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Transcript
Christchurch Earthquake Christchurch Earthquake ‐ New Normal or Old Normal, and Implications for Policy Professor Paul Somerville Chief Geoscientist Risk Frontiers, Macquarie University Outline • The New Normal The New Normal – – Greater earthquake source strength? – More frequent Canterbury earthquakes? More frequent Canterbury earthquakes? • Evidence about source strength from recorded ground motions d ti • Uncertainty about more frequent earthquakes • Implications for policy Definitions of an Earthquake q • Engineer (and everyone else): (and everyone else): “a shaking of the ground” • Geoscientist: G i ti t “a sudden movement on a fault” • In this talk, Earthquake means a sudden movement on a fault, which causes ground motions and other effects Christchurch ‐ The New Normal? Christchurch The New Normal? • Greater Greater source strength ‐ source strength ‐ Are the source Are the source strengths (stress drops) of some categories of New Zealand earthquakes larger than we of New Zealand earthquakes larger than we had thought? • More frequent Canterbury earthquakes – More frequent Canterbury earthquakes Are large earthquakes in the Canterbury Plain going to be much more frequent than Plain going to be much more frequent than before for decades to come? Tectonic Setting and Seismic Hazards Peak acceleration with 475 year ARP GNS Science Risk Frontiers Faults and Tectonics beneath Wellington – the Hikurangi subduction zone GNS Science 1995 Mw 6 9 Kobe Earthquake 1995 Mw 6.9 Kobe Earthquake Improving Building Performance – Kobe Earthquake damage statistics Kobe Earthquake damage statistics Reinforced Concrete Reinforced Concrete Steel Building code changes in 1971 and 1982 were very effective AIJ We Have Only Seen a Few of All the P ibl E h Possible Earthquakes in New Zealand k i N Z l d Earthquake recurrence intervals are hundreds Earthquake recurrence intervals are hundreds of years to tens of thousands of years Japan • • • • Earthquakes expected in the capital – i l Tokyo k Earthquake happened in Kobe in 1995 Previous earthquake occurred in Kobe in 1596 G Generally good performance ll d f of new buildings in 1995 New Zealand • • • • Earthquakes expected in the capital – it l Wellington W lli t Earthquakes happened in Christchurch in 2010‐11 No previous surface faulting Canterbury events in 15kyr G Generally good performance ll d f of new buildings in 2010‐11 except for soil failure Canterbury Plain Earthquake Sequence Canterbury Plain Earthquake Sequence GNS Science Canterbury Earthquake Sequence • The earthquakes occurred on previously unidentified faults that probably have not ruptured in the past 15,000 years • The aftershock sequence has been unusually long, consisting h f h k h b ll l progressive eastward propagation of seismic activity • The 4 Sept 2010 Mw 7.1 Darfield earthquake produced The 4 Sept 2010 Mw 7 1 Darfield earthquake produced expected levels of ground motions in Christchurch for that magnitude and distance, corresponding approximately to g , p g pp y 1/475 building code levels • The 22 Feb 2011 Mw 6.1 Christchurch earthquake produced ground motion levels in Christchurch much larger than expected, for reasons that relate to known seismic source and propagation effects. These levels correspond to an annual i ff Th l l d l probability of exceedance of about 1/2,500 ShakeMaps – Darfield & Christchurch ShakeMaps Darfield & Christchurch USGS Ratio of Christchurch to Darfield Peak Acceleration USGS Coincidence of Ground Motion Intensity and Building Density – Christchurch Event ShakeMap USGS Building Density USGS Ground Motion Model Elements: Source, Path and Site (GMPE) (PHYSICS‐BASED SIMULATION) Intra- Event Ground Motion Variability 2004 Niigata Chuetsu Earthquake H Hanging i wall ll Hiroe Miyake F t wall Foot ll Inter‐Event and Intra‐Event Variabilityy Al Atik et al., al 2014 Did the Christchurch Earthquake have a Higher Source Strength? have a Higher Source Strength? • Higher source strength (stress drop) would h h( d ) ld produce a high inter‐event term • Find out by comparing the recorded ground motions with the predictions of a ground motion prediction model p y earthquake ground q g • Use the pre‐Canterbury motion prediction model of Bradley (2010) Ground Motion vs Closest Distance Ground Motion vs Closest Distance Darfield About the same as or a bit lower than the model Christchurch Higher than model, < 10 km Lower than model, > 10 km Christchurch Ground Motions Not Consistently High at All Distances Inter and Intra Event Variability Al Atik et al., 2014 Feb 22 Christchurch Event Brendon Bradley Did the Christchurch earthquake have a Higher Source Strength? No. • The event term is a source parameter, not a path or site parameter • If the Christchurch event had a high event term, we would expect its ground motions to be large at all distances, which was not the case • This suggests that factors other than source, i.e. propagation path and site response, caused the ti th d it d th unexpectedly large ground motions in the CBD CBD Strong Motion Recording Sites Averaged