal load Fn = 4400 N for thold = 900 s. Pressure sensitive film was placed within the fault beforehand and compressed to estimate the asperity contact points. The fault was unloaded and the film removed and the fault re-initialized to a datum location with respect to the base plate. The non-contact sensor array and fine tuning screws were used to index the slider block at the datum location. The fault was again compressed under the same normal load Fn = 4400 N for thold = 900 s. During Phase II, the normal load was maintained as the rigid loading platen was driven at a constant velocity of v p = mm/s using an electro-mechanical shear actuator. Steady motion of the rigid loading platen was used to simulate the motion of tectonic plates in the far-field causing an accumulation of shear stress along the interface. In Figure 3a we saw that the normal force Fn (blue) remains constant throughout Phase II while an observable increase in shear force Fs (red) occurs due to the motion of the rigid loading plated. As time continues the bulk shear force Fs increases until a stick-slip event (SSE). Figure 3b shows slip displacements from one sensor during three SSEs (SSE1, SSE2 and SSE3) occurring during Phase II. During a SSE the shear force drops accompanied by rapid displacements measured using the slip sensors. This was the result of shear rupture propagating throughout the interface [e.g. 2, 3]. Figure 3c shows in detail the period leading up to a SSE; here the slow slip which accumulated prior to instability from sensor NC4. Slow slip was defined to be when local slip velocities are lower than v local > 4 µm/s (star symbol in Figure 3c), which was evaluated by differentiating the raw slip sensor data a posteriori. Dynamic rupture or rapid slip was always observed when local sliding velocities breached this threshold (v local > 4 µm/s). Superimposed on Figure 3b are the acoustic emission (magenta) measurement that occurred prior to SSE1. Figure 3d shows the high resolution AE signals (PZ1-PZ14) just prior to the SSE3; a total of 24 FS were detected prior to rapid sliding. Figure 3e shows a detailed view of FS7 emphasizing the pulse-like P and S wave arrivals intrinsic to these all FS signals. Fig. 2 a) Schematic demonstrating how optical images of asperities are obtained using the CMOS video camera through the transparent side of the slider block and onto the interface. b) Light was transmitted through the asperities and diffracted in non-contacting regions. c) Raw, unprocessed images.

4 Fig. 3.a) Normal (blue) and shear (red) loads applied to the slider block during Phase II of the experiment. The rigid loading platen moved at v p = mm/s until a stick-slip event (SSE). During a SSE the shear force dropped while slip increased rapidly as seen in b). c) Prior to a SSE we see an accumulation of slow slip and acoustic emission signals (AE) measured just prior to rapid slip. d) The AE signals from piezoelectric sensors (PZ1-PZ13) are shown ms before SSE3. A total of 24 foreshocks were detected and located using the P wave components such as that seen in e). 4. RESULTS 4.1. Slow slip distribution and foreshock location It was found that the real distribution of asperities controls fault behavior. Figure 4a shows estimates asperity location measured using the pressure sensitive film before the direct shear experiment shown in Figure 3. While in reality asperities seen on the pressure sensitive film exhibit convoluted shapes [11], they are represented here as a circular patch with an equivalent area. Figure 4b examines the accumulation of slow slip

5 Fig. 4.a) Asperity locations are shown as circular patches on the interface with areas equivalent to that measured using the pressure sensitive film. b) Slow slip measured along the fault using the slip sensors (NC1-NC7) over the last 10 seconds before rapid slip. c) Locations of foreshocks for SSE2 (red triangles) and SSE3 (blue squares). Size of the symbols are proportional to the size of the event. over all seven slip sensors prior to SSE1. Measurements on all seven slip sensors (NC1-NC7) were interpolated at 50 ms time intervals. The time associated with the onset of rapid slip according to our test, i.e. t fail = 0 s, was the time that any sensors breached the slip velocity threshold of v local > 4 µm/s, as mentioned previously. The grid composed of slip values on all seven sensors (NC1-NC7) were then interpolated in time to provide visual insight into the slow-slip distributions. In all SSEs, slip accelerated and breached the slip velocity threshold firstly at the trailing