Laboratory Earthquakes from the Cornell 3 m apparatus

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https://hdl.handle.net/1813/66670

This data was recorded from a suite of laboratory experiments described in an ongoing set of publications starting with Ke, et al., 2018, Geophysical Research Letters. https://doi.org/10.1029/2018GL080492.

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Data from: Heterogeneous high frequency seismic radiation from complex ruptures
Cebry, Sarah Beth L.; McLaskey, Gregory C. (2024-04-11)
These data are from Laboratory Earthquake Experiments from the Cornell 0.76 m apparatus in support of the following research: To investigate the effect of a normal stress heterogeneity on radiated spectra, we utilized a PMMA laboratory fault with a single, localized bump. By varying the bump prominence (defined here as normal stress on the bump divided by average normal stress across the entire fault), we produced earthquake-like ruptures that ranged from smooth, continuous ruptures to complex ruptures with variable rupture propagation velocity, slip distribution, and stress drop. High prominence bumps produced complex events that radiated more high frequency energy, relative to low frequency energy, than continuous events without a bump. In complex ruptures, the high frequency energy showed significant spatial variation correlated with peak slip rate and maximum local stress drop. Continuous ruptures emitted spatially uniform bursts of high frequency energy. Near-field peak ground acceleration (PGA) measurements of complex ruptures show nearly an order-of-magnitude higher PGA near the bump than elsewhere. We propose that for natural faults, geometric heterogeneities may be a plausible explanation for commonly observed order-of-magnitude variations in near-fault PGA.
Data from: Laboratory earthquake ruptures contained by velocity strengthening fault patches
Song, Jun Young; McLaskey, Gregory C. (2024)
These data are from Laboratory Earthquake Experiments from the Cornell 0.76 m apparatus in support of the following research: To better understand how normal stress heterogeneity affects earthquake rupture, we conducted laboratory experiments on a 760 mm PMMA sample with a 25 mm “bump” of locally higher normal stress. We systematically varied the sample-average normal stress and bump prominence. For bumps with low prominence (bump normal stress over sample average normal stress < 6) the rupture simply propagated through the bump and produced regular sequences of periodic stick-slip events. Bumps with higher prominence (>6) produced complex rupture sequences with variable timing and ruptures sizes, and this complexity persisted for multiple stick-slip supercycles. During some events the bump remained locked and acted as a barrier that completely stopped rupture. In other events, a dynamic rupture front terminated at the locked bump, but rupture reinitiated on the other side of the bump after a brief pause of 0.3-1 ms. Only when stress on the bump was near critical did the bump slip and unload built up strain energy in one large event. Thus, a sufficiently prominent bump acted as a barrier (energy sink) when it was far from critically stressed and as an asperity (energy source) when it was near critically stressed. Similar to an earthquake gate, the bump never acted as a permanent barrier. In the experiments, we resolve the above rupture interactions with a bump as separate rupture phases; however, when observed through the lens of seismology, it may appear as one continuous rupture that speeds up and slows down. The complicated rupture-bump interactions also produced enhanced high frequency seismic waves recorded with piezoelectric sensors.
Data from: Earthquake rupture interactions with a high normal stress bump
Cebry, Sara B. L.; Sorhaindo, Kian; McLaskey, Gergory C. (2023-06-21)
These files contain data supporting all results reported in Cebry et al. Earthquake rupture interactions with a high normal stress bump. In Cebry et al. we found: To better understand how normal stress heterogeneity affects earthquake rupture, we conducted laboratory experiments on a 760 mm PMMA sample with a 25 mm “bump” of locally higher normal stress. We systematically varied the sample-average normal stress and bump prominence. For bumps with low prominence (bump normal stress over sample average normal stress < 6) the rupture simply propagated through the bump and produced regular sequences of periodic stick-slip events. Bumps with higher prominence (>6) produced complex rupture sequences with variable timing and ruptures sizes, and this complexity persisted for multiple stick-slip supercycles. During some events the bump remained locked and acted as a barrier that completely stopped rupture. In other events, a dynamic rupture front terminated at the locked bump, but rupture reinitiated on the other side of the bump after a brief pause of 0.3-1 ms. Only when stress on the bump was near critical did the bump slip and unload built up strain energy in one large event. Thus, a sufficiently prominent bump acted as a barrier (energy sink) when it was far from critically stressed and as an asperity (energy source) when it was near critically stressed. Similar to an earthquake gate, the bump never acted as a permanent barrier. In the experiments, we resolve the above rupture interactions with a bump as separate rupture phases; however, when observed through the lens of seismology, it may appear as one continuous rupture that speeds up and slows down. The complicated rupture-bump interactions also produced enhanced high frequency seismic waves recorded with piezoelectric sensors.
Data from: Creep fronts and complexity in laboratory earthquake sequences illuminate delayed earthquake triggering
Cebry, Sara Beth L.; Ke, Chun-Yu; Shreedharan, Srisharan; Marone, Chris; Kammer, David S.; McLaskey, Gregory C. (2022-09-14)
These data are from Laboratory Earthquake Experiments from the Cornell 0.76 m apparatus in support of the following research: Earthquakes occur in clusters or sequences that arise from complex triggering mechanisms, but direct measurement of the slow subsurface slip responsible for delayed triggering is rarely possible. We investigate the origins of complexity and its relationship to heterogeneity using an experimental fault with two dominant seismic asperities. The fault is composed of quartz powder, a material common to natural faults, sandwiched between 760 mm long polymer blocks that deform the way 10 meters of rock would behave. We observe periodic repeating earthquakes that transition into aperiodic and complex sequences of fast and slow events. Neighboring earthquakes communicate via migrating slow slip, which resembles creep fronts observed in numerical simulations and on tectonic faults. Utilizing both local stress measurements and numerical simulations, we observe that the speed and strength of creep fronts are highly sensitive to fault stress levels left behind by previous earthquakes, and may serve as on-fault stress meters.
Data from: The Role of Background Stress State in Fluid-Induced Aseismic Slip and Dynamic Rupture on a 3-meter Laboratory Fault
Cebry, Sara B. L.; Ke, Chun-Yu; McLaskey, Gregory C. (2022-05)
These files contain data supporting all results reported in: "The Role of Background Stress State in Fluid-Induced Aseismic Slip and Dynamic Rupture on a 3-meter Laboratory Fault" by Cebry et al., where we found: Fluid injection stimulates seismicity far from active tectonic regions, however the details of how fluids modify on-fault stresses and initiate seismic events remains poorly understood. We conducted laboratory experiments using a biaxial loading apparatus with a 3 m saw-cut granite fault and compared events induced at different background shear stress levels. Water was injected at 10 ml/min and normal stress was constant at 4 MPa. In all experiments, aseismic slip initiated on the fault near the location of fluid injection and dynamic rupture eventually initiated from within the aseismic slipping patch. When the fault was near critically stressed, seismic slip initiated only seconds after MP a-level injection pressures were reached and the dynamic rupture propagated beyond the fluid pressure perturbed region. At lower stress levels, dynamic rupture initiated hundreds of seconds later and was limited to regions where aseismic slip had significantly redistributed stress from within the pressurized region to neighboring locked patches. We find that slow slip initiated when local stresses exceeded Coulomb failure criteria, but initiation of dynamic rupture required additional criteria to be met. Even high background stress levels required aseismic slip to modify on-fault stress to meet initiation criteria. We also observed slow slip events prior to dynamic rupture. Overall, our experiments suggest that initial fault stress, relative to fault strength, is a critical factor in determining whether a fluid-induced rupture will "runaway" or whether a fluid induced rupture will remain localized to the fluid pressurized region.
Data from: Seismic swarms produced by rapid fluid injection into a low permeability laboratory fault
Cebry, Sara Beth L.; McLaskey, Gregory C. (2020-12-18)
These data are from Laboratory Earthquake Experiments from the Cornell 0.76 m apparatus in support of the following research: Fluid injection, from activities such wastewater disposal, hydraulic stimulation, or enhanced geothermal systems, decreases effective normal stress on faults and promotes slip. Nucleation models suggest the slip at low effective normal stress will be stable and aseismic—contrary to observed increases in seismicity that are often attributed to fluid injection. We conducted laboratory experiments using a biaxial loading apparatus that demonstrate how an increase in fluid pressure can induce “stick-slip” events along a preexisting saw-cut fault in a poly(methyl methacrylate) (PMMA) sample. We compared slip events generated by externally squeezing the sample (shear-triggered) to those due to direct fluid injection (fluid-triggered) and studied the effects of injection rate and stress levels. Shear-triggered slip events began on a localized nucleation patch and slip smoothly accelerated from slow and aseismic to fast and seismic. Fluid-triggered slip events initiated far more abruptly and were associated with swarms of tiny foreshocks. These foreshocks were able to bypass the nucleation process and jump-start a mainshock resulting in an abrupt initiation. Analysis of these foreshocks indicates that the injection of fluid into a low permeability fault promotes heterogeneous stress and strength which can cause many events to initiate—some of which grow large. We conclude that while a reduction in effective normal stress stabilizes fault slip, rapid fluid injection into a low permeability fault increases multi-scale stress/strength heterogeneities which can initiate small seismic events that have the potential to rapidly grow, even into low stress regions.
Data From: The Earthquake Arrest Zone
Ke, Chun-Yu; McLaskey, Gregory C.; Kammer, David S. (2020-12-16)
These data are from Laboratory Earthquake Experiments from the Cornell 3 m apparatus in support of the following research: Loading a 3-meter granite slab containing a saw-cut simulated fault, we generated slip events that spontaneously nucleate, propagate, and arrest before reaching the ends of the sample. These rupture events have a slip distribution that varies along the fault and make them more similar to natural earthquakes than standard stick-slip events that rupture the entire sample. We propose an analytical crack model that fits our measurements. Similar to natural earthquakes, laboratory measurements show coseismic slip that gradually tapers near the rupture tips. Measured stress changes show roughly constant stress drop in the center of the ruptured region, a maximum stress increase near the rupture tips, and a smooth transition in between, in a region we describe as the earthquake arrest zone. The proposed model generalizes the widely used elliptical crack model by adding gradually tapered slip at the ends of the rupture. Different from the cohesive zone described by fracture mechanics, we propose that the transition in stress changes and the corresponding linear taper observed in the earthquake arrest zone are the result of rupture termination conditions primarily controlled by the initial stress distribution. It is the heterogeneous initial stress distribution that controls the arrest of laboratory earthquakes, and the features of static stress changes. We also performed dynamic rupture simulations that confirm how arrest conditions can affect slip taper and static stress changes. If applicable to larger natural earthquakes, this distinction between an earthquake arrest zone (that depends on stress conditions) and a cohesive zone (that depends primarily on strength evolution) has important implications for how seismic observations of earthquake fracture energy should be interpreted.
Data from: Groove Generation and Coalescence on a Large-Scale Laboratory Fault
Brodsky, Emily E.; McLaskey, Gregory C.; Ke, Chun-Yu (2020)
These data are from Laboratory Earthquake Experiments from the Cornell 3 m apparatus in support of the following research: Faults are the products of wear processes acting at a range of scales from nanometers to kilometers. Grooves produced by wear are a first-order observable feature of preserved surfaces. However, their interpretation is limited by the complex geological histories of natural faults. Here we explore wear processes on faults by forensically examining a large-scale controlled, laboratory fault which has a maximum offset between the sides of 42 mm and has been reset multiple times for a cumulative slip of approximately 140 mm. We find that on both sides of the fault scratches are formed with lengths that are longer than the maximum offset but less than the cumulative slip. The grooves are explained as a result of interaction with detached gouge rather than as toolmarks produced by an intact protrusion on one side of the fault. The density of grooves increases with normal stress. The experiment has a range of stress of 1-20 MPa and shows a density of 10 grooves/m/MPa in this range. This value is consistent with recent inferences of stress-dependent earthquake fracture energy of 0.2 J/m2 21 /MPa. At normal stresses above 20 MPa, the grooves are likely to coalesce into a corrugated surface that more closely resembles mature faults. Groove density therefore appears to be an attractive target for field studies aiming to determine the distribution of normal stress on faults. At low stresses the groove spacing can be measured and contrasted with areas where high stresses produce a corrugated surface.
Data from: Rupture Termination in Laboratory-Generated Earthquakes
Ke, Chun-Yu; McLaskey, Gregory C.; Kammer, David S. (2019)
These data are from Laboratory Earthquake Experiments from the Cornell 3 m apparatus in support of the following research: Loading a 3-meter granite slab containing a saw-cut simulated fault, we generated rupture events that spontaneously nucleate, propagate, and arrest before reaching the ends of the sample. These rupture events have a slip distribution that varies along the fault and make them more similar to natural earthquakes than standard stick-slip events that rupture the entire sample. Through LEFM (Linear Elastic Fracture Mechanics), we showed how the balance between energy release rate and fracture energy governs the termination of a rupture. In our experiments, fracture energy is essentially constant compared to the orders-of-magnitude variations in energy release rate so ruptures terminate because they run out of available strain energy. The utility of the model for both 3-m rock experiments and 200-mm PMMA experiments, and the similarity of fracture energy coefficient between the two materials, verifies the adequacy of PMMA as an analog to crustal rock in this context. Finally, the LEFM-based model provides a framework for linking friction properties and on-fault stress conditions to observable earthquake sequences
Data from: Earthquake Initiation from Laboratory Observations and Implications for Foreshocks
McLaskey, Gregory C. (2019-12)
These data are from Laboratory Earthquake Experiments from the Cornell 3 m apparatus in support of the following research: This paper reviews laboratory observations of earthquake initiation and describes new experiments on a 3 m rock sample where the nucleation process is imaged in detail. Many of the laboratory observations are consistent with previous work that showed a slow and smoothly accelerating earthquake nucleation process that expands to a critical nucleation length scale Lc, before it rapidly accelerates to dynamic fault rupture. The experiments also highlight complexities not currently considered by most theoretical and numerical models. This includes a loading rate dependency where a “kick” above steady state produces smaller and more abrupt initiation. Heterogeneity of fault strength also causes abrupt initiation when creep fronts coalesce on a stuck patch that is somewhat stronger than the surrounding fault. Taken together, these two mechanisms suggest a rate-dependent “cascade-up” model for earthquake initiation. This model simultaneously accounts for foreshocks that are a byproduct of a larger nucleation process and similarities between initial P wave signatures of small and large earthquakes. A diversity of nucleation conditions are expected in the Earth’s crust, ranging from slip limited environments with Lc < 1 m, to ignition-limited environments with Lc > 10 km. In the latter case, Lc fails to fully characterize the initiation process since earthquakes nucleate not because a slipping patch reaches a critical length but because fault slip rate exceeds a critical power density needed to ignite dynamic rupture.

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