Trinidad and Tobago in the current hazard picture for the...

The University of the West Indies Seismic Research Centre email: uwiseismic@uwiseismic.com

Walter Salazar

Lloyd Lynch

Joan Latchman

Richard Robertson

National Consultation on Earthquake Safety in Trinidad and Tobago

July 2010

Trinidad and Tobago in the current hazard picture for

the Eastern Caribbean

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 how

to use the new seismic hazard maps

AGENDA

p.35, 34

• 1. Seismicity Evaluation

• 2. Strong Ground Motion Estimation

• 3. Probabilistic Seismic Hazard Assessment

• 4. Practical example

• 5. Conclusions

Question

• Do you know the expected level of

earthquake shaking at your house or at

your work place in Trinidad ?

• Where do earthquakes occur ?

• What is the size of the earthquakes ?

• How frequently do earthquakes occur ?

1. Seismicity Evaluation

Where do earthquakes occur ? 15 Seismogenic zones

(Project collaboration with EUCENTRE-Italy)

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 6 32.3 Interface Thrust and Strike-Slip

ZONE 7 28.4 Shallow Normal

ZONE 9 24.4 Transition Normal and Strike-Slip

ZONE 10 43.9 Transition/Intraplate Normal and Strike-Slip

ZONE 12 32.5 Crustal Normal and Strike-Slip

ZONE 13 23.3 Crustal Strike slip and Thrust

Seismic

that affect

(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).

(10-11 N)

Trasversal section

-67 -65 -63 -61 -59 -57

Long (km)

Dep (km)

USGS/PDE (1973-2009)

Transversal section

Crustal seismicity

and intra-plate

seismicity

North of Paria

How frequently do earthquakes occur ?

GUTENBERG-RICHTER

RECURRENCE RELATIONSHIPS

• LOG N = a + b * M

N: NUMBERS OF EARTHQUAKES

PER 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

4.5 5.0 5.5 6.0 6.5 7.0 7.5

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

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

63.5º W to 62.3ºW longitude covering

the Araya-Paria Isthmus, and a

moderate seismicity level in Zone 14

that extends from 67.0º to 63.5º W

longitude covering the vicinity of

Caracas to the Araya region.

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

Right Lateral

Normal

Faulting

Transition Zone 10A.

• We define these seismogenic

zones as the intersection

amongst 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 American

plate boundary (Latchman

2009, Weber, 2009 and

Burmester et al 1996). (Morgan

et al., 1988).

Zone 10B: East of Trinidad.

• Russo & Speed (1992)

suggested that the

earthquakes located in this

zone are consistent with the

detachment and bending-

flexure of the South

American slab moving

toward 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

Zone 10

• Transition zones and east of Trinidad:

log N = 2.127 – 0.531 M

M=7.2: 50 years

Log N = a + b M

Zone 12: Trinidad Faults.

Weber et al (2009)

• Weber (2001, 2009) affirms that the

N68ºE oblique trending in the Central

Range Fault is not associated with el

Pilar Fault 90º 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.

Zone 12

• Trinidad Faults:

log N = 2.580 – 0.664 M

M=6.9: 100 years

Log N = a + b M

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 initial

flexure of the down going

Atlantic slab with an average

of westward dipping angle of

50º (Bengoubou-Valeruis et

al, 2008).OLD SUBDUCTION

ZONE (100 m.y)

(12-14 N)

Trasversal section

-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

Zone 3 (interface) and Zone 5 (intraplate):

• Subduction zones:

log N = 3.216 – 0.725 M Interface depth <= 50 km

log N = 2.941 – 0.680 M Intraplate depth > 50 km

M=7.5: 167 years Interface

M=8.3: 505 years Intraplate

Log N = a + b M

Zone 15: South of Trinidad.

• Russo et al. (1993) defined

this zone as a passive

margin edge in the Foreland

basin in North of South

America continent, covering

events with strike slip, mixed

thrust 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

2. Strong Ground Motion Estimation

The strong motion is a potential destructive

movement of the ground that can produce a

significant damage or the collapse of building

structures in the near field.

