On December 26, 2004, a magnitude ~9.2 earthquake struck Sumatra and the Andaman and Nicobar Islands of India (e.g., Park et al., 2005). Within hours, resulting tsunamis inundated coastal communities around the Indian Ocean, killing over 290,000 people. The earthquake ruptured the megathrust between the subducting India-Australia Plate and the overriding Burma-Sunda microplate (Fig. 1). Seismic rupture nucleated offshore Sumatra at ~30-40 km depth and ruptured mostly northwards for ~1300 km over a period of ~550 s (Lay et al., 2005; Ammon et al., 2005; Wu and Koketsu, 2005; Stein and Okal, 2005; Park et al., 2005). While the heterogeneous and exceedingly long rupture process is now the best documented great earthquake, having been recorded on global and regional seismic networks, and continuous GPS stations (e.g., Vigny et al., 2005), the basic tectonic elements of the margin that yield during rupture and control its propagation, versus the elements that terminate rupture and suppress it, are essentially unknown. The tsunami from this event was recorded in deep water as sea-surface height changes by the Jason-1 and TOPEX/Poseidon satellites (e.g., Titov et al., 2005), and by tide gauges around the world. The rupture initiated in the south, near Simeulue Island, and propagated up-dip and northwestward at 2-2.5 km/sec (Ammon et al., 2005; Ishii et al., 2005). Moment release was concentrated in patches; however, despite modern techniques debate still exists over slip distribution patterns (Fig. 2).
In 1996 we began investigating the paleoseismic history of the Cascadia margin based on turbidite records. The possibility that a good earthquake record existed along the margin had been suggested in the late 1960's, and proposed formally in 1990 by John Adams. The 2004 Sumatran earthquake was a great surprise in terms of its breadth, length and magnitude, and revealed that this margin must have a long history of great earthquakes, abut which virtually nothing was known. Kerry Sieh and his colleagurs and students had been pioneering coral head paleoseeismology further to the south on the islands off shore central Sumatra, but the technique is limited to teh past few hundred years. Bui;ding on their ecellent high-precision record, we conducted a 45 day coring cruise in 2007, collecting 109 cores along the margin to investigate a longer record that might be contained in the stratigraphy of the trench and lower slope basins.
Since our Sumatra project began, we have been developing these methods for application to the turbidite history along the Sumatra margin, where the great earthquake of December 26, 2004 struck. The physiography and sedimentation of the Sumatra-Nicobar-Andaman section of the trench is quite similar in some ways, but very different in detail to the nearly ideal Cascadia margin. In the following sections we briefly discuss our methods and results from Cascadia, and the principles we are applying to the Sumatran turbidite record. Continued development of the turbidite paleoseismic technique advances fundamental tectonic and seismic hazard methods that can be applied in any continental margin system, where major fault systems and population centers commonly coincide.
Research supported by the National Science Foundation (awards OCE 1030381 and 0526655). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
A large number of co-investigators, students, technicians and ships crew have been involved in this project since 2007, including: Chris Goldfinger, Ann E. Morey, Jason Patton, John Southon, Chris Moser, Bob Wilson, Lisa McNeill, Russ Wynn, Stefan Ladage, Udrekh Hanifa, Ken Ikehara, Alexis Viscaino, Bran Black, Amy Garrett, and the officers and crew R/V Revelle and the Shipboard Scientific Parties.
For a little entertainment, play the movie at left for an overview of the Sumatra Cruise, courtesy of videographer Chris Aikenhead and Documentary Producer Jerry Thompson. 36 mb file, be patient...For the entire "Shockwave" documentary, follow this link.
The Sumatran Turbidite Record
Sumatra Continental Margin Morphology and Sedimentation
The morphologic details of the continental margin offshore Sumatra and the Andaman Nicobar forearc have now been imaged by numerous expeditions including ours in 2007, and these bathymetry data have been merged to cover much of the northern Sumatran slope (Ladage et al., 2006).
Three important issues that bear on the applicability of turbidite paleoseismology to the Sumatran margin are 1) the presence of channel systems to deliver seismically triggered turbidites 2) the presence of planktonic Forams for radiocarbon dating, and 3) the favorability of physiography for limiting turbidites from other sources.
