2-77

52mm/yr

S F

57mm/yr

60mm/yr

Figure 2.1. Regional tectonic setting of the Sumatran fault. The Sumatran fault (SF) is a trench-parallel, right-lateral strike-slip fault that traverses the hanging wall block of the Sumatran subduction zone from the Sunda Strait to the spreading centers of the Andaman Sea. It separates a forearc sliver plate from the southeast Asian plate. Triangles are active volcanoes of the Sunda arc. Arrows are relative plate motion vectors determined from GPS. Topography and bathymetry are from Smith and Sandwell [1997]. WAF is the West Andaman fault. MF is the Mentawai fault. 2-78

rak L. Toba

Krakatau L. Singka

Nias Siberut

The Sumatran Fault Lake Coverages of 1:250,000 topographic maps Centers of aerial photographs Coverages of 1:50,000 topographic maps

Figure 2.2 Data upon which our map compilation is based. Most of our mapping is based on inspection of 1:50,000-scale topographic maps and produced by BAKOSURTANAL & JANTOP, the national mapping agencies for Indonesia, and 1:100,000-scale aerial photographs. Other data sources include smaller-scale geologic and topographic maps. 2-79

flat-topflat-top ssurfacesurfaces ooff MManinjouaninjou TTuffuff

SF

N 0 2000 m

SF

Figure 2.3. An example of the approximately 1:100,000-scale aerial photographs we used to compile most of our map of the Sumatran fault. These two sets of stereopairs show channel offsets of ~720 m. The channels cut a late Pleistocene pyroclastic flow deposit at about 0.3˚S. The flat upland surfaces are the unincised top of the flow. These offsets yield an average slip rate of ~11 mm/yr. 2-80

N NORTHWEST o o N 5 3

Toba

Alas SEULIMEUM TRIPA RENUN

ACEH BATEE

96 97.5 o E o N E o o o S 2 0 2

BARUMUN SUMPUR Tarutung T O R U Singkarak SIULAK SIANOK Diatas ANGKOLA SUMANI SULITI Sarula Kerinci

Maninjau TORU FOLD-THRUSTS

99 99.5 o o 102 E E E 1 o 03.5 o S o o S 3 5 SOUTHEAST 102 E E

Ranau Suoh Krakatau KETAUN SUNDA DIKIT KUMERING SEMANGKO MUSI MANNA INDIAN OCEAN

Lake Volcanic crater Volcanic edifice major valleys

Figure 2.4. Map of 20 geometrically defined segments of the Sumatran fault system and their spatial relationships to active volcanoes, major graben, and lakes.

2-81

10 S 10 7 S 7 8 S 8 9 S 9

109.5 E normal fault

6 S 6 " depression/basin active volcano active deformation front inferred anticline outer-arc ridge axis outer-arc forearc basin axis bathymetry contour (in meters) thrust

600

!$

108 E

$ 10 S 10

5 S 5

$

"

Sunda Strait Sunda $ 106.5 E S 9

KRAKATAU 4 S 4

$$ 105 E

$$ 8 S 8

&

!

& $ $

103.5 E

Sumatran Fault

4 S 4 7 S 7 5 S 5 6 S 6 3 S 3

Figure 2.5. Sumatran fault and related structures near the Sunda Strait and bathymetric map of the portion of the Sunda Strait and surrounding seafloor. The Sunda segment of the Sumatran fault forms an 1800 m deep graben that widens southward, toward the deformation front. Northwestward movement of the forearc sliver plate along the Sumatran fault appears to have caused thinning of the region between the trench and the strait. Bathymetry is from Digital Elevation Model ETOPO02 and bathymetric surveys [Smith and Sandwell, 1997]. 2-82

GSF

20'S 20'S

o o 4 4

15'S 15'S

o o 4

4

10 km 10

Air Kanan Air

Air Kiri Air N

10'S 10'S

o o 4 4

o 10'E 0 103 Pajarbulan

o 15'E 103

Plate 2-2. Shaded-relief map of the Sumatran fault where it crosses the western flank of a highly dissected volcano at about 4.2˚S. Colored arrows mark several offsets of the Air Kiri and Air Kanan ("river left" and "river right") of ~2.4 km. Created from 1:50,000-scale topographic map. 2-83 raphy is 6 km

