A simple and powerful method for simulating ground motions is to combine parametric or functional descriptions of the ground motion’s amplitude spectrum with a random phase spectrum modified such that the motion is distributed over a duration related to the earthquake magnitude and to the distance from the source. This method of simulating ground motions often goes by the name “the stochastic method.” It is particularly useful for simulating the higher-frequency ground motions of most interest to engineers (generally, f > 0.1 Hz), and it is widely used to predict ground motions for regions of the world in which recordings of motion from potentially damaging earthquakes are not available. This simple method has been successful in matching a variety of ground-motion measures for earthquakes with seismic moments spanning more than 12 orders of magnitude and in diverse tectonic environments. One of the essential characteristics of the method is that it distills what is known about the various factors affecting ground motions (source, path, and site) into simple functional forms. This provides a means by which the results of the rigorous studies reported in other papers in this volume can be incorporated into practical predictions of ground motion.

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Simulation of Ground Motion Using the Stochastic Method

Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves

We present a new empirical ground motion model for PGA, PGV, PGD and 5% damped linear elastic response spectra for periods ranging from 0.01– 10 s. The model was developed as part of the PEER Next Generation Attenuation (NGA) project. We used a subset of the PEER NGA database for which we excluded recordings and earthquakes that were believed to be inappropriate for estimating free-field ground motions from shallow earthquake mainshocks in active tectonic regimes. We developed relations for both the median and standard deviation of the geometric mean horizontal component of ground motion that we consider to be valid for magnitudes ranging from 4.0 up to 7.5–8.5 (depending on fault mechanism) and distances ranging from 0 – 200 km. The model explicitly includes the effects of magnitude saturation, magnitude-dependent attenuation, style of faulting, rupture depth, hanging-wall geometry, linear and nonlinear site response, 3-D basin response, and inter-event and intra-event variability. Soil nonlinearity causes the intra-event standard deviation to depend on the amplitude of PGA on reference rock rather than on magnitude, which leads to a decrease in aleatory uncertainty at high levels of ground shaking for sites located on soil. DOI: 10.1193/1.2857546

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NGA Ground Motion Model for the Geometric Mean Horizontal Component of PGA, PGV, PGD and 5% Damped Linear Elastic Response Spectra for Periods Ranging from 0.01 to 10 s

Many aspects of earthquake mechanics remain an enigma as we enter the closing years of the twentieth century. One potential bright spot is the realization that simple calculations of stress changes may explain some earthquake interactions, just as previous and on going studies of stress changes have begun to explain human-induced seismicity. This paper, which introduces the special section “Stress Triggers, Stress Shadows, and Implications for Seismic Hazard,” reviews many published works and presents a compilation of quantitative earthquake interaction studies from a stress change perspective. This synthesis supplies some clues about certain aspects of earthquake mechanics. It also demonstrates that much work remains before we can understand the complete story of how earthquakes work.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JB01576

Introduction to Special Section: Stress Triggers, Stress Shadows, and Implications for Seismic Hazard