Continental margin morphology in western Sumatra is dominated by the upper plate structure of a Tertiary and Quaternary accretionary prism, structural highs, and forearc basins. Fold and thrust belt topography forms longitudinally linked basins that can be either isolated, or drain to the trench. Canyon systems tend to be short and drainage catchments relatively small. The outer forearc is isolated from Sumatra by the broad, unfilled forearc basin, though input from the offshore islands of Simeulue, Nias, and Siberut Island is possible for some basins in the central and southern Sumatra margin. Similarly, the trench is blocked from input from northern sources by the subducting Ninety-East Ridge, and a large landslide at 14º N (Moore et al., 1976). The trench axis deepens from 4.5 km. to 6.5 km from north to south, and is filled with sediment several km thick in the north from the Nicobar fan, partially burying lower plate bending moment normal faults and fracture zones that trend across the trench. This morphology controls turbidite channel flow within the trench. Where larger lower plate structures like the Investigator fracture zone (IFZ; among several unnamed fracture zones) and the fossil Wharton ridge cross the trench in a northeasterly direction, they block sediment flow southward down the trench axis thus compartmentalizing sediment basins in the trench and restricting sources of input to specific margin segments.
Figure 1. Left: Deformation from at ~30 N, showing landward vergent folds, and slope/abyssal plan channel systems. Right: Five channels systems feeding the abyssal plain between mainland northern Sumatra and Simeulue Island. We intend to focus on these deeper systems, with selected cores in forearc basin sites. Images courtesy of the British Geological Survey/Royal Navy/Southampton Oceanography Centre Team & the United Kingdom Hydrographic Office.
Margin Physiography: A key feature that made Cascadia Basin a good paleoseismic recorder proved to be the relatively wide continental shelf of Oregon and Washington (Goldfinger et al., 2003b; 2004; submitted). Under high-stand conditions, the shelf separates rivers from their associated canyons, which are completely infilled by transgressional sediments. This configuration largely prevents hyperpycnal flows from connecting to canyon systems and generating turbidity currents (Sternberg, 1986) except
where the shelf remains narrow. In Sumatra, the separation is even greater, with a wide forearc basin and forearc high completely separating land sources from lower slope and abyssal plain channels. In addition, the reported blockage of the trench from northern (Himalayan sourced) sediment input appears to limit that source as well. A number of slope basin sites appear to be subject to possible input from the forearc islands (Simeulue, Nias, Siberut, North and South Pagai). The bathymetric compilation has allowed a detailed analysis of drainage pathways at our core sites (Figure 2). This analysis is being done to evaluate the potential sources and drainage areas, as well as sources overlaps for each 2007 core site.
Channel Systems: The available bathymetry, merged with the data we collected in 2007 showed that sediment transport pathways exist, although they tend to be relatively short. Figure 1 shows typical example in a landward vergent region, where the seaward flank of a large fold has been incised by a canyon system that delivers material to an intra-trench basin partially enclosed by the initial thrust ridge, and then to the trench axis. Other similar examples deliver material to completely enclosed basins. Our GIS analysis of the basins and drainages is shown in Figure 1. In our 2007 and subsequent analysis, we found that sediment pathways can de divided into 1) isolated slope basins that have no input from land sources and no exits; 2) slope basins with no input from land sources with exits to the trench or other basins; 3) a few slope basins that may have input from forearc island sources, and 4) trench basins that are partially or completely isolated from other trench segments by basement structures.
Foram abundance: The regional CCD was reported to be ~4500-5100 m depth along the Sumatran margin (Tucker and Wright, 1990; Shulte and Bard, 2003). We found that the regional CCD in our core set (Figure 3) is closer to 4000 m based on foram abundance and preservation in our cores. We discovered this onboard during the 2007 cruise, and developed a strategy to minimize the effect of the shallower than expected CCD.
Figure 2. Catchment basins linked to 2007 core sites, Sumatra margin, from the GIS study of drainage flowpaths we determined the linkage of each core site to its sediment sources.
Paleoseismic Strategy: The strategy that emerged at sea during our 2007 cruise revolved around using slope basin and trench cores to both data turbidites and test the observed stratigraphy for seismic origin. The lack of datable forams in the trench cores meant that all age control would come from basin core sites shallower than ~ 4000 m. We therefore collected cores at sites in pair transects across the slope, trench cores paired with basin cores along the margin. We anticipated that if the cores were of seismic origin, they should correlate between basin and trench sites stratigraphically, and age control would come from the basin cores. Furthermore, basin sites with synchronous earthquake stratigraphy should correlate both stratigraphically and temporally with eachother within seismic segments. This strategy forms the basis of tests of earthquake origin for this project and is summarized in Table 1.
