Abstract: Fault slip and surface deformation patterns are essential factors in seismic hazard assessment. However, slip inversions reveal a widely observed shallow slip deficit (SSD) which has not yet been clearly explained. One possible cause of the SSD is distributed plastic deformation in the fault damage zone near the surface. Roten et al. (2017) performed 3D dynamic rupture simulations of the 1992 M7.3 Landers earthquake in a medium governed by Drucker-Prager plasticity. The study showed that while linear simulations tend to underpredict SSD and off-fault deformation (OFD), nonlinear simulations with moderately fractured rock mass can properly reproduce results that are consistent with the 30-60% SSD and around 46% OFD reported in geodetic observations. Analysis of the Roten et al. (2017) results show that discrepancies between linear and nonlinear simulations are only significant in the top hundreds of meters of the surface-rupturing fault. Although inelastic responses in the fault damage zone provide more realistic representations of earthquake physics, it can be computationally expensive or numerically unfeasible (e.g., in adjoint methods) to include nonlinear effects in ground motion simulations. One solution proposed here is to use an equivalent kinematic source (EKS) model that mimics the fault-zone plastic effects. This method generates source-time-functions by modifying the slip rate time histories based on comparisons of linear and non-linear dynamic rupture models, which are then used as input to kinematic simulations. The EKS model is shown to capture the SSD and OFD patterns observed in dynamic simulations with fault-zone plasticity. Further verification of the method is needed before the anticipated use in practical applications such as the Southern California Earthquake Center (SCEC) CyberShake and Broadband platforms.
Slidecast:
https://vimeo.com/276922362
Abstract: We estimate ground motions in the Pacific Northwest urban areas during M9 subduction scenario earthquakes on the Cascadia megathrust by simulating wave propagation from an ensemble of kinematic source descriptions. Velocities and densities in our computational mesh are defined by integrating the regional Cascadia Community Velocity model (V1.6, Stephenson et al., 2017) including the ocean water layer with a local velocity model of the Georgia and Seattle basins (Molnar et al., 2011) including additional near-surface velocity information. We generate six source realizations, each consisting of a background slip distribution with correlation lengths, rise times and rupture velocities consistent with data from previous megathrust earthquakes (e.g., 2011 M 9 Tohoku or 2010 M 8.8 Maule). We then superimpose M~8 subevents, characterized by short rise times and high stress drops on the background slip model to mimic high-frequency strong ground motion generation areas in the deeper portion of the rupture (Frankel, 2016). The wave propagation is simulated using the discontinuous mesh (DM) version of the AWP finite difference code. We simulate frequencies up to 1.25 Hz, using a spatial discretization of 100 m in the fine grid, resulting in surface grid dimensions of 6,540 x 10,728 mesh points. At depths below 8 km, the grid step increases to 300 m. We obtain stable and accurate results for the DM method throughout the simulation time of 7.5 mins as verified against a solution obtained with a uniform 100 m grid spacing. Peak ground velocities range between 60 and 80 cm/s in downtown Seattle and between 25 and 34 cm/s in downtown Vancouver, while spectral accelerations at 2 s range between 1.7 and 3.6 m/s2 and 1.0 and 1.3 m/s2, respectively. These long-period ground motions are not significantly reduced if plastic Drucker-Prager yielding in shallow cohesionless sediments is taken into account. Broadband synthetics (0-10Hz) are generated by a hybrid technique.
Slidecast:
https://vimeo.com/276921767
Abstract: Sierra Negra volcano is located at the southern portion of Isabela island and exhibits a large caldera (7×9 km). Since 1948, the average eruptive interval time is 11.4 yrs. Using a permanent BB network operated by the Instituto Geofisico, a steady increase in the number and size of earthquakes was detected beginning in Jun 2017. The number of detected events increased from about a dozen local events per day in Jun to a maximum of 107 on 02 Jan, 2018. Since Oct 2017, six events with magnitudes > 3.5 have been detected, with the largest (MLv 4.3) occurring in Jan 2018. This event’s spectrum was characteristic of an LP, even at the closest station just a few km from the epicenter. While most large events have a LP appearance, the majority of smaller events have wide spectra indicative of VTs. Locations have shallow foci presumably above either a horizontal magmatic reservoir or sill. Deformation gleaned from InSAR shows 98 cm of caldera floor uplift detected since early 2017 and 18 cm of uplift in January, 2018. The caldera floor has inflated more than 5 m since the deflationary episode following the 2005 eruption. Combining historical activity with recent seismic and geodetic records, we speculate that a new Sierra Negra eruption will likely occur in the near future.
