1 Study megathrust creep to understand megathrust earthquakes Kelin Wang Pacific Geoscience Centre, Geological Survey of Canada, kelin.wang@canada.ca Introduction Once upon a time, there was a belief that plate subduction was almost purely seismic (e.g., Davies and Brune, 1971), that is, the shallow part of the subduction interface would slip almost only in megathrust earthquakes. This led to the useful but incomplete seismogenic zone concept that emphasizes changes in fault slip behaviour in the dip direction. Globally improved estimates of seismic moment rates led to the notion that aseismic creep might be the primary mode of subduction (Pacheco et al., 1993), but uncertainties were large because of the short time span of instrumental recording as compared to the recurrence intervals of great earthquakes. Today, space geodesy has put it beyond any doubt that many subduction faults creep (Wang and Bilek, 2014). Creep and its spatial and temporal variations hold many keys to understanding fault mechanics and earthquake cycles. Until we understand how and why faults creep, we do not fully understand how and why they produce large earthquakes. However, most of the physics and geology of low-temperature creep is not known. What percentage of the ~43,500 km of the world s subduction faults creep? What controls the duration and rate of creep pulses (slow slip events)? Which creep areas we see today are geologically controlled and thus persistent? What is partial locking (or coupling coefficient < 100%)? Can the same fault patch exhibit both stick-slip and aseismic creep but at different times? How do creeping segments interact with locked segments? What role do they play in controlling earthquake cycles? It is time that the scientific community paid greater respect for creeping megathrusts and made consorted efforts to study and understand them. The geodesy of megathrust creep It is extremely rare to see geodesy-based megathrust locking models that do not feature creeping somewhere along strike. In some cases such as northern Andes, the inferred creeping segments have strike lengths Fig. 1. Modified from Nocquet et al. (2014). Geodetically inferred ~800 km long megathrust creep zone. Green vectors are GPS velocities (with error ellipses). Coupling coefficient describes degree of locking (100% = full locking).
2 comparable to the 1960 Mw 9.5 Chile earthquake (Nocquet et al., 2014; Fig. 1). In other cases such as Cascadia, full locking of a shallow and narrow segment of the fault or a mixture of deeper locking and shallow creep can both fit available data (Schmalzle et al., 2014; Fig. 2). Uncertainties are large because land-based geodetic observations have very poor offshore resolution. To define actual state of creeping or locking, information critically needed for seismic and tsunami risk assessment, there is urgent need to make systematic seafloor geodetic measurements at multiple subduction zones. Limited seafloor geodetic measurements in a few places so far have seen full locking (Gagnan et al., 2005), creep pulses (Wallace et al., 2016), and along-strike alternation of locking and creeping (Yokoda et al., 2016) in the near-trench segment of the megathrust. To choose between the two drastically different Cascadia locking models in Fig. 2, the only practical method is to make seafloor geodetic measurements near the deformation front, especially seafloor GNSS measurements (Wang and Tréhu, 2016). It is likely that low-temperature creep is accomplished predominantly in pulses of variable temporal and spatial scales, for which northern Hikurangi s slow slip events offer one example (Wallace et al., 2016). Geodetically characterizing these scales are important tasks in the study of fault mechanics. Fig. 2. Two locking models of the Cascadia subduction zone from the inversion of land-based geodetic observations (Schamlzle et al., 2014). Gamma and Gaussian are names of functions used to constrain downdip distribution of locking ratio (1 = full locking). Both models fit geodetic data (GPS and leveling) equally well, but (a) features full locking of the shallow megathrust and (b) features creeping of the shallow megathrust with higher degrees of locking at great depths. The geology of megathrust creep There are two end-member types of low-temperature creep: weak creep of smooth faults and strong creep of rough faults (Gao and Wang, 2014), with a spectrum of intermediate modes in between. Most conceptual and numerical models deal with weak creep, assuming a very smooth fault with a gouge typically weakened by hydrous minerals. Less understood is strong creep. Known examples include the southern end of Japan Trench (Fig. 3), northern Hikurangi, Kyushu, and off Peru where the Nazca ridge is subducting. Strong creep appears to be common and is associated with the subduction of large