Northern Sumatra Historic Earthquake Recurrence
Historic Sumatra-Andaman subduction zone (SASZ) earthquakes in this region were much smaller than the 2004 event (1847, 1881, 1941; Bilham, et al., 2005) (Figure 1), with no historical record of very large events similar to 2004. Recent investigation of secondary evidence left behind by tsunami as sand sheets in northern Sumatra (Monecke, at. al., 2007), western Thailand (Jankaew, et al., 2007), and the Andaman-Nicobar Islands (Rajendran, et al., 2008) suggests the penultimate large event most likely occurred 500-700 years ago. Ante-penultimate event ages in Sumatra (Monecke, at. al., 2007), the A-N Isles (Rajendran, et al., 2008), and India (Rajendran, et al., 2007) span an interval ~ 900-1200 years ago.
Figure 3. Core locaitons and historic rupture zones along the margin of western Sumatra. 109 piston, gravity, kasten, and multicores were collected in May-June 1997 aboard the R/V Revelle.
144 Piston/Trigger, Gravity, Multi, and Kasten cores were collected along a 2,000 km transect at paired trench-axis and slope-basin sites. Core sites were located in places that optimized 1) isolation from terrestrial sediment input, 2) sufficient sampling for segment boundary determination, and 3) preservation of sediments that permit our analytical methods. Our strategy was to densely sample both trench and basin sites to test correlations between sites to determine whether observed turbidites satisfy criteria for earthquake generation. 49 cores were located in the region of the 2004 rupture and were considered in this study. The ship was teh R/V Roger Revelle, operated by the Scripps Institution of Oceanography. Failure of the ships wire handling system resuted in our having to pisting core off the stern. This limited out core length for piston cores to ~ 7 m. We used pistong cores, "big Bertha" graity cores, using the same weight as teh piston core, multicores, a 20 x 20 cm x 3m Jumbo Kasten corerm and a small benthos gravity corer for site recon.
Cores were split, described, and scanned at sea with a GEOTEK Multi Sensor Core Logger (MSCL), obtaining P-wave velocity, gamma ray density, resistivity, and loop magnetic susceptibility in 1.5-m maximum length sections. Split cores were imaged with a high resolution line-scan digital camera and the lithostratigraphy was described. High resolution magnetic susceptibility data were collected using a Bartington MS2E point sensor. The cores were imaged with The Oregon State University Aquilion 64 slice CT system with a nominal voxel size of 0.5 mm. Piston-trigger core pairs were analyzed for missing section in the context of the entire core set.
Figure 4. What is most likely the 2004 turbidite is displayed in this figure from cores 96PC and 96TC. A. From left to right, gamma density (g/cc), CT density (greyscale digital number), lithologic log, CT imagery, point and loop magnetic susceptibility (SI x 10-5), and mean grain size (μm, linear scale) are plotted vs. depth (in m) for each core. Moment release (vs. latitude) in red and relative amplitude (vs. time) in green are scaled to match peaks in the loop mag sus data. A light brown tie-line connects the two data sets at a lithologic contact. Particle size distribution data from sample locations found in A are plotted by volume (%) vs. particle size (μm, log scale) with lines generally designating samples’ depth where the lighter lines have a larger mean size and are generally lower in section. Differential volume displays the percent volume of each particle size.
Seismogenic trigger rationale
Adams (1990) and Goldfinger et al. (2003, 2007, 2008, 2010) suggest eight plausible triggering mechanisms for turbidity currents: 1) storm wave loading, 2) earthquake, 3) tsunami wave load (local or distant), 4) sediment load, 5) hyperpycnal flood, 6) volcanic explosion, 7) submarine landslide, and 8) bolide impact. Other mechanisms may reduce slope stability, but are likely random and not regional nor synchronous (Goldfinger et al., 2009, Bryna, et al., 2005). The basis for determining origin of turbidity currents is that regional and synchronous deposition is unlikely to have been generated by a trigger other than an earthquake (Goldfinger et al., 2007, 2008, and 2009).
This paper will test the plausibility of a seismogenic source by 1) using tests for synchronous triggering of sedimentologically isolated turbidite systems and 2) using secondary constraints that consider sedimentologic characteristics of the turbidites. When turbidites can be correlated between cores separated by a large distance or between sites isolated from land sources and from each other, synchronous triggering can be inferred and most other triggering mechanisms can be eliminated (Goldfinger, et al., 2010). Most studies that make a linkage between earthquake shaking and triggering of turbidity currents use this method (Adams, 1990; Nakajima and Kanai, 2000; Gorsline, 2000; Nakajima, 2000; Shiki et al., 1996; 2000a; 2000b; Goldfinger et al., 2003, 2007, 2008, 2010, Gràcia et al., 2010).
