Seismic cycle and rheological effects on estimation of present-day slip rates for the Agua Blanca and San Miguel-Vallecitos faults, northern Baja California, Mexico

Geodesy can be used to infer long-term fault slip rates, assuming a model for crust and upper mantle rheology. We examine the sensitivity of fault slip rate estimates to assumed rheology for the Agua Blanca and San Miguel-Vallecitos faults in northern Baja California, Mexico, part of the Pacific–North America plate boundary zone. The Agua Blanca fault is seismically quiet, but offset alluvial fans indicate young activity. Current seismicity is confined to the nearby San Miguel-Vallecitos fault, a small offset fault better aligned with plate motion. GPS measurements between 1993 and 1998 suggest that both faults are active, with a combined slip rate of 4–8 mm yr. regardless of rheological model. However, slip rate estimates for the individual faults are sensitive to assumed rheology. Elastic half-space models yield 2–3 mm yr. for the Agua Blanca fault, and somewhat faster rates for the San Miguel-Vallecitos fault, 2–4 mm yr., with uncertainties of about 1 mm yr. Models incorporating viscoelastic rheology and seismic cycle effects suggest a faster slip rate for the Agua Blanca fault, 6 ± 1 mm yr, and a slower rate for the San Miguel-Vallecitos fault, 1 ± 1 mm yr, in better agreement with geological data, but these rates are sensitive to assumed rheology. Numerical simulations with a finite element model suggest that for similar rheological and friction conditions, slip on the San Miguel-Vallecitos fault should be favored due to better alignment with plate motion. Long-term faulting processes in the larger offset Agua Blanca fault may have lowered slip resistance, allowing accommodation of motion despite misalignment with plate motion

Seafloor Geodesy in Shallow Water With GPS on an Anchored Spar Buoy

Measuring seafloor motion in shallow coastal water is challenging due to strong and highly variable oceanographic effects. Such measurements are potentially useful for monitoring near‐shore coastal subsidence, subsidence due to petroleum withdrawal, strain accumulation/release processes in subduction zones and submerged volcanoes, and certain freshwater applications, such as volcano deformation in caldera‐hosted lakes. We have developed a seafloor geodesy system for this environment based on an anchored spar buoy topped by high‐precision GPS. Orientation of the buoy is measured using a digital compass that provides heading, pitch, and roll information. The combined orientation and GPS tracking data are used to recover the three‐dimensional position of the seafloor marker (anchor). A test system has been deployed in Tampa Bay, Florida, for over 1 year and has weathered several major storms without incident. Even in the presence of strong tidal currents which can deflect the top of the buoy several meters from vertical, daily repeatability in the corrected three‐component position estimates for the anchor is 1–2 cm or better.Published12116–121401IT. Reti di monitoraggio e sorveglianzaJCR Journa

On the Plate Boundary Forces that Drive and Resist Baja California Motion

The driving forces of microplate transport remain one of the major unknowns in plate tectonics. Our hypothesis postulates that the Baja California microplate is transported along the North America–Pacific plate boundary by partial coupling to the Pacific plate and low coupling to the North America plate. To test this idea, we use numerical modeling to examine the interplate coupling on a multiple-earthquake-cycle time scale along the Baja California–Pacific plate boundary and compare the modeled velocity field with the observed geodetic motion of the Baja California microplate. We find that when the strain can localize along a weak structure surrounding microplate (faults), high interplate coupling, produced by frictional tectonic stresses, can reproduce the observed kinematics of the Baja California microplate as seen from geodetic rigid-plate motions. We also find that the northward motion of Baja California can influence the fault slip partitioning of the major faults in the North America–Pacific plate boundary region north of Baja California

Strain Rate Patterns from Dense GPS Networks

The knowledge of the crustal strain rate tensor provides a description of geodynamic processes such as fault strain accumulation, which is an important parameter for seismic hazard assessment, as well as anthropogenic deformation. In the past two decades, the number of observations and the accuracy of satellite based geodetic measurements like GPS greatly increased, providing measured values of displacements and velocities of points. Here we present a method to obtain the full continuous strain rate tensor from dense GPS networks. The tensorial analysis provides different aspects of deformation, such as the maximum shear strain rate, including its direction, and the dilatation strain rate. These parameters are suitable to characterize the mechanism of the current deformation. Using the velocity fields provided by SCEC and UNAVCO, we were able to localize major active faults in Southern California and to characterize them in terms of faulting mechanism. We also show that the large seismic events that occurred recently in the study region highly contaminate the measured velocity field that appears to be strongly affected by transient postseismic deformation. Finally, we applied this method to coseismic displacement data of two earthquakes in Iceland, showing that the strain fields derived by these data provide important information on the location and the focal mechanism of the ruptures

The Rigidity of the Western Arabian Margin: Extensional Strain Rate Field from GPS Networks

