ALBERT

Description: A crustal normal-faulting earthquake ( ; M w  6.7) occurred in eastern Tohoku, Japan, on 11 April 2011. K-NET and KiK-net stations recorded 82 records from within 100 km of fault rupture. These data and data from associated foreshocks and aftershocks will make a critical contribution to future improvements of ground-motion prediction for normal-faulting earthquakes. Peak ground accelerations (PGA) and peak ground velocities (PGV) are compared with four ground-motion prediction equations (GMPEs) that include the style of faulting as a predictor parameter. For distances under 100 km, and using a network average value of V S 30 , the average ratio of PGA to the selected GMPEs (the event term ) is high by factors of 2.3–3.7. Event terms for PGV are high by factors of 1.4–1.8. Adjusting PGA and PGV with customized site terms ( Kawase and Matsuo, 2004a , b ), the standard deviations of PGA and PGV residuals are reduced from 0.59 to 0.43, and from 0.53 to 0.35, respectively. The event terms decreased to relatively small factors of 1.1–1.8 for PGA and increased slightly to 1.5–2.0 for PGV. Thus, site terms are very important, but positive event terms remain. The remaining positive event terms are not explained by high stress drop, which was typical of crustal events of all mechanisms globally or in Japan. Two subparallel faults ruptured, but source inversions, which we reviewed, revealed that they ruptured sequentially, so simultaneous contributions from the two faults did not cause high motions. Although these observations may tend to suggest that ground motions in large normal-faulting events are larger than predicted by the tested models, we are not aware of any observations from this event that contradict the precarious rock evidence of Brune (2000) that ground shaking is low on the footwall near the rupture.

Description: Using 3D dynamic models, we investigate the effect of fault stepovers on near-source ground motion. We use the finite-element method to model the rupture, slip, and ground motion of two parallel strike-slip faults with an unlinked overlapping stepover of variable width. We model this system as both an extensional and a compressional stepover and compare the results to those of single planar faults. We find that, overall, the presence of a stepover along the fault trace reduces the maximum ground motion when compared to the long planar fault. Whether the compressional or extensional stepover exhibits higher ground motion overall depends on the width of the separation between the faults. There is a region of reduced ground motion at the end of the first fault segment, when the faults are embedded in a homogeneous material. We also experiment with stress fields leading to supershear and subshear rupture velocities, and with different stress drops within those conditions. We find that subshear rupture produces stronger motions than supershear rupture, but supershear ruptures produce that maximum over a larger area than subshear areas, even though the overall area that experiences any shaking at all is not drastically different between the two cases. Lastly, we experiment with placing realistic materials along and around the faults, such as a sedimentary basin in an extensional stepover, a damage zone around the fault, and a soft rock layer on top of bedrock through the entire model area. These configurations alter the pattern of ground motion from the homogeneous case; the peaks in ground motion for the bimaterial cases depend on the materials in question. The results may have implications for ground-motion prediction in future earthquakes on geometrically complex faults. Online Material: MPEG-4 movies of models of dynamic rupture of fault stepovers embedded in heterogeneous material settings.

Description: In the absence of long-term instrumental data, the presence of fragile geologic features near active faults can provide physical limits on the level of ground shaking that could potentially have significant implications for seismic hazards. This paper introduces a multidisciplinary investigation that uses unfractured hoodoos in seismically active regions to constrain the level of ground accelerations at those locations. Although there is a large uncertainty associated with the age of the hoodoos because of their rapidly eroding nature, they can still be useful in providing physical limits on ground motions associated with recent large events. Here, we consider the fragilities of two hoodoos in the Red Rock Canyon region within a few kilometers of the Garlock fault, which is an active strike-slip fault in a transtensional region with at least a few large earthquakes in the Holocene. The hoodoos at these sites could be evidence of median or relatively low ground motions associated with large transtensional strike-slip earthquakes. Results of our field and laboratory tests on two hoodoos provide constraints on peak ground accelerations (PGAs) of and . Using the U.S. Geological Survey’s (USGS) probabilistic seismic hazard (PSH) deaggregation, the dominant earthquake contributing to the hazard at the site of the hoodoos for the recurrence intervals of 475, 975, and 2475 years is located at a distance of 4.8 km and has a magnitude of 7.63, consistent with the observed paleoearthquake evidence on the Garlock fault. The PGAs corresponding to these three return periods are 0.26, 0.40, and , respectively. Therefore, the survival of the more fragile hoodoo during a presumably large event on the Garlock fault in the past 550 years would be consistent with the 2008 seismic hazard level if the ground motions during that event were below the median value.