Standard stratigraphic correlation techniques are employed, including lithostratigraphic description (color, texture, and structure, etc.), visual analysis, CT-image analysis, and core log “wiggle matching” of MSCL data and point magnetic susceptibility data. Core log correlation techniques have been widely developed by oil industry geologists, beginning in 1927 (Schlumberger, 1989) with the first electronic core log. The primary goal of correlating turbidite strata is to establish spatial extent of individual turbidites that are carefully fingerprinted with geophysical data.
Geophysical wiggle matching (fingerprinting: Goldfinger et al., 2007) of turbidites is based on the correlation of identifiable unique stratigraphic characteristics using MSCL core log data: gamma density and magnetic susceptibility. These “fingerprints” commonly show that earthquake triggered strata have similar structure and depositional histories in cores separated by large distances, and correlate in detail at independent sites. The turbidite itself is commonly composed of multiple coarse fraction pulses, probably deposited over minutes to hours. It is these multiple pulse coarse fractions and their geophysical signatures that form much of the basis of the turbidite “fingerprint.” The fact that these cores, in sedimentologically isolated and hydrodynamically unique systems, share turbidites with matching grain size patterns suggests that they also share a common triggering mechanism. Goldfinger et al. (2007, 2008, 2009) have proposed this shared characteristic is, in part, the time history of ground shaking during the earthquake rupture as the seismic waves generate significant excess pore pressures at depth (Biscontin, et al., 2004). Inouchi, et al. (1996) further suggest that the most sensitive criteria for correlating fine grained turbidites (which may not be visible to the naked eye) is the density profile.
In Sumatra the isolation of trench segments from each other, and from isolated slope basins, offer numerous opportunities to compare the stratigraphic sequences in sites that are unique. The basins and trench segments are fed by small canyons that have limited drainage areas on the slope and form small fan/aprons. Slope sites used in this study include cores 108, 104, 103, and 96, which all have isolated sediment sources. In the trench, cores 107, 105, 98, and 94 receive sediment both from upstream in the trench and transported from the continental slope. Correlation of strata is supported by 14C ages of individual turbidites and hemipelagic intervals between events (methods are given in Goldfinger et al., 2008; 2009; 2010).
Considering sedimentologic characteristics, Nakajima and Kanai (2000), Nakajima (2000), Shiki et al., (1996, 2000a, 2000b), and St. Onge, et al. (2004) conclude seismoturbidites may in some cases be distinguished from other sediment gravity flow deposits from non-seismic sources. Characteristics from these studies include 1) wide aerial extent, 2) multiple coarse-fraction structure and amalgamation, 3) upward fining particle size structure, 4) variable mineralogical provenance, 5) greater organic content, and 6) greater depositional mass and coarser particles than storm generated deposits. Hyperpycnites are reported to initially coarsen upwards and then fine upwards, representing the waxing and then waning of the hyperpycnal flow (e.g. St. Onge, et al., 2004, Mulder et al., 2003). Amalgamation is an indicator that the turbidites were deposited from multiple turbidity currents, merging and forming a longitudinal structure reflected in the deposit (Nakajima and Kanai, 2000), possibly from multiple slides on slopes. In Lake Biwa, Nakajima and Kanai (2000) conclude these multiple pulses are the results of synchronous triggering of multiple parts of the canyon system. Cascadia margin cores contain deposits that pass multiple tests of synchronous earthquake origin, and generally contain multiple coarse fining upward sub-units, consistent with other seismo-turbidites (Goldfinger et al., 2008, 2009, 2010). Goldfinger et al. (2010) correlate the number of coarse units to the rupture length of the causative earthquakes. We attempt to evaluate these characteristics in the region of the 2004 SASZ earthquake. Cascadia margin cores contain deposits that appear to coarsen up section, but actually contain multiple coarse pulse sub-units, each fining upwards (Goldfinger et al., 2010). Amalgamation is an indicator that the turbidites were deposited from multiple turbidity currents (Nakajima and Kanai, 2000), possibly from multiple slides on slopes. In Lake Biwa, Nakajima and Kanai (2000) conclude these multiple pulses are the results of synchronous triggering of multiple parts of the canyon system. We attempt to evaluate these characteristics. Sumatra turbidite bases are sharp and generally fine upwards, with some deposits composed of rare multiple upwards fining sub-units, like Cascadia (Goldfinger, et al., 2010). Coarser Sumatra turbidites commonly are amalgamated and this results in incomplete or irregular structure sequences, suggesting multiple turbidity current pulses and a longitudinally heterogeneous turbidity current .
