James R. Rice - US grants

This research interprets seismicity and geodetic data in terms of the geometry and stressing of locked fault zones which will rupture in large earthquakes. The studies will be carried out in the framework of the asperity model which regards a portion of a seismically active fault zone as being effectively locked against slip, except in large earthquakes, while adjacent portions of the zone slip aseismically, or with lower level seismicity, and do not accumulate comparable stress. The initial focus will be on modelling of the Shumagin Islands subduction segment in Alaska, and the Parkfield region along the San Andreas fault. This research is a component of the National Earthquake Hazard Reduction Program.

This award provides one-half the funding required for the purchase of computer equipment to serve the seismology research group in the Department of Earth & Planetary Sciences at Harvard University. Harvard is committed to providing the remaining necessary funds. The equipment includes upgrades of two fileservers to multiprocessor machines, workstation upgrades, acquisition of a color graphics workstation, and the acquisition of a tape archive unit. The seismology research group at Harvard investigates a broad range of problems in seismology including the analysis of very large data sets for the study of earthquake source parameters and the determination of earth structure.

This award will provide partial funding for a systems manager to support the computer system of the geophysics research group in the Department of Geological & Planetary Sciences at Harvard University. The computer facility presently serves 15-20 faculty, postdoctoral research associates, graduate and undergraduate students working in computer-intensive areas of research such as seismic tomography, earthquake seismology, geomagnetics, and other studies of the Earth's deep interior.

9614431
Rice

This research involves a continuation of the study of seismicity in coupled subduction segments and their surroundings (overriding plate, sea floor and subducting slab) to understand the strength and heterogeneity of coupling along the thrust interface. Specific areas of investigation are: 1) investigation of the relations between heterogeneous seismic coupling along the main thrust zone and seismicity in the sea floor and upper plate areas; 2) explanation of seismicity in the descending slab at intermediate depths and its relation to coupling; 3) correlation of seismicity with changes in a Coulomb measure of stress change (shear minus friction coefficient times normal stress change) in a subduction zone and its vicinity; and 4) investigation of compressional earthquakes in zones near the trench and towards the outer rise as signals of the maturity of the earthquake cycle of the thrust interface. This research is a component of the National Earthquake Hazard Reduction Program.

0003718
O'Connell

The general goals of the project are to continue GPS measurements of deformation in the South American Andes, and to integrate deformation with the earthquake cycle and with coupling on the subduction zone and longer-term tectonic deformation. The importance of the research is that it examines fundamental mechanisms behind how subduction created the modern Andes and deformed this continental margin. Specific research tasks include: obtaining and analyzing GPS data from Chile, Argentina, and Bolivia and coordinating with other GPS efforts in South America; expanding the principal investigator's GPS network; looking at back-arc deformation and at the post-seismic effects of the 1960 earthquake; and improving velocity estimates. The P.I.s will model longer term deformation (tectonic deformation) associated with the stress history of subduction and mantle coupling in the wedge. They will model various tractions arising from the edge of the lithosphere, the base of the plate, larger scale mantle flow, plate motions and basal forces. They will examine the distribution and nature of seismic deformation related to subduction in both the lithosphere and crust, especially the 1960 earthquake, and the deformation associated with oblique convergence.

This combination of the GPS measurements and mantle flow modeling is an attempt to understand the fundamental mantle forces that shape convection at an archetypal convergent continental margin. The multifaceted approach (GPS, seismology, modeling) will integrate the various geodynamic components into a comprehensive model of the mountain building process.
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A major problem in earthquake science is to understand rupture through geometrically complex fault systems with bends, branches and stepovers. Such complexities exert major control over the propagation and arrest of rupture. The understanding when and how ruptures stop, which is often associated with such features, is central to understanding seismic risk. This study continues recent developments of the theory and modeling of fault fracture at encounters with kinks, bends and offsets between fault segments. That is done with close reference to explaining rupture patterns as observed in field examples. Those include branches and stepovers in major strike-slip earthquakes (e.g., 2001 Kunlun, Tibet, 2002 Denali, Alaska, and 1992 Landers, California), and splay thrust faulting like that documented for the 1944 Nankai, Japan, and 1964 Alaska subduction zones, with implications for tsunami generation.

