Publications
Does b-value Increase with Higher Pore Pressure?
Presented poster at Annual Meeting of American Geophysical Union 2023, San Francisco, December, 2023
Subsurface energy production in hydrocarbon and geothermal reservoirs commonly involves fluid injection and pore pressure increases, leading to micro-seismicity. Increasing pore pressure reduces normal stress and shear resistance across faults, thereby promoting fault slip. The event-size distribution of induced earthquakes is susceptible to fluid injection, with b-values (the slope of frequency magnitude distribution) decreasing with distance from injection. This decrease is thought to be governed by underlying pore pressure change; however, the general relationship between the change in pore pressure and frequency magnitude distribution is poorly understood due to the lack of direct measurements of pore pressure changes. Here we examine how different pore pressures in laboratory fault zones affect frequency-magnitude distributions and b-values. We conducted frictional sliding experiments on faulted cylindrical Westerly Granite samples at pore-pressures between 0.5 and 35 MPa and confining pressures of 150 MPa. We record acoustic emissions during triaxial compression and stick-slip and analyze event magnitudes. We observe that differences in pore-pressures between experiments are linearly related to b-values. Increasing differential stress prior to slip shows an inverse linear relationship with b-value, which agrees with previous studies. In the fluid-saturated fault experiments, we found that b-value minima occur relatively earlier within the inter-slip period, leading to more extended precursory periods than for dry faults. Moreover, the b-value reduction prior to slip instability is less pronounced for experiments at higher pore pressure. Our experimental results can be applied to fluid-injection-induced seismicity, as pore pressure variations can be used to approximate b-value and thus stress state of the fault, a possible tool for the estimation of seismic hazards due to fluid injection.Repeating Micro-Seismic Events on Laboratory Faults with Different Roughness and Gouge Composition
Presentation at Annual Meeting of American Geophysical Union 2023, San Francisco, December, 2023
Repeating earthquakes, also known as repeaters, occur on the same fault area at varying times. Such events have highly coherent waveforms and may result from the repeated failure of strong fault patches. The degree of localization of repeaters within shear zones can be affected by the gouge material and roughness of the fault. However, shear fabric has been thoroughly documented in the field and synthesized in the laboratory; although, how fault roughness and fault gouge characteristics affect frictional instability and the number of repeating seismic events is poorly understood. We study the relationship between fault roughness and small acoustic emission repeaters via laboratory stick-slip. We integrate two series of frictional sliding experiments: (i) on faulted cylindrical Westerly Granite samples with different fault roughness and (ii) direct shear experiment with different fault gouge compositions. We document acoustic emissions throughout the experiments and use time domain cross-correlation between all the waveforms to detect repeaters. We identified repeating event pairs when they are recorded at least at five common stations and have cross-correlation coefficient thresholds greater than 0.95 between respective stations. Larger fault roughness and frictional properties close to instability enhance number of repeaters. We observed that the smooth fault shows a larger proportion of bigger clusters than rough faults. The strength and size variation of corresponding asperities could be related to such mechanisms. The observation of repeaters prior to most earthquakes is considered evidence of an aseismic triggering process. This information is vital for earthquake prediction, early warning, and mitigation of natural hazards.Dynamic Triggering of Earthquakes in the Central and Eastern USA
Micro-Seismicity Clustering, Aftershock Decay and b-Values During Laboratory Fracture and Stick-Slip Experiments
In Annual Meeting 2023 Seismological Society of America, Seismological Research Letters, volumne 94, Number 2B, April, 2023