CBD Response Spectra Averaged CBD Response Spectra Royal Commission Mw 7 1 Sept 4 2010 Darfield Earthquake Mw 7.1 Sept 4, 2010 Darfield Earthquake Ground Motion vs Ground Motion vs Closest Distance 1 sec Spectral Acc vs R CBD Response Spectra CBD Response Spectra Brendon Bradley Mw 6 2 Feb 22 2011 Christchurch Event Mw 6.2 Feb 22, 2011 Christchurch Event Ground Motion vs Closest Distance 1 sec Spectral Acc vs R CBD Response Spectra Brendon Bradley Christchurch Ground Motions: due to Higher Source Strength? due to Higher Source Strength? • The The Mw 6.2 22 Feb 2011 Christchurch Mw 6 2 22 Feb 2011 Christchurch earthquake ground motions were unusually high within 10 km but at ordinary levels g y beyond 10 km, so cannot be attributed to high source strength (stress drop) • Local conditions in Christchurch may have increased the ground motion levels: – Source: Rupture directivity effects – Path: Basin resonance effects – Site: Soil amplification effects Sit S il lifi ti ff t Large Near‐Fault Directivity Pulses Recorded in both Darfield and Christchurch Events Brendon Bradley Brendon Bradley Directivity y pulse recorded at Lyttelton y in the Darfield Earthquake • The directivity pulse is a shock wave analogous to sonic boom • It only occurs close to the fault and is different from source strength (stress drop) which affects ground motions ti att allll distances di t Ch i h h l Christchurch located on Sedimentary Basin d S di B i Brendon Bradley Basin: Estuarine Sediments Overly the Lyttelton Volcano Lawton et al. 2012 Trapping of Waves in Basins Fl L Flat Layers Basin Edge B i Ed Robert Graves Basin Waves – Christchurch Earthquake Basin Waves – Christchurch Earthquake Schematic Geology Schematic Geology Recorded Waveforms Recorded Waveforms Christchurch – on Quaternary basin Lyttelton – on volcanic rock Brendon Bradley Lyttelton, on bedrock, has just the directivity pulse. ChCh also has basin waves Basin Waves – Darfield Earthquake Basin Waves Darfield Earthquake Christchurch - CHHC Lyttelton - LPCC Lyttelton, on bedrock, has just the directivity pulse. ChCh also has basin waves Focusing of Energy by Rupture Directivity and Basin Effects Puente Hills Blind Thrust Los Angeles Directivity Long Beach – Basin Effect Robert Graves Soft Shallow Soils Soft Shallow Soils Tonkin & Taylor Difference between Rock and Soil Response Spectra ‐ Lyttelton The ground motion that enters the soil from below is presumably similar to that recorded on the adjacent rock site Brendon Bradley Conclusions: Christchurch Ground Motions • Local conditions in Christchurch may h have increased the ground motion i d h d i levels: – Source: Rupture directivity effects – Path: Basin resonance effects – Site: Soil amplification effects Higher Source Strength ‐ The New Normal? • Earthquake source strength ‐ Are source strengths of some categories of New Zealand earthquakes uniformly higher than we had thought? – No – This conclusion is relevant to all of New Zealand Thi l i i l ll f N Z l d – But we need to fully understand the conditions that caused the locally high ground motions in the Christchurch y g g earthquake and then assess where else in New Zealand such conditions may exist More Frequent Canterbury Earthquakes? q y q • Seismic hazard analysis usually assumes spatial and temporal randomness in earthquake occurrence • Evidence shows that earthquakes occur in d h h h k spatial and temporal clusters • GNS Science applied temporal and spatial GNS S i li d t l d ti l clustering to time‐varying seismic hazard analysis for Christchurch – the first such analysis for Christchurch the first such application worldwide GNS Time Dependent Hazard Model – p Christchurch (Gerstenberger et al., 2012) • An increased rate of earthquakes is expected to last for decades and far exceed the rate of earthquake occurrence in the previous 170 years h k h (by a factor of about 20) • 1/475 year peak acceleration increased from 1/475 k l ti i df 0.22g to 0.65g, higher than Wellington (0.4g) • 1/475 building code peak acceleration increased 1/475 building code peak acceleration increased from 0.22g to 0.35g; a compromise GNS and Building Code Spectra ‐ G S d ildi C d S Christchurch Gerstenberger et al., 2012 Alternative Seismic Source Models • Gerstenberger g et al. (2012) ( ) – Time‐varying; includes short term (aftershocks) and long term earthquake clustering • Bradley (2015) – Time‐varying; includes aftershock model • GNS (2014) GNS (2014) – Time‐independent GNS model with added aftershocks derived from Gerstenberger g et al (2012) ( ) • Calculations using these alternative models (next slide) produce very different results; this issue remains unresolved Response Spectra for 475 year ARP Response Spectra for 475 year ARP Calculated from Alternative Models Which model to use remains unresolved More Frequent Canterbury Events q y ‐ The New Normal? • More Frequent Canterbury Earthquakes – Are large earthquakes in the Canterbury Plain going to be much more frequent than before for decades to come? – D Don’t know ’ k – Has immediate implications for Canterbury Plain – Rather than simply apply a statistical