Frictional properties of sediments entering the Costa Rica subduction zone offshore the Osa Peninsula: implications for fault slip in shallow subduction zones
© Namiki et al.; licensee Springer. 2014
Received: 31 December 2013
Accepted: 4 July 2014
Published: 16 July 2014
We examined the frictional properties of sediments on the Cocos plate offshore the Osa Peninsula, Costa Rica, and explored variations in the intrinsic frictional properties of the sediment inputs to the Costa Rica subduction zone. Sediment samples were collected at Site U1381A during the Integrated Ocean Drilling Program Expedition 334, and include hemipelagic clay to silty clay material (Unit I) and pelagic silicic to calcareous ooze (Unit II). The frictional properties of the samples were tested at a normal stress of 5 MPa under water-saturated conditions and with slip velocities ranging from 0.0028 to 2.8 mm/s for up to 340 mm of displacement. The experimental results reveal that the steady-state friction coefficient values of clay to silty clay samples are as low as ~0.2, whereas those of silicic to calcareous ooze samples are as high as 0.6 to 0.8. The clay to silty clay samples show a positive dependence of friction on velocity for all tested slip velocities. In contrast, the silicic to calcareous ooze samples show a negative dependence of friction on velocity at velocities of 0.0028 to 0.28 mm/s and either neutral or positive dependence at velocities higher than 0.28 mm/s. Given the low frictional coefficient values observed for the clay to silty clay samples of Unit I, the décollement at the Costa Rica Seismogenesis Project transect offshore the Osa Peninsula likely initiates in Unit I and is initially very weak. In addition, the velocity-strengthening behavior of the clay to silty clay suggests that faults in the very shallow portion of the Costa Rica subduction zone are stable and thus behave as creeping segments. In contrast, the velocity-weakening behavior of the silicic to calcareous ooze favors unstable slip along faults. The shallow seismicity occurred at a depth as shallow as ~9 km along the Costa Rica margin offshore the Osa Peninsula (Mw 6.4, June 2002), indicating that materials characterized by velocity-weakening behavior constitute the fault zone at the depth of the seismicity. Fault slip nucleating along a fault in Unit II would be a likely candidate for the source of the shallow earthquake event.
Subduction zone earthquakes exhibit spatial variability in seismic behavior along megathrusts, ranging from stable sliding or episodic slow slip events (SSEs) to large earthquakes (Ide et al. 2007). This variation likely reflects spatial variations in frictional properties along the seismogenic portion of plate-boundary megathrusts (e.g., Bilek and Lay 1998; Scholz 1998; Bilek 2007; Wang and He 2008; Rushing and Lay 2012).
An increasing number of experimental studies have found that the frictional properties of subduction-zone material vary depending on mineralogy, such as clay content, emphasizing the expected strong material control on frictional properties along seismogenic subduction zone thrusts (Vrolijk 1990; Brown et al. 2003; Kopf and Brown 2003; Ikari et al. 2009a; Ujiie and Tsutsumi 2010; Tsutsumi et al. 2011; Ikari et al. 2013; Saito et al. 2013; Takahashi et al. 2013; Ujiie et al. 2013). Given the diversity in subduction-zone stratigraphy (Underwood 2007), it is essential to characterize the material inputs to subduction zones with respect to their intrinsic frictional properties if fault behavior within seismogenic subduction zones is to be better understood (Vrolijk 1990; Underwood 2007). However, available experimental data have thus far been limited mostly to clayey subduction-zone materials.
In this study, to explore variations in the intrinsic frictional properties of subduction zone sediment inputs that may be manifested in earthquake behavior along an erosional subduction zone interface, we examined the frictional properties of such sediments on the Cocos plate offshore the Osa Peninsula, Costa Rica. Our results reveal that variations in material inputs should produce variations in frictional properties along faults in the shallow Costa Rica subduction zone, providing an important constraint on faulting along the Costa Rica subduction zone interface.
The location of shallow earthquakes along the plate interface varies along strike: earthquakes occur shallower and closer to the trench in the CNS crust than in the EPR crust (Protti et al. 1994; Newman et al. 2002). In June 2002, a Mw 6.4 earthquake occurred at a depth as shallow as ~9 km along the Costa Rica margin offshore the Osa Peninsula (Arroyo et al. 2009). The subduction zone beneath the Caribbean plate is also characterized by strong variations in fault-slip behavior. Recently, the network of GPS stations around the Nicoya Peninsula has detected SSEs accompanied by seismic tremors in the Costa Rica subduction zone, not only at depths of 25 to 30 km near the down-dip limit of the seismogenic zone but also at depths of ~6 km near the up-dip limit (Outerbridge et al. 2010; Jiang et al. 2012). However, SSEs are not observed around the Osa Peninsula.