Age control for stratigraphy is provided by Accelerator Mass Spectrometer (AMS) 14C and 210Pb radiometric techniques. 14C data is based on decay with a half life of 5,730 years and is useful for strata between ~300 - ~35,000 years old. 210Pb data, based on a shorter half-life of 22 years (Noller, 2000), provides information about sedimentary deposition for the past ~150 years (Robbins and Edgington, 1975). We use 210Pb age data to constrain the timing of deposition for the most recently deposited sediments.
For 14C ages we utilized planktic foraminiferid species as they most closely represent the age of the youngest sea water, the surface water that is most closely in 14C equilibrium with the atmosphere. Trench core sites were deeper than the Carbonate Compensation Depth (CCD), the depth below which foraminiferid CaCO3 tests dissolve. Foraminiferid abundance was nil in trench core sediments, so 14C age control applies only to the slope cores.
14C ages are calibrated (Bronk Ramsey, 1994) and a marine reservoir correction of 16±11 years is made using the INTCAL04 database (Stuiver and Reimer, 1998). Only two delta R values are available for the Sumatra area, and while constraints are few on this calibration, we here are correlating marine sites to other nearby marine sites, thus the local correlations are valid while absolute ages may contain additional uncertainty.
The primary sources of error include variation of 1) age in surface and near surface sea water, 2) sedimentation rate, and 3) basal erosion during turbidite emplacement. The most significant of these in the context of this paper is basal erosion. While we can evaluate this visually to some extent, and differential erosion can be inferred between nearby cores from differences in hemipelagic thickness and the 14C ages (Goldfinger et al., 2010), there will likely be undetected erosion in these data. Sedimentation rates are calculated using 14C age estimates and thickness of hemipelagic sediment. Sedimentation rates are used to calculate ages for turbidites that have no direct age control and to correct other ages for sample thickness.
Recurrence of great earthquakes (7 ka, years before present, BP, 1950) is estimated based on turbidite stratigraphy (representing earthquake events) correlated between 49 deep sea sediment cores in the region of the 2004 rupture. We apply criteria developed in Cascadia, Japan, and in Sumatra thus far to discriminate such events from those triggered by other mechanisms by testing the turbidite stratigraphy for synchronous triggering of turbidity currents between sedimentologically isolated basin core sites and deeper trench sites using radiocarbon, multiple proxies, and ash stratigraphy.
Nineteen turbidites are interpreted to have been triggered during strong ground shaking from earthquakes over the past ~7,000 years. The youngest turbidite is most likely the result of the 2004 earthquake. Calibrated probability density function peak ages for events 4 – 13 are 560±60, 710±80, 1100±110, 1420±80, 1480±60, 2210±80, 2580±60, 3580±90, 4400±80, 4760±60 years BP and events 15 – 19 are 5040±120, 5320±110, 5660±100, 6430±90, and 7000±70 years BP. The turbidite record is also compatible with the developing onshore record of paleoearthquakes in Aceh, Thailand, Sumatra, forearc Islands (Simeulue, Siberut, Sipora, and the Pagai Islands), and the Andaman Islands, but the terrestrial record is less complete. The recurrence interval (RI) estimate for earthquakes in the 2004 rupture region for the last 7 ka is 390±70 years. The recurrence pattern appears to include significant clustering through the Holocene, with three apparent clusters, and two gaps of 700-1000 years.
Alternative Explanations? Hyperpycnal flow
Hyperpycnal flow is the density driven underflow from storm discharge of rivers into marine or lacustrine systems, and has been proposed as a link to turbidity currents in a variety of settings.
In some proximal settings such as large lakes, shelf basins, and fjords, records of both earthquakes and storm deposits are found. In one of the best comparisons, St.-Onge et al (2004) show that details of both seismic and hyperpycnal deposition in the Saguenay Fjord in eastern Canada are diagnostic, and argue that hyperpycnal deposits are distinguished by reverse grading at the base, followed by normal grading. The diagnostic reverse-then-normal grading for hyperpycnal deposits has been widely reported, and is the result of waxing, then waning flow associated with the storm. In the Saguenay Fjord, six events have normal grading, and are inferred to be earthquake generated. Four others have similar basal units, but are topped by a reverse graded unit, and then a normally graded unit, with no evidence of hemipelagic sediment between these units. These events are interpreted as an earthquake, followed by a hyperpycnite that resulted from the breaching of a landslide dam caused by the original earthquake. The dam breaching is a variant of the more common hyperpycnal scenario involving waxing and waning depletive flow (Kneller, 1995), but would likely produce a similar flow hydrograph (St-Onge et al., 2004).