3 geometrical irregularities such as seamounts and aseismic ridges (Wang and Bilek, 2014; Basset and Watts, 2015). These irregularities generate fracture systems as they push ahead against the resistance of brittle rocks. The resultant heterogeneous stress and structural environment makes it very difficult to lock the fault. The geodetically observed creep under such conditions is accomplished by the complex deformation of a 3D damage zone. Although an integrated frictional strength of the fault is still a useful concept, the creeping mechanism is very different from frictional slip of a velocity-strengthening smooth fault. Cataclasis and pressure-solution creep in the fracture systems must be important processes in strong creep. Near-field geophysical monitoring of modern creeping faults and studies of exhumed ancient faults will help understand the physical mechanism of strong creep. Heat flow measurements are needed to better constrain energy dissipation of strong creep (Gao and Wang, 2014). Identification of geologically controlled, persistent creep zones is of vital importance to accessing seismic and tsunami hazards. The importance of low-temperature creep to fault mechanics can be understood in more general terms. Rate-and-state friction in models or laboratory settings is an extreme form of shear localization. Less mature faults host a shear zone, of which strong creep is an extreme example. Distributed shear in parts of this zone gives rise to fault creep, although other parts of the same zone may host more localized and even seismic slip (Rowe et al., 2013). To gain a fuller picture of fault slip, laboratory experiments need to be devised that can accommodate a thick zone of gouge, granular materials, or 3D structure. Fig. 3. Creeping zone in southernmost Japan Trench associated with subducting seamounts. Left (Wang and Bilek, 2014): Rupture area of 2011 Tohoku-oki earthquake from four different models; star shows the epicentre. Right: Locking/creep distribution of megathrust inferred from land-based geodetic measurements (red = locking). The reliability of the model is poor in the dip direction but OK along strike. The seismology of megathrust creep Low-temperature fault creep is usually accompanied with small earthquakes. The present assumption of full locking along most of Cascadia, that is, the preference of locking models like Fig. 2a over that of Fig. 2b, is based on the paucity of interplate seismicity (Wang and Tréhu, 2016), but it is yet to be determined whether certain types of low-temperature creep can occur without producing small earthquakes. For weak creep, the small earthquakes are understood to be the rupture of isolated small seismogenic patches imbedded in the creep zone. Repeated
4 rupture of the same patch results in mini- earthquake cycles that can be used to estimate the creep rate of the hosting creep zone (e.g., Uchida and Matsuzawa, 2013). For strong creep, offfault seismicity is common, with a variety of focal mechanisms. The strong creep at Kyushu produces massive tremor and very-low-frequency earthquakes throughout the shallow, potentially seismogenic part of the subduction fault, although the thickness of the tremor band is unknown (Yamashita et al., 2015). Near-field OBS monitoring is needed to characterize seismicity associated with creep, relevant scaling relationship (e.g., b-value), and any variations with time. Controlled-source seismology plays an irreplaceable role in understanding creep. Highresolution mapping of subducting geometrical irregularities and the 3D structure of the associated fracture systems, combined with studies of exhumed fault zones, provides basic information for the mechanical state of the fault zone. The geodynamics of megathrust creep For generating large megathrust earthquakes, locking and rupturing is only half of the story. The driving force for megathrust earthquakes is fundamentally far-field, but creeping zones provide local, near-field loading for adjacent locked patches. The relative size and spatial arrangement of the locked and creeping patches to a large degree control the recurrence interval of megathrust rupture, for which repeating earthquakes provide a miniature illustration. This perspective is lacking in the 2D seismogenic zone concept. In fact, creeping zones of sufficient size that serve to separate locked patches along strike enable the definition of future rupture zones using interseismic geodetic observations (Protti et al., 2014). In a viscoelastic Earth, if the locked zone is very long along strike, it is not possible to infer the downdip limit of locking from interseismic geodetic observations alone (Wang and Tréhu, 2016). The study of megathrust creep is important to understanding stress shadowing, which refers to the effect of a locked patch hindering the motion of its neighbouring fault areas. An example is the shallow, near-trench segment of the megathrust updip of a locked seismogenic zone. One may envision that this zone is in the stress shadow of the locked zone, but one can also produce rate-state friction models that let this zone slowly creep for the entire interseismic period. What needs to be known is the influence range of the stress shadow of the locked zone and its time evolution. The most effective way to gain this knowledge is to monitor near-trench motion with seafloor geodesy. Stick-slip and creeping subduction faults influence long-term deformation of plate boundary zones in very different ways. This is especially true in a viscoelastic Earth (Wang et al., 2012). The study of megathrust creep is thus important also to large-scale geodynamics. References Bassett, D., and A. B. Watts (2015), Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief, Geochem. Geophys. Geosyst., 16, 1508 1540.
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