Abstract: Physically meaningful characterization of seismic-wave attenuation is critical in theoretical and observational seismology. However, currently, descriptions of “attenuation” are somewhat mystified by reliance on empirical concepts, such as time-delayed strain-stress responses and frequency-dependent material properties. Attenuation is often treated as abstract “mechanical energy dissipation” measured by the quality factor (Q) of the medium. Nevertheless, in physics, no such medium properties exist, and energy dissipation rate represents only one aspect of the process of deformation. For example, compare the four commonly used attenuation measures: the geotechnical damping ratio (x), spectral decay parameter (k) in site characterization studies, and 1/Q and t* in seismology. Of these measures, only x is a true medium property, which is the viscosity of the near-surface mechanical resonator. By contrast, quantities k, 1/Q and t* are parameters of certain types of transfer functions, such as spectral or stress/strain ratios or logarithmic decrements of amplitudes measured in certain experiments. This means that these properties are apparent, and they may (and typically do) depend on boundary conditions, measurement procedures and assumptions. Hypothesizing that such properties (for example, the P- and S-wave, Young’s-modulus, surface-wave, or free-oscillation Qs) are mutually related as predicted by the viscoelastic model is generally incorrect. To overcome these problems, the “attenuation” phenomenon should be approached ab initio, by starting from material properties, differential equations of wave mechanics and the appropriate boundary and initial conditions. Such physical approaches are well known in poro- and thermoelasticity and hydro-mechanics, but they are still poorly utilized in seismology. Several examples of anelastic (but non-Q) forward and inverse physical models of laboratory and field experiments are used to illustrate the above statements.
Slidecast:
https://vimeo.com/276930160
Abstract: Typical subduction zones are characterised by curved thrust fault geometries that merge with the bathymetry under very shallow angles of narrow subduction wedges. Additionally, complicated networks of fault branches at high angles to the megathrust in the overriding and oceanic plates potentially generate strong gaining effects of vertical sea-floor displacements, making tsunami generation more likely. We present high-resolution physics-based numerical simulations of the 2004 Sumatra-Andaman earthquake, including non-linear frictional failure on a megathrust-splay fault system, off-fault plasticity, seismic wave propagation up to 2.2 Hz in 3D media and bathymetry. We specifically analyse splay fault slip transfering into vertical sea-floor displacement which may be required to to generate large events despite the fact that observations suggest high-fluid pressure and low stress drops limiting the available energy. The earthquake scenario matches coseismic slip and horizontal and vertical surface displacements inferred from observations. We find a high sensitivity of splay fault activation and plasticity effects to the orientation of the background stress field in conjuncture with fault geometry. Considering the geometric complexity of subduction zones and their potentially characteristically long rupture duration, invariably leads to huge differences in element sizes and many thousands of time steps. Our largest model consists of up to 221 million elements and 111 billion degrees of freedom. Such high fidelity simulations were performed using recent advances applied to SeisSol (www.seissol.org), specifically through a novel clustered local-time-stepping scheme extended to the dynamic rupture implementation (Uphoff et al., SC17).
Slidecast:
https://vimeo.com/276934728
Abstract: Southern Mexico was struck by four earthquakes with Mw > 6 and numerous smaller earthquakes in September 2017, starting with the 8 September Mw 8.2 Tehuantepec Earthquake beneath the Gulf of Tehuantepec offshore Chiapas and Oaxaca. We study whether this M8.2 earthquake triggered the three subsequent large M>6 quakes in southern Mexico to improve understanding of earthquake interactions and time-dependent risk. All four large earthquakes were extensional despite the subduction of the Cocos plate at the convergent plate boundary. The traditional definition of aftershocks: likely an aftershock if it occurs within two rupture lengths of the main shock soon afterwards. Two Mw 6.1 earthquakes, half an hour after the M8.2 beneath the Tehuantepec gulf and on 23 September near Ixtepec in Oaxaca, both fit as traditional aftershocks, within 200 km of the main rupture. The 19 September Mw 7.1 Puebla earthquake was ~600 km away from the M8.2 shock, outside the standard aftershock zone. Geodetic measurements from interferometric analysis of synthetic aperture radar (InSAR) and time-series analysis of GPS station data constrain finite fault static slip models for the M8.2, M7.1, and M6.1 Ixtepec earthquakes. We include open-ocean tsunami waveforms for the M8.2 inversions. We analyzed InSAR data from Copernicus Sentinel-1A and -1B satellites and JAXA ALOS-2 satellite. Our Bayesian (AlTar) static slip model for the M8.2 quake shows significant slip extended > 150 km and possible 220 km NW from the hypocenter and there is a high probability that the slip extended to depths of at least 70 km indicating slab pull stress state. Our slip model for the M7.1 earthquake is similar to the v2 NEIC FFM. Inversions for the M6.1 Ixtepec quake confirm shallow depth in the upper-plate crust and show centroid is about 30 km SW of the preliminary NEIC epicenter but consistent with cluster relocations. The NEIC updated epicenter and Mexican SSN location are closer to the InSAR-constrained location.
Slidecast:
https://vimeo.com/276974759
Abstract: There has been significant public and scientific interest in the observation of changed seismicity rates in North America since 2008, possibly due to human activities. Van der Baan and Calixto (Geochemistry, Geophysics, Geosystems, 2017) find that the seismicity rate in Oklahoma between 2008 and 2016 is strongly correlated to increased hydrocarbon production. The possibility of systematic correlations between increased hydrocarbon production and seismicity rates is a pertinent question since the United States became the world’s largest hydrocarbon producer in 2013, surpassing both Saudi Arabia’s oil production and Russia’s dry gas production. In most areas, increased production is due to systematic hydraulic fracturing which involves high-pressure, underground fluid injection. Increased hydrocarbon production also leads to increased salt-water production which is often disposed of underground. Increased underground fluid injection in general may cause increased seismicity rates due to facilitated slip on pre-existing faults. Contrary to Oklahoma, analysis of oil and gas production versus seismicity rates in six other states in the United States and three provinces in Canada finds no state- or province-wide correlation between increased seismicity and hydrocarbon production, despite 8- to 16-fold increases in production in some states. However, in various areas, seismicity rates have increased locally. A comparison with seismic hazard maps shows that human-induced seismicity is less likely in areas that have historically felt fewer earthquakes. The opposite is not necessarily true.
Slidecast:
https://vimeo.com/276939147
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
