Commentary by Dr Peter Stafford on the second Canterbury earthquake

1. The
‘second’
Christchurch
earthquake,
February
22,
2011
Dr
Peter
J.
Stafford
Willis
Research
Fellow
RCUK
Fellow
/
Lecturer
in
Modelling
Engineering
Risk
Department
of
Civil
&
Environmental
Engineering
Imperial
College
London
Note:
Dr
Stafford
was
born
in
Christchurch
and
has
family
and
friends
living
in
the
city.
He
completed
his
undergraduate
degree
and
his
PhD
at
the
University
of
Canterbury,
in
Christchurch.
Widespread
damage
has
been
caused
by
the
ground
shaking
induced
by
the
ML6.3
magnitude
earthquake
located
just
to
the
south
of
Christchurch.
The
event
is
much
closer,
and
much
shallower,
than
the
larger
ML7.1
earthquake
that
occurred
in
September
2010.
This
is
the
primary
reason
why
the
levels
of
ground
shaking
that
have
been
observed
are
so
severe.
Initial
reports
from
GNS
Science
in
New
Zealand
(a
government
research
institute
looking
at
Geological
and
Nuclear
Sciences
–
GNS)
indicate
that
the
levels
of
shaking
that
were
observed
in
and
around
Christchurch
are
significantly
greater
than
the
levels
that
structures
are
typically
designed
to
withstand.
It
is
therefore
no
surprise
that
many
structures
have
suffered
extensive
levels
of
damage,
and
that
some
have
collapsed.
http://www.geonet.org.nz/var/storage/images/media/images/news/2011/lyttelton_pga/57
159-­‐1-­‐eng-­‐GB/lyttelton_pga.png

2. Images
such
as
that
above,
taken
from
the
GeoNet
website,
suggest
that
peak
ground
accelerations
reached
as
high
as
1.9
times
the
acceleration
due
to
gravity
(although
it
is
not
clear
whether
these
are
recorded
in
the
horizontal
or
vertical
direction
–
horizontal
accelerations
are
those
most
relevant
for
estimating
damage
to
buildings).
By
any
measure,
accelerations
of
this
level
are
considered
to
be
very
large.
For
emergency
response
teams
it
is
now
crucial
to
ensure
that
survivors
currently
trapped
in
debris
from
collapsed
buildings
are
extracted
as
soon
as
possible.
Teams
of
people
will
be
working
through
the
night,
in
very
trying
conditions
involving
further
aftershocks,
but
the
likelihood
of
people
surviving
will
reduce
rapidly
with
time.
This
event
is
almost
certainly
an
aftershock
of
the
larger
event
that
occurred
in
September.
This
large
event
caused
damage
to
structures
throughout
the
city
that
was
thought
to
be
primarily
superficial
(affecting
what
structural
engineer’s
refer
to
as
non-­‐structural
elements,
e.g.,
facades,
in-­‐fill
panels,
non-­‐load-­‐bearing
walls
and
partitions
etc).
However
it
is
too
early
to
say
whether
some
of
the
collapses
that
have
occurred
have
resulted
from
undetected
damage
caused
by
the
first
major
event.
The
recovery
efforts
following
the
first
major
event,
while
sustained
and
concerted,
are
far
from
compete
and
a
lot
of
work
remained
in
order
to
bring
the
city
back
to
a
normal
state
even
prior
to
the
more
recent
event
that
occurred
this
morning.
The
problem
for
many
home-­‐
owners
now
will
be
associated
with
the
uncertainty
regarding
how
and
when
their
homes
will
be
repaired.
The
New
Zealand
Earthquake
Commission
(EQC)
must
again
provide
financial
compensation
for
the
losses
sustained
by
the
earthquake
and
this
will
place
a
heavy
burden
upon
them.
With
estimates
of
direct
losses
on
the
order
of
2
billion
NZD
resulting
from
the
first
event,
it
is
likely
that
this
new,
more
damaging,
event
will
send
direct
losses
many
times
above
this
level.
And,
of
course,
the
indirect
cost
to
businesses
is
also
likely
to
be
very
significant.
Aftershocks
of
this
size
are
to
be
expected
following
a
major
earthquake.
As
a
general
‘rule-­‐of-­‐
thumb’
the
largest
expected
aftershock
in
a
sequence
will
typically
have
a
magnitude
that
is
roughly
one
unit
of
magnitude
lower
than
the
mainshock.
Note,
however,
that
a
unit
change
in
magnitude
relates
to
an
approximately
30-­‐fold
difference
in
the
amount
of
energy
that
is
released
from
the
earthquake
source.
Aftershock
events
are
defined
as
being
dependent
upon
the
mainshock.
However,
it
is
not
always
clear
whether
an
earthquake
is
really
an
aftershock
or
not.
The
relevance
of
this
point
is
that
in
many
countries,
insurance
policies
are
tied
to
covering
one
‘act-­‐of-­‐God’
type
event
per
annum.
If
the
damage
caused
by
the
aftershock
is
deemed
to
be
part
of
the
main
event
then
one
may
be
covered
by
insurance,
if
the
event
is
deemed
to
be
independent
then
coverage
may
not
be
guaranteed.
The
implications
of
this
are
great
for
events
such
as
this
where
the
aftershock
is
going
to
be
responsible
for
significantly
greater
degrees
of
loss
than
the
mainshock
event.
Fortunately,
for
those
affected
by
this
event,
their
coverage
through
the
EQC
is
not
structured
in
this
way.
A
very
good
idea
of
the
types
of
events
that
the
citizens
are
experiencing
now
can
be
seen
from
the
website
below
that
plots
the
aftershocks
that
have
occurred
over
recent
times.
http://www.christchurchquakemap.co.nz/