Christie D. Rowe

Structure and composition of the plate-boundary slip zone for the 2011 Tohoku-Oki earthquake.

Deep Drilling for Earthquake Clues The 2011 Mw 9.0 Tohoku-Oki earthquake and tsunami were remarkable in many regards, including the rupturing of shallow trench sediments with huge associated slip (see the Perspective by Wang and Kinoshita). The Japan Trench Fast Drilling Project rapid response drilling expedition sought to sample and monitor the fault zone directly through a series of boreholes. Chester et al. (p. 1208) describe the structure and composition of the thin fault zone, which is predominately comprised of weak clay-rich sediments. Using these same fault-zone materials, Ujiie et al. (p. 1211) performed high-velocity frictional experiments to determine the physical controls on the large slip that occurred during the earthquake. Finally, Fulton et al. (p. 1214) measured in situ temperature anomalies across the fault zone for 9 months, establishing a baseline for frictional resistance and stress during and following the earthquake. The Tohoku-Oki earthquake occurred along a thin, clay-rich fault zone in the basal strata of the subducting plate. The mechanics of great subduction earthquakes are influenced by the frictional properties, structure, and composition of the plate-boundary fault. We present observations of the structure and composition of the shallow source fault of the 2011 Tohoku-Oki earthquake and tsunami from boreholes drilled by the Integrated Ocean Drilling Program Expedition 343 and 343T. Logging-while-drilling and core-sample observations show a single major plate-boundary fault accommodated the large slip of the Tohoku-Oki earthquake rupture, as well as nearly all the cumulative interplate motion at the drill site. The localization of deformation onto a limited thickness (less than 5 meters) of pelagic clay is the defining characteristic of the shallow earthquake fault, suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes.

Stress State in the Largest Displacement Area of the 2011 Tohoku-Oki Earthquake

Stressed Out Large seismic events such as the 2011 magnitude 9.0 Tohoku-Oki earthquake can have profound effects not just on the severity of ground motion and tsunami generation, but also on the overall state of the crust in the surrounding regions. Lin et al. (p. 687) analyzed the stress 1 year after the Tohoku-Oki earthquake and compared it with the estimated stress state before the earthquake. In situ resistivity images were analyzed from three boreholes drilled into the crust across the plate interface where the earthquake occurred. Stress values indicate a nearly complete drop in stress following the earthquake such that the type of faulting above the plate boundary has changed substantially. These findings are consistent with observations that the sea floor moved nearly 50 meters during the earthquake. Borehole stress measurements indicate a nearly total stress drop in the region of largest slip. The 2011 moment magnitude 9.0 Tohoku-Oki earthquake produced a maximum coseismic slip of more than 50 meters near the Japan trench, which could result in a completely reduced stress state in the region. We tested this hypothesis by determining the in situ stress state of the frontal prism from boreholes drilled by the Integrated Ocean Drilling Program approximately 1 year after the earthquake and by inferring the pre-earthquake stress state. On the basis of the horizontal stress orientations and magnitudes estimated from borehole breakouts and the increase in coseismic displacement during propagation of the rupture to the trench axis, in situ horizontal stress decreased during the earthquake. The stress change suggests an active slip of the frontal plate interface, which is consistent with coseismic fault weakening and a nearly total stress drop.

The thickness of subduction plate boundary faults from the seafloor into the seismogenic zone

The thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior. We have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges. We describe a general geometric model for subduction thrust decollements, which includes multiple simultaneously active, anastomosing fault strands tens of meters thick. The total thickness encompassing all simultaneously active strands increases to ~100–350 m at ~1–2 km below seafl oor, and this thickness is maintained down to a depth of ~15 km. Thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens-ofmeters- thick fault strands. Although fl attening, primary inherited chaotic fabrics, and fault migration through subducting sediments or the frontal prism may build melange sections that are much thicker (to several kilometers), this thickness does not describe the active fault at any depth. These observations suggest that models should treat the subduction thrust plate boundary fault as <1–20 cm thick during earthquakes, with a concentration of postseismic and interseismic creep in single to several strands 5–35 m thick, with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands.

Silica gel formation during fault slip: Evidence from the rock record

Dynamic reduction of fault strength is a key process during earthquake rupture. Many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments, including silica gel lubrication. However, few have been observed in nature. Here we report on the fi rst documented occurrence of a natural silica gel coating a fault surface. The Corona Heights fault slickenside in San Francisco, California, is covered by a shiny layer of translucent silica. Microstructures in this layer show fl ow banding, armored clasts, and extreme comminution compared to adjacent cataclasites. The layer is composed of ~100 nm to 1 µm grains of quartz, hydrous crystalline silica, and amorphous silica, with 10‐100 nm inclusions of Fe oxides and ellipsoidal silica colloids. Kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fl uid during fault slip. The layer therefore represents a relict silica gel that formed during fault motion, and which could have resulted in frictional instability. These observations confi rm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions.

