Hadi Ghofrani

Hydraulic Fracturing and Seismicity in the Western Canada Sedimentary Basin

The development of most unconventional oil and gas resources relies upon subsurface injection of very large volumes of fluids, which can induce earthquakes by activating slip on a nearby fault. During the last 5 years, accelerated oilfield fluid injection has led to a sharp increase in the rate of earthquakes in some parts of North America. In the central United States, most induced seismicity is linked to deep disposal of coproduced wastewater from oil and gas extraction. In contrast, in western Canada most recent cases of induced seismicity are highly correlated in time and space with hydraulic fracturing, during which fluids are injected under high pressure during well completion to induce localized fracturing of rock. Furthermore, it appears that the maximum-observed magnitude of events associated with hydraulic fracturing may exceed the predictions of an often-cited relationship between the volume of injected fluid and the maximum expected magnitude. These findings have far-reaching implications for assessment of inducedseismicity hazards.

Impact of Induced Seismicity on the Evaluation of Seismic Hazard: Some Preliminary Considerations

A case study of seismicity induced by hydraulic fracturing operations near Fox Creek, Alberta, is used to evaluate the extent to which the potential for induced seismicity at a site alters the pre-existing hazard from natural seismicity. We find that in low-to-moderate seismicity environments, the hazard from an induced-seismicity source, if one is activated in close proximity to a site, can greatly exceed the hazard from natural background seismicity at most probabilities of engineering interest, over a wide frequency range. The most important parameters in determining the induced-seismicity hazard are the activation probability and the b-value of the initiated sequence. Uncertainty in the value of the key input parameters to a hazard analysis implies large uncertainty (more than an order of magnitude) in the likelihood of strong shaking.

ML and MW Scales in the Iranian Plateau Based on the Strong-Motion Records

The availability of a large amount of strong-motion data recorded by the National Strong Motion Network of Iran (nsmni) has motivated this study to develop relations for routine determination of M L and M W from digital horizontal components of the strong-motion records. The dataset comprises 861 two-component horizontal acceleration time series recorded for 125 earthquakes with magnitudes of 4.5 and larger. The M L scale is based on the horizontal synthesized Wood–Anderson seismograms. We have applied the Monte Carlo technique to evaluate distance correction curves for use in determining the local magnitude, M L , in Iran and in its northern, eastern, and Zagros subregions. Results indicate that the distance correction curves show trilinear behavior for geometrical spreading. The resulting coefficients evaluated for Iran as a whole are: R 1 = 96 ± 5 km; R 2 = 131 ± 5 km; n 1 = 1.01 ± 0.02; n 2 = −0.14 ± 0.1; n 3 = 0.14 ± 0.03; k = 0.00020 ± 0.00008. The distances less than R 1 correspond to attenuation of the direct waves. Between R 1 and R 2 is the distance where the multiply reflected and refracted shear waves from Moho dominate the arrivals. n 1 , n 2 , and n 3 are the coefficients of geometrical spreading for distances from the source to R 1 , R 1 to R 2 , and beyond R 2 . k is the coefficient of inelastic attenuation. For estimating the M W scale from the strong-motion data, we used the method proposed by Andrews (1986). To find the best correlation between the moment magnitudes measured from the strong-motion data and those measured from teleseismic data, we examined several time windows (e.g., whole trace, S -wave coda, and source time durations). The regressions show that the M W estimates from different time windows are all equally well correlated with the corresponding reported values with nearly identical standard deviations. Finally, relations between the estimates of local and moment magnitudes for the regions show that for earthquakes with magnitudes larger than about 6.0, the M L scale gradually becomes saturated and, therefore, it gives smaller values than those obtained by the M W scale. However, for smaller earthquakes, the M L scale overestimates the M W scale. This discrepancy occurs mainly because the frequency contents of the waveforms employed in these scales are different.

Stochastic Finite-Fault Modeling of Strong Ground Motions from the 26 December 2003 Bam, Iran, Earthquake

Equation (6) of Shoja-Taheri and Ghofrani (2007) was inadvertently deleted from page 1956. BSSA regrets the error. The equation is included here, along with surrounding text. Corrected text: To compare the attenuation of the synthetic ground motions with the attenuation relation for the region (Shoja-Taheri et al., 2005), we simulated accelerograms for nearly 5000 nodes in a mesh covering the area shaken by this earthquake. We assumed that all the stations in the area are located on rock. We fitted the simulated PGAs using

A preliminary statistical model for hydraulic fracture‐induced seismicity in the Western Canada Sedimentary Basin

We characterize the statistical relationship between hydraulic fracturing and seismicity in western Canada by using the concept of cellular seismicity. We determine the regionally averaged probability that hydraulic fracture operations will be associated with M ≥ 3 seismicity within a 10 km grid cell. This rate is 0.01 to 0.026 at the 95th percentile confidence limit. Monte Carlo simulations confirm that the observed correlations are extremely unlikely (≪1%) to have been obtained by chance. Proximity to a disposal well and proximity to the Swan Hills Formation, which has been suggested as a proxy for basement controlled faults, appear to increase the likelihood that hydraulic fracturing will trigger seismicity.