earthquake forecast Rather than simply apply a statistical earthquake forecast model it would be preferable to seek physical evidence for changes (e.g. in stress level and orientation) and try to identify where earthquakes might occur identify where earthquakes might occur Effect of More Frequent Canterbury E th Earthquakes k • More frequent earthquakes increase the hazard level by causing a higher probability of experiencing larger and closer earthquakes, and more severe ground motions (above the median ground motion level) for motions (above the median ground motion level) for that magnitude and distance • Considerations related to existing buildings: Considerations related to existing buildings:* – Building codes do not require design for the strongest possible ground motions – Well engineered buildings have the capacity to withstand ground motions beyond the design level ground motions – this was demonstrated in the Christchurch earthquake this was demonstrated in the Christchurch earthquake *From this point I am expressing opinions about engineering; I am not an engineer Goal of Earthquake Engineering Maximise utility, defined as maximising Maximise utility defined as maximising total benefit, human‐centered on a moral f foundation, by balancing: d b b l – Demand vs. Capacity p y – Cost vs. Benefit Earthquake as Base Shear Demand Joe’s Beer! Food! ZICS W V R 2001 PEER Annual Meeting Joe’s Bar and Grill courtesy of Ron Hamburger Capacity of Building to Incur Drift (Lateral Displacement of Roof) due to Demand Base Shear Demand Joe’s Beer! Food! Beer! Food! Very rare events (2%/50yrs) Rare events (10%/50yrs) Occasional events (20%/50yrs) Frequent events (50%/50yrs) PEER Operational Life Safe Structurallyy Stable Lateral 2001 PEER Annual MeetingDeformation Performance Based Earthquake Engineering Performance‐Based Earthquake Engineering (PBEE) – Probability Framework Equation vDV G DV DM | dG DM EDP | dG EDP IM | d ( IM ) Impact Hazard Performance (Loss) Models and Simulation • DV - Decision Variable ($ loss, downtime, life-safety) • DM - Damage Measure (condition, necessary repairs,… ) • EDP – Engineering E i i D Demand dP Parameter t (drift, (drift acceleration acceleration, ...)) • IM - Intensity Measure (Sa, Sv, duration …) PEER Earthquake Source Strength – Implications for Policy • Existing ground motion prediction models are validated • Look for conditions in other urban areas that may resemble those that amplified the Ch i t h h Christchurch ground motions d ti • Continue orderly measures to reduce building vulnerability based on Christchurch data vulnerability based on Christchurch data, current knowledge and capabilities Earthquake Source Strength – q g Implications for Policy • The 22 Feb 2011 Christchurch earthquake was quite small magnitude 6.2 quite small – magnitude 6 2 • Earthquakes this small typically do not break the ground surface so the faults on which they could ground surface, so the faults on which they could potentially occur may be difficult to identify • Focus on identifying such faults in urban regions Focus on identifying such faults in urban regions More Frequent Canterbury Earthquakes More Frequent Canterbury Earthquakes – Implications for Policy • Building code officials need to clearly understand the basis of time varying hazard and understand the basis of time‐varying hazard and consider whether its use is suitable for their p p purposes • Abrupt large changes in seismic hazard level are difficult to reconcile with desired continuity in building codes and with existing building stock Residual Capacity – p y Policy Implications y p • The Christchurch earthquake has shown again how q g difficult it can be to relate ground shaking level to damage, and damage to residual capacity • Focus research on assessment of residual capacity of damaged buildings to meet requirements for: – Safety tagging and safety assessment – Insurance loss assessment – Decision to repair or demolish – Code mandated repair – Seismic certification of buildings Damage Control – g Policy Implications y p • The Christchurch earthquake has shown that building codes aimed primarily at life safety have building codes aimed primarily at life safety have been largely successful in that goal, but have not been effective at preventing losses • Focus research on design and retrofit innovations aimed at reducing losses as well as enhancing life safety: – Performance based design: the owner specifies acceptable damage states for various levels of t bl d t t f i l l f annual probability – Protective systems (base isolation) Protective systems (base isolation) – Self‐centering structural systems Cook Strait and Lake Grassmere Coo St a t a d a e G ass e e Earthquake Sequences, 2013 Earthquake Sequences GNS Science Grassmere Ground Motions GNS Science Fourth Announcement 10 th Pacific Conference on Ea r t h q u a ke E n g i n e e r i n g {& Annual Meeting of the AEES} Australian Earthquake Engineering Society Conference website aees.org.au/10pcee BUILDING AN EARTHQUAKE‐RESILIENT PACIFIC Sydney, Australia 6‐8 November 2015 New Zealand Society for Earthquake Engineering