Charles Sammis

University of Southern California | USC · Department of Earth Sciences

Most earthquake ruptures propagate at speeds below the shear wave velocity within the crust, but in some rare cases, ruptures reach supershear speeds. The physics underlying the transition of natural subshear earthquakes to supershear ones is currently not fully understood. Most observational studies of supershear earthquakes have focused on determ...

Supershear earthquakes are rare but powerful ruptures with devastating consequences. How quickly an earthquake rupture attains this speed, or for that matter decelerates from it, strongly affects high-frequency ground motion and the spatial extent of coseismic off-fault damage. Traditionally, studies of supershear earthquakes have focused on determ...

The 2004 M-w 6 Parkfield, California, earthquake was preceded by a 4-year period of anomalously high seismicity adjacent to, but not on, the San Andreas fault. The rate of small events (M-w < 3) at distances between 1.5 and 20 km from the fault plane and at depths >8 km, increased from 6 events per year prior to 2000 to 20 events per year between t...

Fault slip distributions provide important insight into the earthquake process. We analyze high-resolution along-strike co-seismic slip profiles of the 1992 Mw = 7.3 Landers and 1999 Mw = 7.1 Hector Mine earthquakes, finding a spatial correlation between fluctuations of the slip distribution and geometrical fault structure. Using a spectral analysi...

It is widely assumed that the boundary layer above the core is the source of intraplate volcanoes such as Hawaii, Samoa, and Yellowstone, and that the sub-plate boundary layer at the top of the mantle is thin and entirely subsolidus. In fact, this upper layer is thicker and has higher expansivity, buoyancy, and insulating power than the lower one,...

Co-seismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods (e.g., InSAR, lidar differencing, and GPS). Current methods, however, either fail to capture patterns of near-field co-seismic surface deformation or lack pre-event data. Consequently, the characteristics of off-fault d...

Variations in fault structure, for example, surface roughness and deformation zone width, influence the location and dynamics of large earthquakes as well as the distribution of small seismic events. In nature, changes in fault roughness and seismicity characteristics can rarely be studied simultaneously, so that little is known about their interac...

The motion along upper crustal faults in response to tectonic loading is controlled by both loading stresses and surface properties, for example, roughness. Fault roughness influences earthquake slip distributions, stress-drops and possible transitions from stable to unstable sliding which is connected to the radiation of seismic energy. The relati...

The statistics of large earthquakes commonly involve large uncertainties due to the lack of long-term, robust earthquake recordings. Small-scale seismic events are abundant and can be used to examine variations in fault structure and stress. We report on the connection between stress and microseismic event statistics prior to the possibly smallest...

The low velocity zone of the upper mantle is due to the presence of melt.

We show that microseismic events—earthquakes with small magnitudes—can be fruitfully used to gain insight into the properties of the fracture network of large-scale porous media, such as oil, gas, and geothermal reservoirs. As an example, we analyze extensive data for the Geysers geothermal field in northeast California. Injection of cold water int...

A system of non-linear coupled relaxation oscillators is used to show how the communication between large earthquakes on a global scale can align their seismic cycles to produce a worldwide clustering of large events. Our model builds on recent observations that the seismic waves from a large earthquake can trigger an episode of non-volcanic tremor...

Fault zones contain structural complexity on all scales. This complexity influences fault mechanics including the dynamics of large earthquakes as well as the spatial and temporal distribution of small seismic events. Incomplete earthquake records, unknown stresses, and unresolved fault structures within the crust complicate a quantitative assessme...

We provide estimates of temperature changes produced in fault damage zones during brittle deformation associated with distributed cracking and pulverization. In contrast to localized faulting accompanied by significant frictional weakening, the relatively high friction coefficient on the multitudinous small cracks generated in the fracturing proces...

The micromechanical damage mechanics formulated by Ashby and Sammis (Pure Appl Geophys 133(3) 489–521, 1990) has been shown to give an adequate description of the triaxial failure surface for a wide variety of rocks at low confining pressure. However, it does not produce the large negative curvature in the failure surface observed in Westerly grani...

