Technical Updates: 2015, First Quarter


    Various scientific NGA-East tasks have progressed and have been completed, as outlined in subsequent sections of this quarterly report. The NGA-East Technical Integration team (TI team) continued its evaluation and integration of proponent ground-motion models (GMMs), the two essential tasks of the Ground Motion Characterization (GMC) model building process. The TI team is also making use of products from various completed scientific tasks, such as the Database, Regionalization and Simulations tasks. This issue of the Technical Updates summarizes progress of the model building process, Project Plan tasks H, J, K, and L (see below).
    Numerous technical meetings were hosted in 2015’s first quarter. In addition to their weekly conference calls, the TI team invited the Participatory Peer Review Panel (PPRP) and the Joint Management Committee (JMC) to a Working Meeting on January 20-22, 2015. NGA-East’s SSHAC Workshop 3B was also hosted at UC Berkeley on March 4-5, 2015 and presentations from this event are available here.

Development of Median Ground-Motion Models (Task H)

    The median GMM developers finalized their models in January. A total of 20 new GMMs were completed and submitted along with the final model predictions and a short summary of the methods.

    A comprehensive PEER report documenting the new GMMs’ development was published in early April and is available here. Chapter 1 of the report presents an introduction and each of the following chapter documents the various models, authored by the numerous contributors, as summarized in Figure 1.

    PEER GGM Report organizationFigure 1. Organization of PEER Report 2015/04 – NGA-East: Median Ground-Motion Models for the Central and Eastern North America Region.

    Additional median ground motion tasks have been initiated:

    • Development of Gulf Coast adjustment models. Two teams have developed preliminary models: the PEER team (Hollenback et al.) and the DASG team (Darragh et al.).
    • Consideration of source depth as a modeling parameter. Drs. Hollenback (PEER) and Boore (USGS) are working on complementary tasks with support from various researchers in the NGA-East team.
    • Development and documentation of site amplification factors for VS30 of 760 m/s to 3000 m/s. This task has been completed and a PEER report is under preparation.

Sigma (Standard Deviation) (Task J)

    The Sigma Working Group has further developed Sigma models for CENA, borrowing extensively from data-rich regions and models (e.g. NGA-West2). Findings show that although some components of the variability (Tau, PhiSS) may be portable across a wide range of regions, the epistemic site-to-site variability (PhiS2S) must be consistent with the region-specific surface geology characteristics. Following discussions during Workshop 3B, refinements to the models were considered and implemented. The models are currently being reviewed by the TI team who will present their proposed selected model at Workshop 3C in June 2015.

Evaluation of Epistemic Uncertainty (Task K) and Logic Tree building (Task L)

    The Logic Tree building process for median ground motions involves several evaluation and integration tasks. These tasks and the quantification of epistemic uncertainty are related and the TI team chose the Sammon’s maps as a key tool for model building. The project continues refining the details of the Sammon’s maps-based approach. The latest sets of presentations on this topic are available on the Workshop 3B page. The TI team tasks can be grouped into four categories, summarized in Figure 2.

    • Screen GMMs and develop initial Sammon’s maps. In this task, the available GMMs (10 previously published and 20 new ones from task H) are evaluated for use as seed models to generate initial Sammon’s maps.
    • Populate ground-motion Sammon’s map space. Initial sets of weights are assigned to develop a covariance matrix from the seed models. A large number of candidate GMMs are developed by sampling the covariance matrix.
    • Rediscretize ground-motion space. Generated GMMs are evaluated and a subset is selected as representative of the ground-motion space.
    • Assign weights. Selected GMMs are assigned weights by the TI team.


    Median GMC model buildingFigure 2. Summary of TI team evaluation and integration tasks for median ground motions. Yellow trapezoids are products, grey rectangles are automated processes and computations and purple diamonds represent decision points by the TI team.