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60 65 70 75
60 65 70 75
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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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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
40
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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
T
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
- L
AC
CA
DIV
E P
LAT
EA
U
N I
N E
T Y
E A
S T
R
I D
G E
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
40
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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)
0
20
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80
100
120
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180
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300
340
360
380
400
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480
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.
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