60 65 70 75

60 65 70 75

10

5

0

5

10

5

0

5

C A R L S B E R G R I D G E

M A L D I V E S

I N D I A NO C E A N

S O M A L I P L A T E

I N D I A N P L A T E

A U S T R A L I A N P L A T E

B R I T I S HI N D I A N O C E A N

T E R R I T O R Y

M a l d i v e I s l a n d s

Addu Atoll

HadhdhunmathiAtoll

Huvadhu Atoll

KolhumadhuluAtoll

MulaakatholhuAtoll

Ari Atoll

Male AtollMale

C h a g o sA r c h i p e l a g o

Diego Garcia

M I D

- I N D

I A N

R I D

G E

S a y a d e M a l h aB a n k

Vema

Tren

ch

V e m

a F

r a c

t u r e

Z o

n e

C H

A G

O S

- L A C

C A

D I V

E R

I D G

E

Epicentral Area

Hitaddu

Point MarianneEast Point Plantation

Map prepared by U.S. Geological SurveyNational Earthquake Information Center2 September 2003Map not approved for release by Director USGS

DATA SOURCES

EARTHQUAKES AND SEISMIC HAZARD USGS, National Earthquake Information Center NOAA, National Geophysical Data Center IASPEI, Centennial Catalog (1900 - 1999) Handbook of Seismology and Earthquake Engineering Global Seismic Hazard Assessment Program

PLATE TECTONICS Smithsonian Institution, Global Volcano Program

BASE MAP NIMA and ESRI, Digital Chart of the World USGS, EROS Data Center

NEWS REPORT Haveeru Daily Online, Male, Maldives

EARTHQUAKE SUMMARY MAP XXXU.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

DISCLAIMER

Base map data, such as place names and politicalboundaries, are the best available but may not be current or may contain inaccuracies and thereforeshould not be regarded as having official significance.

M7.6 Carlsberg Ridge Earthquake of 15 July 2003

50 55 60 65 70 75 80 85 90

50 55 60 65 70 75 80 85 90

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15

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I N D I A N O C E A N

SRI LANKAINDIA

MADAGASCAR

MAURITIUS

SEYCHELLES

SOMALIA

MALDIVES

BRITISH INDIAN OCEAN TERRITORY

M I D

- I N D

I A N

R I D

G E

C A

R L S B E R G R I D G E

C H

A G

O S

- L A C

C A

D I V

E P

L A T

E A

U

MA S C A R E N E P L A T E A

U

S O M A L IB A S I N

A R A B I A N B A S I N

M A S C A R E N EB A S I N

M I D - I N D I A NB A S I N

N I

N E

T Y

- E

A S

T

R I

D G

E

Cha

gos

Tre

nch

Vema

Frac

ture Z

one

Owe

n Fr

actu

re Z

one

Rodrigues

Diego Garcia

MaldiveIslands

ChagosArch.

Seychelles

Tectonic Setting

DETAILED MAP

I N D I A N P L A T E

S O M A L I P L A T E

A U S T R A L I A N P L A T E

Amirante Tre

nch

Saya deMalhaBank

Male

Colombo

33 mm/yr

38 mm/yr

0 500 1,000 1,500 2,000250

Kilometers

SCALE 1:22,500,000 at the EquatorMercator Projection

CARLSBERG RIDGE, INDIAN OCEAN

15 July 2003 20:27:50.21 UTC2.562 S., 68.300 E.Depth 10 kmMw = 7.6 (USGS)

This north- central Indian Ocean earthquake was felt strongly in the Republicof the Maldives. The shock was felt most strongly in the southernmost atolls of the Maldives (Gaafu, Dhalalu, Fuvah Mulah, and Addu) and in Male', the capital city; no injuries or damage were reported.

