THE TYPE of earthquake and
amount of shaking expected
are factors in building code
requirements on the North
Shore and elsewhere.

QUAKE TIPS
By Ross Peterson

The two types of earthquakes
we should plan for have different
strengths and duration of shaking:

The first is the Joca) fault carth-
quake which scientists say should
be expected every few decades in
southwestern British Columbia. .

This carthquuke is caused by the
slipping or fracturing of rock layers
beneath us, Shaking could last a
few seconds to a minute or more.

This type of earthquake would
produce peak yround accelerations
(how quickly the ground moves
back and forth or up and down) of
0.2 times gravitational accelera- -
tion. Except for any sinking or stid-
ing of soils, it is this sideways
shaking which will cause damage
to buildings and structures,

This level of motion is consid-
ered in the National Building Code
of Canada, the B.C. Building Code
and local municipal code require-
ments for acceptable building
design. This type of earthquake is
therefore called the Design Level
Earthquake,

Importantly, the building code
requirements do not protect against
all damage to buildings from an
earthquake of this strength, but
rather they have been established
to save lives by requiring that
buildings stay structurally intact to
allow the safe evacuation of its
occupants.

The building itself may be non-
functional or non-inhabitabie after
the earthquake because of non-

' structural damage, including the
loss of its services.

There is no retroactivity to the
code requirements; even with peri-
Gdic upgrading of the codes, older
buildings would not be expected to
survive as well as newer ones.

The public can expect more
stringent building code require-
ments in the future as more is
learned about earthquake risk and
ways to reduce damage.

The second type, the offshore
subduction earthquake, occurs
much less frequently, but has a

STRAIT OF GEGRGIA

BOUNDARY
BAY

o_|
NEWS graphic Myra Alavoine

DEGREES OF danger for Lower Mainland residents differ pari-
ly because of the type of soil the area has. This map shows
areas of soil types from bedrock (lightest) to loose or soft soils
(darkest), The more darkly shaded areas are more likely to

break up during an earthquake.

much greater impact.

Shaking could last up to three or
four minutes, and have peak accel-
erations of 0.5 times gravity, which
is beyond the current building code
requirements.

Geoscientists believe that this
off-shore earthquake eecurs about
every 300-600 years, with the last
one 300 years ago. We are, there-
fore, about due for another one.

What do we know about our
buildings and what is being done to
improve their survivability?

Most of our public buildings
have been assessed for their seis-
mic strength; including public
schools, municipal halls, fire halls,
hospitals and police buildings.

This work will satisfy the cur-
rent requirements to withstand the
design level earthquake. The likely
damage from the larger and longer
duration subduction earthquake is
not clear, but we should be pre-
pared for more extensive damage
from this less frequent but perhaps
overdue event.

The North Shore municipalities
are also looking at other structures
such as municipal bridges and
retaining walls for retrofits or
replacement.

The public should be aware that
a number of utility companies and
other agencies are carrying out
similar programs for provincial
bridges, hydroelectric. telephone

and natural gas systems, dams,

water and sewer lines, railroads,
and port facilities.
Completion of these strengthen-

ing projects will give us a greater

assurance of minimal impact and
down time from at least the design
level earthquake.

The North Shore municipalities 9

will be encouraging the owners of
private buildings (including apart-
ment buildings, retail. office and
commercial buildings) to assess
their survivability as well.
Because ground shaking is
expected to vary in different parts

of the Lower MainJand, mainly

because of different soil types,
code requirements for buildings are
different for the North Shore com-
pared to Richmond or Delta.

In the future, we expect to be
able to make building requirements
specific to conditions found in dif-
ferent parts of the North Shore, in
order to protec! life and property
while being as fair as possible in
building regulations.

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