edge (TE), then moving towards the leading edge (LE). Figure 4b shows the distribution of slow slip along the strike of the fault (x-direction) ten seconds before rapid sliding. Results are similar to those found by Selvadurai and Glaser [17] in that sliding was non-uniformly distributed over the interface and a depreciate amount of slip, relative to the other slip sensors, occurred near sensor NC6. This was likely caused by larger and denser distributions of asperities, graphically represented by the hatched region in Figure 4a. Foreshocks occurred only in this region, as seen in Figure 4c. Figure 4c shows the foreshock catalogue for SSE2 (red triangles) and SSE3 (blue squares) from Figure 3 and the field of view (FOV) of the camera is shown here in gray. The size of the symbols are proportional to the peak ground displacements (PGD) averaged over the closest five AE sensors used to locate the foreshock Optical results The timing of the camera and the AE data acquisition was not synchronized in this experiment. We relied solely on the frame to frame images and the changes in light intensity between them. During the slow slip portion of the experiment changes in the FOV occur slowly enough that images remain focused. During the transition and acceleration of shear rupture the image suddenly became blurred. The first frame which was blurred was called the failure frame F tfail. The frames prior to these are referred to as F tfail - 1, F tfail - 2, F tfail - 3, etc. that occurred at approximate times of t fail ms, t fail ms and t fail ms, respectively. In this section we examined the changes in light intensity in frames leading up to failure (t fail ) during which foreshock sequences were observed (see Figure 3d). We consider only two foreshock sequences: SSE2 and SSE3 (Figure 4c). The AE data showed that foreshock sequences for SSE2 and SSE3 occurred over a time span of s

6 and s, respectively. Dividing by Δt frame gives 4.8 Fig. 5. Images from video camera at frame Ftfail for SSE2 (top) and SSE3 (bottom). Non-uniform grid was constructed using ruler parallels and foreshocks were superimposed on image. Shear loading was from left to right. and 6.1 so there was a minimum of 4 and 6 frames within the time period in which foreshocks occur for sequences associated with SSE2 and SSE3, respectively. Figure 5 shows the failure frame, F tfail, for SSE2 on the top position and SSE3 on the bottom. Images have been converted from truecolor image RGB to a grayscale intensity I image (I = *R *G *B) and inverted to enhance the contrast of the asperities. The ruler placed at the bottom of the image appeared blue (darker) and there was a small gap between the ruler and the slider sample where the base plate shows (hotter). The interface has a non-uniform grid overlaid which runs parallel to the graduation lines on the ruler. All other regions are omitted in our analysis and this discussion. In these images the direction of shear loading was from left to right and the location of the interface on the x-axis, referring to the coordinate system in Figure 4, was displayed above the grid. Superimposed along the interface are the respective foreshocks sequences for SSE2 (above) and SSE3 (below) present in the FOV. They are represented as black triangles and their size was proportional to the size of the event, i.e. PGD ranging from to nm. The portion of foreshock catalogue which fell into Fig. 6.a) AE signals from PZ5 for five foreshocks in SSE2 sequence which were located in the field of view of the camera. b) Left hand side shows a schematic of the frame separation of the camera along time in the y-direction. Foreshocks shown in a) are placed on the time axis on the right hand side. Changes in light intensity due to foreshocks between frames allowed us to initialize the timing of the camera. the FOV of the camera are shown for both sequences. We saw foreshocks align with regions of higher light intensity assumed to represent asperities for both foreshock sequences. From this point forward we will only discuss the foreshock sequence SSE2 and the changes observed using the video camera; the sequence SSE3, which consisted of twice the amount of foreshocks, will be discussed in future work. We focus on the capabilities (and limitations) of this novel technique. In Figure 6 we look at the signals and attempt to reconcile the timing of foreshocks with respect to the images from the video

