On 20th April, 2013, another dreadful earthquake disaster occurred in Sichuan, China. The earthquake was a moment magnitude (Mw) 6.6 event, originated from the Longmenshan fault (http://earthquake.usgs.gov/earthquakes/eventpage/usb000gcdd#summary). As of April 27, the number of fatalities (including those missing) was 217. Besides, more than 13,000 were injured and more than 2 million were affected by the earthquake. The earthquake source region was the same as the catastrophic 12th May 2008 Mw7.9 Wenchuan earthquake (total number of deaths exceeded 69,000).
From an earthquake risk management perspective, this tragic event reminds us of two important facts. Firstly, earthquake damage is the consequence from implemented seismic protection and experienced seismic hazard. In simple terms, damage is caused when seismic demand exceeds seismic capacity of a structure. Plenty of reports and news have indicated that seismic design provisions for buildings and infrastructure and their implementation in the Sichuan province were not sufficient even after the 2008 earthquake. Newly constructed buildings were severely damaged or collapsed during the recent event. As adequate techniques and devices to mitigate seismic damage of physical structures (e.g. ductile connections, dampers, isolation, etc.) are available, the damage and loss could have been reduced significantly. It is also important to recognise that recovery and reconstruction from disasters provide us with opportunities to fix the problems fundamentally – we should take advantages of those events in a proactive manner.
Secondly, major earthquakes occur repeatedly in the same seismic region, and triggered events can cause significant damage. The 2013 event may not be directly triggered by the 2008 event. Nonetheless, it is suspected that the previous event have had some influence on the occurrence of the recent one through complex stress interaction within the Longmenshan fault system (this is possible in light of the triggering of the 1999 Mw7.1 Hector Mine earthquake due to the 1992 Mw7.3 Landers earthquake in California; Felzer et al., 2002). The destructiveness of repeated earthquakes has also been highlighted in recent earthquake disasters, particularly in the 2010-2011 Canterbury sequences (Shcherbakov et al., 2012). The mainshock was the 4thSeptember 2010 Mw7.1 Darfield earthquake, occurred about 40 km from the city centre of Christchurch. Following this event, aftershocks continued and migrated toward the city. On 22nd February, 2011, a major Mw6.3 Christchurch earthquake struck very near to the downtown Christchurch at a shallow depth, causing severe damage to buildings and infrastructure.
The current probabilistic seismic hazard and risk analysis framework does not address aftershock impact in the assessment. It is mainly focused upon the effects due to mainshocks, which are modelled by stationary Poisson processes (or some renewal processes). Clearly, in a post-mainshock situation, aftershock generation processes are no longer time-invariant. As part of PURE-CREDIBLE projects, I have been working on this issue by analysing world-wide aftershock sequences to develop simple statistical models, such as the Gutenberg-Richter law, modified Omori law, and Bath’s law (Shcherbakov et al., 2013). The main aim is to evaluate the fitness of the three seismological models to actual mainshock-aftershocks data and the uncertainty (variability) of the model parameters. These models facilitate the quantification of aftershock hazards, following a major mainshock, and the simulation of artificial mainshock-aftershocks sequences. Building upon this, I have further investigated how to assess the nonlinear seismic damage potential on structures due to aftershocks, in addition to mainshocks (Goda and Taylor, 2012; Goda and Salami, 2013). Extensive nonlinear dynamic simulations of structural behaviour subjected to real mainshock-aftershocks sequences were conducted to establish empirical benchmark for the aftershock effects on structures. Moreover, an artificial procedure for generating mainshock-aftershocks sequences was devised, and the equivalence of the nonlinear damage potential due to real and artificial sequences was verified. With the newly developed seismological and engineering tools, one can assess the aftershock hazard and risk quantitatively; these tools can be used as a starting model to evaluate the aftershock impact in low-to-moderate seismic regions where real mainshock-aftershocks sequences are not available. Importantly, the extended seismic hazard and risk analysis methodology takes into account triggered/induced hazards more comprehensively (in this regard, many improvements should be made by including geotechnical hazards and tsunamis), and envisages a dynamic, rather than quasi-static, risk assessment framework. It is noteworthy that major challenges in characterising aftershock generation remain. The above developments are focused upon temporal features of aftershock occurrence (e.g. decay of seismic activities since a mainshock), while spatial dependence of aftershocks is not explicitly considered. A workable technique is an ETAS (Epidemic Type Aftershock Sequence) model (Ogata and Zhang, 2006); it allows ‘individual aftershocks generate their own aftershock sequences’ and ‘aftershock occurrence rates vary in space’. Prof. Ian Main in the PURE-RACER consortium is actively working on this issue.