Earthquakes rarely occur in isolation but rather as sequences of events, clustered in space and time. We study seismic event clustering in controlled laboratory experiments where fault zone properties and stress can directly be monitored. We employ recently developed statistical methods (e.g., nearest neighbor clustering, R-statistic, Bi-statistic) to resolve seismic event interactions in series of experiments on in-tact and faulted Westerly granite samples. The samples exhibit deferent heterogeneity and roughness which strongly impact seismicity clustering. Our result show that heterogeneity in intact-samples promotes spatial clustering of seismic events albeit without temporal (Omori-type) correlations. Aftershock-like clustering is absent even during fracture nucleation and propagation close to peak stress. Aftershock-like triggering occurs during stable sliding on freshly formed fractures and in the presence of large-scale stress heterogeneity. The detected aftershocks in these cases can be described by standard seismological relationships such as a modified Omori-Utsu relation and its associated inter-event time distribution and productivity relation. Similarly, stick-slip on rough faults is associated with notable spatial-temporal seismicity clustering and Omori-decay mirroring natural seismicity statistics. Homogenous, planar surfaces, on the other hand, produce few aftershocks after unstable slip. Fault roughness also governs b-values and focal mechanisms variability. Rough faults lead to more heterogeneous focal mechanisms, spatially distributed seismicity and high b-values. The variability in focal mechanisms can be explained by heterogeneous, underlying stress fields which limit rupture size and promote high energy release within aftershock sequences. We conclude that roughness and heterogeneity strongly affect events sizes, clustering, and seismic energy partitioning between fore, main and aftershocks.Surface Roughness Evaluation in Anisotropic Rocks
Presentation at Annual Meeting of American Geophysical Union 2023, San Francisco, December, 2023
Roughness enables a better understanding of seismic slip, stress state, and aftershock activity in earthquake physics; strength and failure behavior of rocks in geomechanics. Surface roughness estimation is of great importance in studying rock deformation, rock strength, and fracturing behavior. Here we present an iterative image analysis algorithm for determining the surface roughness of the rocks. Our experiments incorporate gneiss and psammitic schist rock samples from the Lesser and Higher Himalayas of Central Nepal; fractured in the Uniaxial Compressive Strength (UCS) test to produce a natural rough surface. We conducted UCS tests with loading angles 0˚, 15˚, 30˚, 45˚, 60˚, 75˚ and 90˚ to the fractured plane of rock samples. We record the roughness of the rock based on the mineral distribution and pattern analysis from the photomicrographs. We observe that the asperity of fractured rock samples is controlled by mineral arrangement. The variations in melting and crystallization, location, and temperature of the minerals have a significant effect on the texture of rocks, resulting in the presence of dark and white mineral bands. Form, waviness, and roughness are terms designating low, medium, and high-frequency range variations of asperity, respectively. This approach incorporates the texture of a rock by analyzing the wavelength and frequency of mineral arrangement patterns. The iterative image analysis algorithm cuts off the waviness and form and extracts roughness from the asperity of rocks. This approach improves understanding of roughness from functional roughness by utilizing a simple curve made by mineral arrangement inside the anisotropic rocks in geomechanics, earthquake physics, and rock engineering with the use of images from simple microscopy.2023
Frequency Dependent Damage Pattern in Kathmandu Valley Due to Mw 7.8 Gorkha Earthquake
Journal of Geology & Geophysics
The Mw 7.8 Gorkha Earthquake (25th April 2015) is powerful earthquake ripped through Central Nepal occurs about 77 Km northwest of Kathmandu Valley. Several studies reveal the fact that comparatively larger earthquake damage in the Kathmandu valley are associated with the valley ground structure. Study focus on reason behind clustering of damages due to mainshock (7.8 Mw) inside Kathmandu valley in certain pattern and its dependency with frequency content of the shattered waves. Data used to meet objective of present research are ground motion data and damage data, for ground motion data seismic stations inside the valley are use. The damage data are collected by both primary and secondary sources. Frequency domain spectral analysis is incorporated in research and found that the maximum power and amplitude, associated, and attributed for particular narrow frequency band. Spatial component of frequency is wavelength which may indicate periodic repetition of maximum power with crest and trough. To estimate spatial distribution of maximum amplitude simplified wave relation is used. Study reveals that the lateral extension of the peak destruction zone as fourth of wavelength and the successive distance between peak destruction zones is half of wavelength. Peak destruction zone, the zone where the damage is maximum and lies either on crest or trough of the propagated wave. Study reveals that propagation of waves is S45oE form the epicenter of Gorkha Earthquake. Heterogeneity in damage on peak destruction zone can be contributed by the variation in geology of Kathmandu Valley.