Katsuichiro Goda

The 2015 Gorkha Nepal Earthquake: Insights from Earthquake Damage Survey

The 2015 Gorkha Nepal earthquake caused tremendous damage and loss. To gain valuable lessons from this tragic event, an earthquake damage investigation team was dispatched to Nepal from 1 May 2015 to 7 May 2015. A unique aspect of the earthquake damage investigation is that first-hand earthquake damage data were obtained 6 to 11 days after the mainshock. To gain deeper understanding of the observed earthquake damage in Nepal, the paper reviews the seismotectonic setting and regional seismicity in Nepal and analyzes available aftershock data and ground motion data. The earthquake damage observations indicate that the majority of the damaged buildings were stone/brick masonry structures with no seismic detailing, whereas the most of RC buildings were undamaged. This indicates that adequate structural design is the key to reduce the earthquake risk in Nepal. To share the gathered damage data widely, the collected damage data (geo-tagged photos and observation comments) are organized using Google Earth and the kmz file is made publicly available.

Sensitivity of tsunami wave profiles and inundation simulations to earthquake slip and fault geometry for the 2011 Tohoku earthquake

In this study, we develop stochastic random-field slip models for the 2011 Tohoku earthquake and conduct a rigorous sensitivity analysis of tsunami hazards with respect to the uncertainty of earthquake slip and fault geometry. Synthetic earthquake slip distributions generated from the modified Mai-Beroza method captured key features of inversion-based source representations of the mega-thrust event, which were calibrated against rich geophysical observations of this event. Using original and synthesised earthquake source models (varied for strike, dip, and slip distributions), tsunami simulations were carried out and the resulting variability in tsunami hazard estimates was investigated. The results highlight significant sensitivity of the tsunami wave profiles and inundation heights to the coastal location and the slip characteristics, and indicate that earthquake slip characteristics are a major source of uncertainty in predicting tsunami risks due to future mega-thrust events.

Estimation of Seismic Loss for Spatially Distributed Buildings

A simulation-based framework for assessing seismic risk of spatially distributed buildings is developed by taking the spatial correlation of seismic excitations into account. For each of seismic events compiled in a synthetic earthquake catalog, inelastic seismic demand on buildings that are approximated by bilinear single-degree-of-freedom systems is compared with uncertain structural capacity to evaluate seismic damage severity. The proposed framework is employed to investigate the sensitivity of the estimated seismic risk of sets of buildings to the degree of spatially correlated and simultaneously occurring seismic excitations. In particular, four correlation levels—no correlation, full correlation, and partial correlation with/without intra-event components—are considered. The assignment of the partial correlation is based on a recently developed spatial correlation model, and the sets of hypothetical buildings mimic existing building stocks in downtown Vancouver. The analysis results highlight that underestimation or overestimation of correlation of seismic demand could lead to very different probabilistic characteristics of aggregate seismic loss although its mean is unaltered. The sensitivity analysis results suggest that uncertainty in structural capacities as well as average local soil conditions is of relative importance.

Effects of seismicity models and new ground motion prediction equations on seismic hazard assessment for four Canadian cities

Seismicityratesandground-motionpredictionequations(GMPEs)arethe key uncertainties in probabilistic seismic hazard analysis (PSHA). We explore the impact of new findings and knowledge from seismological and ground-motion studies on seismic hazard assessment for eastern and western Canada. Updated information includes the reevaluation of seismicity rates and their interpretation in terms of seismic source zones and the use of new GMPEs. We refer to our model as an interim updated seismic hazard model, as it does not treat all uncertainties comprehensively; rather, we address the impact of key uncertainties. Based on our updated interim seismic hazard model, uniform hazard spectra (UHS) at four major cities across Canada are obtained and compared with UHS in the current seismic hazard maps of Canada (2005/2010), whicharebasedona1995seismichazardmodeldevelopedbytheGeologicalSurveyof Canada. Sensitivity analysis highlights the significant impact of seismicity smoothing in low-to-moderate seismic regions (eastern Canada), while GMPEs are important for all regions. Moreover, our interim updated seismic hazard model can readily produce seismic hazard curves as well as seismic hazard deaggregation results for various site conditionsandformultipleprobabilitylevels;thiscapabilityisessentialforcarryingout advanced earthquake engineering analyses.

