Seismic
Hazard
Assessment
of Metro
Mania
(A
Foundation
Engineering
Analysis)
Arthur
Saldivar-Sali,
MSc, DIC,
FGS, PhD
Consulting
Geological
Engineer
Abstract
To the
construction
engineer,
a
critical
step in
the
design
of a
building
or in
the
choice
of a
housing
development
site in
an
earthquake
area is
the
selection
of a
suitable
location.
For this
purpose,
a number
of
earthquake-related
hazards
should
always
be
considered
including
soil
liquefaction,
ground-shaking
amplification,
large
permanent
ground
deformation
and
slope
failure.
Metro
Manila’s
susceptibility
to
seismic
hazards
has been
empirically
proven
time and
again.
With its
“young”,
variable
geology
and
relatively
flat
terrain,
these
hazards
need to
be
evaluated
on the
basis of
the
influence
of the
subsurface
conditions
on the
ground
surface
motion
that
buildings
may
expect
to
experience
in an
earthquake.
This
paper
identifies
and maps
the
effect
of
earthquakes
on the
stability
of
building
foundations
within
the
metropolis
as a
result
of this
pronounced
variation
in
geological
conditions
immediately
below
subsurface.
The
critical
local
geological
conditions
that
influence
the
suitability
of a
site in
Metro
Manila
are:
-
soil
conditions
which
can
amplify
ground
motions
in
certain
frequency
ranges
and
-
susceptibility
to
liquefaction,
subsidence,
and
slope
failure.
Based on
the
correlation
of
surface
geology
with
historical
records
of
damage
wrought
by
earthquakes,
Metro
Manila
is
divided
into
four
zones of
varying
susceptibility
to
seismic
hazards.
These
range
from
very
low-risk
(large
parts of
Quezon
City,
Makati,
and
Paranaque),
low-risk
(Pasay,
East
Marikina),
high-risk
(Ermita,
Navotas,
South
Marikina)
and
very
high-risk
(major
sectors
of
downtown
Manila,
south
Marikina
and
Pateros)
zones.
On
record,
the
highest
concentration
of
damage
to first
class
buildings
during
the
August
1968
earthquake
(M6.8)
and July
1990
earthquake
(M7.8)
was in
the
vicinity
of the
Pasig
River
delta.
It is
also
shown by
the
severity
of
damage
that the
greater
the
intensity
of an
earthquake
the more
pronounced
are
these
differences
in
susceptibility
among
the four
identified
seismic
microzones.
Finally,
a
land-use
and
building
constraint
map is
proposed
for
Metro
Manila
based on
the
established
seismic
microzones.