S U M M A R YThe northeastern Caribbean provides a natural laboratory to investigate strain partitioning, its causes and its consequences on the stress regime and tectonic evolution of a subduction plate boundary. Here, we use GPS and earthquake slip vector data to produce a present-day kinematic model that accounts for secular block rotation and elastic strain accumulation, with variable interplate coupling, on active faults. We confirm that the oblique convergence between Caribbean and North America in Hispaniola is partitioned between plate boundary parallel motion on the Septentrional and Enriquillo faults in the overriding plate and plateboundary normal motion at the plate interface on the Northern Hispaniola Fault. To the east, the Caribbean/North America plate motion is accommodated by oblique slip on the faults bounding the Puerto Rico block to the north (Puerto Rico subduction) and to the south (Muertos thrust), with no evidence for partitioning. The spatial correlation between interplate coupling, strain partitioning and the subduction of buoyant oceanic asperities suggests that the latter enhance the transfer of interplate shear stresses to the overriding plate, facilitating strike-slip faulting in the overriding plate. The model slip rate deficit, together with the dates of large historical earthquakes, indicates the potential for a large (M w 7.5 or greater) earthquake on the Septentrional fault in the Dominican Republic. Similarly, the Enriquillo fault in Haiti is currently capable of a M w 7.2 earthquake if the entire elastic strain accumulated since the last major earthquake was released in a single event today. The model results show that the Puerto Rico/Lesser Antilles subduction thrust is only partially coupled, meaning that the plate interface is accumulating elastic strain at rates slower than the total plate motion. This does not preclude the existence of isolated locked patches accumulating elastic strain to be released in future earthquakes, but whose location and geometry are not resolvable with the present data distribution. Slip deficit on faults from this study are used in a companion paper to calculate interseismic stress loading and, together with stress changes due to historical earthquakes, derive the recent stress evolution in the NE Caribbean.

The Hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace. Dislocation models of the surface deformation adjacent to the Hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at approximately 7 millimeters per year to the bottom of the seismogenic zone along a approximately 20-kilometer-long northern fault segment. Microearthquakes with the same waveform repeatedly occur at 4- to 10-kilometer depths and indicate deep creep at 5 to 7 millimeters per year. The difference between current creep rates and the long-term slip rate of approximately 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern Hayward fault adjacent to the locked 1868 earthquake rupture, which broke the southern 40 to 50 kilometers of the fault. The potential for a major independent earthquake of the northern Hayward fault might be less than previously thought.

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S U M M A R YThe Northeastern Caribbean region accommodates ∼20 mm yr −1 of oblique convergence between the North American and Caribbean plates, which is distributed between the subduction interface and major strike-slip faults within the overriding plate. As a result, this heavily populated region has experienced eleven large (M ≥ 7.0) earthquakes over the past 250 yr.In an effort to improve our understanding of the location and timing of these earthquakes, with an eye to understand where current seismic hazards may be greatest, we calculate the evolution of Coulomb stress on the major faults since 1751 due to coseismic, postseismic, and interseismic deformation. Our results quantify how earthquakes serve to relieve stress accumulated due to interseismic loading and how fault systems communicate with each other, serving both to advance or retard subsequent events. We find that the observed progressive westwards propagation of earthquakes on the Septentrional and Enriquillo strike-slip faults and along the megathrust was encouraged by coseismic stress changes associated with prior earthquakes. For the strike-slip faults, the loading of adjacent segments was further amplified by postseismic relaxation of a viscoelastic mantle in the decades following each event. Furthermore, earthquakes on the Septentrional fault relieve a small level of Coulomb stress on the parallel Enriquillo fault to the south (and vice versa), perhaps explaining anticorrelated timing of events on these respective fault systems. The greatest net build-up of Coulomb stress changes over the past 250 yr occurs along the central and eastern segment of the Septentrional and the Bowin strike-slip faults (65 • -71 • W), as no recent earthquake has relieved stress in these regions. For oblique thrust faults, net stress build-up over the past 250 yr is the largest on the North American/Caribbean megathrust west of 70.5 • W. High Coulomb stress has also developed east of 65.5 • W, where no historic events have been inferred to have relieved stress, though uncertainties in fault slip rates from our block model associated with a lack of GPS observations in this region may have led to an over-estimation of stress changes.

