TSK 11 Göttingen 2006 Roeser et al.
 Geotechnical characterization
 of trench- and slope sed-
 iments off Southern Chile:
 preliminary results Vortrag
 Georg Roeser1 Jan H. Behrmann1
 Achim Kopf2
 Introduction
 To understand seismogenesis in shal-
 low parts of subduction zones, it is vi-
 tal to know about strength and fric-
 tional parameters of subducted sedi-
 ment. For this purpose, PETROTEC,
 as part of the TIPTEQ-Project, gath-
 ers geotechnical data for sediments de-
 posited on the incoming Nazca Plate,
 the trench and the slope off the southern
 Chilean coast during the last 5Ma, and
 whose equivalents are now being under-
 thrusted into the seismogenic zone be-
 neath South America (Fig. 1).
 Figure 1: Principle of sampling the south-
 ern Chile Trench for geotechnical purposes.
 Material comes from gravity cores col-
 lected during R/V SONNE Cruises
 SO181 (Flüh E.& Grevemeyer I (Edi-
 tors) 2005), SO102 (Hebbeln D, Wefer
 G, et al. 1995) and SO156 (Hebbeln D,
 et al. 2001), as well as from ODP Leg
 141 (Behrmann, JH et al. 1992) drill
 cores (Fig. 2).
 1 University of Freiburg, Albertstrasse 23b,
 D-79104 Freiburg i. Brsg., Germany 2 DFG
 research center ocean margins, University of
 Bremen, Loebener Str., D-28359 Bremen
 Figure 2: Swath bathymetric map of the
 southern Chile Trench between 36°S and
 47°S, after Flüh E & Grevemeyer I (Eds)
 (2005), with gravity core sampling locali-
 ties, and ODP Leg 141 drill sites.
 Sediment strength and frictional prop-
 erties are determined by triaxial testing,
 ring shear testing and direct shear test-
 ing.
 First results from triaxial testing show
 that Young’s moduli are much lower
 (3–20 kPa) in comparison to diatom-
 rich muds from equivalent depths in the
 Japan Trench area (180–240 kPa; Roller
 et al 2003). Internal angles of fric-
 tion from ring shear testing, direct shear
 testing, and triaxial testing yielded co-
 herent results. Values from ring shear
 testing differ depending on material,
 normal stress and shear velocity, and
 1
Roeser et al. TSK 11 Göttingen 2006
 Figure 3: Shear stress vs. time diagrams, derived from ring shear testing, showing the
 rate- and state-variable friction law (Box after Scholz (1998)).
 vary from 9° to 34°. With increasing
 shear velocity, there is at first a decrease
 of the internal angle of friction, followed
 by an increase. Values from direct shear
 testing range up to 27° and angles of
 friction determined from triaxial testing
 show values of about 10° to 20°.
 Results from ring shear testing show
 compliance to the rate- and state-
 variable friction law as described by
 Scholz (1998). Fig. 3 shows two ex-
 amples of shear stress vs. time dia-
 grams. In diagram a), the material
 shows velocity weakening behaviour at
 low normal stress (up to 4MPa). Ma-
 terial from another specimen shown in
 2
TSK 11 Göttingen 2006 Roeser et al.
 diagram b), however, shows velocity
 strengthening behaviour at low (1MPa)
 and high (8MPa) normal stress, but
 velocity weakening behaviour at inter-
 mediate normal stress values (4MPa).
 Preliminary data show no pattern or
 a connection between velocity weaken-
 ing/strengthening behaviour and differ-
 ent normal stress values. Stick-slip be-
 haviour is observed at high shear ve-
 locities, not depending on normal stress
 (Fig. 3b).
 In addition to sediment strength prop-
 erties, permeabilities were determined
 on the basis of consolidation data dur-
 ing triaxial testing. Values range from
 1.0953× 10−8 to 8.7× 10−10 ms−1. Av-
 erage permeability is 10−9 ms−1, an ex-
 pected value for marine clays to silts.
 We discuss possibilities to extrapolate
 laboratory data (up to 40MPa effec-
 tive stress, equivalent to 1.5 km depth)
 to upper seismogenic zone conditions
 (equivalent to 100–150MPa effective
 stress).
 Results so far raise questions: what
 causes different velocity strengthen-
 ing/weakening behaviour at varying
 normal stress values and in which way
 is seismogenesis affected?
 References
 Behrmann JH, et al. (1992) Chile Triple Junc-
 tion. Proc. ODP, Init. Rept. (Pt. A) 141,
 1–708
 Flüh E & Grevemeyer I (ed) (2005) FS Sonne
 Cruise Report SO181. IFM Geomar Report
 Nr. 2 March 2005, p 102
 Hebbeln D, Wefer G, et al. (1995) Cruise Re-
 port of R/V SONNE Cruise 102, Valparaiso
 — Valparaiso, 9.5.-28.6.1995. Berichte,
 Fachbereich Geowissenschaften, Universität
 Bremen, 68, pp 126
 Hebbeln D, et al. (2001) PUCK: Report
 and preliminary results of R/V SONNE
 Cruise SO156, Valparaiso — Talcahuano,
 29.3-14.5.2001. Berichte, Fachbereich Ge-
 owissenschaften, Universität Bremen, 182,
 pp 195
 Roller S, Pohl C & Behrmann JH (2003) Data
 report: Triaxial shear strength investigations
 of sediments and sedimentary rocks from the
 Japan Trench, ODP Leg 186. In: Suyehiro
 K, Sacks IS, Acton GD, & Oda M (ed) Proc.
 ODP, Sci. Results, 186, 1–19 [Online]
 Scholz CH (1998) Earthquakes and friction
 laws. Nature 391, 37–42
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