Anomalously low values of friction observed in layers of submicron particles deformed in simple shear at high slip velocities are explained as the consequence of a one nanometer thick layer of water adsorbed on the particles. The observed transition from normal friction with an apparent coefficient near μ=0.6 at low slip speeds to a coefficient nea...

The micromechanical damage mechanics formulated by Ashby and Sammis [PAGEOPH, 1990] and generalized by Deshpande and Evans [J. Mech. Phys. Solids, 2008] has been extended to allow for a more generalized stress state and to incorporate an experimentally motivated crack growth (damage evolution) law that is valid over a wide range of loading rates. T...

Seismicity clusters within fault zones can be connected to the structure, geometric complexity and size of asperities which perturb and intensify the stress field in their periphery. To gain further insight into fault mechanical processes, we study stick-slip sequences in an analog, laboratory setting. Analysis of small scale fracture processes exp...

The micromechanical damage mechanics formulated by Ashby and Sammis [1] and generalized by Desh- pande and Evans [2] has been extended to allow for a more generalized stress state and to incorporate an ex- perimentally motivated new crack growth (damage evo- lution) law that is valid over a wide range of loading rates. This law is sensitive to both...

The interaction between a dynamic mode II fracture on a fault plane and off-fault damage has been studied experimentally using high-speed photography and theoretically using finite element numerical simulations. In the experimental studies, fracture damage was created in photoelastic Homalite plates by thermal shock in liquid nitrogen and rupture v...

The central tenant of the critical point model for earthquakes is that a large earthquake on a fault or network of faults is only possible when the crust is in a critical state defined by a long-range spatial correlations of high stress patches that allow a rupture, once nucleated, to grow into a major event. The earthquake then lowers the regional...

The area where microseismic events occur may be correlated with the fracture network at a geothermal field. For an Enhanced Geothermal System (EGS) reservoir, an extensive fracture network with a large aerial distribution is required. Pore-pressure increase, temperature changes, volume change due to fluid withdrawal/injection and chemical alteratio...

High-speed digital photography was used to study rupture propagation on the interface between transparent damaged and undamaged photoelastic plates. Bilateral ruptures were nucleated on pre-machined faults at an angle α to the uniaxial loading axis. Stress concentration at the crack tips produced fringes in polarized laser light that allowed their...

We investigate asymmetric rupture propagation on an interface that combines a bulk elastic mismatch with a contrast in off-fault damage. Mode II ruptures propagating on the interface between thermally shocked (damaged) Homalite and polycarbonate plates were studied using high-speed photographs of the photoelastic fringes. The anelastic asymmetry in...

Considerable progress has been made recently in quantifying geometrical and physical properties of fault surfaces and adjacent fractured and granulated damage zones in active faulting environments. There has also been significant progress in developing rheologies and computational frameworks that can model the dynamics of fault zone processes. This...

We review the results of a recent series of papers in which the interaction between a dynamic mode II fracture on a fault plane and off-fault damage has been studied using high-speed photography. In these experiments, fracture damage was created in photoelastic Homalite plates by thermal shock in liquid nitrogen and rupture velocities were measured...

The brittle portion of the Earth’s lithosphere contains a distribution of joints, faults and cataclastic zones that exist on a wide range of scale-lengths and usually have complex geometries including bends, jogs, and intersections. The material around these complexities is subjected to large stress concentrations, which lead during continuing defo...

Structural analysis of key outcrops from ~5 to ~25 km exhumation depth along the Salzach-Ennstal-Mariazell-Puchberg (SEMP) fault-zone in Austria reveal highly localized deformation in the seismogenic crust down through the brittle-ductile transition (BDT), widening into a 2-km-wide mylonite at mid-crustal levels. Specifically, grain-size distributi...

Fault-slip data from the granitic hanging-wall of the Sierra Madre fault near La-Canada, California, show a steeply dipping conjugate set of cm- to decimeter scale slip surfaces (115 data samples) with moderate to strong inclinations of slip vectors. These off-fault damage elements may be associated with Mohr-Coulomb slip in the stress field of a p...

Dynamic models of earthquakes have shown that the stress concentration at the rupture tip is capable of fracturing rock to distances of meters from the fault plane. The generation of this off-fault damage and its effect on rupture propagation has been studied theoretically using Coulomb plasticity and continuum damage mechanics, and experimentally...

