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Slip Weakening As a Mechanism for Slow Earthquakes

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Slip Weakening As a Mechanism for Slow Earthquakes LETTERS PUBLISHED ONLINE: 12 MAY 2013 | DOI: 10.1038/NGEO1818 Slip weakening as a mechanism for slow earthquakes Matt J. Ikari1,2*, Chris Marone2, Demian M. Saffer2 and Achim J. Kopf1 Slow slip forms part of the spectrum of fault behaviour between splay fault (Site C0004; ∼300 m depth) and the décollement (Site stable creep and destructive earthquakes1,2. Slow slip occurs 1174; ∼800 m depth) were penetrated during Integrated Ocean near the boundaries of large earthquake rupture zones3,4 and Drilling Program Expedition 316 and Ocean Drilling Program Leg may sometimes trigger fast earthquakes2. It is thought to 190, respectively13,14 (Fig. 1). Brecciated hemipelagic silty claystone occur in faults comprised of rocks that strengthen under fast samples from within zones of active shearing contain 37–65% clay slip rates, preventing rupture as a normal earthquake, or on minerals, up to 40% quartz+plagioclase and little calcite13,14. faults that have elevated pore-fluid pressures5–7. However, Fault slip instability and earthquake nucleation are typically the processes that control slow rupture and the relationship evaluated by combining elastic dislocation theory with friction between slow and normal earthquakes are enigmatic. Here we laws16–19. The criterion for slip instability is derived from a force use laboratory experiments to simulate faulting in natural rock balance, in which the elastic stiffness of the loading system K samples taken from shallow parts of the Nankai subduction (stress/length) must be smaller than a critical stiffness Kc defined zone, Japan, where very low-frequency earthquakes—a form of by the properties of the fault. Within the framework of rate- slow slip—have been observed8–10. We find that the fault rocks and state-dependent (RSF) friction, the instability criterion can be exhibit decreasing strength over millimetre-scale slip distances written to first order as: rather than weakening due to increasing velocity. However, − − 0 (a b)σn the sizes of the slip nucleation patches in our laboratory K < Kc D (1) simulations are similar to those expected for the very low- Dc 0 frequency earthquakes observed in Nankai. We therefore where σn is the effective normal stress and Dc is a critical slip distance suggest that this type of fault-weakening behaviour may over which fault strength evolves18. The parameter a−b is defined generate slow earthquakes. Owing to the similarity between by the experimentally measured change in steady-state friction in the expected behaviour of slow earthquakes based on our data, response to a change in sliding velocity from Vo to V (Fig. 2): and that of normal earthquakes during nucleation, we suggest that some types of slow slip may represent prematurely 1µss a−b D arrested earthquakes. ln(V =Vo) Most slow fault slip has been observed near the downdip limit of the seismogenic zone at depths of ∼25–45 km (refs 2,11), but in Positive values of a−b define velocity-strengthening behaviour, well-instrumented areas it has also been detected at shallow depths which indicates that stable sliding should be expected. Negative (<10 km), updip of large earthquake ruptures8–10. Slow slip events values of a − b (velocity weakening) are a requirement for the at tectonic boundaries probably occur at the plate interface2,10, nucleation of slip instability17,18. and therefore should be controlled by the frictional properties and In general, the mechanism(s) that allow slow slip that is conditions that characterize fault zones. For deep slow slip events, sustained and quasi-dynamic is poorly understood12,19. Many laboratory testing of natural material under in situ conditions is forms of slow slip are considered to be aseismic (for example, not feasible at present. Therefore, there is very little evidence to creep events, earthquake afterslip) and therefore are expected constrain the boundary conditions and friction laws that govern only on velocity-strengthening faults, and typically in response how these events nucleate, propagate and ultimately arrest12. Less to a perturbation20. These non- or quasi-dynamic events would attention has been paid to slow and transient slip at shallow depths; be undetected by seismometers and instead must be identified however, these regions are accessible to drilling and sampling. Here, geodetically21–23. Others, such as VLF events, are detected seis- we focus on the Nankai Trough, Japan, where mudstone samples mically (that is, they radiate seismic energy), indicating a dy- have been recovered from major shallow fault zones13,14. namic stress drop8,9. Laboratory work shows consistent velocity Very low-frequency (VLF) earthquakes observed at Nankai strengthening for samples from the shallow Nankai megasplay have reverse-faulting focal mechanisms, and occur on splay faults and