7 Fig. 7. Final 5 frames prior to SSE2 detailing the images taken in the location of each foreshock shown in Figure 6a. Changes in light intensity occurring between frames was highlighted by black boxes. Frame F tfail +1 was not shown since it was blurred. White boxes give an estimate of the regions in which light intensity changed. camera. Figure 6a shows the AE signals from sensor PZ5 for five foreshocks (FS2, FS6, FS10 and FS12) within the cameras field of view during SSE2. These foreshocks have also been marked spatially in Figure 5 (top). We observe similar signals for each foreshock but source dynamics are omitted from this discussion. Foreshock signals have been aligned about the peak P- wave arrival and in ascending order of occurrence along the increasing y-axis. We assumed that these signals are created from the sudden failure of asperities along the interface [20] - an assumption commonly made in seismological studies [21] where earthquake are modeled using the exterior fracture around a strong circular/elliptical asperity [22-24]. If this was the case we would expect to see changes in the light intensity passing through the asperity after the foreshock occurred. In Figure 6b the right hand side shows the timing of foreshock determined using the acoustic data and the time between each foreshock was shown (Δt). To the left the images taken from the camera from frame F tfail to F tfail -4 are shown and spacing between each frame was Δt frame (~33 ms). Now we must examine the specific regions on the interface where the foreshocks were located which was shown in Figure 7. For example, foreshock FS6 and FS9 occur between frame F tfail to F tfail -1 (according to Figure 6b) and if we look at these frames we note a definitive change in light intensity near to the location of the event. Having located an event with respect to the film we can move backwards and forwards in time while shifting the camera timing a maximum of Δt frame Δ(FS6 to FS9), i.e. 33-(9) = 24 ms. More foreshocks spaced consistently over a longer time interval would more accurately constrain the timing of the camera. Examining the changes in light intensity along the interface at specific foreshock locations gives a possible upper estimate on the true area of the region causing this dynamic signal (Figure 6a). Initial results show that the change in high intensity pixels, i.e. pixel value greater than 0.65, from frame to frame (black boxed images in Figure 7) show a correlation to the size of the dynamic signal observed on sensor PZ5 (Figure 6a). The area was estimated and shown using the white boxes in Figure DISCUSSION As commonly observed in the field, our fault undergoes non-uniform slow slip prior to the larger stick-slip event (Figure 4b). We suspect that this was due to rheological differences along the interface due to the differences in asperity distribution as measured using the pressure sensitive film (Figure 4a). The region which experienced the least amount of slow slip prior to rapid

8 sliding was referred to as the slip deprived region and was approximated as the hatched region in Figure 4a. Within this region, small high-frequency acoustic emission which posed both P and S-wave characteristics were emitted from this region. A CMOS digital video camera was focused about this region throughout the experiments recording at 30 frames per second. Our hypothesis was that foreshocks present within the slip deprived region as measured by the AE signals may also be observed (albeit a less of a temporal resolution) by examining changes in light intensity from these locations from frame to frame. In this discussion we analyzed the video evidence of asperity failure and its relation to the elastodynamic signals emitted during a foreshock. The benefits of being able to visualize changes in asperities may prove beneficial in understanding the nucleation processes along the fault [25, 26]. Our current analysis provided no estimates of the source dynamics but initial estimates of source-time functions from the foreshocks suggest mechanisms similar to those at scale in nature (i.e. double-couple [27]). Static and dynamic stress changes on faults in nature are not well understood; a large portion of the seismological effort has been studying how stress changes along the interface vary due to small earthquakes, non-volcanic tremor and foreshocks prior to larger more energetic events. Having more accurate asperity failure models, i.e. understanding how asperities fail in relation to each other and in spatial and temporal relations to a locked section of the fault may help better forecast and minimize seismic hazard in the future. Developing analogous experiments in the laboratory to conditions