The quasi-static micromechanical damage mechanics originally formulated by Ashby and Sammis (PAGEOPH, 1990) has been expanded in three important ways: (1) An energy density function has been derived that allows a self-consistent inclusion of the effects of dynamic damage evolution on the elastic and anelastic response. (2) whereas the Ashby/Sammis...

High-pass filtering (> 20 Hz) of acceleration records from the 1999 Chi-Chi, Taiwan, and 2004 Parkfield, California, earthquakes reveals a series of bursts that occur only during strong shaking. Initially interpreted as originating from asperity failure on the Chelungpu fault, bursts observed during the Chi-Chi earthquake were subsequently determin...

The interaction between a dynamic mode II fracture on a fault plane and off-fault damage has been studied using high-speed photography. Fracture damage was created in photoelastic Homalite plates by thermal shock in liquid nitrogen, and rupture velocities were measured by imaging fringes at the tips. Three experimental configurations were investiga...

Analysis of a strike-slip fault exhumed from midseismogenic depths reveals that the fault experienced progressive strain localization toward a high-strain fault core. We focus on the Ennstal segment of the 400-km-long Salzach-Ennstal-Mariazell-Puchberg (SEMP) strike-slip fault system in the Eastern Alps, which accommodated ∼60 km of left lateral di...

Nuclear monitoring agencies often use seismic amplitudes to estimate the yields of underground nuclear tests. Any emplacement phenomena that can alter those amplitudes and lead to bias in estimated yields must be considered in the analysis. One condition that might cause such a bias is detonation in frozen rock. Laboratory analy-ses (Mellor, 1971;...

Real earthquake faults are surrounded by fractured zones whose effect on earthquake rupture is investigated. We first performed a series of dynamic photoe-lasticity experiments of a dynamic shear rupture along a frictional interface bounded on one side by an intact material and on the other side by a damaged material of the same or different undama...

Real earthquake faults are surrounded by fractured zones whose effect on earthquake rupture is investigated using a micro-mechanics based damage constitutive description, for the off-fault material, with friction on the fault governed by coulomb like slip weakening law. The micro-mechanics based damage model is an extension of the Ashby and Sammis...

One of the most exciting frontiers in earthquake science is the linkage between the internal structure and mechanical behavior of fault zones. Little is known about how fault-zone structure varies as a function of depth, yet such understanding is vital if we are to understand the mechanical instabilities that control the nucleation and propagation...

High-pass filtering (g.t. 20Hz) of acceleration records from the 1999 Chi-Chi Taiwan and 2004 Parkfield, California earthquakes reveal a series of bursts that occur only during strong shaking [Fischer et. al, 2008a,b]. Initially interpreted as originating from asperity failure on the Chelungpu fault [Chen et. al, 2005], bursts observed during the C...

High-pass filtering (30Hz) of acceleration records from the USGS Parkfield Dense Seismograph Array (UPSAR) reveals a series of bursts that occur only during the strong shaking of the 2004 M6 Parkfield earthquake (Mw 6.0) and its large aftershocks. These high-frequency bursts are probably associated with events occurring very near each station, and...

1] The effect of off-fault damage on the speed of ruptures propagating on faults in photoelastic Homalite plates was measured using high-speed digital photography. The off-fault damage was composed of a network of fractures introduced by thermally shocking the Homalite in liquid nitrogen. The mode II rupture speed measured in damaged Homalite was s...

High-frequency band-pass filtering of acceleration records from the 1999 Chi-Chi, Taiwan, earthquake (M-w 7: 6) resolves the continuous signal into a series of relatively short-duration discrete-energy bursts. We hypothesize that these bursts originate near the individual stations as small, shallow events that have been dynamically triggered by the...

1] Recent observations of nanometer-scale particles in the cores of exhumed fault zones raise questions about how such small particles are formed and how they survive, especially if significant shear heating is produced during an earthquake. Commercial crushing and grinding operations encounter a grind limit near 1 mm below which particles deform p...

There is mounting field evidence that the size distribution of particles in fault gouge and breccia is power law with a slope (fractal capacity dimension D) that increases with increasing strain from values near D=2.6 at low strain to values approaching D=3.0 at large strains. This trend is also observed in 3-dimensional computer simulations that a...