Record of mega-earthquakes in subduction thrusts: The black fault rocks of pasagshak point (Kodiak Island, Alaska)

On Kodiak Island, Alaska, decimeterthick black fault rocks are at the core of foliated cataclasites that are tens of meters thick. The cataclasites belong to melange zones that are regarded as paleodecollements active at 12–14 km depth and 230–260 °C. Each black layer is mappable for tens of meters along strike. The black fault rocks feature a complex layering made at microscale by alternation of granular and crystalline micro textures, both composed of micronscale subrounded quartz and plagioclase in an ultrafi ne, phyllosilicate-rich matrix. In the crystalline microlayers, tabular zoned micro lites of plagioclase make up much of the matrix. No such feldspars have been found in the cataclasite. We interpret these crystalline microlayers as pseudotachylytes. The granular microlayers show higher grainsize variability, crushed microlites, and textures typical of fl uidization and granular fl ow deformation. Crosscutting relationships between granular and crystalline microlayers include fl ow and intrusion structures and mutual brittle truncation. This suggests that each decimeters-thick composite black fault rock layer records multiple pulses of seismic slip. In each pulse, ultracomminuted fl uidized material and friction melt formed and deformed together in a ductile fashion. Brittle truncation by another pulse occurred after solidifi cation of the friction melt and the fl uidized rock. X-ray powder diffraction (XRPD) and X-ray fl uorescence (XRF) analyses show that black fault rocks have similar mineral composition and chemical content as the cataclasites. The observed systematic chemical differences cannot be explained by bulk or preferential melting of any of the cataclasite components. The presence of an open, fl uidinfi ltrated system with later alteration of black fault rocks is suggested. The geochemical results indicate that these subductionrelated pseudotachylytes differ from those typically described in crystalline rocks and other tectonic settings.

A geological fingerprint of low-viscosity fault fluids mobilized during an earthquake

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, B01303, doi:10.1029/2008JB005633, 2009 A geological fingerprint of low-viscosity fault fluids mobilized during an earthquake E. E. Brodsky, 1 C. D. Rowe, 2 F. Meneghini, 1 and J. C. Moore 1 Received 14 February 2008; revised 15 September 2008; accepted 10 October 2008; published 9 January 2009. [ 1 ] The absolute value of stress on a fault during slip is a critical unknown quantity in earthquake physics. One of the reasons for the uncertainty is a lack of geological constraints in real faults. Here we calculate the slip rate and stress on an ancient fault in a new way based on rocks preserved in an unusual exposure. The study area consists of a fault core on Kodiak Island that has a series of asymmetrical intrusions of ultrafine- grained fault rock into the surrounding cataclasite. The intrusive structures have ductile textures and emanate upward from a low-density layer. We interpret the intrusions as products of a gravitational (Rayleigh-Taylor) instability where the spacing between intrusions reflects the preferred wavelength of the flow. The spacing between intrusions is 1.4 ± 0.5 times the thickness of the layer. This low spacing-to-thickness ratio cannot be explained by a low Reynolds number flow but can be generated by one with moderate Reynolds numbers. Using a range of density contrasts and the geometry of the outcrop as constraints, we find that the distance between intrusions is best explained by moderately inertial flow with fluid velocities on the order of 10 cm/s. The angle that the intrusions are bent over implies that the horizontal slip velocity was comparable to the vertical rise velocity, and therefore, the fault was slipping at a speed of order 10 cm/s during emplacement. These slip velocities are typical of an earthquake or its immediate afterslip and thus require a coseismic origin. The Reynolds number of the buoyant flow requires a low viscous stress of at most 20 Pa during an earthquake. Citation: Brodsky, E. E., C. D. Rowe, F. Meneghini, and J. C. Moore (2009), A geological fingerprint of low-viscosity fault fluids mobilized during an earthquake, J. Geophys. Res., 114, B01303, doi:10.1029/2008JB005633. 1. Introduction [ 2 ] In order to successfully model the initiation, propa- gation or arrest of an earthquake, quantitative measures of the absolute resisting stresses on faults are necessary. Unfortunately, these stresses are still uncertain. Seismology alone cannot measure absolute stresses [Abercrombie et al., 2006]. Rock mechanics suggests that the absolute stress between two solid rocks should be on the order of the lithostatic stress [Byerlee, 1970]. However, heat flow studies fail to measure the implied frictional heating [Lachenbruch and Sass, 1980, 1992]. Furthermore, recent laboratory and theoretical work suggests that qualitatively different processes are active at high slip rates that funda- mentally change the resisting stresses [Andrews, 2002; Brodsky and Kanamori, 2001; Fialko and Khazan, 2005; Nielsen et al., 2008; Rice, 2006; Spray, 2005; Yuan and Prakash, 2008]. Ground truth for these proposals is limited because there are few tools to determine the slip rate of Department of Earth and Planetary Science, University of California, Santa Cruz, California, USA. Department of Geological Sciences, University of Cape Town, Cape Town, South Africa. Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JB005633

