Trinidad and Tobago in the current hazard picture for the Eastern Caribbean

8/10/2019 Trinidad and Tobago in the current hazard picture for the Eastern Caribbean

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The University of the West Indies Seismic Research Centre email: [email protected]

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)


7/55The University of the West Indies Seismic Research Centre email: [email protected]

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 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



1

2

3

4

5

6

7

8

Seismic

Zones

that affect

T&T



(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

-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



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)



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



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.



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



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).



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.



Zone 10

Transition zones and east of Trinidad:

log N = 2.127 0.531 M


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


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





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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Seismic Hazard maps


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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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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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45

Ss=1.85 g


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46

S1=0.375 g


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47

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 ?

Documents

Trinidad and Tobago in the current hazard picture for the Eastern Caribbean