One of the most exciting frontiers in earthquake science is the linkage between the internal structure and mechanical behavior of fault zones. Little is known about how fault-zone structure varies as a function of depth, yet such understanding is vital if we are to understand the mechanical instabilities that control the nucleation and propagation...

Spontaneous bilateral mode II shear ruptures were nucleated on faults in photoelastic Homalite plates loaded in uniaxial compression. Rupture velocities were measured and the interaction between the rupture front and short fault branches was observed using high-speed digital photography. Fault branches were formed by machining slits of varying leng...

Paleoseismological data reveal four clusters of large earthquakes in the Los Angeles region during the past 12,000 years. The historic period is part of an ongoing, >=1,000-year-long lull between clusters. These Los Angeles-region clusters have occurred during the lulls between similar clusters observed on the eastern California shear zone (ECSZ) i...

The method of long waves is used to compute the volume dependence of the elastic properties of MgO, γ-Al2MgO4 (spinel), and γ-Mg2SiO4 (spinel structure) from a rigidion, central force model. A comparison of theoretical and experimental elastic constants for the first two compounds shows that the neglect of non-central forces and ionic polarization...

A nearest neighbor fragmentation model, previously developed to explain observations of power law particle distributions with fractal dimension Df=2.6 in low-strain fault gouge and breccia, is extended to the case of large strains to explain recent observations of Df=3.0 in highly strained cataclasites. At low strains, the elimination of same-sized...

High-frequency band-pass filtering of acceleration records from the 1999 Chi-Chi, Taiwan earthquake (Mw = 7.5) resolves the continuous signal into a series of relatively short duration discrete energy bursts. We hypothesize that these bursts originate at small shallow events near the individual stations that have been dynamically triggered by the P...

Fault zone fracture damage is expected to lower the propagation velocity of earthquake ruptures for two reasons. First, the limiting Rayleigh speed of the rupture [Vr = 0.92 Vs for Mode II or Vr = Vs for Mode III] is lowered to the extent that the damage lowers the shear wave speed Vs. Second, the rupture speed is further reduced because the fractu...

We test the Bowman and King [Bowman, D.D., King, G.C.P., 2001a, Accelerating seismicity and stress accumulation before large earthquakes. Geophys. Res. Lett., 28 (21), 4039–4042, Bowman, D.D., King, G.C.P., 2001b. Stress transfer and seismicity changes before large earthquakes. C. R. Acad. Sci. Paris, 333, 591–599] Stress Accumulation model by exam...

One of the most exciting and important frontiers in earthquake science is the linkage between the internal structure and the mechanical behavior of fault zones. In particular, little is known about how fault-zone structure varies as a function of depth, from near-surface conditions down through the seismogenic crust and into the ductile lower crust...

Rice et al. (2005) formulated an analytical model for dynamic propagation of a slip-pulse on a fault plane. Using earthquake parameters analyzed by Heaton (1990), they found that stress concentration at the rupture front should produce granulation of fault rock to a distance of a few meters and wall rock fracture damage to 10s of meters. This off-f...

It is generally accepted that each earthquake changes the probability of successive earthquakes in a region the size of which scales with its magnitude, the shape of which is determined by its focal mechanism, and by an amount that can be estimated using the Gutenberg-Richter magnitude distribution and the Omori law for the rate of aftershocks, whe...

High frequency, band-pass filtered waveforms from the 1999 Mw = 7.6 Chi-Chi, Taiwan earthquake show a multitude of distinct, short duration energy bursts. Chen et al. (B.S.S.A., 2005, accepted for publication) assumed the sources of these bursts were slip patches on or near the Chelungpu fault plane and devised a location algorithm that resolved ab...

We develop a 2D slip-weakening description of a self-healing slip pulse that propagates dynamically in a steady-state configuration. The model is used to estimate patterns of off-fault secondary failure induced by the rupture, and also to infer fracture energies G for large earthquakes. This extends an analysis for a semi-infinite rupture (Poliakov...

The fault zone of a mature large-displacement fault may be idealized as a nested hierarchical structure consisting of a core of extremely fine grained material surrounded by coarser granulated gouge and breccia which is in turn bordered by fracture-damaged wall rock in which the fracture density decreases with distance to a regional background leve...