Nonlinear Response Potential of Mainshock–Aftershock Sequences from Japanese Earthquakes

Abstract A large mainshock triggers numerous aftershocks, exposing evacuees and residents to significant risk and hampering building reoccupation and restoration activities in a post‐disaster situation. It is thus important to take into account the seismic effects of mainshock–aftershock (MSAS) sequences, not just those of mainshocks. To assess the nonlinear damage potential caused by aftershocks, this study investigates the effects of aftershocks on peak ductility demand of inelastic single‐degree‐of‐freedom systems using real as well as artificial MSAS sequences. The real sequences are constructed from the K‐NET and KiK‐net databases for Japanese earthquakes. Comparison of peak ductility demand due to real mainshock events alone and real MSAS sequences renders empirical assessment of the aftershock impact on peak ductility demand. Moreover, time‐history data of artificial MSAS sequences are generated based on the generalized Omori’s law and suitable aftershock record selection procedure that takes into account key characteristics of aftershock records (magnitude, distance, and site classification). The validity of artificially generated MSAS sequences is evaluated by comparing probabilistic characteristics of peak ductility demand caused by artificial sequences with those caused by real sequences. The results indicate that peak ductility demands from real and artificial sequences are similar; thus, artificial sequences can be substituted for real sequences. Such calibration is particularly useful when an extensive data set of real MSAS sequences is not available. Online Material: Table of mainshock–aftershock sequences from Japanese earthquakes.

Ground motion characteristics and shaking damage of the 11th March 2011 Mw9.0 Great East Japan earthquake

A catastrophic Mw9.0 earthquake and subsequent giant tsunami struck the Tōhoku and Kanto regions of Japan on 11th March 2011, causing tremendous casualties, massive damage to structures and infrastructure, and huge economic loss. This event has revealed weakness and vulnerability of urban cities and modern society in Japan, which were thought to be one of the most earthquake-prepared nations in the world. Nevertheless, recorded ground motion data from this event offer invaluable information and opportunity; their unique features include very strong short-period spectral content, long duration, and effects due to local asperities as well as direction of rupture/wave propagation. Aiming at gaining useful experience from this tragic event, Earthquake Engineering Field Investigation Team (EEFIT) organised and dispatched a team to the Tōhoku region of Japan. During the EEFIT mission, ground shaking damage surveys were conducted in Sendai, Shirakawa, and Sukagawa, where the Japan Meteorological Agency intensity of 6+ was observed and instrumentally recorded ground motion data were available. The damage survey results identify the key factors for severe shaking damage, such as insufficient lateral reinforcement and detailing in structural columns from structural capacity viewpoint and rich spectral content of ground shaking in the intermediate vibration period range from seismic demand viewpoint. Importantly, inclusion of several ground motion parameters, such as nonlinear structural response, in shaking damage surveys, can improve the correlation of observed ground motion with shaking damage and therefore enhance existing indicators of potential damage.

Uncertainty modeling and visualization for tsunami hazard and risk mapping: a case study for the 2011 Tohoku earthquake

This study presents a rigorous computational framework for visualizing uncertainty of tsunami hazard and risk assessment. The methodology consists of three modules: (i) earthquake source characterization and stochastic simulation of slip distribution, (ii) tsunami propagation and inundation, and (iii) tsunami damage assessment and loss estimation. It takes into account numerous stochastic tsunami scenarios to evaluate the uncertainty propagation of earthquake source characteristics in probabilistic tsunami risk analysis. An extensive Monte Carlo tsunami inundation simulation is implemented for the 2011 Tohoku tsunami (focusing upon on Rikuzentakata along the Tohoku coast of Japan) using 726 stochastic slip models derived from eleven inverted source models. By integrating the tsunami hazard results with empirical tsunami fragility functions, probabilistic tsunami risk analysis and loss estimation are carried out; outputs from the analyses are displayed using various visualization methods. The developed framework is comprehensive, and can provide valuable insights in promoting proactive tsunami risk management and in improving emergency response capability.