The studies open new frontiers in rupture dynamics and the physics of earthquakes. Those include a basic understanding of how rupture paths are chosen through complex fault systems, and of the formulation of appropriate computational models (based on dynamic finite element and boundary integral equation methodology) to analyze slip propagation through kinks and branches. In such cases there are significant, coupled, dynamic changes in both the normal and shear stress components supported by the fault, which pose new challenges to representing fracture propagation. Progress in correlating theory with field (and sometimes lab) examples is providing new ways of looking at fault geometry and evidence about prestress states, and translating that into predictions about rupture paths. An important issue under study is whether relic fault geometries with branches and other complexities can be used to infer the direction of rupture in past events, which is important for identifying regions of most severe ground motion. Also, the work addresses how damage zones along faults evolve by successive ruptures, and how inelastic processes within such zones may interact back with stress transmission to the rupture front and with the dynamics of propagation to generate high frequency seismic wave emission.

The project during the previous funding cycle was also effective in aiding the participation of women in research. That includes the co-PI, a graduate student research assistant, and three visiting student interns who completed research on fault rupture as capstone projects in completion of their degree programs elsewhere.

Physically based descriptions of fault constitutive response are being used to address key problems in the dynamics of earthquakes. There are two main topics:
(1) In current attempts to devise models of crustal earthquake sequences, using experimentally motivated temperature variation (hence depth variation) of rate and state constitutive parameters, it has been noticed that important new features emerge as the state-evolution slip distance L is decreased towards values in the laboratory range. These are the emergence of a population of small events that is clustered towards the base of the seismogenic zone, and the effect of the resulting heterogeneous residual stress patterns from those events on the earliest phases of seismic radiation in large events. This process seems promising to explain the initially hesitant radiation in many large events, known as the "seismic nucleation phase". To fit such calculations on present computers, L must be made much larger than laboratory values, which are of order of magnitude 10 microns, since the required numerical grid size scales with a large factor (of order 10^5) times L. Yet interesting behavior is emerging as L is reduced towards those values. This project addresses the small L range by a combination of new numerical studies, coordinated with asymptotic analysis of simplified models, as L is decreased in size, to provide a basis for interpretation and extrapolation.
(2) Thermal weakening effects are thought to occur during rapid slip in major earthquakes, causing the effective friction coefficient to diminish from lab-like values, present when a propagating rupture front first reaches a point on a fault, to much lower values when rapid and large slip occurs. This problem is being addressed by building on initial studies that focus separately on the earliest phases of sliding, before melting occurs, and on the mature stage of active pseudotachylyte development. Flash heating at asperity contacts is expected to be the primary thermal weakening process when slip rates are high (> 1 m/s) but total slip is still small, in a sense that can be quantified. A preliminary analysis captures some features of available experiments. Some ideas are being developed on how to address the much larger slip range when partial melting occurs. These include a view, supported by pseudotachylyte observations, that a granular fault gouge becomes liquefied through development of small amounts of partial melt, and that a self-regulated process of velocity-weakening character develops as this highly pressurized phase permeates into the adjoining fault walls. These concepts are being developed for purposes of integrating them with numerical simulations and theory on how the mode of rupture depends on constitutive response, to examine consequences for rupture dynamics. That will contribute to understanding how major fault systems can operate at realistically low overall driving stresses, even when the stress needed locally to initiate slip is much larger, and to quantifying the minimum average stress level for which a rupture, once initiated at a location of locally high shear stress or low effective normal stress, can propagate over large distances.

Recent observations by GPS have revealed transient but aseismic deformation episodes in shallow subduction zones, thus far in the Pacific Northwest of the US, the Guerrero region along the southwestern coast of Mexico, and the Nankai region in Japan. Those seem to involve episodic slips, accumulating over time windows of a month to a few years, along deeper regions of the subduction interface that would not be expected to nucleate regular earthquakes or to move appreciably during such earthquakes.
Those transients pose significant questions as to their origin, and also relative to existing concepts of how locked portions of the fault zone (which will ultimately fail in earthquakes) are loaded. That is, the recent results imply an episodic component of the loading, which has consequences for improved predictability of earthquakes.
The research in this project addresses the physical causes of transients. Major questions involve how they start and what controls their migration, in the dip direction and along strike. They occur in regions where there is metamorphic fluid release from the subducted seafloor materials, and thus we focus especially on pore water, at very high pressure, as a possibly essential component of understanding transients. More generally we want to understand how they depend on tectonic parameters characteristic of a particular subduction zone (geometry, convergence rate, thermal structure, pattern of metamorphic reactions).
A major component of our work is the development of a 3D numerical model of a subduction fault with temperature-dependent, hence depth-dependent, frictional properties, involving a transition to stable friction downdip from the cooler, locked part of the fault zone. Remarkably, in simulations of long tectonic loading sequences with multiple earthquakes along strike, we found that aseismic transient slip episodes emerged spontaneously, with features like in some of the natural observations.
In the planned work, we will incorporate into the modeling a more complete physical description of fluid release, transport and pore pressurization, to develop descriptions of transients in a manner that is also consistent with observations of non-volcanic tremors associated with them in some subduction zones. As a way to check and constrain the assumed rheology of the downdip aseismic fault zone in the model, we plan to investigate the fitting of the model to GPS and InSAR constraints on post-seismic slip from the northern Chile subduction zone.