Integrated Ocean Drilling Program (IODP) Expedition 334, which also constituted a part of the Costa Rica Seismogenesis Project (CRISP), was established to understand the processes that control the nucleation and seismic rupture of large earthquakes along the Costa Rica erosive margin offshore the Osa Peninsula (Vannucchi et al. 2012). The primary objective of Expedition 334 was to both sample and quantify the sediment inputs to the seismogenic zone of an erosive subduction margin. In this expedition, Site U1381, which is located 50 km offshore the Osa Peninsula (Figure 1), is a key site for characterizing the material inputs to the subduction zone. The samples analyzed in the present study were recovered from this site.
Biogenic components are observed in most of the samples. However, Unit II is distinguished from Unit I by its abundant biogenic components (Expedition 334 Scientists 2012). The clay to silty clay samples contain foraminifers, calcareous nannofossils, a few sponge spicules, and minor amounts of small silicic shells of organisms such as diatoms (Figure 2a). The silicic to calcareous ooze samples consist mainly of radiolarians, diatoms, silicoflagellates, sponge spicules, foraminifers, and calcareous nannofossils (Figure 2c). The tephra samples also include foraminifers and calcareous nannofossils.
Figure 2 shows X-ray diffraction (XRD) patterns obtained from the samples. The clay to silty clay samples consist of quartz, plagioclase, pyrite, phyllosilicate, and biogenic components. In contrast, the silicic to calcareous ooze samples of Unit II are almost free of clay minerals and are instead composed mainly of calcite and possibly amorphous silica. The existence of amorphous silica is indicated by a broad peak detected at 2θ = 20° to 30°. The tephra samples also show similar broad peaks that indicate an abundance of glass.
Friction experiments were conducted on the samples using a rotary-shear, intermediate- to high-velocity friction-testing machine. The collected samples were dried at 60°C for 24 h and disaggregated gently using a pestle. For each experiment, 0.5 g of the sample was placed between a pair of gabbro cylinders, each measuring 24.98 mm in diameter. Distilled water was added to the experimental gouge layer to saturate it. The experimental fault was surrounded by a Teflon® (DuPont, Wilmington, DE, USA) ring to avoid gouge expulsion during experiments. The gouge was axially pre-compacted under the test conditions (5 MPa) for 30 min.
In the friction experiments, all the pre-compacted samples were subjected to an initial slip velocity of 0.28 mm/s until ~170 mm of displacement (three rotations of the sample cylinder), during which time the friction value either increased or decreased to attain a roughly steady-state value. After this pre-sliding, a sequence of velocity stepping by a factor of 10 was imposed to examine the velocity dependence of friction for loading velocities of 0.0028 to 2.8 mm/s and displacements of >250 mm.
Figure 4a shows the initial peak and steady-state μ values measured for all the samples. The values of friction coefficient μ of the sediments at the onset of sliding at V = 0.28 mm/s, except for the tephra samples, are roughly in the same range of 0.4 to 0.6. In contrast, the steady-state μ values display clear differences between samples from different lithostratigraphic units. It is notable that the values of μ of the silicic to calcareous ooze samples decrease with greater sampling depth.
The samples tested in this study can be divided into two groups with regard to the velocity dependence of steady-state friction. Two representative results are shown in Figure 3a,c, in which V was changed in a stepwise manner between 0.028 and 0.28 mm/s. Sample U1381A-6R-2 W (Figure 3a), representative of the clay to silty clay samples of Unit I, shows consistently positive velocity dependence at these velocities, whereas sample U1381A-8R-1 W (Figure 3c), representative of the silicic to calcareous ooze samples of Unit II, shows mostly negative velocity dependence. The critical slip distances Dc at these velocities also differ between the two samples: the value of sample U1381A-6R-2 W is ~0.1 mm, whereas that of sample U1381A-8R-1 W is ~0.05 mm. At higher velocities of >0.28 mm/s, the velocity dependence of the clay to silty clay is still positive (Figure 3b), whereas that of the silicic to calcareous ooze is either neutral or positive (Figure 3d).
Values of (a - b) are plotted as a function of up-step velocities in Figure 3e. Values of (a - b) of the clay to silty clay samples are low (<0.005) at lower velocities but increase to ~0.01 at the highest velocities. The absolute values of (a - b) of the silicic to calcareous ooze samples are mostly low (<0.005) at the lowest velocities of 0.0028 to 0.028 mm/s. At intermediate velocities of 0.028 to 0.28 mm/s, absolute values of (a - b) become smaller and approach zero, implying that the silicic to calcareous ooze samples exhibit an almost neutral velocity dependence of friction at these velocities (Figure 3e).
Similar contrasting frictional properties of subduction zone material have recently been reported by Ikari et al. (2013) from an experimental study of the sediments entering the Costa Rica subduction zone offshore the Nicoya Peninsula. In the Nicoya Peninsula region, calcareous nannofossil chalk is frictionally strong (μ = 0.71 to 0.88) and shows velocity-weakening behavior, whereas the hemipelagic clayey sediment is weak (μ = 0.22 to 0.35) and exhibits mostly velocity-strengthening behavior.