Variability of tsunami inundation footprints considering stochastic scenarios based on a single rupture model: Application to the 2011 Tohoku earthquake

The sensitivity and variability of spatial tsunami inundation footprints in coastal cities and towns due to a megathrust subduction earthquake in the Tohoku region of Japan are investigated by considering different fault geometry and slip distributions. Stochastic tsunami scenarios are generated based on the spectral analysis and synthesis method with regards to an inverted source model. To assess spatial inundation processes accurately, tsunami modeling is conducted using bathymetry and elevation data with 50 m grid resolutions. Using the developed methodology for assessing variability of tsunami hazard estimates, stochastic inundation depth maps can be generated for local coastal communities. These maps are important for improving disaster preparedness by understanding the consequences of different situations/conditions, and by communicating uncertainty associated with hazard predictions. The analysis indicates that the sensitivity of inundation areas to the geometrical parameters (i.e., top-edge depth, strike, and dip) depends on the tsunami source characteristics and the site location, and is therefore complex and highly nonlinear. The variability assessment of inundation footprints indicates significant influence of slip distributions. In particular, topographical features of the region, such as ria coast and near-shore plain, have major influence on the tsunami inundation footprints.

New Scaling Relationships of Earthquake Source Parameters for Stochastic Tsunami Simulation

New scaling relationships of key earthquake source parameters are developed by uniformly and systematically analyzing 226 finite-fault rupture models from the SRCMOD database (http://equake-rc.info/srcmod/). The source parameters include the fault width, fault length, fault area, mean slip, maximum slip, Box-Cox power, correlation lengths along dip and strike directions, and Hurst number. The scaling relationships are developed by distinguishing tsunamigenic models from non-tsunamigenic models; typically, the former occurs in ocean and has gentler dip angles than the latter. The new models are based on extensive data, including recent mega-thrust events, and thus Eire more reliable. Moreover, they can be implemented as multivariate probabilistic models that take into account uncertainty and dependency of the multiple source parameters. The comparison between new and existing models indicates that the new relationships are similar to the existing ones for earthquakes with magnitudes up to about 8.0, whereas the relationships for the fault width and related parameters differ significantly for larger mega-thrust events. An application of the developed scaling relationships in tsunami hazard analysis is demonstrated by synthesizing stochastic earthquake source models in the Tohoku region of Japan. The examples are aimed at providing practical guidance as to how the developed scaling relationships can be implemented in stochastic tsunami simulation. The numerical results indicate that the effects of magnitude scaling of the source parameters and their uncertainties have major influence on the tsunami hazard assessment.

Seismic Vulnerability of Reinforced Concrete Frame with Unreinforced Masonry Infill Due to Main Shock–Aftershock Earthquake Sequences

This paper presents a parametric study on the inelastic response of six-story reinforced concrete (RC) frames subjected to main shock–aftershock (MS-AS) ground motions. For this purpose, one bare frame (BF) and four masonry RC frames with two infill thicknesses (75 mm or 125 mm) and two infill patterns (open ground story or fully infilled) are considered. They are situated at site class C in Vancouver, Canada, and are designed for office occupancy according to the 2005 National Building Code of Canada. The five frames are subjected to 100 ensembles of MS-AS ground motions scaled to seismic hazard level corresponding to the return period of 2,500 years. For each sequence of earthquakes, change in the fundamental period (T1) and inter-story drift (ISD) for both MS-AS sequences are quantified. The analysis results show that the period change and ISD were significant for BF, whereas the infilled frames sustained small damage with negligible change in T1.