Rice 0739444

This award supports a project to study the mode of formation and causes of glacial earthquakes. The paradigm for glacial flow has been that glaciers flow in a viscous manner, with major changes in the force balance occurring on the decade timescale or longer. The recent discovery of a number of even shorter timescale events has challenged this paradigm. In 2003, it was discovered that Whillans Ice Stream in West Antarctica displays stick-slip behavior on the 10-30 minute timescale, with ice stream speed increasing by a factor of 30 from already high speeds. In the past year, the minimum timescale has been pushed shorter by recognition that a class of recently discovered 50-second-long, magnitude-5 earthquakes are closely associated with changes in the force balance near the calving fronts of large outlet glaciers in both Greenland and East Antarctica. With no adequate theory existing to explain these relatively large earthquakes associated with outlet glaciers, we have begun to investigate the physical mechanisms that must be involved in allowing such a response in a system traditionally not thought capable of generating large variations in forces over timescales less than 100 seconds. The intellectual merit of the work is that large-amplitude, short-timescale variability of glaciers is an important mode of glacier dynamics that has not yet been understood from a first-principles physics perspective. The proposed research addresses this gap in understanding, tying together knowledge from numerous disciplines including glaciology, seismology and fault rupture dynamics, laboratory rock physics, granular flow, fracture mechanics, and hydrogeology. The broader impacts of the work are that there is societal as well as general scientific interest in the stability of the major ice sheets. However, without an understanding of the physical processes governing short time scale variability, it is unlikely that we will be able accurately predict the future of these ice sheets and their impact on sea level changes. The project will also contribute to the development and education of young scientists.

The project group focuses on a major problem in earthquake science, namely, to understand the interaction of seismic slip-rupture with geometrical and structural complexities of fault zones. Such interactions include transitions of the failure path among fault strands at bends, branches and stepovers, rupture arrest, and induced inelastic deformations in fault border zones, which are generally damaged (highly cracked and/or granulated) and fluid-saturated. Prior work of the group, on which the current studies build, provided new understanding of how rupture paths are chosen at branch-like geometric complexities, and of how inelastic response of the fault bordering zone affects rupture propagation and shear localizations.

The new areas for theory and modeling in current work are as follows: (1) Understanding how interactions of deformations with ground fluids and frictional elastic-plastic responses in the damage zone couple to the dynamics of rupture propagation. That includes explaining the effects of different types of across-fault material dissimilarity (in elastic properties, strength and extent of damage, and near-fault permeability to fluids); such dissimilarities are common for mature, highly slipped faults. (2) Assessing if, and to what extent, current understanding of how rupture paths are chosen at branch intersections, and of whether rupture passes through or arrests at step-overs, is affected by the presence of extensively damaged material, capable of elastic-plastic response, near such fault junctions. (3) Determining how residual stress states imprinted in fault-border material by the previous rupture affects response in the next event, and how that depends on rupture directivity in the past and pending events; (4) Devising procedures to rigorously analyze strain localizations that arise in modeling inelastic response of damaged/granulated fault border zones, by imposing localization-limiting procedures that eliminate grid dependence, thus ultimately evolving a methodology that can predict spontaneous development of localized fault-rupture paths through damaged material.

Correlation of theory and modeling with field examples and lab experiments is a hallmark of the group's work, and new thrusts in that direction are as follows: (I) Adopting methodology like in (4) above to understanding when a damaged pull-apart stepover, like in the 1992 Landers earthquake between the Johnson and Homestead Valley Faults, and between the Homestead Valley and the Emerson Fault, is breeched by a through-going rupture, and similarly for the 1920 M8 Haiyuan, China event, which ruptured through a sequence of pull-aparts. (II) Understanding mega-branches of great thrust ruptures onto splay faults through the sediment cover of accretionary subduction zones, like documented or suspected at Alaska, Cascadia, Nankai and Sumatra, as well as when and by what processes branching onto landward- versus seaward-vergent splays can occur, and what that means for tsunami generation. (III) Testing the evolving theoretical understanding of rupture branching and interactions with damaged border zones against results of lab experiments (conducted by colleagues elsewhere) which are devised to address the same issues.

The understanding of when and how earthquake ruptures stop, which often involves geometric complexities of the type we address, is central to understanding seismic risk. New ways of using relic fault geometries to constrain directivity and other features of past events is also a potentially valuable outcome.

Earthquakes on the well-established and highly slipped fault zones which host major events seem to occur at overall levels of shearing stress which are notably lower than "static friction" stress levels required to initiate slow frictional sliding between the fault walls. If those static friction stresses prevailed during earthquake slip, they would produce perceptible localized heat outflows along faults and leave abundant signs of melting and re-solidification, even at shallow crustal depths. Neither are generally found. Also, recent field and lab observations show that the majority of deformation during rapid shear is generally localized to a remarkably thin principal shear zone along the fault, often less than a millimeter to a centimeter wide, with that feature forming within a much broader, say, one to a hundred meters wide, zone of granulated and damaged rock. Our aim in the planned study is to understand the materials and thermal physics responsible for those features of fault zone response, and to establish some of their consequences for the manner by which slip-ruptures propagate along faults in major earthquakes. It is hoped that such basic understanding of the physics of earthquakes may ultimately have payoffs in the improved predictability of seismic phenomena and effects.

We have developed the concept that thermal heating of groundwater-saturated fault gouge during shear leads to strong localization of strain into realistically narrow zones. That focuses further heating and temperature rise, but rather than leading directly to melting, weakening mechanisms are triggered that sufficiently limit strength, and hence continued heating, so as to make bulk melting of the fault zone rare, at least at shallow crustal depths. A relatively universal form of weakening is that groundwater thermally expands much more than its mineral host, causing the mineral constituents to push less strongly against one another, and hence to have low frictional strength. A variant of this process is that thermal decomposition of common fault constituents such as carbonates and hydrated clays occurs, at temperatures far below melting, and creates a highly pressurized volatile product phase (CO2 or H2O, respectively) which similarly reduces strength. Further weakening processes, of which the physical details are still unclear, relate to the nanometer size range of the solid decomposition and wear products. We will model how such weakening processes influence features of propagating earthquake ruptures (e.g., crack vs. slip pulse, rupture velocity, stress drop, total slip), how rupture relates to the fault mineralogy and depth, and how different dynamic weakening processes might be identified in seismic observations. Hypotheses to be tested are that thermal decomposition combined with variation in fault mineralogy could explain how rupture stops at the base of the seismogenic zone, and that thermal decomposition could provide a mechanism for occasional extreme earthquakes on faults that generally experience smaller events. We will model the material lying outside the narrow highly-deforming fault core as an elastic or an elastic-brittle-plastic solid, and use our analyses of the localized shearing processes within the deforming fault core as the basis for imposing boundary conditions along the fault surfaces in the larger analysis. The study should contribute towards a unified overall understanding of seismic processes. It will have inputs from fine scale materials physical/chemical theory, geologic fault core studies, rock mechanics lab friction experiments, spontaneous rupture simulations, seismic observations of the slip mode and extent of seismic ruptures, and large scale constraints, by heat flow, topography support and related studies, of the stress regimes under which major earthquakes occur.

Rice/1341499

Flow of glacial ice along the Siple Coast, West Antarctica, localizes into fast-flowing ice streams of 20-80 km width, moving at speeds of 100s of m/yr as the ice streams approach the sea. These ice streams are bordered by ridges of nearly stagnant ice and no topographic feature in the ice sheet bed has been identified as guiding that width, which instead seems to be chosen dynamically. Using theoretical modeling and computational physics, this study aims to understand (1) the mechanical, hydrologic and thermal processes active within the ice streams, (2) the origin of the stream morphology and what controls the margin locations of the fast-flowing ice, and (3) how ice discharge from the West Antarctic Ice Sheet will respond to climate-related changes in atmospheric and ocean temperatures and precipitation. Preliminary studies suggest that the margins of all active ice streams in the Siple Coast have substantial ?temperate zones?, i.e., ice is at the melting point for several dozens to hundreds of meters above the bed. Temperate ice contains melt water, and thus a first focus is to understand how water generation and transport near the bed in the shear margins might partially stabilize the margin locations and control the speed of ice discharge within the ice stream.

The intellectual merit of these studies is that it will contribute to understanding what controls the channelization of ice flow in the West Antarctic Ice Sheet (WAIS) and the rate at which its ice is discharged to the adjacent ocean. The broader impacts of the work are that advancing our knowledge of ice-stream dynamics is crucial for predicting how ice discharge will be impacted by future variations in atmospheric and ocean temperatures, ocean currents, precipitation, and solar radiation. Based on the results from this study, the investigators will attempt to develop a mathematical model of ice stream and their shear margins, that can be included in larger scale numerical simulations of ice sheet processes and climate models. This project will contribute to the training of two graduate students.

This award does not have any field work in Antarctica.