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Walter Salazar
Lloyd Lynch
Joan Latchman
Richard Robertson
National Consultation on Earthquake Safety in Trinidad and Tobago
July 2010
Trinidad and Tobago in thecurrent hazard picture for
the Eastern Caribbean
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Objectives
To present the level of Seismic Hazard
in terms of earthquake shaking for the
Eastern Caribbean region and Trinidad
and Tobago
To present a practical example of howto use the new seismic hazard maps
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AGENDA
p.35, 34
1. Seismicity Evaluation
2. Strong Ground Motion Estimation
3. Probabilistic Seismic Hazard Assessment
4. Practical example
5. Conclusions
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Question
Do you know the expected level of
earthquake shaking at your house or at
your work place in Trinidad ?
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Where do earthquakes occur ?
What is the size of the earthquakes ?
How frequently do earthquakes occur ?
1. Seism ic i ty Evaluation
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Where do earthquakes occur ? 15 Seismogenic zones
(Project collaboration with EUCENTRE-Italy)
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Main characteristics of the seismogenic zones
Depth (km) Type Main Focal Mechanism
ZONE 1 19.1 Upper-crustal Normal and Strike-Slip
ZONE 2 29.6 Interface Thrust (Inverse)
ZONE 3 29.4 Interface Thrust (Inverse)
ZONE 4 86.0 Intraplate Normal
ZONE 5 97.9 Intraplate NormalZONE 6 32.3 Interface Thrust and Strike-Slip
ZONE 7 28.4 Shallow Normal
ZONE 8 74.5 Intraplate Normal
ZONE 9 24.4 Transition Normal and Strike-Slip
ZONE 10 43.9 Transition/Intraplate Normal and Strike-Slip
ZONE 11 99.5 Intraplate Normal
ZONE 12 32.5 Crustal Normal and Strike-Slip
ZONE 13 23.3 Crustal Strike slip and Thrust
ZONE 14 14.7 Crustal Strike slip and Thrust
ZONE 15 57.3 Crustal Strike slip and Thrust
1
2
3
4
5
6
7
8
Seismic
Zones
that affect
T&T
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(Russo et al., 1993)
Zone 11: North of Paria Peninsula
This zone constitutes a
subducting detached oceanic
lithosphere with depth ranging
from 50 to 300 km and represents
one of the most active
seismogenic sources in the
Eastern Caribbean (Russo et al.
1993; SRC, 2009b).
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(10-11 N)
Trasversal section
-250
-200
-150
-100
-50
0
-67 -65 -63 -61 -59 -57
Long (km )
Dep (km)
USGS/PDE (1973-2009)
Transversal section
Crustal seismicity
and intra-plateseismicity
North of Paria
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How frequently do earthquakes occur ?
GUTENBERG-RICHTER
RECURRENCE RELATIONSHIPS LOG N = a + b * M
N: NUMBERS OF EARTHQUAKESPER YEAR ABOVE
MAGNITUDE M
a: LEVEL OF ACTIVITY
b: is the measure of the relative abundance of large
to small shocks
ZONE 1
y = -1.0122x + 4.7943
R2= 0.9882
-4.000
-3.500
-3.000
-2.500
-2.000
-1.500
-1.000
-0.500
0.000
0.500
4.5 5.0 5.5 6.0 6.5 7.0 7.5
MW
log(Mw)
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Zone 11
North of Paria Peninsula:
log N = 3.643 0.783 M
Recurrence interval* (T=1/N):
M=7.7: 243 years + 1766 = 2009
last big earthquake
M = 8.3 : 718 years
Log N = a + b M
Size of the earthquakes
*Recurrence interval: refers to the average time between earthquakes of a particular
magnitude or larger in a given seismic source
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12
Zones 13 and 14: El Pilar fault.
These zones comprise the boundary
between the Caribbean and the
South American plate. The events
that have their origin in the fault are
shallow - less that 50 km depth - and
they are characterized mainly by right
lateral strike slip mechanism in the
northern coast of South America.