observed along natural fault was the primary focus of our efforts Rise time and source dimensions Our current interpretation and understanding of signals produced in nature and studied in seismology is quite limited. Many of the conclusions surrounding faults and their rheology which control the dynamics (or transition to) is limited to the study of seismic signals, geodetic motions, geologic outcrops and laboratory experiments [e.g. 28]. In many studies which attempt to reconcile slow fault motion and location where shear stress accumulates (i.e. locked regions) rely on the strong asperity model [29, 30]. This model uses a simple geometry in the form of the exterior crack problem to interpret observed seismic data and been used quite extensively to understand the creeping motion of fault over the last fifteen years [14, 21, 31]. However, our understanding of the physics surrounding the true nature of these signals are usually limited (in both laboratory and field studies) due to our general lack of knowledge of the composition of fault structures, i.e. characterization of thickness of the seismogenic zone, the material properties of fault gouge, and the properties of contact surfaces within the slipping zone. Our Fig. 8. Variations P-wave rise times measured by sensor PZ5 for the foreshocks shown in Figure 6a. The signals have been centered about the peak ground displacement of the incoming P-wave. facilities presented here are attempting to further understand the latter. Using both pressure sensitive film and the video camera, we gained intimate knowledge of the local contact conditions both in the static state (under simple normal loading) and dynamic (subjected to normal and shearing forces). The transparent nature of PMMA gave us visual insight into the changing state of asperities which seemed to generate the high-frequency foreshocks an observation that, to the knowledge of the authors, has never been investigated in the laboratory and field studies (primarily due to the non-transparent nature of rock). Moving to expand on our current understanding of strong asperity failure we first examine the Brune [22] relationship which allowed us to estimate source dimensions from the foreshocks. It uses the corner frequency of the radiated seismic energy to calculate the source radius R 0 of the given event. This relation was given as R 0 = 2.35β/(2πf 0 ), where β was the shear velocity of PMMA (1390 m/s) and f 0 was the corner frequency given in khz. We approximate the corner frequency to be the inverse of the rise time, i.e. ~1/t 0 [32]. Rise times discussed here are from the foreshocks in Figure 6a. Figure 8 shows a detailed view of the P-wave arrival for the five foreshocks. We observed rise times of t 0 = 0.45, 0.58, 0.80, 1.23 and 1.55 µs for foreshocks FS2, FS9, FS10, FS6 and FS12, respectively. Using the Brune model this gave source dimensions between R 0 = 0.81 to 0.23 mm. These estimates of source dimension are corroborated by the images in Figure 7. FS12 has a theoretical source radius of approximately 0.81 mm (Brune estimate) and experimental images suggest a length scale (i.e. half the base length of the white box in Figure 7) of approximately mm (560 µm). While we note that discrepancy may arise from the loosely constrained time stamps of the frame of interest; individual image may

9 Fig. 9. a) Foreshock signals FS6 (left) and FS9 (right) from SSE2. b) Spectrograms were calculated over time between a range of frequencies 0 to 1 MHz. Portions of the signal (red) and spectrogram (white) are highlighted following the maximum P-wave arrival. have been taken mid-rupture causing an asperity to appear smaller than expected. Understanding the earthquake rupture process was central to our understanding of fault systems and earthquake hazards. The manner in which earthquakes are sized predominantly rely on the assumption that rupture has ceased before estimation of size can be made. Many studies have attempted to understand the relation of rise time to the eventual size of the earthquake [33, 34] and whether this is deterministic of the eventual magnitude is unclear and thus far no unifying theory has emerged. If it is deterministic then the eventual size of the earthquake would be measurable before rupture has ceased. Preliminary results indicate that asperity failure which created the foreshock signals, are indeed deterministic the eventual size of the earthquake was proportional to the rise time. In Figure 8, the detailed views of the P-wave arrivals, for sensor PZ5, for each foreshock in Figure 6a. The foreshocks