In another well studied example, a major river flood in the1969 El Nino input ~25 million tons of sediment (20X the yearly Columbia sediment load) to the Santa Ana River in Southern California over a 24 hour period, in close proximity to nearby canyon heads. Sediment from this extreme flood did not continue down canyons as hyperpycnal flow, but deposited as a distinct yellow unit on the shelf and upper slope. Over the next 10 years it moved downslope as turbid layer transport caused by storm wave resuspension, similar to that observed by Puig et al. (2004) for the Eel system, and deposited as yellow layers between varves of the Santa Barbara Basin (Drake et al., 1972).
Other examples of reverse-then-normal grading for hyperpycnites have been reported and compared to normally graded failure deposits in the Var system (Mulder et al., 2001), Lake Biwa (Shiki et al, 2000a), and modeled by Mulder et al. (1998b), Felix (2002) and others. The dynamics of longitudinal and temporal variability and their effects have been discussed in detail by Kneller and McCaffrey, 2003, and Mulder et al., 2003. Hyperpycnites are also commonly very organic rich as compared to seismic turbidites, having their sources in floods rather than in resuspension of older canyon wall material as in earthquake triggering (Mulder et al., 2001; Shiki et al., 2000; Nakajima et al, 2000). It has been suggested that this distinction may be used as a basis for distinguishing earthquake and storm deposits using OSL dating.
Documentation of hyperpycnal flows into lakes and shelf basins is abundant, however evidence of such flows entering canyons systems and moving into deep water is sparse. Most, if not all examples involve very short distances between the river mouth and canyon head, either during Pleistocene low-stand conditions, or in systems that have very narrow shelves during high stand conditions. Hyperpycnal flows extend further from river mouths with high discharge (Alexander and Mulder, 2002), but documentation is sparse. Wright et al. (2001) infer that hyperpycnal flow is strongly affected by ambient currents, and generally deliver sediment to the slope only upon relaxation of longshore currents. Most investigators cite Pleistocene examples when referring to flows reaching the abyssal plain or lower fan reaches (e.g. Mulder et al., 2003; Normark et al., 1998; Normark and Reid, 2002; Piper et al., 1999). Under low stand conditions, rivers and canyons are directly connected, and such flows are expected. These authors relate the deep water deposition of hyperpycnites to sea level control, or alternatively to climate shifts. An example of high stand hyperpycnal flow has been reported for the Var River, in which the canyon and river mouth are less than 1 km apart (Klaucke et al., 2000; Mulder et al., 1998a). Many large river systems deposit most of their load in river mouth bars, with lesser quantities making it past such bars in to a delta front slope (eg. Yellow River, Li et al., 1998). Whether hyperpycnal flows can reach the deep sea via canyon systems incised during the Pleistocene appears to be a function of shelf width, river peak storm discharge, Holocene aggradation of Pleistocene canyons, and the wave and current climate during peak storm discharge. However, the requirements for and evidence of hyperpycnal flows to the deep ocean under high-stand conditions (excepting very narrow shelves) remain unknown at best (Mulder et al., 2001). In Sumatra, a wide and unfilled forearc basin separates teh mainland from the outer slope and trench core sites. There remains however the possibility of sources on the forearc islands contaminating the deep water record.
Sumatran Turbidite Characteristics
In our work,Sumatran turbidites show rare to no evidence of the characteristic reverse grading and high organic content observed for hyperpycnal deposits. We observe normally graded sequences with multiple amalgamated pulses, sharp erosional bases, and normal grading of individual pulses when present. The number and form of these multiple pulses persists over wide areas, and are the basis for individual event correlation.
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7.5 KA Earthquake Recurrence History in the Region of the2004 Sumatra-Andaman Earthquake, Geological Society of America Annual Meeting, 2009
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—, 2003, Holocene Earthquake Records From the Cascadia Subduction Zone and Northern San Andreas Fault Based on Precise Dating of Offshore Turbidites: Annual Reviews of Earth and Planetary Sciences, v. 31, p. 555-577.
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