We observed a high level seismic
output in Zone 13 that extends from
63.5 W to 62.3W longitude covering
the Araya-Paria Isthmus, and a
moderate seismicity level in Zone 14that extends from 67.0 to 63.5 W
longitude covering the vicinity of
Caracas to the Araya region.
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Zone 13
El Pilar Fault:
log N = 3.392 -0.747 M
Recurrence interval (T=1/N):M=7.2: 97 years
Log N = a + b M
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14
Right Lateral
Normal
Faulting
Transition Zone 10A.
We define these seismogenic
zones as the intersectionamongst the transform faults
and subduction zones with the
Lesser Antilles Arc located at
the South of the Eastern
Caribbean. Zone 10A includes the shallow
seismic activity in the South
part of the island of Tobago
which we consider within the
Caribbean-South Americanplate boundary (Latchman
2009, Weber, 2009 and
Burmester et al 1996). (Morgan
et al., 1988).
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15
Zone 10B: East of Trinidad.
Russo & Speed (1992)
suggested that theearthquakes located in this
zone are consistent with the
detachment and bending-
flexure of the South
American slab movingtoward the collision zone.
The zone covers mainly
normal faulting mechanism
with ENE-WSW striking
planes and strike slip faults
with an average depth of 45
km.
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Zone 10
Transition zones and east of Trinidad:
log N = 2.127 0.531 M
Recurrence interval (T=1/N):M=7.2: 50 years
Log N = a + b M
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Zone 12: Trinidad Faults.
Weber et al (2009)
Weber (2001, 2009) affirms that theN68E oblique trending in the Central
Range Fault is not associated with el
Pilar Fault 90 trending of pure
wrenching. The Central range fault
could be locked rather than creeping.
The Northern Range and the Arima
Fault comprises a complex fault
system with lateral strike-slip, thrust
and normal faulting.
Los Bajos Fault: south of Trinidad.
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Zone 12
Trinidad Faults:
log N = 2.580 0.664 M
Recurrence interval (T=1/N):M=6.9: 100 years
Log N = a + b M
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Zone 2-5: Subduction in the Lesser-Antilles.
We include in Zones 3 all the
shallow focus earthquakes(depth 50 km) along the
inclined inter-face seismic
zone that yields underthrust
focal mechanisms (Byrne et
al. 1988).
We include in Zone 5 deeper
intra-plate events (>50 km)
indicate that there is a normal
faulting resulting from initialflexure of the down going
Atlantic slab with an average
of westward dipping angle of
50 (Bengoubou-Valeruis et
al, 2008).OLD SUBDUCTION
ZONE (100 m.y)
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(12-14 N)
Trasversal section
-250
-200
-150
-100
-50
0
-67 -65 -63 -61 -59 -57
Long (km )
Dep (km)
USGS/PDE (1973-2009)
Transversal section
Upper Crustal seismicity and
inter and intra-plate
subduction seismicity
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Zone 3 (interface) and Zone 5 (intraplate):
Subduction zones:
log N = 3.216 0.725 M Interface depth 50 km
Recurrence interval (T=1/N):
M=7.5: 167 years Interface
M=8.3: 505 years Intraplate
Log N = a + b M
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Zone 15: South of Trinidad.
Russo et al. (1993) defined
this zone as a passivemargin edge in the Foreland
basin in North of South
America continent, covering
events with strike slip, mixedthrust and strike slip, and
thrust mechanism around the
Orinoco-Delta region in
Venezuela, with an average
depth of 50 km and a
maximum magnitude of 6.6
(Mw).
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2. Strong Ground Mot ion Est imat ion
The strong motion is a potential destructivemovement of the ground that can produce a
significant damage or the collapse of building
structures in the near field.