Mathematical Handbook for Scientists and Engineers

The high-frequency seismic field near the epicenter of a large earth- quake is modeled by subdividing the fault plane into subelements and summing their contributions at the observation point. Each element is treated as a point source with an o92 spectral shape, where co is the angular frequency. Ground-motion contributions from the subsources are calculated using a stochastic model. Attenuation is based on simple geometric spreading in a whole space, coupled with regional anelastic atten- uation (Q operator). The form of the co n spectrum with natural n follows from point shear-dislocation theory with an appropriately chosen slip time function. The seismic moment and comer frequency are the two parameters defining the point-source spectrum and must be linked to the subfanlt size to make the method complete. Two coefficients, Aa and K, provide this link. Assigning a moment to a subfault of specified size introduces the stress parameter, Ao-. The relationship between comer frequency (dislocation growth rate) and fault size is established through the coefficient K, which is inherently nonunique. These two parameters control the number of subsources and the ampli- tudes of high-frequency radiation, respectively. Derivation of the model from shear- dislocation theory reveals the unavoidable uncertainty in assigning ogn spectrum to faults with finite size. This uncertainty can only be reduced through empirical vali- dation. The method is verified by simulating data recorded on rock sites near epicenters of the M8.0 1985 Michoacan (Mexico), the M8.0 1985 Valparaiso (Chile), and the M5.8 1988 Saguenay (Qurbec) earthquakes. Each of these events is among the largest for which strong-motion records are available, in their respective tectonic environ- ments. The simulations for the first two earthquakes are compared to the more de- tailed modeling of Somerville et al. (1991), which employs an empirical source function and represents the effects of crustal structure using the theoretical impulse response. Both methods predict the observations accurately on average. The precision of the methods is also approximately equal; the predicted acceleration amplitudes in our model are generally within 15% of observations. An unexpected result of this study is that a single value of a parameter K provides a good fit to the data at high frequencies for all three earthquakes, despite their different tectonic environments. This suggests a simplicity in the modeling of source processes that was unanticipated.

Modeling finite-fault radiation from the ωn spectrum

Numerous observations accumulated principally during the last 40 years show that seismic waves generated from earthquakes and cultural noise may alter water and oil production In some cases wave excitation may appreciably increase the mobility of fluids The effect of elastic waves on the permeability of saturated rock has been confirmed in numerous laboratory experiments Two related applications have arisen from these findings In the first application, high-power ultrasonic waves are applied for downhole cleaning of the near-wellbore in producing formations that exhibit declining production as a result of the deposition of scales and precipitants, mud penetration, etc In many cases, ultrasound effectively removes the barriers to oil flow into the well The ultrasonic method is reported to be successful in 40-50 percent of the cases studied In the case of successful treatment, the effect of improved permeability may last up to several months Whereas this method has a very local effect, a second application is used to stimulate the reservoir as a whole Here seismic frequency waves are applied at the earth’s surface by arrays of vibroseis-type sources This method has produced promising results; however, further testing and understanding of the mechanisms are necessary

Elastic-wave stimulation of oil production; a review of methods and results

CHANGES IN PERMEABILITY CAUSED BY TRANSIENT STRESSES: FIELD OBSERVATIONS, EXPERIMENTS, AND MECHANISMS Michael Manga, 1 Igor Beresnev, 2 Emily E. Brodsky, 3 Jean E. Elkhoury, 4 Derek Elsworth, 5 S. E. Ingebritsen, 6 David C. Mays, 7 and Chi-Yuen Wang 1 Received 7 November 2011; revised 15 February 2012; accepted 10 March 2012; published 12 May 2012. [ 1 ] Oscillations in stress, such as those created by earth- quakes, can increase permeability and fluid mobility in geo- logic media. In natural systems, strain amplitudes as small as 10 A6 can increase discharge in streams and springs, change the water level in wells, and enhance production from petroleum reservoirs. Enhanced permeability typically recovers to prestimulated values over a period of months to years. Mechanisms that can change permeability at such small stresses include unblocking pores, either by breaking up permeability-limiting colloidal deposits or by mobilizing droplets and bubbles trapped in pores by capillary forces. The recovery time over which permeability returns to the prestimulated value is governed by the time to reblock pores, or for geochemical processes to seal pores. Monitor- ing permeability in geothermal systems where there is abun- dant seismicity, and the response of flow to local and regional earthquakes, would help test some of the proposed mechanisms and identify controls on permeability and its evolution. Citation: Manga, M., I. Beresnev, E. E. Brodsky, J. E. Elkhoury, D. Elsworth, S. E. Ingebritsen, D. C. Mays, and C.-Y. Wang (2012), Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms, Rev. Geophys., 50, RG2004, doi:10.1029/2011RG000382. INTRODUCTION [ 2 ] The permeability of Earth’s crust is of great interest because it largely governs key geologic processes such as advective transport of heat and solutes and the generation of elevated fluid pressures by processes such as physical com- paction, heating, and mineral dehydration. For an isotropic Department of Earth and Planetary Science, University of California, Berkeley, California, USA. Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, USA. Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA. Department of Civil and Environmental Engineering, University of California, Irvine, California, USA. Department of Energy and Mineral Engineering, Center for Geomechanics, Geofluids, and Geohazards, EMS Energy Institute, Pennsylvania State University, University Park, Pennsylvania, USA. U.S. Geological Survey, Menlo Park, California, USA. Department of Civil Engineering, University of Colorado Denver, Denver, Colorado, USA. Corresponding author: M. Manga, Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720, USA. (manga@seismo.berkeley.edu) material, permeability k is defined by Darcy’s law that relates the fluid discharge per unit area q to the gradient of hydraulic head h, q ¼A kgr rh; m where r is the fluid density, m the fluid viscosity and g is gravity. The permeability of common geologic media varies by approximately 16 orders of magnitude, from values as low as 10 A23 m 2 in intact crystalline rock, intact shales, and fault cores, to values as high as 10 A7 m 2 in well-sorted gravels. Nevertheless, despite being highly heterogeneous, perme- ability can be characterized at the crustal scale in a manner that provides useful insight [e.g., Gleeson et al., 2011]. [ 3 ] The responses of hydrologic systems to deformation provide some insight into controls on permeability, in par- ticular its evolution in time. For example, the water level in wells and discharge in rivers have both been observed to change after earthquakes. Because earthquakes produce stresses that can change hydrogeologic properties of the crust, hydrologic responses to earthquakes are expected, especially in the near field (within a fault length of the Copyright 2012 by the American Geophysical Union. Reviews of Geophysics, 50, RG2004 / 2012 1 of 24 Paper number 2011RG000382 8755-1209/12/2011RG000382 RG2004