Biomarkers heat up during earthquakes: New evidence of seismic slip in the rock record

09.00 ancient faults and almost no recognized record of the seismic stresses [Cowan, 1999]. [ 3 ] Here we present observations and analysis of an unusual site that can help constrain the local absolute stresses on a fault. The faulting preserved at the key outcrop occurred at 12– 14 km depth in a megathrust in an accre- tionary prism that is now exposed on Kodiak Island, Alaska [Rowe et al., 2005; Rowe, 2007]. The fault core is composed of a

Structure and lithology of the Japan Trench subduction plate boundary fault

3.5 cm layer of ultrafine-grained black rock that is embedded in a 13.5m-thick cataclasite that is interpreted as a subduction thrust by Byrne [1984] (Figures 1 and 2). A series of cusped intrusions that extend upward from the core are interpreted as buoyant intrusions into the overlying layer. Such gravitational (Raleigh-Taylor) instabilities are general features of layered materials whenever a higher- density layer overlies a lower density layer and both layers behave ductilely. This study analyzes the spacing of the intrusions using a fluid dynamic model in order to: (1) determine the emplacement speed and thus provide a new geological tool to identify seismogenic faults in the geolog- ical record and (2) constrain the rheology and local stress on the fault during emplacement. [ 4 ] Ultrafine-grained fault rocks like the one studied here have been the subject of intense scrutiny in previous work [Sibson and Toy, 2006]. The major debate revolves around whether or not the unit is a frictional melt (pseudotachylyte) or an ultracataclasite [Di Toro et al., 2005; Magloughlin and B01303 1 of 14

Textural record of the seismic cycle: strain-rate variation in an ancient subduction thrust

During earthquakes, faults heat up due to frictional work. However, evidence of heating from paleoearthquakes along exhumed faults remains scarce. Here we describe a method using thermal maturation of organic molecules in sedimentary rock to determine whether a fault has experienced differential heating compared to surrounding rocks. We demonstrate the utility of this method on an ancient, pseudotachylyte-hosting megathrust at Pasagshak Point, Alaska. Measurements of the ratio of thermally stable to thermally unstable compounds (diamondoids/n-alkanes) show that the melt-bearing rocks have higher thermal maturity than surrounding rocks. Furthermore, the mineralogy of the survivor grains and the presence of any organic molecules allow us to constrain the temperature rise during the ancient earthquakes to 840–1170 °C above ambient temperatures of ~260 °C. From this temperature rise, we estimate that the frictional work of the earthquake was ~105–228 MJ/m 2 . Using experimental friction measurements as a constraint, we estimate that the minimum slip necessary for heating was ~1–8 m. This paper demonstrates that biomarkers will be a useful tool to identify seismic slip along faults without frictional melt.

Emplacement and dewatering of the world's largest exposed sand injectite complex

The 2011 Mw9.0 Tohoku-oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343T sailed 1 year after the earthquake and drilled into the plate boundary ∼7 km landward of the trench, in the region of maximum slip. Core analyses show that the plate boundary decollement is localized onto an interval of smectite-rich, pelagic clay. Subsidiary structures are present in both the upper and lower plates, which define a fault zone ∼5–15m thick. Fault rocks recovered from within the clay-rich interval contain a pervasive scaly fabric defined by anastomosing, polished, and lineated surfaces with two predominant orientations. The scaly fabric is crosscut in several places by discrete contacts across which the scaly fabric is truncated and rotated, or different rocks are juxtaposed. These contacts are inferred to be faults. The plate boundary decollement therefore contains structures resulting from both distributed and localized deformation. We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity-strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity-weakening behavior. The presence of multiple discrete faults resulting from seismic slip within the decollement suggests that rupture to the trench may be characteristic of this margin.