Christchurch Earthquake Christchurch Earthquake ‐ New Normal or Old Normal, and Implications for Policy Professor Paul Somerville Chief Geoscientist Risk Frontiers, Macquarie University
Outline 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): Engineer (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 source strength ‐ Are the source • Greater source strength ‐ 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 Risk Frontiers GNS Science
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 Steel AIJ Building code changes in 1971 and 1982 were very effective
We Have Only Seen a Few of All the Possible Earthquakes in New Zealand P ibl E h 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 New Zealand • Earthquakes expected in the Earthquakes expected in the • capital – Wellington it l W lli t capital – Tokyo i l k • Earthquakes happened in • Earthquake happened in Kobe Christchurch in 2010 ‐ 11 in 1995 • No previous surface faulting • Previous earthquake occurred Canterbury events in 15kyr in Kobe in 1596 G Generally good performance ll d f • • G Generally good performance ll d f of new buildings in 2010 ‐ 11 of new buildings in 1995 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 USGS
Ratio of Christchurch to Darfield Peak Acceleration USGS
Coincidence of Ground Motion Intensity and Building Density – Christchurch Event ShakeMap Building Density USGS USGS
Ground Motion Model Elements: Source, Path and Site (GMPE) (PHYSICS ‐ BASED SIMULATION)
Intra- Event Ground Motion Variability 2004 Niigata Chuetsu Earthquake F Foot wall t ll H Hanging wall i ll Hiroe Miyake
Inter ‐ Event and Intra ‐ Event Variability Al Atik et al Al Atik et al., 2014 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 • Use the pre ‐ Canterbury earthquake ground p y q g motion prediction model of Bradley (2010)
Ground Motion vs Closest Distance Ground Motion vs Closest Distance Darfield Christchurch About the same as or a bit Higher than model, < 10 km lower than the model Lower than model, > 10 km
Christchurch Ground Motions Not Consistently High at All Distances Inter and Intra Event Variability Feb 22 Christchurch Event Brendon Bradley Al Atik et al., 2014
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 CBD Response Spectra CBD Response Spectra Closest Distance Brendon Bradley 1 sec Spectral Acc vs R
Mw 6 2 Feb 22 2011 Christchurch Event Mw 6.2 Feb 22, 2011 Christchurch Event Ground Motion vs CBD Response Spectra Closest Distance Brendon Bradley 1 sec Spectral Acc vs R
Christchurch Ground Motions: due to Higher Source Strength? due to Higher Source Strength? • The Mw 6 2 22 Feb 2011 Christchurch • The 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 pulse recorded at Lyttelton in the Darfield Earthquake y y • The directivity pulse is a shock wave analogous to sonic boom sonic boom • It only occurs close to the fault and is different from source strength (stress drop) which affects ground motions at all distances ti t ll di t
Ch i Christchurch located on Sedimentary Basin h h l d S di B i Brendon Bradley
Basin: Estuarine Sediments Overly the Lyttelton Volcano Lawton et al. 2012
Trapping of Waves in Basins Fl Flat Layers Basin Edge L B i Ed Robert 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 Puente Hills Blind Thrust Thrust Los Angeles - Directivity Long Beach – Basin Effect Robert 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 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
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