Slidecast:
https://vimeo.com/276926562
Abstract: This study addresses whether knowing the slip rate on a fault improves estimates of magnitude (MW) of shallow, continental surface-rupturing earthquakes. Based on 43 earthquakes from the database of Wells and Coppersmith (1994), Anderson et al. (1996) previously suggested that estimates of MW from rupture length (LE) are improved by incorporating the slip rate of the fault (SF). We re-evaluate this relationship with an expanded database of 80 events, that includes 56 strike-slip, 13 reverse, and 11 normal faulting events. When the data are subdivided by fault mechanism, magnitude predictions from rupture length are improved for strike-slip faults when slip rate is included; a slip rate term does not improve magnitude fits for reverse or normal faults. Whether or not the slip rate term is present, a linear model with MW ∼ log LE over-all rupture lengths implies that the stress drop depends on rupture length – an observation that is not supported by teleseismic observations. We consider two other models, including one adapted from Chinnery (JGR, 1964) which we prefer because it has constant stress drop over the entire range of LE for any constant value of SF and because fits the data as well as the linear model. The dependence on slip rate for strike-slip faults is a persistent feature of all considered models. The observed dependence on SF supports the conclusion that for strike-slip faults of a given length, the static stress drop, on average, tends to decrease as the fault slip rate increases.
Slidecast:
https://vimeo.com/276939612
Abstract: On September 19th 2017, a magnitude 7.1 earthquake occurred between the states of Morelos and Puebla, Mexico. The event was a normal-faulting intraplate earthquake with a focal depth of 57 km. Although intermediate depth earthquakes (IDE) of this kind are relatively frequent across the globe, the physics of their source process is still not well understood. Due to the high confining pressure and temperature at depths below 50 km, rocks ought to deform by ductile flow rather than the brittle failure governing most of shallow, interplate earthquakes. We performed a dynamic source inversion of the M7.1 event using six strong motion records with epicentral distances smaller than 110 km. We implemented a new Particle Swarm Optimization algorithm for this purpose that takes advantage of parallel computing and allows a statistical analysis of the solution. Consistently with similar Mexican earthquakes (Díaz-Mojica et al., 2014), the inversion of the M7.1 event revealed that the rupture speed (Vr/Vs ~ 0.3-0.5) and radiation efficiency (0.02-0.28) are low. Besides, as expected for intraslab earthquakes, the stress drop (~20 MPa) is high. Similar results where recently found using an independent method for an IDE below the Wyoming Craton in US (Prieto et al., 2017) suggesting that slow, inefficient source processes may characterize earthquake ruptures below the brittle-ductile transition of the lithosphere. Although such rupture properties are typical of tsunami earthquakes, the M7.1 shock produced Fourier accelerations about two times larger than those observed between 1 and 2 s for earthquakes with similar magnitude reduced to the same hypocentral distance (Singh et al., 2018). It is possible that rupture directivity contributed to this observation. Our results also show that ~72% of the total energy change produced by the event was not radiated. This means that the specific fracture energy was close to 2 × 107 J/m2 in average, which is about 10 times larger than expected for shallow crust earthquakes. Recent studies suggest that thermal shear runaway is the leading rupture mechanism of IDEs (Prieto et. al., 2013). This mechanism produces a highly localized ductile deformation in the fault zone inhibiting brittle fracture but allowing large particle accelerations.
Slidecast:
https://vimeo.com/276971642
Abstract: We have investigated seismicity associated with hydraulic fracturing (HF) in Ohio since 2013, which provides an ideal setting for studying the relations between high pressure injection and earthquakes due to isolation from other injection activities. Our analysis using an array of local stations in Harrison County revealed 2 distinct groups: 1) deeper earthquakes in Precambrian basement, with larger magnitudes (M>2), b-values < 1, and many post shut-in earthquakes, versus 2) shallower earthquakes in Paleozoic rocks ~400 m below HF, with smaller magnitudes (M<1), b-values > 1.5, and few post shut-in earthquakes. Based on geologic history, laboratory experiments, and fault modeling, we interpreted the deep seismicity as slip on mature faults in older crystalline rocks and the shallow seismicity as slip on immature faults in younger sedimentary rocks. Wells inducing deeper seismicity produced more water than wells with shallow seismicity, indicating more extensive hydrologic connections outside the target formation, consistent with pore pressure diffusion influencing seismicity. However, the 2-3 hours between onset of HF and seismicity is too short for typical fluid pressure diffusion rates across distances of ~1 km and argues for poroelastic stress transfer also having a primary influence on seismicity. We now extend our analysis to other cases of HF induced seismicity in the Appalachian Basin. While these cases did not have publicly available local arrays, a broader set of stations operated by Miami University, ODNR, USGS, and IRIS is sufficient to identify the primary patterns of these seismic sequences. We employ multistation template matching to improve detection, waveform correlation to improve phase arrivals, and double difference relocation to improve the hypocentral characterization. We also examine frequency-magnitude distributions and well production patterns to compare with the geologic and operational interpretations made in the Harrison County study.
Slidecast:
https://vimeo.com/276939431