Discussion and conclusions
The observed weakness of the clay to silty clay sediments suggests that the frictional strength of faults is controlled mainly by the content of clay minerals, as has been demonstrated in many experimental studies (e.g., Summers and Byerlee 1977; Morrow et al. 1992, 2000; Brown et al. 2003; Kopf and Brown 2003; Moore and Lockner 2004; Ikari et al. 2009a,[b]; Tembe et al. 2010; Saito et al. 2013; Takahashi et al. 2013). In contrast, the steady-state μ values of the silicic to calcareous ooze sediments from Unit II are high, measuring ~0.8 and are comparable to the values reported for crustal rock samples (Byerlee 1978). The strength of the silicic to calcareous ooze samples probably arises because they are almost free of weak clay minerals.
The initial peak and steady-state μ values of the silicic to calcareous ooze samples decrease systematically with increasing depth (Figure 4a). No systematic change in physical properties (e.g., P-wave velocity, porosity, and shear strength) that would correlate to the variation in μ values has been reported for the Unit II cores (Expedition 334 Scientists 2012). However, it is interesting to note that some geochemical constituents of the pore fluid of the cores, such as the contents of Ca and SO4, change monotonically with depth within Unit II (Expedition 334 Scientists 2012). This suggests the possible influence of fluid chemistry on the frictional properties of the fault material.
In the toe region of subduction zones, décollements are likely to initiate within, or shift their positions to, zones of weaker stratigraphic intervals (Underwood 2007). Given the low frictional coefficient values observed for the clay to silty clay samples of Unit I, the décollement at the CRISP transect offshore the Osa Peninsula likely initiated in Unit I and was initially very weak. It should be noted that our velocity-step tests reveal that clay to silty clay exhibits only positive frictional velocity dependence (Figure 3). The observed velocity-strengthening behavior and the very low steady-state μ values of the Unit I sediments suggest that the faults in the shallow portion of the Costa Rica subduction zone are stable and thus behave as creeping segments.
In contrast, the silicic to calcareous ooze samples from Unit II, which are characterized by higher μ values, exhibit mostly velocity-weakening behavior for slip velocities of 0.0028 to 0.28 mm/s (Figure 3e). Velocity-weakening behavior implies potentially unstable fault motion (Scholz 1998). In addition, the lower Dc values of silicic to calcareous ooze samples may mean that faults that develop in Unit II are more unstable. The difference in Dc values between Unit I and Unit II may be caused by different clay mineral contents. The June 2002 Mw 6.4 earthquake implies the existence of velocity-weakening material in this focal depth region. Fault slip that nucleated along a fault in Unit II would be a likely candidate for the source of the shallow earthquake event.
Variation in the distribution and type of earthquakes along the Costa Rica subduction zone, such as the shallow earthquake that occurred at Osa Peninsula (the 2002 Mw 6.4 event, Figure 1a) and the SSEs at Nicoya Peninsula, is suggestive of variations in fault zone material along the plate interface. The simple stratification signature of the material inputs (Figure 1b) may help us to better understand the processes leading to a heterogeneous distribution of fault material along the subduction channel. For instance, the topography of the seafloor may contribute to generating a heterogeneous distribution of materials along the plate boundary (Ikari et al. 2013). Considering the possible association of seismicity with the dewatering reaction along the plate interface (e.g., Spinelli et al. 2006; Audet and Schwartz 2013), the stabilizing effect of pore fluid pressure in the vicinity of velocity-weakening faults must be taken into account. High pore fluid pressure may produce a conditionally stable region along a velocity-weakening fault with a small value of (a - b) by reducing the effective normal stress (Scholz 1998). A velocity-weakening fault in Unit II would be stable under quasistatic loading but may become unstable under sufficiently strong dynamic loading. Velocity-strengthening behavior, as observed for the Unit II samples at higher velocities of 0.28 to 2.8 mm/s (Figure 3e), may also act to stabilize the propagation of earthquake ruptures, and may influence slip patterns along faults or the magnitudes of earthquakes. As suggested by Saito et al. (2013), such velocity-strengthening behavior, in addition to variation in pore fluid pressure, is important for understanding the behavior of faults in subduction zones.
We thank two anonymous reviewers for their constructive comments and suggestions, which led to improvements in the manuscript. We used samples and data provided by the Integrated Ocean Drilling Program (IODP). We thank IODP Expedition 334 shipboard scientific personnel for the helpful discussions at various stages of this work. This research was supported by the IODP after-cruise research program, JAMSTEC, and MEXT grant 21107004. The study was also supported by MEXT Japan, under its Observation and Research Program for Prediction of Earthquakes and Volcanic Eruptions project. AT was supported partly by JSPS grant 24340104.
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