EARTHSCOPE INSTITUTE: THE SPECTRUM OF FAULT SLIP BEHAVIORS

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1 EARTHSCOPE INSTITUTE: THE SPECTRUM OF FAULT SLIP BEHAVIORS Intellectual Merit Recent studies using data from the EarthScope Facility as well as other national and international sources show that tectonic faults exhibit a broad spectrum of slip behaviors ranging from creep to earthquakes, including several phenomena that were unknown as recently as 10 years ago. We propose a workshop to foster critical thinking about the nature of such behaviors. Transient fault slip (TFS) includes slow and silent earthquakes, episodic tremor and slip, creep events, low frequency earthquakes, tsunamigenic earthquakes, and postseismic fault afterslip. There appears to be a continuous spectrum of slip modes ranging from aseismic creep to earthquakes. These observations challenge the standard view of brittle faulting that has existed for the past 40 years, in which frictional slip occurs in one of only two modes: stick-slip or stable creep. This proposal is for a workshop focused on emerging views of TFS and related phenomena. The primary goals of the workshop are: 1) to provide intellectual leadership and foster critical thinking about the underlying mechanisms and physical processes responsible for TFS, 2) to promote broad, community-based interest in understanding TFS, and 3) to discuss novel approaches for identifying new fault slip phenomena and new tectonic settings that may host transient strain release. Modern friction constitutive laws predict that fault slip will exhibit a range of transient behaviors. However, the underlying processes are poorly understood and key constitutive parameters are poorly constrained. Moreover, existing frictional models require special conditions to produce selfsustained modes of slow slip, and phenomena such as tremor are not well described, which may indicate that additional processes are needed in the models. Interest in TFS has grown at an unprecedented rate. Recent advances include the ability to locate tremor and the observation that TFS occurs in a variety of tectonic settings. These works have both honed our appreciation of the spectrum of fault slip behaviors and highlighted how little we know about their mechanisms. The workshop will aim to address these issues by: 1) improving understanding of TFS, 2) illuminating the relation between the spectrum of slip behaviors, and 3) seeding collaborations between observational-, theoretical-, and laboratory-based research programs. Broader Impacts of the Proposed Research We envision a group of ~100 researchers including post-docs, students and faculty coming together for a three day meeting in Portland Oregon. The format will include presentations of scientific results, ample time for integrative discussion, and a focus on regions where TFS has been observed, including Cascadia, California, Alaska, Central America and Japan. The Institute will foster education and communication across disciplines ranging from geodesy and seismology to rock mechanics, petrology, hydrogeology, tectonics, and geodynamics. The workshop will provide an opportunity for young researchers to broaden their understanding of a high-profile, emerging problem in geophysics and to develop ideas for how they might become involved. Anticipated results include broad, community-based appreciation of the nature of TFS and the key problems that need to be addressed. Workshop participants will focus on links between the phenomena that constitute TFS, including the potential role of slow and transient slip for earthquake triggering and seismic hazard assessment. We anticipate that the workshop will be the initial step in fostering broad, multidisciplinary collaboration on TFS and that it will provide the foundation for a virtual on-line Institute that will be facilitated by the EarthScope National Office. Marone, Freymueller, Vidale and Trehu EarthScope Institute on Transient Fault Slip