100.5 E Sumani Segment Sumani 0

North

L A K E S I N G K A R A K A R A K G N I S E K A L

0.5 S 0.5

t Sianok Segmen Sianok Paddy Field Streams Volcanic Crater Volcanic Active Strike Slip Fault Active Active Fault with significant Fault Active Normal Component Active Faults, dashed where Faults, Active buried where dotted approximated, Map of the Sumani and Sianok segments near the Singkarak graben. The youthful appearance of the bounding normal faults The youthful appearance of the bounding normal faults near the Singkarak graben. Map of the Sumani and Sianok segments Marapi Volcano Marapi from 1:50,000-scale quadrangles. The fault geometry was compiled from the topographic maps and stereo aerial photographs. geometry was The fault from 1:50,000-scale quadrangles. Plate 2-3. Topog slip. accumulate dextral suggests that accommodation space continues to form as the en echelon Sumani and Sianok segments 2-84

N 5 o o N 8 6 4 6 3 7 9 2 3 10 1 b c ) a * + o 0

Northern part of Sumatra d

Selected Large Offsets Selected Small Offsets 17 Observed channel offsets GSF Lakes River . Volcano (see Table 2.3) b (See Table 2.2) Lines pointing to the exit

11 o S 12 0 16 o 13 14 17 3 32 18 e 15 21 23 24 31 19 22 25 28 f 20 26 30 27 29 g j , - . i h

Southern part of Sumatra o S 6

Figure 2.6. Map of small and large geomorphic offsets along the Sumatran fault. See Tables 2.2 and 2.3 for more information. The largest offsets indicate that total slip across the fault is at least 20 km. 2-85 reubo ffset by this ffset

97 E

Valleys F several million years ago.

offset (km) offset 21 Drainage Divide 21

Syncline

Kuala Tripa Kuala

Tripa Tadu

NORTH SUMATRA NORTH

S e volcanoes centers and cones of active u

n

a Anticline

g zone fault active large river large

21

a small river n

97 E Meureubo

21

Woyla

5 N 5 Geumpang

100 km

Baro

20

1 DD-1 95 E Andaman Sea 020 96 E Aceh

. Two of the most compelling large geomorphic offsets along the Sumatran fault, the 21 km dextral offsets of the Tripa and Meu Tripa of the along the Sumatran fault, 21 km dextral offsets geomorphic offsets of the most compelling large Two .

6 N 6 7 N 7 95 E Figure 2.7 Figure Rivers in north Sumatra. The headwaters of the nearby Woyla River and folded Quaternary sediments near 6 N also appear to be o Woyla The headwaters of the nearby Rivers in north Sumatra. since initial uplift of this part the Barisan Mountains appear to represent the total dextral offset These offsets amount. 2-86

t X'

Sumani Segmen X' 23 km offset Singkarak X Lake

egment X Sianok S 18 km offset 13 km offset 8 km offset 20 km time 10 3 km offset 0

Figure 2.8. Hypothetical evolution of the Singkarak graben and bounding normal faults showing how the length of the normal-oblique faults might represent the total offset on the Sianok and Sumani segments. Profile E shows the current geometry of the graben. 2-87

SF B A

SF N 3S

0.5N

102E

100E

103E C 0 80 km

Young Volcanic Cover

Bengkulu SF Tertiary Andesites 4S Paleogene Triassic INDONESIAN OCEAN Permo-Carboniferous Granites Geological offsets about 20 to 25 km

Plate 2-4. Geologic maps of offset bedrock units along three sections of the Sumatran fault suggesting that the total offset across the fault is only ~20 km. Reproduced from Katili and Hehuwat [1967]. 2-88

103.5 E 106.5 E 109.5 E 105 E 108 E

4 S 5 S 6 S deformation front 3 S N 7 S outer-arc ridge axis forearc basin axis

600 bathymetry contour Sumatran Fault (in meters) 4 S depression/basin 8 S 600 50-km isoba KRAKATAU active volcano th 1800 Sunda Strait 600 Stretched Region

5 S 9 S

1800 2400 356 km 2400 3000 6 S 3600 10 S 6600 6000

6000 6000 6600

7 S 8 S 9 S 10 S

Figure 2.9. Stretching of the forearc sliver plate near the Sunda Strait, which appears to have thinned the forearc wedge perpendicular to the deformation front. By volumetric balancing, we calculate that ~100 km of stretching of the forearc sliver has occurred parallel to the Sumatran fault since formation of the outer-arc ridge and forearc basin. This would be a maximum value for northwestward translation of the part of the forearc sliver plate that is south of the equator. 2-89