0 100 200 300 40050

Kilometers

SCALE 1:4,500,000 at the EquatorMercator Projection

30 40 50 60 70 80 90 100 110 120

30 40 50 60 70 80 90 100 110 120

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I N D I A N O C E A N

A U S T R A L I A N P L A T E

S O MA L I P L A T E

A F R I C A N P L A T E

I N D I A N P L A T E

E U R A S I A NP L A T E

M I D

- I N D

I A N

R I D

G E

S O U

T H W

E S

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I N D

I A

N R

I D G E

C A R L S B E R G R

I D G

E CH

AG

OS

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AC

CA

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LAT

EA

U

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S T

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I D

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A N T A R C T I CP L A T E

M A S C A R E N

E P

L A T E A U

BROKEN RIDGE

DETAILED MAP

Indian Ocean Plate Tectonics

A R A B I A N P L A T E

0 1,000 2,000 3,000 4,000500

Kilometers

SCALE 1:55,000,000 at the EquatorMercator Projection

EXPLANATIONMain Shock

15 July 2003

Largest Earthquakes 1900 - 2002

0 - 69 km

70 - 299

300 - 700

Plate Boundaries

Convergent

Divergent

Transform

Diffuse

Volcanoes

30 40 50 60 70 80 90 100 110 120

30 40 50 60 70 80 90 100 110 120

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I N D I A N O C E A N

A U S T R A L I A N P L A T E

S O MA L I P L A T E

A F R I C A N P L A T E

I N D I A N P L A T E

E U R A S I A NP L A T E

A N T A R C T I CP L A T E

DETAILED MAP

Generalized Seismic Hazard

A R A B I A N P L A T E

EXPLANATIONSeismic Hazard

0.0 - 0.2 m/sec

0.2 - 0.4

0.4 - 0.8

0.8 - 1.6

1.6 - 3.2

3.2 - 64.

6.4 - 9.8

Main Shock

15 July 2003

Largest Earthquakes 1900 - 2002

0 - 69 km

70 - 299

300 - 700

Plate Boundaries

Convergent

Divergent

Transform

Diffuse

Volcanoes

0 1,000 2,000 3,000 4,000500

Kilometers

SCALE 1:55,000,000 at the EquatorMercator Projection

Seismic hazard is expressed as peak ground acceleration (PGA) on firm rock,in meters/sec, expected to be exceededin a 50-yr period with a probability of 10 percent.

EXPLANATIONMain Shock

15 July 2003

Aftershocks 15 - 28 July 2003

4.5 - 4.8

5.0 - 5.4

5.5 - 5.9

Plate Boundaries

Divergent

Transform

Diffuse

Populated Places

Hitaddu

EXPLANATIONMain Shock

15 July 2003

Plate Boundaries

Divergent

Transform

Diffuse

DISCUSSION

This earthquake occurred on the Carlsberg Ridge, a mid- ocean ridge system that is located in the Arabian sea between India and Northern Africa. The ridge marks the boundary between the Indian and African plates. Near the epicenter the Indian plate is moving away from the African Plate at a rate of 33 mm/yr in a northeasterly direction. The Carlsberg Ridge is a slow- spreading ridge with rough topography and a depth that varies from 1700- 4400 meters. Mid- ocean ridges are divergent plate boundaries, where two tectonic plates move apart from each other. New oceanic crust is formed as magma rises up between the two diverging plates. Active spreading ridges are offset by zones known as transform faults, where plates slide horizontally past each other neither destroying nor forming crust. This gives the plate boundary a zigzag pattern. Ocean ridges repre-sent the longest, linear uplifted features of the earth's surface and are marked by a belt of shallow earthquakes. Earthquakes can be caused by the release of tensional stress in the uplifted ridge or by the horizon-tal movement of plates along the transform faults.

Finite Fault Model

Strik

e S3

4W

Down-Dip 700 20 40 60 8010

Kilometers

EXPLANATIONAmount of Slip (cm)

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FINITE FAULT MODEL (C. Ji, California Institute of Technology)

Distribution of the amplitude and direction of slip for subfault elements (small rectangles) of the fault rupture model are determined from the inversion of teleseismic body waveforms. Arrows indicate the amplitude and direction of slip (or rake vector); the slip amount is also color-coded as shown. The view is from above, perpendicular to the fault plane.

The fact that the orientation of the finite fault plane may differ from the corresponding best double couple nodal plane depicted in the focal mechanism diagram on the main map reflects differing methods of analysisand data sets.

The strike of the fault rupture plane is S34W and the dip is 70. The dimensions of the subfault elementsare 10 km in the strike direction and 5 km in the dipdirection.

FINITE FAULT MODEL

SCALE 1:1,500,000

RELATIVE PLATE MOTIONS

The relative motion of adjacent tectonicplates is depicted on the map by short vectors located at selected locationson the plate boundary. In this present-ation, one plate is assumed to be rigid and fixed. The vector therefore repre-sents the direction of the moving platerelative to the fixed plate. The rate of relative motion is labelled next tothe vector.

The components of the vector perpen-dicular and parallel to the plate margin approximate convergent/divergent and transverse direction of motion between the plates, respectively. As viewed from the fixed plate, an inward directed com-ponent suggests convergence at and near the plate boundary that may be ex-pressed as crustal folding, uplift, thrustfaulting, or plate subduction. Similarly, an outward directed component sug-gests plate divergence such as would be expected at a zone of crustal spread-ing. Transcurrent or transform faultingwould be expected when the predom-inant vector component is parallel to theplate margin.