2 EARTHSCOPE INSTITUTE: THE SPECTRUM OF FAULT SLIP BEHAVIORS Convened by: Chris Marone, Jeff Freymueller, John Vidale and Anne Trehu Introduction A large number of recent studies using data from the EarthScope Facility as well as other national and international sources show that tectonic faults exhibit a broad spectrum of slip behaviors ranging from creep to earthquakes. Such transients include several phenomena that were unknown as recently as 10 years ago (e.g., Obara, 2002; Rogers and Dragert, 2003). To date, several distinct types of transient fault slip (TFS) have been identified, including creep events and strain transients, slow and silent earthquakes, low frequency earthquakes, tectonic fault tremor, and post-earthquake afterslip. The level of interest in TFS and the rate of scientific progress as evidenced by recent works on tremor and low frequency earthquakes is high and perhaps unprecedented (e.g., Amoruso and Crescentini, 2009; Brown et al., 2009; Johnson et al., 2009; Lambert et al., 2009; La Rocca et al., 2009; Liu et al., 2009; Nadeau and Guilhem, 2009; Peng et al., 2009; Rubinstein et al., 2009, 2010; Shelly, 2009, 2010; Shelly et al., 2009; Smith and Gomberg, 2009; Taira et al., 2009; Thomas et al., 2009; Uchide et al., 2009; Wdowinski, 2009). TFS involves processes with time constants from 10's of seconds to 10's of days and occurs on or in association with faults that host dynamic rupture with time constants of fractions of a second to 10's of sections (Figure 1). The spectrum of observed fault slip behaviors challenges the standard model of brittle faulting that was originally suggested by Bridgeman (1936) and has been widely accepted since it was demonstrated by Brace and Byerlee (1966). In the standard model, tectonic fault slip occurs in one of only two modes: earthquakes and aseismic creep. A stability bifurcation was thought to separate earthquakes from aseismic slip, with no possible slip modes between the end members. Several aspects of this model have proven quite useful. For example, frictional instability is understood as the result of frictional weakening and dynamic stress release; whereas, stable aseismic creep is understood as the result of inelastic slip at constant frictional yield strength. The time constants for earthquake ruptures are governed by elastic wave speeds and the massive energy release associated with frictional weakening and elastic strain release. In the context of the classical model of Brace and Byerlee (1966), fault slip may accelerate from plate tectonic rates to earthquake rates within a few seconds. Thus, fault slip is expected to occur in two modes: very fast or very slow. We now know that this view is oversimplified. Faults exhibit a spectrum of slip behaviors with time constants between earthquakes and creep events (Figure 1). In retrospect, there were many clues to this more complex behavior. Observations of postearthquake afterslip date from the 1966 Parkfield earthquake (Smith and Wyss, 1968). One can find instrumental observations of transient fault slip in various forms that extend back to the 1990's (e.g., Beroza and Jordan, 1990; Ihmlé et al., 1993; Linde et al., 1996; Marone, Freymueller, Vidale and Trehu EARTHSCOPE INSTITUTE on Transient Fault Slip p. 1

3 Bürgmann et al., 1997; Lienkaemper et al., 1997) and earlier (e.g., Tocher, 1960; King et al., 1973). Recent work has produced an explosion of observations that provide important constraints on the strain release, size, and location of TFS (e.g., Miller et al., 2002; Shelly et al., 2006, 2007; Brudzinski and Allen, 2007; Ide et al., 2007; Peng et al., 2008; Suito and Freymueller, 2009; Johnson et al., 2009). These studies show that most, if not all, of the transient phenomena occur on tectonic faults and that TFS occurs in a variety of tectonic settings and at a variety of depths. Existing studies suggest that fault slip behaviors may cover a continuous spectrum of behaviors with time constants ranging from those of earthquakes to plate tectonic rates. Figure 1. Upper panel shows a range of fault slip behaviors illustrated in a subduction zone setting. Episodic tremor and slip (ETS) are often observed at the downdip end of the seismogenic zone. Earthquakes that are deficient in high frequency energy, so-called low (or very low) frequency earthquakes VLF occur in a variety settings and possibly in association with slow earthquakes or aseismic transients. Lower panel shows the corresponding range of characteristic durations for TFS and normal earthquakes. (from Saffer, Marone and Bilek: NSF-MARGINS Decadal Review 2009) Marone, Freymueller, Vidale and Trehu EARTHSCOPE INSTITUTE on Transient Fault Slip p. 2

4 In the context of the friction laws and stability models that were in use during the 1970's, the observed range of fault slip behaviors is unexpected. However, modern friction constitutive laws are capable of describing a broad range of TFS behaviors (e.g., Marone, 1998; Scholz, 1998; Liu and Rice, 2007, 2009; Rubin, 2008). Earthquake simulations that incorporate rate and state friction laws are capable of describing slip nucleation, dynamic rupture propagation and complex elastic-frictional interactions (e.g., Ben-Zion and Rice, 1997; Ben-Zion and Andrews, 1998; Kaneko et al., 2008; Perfettini and Ampuero, 2008). The rate and state laws have also been used to model transient earthquake afterslip and creep (e.g., Marone et al., 1991; McGuire and Segall, 2003; Perfettini and Avouac, 2004). However these friction laws remain largely empirical and the underlying processes are poorly understood. Key constitutive parameters are poorly constrained by laboratory data and there are relatively few studies in which modeling of field observations have produced constraints on the constitutive parameters. Several modes of transient fault slip, including tremor and low frequency earthquakes, are not well explained by current models. Moreover, proper scaling of laboratory-derived constitutive parameters to tectonic faults remains an open question. Clearly there is much work that needs to be done to better understand how friction constitutive laws can be used to help understand the observed range of fault behaviors. Rationale for an EarthScope Institute The EarthScope facility is now largely in place and the program is making the transition from installation to data collection and science. A key goal in this transition is to engage the scientific community on broad, emerging problems with transformative potential. One mechanism to achieve this goal is to support scientifically thematic EarthScope Institutes. Our proposal is to support such an Institute. We intend to follow the model of previous, highly successful NSF-sponsored Institutes such as those supported by the MARGINS program. The Institute we propose will serve to focus and energize the allied experimental, theoretical, and field-based studies of TFS. We propose to support emerging multi-disciplinary studies of TFS by bringing together modelers, experimentalists, and field based researchers. A central theme of the Institute will be to crystallize thinking about observations and underlying physical processes of TFS. We will build on results of the USGS, PGC and EarthScope-sponsored Workshop at Dunsmuir Lodge, Sidney, British Columbia, 2008: Aseismic Slip, Non-Volcanic Tremor, and Earthquakes, but with a broader scientific scope and a broader group of participants. Goals of the Institute on Transient Fault Slip The principle goal of this Institute will be to improve understanding of the mechanisms responsible for the spectrum of observed fault slip behaviors. We will work to seed collaborations between observational efforts, numerical and theoretical approaches, and laboratory based research programs focused on TFS. The Institute will Marone, Freymueller, Vidale and Trehu EARTHSCOPE INSTITUTE on Transient Fault Slip p. 3

5 foster critical thinking about the underlying mechanisms and physical processes responsible for TFS and promote broad, community-based interest in understanding TFS. The following are key questions to be addressed: What are the mechanisms and physical processes that produce tectonic fault tremor? What are the processes that govern TFS? What are the physics that govern the transition from slow to fast TFS? How are the physics of slow, intermediate, and fast TFS phenomena related? Are some TFS phenomena more readily triggered by dynamic stresses than others? Do tremor and slow TFS phenomena occur on the same segments of the fault that host dynamic rupture? Under what conditions can we identify the slip plane with sufficient resolution to answer this question? How best can we foster interdisciplinary connections between researchers in geodesy, seismology, rock mechanics, petrology, hydrogeology, tectonics, and geodynamics? Are post-earthquake afterslip and episodes of creep that do not follow earthquakes two examples of the same fundamental process? What are the mechanisms that inhibit fast, dynamic rupture and allow slow earthquakes? Does tremor occur on oceanic faults and/or in association with ocean-ocean subduction zones? Can creep events and dynamic stresses released by tremor trigger large earthquakes? Does tremor activity evolve during the seismic cycle of nearby large earthquakes; more specifically, is precursory tremor activity identifiable? Do slow TFS processes alter the frictional properties of the slip patch on which they occur? Do tremor and other forms of TFS promote or trigger dynamic rupture and vice versa? Is ETS mapping a useful way to map asperities that will fail in future earthquakes? Is there a minimum size patch size for tremor, slow earthquakes and other forms of TFS? What controls the migration velocity of the various forms of TFS? Can tremor be continuous or is it always episodic? Marone, Freymueller, Vidale and Trehu EARTHSCOPE INSTITUTE on Transient Fault Slip p. 4

6 Is it reasonable to think of tremor and some low frequency earthquakes as subcritical slip patches that are too small to nucleate dynamic frictional instability? Can the range of TFS phenomena be described by one set of constitutive equations? Scientific Organizing Committee Scientific sessions at the workshop will be organized by a committee consisting of Chris Marone (rock mechanics, Penn State University), Jeff Freymueller (geodesy, University of Alaska), John Vidale (seismology, University of Washington), Anne Trehu (marine geology & geophysics, Oregon State University), Greg Beroza (seismology, Stanford University), Mike Brudzinski (seismology, Miami University), Joan Gomberg (seismology, USGS), and Jeff McGuire (seismology and geodesy, Woods Hole Oceanographic Institution). Personnel from the EarthScope office will provide logistical support for the meeting, including help with participant registration, abstract submission, as well as additional on-site support. Structure of the Workshop to Initiate the EarthScope Institute The EarthScope Institute on The Spectrum of Fault Slip Behaviors will begin with a workshop meeting to be held in Portland Oregon on October 11-14, The workshop will include presentations of scientific results, ample time for integrative discussion, and a focus on regional examples of TFS. Scientific sessions will be organized around five main themes: 1) The Spectrum of Fault Slip, 2) Seismic and Geodetic Observations, 3) Lab and Field Observations, 4) Theory & Models, and 5) The Path Ahead. Each session will involve keynote talks, to set the stage and provide a broad perspective, followed by shorter talks, discussion and poster presentations. After the workshop, a webpage will be established that includes presentations and breakout group summaries presented at the workshop, as was done after the Workshop for an EarthScope Science Plan. This web site will be the seed for the on-line Virtual Institute dedicated to studying the spectrum of slip on faults. Workshop Participants The workshop will be limited to 100 funded participants and up to 40 additional scientists who may fund their own participation. In order to insure that the distribution and quality of expertise present at the meeting is sufficient to generate lively, knowledgeable, and incisive discussion, we expect to invite participants in addition to the Keynote speakers, moderators and conveners. One emphasis of the plan will be to invite practitioners from each of the key specialties (theory, field-observation, laboratory) and primary tectonic-geographic thematic areas in order to: 1) summarize the scientific state-of-the-art on TFS, 2) identify the key outstanding problems in at least the two categories of 'low hanging fruit' and longer-term objectives, 3) bring fresh thinking and expertise to bear on the key scientific problems, and 4) build community consensus on Marone, Freymueller, Vidale and Trehu EARTHSCOPE INSTITUTE on Transient Fault Slip p. 5

7 technical issues, equipment deployment, and programmatic directions that can promote scientific progress. We have developed a preliminary list of individuals to invite, and further refinement of the list will occur over the coming months. The complete list has been withheld from this document so as not to compromise impartiality of the review process. The Institute will be advertised widely via , websites, and in AGU s EOS. Approximately 35 slots will be reserved for participation of interested individuals who respond to the advertisement. Selection (if necessary) of non-invited individuals will be done by the conveners with preference given to young researchers and students in addition to submitted abstracts and relevance of research interests. We will reserve roughly 20 slots for students. Workshop Venue The workshop will be held in Portland, OR at the World Trade Center, with gatherings and informal discussion at the nearby Hotel Fifty, where participants will stay. The main scientific sessions will take place in the auditorium at the World Trade Center, with discussion, and poster presentations in adjoining rooms. We anticipate that the workshop will be the initial step in fostering broad, multidisciplinary collaboration on TFS and that it will provide the foundation for a virtual on-line Institute that will be facilitated by the EarthScope National Office. Education, Human Resources, and Broader Impacts The proposed Institute will involve a group of ~100 researchers including postdocs, students and faculty. The format will foster education and communication across disciplines ranging from geodesy and seismology to rock mechanics, petrology, hydrogeology, tectonics, and geodynamics. The workshop will provide an opportunity for young researchers to broaden their understanding of a high-profile, emerging problem in geophysics and to develop ideas for how they might become involved. Anticipated results include broad, community-based appreciation of the nature of TFS and the related outstanding problems in fault and earthquake mechanics. Workshop participants will focus on links between the phenomena that constitute TFS, with significant attention to earthquake triggering and seismic hazard assessment. A substantial portion of the budget will be reserved for graduate student support. For many of these students, participation in a workshop with internationally-recognized experts, with ample time for discussion and detailed explanation of underlying concepts, will provide a capstone experience in their professional development. Marone, Freymueller, Vidale and Trehu EARTHSCOPE INSTITUTE on Transient Fault Slip p. 6

8 References Amoruso, A.; Crescentini, L. Slow diffusive fault slip propagation following the 6 April 2009 L'Aquila earthquake, Italy Geophys. Res. Lett., Vol. 36, L Ben-Zion Y, Rice JR Dynamic simulations of slip on a smooth fault in an elastic solid. J. Geophys. Res. 102: Ben-Zion, Y. and D. J. Andrews, Properties and Implications of Dynamic Rupture Along a Material Interface, Bull. Seismol. Soc. Amer., 88, , Beroza GC, Jordan TH Searching for slow and silent earthquakes using free oscillations. J. Geophys. Res. 95: Bürgmann, R., P. Segall, M. Lisowski, and J. Svarc (1997), Postseismic strain following the 1989 Loma Prieta earthquake from GPS and leveling measurements, J. Geophys. Res., 102(B3), Brace, W.F., and J.D. Byerlee, Stick-slip as a mechanism for earthquakes, Science, 153, 990, Bridgeman, P.W., Shearing phenomena at high pressure of possible importance to geology. J. Geol., 44: Brown, Justin R.; Beroza, Gregory C.; Ide, Satoshi; Ohta, Kazuaki; Shelly, David R.; Schwartz, Susan Y.; Rabbel, Wolfgang; Thorwart, Martin; Kao, Honn Deep lowfrequency earthquakes in tremor localize to the plate interface in multiple subduction zones Geophys. Res. Lett., Vol. 36, L Brudzinski, M.R. (2008) Do Faults Shimmy Before They Shake?, Nature Geoscience, 1, Brudzinski, M.R. and Allen, R.M. (2007) Segmentation in Episodic Tremor and Skip all Along Cascadia, Geology, 35, Chen, W.-P. and Brudzinski, M.R. (2007) Repeating Earthquakes, Episodic Tremor and Slip: Emerging Patterns in Complex Earthquake Cycles?, Complexity, 12, p Ide, S., Beroza, G. C., Shelly, D. R. & Uchide, T. A scaling law for slow earthquakes. Nature, 447, 76 79, Ide, S., Shelly, D. R. & Beroza, G. C. Mechanism of deep low frequency earthquakes: Further evidence that deep non-volcanic tremor is generated by shear slip on the plate interface, Geophys. Res. Lett., 34, doi: / 2006GL028890, Ihmlé PF, Harabaglia P, Jordan TH Teleseismic detection of a slow presursor to the great 1989 Macquarie ridge earthquake. Science 261:177 Ihmlé PF, Jordan TH Teleseismic search for slow precursors to large earthquakes. Science 266: Marone, Freymueller, Vidale and Trehu EARTHSCOPE INSTITUTE on Transient Fault Slip p. 7

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