DATE MW H EARTHQUAKE TYPE

NOVEMBER 29, 2007 7.4 148.0 INTRAPLATE

OCTOBER 4, 2000 6.1 110.4 INTRAPLATE

JANUARY 25, 2001 4.6 85.5 INTRAPLATE

JUNE 8, 1999 5.8 52.4 INTERFACE

DECEMBER 2, 2004 5.8 48.2 CRUSTAL

DECEMBER 3, 2004 5.4 40.5 CRUSTAL

JUNE 21, 2003 5.3 10.0 SHALLOW

NOVEMBER 21, 2004 6.3 21.2 UPPER CRUSTAL

VOLCANIC ARC

• Seismic Research Centre (SRC)

• BRGM (Bureau de Recherches

Géologiques et Minières - France)

Compare available recordings of

acceleration with available Ground

Motion Prediction Equations (GMPEs)

-North of Paria Peninsula

-Trinidad Faults

-Transition zone

3. SEISMIC HAZARD MAPS

Logic Tree Formulation

Zone and Free-Zone Methods

HAZARD MAP 2475 YEARS RETURN PERIOD

PEAK GROUND ACCELERATION

Port of Spain

San Fernando

Point Fortin

HAZARD MAP 475 YEARS RETURN PERIOD

PEAK 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)

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 ?

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

January 13, 2001

BOTH ARE INTRAPLATE SHOCKS

SAME DISTANCE (100 km) AND MAGNITUDE

What could happen if the earthquake occur

during the rainy season ?

El Salvador

Northern Range

Landslides in the photos

Damaged house in Comasagua (La Libertad) with reinforced concrete frames

and reinforced brick walls – El Salvador Earthquake 2001.

Peak Ground Acceleration 0.6 g

Collapsed ADOBE house in Santiago de María(Usulután)

El Salvador Earthquake 2001.

• Return period: the mean (average) timebetween occurrences of a seismic hazard –for example, a certain ground motion at asite

• Recurrence interval: refers to the averagetime 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)

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

Seismic Hazard maps

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

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.

1s SA (g)

Port of Spain

San Fernando

Point Fortin

North of Paria Peninsula

4. PRACTICAL EXAMPLE

Practical Example

• Get the design response spectrum and the

seismic coefficients Cs for the following

Scarborough -Tobago (Hotel – 20 stories)

LIFE TIME = 50 YEARS AND

PROBABILITY OF EXCEEDANCE 2% :

2475 YEARS RETURN PERIOD

Tobago

Design spectral acceleration parameters IBC – ASCE 7_05

• SDS = 2/3*Fa * Ss

• SD1 = 2/3*Fv * S1

Fa and Fv: depends on soil conditions

For 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

Spectral acceleration for 0.2 s

Ss=1.85 g

S1=0.375 g

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

1.23a DSS S g

0.492 18.45aS T

0.13 g

48ETABS STRUCTURAL MODEL

• 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 the

highest 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 for RP=475 years;

for rock site conditions.

Influence of seismogenic zones

Island of Trinidad

Cornell-McGuire approach

Hazard dominated by Zone 11

Port of Spain (Trinidad)

RP=2475 years

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Transition zone

North of Paria P.

Trinidad Faults

• What is the next job at UWI - SRC with

a possible collaboration of T&T

government ?

Rodriguez (2003)

Site effect

SITE EFFECTS EVALUATION

• Site effects: amplification of motion due to the presence of sedimentation (soil) - geotechnical hazard

We need a SEISMIC MICROZONATION:

A map of a small within a country, e.g. a City, that details the different levels of a specified

geotechnical hazard that may be triggered by an earthquake constitutes a Seismic Microzonation.

Collaboration project with Ministry of Planning

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.

GEOPHON

LINE OF

GEOPHONES

SEISMIC REFRACTIONMICROTREMORS ARRAY

STRONG

MOTION

INSTRUMENTGPS

SCIENTIFIC EQUIPMENT

THANK YOU

Questions ?

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