Download Christchurch Earthquake

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
vDV    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