— The Load Unload Response Ratio theory (LURR) puts forward the idea that the ratio of seismicity during times of increased tidal loading to times of decreased tidal loading takes on anomalously large values as the preparatory region of the earthquake approaches a critical state. We repeated the calculations of LURR for several earthquakes in Calif...

— An algorithm recently developed by RUNDLE et al. (2002) to find regions of anomalous seismic activity associated with large earthquakes identified the location of an M w = 5.6 earthquake near Calexico, Mexico. In this paper we analyze the regional seismicity before this event, and a nearby M w = 5.7 event, using time-to-failure algorithms develop...

High stresses in the source volume of an underground explosion produce a shell of crushed and fractured rock surrounding the shot point. We have modeled this process using a micro-mechanical damage mechanics in an effective medium source model to calculate the nucleation, growth, and interaction of fractures from an array of preexisting flaws. We h...

One of the most exciting and important frontiers in earthquake science is the linkage between the internal structure and the mechanical behavior of fault zones. In particular, little is known about how fault-zone structure varies as a function of depth, from near-surface conditions down through the seismogenic crust and into the ductile lower crust...

Rice, Sammis and Parsons \(BSSA, 2004\) derived analytical expressions for a dynamical stress field in the vicinity of a propagating slip pulse on a fault plane. They found that the magnitude of off-fault stresses was primarily determined by the velocity of the propagating slip pulse and the normal and shear stresses applied to the fault. Coulomb f...

High-frequency band-pass filtering of broadband strong-motion seismograms recorded immediately adjacent to the fault plane of the 1999 Chi-Chi, Taiwan earthquake reveals a sequence of distinct bursts, each of which can be considered as a sub-event from an asperity source of the Chi-Chi mainshock. These bursts collectively make up the entire mainsho...

We tested the hypotheses (1) that the fractal dimension, D, of hypocenters are different in a locked and a creeping segment of the San Andreas fault and (2) that the relationship D ≈ 2b holds approximately, where b is the slope of the frequency-magnitude relationship. The test area was the 30- to 50-km fault segment north of Parkfield for which two...

Recent measurements of slip profiles on normal faults have found that they are usually triangular in shape. This has been explained to be a consequence of on-fault processes such as slip-dependent friction. However, the recent observation that cumulative slip profiles on normal faults and fault systems in Afar are both triangular and self-similar e...

In recent years there has been renewed interest in observations of accelerating moment release before large earthquakes, as well as theoretical descriptions of seismicity in terms of statistical physics. Most aspects of these works are encompassed by a concept called intermittent criticality in which a region alternately approaches and retreats fro...

Mellor (1973) measured the uniaxial compressive and tensile strengths of granite, limestone, and sandstone over a range of temperatures from 20oC to -197oC. For each of the three rock types, he tested both air-dry and water- saturated samples. These data were modeled using the micromechanical damage model. This model was formulated by Ashby and Sam...

1] Earthquakes and the faults upon which they occur interact over a wide range of spatial and temporal scales. In addition, many aspects of regional seismicity appear to be stochastic both in space and time. However, within this complexity, there is considerable self-organization. We argue that the occurrence of earthquakes is a prob-lem that can b...

We test the Bowman and King [GRL, 2001] "stress recovery" model by examining the pattern of strain released by small events prior to the 1992 Landers earthquake. The stress recovery (SR) model was developed to explain observations of accelerating seismicity preceding large earthquakes. The model proposes that the accelerating seismicity sequences r...

Fault breccia from the Sierra Madre Fault Zone is imaged using a new technique, resolving a highly detailed structure of internal slip surfaces with characteristic distribution of orientations and senses of slip. We excavated a 25x28x35 cm3 sample of brecciated granite from the hanging wall of the Sierra Madre fault near JPL, La Canada, California....

Studies of Coulomb stress changes associated with large earthquakes (greater than M6.0) demonstrate that aftershocks commonly fall within zones of increased stress and that seismic hazard from additional large events increases within these zones. We are developing a system that will automate stress change calculations and integrate the results with...