4 Ma Zone of transtension to 2 Ma N = Nias Sb = Siberut 37 mm/yr (74 km) Sm = Simeulue

37 mm/yr (74 km) Sm N Sb

42 mm/yr 57 mm/yr 59 mm/yr (118 km) (80 km) (114 km) A

37 42 59 53 57 mm/yr mm/yr mm/yr

2 Ma Zone of transtension to N = Nias 0.1 Ma Sb = Siberut 40 mm/yr (76 km) Sm = Simeulue

10 mm/yr (19 km) 10 mm/yr (19 km) 40 mm/yr (76 km) N 40 mm/yr Sm Sb (76 km) 30 mm/yr (57 km) 42 mm/yr 47 mm/yr 48 mm/yr (80 km) B (89 km) (91 km)

40 40 40 42 47 48 53 57 59 mm/yr mm/yr mm/yr

Zone of transtension 37 mm/yr (3.7 km) 0.1 Ma N = Nias to Sb = Siberut present Sm = Simeulue 25 mm/yr (2.5 km) 10 mm/yr ? (1.0 km) 12 mm/yr 15 mm/yr 10 mm/yr N (1.5 km) (1.0 km) Sm Sb 43 mm/yr (4.3 km) 53 mm/yr C (5.3 km) 55 mm/yr (5.5 km) 25 10 10 43 53 55 53 57 59 mm/yr mm/yr mm/yr

Figure 2.10. (opposite) A plausible (but nonunique) history of deformation along the obliquely convergent Sumatran plate margin, based upon our work and consistent with GPS results and the timing of deformation in the forearc region. (a) By about 4 Ma, the outer-arc ridge has formed. The former deformation front and the Mentawai homocline provide a set of reference features for assessing later deformations. From 4 to 2 Ma, partitioning of oblique plate convergence occurs only north of the equator. Dextral-slip faults on the northeast flank of the forearc sliver plate parallel the trench in northern Sumatra but swing south and disarticulate the forearc basin and outer-arc ridge north of the equator. (b) Slip partitioning begins south of the equator about 2 Ma, with the creation of the Mentawai and Sumatran faults. Transtension continues in the forearc north of the equator. (c) In perhaps just the past 100 yr, the Mentawai fault has become inactive, and the rate of slip on the Sumatran fault north of 2˚N has more than doubled. This difference in slip rate may be accommodated by a new zone of transtension between the Sumatran fault and the deformation front in the forearc and outer-arc regions. 2-90 98E 96E 94E

W A F NorthernN Domain o r th e 6N rn D o m a in 100E U D

4N S F Central Domain North 102E

2N

Equator 104E

S F Southern Domain

2S I 106E F Z M F Z D 4S E F 20020 15015 0 O 0 10010 R 505 km 0 0 M km

A T 6S I O N

F R O N 8S T

Plate 2-5. Geometric and structural details of the Sumatran fault, the forearc basin, outer-arc ridge, and volcanic arc, suggesting the division of the Sumatran plate margin into northern, central, and southern domains. The simplest outer arc, forearc, and Sumatran fault geometries are in the southern domain. The coincidence of this structural domain with the source region of the giant (Mw 9) subduction earthquake of 1833 suggests that geometrical simplicity encourages large ruptures. The central domain appears to have been the source region of the great (Mw 8.4) subduction earthquake of 1861. Fragmentation of the central domain appears to have been caused by subduction of the Investigator fracture zone during the past 5 Myr. The locus of impingement of the fracture zone on the deformation front was calculated by assuming the current relative plate motion vector and the forearc deformation history of Figure 10. Contours in red are the top of the Benioff-Wadati zone. Bathymetric contour interval is 200 m. volcanoes mentionedinthetext arenamed. the fault. Large (15kmdiameter) volcanic edifices arelistedalongthehorizontalaxis.Smaller are within2kmofthefault. Mostoftheseareassociatedwithextensional (right)stepovers in volcanic archasnotinfluencedthelocationof fault. However, 9ofthe50volcanic centers Figure 2.11. SW Distance from Great Sumatran fault (km) NE NW 100 40 20 20 40 60 80 0

Plotofthedistancevolcanic centersfromthe Sumatranfault showing thatthe Pulau Weh 5.5˚N Kapal

Toba Sibual-buali 2˚N Sorik Merapi

Talakmau 2-91 Maninjou Equator Marapi 0.4˚S Talang

Kerinci Volcano diameter Tujuh Kunyit large (>15km) medium small (<10km) Kumbang Dipatiampat Kaba Dempo

Ranau

SE Suoh 5.5˚S Ð 2 k m Krakatau