have been amplified for clarity and the signals have been centered about the peak ground displacement caused by the P-wave. We observed that the rise time seems to take longer for the larger event (FS12) and becomes increasingly smaller as the overall size of the event decreased Frequency content of foreshock signals Figure 9a shows two foreshocks FS6 (left) and FS9 (right) from SSE2. Spectrograms (Figure 9b) were calculated using normalized values of the individual foreshock. These foreshocks were normalized about the maximum amplitude of the s-wave arrival. Spectrograms allowed us to view the frequency content of the signal throughout time. Frequencies between 0 and 1 MHz were calculated using power spectral density (PSD) estimates over 10 µs time windows with 50% overlap of the moving windows. The power of the signal was displayed between 0 to -100 db and the frequency response of the sensors are flat between ~0.08 to 2.5 MHz. The signal directly after the incoming P- wave was highlighted by red circles in the time domain of the FS signals (top). It seemed that variations in the time domain signal (in frequency ranges that are flat according to the sensor calibration) caused an increase in high-frequency content between ~500 to 800 khz for FS9 (right) and not in FS6 (left). Looking at the dynamic asperity images from Figure 7 we saw that they have dramatically different distributions of high intensity pixels just prior to failure. FS6 has a more densely distributed set of high intensity pixels while FS9 has multiple smaller asperities (see Figure 7). We believe that the additional high-frequency content to in the AE signal was attributed to the dynamic failure of the smaller asperities in close proximity to an initially failed asperity. This hypothesis cannot be confirmed unequivocally in this experiment but tests with increased spatial and temporal image resolution will help understand the complex dynamics embedded in the foreshock signals and provide us with a more accurate understanding of strong asperity failure in general. 6. CONCLUSION Here we presented a novel experimental methodology which allows us to study the dynamic changes occurring along a frictional fault in the laboratory. Our fault experiences similar slow motions as faults in nature; non-uniform distributions of slip accrued in the quasistatic phase. A slip deprived region along the fault may be similar to locked region on a natural faults. Within this locked region, moments prior to fast rupture,

10 foreshocks were detected. These foreshocks had similarities to earthquakes in nature. A camera focused along the interface showed that light passing through asperities populating the slip deprived region changes at the same approximate time and location as when the foreshocks were generated. Changes in light intensity transmitted through asperities was concomitant to the generation of foreshocks detected and located acoustically. Frame to frame changes in asperities allowed us to estimate the source radius causing the foreshock which was comparable to the Brune corner frequency approximation. We observed that larger patches produced larger foreshocks with increased rise time of the P-wave. In the future, the relation between size of earthquake and rise time, notably whether it is a deterministic process, may help forecast the overall size of the earthquake before rupture has ceased and more accurately constrain the process using the actual images of the failing asperities. Asperity interaction is complex and difficult to determine the change between static and dynamic asperity failure. Images suggest that increased high-frequency content of the foreshocks signal, following the P-wave arrival, may have been produced by dynamic failure of local asperities near to the larger initially failed asperity. A better understanding of the dynamics process surrounding asperity failure could help increase our understanding of earthquakes, specifically models inherently used in studying nucleation. With more knowledge of the spatio-temporal distributions of foreshocks and more constrain on the physical mechanism(s) leading up to or during strong asperity failure; it may be possible to develop scaling relations from the laboratory to the field and better estimate seismic hazard in the future. REFERENCES 1. Okubo, P. G. and J. H. Dieterich Effects of physical fault properties on frictional instabilities produced on simulated faults, J. Geophys. Res. 89(B7): Ohnaka, M. & Shen, L. F Scaling of the shear rupture process from nucleation to dynamic propagation: Implications of geometric irregularity of the rupturing surfaces, J. Geophys. Res. 104(B1): McLaskey, G. C. and B. D. Kilgore Foreshocks during the nucleation of stick-slip instability, J. Geophys. Res. 118(6): Dieterich, J. H Preseismic fault slip and earthquake prediction, J. Geophys. Res. 83(B8): Marone, C Laboratory-derived friction laws and their application to seismic faulting, Annu. Rev. Earth Planet. Sci., 26: Lapusta, N., J. R. Rice, Y. Ben-Zion, and G. Zheng Elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rateand state-dependent friction, J. Geophys. Res. 105(B10): 23,765 23, Kaneko, Y. and J.