5-31. Cambodia. Vietnam. South China S ea. Malaysia. Kuala L umpur. Singapore. Kalimantan. Sunda S trait. Jakarta. Java 98E. Sumatra. Pini.

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1 5-31 Ridge Ninetyeast 52mm/yr Wharton For e S umatran Ridge Indian Andaman Sea arc 1861 Burma Ridge 1833 Thailand Sumatran Sliver Subduction Southeast Asian P late Malaysia Fault P Cambodia Kuala L umpur late O cean Vietnam Singapore South China S ea Jakarta Sunda S trait Kalimantan Java Figure 5.1a Map of the principal tectonic elements of the Sumatran plate boundary, and on ETOPO2 shaded relief base map. Oblique subduction of the Indian and Australia plates beneath Sumatra is accommodated principally by slip on the Sumatran subduction and the dextral Sumatran fault. Volcanoes are triangles. Dotted-black line ellipsoids are estimated source ruptures of the 1833 and 1861 giant earthquake. A square is an index location for Fig.5.1b. Adapted from Chapter 2. Investigator 6mm/yr Cocos Island Indo-Australian P late Nias 98E 98E -6-2 Pini Basin -6 Seismic source Tanamasa Tana bala Badgugu Pini Sumatra Equator 5-km isobath -12 Figure 5.1b Index map and the regional geological structures. Badgugu site is located at the southern edge of the seismic source determined from the analysis of the teleseismic data from Rivera et al.[22]. 1S Mentawai Fault 6mm/yr Siberut Investigator Ridge 2S

2 5-32 (a) (b) view to south view to west Figure 5.2b Photographs of the two fossil microatolls that constitute the majority of the Badgugu record, taken as we were cutting the larger head in mid-2. Both photos were taken during low tide, when the flat upper surfaces of the heads were well above water. (a) Head #14, with slot showing the location of slab sample Bdg99A1. View is toward the South and the open bay. About m in distance is the outer edge of the intertidal platformnote the shallowness of the lagoon, other fossil heads in the middle ground, and the rubble rampart in the background. (b) Head #, while cutting of slab sample BdgA1 is in progress. The beach and houses of Badgugu village are about 1 m to the west.

3 5-33 Beach Intertidal reef Subtidal reef Grass Coconut orchard and other trees boulder and cobble field Zone of modern heads 1-6 Older heads 7-23 Heads died in 24-3 Modern goniastrea Modern porites North m dated sample of coral block +2 contours are in cm Figure 5.3 West Badgugu site map shows the wide and shallow intertidal reef platform, with its lagoon and outer rubble rampart. The large breach in the rampart allows circulation of seawater between the bay and the lagoon. The lagoon is a graveyard of fossil microatolls, most of which died from emergence of about 8 cm in.

4 5-34 Relative elevation (cm) Older heads Head that died in Bdg99A1 BdgA Head# Modern Goniastrea Modern Porites Bdg99A3 Figure 5.4. Relative elevations of perimeter crests of 46 microatolls. The datum is the average HLS of modern Porites heads. Modern Goniastrea heads are about 1 cm higher. The Porites heads that died in are about 35 cm higher than their modern equivalents. Little is known about the older population of microatolls.

5 / /-4 e r o d / / / / / Figure 2.2.3a 75 e d / BdgB1 SE / /-6 low-low tides in July /-8 Substrate A N S 5 1 centimeter S B Bdg99A3 Bdg99A1 NW C / / / / / / / / / / / / / /-27 BdgA1 D / /-.2 mm/y / /-.6 mm/y / BdgA1: Bdg99A1: Bdg99A3: BdgB1: preserved HLS eroded HLS minimum HLS Y e a r E N 19 Relative HLS Elevation (cm) Figure 5.5 Four slabs from the west and east Badgugu sites. A. Slab of fossil microatoll from east Badgugu site. B. Slab of the modern head near the outer edge of the intertidal flat in the west Badgugu site C. Slab of the smaller "pancake-shaped" fossil microatoll in the west Badgugu. D. Slab of the larger fossil microatoll. E. The composite HLS history from the four slabs. The record is dominated by the 8 cm emergence in. A very slow submergence operated for two centuries prior to, interrupted by significant disturbances in about 1743, 1797, 1833, 1861, 1876, and A fast submergence rate followed the event. The rate decelerated after the 1962 event but higher still than that of before.

6 / /-.2 mm/y / /-.6-1 HLS (cm) - baseline is perimeter elevation West Badgugu site Distance from trench axis: km Memong Site Distance from the trench axis:124 km 1.3 +/-.4 mm/y -8.4+/-.9.5+/ / Lago site Distance from the trench: 138 km 2.+/-.6 mm/y Years Figure 5.6 The HLS histories depicted from three longest coral slabs from the sites around the equator (i.e., Badgugu, Memong, and Lago). The records suggest that the physical behavior of the subduction before appears to have unchanged since 18.

7 5-37 Cumulative slip on the interface(cm) West Submergence Outboard interface slip deficit since Seismic Event locked patch slip at 1962 Emergence plate rate stable down-dip interface East slip accumulated at the plate rate Distance from the trench axis (km) Figure 5.7 A graphic presentation of a complex history of seismic and aseismic slip on the subduction interface for the last 2 years, based on the modeling in Chapter 4, Fig The steady-state subduction on the downdip interface that slips continuously at plate rate (4 cm/yr) is represented by the graph on the right (east) side. The slip history on the shallow portion of the interface (closer than 13 km to the trench axis) is dominated by quasi-stable sliding. Within this portion, the red and yellow patches represent the strain releases during the and the 1962 events. The strain accumulations (slip deficits) are shown by the other patches with cooler colors. Note that, Slip deficit sin 1797 on the patch, 88 to 127 km from the trench, equal to the predicted slip event in. The slips on the updip section ( 9 km) are entirely aseismic, but include that of the 1962 slow-slip event.