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Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms

INTRODUCTION 

Ground motions from earthquakes are created by ruptures on tectonic faults. The causative faults can be considered point sources at distances large compared to the fault dimensions. At closer distances, the finite-fault effects become important. These effects are primarily related to the finite speed of rupture propagation, which causes certain parts of the fault to radiate energy much earlier than do other parts; the delayed waves then interfere, creating significant directivity effects. The duration and amplitude of ground motion become dependent on the angle of observation.

Finite-source modeling has been an important part of ground-motion prediction near the epicenters of large earthquakes (Hartzell, 1978; Irikura, 1983; Joyner and Boore, 1986; Heaton and Hartzell, 1989; Somerville et al., 1991; Hutchings, 1994; Tumarkin and Archuleta, 1994; Zeng et al. , 1994). In the approach adopted in most studies, the fault plane is discretized into elements, each element is treated as a small...

FINSIM--a FORTRAN Program for Simulating Stochastic Acceleration Time Histories from Finite Faults

Oscillations in stress, such as those created by earthquakes, can increase permeability and fluid mobility in geologic media. In natural systems, strain amplitudes as small as 10 -6 can increase discharge in streams and springs, change the water level in wells, and enhance production from petroleum reservoirs. Enhanced permeability typically recovers to pre-stimulated values over a period of months to years. Mechanisms that can change permeability at such small stresses include unblocking pores, either by breaking up permeability-limiting colloidal deposits or by mobilizing droplets and bubbles trapped in pores by capillary forces. The recovery time over which permeability returns to the pre-stimulated value is governed by the time to re-block pores, or for geochemical processes to seal pores. Monitoring permeability in geothermal systems where there is abundant seismicity, and the response of flow to local and regional earthquakes, would help test some of the proposed mechanisms and identify controls on permeability and its evolution.

Igor A. Beresnev

Papers

Modeling finite-fault radiation from the ωn spectrum

Elastic-wave stimulation of oil production; a review of methods and results

Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms

FINSIM--a FORTRAN Program for Simulating Stochastic Acceleration Time Histories from Finite Faults

Changes in permeability caused by transient stresses: field observations, experiments, and