-P. Ampuero A mechanism for preseismic steady rupture fronts observed in laboratory experiments, Geophys. Res. Lett. 38(21): L Selvadurai, P. A. and S. D. Glaser Experimental evidence of micromechanical processes that control localization of shear rupture nucleation, in 46th U. S. Rock Mechanics/Geomechanics Symposium, San Fransisco, CA. 9. Schmittbuhl, J., G. Chambon, A. Hansen, and M. Bouchon Are stress distributions along faults the signature of asperity squeeze?, Geophys. Res. Lett. 33(13): L Candela, T., F. Renard, M. Bouchon, J. Schmittbuhl, and E. E. Brodsky Stress drop during earthquakes: Effect of fault roughness scaling, Bull. Seis. Soc. Am. 101(5): Selvadurai, P. A., and S. D. Glaser Direct Measurement of Contact Area And Seismic Stress Along a Sliding Interface, in 46th U. S. Rock Mechanics/Geomechanics Symposium, Chicago, Il. 12. Bürgmann, R., D. Schmidt, R. M. Nadeau, M. d Alessio, E. Fielding, D. Manaker, T. V. McEvilly, and M. H. Murray Earthquake potential along the northern Hayward fault, California, Science 289(5482): Obara, K Nonvolcanic deep tremor associated with subduction in southwest Japan, Science 296(5573): Shirzaei, M. and R. Bürgmann Time-dependent model of creep on the Hayward fault from joint inversion of 18 years of InSAR and surface creep data, J. Geophys. Res. 118: McLaskey, G. C. and S. D. Glaser Hertzian impact: experimental study of the force pulse and resulting stress waves, J. Acous. Soc. Am. 128: McLaskey, G. C. and S. D. Glaser Acoustic emission sensor calibration for absolute source measurements, J. Nondest. Eval. 31(2): Selvadurai, P. A. and S. D. Glaser Laboratorydeveloped contact models controlling instability on frictional faults, in preparation for J. Geophys. Res. 18. Dieterich, J. H. and B. D. Kilgore Imaging surface contacts: power law contact distributions and contact stresses in quartz, calcite, glass and acrylic plastic, Tectono. 256: Hetch, E Optics, Pearson Education Limited. 20. McLaskey, G. C. and S. D. Glaser Micromechanics of asperity rupture during laboratory

11 stick slip experiments, Geophys. Res. Lett. 38(12): L Nadeau, R. M. and L. R. Johnson Seismological studies at Parkfield VI: Moment release rates and estimates of source parameters for small repeating earthquakes, Bull. Seis. Soc. Am. 88(3): Brune, J. N. 1970, Tectonic stress and spectra of seismic shear waves from earthquakes, J. Geophys. Res. 75(26): Das, S. and B. V. Kostrov Breaking of a single asperity rupture process and seismic radiation, J. Geophys. Res. 88(B5): Das, S. and B. V. Kostrov An elliptical asperity in shear: fracture process and seismic radiation, Geophys. J. Int., 80(3): Ellsworth, W. L. and G. C. Beroza Seismic evidence for an earthquake nucleation phase, Science 268(5212): Dodge, D. A., G. C. Beroza, and W. L. Ellsworth Detailed observations of California foreshock sequences: Implications for the earthquake initiation process, J. Geophys. Res. 101(B10): Aki, K. & P. G. Richards Quantitative Seismology, 2nd Ed., University Science Books 28. Rice, J. R. and M. Cocco Seismic fault rheology and earthquake dynamics, MIT Press, Cambridge, Mass., Chapter 5, pp Sammis, C. G. and J. R. Rice Repeating earthquakes as low-stress-drop events at a border between locked and creeping fault patches, Bull. Seis. Soc. Am. 91(3): Johnson, L. R. and R. M. Nadeau Asperity Model of an Earthquake: Static Problem, Bull. Seis. Soc. Am. 92(2): Waldhauser, F. and W. L. Ellsworth Fault structure and mechanics of the Hayward fault, California, from double-difference earthquake locations, J. Geophys. Res. 107( B3): ESE 3-1 ESE Beresnev, I. A What we can and cannot learn about earthquake sources from the spectra of seismic waves, Bull. Seis. Soc. Am. 91(2): Iio, Y Observations of the slow initial phase generated by microearthquakes: Implications for earthquake nucleation and propagation, J. Geophys. Res. 100(B8): Olson, E. L. and R. M. Allen The deterministic nature of earthquake rupture, Nature 438: