Competition between preslip and deviatoric stress modulates precursors for laboratory earthquakes

Abstract Variations in elastic wave velocity and amplitude prior to failure have been documented in laboratory experiments as well as in a limited number of crustal earthquakes. These variations have generally been attributed to fault zone healing, changes in crack density, or pore fluid effects modulated dilatation or fault slip. However, the relationships between amplitude and velocity variations during the seismic cycle, and the underlying mechanisms of precursors to failure remain poorly understood. Here, we perform frictional shear experiments and measure the evolution of elastic wave velocity and amplitude throughout the laboratory seismic cycle. We find that elastic amplitudes and velocities undergo clear preseismic variations prior to fault failure. While preseismic amplitude reduction occurs early in the interseismic period, wave speed reduces later, just prior to failure. We perform a complementary set of stress oscillation experiments to quantify the response of seismic amplitudes and velocities to variations in the stress tensor. Taken together, our results indicate that preseismic amplitude variations are primarily controlled by fault slip rate and acceleration. On the other hand, elastic velocity responds to a combination of fault preslip which reduces seismic wavespeed and increasing stress in the wallrock, which increases wavespeed. Our data show that precursory changes in seismic wave speed may be more common than previously thought because they are masked by changes in wallrock stress. These results underscore the importance of continuous and long-term time-lapse monitoring of crustal faults for seismic hazard assessment and potential precursors to failure

Asymmetric brittle deformation at the Pāpaku Fault, Hikurangi Subduction Margin, NZ, IODP Expedition 375

Quantifying fault damage zones provides a window into stress distribution and rheology around faults. International Ocean Discovery Program (IODP) Expeditions 372/375 drilled an active thrust splay fault within the Hikurangi subduction margin. The fault, which is hosted in Pleistocene clastic sediments, is surrounded by brittle fractures and faults as well as ductile deformation features. We find that fracture density in the damage zone enveloping the fault is asymmetric, with the hanging wall showing greater overall fracture density and at greater distances from the fault than the footwall. Furthermore, the peak in fracture density occurs within an area of mesoscale folding and localized slip in the hanging wall rather than adjacent to the main fault zone. We attribute the asymmetry in damage to disparate deformation histories between the hanging wall and footwall, greater ductile deformation within the footwall, and/or dynamic stress asymmetry around a propagating rupture. Damage asymmetry is common at shallow depths in subduction zones and influences the mechanical and hydrological properties of the fault, such as channelized fluid flow and fault stability. Finally, we demonstrate that subduction zone faults show similar damage-displacement scaling as continental faults

Slow slip source characterized by lithological and geometric heterogeneity

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust

Stability Investigations of Tunnels in a Coal Mine in China Through 3D-Discontinuum Numerical Modeling and Field Deformation Monitoring Data

An imperative task for successful underground mining is to ensure the stability of underground structures, since it influences the safety, and in turn, the production capacity and economic performance of the mine. This is more so for deep excavations in soft rock which may be under significantly high stresses. In this thesis, stability studies on two tunnels, a horseshoe-shaped and an inverted arch-shaped tunnel, have been presented. The tunnels, running at a depth of 1325 m, are part of the Xiezhuang Coal Mine, in the Xinwen mining area, in China. Using the available information on stratigraphy, geological structures, in-situ stress measurements and geo-mechanical properties of intact rock and discontinuity interfaces, a three-dimensional numerical model has been built using the 3DEC 3-Dimensional Distinct Element Code to simulate the stress conditions around the tunnels. Based on available discontinuity geometry constraints, the rock mass has been modelled as a mixture of a discontinuum medium close to the tunnels and as an equivalent-continuum in the far field. Due to the unavailability of field measurements for rock mass mechanical parameters, the parameters have been estimated by incorporating the available intact rock mechanical properties and field deformation monitoring data into a strength reduction model calibration procedure. This back-analysis (calibration) has been carried out through a pseudo-time dependent support installation routine which incorporates the effect of time through a stress-relaxation mechanism. The results from the back-analysis indicate that the rock mass cohesion, tensile strength, uniaxial compressive strength, and elastic modulus values are about 35-45 % of the corresponding intact rock property values. Additionally, the importance of incorporating stress relaxation before support installation in numerical modeling has been illustrated, for the first time in literature, through the increased support factors of safety and reduced grout failures. The calibrated models have been analyzed for different supported and unsupported cases in an attempt to quantify the effect of supports in stabilizing the tunnels and to estimate the adequacy of the existing supports being used in the mine. A direct outcome is that the findings indicate that longer supports may be better suited for the existing geo-mining conditions around the tunnels since they have fractured zones that are larger than the supports currently in use at the mine. The effects of supports have been demonstrated using changes in deformations and yield zones around the tunnels, and changes in the average factors of safety and grout failures of the supports. The use of longer supports and floor bolting has provided greater stability for the rock masses around the tunnels. A comparison between the closure strains in the two differently shaped tunnels indicates that the inverted arch tunnel may be more efficient in reducing roof sag and floor heave for the existing geo-mining conditions. Additional analyses focusing on parametric sensitivity studies on the rock and joint mechanical properties show that the tunnel stability is highly sensitive to changes in cohesion and internal friction angle of the intact rock, and changes in joint basic friction angle. Tunnel stability is seen to not be very sensitive to changes in intact rock tensile strength and joint shear stiffness for the tunnels being studied. Finally, support optimization studies conducted by studying the effect of changing cable diameters and grout uniaxial compressive strengths on support factors of safety and grout failures show the trade-off that is necessary in selecting cable strength vis-à-vis grout strength. The results indicate that simply increasing either one of cable or grout strength parameters without considering their interactions and compatibilities could be detrimental to the stability of the support system

3-D discontinuum numerical modeling of subsidence incorporating ore extraction and backfilling operations in an underground iron mine in China

An underground iron mine in China has been used as a case-study to research the subsidence due to ore extraction and backfilling during open stoping operations. A 3-D discontinuum numerical model was built incorporating geologic complexities including faults and interfaces between different lithologies, and the stoping and backfilling sequence adopted from the mine plans. The stoping was carried out in two vertically stacked horizontal layers, with a total of 16 stopes. Large displacements of up to 50 cm were observed along the roof of the stopes, and a maximum surface subsidence of 22.5 cm was observed. Backfilling was found to eliminate subsequent displacements and subsidence. The extraction of the upper orebody was found to influence displacements in the lower orebody. Finally, a subsidence profile was constructed to show the subsidence at all locations along the length of the surface and region of influence on the surface. Keywords: Underground iron mine, Discontinuum numerical modeling, Open stoping, Backfill, Subsidenc

Frictional and Lithological Controls on Shallow Slow Slip at the Northern Hikurangi Margin

Abstract Slow slip events (SSEs) have been identified at subduction zones globally as an important link in the continuum between elastodynamic ruptures and stable creep. The northern Hikurangi margin is home to shallow SSEs which propagate to within 2 km of the seafloor and possibly to the trench, providing insights into the physical conditions conducive to SSE behavior. We report on a suite of friction experiments performed on protolith material entering the SSE source region at the Hikurangi margin, collected during the International Ocean Discovery Program Expedition 375. We performed velocity stepping and slide‐hold‐slide experiments over a range of fault slip rates, from plate rate (5 cm/yr or 1.6 × 10−9 m/s) to ∼1 mm/s (10−3 m/s) and quantified the frictional velocity dependence and healing rates for a range of lithologies atdifferent stresses. The frictional velocity dependence (a‐b) and critical slip distance DC increase with fault slip rate in our experiments. We observe atransition from velocity weakening to strengthening at slip rates of ∼0.3 µm/s. This velocity dependence of DC could be due to a combination of dilatant strengthening and a widening of the active shear zone at higher slip rates. We document low healing rates in the clay‐rich volcaniclastic conglomerates, which lie above the incoming plate basement at least locally, and relatively higher healing rates in the chalk lithology. Finally, our experimental constraints on healing rates in different input lithologies extrapolated to timescales of 1–10 years are consistent with the geodetically inferred low stress drops and healing rates characteristic of the Hikurangi SSEs

Laboratory Estimation of Rock Joint Stiffness and Frictional Parameters

Numerical modeling of complex rock engineering problems involves the use of various input parameters which control usefulness of the output results. Hence, it is of utmost importance to select the right range of input physical and mechanical parameters based on laboratory or field estimation, and engineering judgment. Joint normal and shear stiffnesses are two popular input parameters to describe discontinuities in rock, which do not have specific guidelines for their estimation in literature. This study attempts to provide simple methods to estimate joint normal and shear stiffnesses in the laboratory using the uniaxial compression and small-scale direct shear tests. Samples have been prepared using rocks procured from different depths, geographical locations and formations. The study uses a mixture of relatively smooth natural joints and saw-cut joints in the various rock samples tested. The results indicate acceptable levels of uncertainty in the calculation of the stiffness parameters and provide a database of good first estimates and empirical relations which can be used for calculating values for joint stiffnesses when laboratory estimation is not possible. Joint basic friction angles have also been estimated as by-products in the small scale direct shear tests

Data from: Creep fronts and complexity in laboratory earthquake sequences illuminate delayed earthquake triggering

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.This work was sponsored by National Science Foundation grants EAR-1763499, EAR-1847139, and EAR-1763305, European Research Council Advance Grant 835012 (TECTONIC), and US Department of Energy grants DE- SC0020512 and DE-EE0008763

Creep fronts and complexity in laboratory earthquake sequences illuminate delayed earthquake triggering

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.ISSN:2041-172

The Hidden History of the South‐Central Cascadia Subduction Zone Recorded on the Juan de Fuca Plate Offshore Southwest Oregon

Abstract New seismic reflection data collected and processed as part of early career scientist training at sea and in classroom projects fill gaps in seismic coverage of the Cascadia subduction zone and provide new insights into anomalous subduction behavior and mass wasting along the south‐central Cascadia Subduction Zone (CSZ) between 42°20’N and 44°15’N. The data reveal at least six distinct buried horizons of folded and faulted sediments similar to strata recently interpreted to result from in situ deformation induced by the load imposed by a large blocky mass transport deposit known as the 44°N slide. Although our results support prior studies indicating that the south‐central CSZ has experienced large slope instabilities, they indicate that the slides were more frequent but volumetrically smaller than previously thought. Similar strata have not been observed elsewhere beneath the abyssal plain adjacent to the Cascadia subduction zone. The structure of the deformation front along this segment is also indistinct, in contrast to the clear frontal faults outboard of folded trench strata observed immediately to the north and south (and generally throughout the rest of Cascadia). We attribute the anomalous nature of this segment of the margin to past subduction of shallow and rough seafloor, which resulted in greater uplift of the forearc than elsewhere along the margin. A consequence of this postulated history would be the shedding of older, more consolidated blocks onto the Juan de Fuca plate, resulting in the observed distinctive stratigraphy offshore southern Oregon