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DATE MW H EARTHQUAKE TYPE
NOVEMBER29, 2007 7.4 148.0 INTRAPLATE
OCTOBER4, 2000 6.1 110.4 INTRAPLATE
OCTOBER28, 2005 5.5 80.9 INTRAPLATE
NOVEMBER15, 2006 5.2 98.9 INTRAPLATE
OCTOBER24, 2005 5.1 137.7 INTRAPLATE
NOVEMBER17, 2006 4.9 135.8 INTRAPLATE
JANUARY25, 2001 4.6 85.5 INTRAPLATE
JUNE8, 1999 5.8 52.4 INTERFACE
DECEMBER2, 2004 5.8 48.2 CRUSTAL
DECEMBER3, 2004 5.4 40.5 CRUSTAL
JUNE21, 2003 5.3 10.0 SHALLOW
NOVEMBER21, 2004 6.3 21.2 UPPER CRUSTAL
VOLCANIC ARC
24
Seismic Research Centre (SRC)
BRGM (Bureau de Recherches
Gologiques et Minires - France)
Compare available recordings of
acceleration with available Ground
Motion Prediction Equations (GMPEs)
-North of Paria Peninsula
-Trinidad Faults
-Transition zone
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3. SEISMIC HAZARD MAPS
Logic Tree Formulat ion
Zone and Free-Zone Methods
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HAZARD MAP 2475 YEARS RETURN PERIODPEAK GROUND ACCELERATION
Port of Spain
San Fernando
Point Fortin
HAZARD MAP 475 YEARS RETURN PERIODPEAK GROUND ACCELERATION
Port of Spain
San Fernando
Point Fortin
Rock conditions Rock conditions
(0.33g)
(0.28g)
(0.26g)
(0.58g)
(0.52g)
(0.48g)
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Gravity constant: g = 980.1 cm/s/s
Peak ground acceleration* = 0.58*980.1 = 569 cm/s/s
Acceleration: How fast the rate of shaking changes
In 1 second there is a change of 569 cm/s : 21 km/h
*Peak Ground Acceleration (PGA): The maximum recorded
acceleration value in a particular site during and earthquake
What does 0.58 g mean ?
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Peak Ground Acceleration 0.5 g Peak Ground Acceleration 0.3 g
Las Colinas
Curva La Leona
Salazar and Seo (2003)
El Salvador M=7.8 Jan. 13, 2001
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M=7.8January 13, 2001
BOTH ARE INTRAPLATE SHOCKS
SAME DISTANCE (100 km) AND MAGNITUDE
7.8
What could happen if the earthquake occur
during the rainy season ?
El Salvador
Northern Range
Landslides in the photos
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Damaged house in Comasagua (La Libertad) with reinforced concrete frames
and reinforced brick walls El Salvador Earthquake 2001.
Peak Ground Acceleration 0.6 g
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Peak Ground Acceleration 0.6 g
Collapsed ADOBE house in Santiago de Mara(Usulutn)
El Salvador Earthquake 2001.
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Return period: the mean (average) timebetween occurrences of a seismic hazard
for example, a certain ground motion at asite
Recurrence interval: refers to the average
time between earthquakes of a particularmagnitude or larger in a given seismicsource
Life time (L): it refers to the effective life ofoccupancy of a building (generally 50 or 100years)
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34
Seismic Hazard maps
RP=95 years (10% in 10 years)PGA (g) 0.2s SA (g) 1s SA (g)
Resolution: every 0.025 degress = 2.8 km
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RP=475 years (10% in 50 years)
PGA (g) 0.2s SA (g) 1s SA (g)
Seismic Hazard maps
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RP=975 years (10% in 100 years)
PGA (g) 0.2s SA (g) 1s SA (g)
Seismic Hazard maps
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RP=2475 years (2% in 50 years)
PGA (g) 0.2s SA (g) 1s SA (g)
Seismic Hazard maps
DESIGN RESPONSE SPECTRUM
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0.2s
Spectral values
to be read in
the seismic
hazard maps
DESIGN RESPONSE SPECTRUM
Mapped Acceleration ParametersSs: is the acceleration for 0.2 s 5% damping
S1: is the acceleration for 1.0 s 5% damping
Ss
S1
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Why the hazard maps are
presented in terms of gravity g
EARTHQUAKE INERTIA FORCES: F = m * a
m: mass of the building; m = W/g
a : acceleration
Engineers use Weight W for structural calculations:
If W = 130 ton and a=0.4 g
F= m * a = (130 ton/g) * 0.4 g = 52 ton.
