Patrick M. Fulton

Low Coseismic Friction on the Tohoku-Oki Fault Determined from Temperature Measurements

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 frictional resistance on a fault during slip controls earthquake dynamics. Friction dissipates heat during an earthquake; therefore, the fault temperature after an earthquake provides insight into the level of friction. The Japan Trench Fast Drilling Project (Integrated Ocean Drilling Program Expedition 343 and 343T) installed a borehole temperature observatory 16 months after the March 2011 moment magnitude 9.0 Tohoku-Oki earthquake across the fault where slip was ~50 meters near the trench. After 9 months of operation, the complete sensor string was recovered. A 0.31°C temperature anomaly at the plate boundary fault corresponds to 27 megajoules per square meter of dissipated energy during the earthquake. The resulting apparent friction coefficient of 0.08 is considerably smaller than static values for most rocks.

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.

Re-evaluation of heat flow data near Parkfield, CA: Evidence for a weak San Andreas Fault

[1] Improved interpretations of the strength of the San Andreas Fault near Parkfield, CA based on thermal data require quantification of processes causing significant scatter and uncertainty in existing heat flow data. These effects include topographic refraction, heat advection by topographically-driven groundwater flow, and uncertainty in thermal conductivity. Here, we re-evaluate the heat flow data in this area by correcting for full 3-D terrain effects. We then investigate the potential role of groundwater flow in redistributing fault-generated heat, using numerical models of coupled heat and fluid flow for a wide range of hydrologic scenarios. We find that a large degree of the scatter in the data can be accounted for by 3-D terrain effects, and that for plausible groundwater flow scenarios frictional heat generated along a strong fault is unlikely to be redistributed by topographically-driven groundwater flow in a manner consistent with the 3-D corrected data. INDEX TERMS: 8130 Tectonophysics: Heat generation and transport; 8150 Tectonophysics: Plate boundary—general (3040); 8164 Tectonophysics: Stresses—crust and lithosphere. Citation: Fulton, P. M., D. M. Saffer, R. N. Harris, and B. A. Bekins (2004), Re-evaluation of heat flow data near Parkfield, CA: Evidence for a weak San Andreas Fault, Geophys. Res. Lett., 31 , L15S15, doi:10.1029/2003GL019378.

Long-term temperature records following the Mw 7.9 Wenchuan (China) earthquake are consistent with low friction

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Does hydrologic circulation mask frictional heat on faults after large earthquakes

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B09402, doi:10.1029/2009JB007103, 2010 Does hydrologic circulation mask frictional heat on faults after large earthquakes? Patrick M. Fulton, 1 Robert N. Harris, 1 Demian M. Saffer, 2 and Emily E. Brodsky 3 Received 2 November 2009; revised 17 March 2010; accepted 30 April 2010; published 3 September 2010. [ 1 ] Knowledge of frictional resistance along faults is important for understanding the mechanics of earthquakes and faulting. The clearest in situ measure of fault friction potentially comes from temperature measurements in boreholes crossing fault zones within a few years of rupture. However, large temperature signals from frictional heating on faults have not been observed. Unambiguously interpreting the coseismic frictional resistance from small thermal perturbations observed in borehole temperature profiles requires assessing the impact of other potentially confounding thermal processes. We address several issues associated with quantifying the temperature signal of frictional heating including transient fluid flow associated with the earthquake, thermal disturbance caused by borehole drilling, and heterogeneous thermal physical rock properties. Transient fluid flow is investigated using a two‐dimensional coupled fluid flow and heat transport model to evaluate the temperature field following an earthquake. Simulations for a range of realistic permeability, frictional heating, and pore pressure scenarios show that high permeabilities (>10 −14 m 2 ) are necessary for significant advection within the several years after an earthquake and suggest that transient fluid flow is unlikely to mask frictional heat anomalies. We illustrate how disturbances from circulating fluids during drilling diffuse quickly leaving a robust signature of frictional heating. Finally, we discuss the utility of repeated borehole temperature profiles for discriminating between different interpretations of thermal perturbations. Our results suggest that temperature anomalies from even low friction should be detectable at depths >1 km 1 to 2 years after a large earthquake and that interpretations of low friction from existing data are likely robust. Citation: Fulton, P. M., R. N. Harris, D. M. Saffer, and E. E. Brodsky (2010), Does hydrologic circulation mask frictional heat on faults after large earthquakes?, J. Geophys. Res., 115, B09402, doi:10.1029/2009JB007103. 1. Introduction [ 2 ] Frictional resistance along faults is an important parameter controlling earthquake nucleation and propaga- tion. Because friction is central to earthquake mechanics, considerable effort has gone into characterizing fault zone friction both in the laboratory and in situ [e.g., Scholz, 2002]. Laboratory measurements suggest that the intrinsic low‐ speed friction coefficient for most rocks is approximately 0.60−0.85 [Byerlee, 1978]. This magnitude of friction is hypothesized to generate large thermal anomalies on natural faults with large slip rates and/or large total displacements, assuming hydrostatic pore pressure. Curiously, analysis of surface heat flow data [e.g., Brune et al., 1969; Lachenbruch and Sass, 1980; Wang et al., 1995] and subsurface temper- College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA. Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA. Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA. Copyright 2010 by the American Geophysical Union. 0148‐0227/10/2009JB007103 ature profiles [Yamano and Goto, 2001; Kano et al., 2006; Tanaka et al., 2006, 2007] that cross fault zones do not show substantial, unequivocal anomalies from frictional heating. These observations prompt two questions: (1) could the fric- tional resistance be as large as expected from Byerlee’s law and hydrostatic pore pressure, but the heat signal is masked or dissipated by other processes? (2) If not, what is the in situ value of frictional resistance during fault slip? [ 3 ] Much effort has been spent recently on the second of these questions resulting in theoretical models supported by both laboratory and field observations that suggest that coseismic friction may be quite low [e.g., Brodsky and Kanamori, 2001; Di Toro et al., 2004; Rice, 2006; Ma et al., 2006], but considerably, less work has been con- ducted on the first question. Studies of processes that may mask or dissipate the frictional heat signal have focused on steady state topographically driven or buoyancy‐driven groundwater flow [Williams and Narisimhan, 1989; Saffer et al., 2003; Fulton et al., 2004] and the effects of heteroge- neous thermal properties [Tanaka et al., 2007; Fulton and Saffer, 2009a]. One candidate for obscuring a frictionally generated thermal signal that has not been fully explored is transient groundwater flow following an earthquake [e.g., Kano et al., 2006; Scholz, 2006]. B09402 1 of 13