Another notable development/news in the field of engineering seismology and earthquake engineering is the (near) completion of a so-called NGA-WEST2 project (see http://peer.berkeley.edu/ngawest2/ for general information) on ground motion prediction models for global shallow crustal earthquakes. Various research outcomes from the NGA-WEST2 project were presented during the 2013 Seismological Society of America annual meeting at Salt Lake City. A new set of ground motion models will replace the 2008 models eventually. The new suite improves modelling of directivity and directionality (e.g. near-fault motions), extension of the applicable range to a lower magnitude range (i.e. magnitude scaling), model development for vertical motions, quantification of epistemic uncertainty, and evaluation of soil amplification factors (including non-linear site responses). All these refinements are important and relevant for seismic hazard assessment in Europe and the U.K. Particularly, more robust magnitude scaling of the predicted ground motion will enhance the accuracy of the seismic hazard assessment (note: an on-going NGA-EAST project will have greater influence on the seismic hazard and risk assessment in the U.K.).
Lastly, I would like to draw attention to a publication of ‘Handbook of seismic risk analysis and management of civil infrastructure systems’ (Woodhead Publishing, Cambridge, U.K.;http://www.woodheadpublishing.com/en/book.aspx?bookID=2497), which I have co-edited. The contributors of the handbook are international (U.K., Italy, Germany, Greece, Turkey, Canada, U.S.A., Japan, Hong Kong, New Zealand, and Colombia), having diverse professional backgrounds and interests. The handbook covers a wide range of topics related to earthquake hazard assessment, seismic risk analysis, and risk management for civil infrastructure. One of the main focuses is to provide the state-of-the-art overview of seismic risk analysis and uncertainty quantification. I hope that this is an invaluable guide for professionals requiring understanding of the impact of earthquakes on buildings and lifelines, and the seismic risk assessment and management of buildings, bridges and transportation.
Felzer, K.R., Becker, T.W., Abercrombie, R.E., Ekstrom, G., Rice, J.R. (2002). Triggering of the 1999MW 7.1 Hector Mine earthquake by aftershocks of the 1992 MW 7.3 Landers earthquake. Geophys. Res.: Solid Earth, 107, ESE 6-1-ESE 6-13.
Goda, K., Taylor, C.A. (2012). Effects of aftershocks on peak ductility demand due to strong ground motion records from shallow crustal earthquakes. Earthquake Eng. Struct. Dyn., 41, 2311-2330.
Goda, K., Salami, M.R. (2013). Cloud and incremental dynamic analyses for inelastic seismic demand estimation subjected to mainshock-aftershock sequences. Bull. Earthquake Eng. (in review).
Ogata, Y., Zhuang, J. (2006). Space–time ETAS models and an improved extension. Tectonophysics, 413, 13-23.
Shcherbakov, R., Nguyen, M., Quigley, M. (2012). Statistical analysis of the 2010 Mw 7.1 Christchurch, New Zealand, earthquake aftershock sequence. New Zealand J. Geol. Geophys., 55, 305-311.
Shcherbakov, R., Goda, K., Ivanian, A., Atkinson, G.M. (2013). Aftershock statistics of major subduction earthquakes. Bull. Seismol. Soc. Am. (in review).