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RP=2475 years (2% in 50 years)
1s SA (g)
Port of Spain
San Fernando
Point Fortin
North of Paria Peninsula
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4. PRACTICAL EXAMPLE
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Practical Example
Get the design response spectrum and theseismic coefficients Cs for the following
site:
Scarborough -Tobago (Hotel 20 stories)
LIFE TIME = 50 YEARS AND
PROBABILITY OF EXCEEDANCE 2% :
2475 YEARS RETURN PERIOD
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Tobago
Hotel
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44
Design spectral acceleration parameters IBCASCE 7_05
SDS = 2/3*Fa * Ss
SD1 = 2/3*Fv * S1
Fa and Fv: depends on soil conditionsFor rock site conditions CLASS B
It Corresponds to a shear wave velocity Vs = 760 m/s:
Fa = 1.0 and Fv = 1.0
Spectral acceleration for 1.0 s
Seismic
Hazard
maps
Spectral acceleration for 0.2 s
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Ss=1.85 g
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S1=0.375 g
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The Seismic Coefficient Cs
Fundamental Period:
T = 0.1 n = 0.1 (20 ) = 2.0 s
n: number of stories
Fundamental Period T = 2.0 s
Cs = 0.13g
Reduction factor Ductility
and Overstrengh R= 8.0
Cs = 0.13g/8
Cs = 0.016 g
1 0.25Da
S gST T
1.23a DSS S g
0.20s
0.04s
0.492 18.45a
S T
0.13 g
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48ETABS STRUCTURAL MODEL
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Conclusions
- The complex tectonics of the Eastern Caribbean suggests
a detail seismogenic sources delimitation to be
incorporated in the hazard assessment, which is
dominated by intra-plate seismicity in most of the islands,
including TRINIDAD AND TOBAGO
- South West of Tobago, Toco and Chaguaramas have thehighest seismic hazard level in terms of Peak Ground
Acceleration (0.6 g for a RP=2475 years; 0.33 g for a
RP=475 years) for rock site conditions.
- For Port of Spain the expected peak ground acceleration
level is 0.58 g RP=2475 years; 0.33 g forRP=475 years;
for rock site conditions.
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50
In f luence of seismogenic zones
Island of Trinidad
Cornell-McGuire approach
Hazard dominated by Zone 11
Port of Spain (Trinidad)
RP=2475 years
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
0.00 0.50 1.00 1.50 2.00 2.50 3.00
T(s)
SA(
g)
SZ01
SZ02
SZ03
SZ04
SZ05
SZ06
SZ07
SZ08
SZ09
SZ10
SZ11
SZ12
SZ13
SZ14
SZ15
ALL
Transition zone
North of Paria P.
Trinidad Faults
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What is the next job at UWI - SRC with
a possible collaboration of T&Tgovernment ?
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52
Rodriguez (2003)
Site effect
SITE EFFECTS EVALUATION
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SITE EFFECTS EVALUATION
Site effects: amplification of motion due to the
presence of sedimentation (soil) - geotechnicalhazard
We need a SEISMIC MICROZONATION:
A map of a small within a country, e.g. a City, thatdetails the different levels of a specified
geotechnical hazard that may be triggered by anearthquake constitutes a Seismic Microzonation.
Collaboration project with Ministry of Planning
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Proposed single-mobile microtremors measurement sites (red points) at Port of Spain, Trinidad.
Applying the well-known H/V ratio technique proposed by Nakamura (1989) we can obtain the
fundamental period of the soil profile
at each measuring site.
SCIENTIFIC EQUIPMENT
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GEOPHON
E
LINE OF
GEOPHONES
12
3
4
5
6 7
SEISMIC REFRACTIONMICROTREMORS ARRAY
STRONGMOTION
INSTRUMENTGPS
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THANK YOU
Questions ?