A permeability and compliance contrast measured hydrogeologically on the San Andreas Fault

Hydrogeologic properties of fault zones are critical to faulting processes; however, they are not well understood and difficult to measure in situ, particularly in low-permeability fractured bedrock formations. Analysis of continuous water level response to Earth tides in monitoring wells provides a method to measure the in situ hydrogeologic properties. We utilize four monitoring wells within the San Andreas Fault zone near Logan Quarry to study the fault zone hydrogeologic architecture by measuring the water level tidal response. The specific storage and permeability inferred from the tidal response suggest that there is a difference in properties at different distances from the fault. The sites closer to the fault have higher specific storage and higher permeability than farther from the fault. This difference of properties might be related to the fault zone fracture distribution decreasing away from the fault. Although permeability channels near faults have been documented before, the difference in specific storage near the fault is a new observation. The inferred compliance contrast is consistent with prior estimates of elastic moduli in the near-fault environment, but the direct measurements are new. The combination of measured permeability and storage yields a diffusivity of about 10 m/s at all the sites both near and far from the fault as a result of the competing effects of permeability and specific storage. This uniform diffusivity structure suggests that the permeability contrast might not efficiently trap fluids during the interseismic period.

Drilling Into Faults Quickly After Earthquakes

What will it take to advance from current empirical models of earthquake initiation and fault slip to a full physics-based understanding of rupture processes? The most important requirements include knowledge of absolute stress levels on the fault during an earthquake, how stresses recover afterward to prepare for the next event, how one earthquake promotes or inhibits another, and how material properties of a particular fault affect its propensity to fail catastrophically rather than creep. Immediately after a large earthquake, an opportunity exists to fill these knowledge gaps. For a few years after a major earthquake, the fault is observably changing and a deep borehole can capture measurable signals to address the key questions.

In situ observations of earthquake-driven fluid pulses within the Japan Trench plate boundary fault zone

Transient fluid flow within faults is suspected to be an important component of the earthquake cycle and subduction zone evolution. However, an understanding of the mechanisms and time scales involved has been limited due to a paucity of direct measurements. Here we report on in situ observations that appear to capture the thermal signature of earthquake-driven fluid pulses within the damage zone of the Japan Trench plate boundary fault. The data are from a sub-seafloor temperature observatory installed through the fault following the March 2011 Mw 9.0 Tohoku-oki earthquake as part of the Integrated Ocean Drilling Program’s Japan Trench Fast Drilling Project (JFAST). High-resolution temperature time series data reveal spatially correlated transients in response to earthquakes that are indicative of advection by transient fluid flow. We interpret the observed phenomenon as reflecting pressure redistribution in a fault zone and a potential mechanism for earthquake triggering and episodic heat and chemical transport.

The Post-Earthquake Stress State on the Tohoku Megathrust as Constrained by Reanalysis of the JFAST Breakout Data

The JFAST drilling project endeavored to establish the stress state on the shallow subduction megathrust that slipped during the M9 Tohoku earthquake. Borehole breakout data from the drillhole can constrain both the orientation and magnitude of the principal stresses. Here we reanalyze that data to refine our understanding of the stress state on the fault. In particular, we: (1) Improve the identification of breakouts, (2) Consider a fuller range of stress states consistent with the data, and (3) Incorporate new and more robust laboratory constraints on rock strength. The original conclusion that the region is in a normal faulting regime after the earthquake is strengthened by the new analysis. The combined analysis suggests the earthquake released sufficient elastic strain energy to reset the local stress field.

Erratum: Limited latitudinal mantle plume motion for the Louisville hotspot (Nature Geoscience (2012) 5 (911-917))

Nature Geoscience 5, 911–917 (2012); published online 25 November 2012. In the print version of this Article originally published, the present address for Toshitsugu Yamazaki was erroneously omitted. It is as follows: Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.