There is mounting paleoseismological evidence that large earthquakes on a given fault network tend to occur in temporal clusters. Examples include the southern San Andreas system in the Imperial Valley (Rockwell et al., in prep, 2003), the Eastern California Shear Zone (Rockwell et al., BSSA, 2000), the Garlock system (Dawson et al, in prep., 2003)...

We characterize average slip distributions on earthquakes beyond their individual heterogeneity. For that, we analyze a large number of seismic slip distributions both measured at the surface after earthquakes (44 profiles) and derived from slip inversion models (76 models). Investigating the overall shape of these slip profiles, we find that they...

A quasi-static numerical method was used to simulate the failure of a strong stuck asperity on an otherwise creeping fault plane. The numerical model produced the same slip distribution as analytical asperity models, which, for constant loading rate, produced repeating events having a period T that scales with moment M0 as , the scaling relation ob...

Many large earthquakes are preceded by a regional increase in seismic energy release. This phenomenon, called ``accelerating moment release'' (AMR), is due primarily to an increase in the number of intermediate-size events in a region surrounding the mainshock. While AMR has been observed before large earthquakes in many locations, most of these ob...

There are currently three major competing views on the essential geometrical, mechanical, and mathematical nature of faults. The standard view is that faults are (possibly segmented and heterogeneous) Euclidean zones in a continuum solid. The continuum-Euclidean view is supported by seismic, gravity, and electromagnetic imaging studies; by successf...

It has been recognised for some time that oceanic ridges propagate and that the process involves the transfer and concentration of stored elastic energy to the site where new rift is being created. Together with flexure associated with volcano loading and subduction, the evidence that oceanic lithosphere has long-term elastic strength seems convinc...

The non-linear source region of an underground explosion can be divided into a series of nested spherical shells, each characterized by a different deformation rheology. Between the detonation point and the cavity radius, rc, rock is vaporized. Between rc and the "failure radius", rf , rock is fragmented. Between rf and the "damage radius" rd , new...

We examine spatio-temporal features of the strain pattern in relation to the large earthquake history of southern California in the last 20 years. Our analysis specifically focuses on the 1992 Landers and 1994 Northridge earthquakes. Recent studies disagree over whether or not maximum horizontal stress directions in the vicinity of these two earthq...

We investigate regional seismicity patterns as a function of space, time and magnitude preceding the 1992 Landers California earthquake using both raw and declustered catalogs. In addition to the previously documented increase of intermediate earthquakes (and associated acceleration of seismic moment release), the occurrence of small earthquakes do...

The analysis of the stress field near the tip of a semi-infinite dynamic mode II rupture (Poliakov et al., JGR, in press, 2002; http://esag.harvard.edu/dmowska/PDR.pdf), with slip-weakening failure, is extended to the case of a dynamic slip pulse of finite length. Slip-weakening occurs over distance R at the tip, and slip itself takes place over a...

Regional seismicity is characterized by power law distributions in magnitude (the Gutenberg-Richter Law), space (a fractal distribution) and time (the Omori aftershock law). Since such power-laws are characteristic of systems in, or near, a critical state, the concept of self organized criticality (SOC) presents an attractive framework. In the SOC...

The growth ofpre-existing fractures is an important physical process that contributes to the generation of highfrequency seismic waves by explosions. Characterized as damage, this crack growth affects the seismic source in two important ways. First, increased damage weakens the rock by lowering its elastic modulus. Second, the growing fractures act...

We review the "critical point" concept for large earthquakes and enlarge it in the framework of so-called "finite-time singularities". The singular behavior associated with accelerated seismic release is shown to result from a positive feedback of the seismic activity on its release rate. The most important mechanisms for such positive feedback are...

Earth is a dynamic planet. Solid state convection in the deep interior is coupled to the motion of about a dozen rigid plates at the surface. Earthquakes, volcanoes and mountains are located mainly at the boundaries between plates and reflect the relative motion between them. The associated deformation processes span a wide range of regimes from hi...

The numerical model developed by Ben-Zion and Rice (1993) is used to simulate a cluster of asperities on a creeping fault plane. For constant loading rate, the cluster produces repeating events having a period T that scales with moment M0 as T ~M01/6. This is the same scaling observed by Nadeau and Johnson (1998) at Parkfield and derived analytical...