The Calaveras and Hayward faults are major components of the San Andreas fault system in the San Francisco Bay region. Dextral slip is presumed to transfer from the Calaveras fault to the Hayward fault in the Mission Hills region, an area of uplift in the contractional stepover between the two faults. Here the estimated deep slip rates drop from 15 to 6 mm/yr on the Calaveras fault, and slip begins on the Hayward fault at an estimated 9 mm/yr. A lineament of microseismicity near the Mission fault links the seismicity on the Calaveras and Hayward faults and is presumed to be related directly to this slip transfer. However, geologic and seismologic evidence suggest that the Mission fault may not be the source of the seismicity and that the Mission fault is not playing a major role in the slip transfer.We perform a joint inversion for hypocenters and the 3D P-wave velocity structure of the stepover region using 477 earthquakes. We find strong velocity contrasts across the Calaveras and Hayward faults, corroborated by geologic, gravity, and aeromagnetic data. Detailed examination of two seismic lineaments in conjunction with the velocity model and independent geologic and geophysical evidence suggests that they represent the southern extension of a northeasterly dipping Hayward fault that splays off the Calaveras fault, directly accounting for the deep slip transfer. The Mission fault appears to be accommodating deformation within the block between the Hayward and Calaveras faults. Thus, the Calaveras and Hayward faults need to be considered as a single system for developing rupture scenarios for seismic hazard assessments.Online material: 3D interactive visualizations of the Mission and Alum Rock hypocenters.

[1] The Calaveras fault is a major component of the San Andreas fault system in the San Francisco Bay area, that generated 13 earthquakes of M L > 5 since 1850. In most recent M L > 5 events, premain shock and postmain shock microseismicity is sparse in the region of coseismic slip. These aseismic areas are believed to represent locked patches of the fault that are accumulating strain to be released in M L > 5 events. We analyze geodetic data to better characterize the spatial distribution of interseismic slip rates on the Calaveras fault, modeling the slip distribution in the seismogenic zone by inversion of over 25 years of surface deformation data. We use a regional fault model with the seismogenic zone of the Calaveras fault discretized into $6 km  3 km elements, employing a weighted least squares approach with smoothing and positivity constraints. Our discretized fault slip model consistently identifies regions of slip deficit in the seismogenic zone of the Calaveras fault that generally correspond to regions of decreased microseismicity and ruptures of previous moderate earthquakes. In particular, we find correspondence with the 1979 Coyote Lake and 1984 Morgan Hill events, as well as regions where historical earthquakes on the Coyote and the Sunol-San Ramon segments have occurred. Moment magnitude calculations based on the estimated slip deficit, fault area, and recurrence intervals agree with measured magnitudes of modern events and interpreted historical magnitudes. The results suggest that a combination of geodetically derived fault slip models and microseismicity distribution can be used to characterize seismic hazard.INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements-interplate (8155); 1243 Geodesy and Gravity: Space geodetic surveys; 7223 Seismology: Seismic hazard assessment and prediction; KEYWORDS: Calaveras fault, fault models, earthquakes, California Citation: Manaker, D. M., R. Bürgmann, W. H. Prescott, and J. Langbein, Distribution of interseismic slip rates and the potential for significant earthquakes on the Calaveras fault, central California,

S U M M A R YDuctile behaviour in rocks is often associated with plasticity due to dislocation motion or diffusion under high pressures and temperatures. However, ductile behaviour can also occur in brittle materials. An example would be cataclastic flow associated with folding at shallow crustal levels. Engineers utilize damage mechanics to model the continuum deformation of brittle materials. In this paper we utilize a modified form of damage mechanics that includes a yield stress. Here, damage represents a reduction in frictional strength. We use this empirical approach to simulate bending of the lithosphere through the problem of plate flexure.We use numerical simulations to obtain quasi-static solutions to the Navier equations of elasticity. We use the program GeoFEST v. 4.5 (Geophysical Finite Element Simulation Tool), developed by NASA Jet Propulsion Laboratory, to generate solutions for each time step. When the von Mises stress exceeds the critical stress on an element we apply damage to reduce the shear modulus of the element. Damage is calculated at each time step by a power-law relationship of the ratio of the critical stress to the von Mises stress and the critical strain to the von Mises strain. This results in the relaxation of the material due to increasing damage. To test our method, we apply our damage rheology to a semi-infinite plate deforming under its own weight. Where the von Mises stress exceeds the critical stress, we simulate the formation of damage and observe the time-dependent relaxation of the stress and strain to near the yield strength. We simulate a wide range of behaviours from slow relaxation to instantaneous failure, over timescales that span six orders of magnitude. Using this method, stress relaxation produces perfectly plastic behaviour in cases where failure does not occur. For cases of failure, we observe a rapid increase in damage, analogous to the acceleration of microcrack formation and acoustic emissions prior to failure. Thus continuum damage mechanics can be used to simulate the irreversible deformation of brittle materials.

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