Description: The Fort Sage Mountains fault zone is a normal fault in the Walker Lane of the western Basin and Range that produced a small surface rupture (〈20 cm) during an M L  5.6 earthquake in 1950. We investigate the paleoseismic history of the Fort Sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950. Rupture of the Fort Sage fault ~5.6 ka resulted in surface displacements of at least 0.8–1.5 m, implying earthquake moment magnitudes ( M w ) of 6.7–7.1. An older rupture at ~20.5 ka displaced the ground at least 1.5 m, implying an earthquake of M w  6.8–7.1. A field of precariously balanced rocks (PBRs) is located less than 1 km from the surface-rupture trace of this Holocene-active normal fault. Ground-motion prediction equations (GMPEs) predict peak ground accelerations (PGAs) of 0.2–0.3 g for the 1950 rupture and 0.3–0.5 g for the ~5.6 ka paleoearthquake one kilometer from the fault-surface trace, yet field tests indicate that the Fort Sage PBRs will be toppled by PGAs between 0.1–0.3 g . We discuss the paleoseismic history of the Fort Sage fault in the context of the nearby PBRs, GMPEs, and probabilistic seismic hazard maps for extensional regimes. If the Fort Sage PBRs are older than the mid-Holocene rupture on the Fort Sage fault zone, this implies that current GMPEs may overestimate near-fault footwall ground motions at this site.

Description: Using 3D dynamic models, we investigate the effect of fault stepovers on near-source ground motion. We use the finite-element method to model the rupture, slip, and ground motion of two parallel strike-slip faults with an unlinked overlapping stepover of variable width. We model this system as both an extensional and a compressional stepover and compare the results to those of single planar faults. We find that, overall, the presence of a stepover along the fault trace reduces the maximum ground motion when compared to the long planar fault. Whether the compressional or extensional stepover exhibits higher ground motion overall depends on the width of the separation between the faults. There is a region of reduced ground motion at the end of the first fault segment, when the faults are embedded in a homogeneous material. We also experiment with stress fields leading to supershear and subshear rupture velocities, and with different stress drops within those conditions. We find that subshear rupture produces stronger motions than supershear rupture, but supershear ruptures produce that maximum over a larger area than subshear areas, even though the overall area that experiences any shaking at all is not drastically different between the two cases. Lastly, we experiment with placing realistic materials along and around the faults, such as a sedimentary basin in an extensional stepover, a damage zone around the fault, and a soft rock layer on top of bedrock through the entire model area. These configurations alter the pattern of ground motion from the homogeneous case; the peaks in ground motion for the bimaterial cases depend on the materials in question. The results may have implications for ground-motion prediction in future earthquakes on geometrically complex faults.

Description: We calculate near-source broadband (0-10 Hz) seismograms by combining low-frequency three-dimensional (3D) finite-difference seismograms (0-0.5 Hz) computed in a 3D velocity model using site-specific scattering Green's functions for random, isotropic scattering media. The scattering Green's functions are convolved with a slip-rate function to form local scattering operators (scatterograms), which constitute the high-frequency scattered wave field. The low-frequency and high-frequency scatterograms are then combined in the frequency domain to generate broadband waveforms. Our broadband method extends the Mai et al. (2010) approach by incorporating dynamically consistent source-time functions and accounting for finite-fault effects in the computation of the high-frequency waveforms. We used the proposed method to generate broadband ground motions at 44 sites located 5-100 km from the fault, for M (sub w) 7.7 earthquake scenarios (TeraShake) on the southern San Andreas fault, which include north-to-south, south-to-north, and bilateral rupture propagation from kinematic and spontaneous dynamic rupture models. The broadband ground motions computed with the new method are validated by comparing peak ground acceleration, peak ground velocity, and spectral acceleration with recently proposed ground-motion prediction equations (GMPEs). Our simulated ground motions are consistent with the median ground motions predicted by the GMPEs. In addition, we examine overturning probabilities for 18 precariously balanced rock sites (PBR). Our broadband synthetics for the M (sub w) 7.7 TeraShake scenarios show no preferred rupture direction on the southern San Andreas fault but are inconsistent with the existence of PBRs at several of the sites analyzed.

Description: A crustal normal-faulting earthquake (Formula ; M (sub w) 6.7) occurred in eastern Tohoku, Japan, on 11 April 2011. K-NET and KiK-net stations recorded 82 records from within 100 km of fault rupture. These data and data from associated foreshocks and aftershocks will make a critical contribution to future improvements of ground-motion prediction for normal-faulting earthquakes. Peak ground accelerations (PGA) and peak ground velocities (PGV) are compared with four ground-motion prediction equations (GMPEs) that include the style of faulting as a predictor parameter. For distances under 100 km, and using a network average value of V (sub S30) , the average ratio of PGA to the selected GMPEs (the event term ) is high by factors of 2.3-3.7. Event terms for PGV are high by factors of 1.4-1.8. Adjusting PGA and PGV with customized site terms (Kawase and Matsuo, 2004a,b), the standard deviations of PGA and PGV residuals are reduced from 0.59 to 0.43, and from 0.53 to 0.35, respectively. The event terms decreased to relatively small factors of 1.1-1.8 for PGA and increased slightly to 1.5-2.0 for PGV. Thus, site terms are very important, but positive event terms remain. The remaining positive event terms are not explained by high stress drop, which was typical of crustal events of all mechanisms globally or in Japan. Two subparallel faults ruptured, but source inversions, which we reviewed, revealed that they ruptured sequentially, so simultaneous contributions from the two faults did not cause high motions. Although these observations may tend to suggest that ground motions in large normal-faulting events are larger than predicted by the tested models, we are not aware of any observations from this event that contradict the precarious rock evidence of Brune (2000) that ground shaking is low on the footwall near the rupture.