Earthquake Science Center Seminars

Ground motion simulations are an important resource for augmenting recorded motions, assessing impacts for scenario earthquakes, and exploring parametric sensitivity. Providing confidence that simulations are realistic requires demonstrating that they not only reproduce motions from past earthquakes, but also that they can predict motions for future events. Ideally, simulations should capture effects due to complexities in the rupture process, as well as effects due to large-scale (e.g., basins) and small-scale (e.g., scattering or site effects) variations in the seismic velocity structure. Accounting for these features is challenging due to uncertainty in the expected ranges of the required parameters. Furthermore, adding increased detail to the model space increases the computational requirements. Simulation approaches have traditionally been classified as "deterministic" or "stochastic" depending on the level of complexity used to describe earthquake rupture and wave propagation effects. Because of knowledge limitations and computational cost, the deterministic approach is typically employed for lower frequencies (< 1 Hz) and the stochastic approach at higher frequencies (> 1Hz). This distinction naturally leads to a hybrid approach, where the separate low- and high-frequency responses are combined to produce a broadband response. In my talk, I will describe the features behind the hybrid simulation approach along with examples of its application to model recorded earthquake ground motions. I will also describe refinements to the deterministic approach that extend its range of applicability to higher frequencies. Finally, I will summarize those cases where simulations are most beneficial and explore some frontiers scientists are facing in ground motion simulations.

Subduction zones host the largest earthquakes and associated tsunamis. Several subduction earthquakes have been preceded by a precursory activity, namely an increase of the seismicity rate before the mainshock.

In this seminar I will discuss examples of how this accelerated seismicity can be associated with other deformation or seismological transient signals at various time and space scales: from localized preseismic slip on the subduction interface before the earthquake, to intriguing long-term and/or large-scale interactions between seismicity and deformation.

Motivated by the many false alarms during the 2011 Tohoku earthquake sequence in Japan, we developed a probabilistic approach to handle multiple concurrent earthquakes in earthquake early warning (EEW) system, called the integrated particle filter (IPF) method. We formulated a likelihood model that exploits information of both triggered and not yet triggered seismic stations, as well as the spatial distribution of seismic stations. A particle filter approach is used to perform real-time updating of the probabilistic estimations of source parameters, and the prediction uncertainty obtained in our model is used in an approximate Bayesian model class selection algorithm to estimate the number of concurrent events. Our algorithm results in over 90 per cent reduction in the number of incorrect warnings compared to the existing EEW system operating in Japan, and have shown good performance in places that do not have a dense seismic network.

Unconventional reservoirs are becoming increasingly important on the world energy stage. Such reservoirs need stimulation in order to be produced in an economically viable way, unlike conventional sources. Nevertheless, unconventional recovery factors are still considerably low, and further research contributions are needed to better understand how these reservoirs respond to stimulation. In this talk, I present work done at Stanford University under the supervision of Prof. Mark D. Zoback as part of my doctoral degree research. In this project, we analyzed data from two stimulated horizontal wells the Barnett Shale with different seismic surveys acquired before and after stimulation and gas production. We used 3D seismic attribute analysis to identify fractures in and around injection wells to evaluate the methods used, namely azimuth-dependent Amplitude Vs. Offset (AzAVO) and Velocity Vs. Azimuth (VVAz), and to understand how stimulation and production affected fractures in this area of the Barnett. Results showed that fracture density, rather than orientation, is more affected by the pressure introduced during stimulation and by pressure reduction with injected fluid removal and production.

After 1994, what legal elements have evolved to induce owners to improve the seismic performance of their high-rises in urban centers?

First, the performance target for high-rises has expanded from preventing loss of life during foreseeable earthquakes to reduction of property damage. See, e.g., the NEHRP legislation signed into law on December 11, 2018.

A second evolutionary incentive for owners to improve seismic performance is the strengthening of the owner's duty to protect the personal and property interests of third parties. See, e.g., the Millennium litigation.

A third evolutionary incentive is the strengthening of the owner's duty to assess whether planned or existing facilities suffer from vulnerabilities that would yield substandard performance during a foreseeable earthquake.

The Next Generation Attenuation Subduction (NGA-Sub) project is a multi-year, multidisciplinary project with the goal of developing an earthquake ground motion database and ground motion models for global subduction zone earthquakes, including those in Japan, Taiwan, Cascadia, Alaska, New Zealand, Mexico, and Central and South America. A combination of data inspection, regression techniques, ground motion simulations, and geometrical constraints are used to develop regionalized models for shaking intensity from interface and intraslab events. Complex path effects are observed in the data, including differences in distance-scaling between interface and intraslab events, forearc and back-arc effects, and regional variations. Our approach to model development was to first constrain a path term capturing these effects, then to subsequently investigate magnitude scaling, source depth scaling, and site effects. Regionalized terms of the ground-motion model include the model amplitude, anelastic attenuation, magnitude-scaling breakpoint, VS30-scaling, and sediment depth terms. This seminar will contain an overview of model development, and highlights the event-type and regional variations across the models, with a particular focus on Cascadia.

Although the magnitude of the Nov 30, 2018 Anchorage Earthquake was relatively high, with short epicentral distance to centers of population like Anchorage and Eagle River, the observed damage and absence of fatalities or serious injuries were less than anticipated from such a large event. The depth of the event (27 miles) may have dissipated much of its energy along its long hypo-central distance. However, widespread, operation-disruptive non-structural damage took place in different types of buildings ranging from cosmetic to heavy non-structural and equipment damage and water flooding. Major school, hospital and business closure and disruption resulted from such damage. Structural damage to non-engineered residential buildings was substantial. Existing buildings lacking seismic details experienced minor to moderate structural damage. Widespread geotechnical and foundation damage was observed due to the nature of soil in the region. Minor structural damage and moderate geotechnical related damage occurred in bridges. This seminar focuses on the structural and non-structural damage results from EERI Field Reconnaissance conducted in Southcentral Alaska following the Earthquake.

At the down-dip edge of some subduction seismogenic zones, slow slip events (SSE) have been discovered with

continuous GPS measurements. When accompanied by low amplitude seismic signals called tremors, the combined

phenomenon is named episodic tremor and slip (ETS). In some places, episodic tremors are observed without clear

signal on the geodetic time series. In this presentation, I show that we can use the timing of tremor signals to extract

a geodetic signature of ETS in the case of the San Andreas Fault (SAF) and the Alaska subduction zone. On the

Parkfield segment of the SAF, we extract an average Mw 4.9 SSE localized at about 18 km depth, at the transition

between the creeping section of the SAF and the Parkfield locked asperity. In Alaska, we show that the five year-

long upper cook inlet SSE described as a smooth slip event in time by previous studies, is in reality a cluster of

short term SSEs colocalized with tremor bursts. In both contexts, these observations shed light on the slip dynamics

during the interseismic phase of the seismic cycle and help understand the seismogenic zones loading processes.