Nonlinear Response Potential of Real versus Simulated Ground Motions for the 11 March 2011 Tohoku-oki Earthquake

This study compares the nonlinear response potential of generic inelastic single-degree-of-freedom systems subjected to three sets of ground motion records for the 2011 Tohoku main shock. The compared record sets, all for the same sites, are: (1) observed accelerograms at 48 KiK-net strong motion stations; (2) time-histories simulated from the empirical Greens function method; and (3) time-histories simulated using the stochastic finite-fault method (with multiple sub-events). The adopted techniques can capture a realistic source rupture process involving multiple strong motion generation areas in simulations. Statistical analysis of computed peak ductility demands for the three record sets is conducted via cloud and stripe analyses. Results indicate that for the 2011 Tohoku main shock, different record sets produce similar average trends of the inelastic seismic demand curves. This conclusion is applicable to both cloud and stripe approaches and to structural systems with degrading and pinching hysteresis.

Using Borehole Records to Estimate Magnitude for Earthquake and Tsunami Early-Warning Systems

This paper presents a new application of the ground‐motion prediction equations (GMPEs) to estimate the event magnitude for earthquake and tsunami early warning systems. This technique incorporates borehole strong‐motion records along with surface recordings provided by Kiban Kyoshin network (KiK‐net) stations. We analyzed strong ground motion data from earthquakes with moment magnitude ( M ) ranging from 5.0 to 8.1 recorded by KiK‐net stations provided by Japan’s National Research Institute for Earth Science and Disaster Prevention (NIED) over the interval of 1998 to 2010. We used 2160 strong ground motion accelerograms with peak ground acceleration (PGA) larger than 10  cm/s2 recorded by borehole seismographs, and 890 waveforms with PGA larger than 80  cm/s2 recorded by surface seismographs to derive GMPEs for PGA and peak ground velocity (PGV) in Japan. These GMPEs are used as the basis for regional magnitude determination. Predicted magnitudes from PGA values (MPGA) and predicted magnitudes from PGV values (MPGV) were defined separately for borehole and surface recordings. MPGA and MPGV strongly correlate with the moment magnitude of the event, provided that at least 20 records for each event are available. The results show that MPGV from borehole data has the smallest standard deviation among the estimated magnitudes and provides an accurate early assessment of earthquake magnitude. We demonstrate that incorporation of borehole strong ground motion records immediately available after the occurrence of large earthquakes significantly increases the accuracy of earthquake magnitude estimation and improves earthquake and tsunami early warning systems performance.

Overview of Ground-Motion Issues for Cascadia Megathrust Events: Simulation of Ground-Motions and Earthquake Site Response

Ground motions for earthquakes of M7.5 to 9.0 on the Cascadia subduction interface are simulated based on a stochastic finite-fault model, and used to estimate average response spectra for reference firm soil conditions. The simulations are first validated by modeling the wealth of ground-motion data from the the 2011 M9.0 Tohoku earthquake of Japan. Adjustments to the calibrated model are then made to consider average source, attenuation and site parameters for the Cascadia region. This includes an evaluation of the likely variability in stress drop for large interface earthquakes, and an assessment of regional attenuation and site effects. We perform best-estimate simulations for a preferred set of input parameters. Typical results suggest mean values of 5%-damped pseudo-acceleration in the range from about 100 to 200 cm/s2, at frequencies from 1 to 4 Hz, for firm-ground conditions in Vancouver. Uncertainty in most-likely value of the parameter representing stress drop causes variability in simulated response spectra of about ±50%. Uncertainties in the attenuation model produce even larger variability in response spectral amplitudes – a factor of about two at a closest distance to the rupture plane (Rcd) of 100 km, becoming even larger at greater distances. It is thus important to establish the regional attenuation model for ground-motion simulations, and to bound the source properties controlling radiation of ground motion. We calculate theoretical 1D spectral amplification estimates for four selected Fraser River Delta sites to show how the presence of softer sediments in the region may alter the predicted ground motions. The amplification functions are largely consistent with observed spectral amplification at Fraser River delta sites, suggesting amplification by factors of 2.5 to 5 at the peak frequency of the site; we note that deep sites in the delta have a low peak frequency, ~0.3 Hz. This work will aid in seismic hazard assessment and mitigation efforts in the active Cascadia region of southwestern B.C. An important consideration is that the uncertainties are large and present a dominant unknown when assessing seismic risk. We find that variability in the expected motions exceeds a factor of two even on rock-like sites, with uncertainty