Deep rock damage in the San Andreas Fault revealed by P- and S-type fault-zone-guided waves

Damage to fault-zone rocks during fault slip results in the formation of a channel of low seismic-wave velocities. Within such channels guided seismic waves, denoted by Fg, can propagate. Here we show with core samples, well logs and Fg-waves that such a channel is crossed by the SAFOD (San Andreas Fault Observatory at Depth) borehole at a depth of 2.7 km near Parkfield, California, USA. This laterally extensive channel extends downwards to at least half way through the seismogenic crust, more than about 7 km. The channel supports not only the previously recognized Love-type(FL) and Rayleigh-type(FR) guided waves, but also a new fault-guided wave, which we name FF. As recorded 2.7 km underground, FF is normally dispersed, ends in an Airy phase, and arrives between the Pand S-waves. Modelling shows that FF travels as a leaky mode within the core of the fault zone. Combined with the drill core samples, well logs and the two other types of guided waves, FF at SAFOD reveals a zone of profound, deep, rock damage. Originating from damage accumulated over the recent history of fault movement, we suggest it is maintained either by fracturing near the slip surface of earthquakes, such as the 1857 Fort Tejon M 7.9, or is an unexplained part of the fault-creep process known to be active at this site. When a fault moves in the ‘elastico-frictional’ regime in the Earth’s crust, it both releases elastic energy stored in the surrounding rocks and reduces the strength of the fault through non-elastic processes (Sibson 1977). The resulting brittle shearing and fracturing are not limited to a sliding surface, and this results in a damage zone containing a mixed suite of gouges, cataclasites and breccias (Sibson 1977). Surface and mine exposures of exhumed fault zones has led to the development of a canonical model of crustal faults (Chester & Chester 1998; Faulkner et al. 2003; Sibson 2003; Chester et al. 2004). This model is characterized by one or more very narrow slip zones (millimetres to decimetres) where most of the fault displacement occurs. This plane is embedded in a broader damage zone that is tens to hundreds of metres wide. The damage zone is thought to form as a consequence of several causes. These include: the changing geometry of the principal slip zone at jogs and stepovers (Sibson 1986); preferential shearing in less-competent lithologies within the fault zone (Fagereng & Sibson 2010); and earthquake-concentrated stresses exceeding the rock strength adjacent to the fault surface (Andrews 2005). This latter mechanism operates along straight fault segments as well as at geometrical complexities. As a result of these mechanisms, macroscopic and microscopic fracturing in the fault zone increases. Consequently, seismic-wave velocities are reduced and attenuation (1/Q, where Q is the quality factor) is increased within the damage zone. The resulting low-velocity, low-Q zone forms a waveguide that follows the fault. Within this waveguide special seismic waves, denoted by Fg, can propagate (Ben-Zion & Aki 1990; Li & Leary 1990; Li et al. 1990; Malin et al. 1996). These include low-frequency SH/Love-type (FL) and PSV/Rayleigh-type (FR) fault-zone-guided waves. These waves are routinely observed along surface ruptures of major earthquakes; as, for example, after the 1992 Landers, California earthquake (Li & Vidale 2001). The depth extent of the low-velocity channel, however, has been controversial. Some researchers argue that it is a near-surface feature, reaching only down to the top of the seismogenic zone: less than about 3–5 km (e.g. Ben-Zion et al. 2003; Peng et al. 2003; Lewis et al. 2005; Lewis & Ben-Zion 2010). Others argue that it extends well into the elastico-frictional regime: approximately 5–10 km (e.g. Korneev et al. 2003; Li & Malin 2008; Wu et al. 2010). In some cases, different wave types from the same recordings have been used to argue for shallow (Yang & Zhu 2010) or deep (Li & Vernon 2001) damage. From: Fagereng, Å., Toy, V. G. & Rowland, J. V. (eds) Geology of the Earthquake Source: A Volume in Honour of Rick Sibson. Geological Society, London, Special Publications, 359, 39–53. DOI: 10.1144/SP359.3 0305-8719/11/$15.00 # The Geological Society of London 2011. by guest on January 18, 2012 http://sp.lyellcollection.org/ Downloaded from