décollement24, which is seemingly incompatible with the oc- within the accretionary prism or along the shallow décollement8–10. currence of shallow VLF events, although an observed minima They seem to occur in regions of elevated pore-fluid pressure in a − b in the velocity range ∼0.3-3 µm s−1 has been predicted and therefore decreased effective stress11,15. The event locations to favour slow slip24. correlate approximately with the estimated updip extent of rupture One aspect of friction that is typically overlooked is the ef- from the 1944 and 1946 Mw > 8 great earthquakes, and with the fect of long-term slip hardening or weakening. In the standard location of scientific drillsites (Fig. 1). Shallow portions of a major RSF formulation, the slip over which friction evolves following a 1MARUM, Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany, 2Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA. *e-mail: [email protected]. NATURE GEOSCIENCE j ADVANCE ONLINE PUBLICATION j www.nature.com/naturegeoscience 1 © 2013 Macmillan Publishers Limited. All rights reserved. LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1818 a Tokai Japan Kii Peninsula 1944 34° N Shikoku Island Site C0004 ° 33 N K yushu Kumano Site 1174 Transect 1946 32° N Muroto Transect Nankai Trough 0100 200 deformation front km 132° E134° E 136° E 138° E b Site C0004 mbsf 2 Site C0004 Forearc basin 200 4 Megasplay Décollement 300 Megasplay fault zone 6 Accretionary fault prism 400 8 Depth (km) 10 12 Kumano transect Line 5 60 50 40 30 20 10 0 Distance from deformation front (km) c 0 Site 1174 mbsfSlope 700basin Frontal 2 thrust Site Décollement Frontal 800 1174 4 Accretionary prism zone OOST 900 6 Depth (km) 1,000 Décollement 8 1,100 Décollement Muroto transect Line 141-2D 10 60 50 40 30 20 10 0 Distance from deformation front (km) Figure 1 j Geologic setting of the Nankai subduction zone. a, Map of the Nankai area, offshore Japan, showing the locations of drilling sites C0004 (megasplay) and 1174 (décollement). Grey ovals indicate clusters of VLF earthquakes identified in ref. 8. Also shown are rupture areas (dashed boxes) and epicentres (stars) of the 1944 Tonankai and 1946 Nankaido earthquakes. b, Cross-section of the Kumano transect, showing the location of samples tested from the megasplay fault zone. c, Cross-section of the Muroto transect, showing the location of samples tested from the décollement zone. Figure modified from ref. 24. perturbation, Dc, is usually ∼≤100 µm (ref. 17). Slip-dependent is overcome by longer term slip weakening beyond a critical friction trends over sliding distances of ∼1 mm are observed ex- sliding distance W : perimentally, but typically assumed to reflect background trends that do not affect sliding stability25. We investigate the effects (a−b)ln(V =Vo) of these long-term trends by measuring the slip dependence of W D −η friction, defined by the parameter η D dµ/dx (mm−1) measured from a linear fit to the last ∼300 µm of each 800 µm velocity (Fig. 2). As W is calculated from a − b and η, it also exhibits step (Fig. 2c). In our experiments, this slip weakening occurs minimum values in the range 0.3–3 µm s−1 (Fig. 3c). If a − b beyond the initial effects of slip velocity perturbations; therefore, and/or W are small, unloading of the fault zone during slip is although η is determined from velocity steps it is distinct from dominated by the long-term slip-weakening rate η rather than standard RSF behaviour. by RSF parameters. In this case, a stiffness criterion analogous to We find that negative values of η are common for samples from equation (1) may be expressed as: Nankai, indicating slip-weakening behaviour (Fig. 3b). Minimum values of η coincide with the minima in a − b in the velocity K < K ∗ D −ησ 0 c n range ∼0.3–3 µm s−1. Negative η could result in net weakening ∗ and slip instability, if initial velocity-strengthening behaviour where Kc is the critical stiffness for slip instability by slip weakening. 2 NATURE GEOSCIENCE j ADVANCE ONLINE PUBLICATION j www.nature.com/naturegeoscience © 2013 Macmillan Publishers Limited. All rights reserved. NATURE GEOSCIENCE DOI: 10.1038/NGEO1818 LETTERS a a 0.008 μ Megasplay V V o Décollement 0.006 b*ln(V/Vo) a*ln(V/Vo) Δμ ss 0.004 a ¬ b Coefficient of sliding friction (Coefficient ) 0.002 Displacement (x), mm b 0.000 μ 0.01 0.10 1 10 100 V V o Load point velocity (μm s¬1) W b 0.015 0.010 Δx Δμ Δμ ss 0.005 Coefficient of sliding friction (Coefficient ) ) ¬1 0.000 Displacement (x), mm mm ( c 0.355 η ¬0.005 μ ¬1 μ ¬1 Vo = 0.3 m s V = 1 m s ¬0.010 0.350 a = 0.0053 b = 0.0026 ¬0.015 μ Dc = 13.2 m Δ a ¬ b = 0.0027 ¬0.020 μ 0.01 0.10 1 10 100 0.345 Load point velocity (μm s¬1) Δx W = 165 μm μ c ¬1 100 ηο = ¬0.0127 mm 0.340 Δμ 10 0.335 Experimental data Model inversion Δx P2068: C0004D 34R-1 ¬1 ηο = ¬0.0162 mm 1 (mm) 0.330 W 10.4 10.6 10.8 11.0 11.2 11.4 11.6 Load point displacement (mm) 0.10 Figure 2 j Determination of important friction parameters.
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