Previous analysis of the deformation field of the Arabian margin using then the available network of Global Navigation Satellite System (GNSS) stations found that an assumption of a rigid Arabia satisfied the observations as reported by ArRajehi et al. (Tectonics 29, no. 3, 2010). However, an Mw 5.4 intra-plate earthquake occurred in 2009 in the Alshaqqah Harrat as discussed by Pallister et al. (Nature Geoscience 3, no. 10:705, 2010) suggesting that there is ongoing active deformation within the western Arabian margin. We evaluate the Arabian margin deformation field using an expanded network totaling 87 GPS stations distributed within Saudi Arabia. With the addition of these GPS stations, we found evidence of extensional strain within the Arabian margin, in regions that were unsampled by the previous geodetic networks. Although, due to the short time series, the signal at these new GPS sites is very close to the limit of the resolution, this study provides the first indication, derived from GPS velocities, of the location and extent of active internal deformation within the Arabian plate

Dynamic Uplift in a Transpressional Regime: Numerical Model of the Subduction Area of Fiordland, New Zealand

Bending of the downgoing plate in subduction zone typically leads to an offshore peripheral bulge. This leads to dynamic uplift generated by the elastic bending of the subducted slab, and is generally enough to support the topography of the bulge in a non-isostatic manner and produce a positive gravity anomaly. The Southwest region of the South Island of New Zealand, Fiordland, is characterized by high elevation and a large positive Bouguer gravity anomaly. This combination of high topography with high Bouguer gravity argues against isostatic equilibrium and suggests an additional support mechanism. Earthquakes as deep as 150 km, a deformed Benioff zone and inferences from plate reconstructions all support a tectonic model where the eastern margin of the Australian plate is subducting beneath Fiordland and is sharply bent. This bending of the Australian plate provides the needed non-isostatic support for Fiordland topography and generates the observed gravity anomaly. Although the peripheral bulge in subduction zones is generally localized offshore, the positive gravity anomaly (Bouguer and free air) in Fiordland is onshore, close to the shoreline, and generally corresponds spatially with high elevations. Here we propose a mechanism that allows the subducted sliver of slab to be decoupled from the main Australian plate and strongly bent beneath Fiordland. We test this scenario with a finite-element model. The model allows us to study the flexural response of a subducting elastic slab bent by lateral compression into a shape similar to the one inferred from seismicity. We test how different plate geometries and plate boundary forces influence the flexural dynamic support of Fiordland topography, providing important constraints on the local plate dynamics. The model results show that for a tectonically reasonable combination of plate geometries and boundary forces, the deformation of the lithosphere produces the observed topography and gravity signature. In particular we find that the bending of the subducted Australian plate can supply the needed uplift and support for the topography of Fiordland. However, a weak area west of but nearby the Fiordland shoreline, perhaps a fault or tear, is needed to decouple the subducted sliver, confine the bulge, and localize the uplift within Fiordland

Quantifying Rates of “Rifting while Drifting” in the Southern Gulf of California: The role of the Southern Baja California Microplate and its Eastern Boundary Zone

The southern Baja California (Mexico) microplate has been rapidly moving away from the North America plate since ca. 12 Ma. This relative motion toward the northwest developed an oblique-divergent plate boundary that formed the Gulf of California. The rift-drift hypothesis postulates that when a continent ruptures and seafloor spreading commences, rifting on the plate margins ceases, and the margins start to drift, subside, and accumulate postrift sediments, eventually becoming a passive margin. In contrast to this hypothesis, the southern part of the Baja California microplate (BCM), and in particular its actively deforming eastern boundary zone, has continued significant rifting for millions of years after seafloor spreading initiated within the southern Gulf of California at 6–2.5 Ma. This is a process we call “rifting-while-drifting.” Global positioning system (GPS)–based data collected from 1998 to 2011 show relative motion across the eastern boundary zone up to ~2–3.2 mm/yr with respect to a stable BCM. Furthermore, the velocity directions are compatible with normal faulting across the eastern boundary zone nearly perpendicular to the trend of the plate boundary at the latitude of La Paz and therefore a highly strain partitioned domain. North of 25°N latitude up to the Loreto area, there is a domain with no strain partitioning, and northwestdirected transtensional deformation dominates. From long-term geologic and paleoseismology studies, late Quaternary faulting rates are equal to or less than the GPS-derived rates, while geologic rates older than 1–2 Ma are commonly much higher. We suggest that the “rifting-while-drifting” process may be caused by the large topographic relief across the BCM margin, which created a significant gradient in gravitational potential energy that helps in driving continued relatively slow faulting. The relief was inherited from the much faster faulting of the BCM eastern boundary zone before plate motions largely localized along the modern transform–spreading centers in the axis of the Gulf of California. The low sediment flux from the small drainages and arid climate on the southern Baja California Peninsula result in the maintenance of underfilled to starved basins, and the relatively slow late Quaternary active faulting promotes continued topographic relief over millions of years

Modeling Fault Creep on the Hayward Fault and Implications of Seismicity for Defining Patterns of Fault Creep

The Hayward fault is considered to be one of the primary hazards in the San Francisco Bay region. Although it is documented to undergo significant creep, with some creeping patches accommodating 50% or more of the long-term fault displacement, the fault also experiences moderate to large earthquakes (most recent M ~6.8 in 1868). Under the assumption that the seismic hazard associated with a fault is related to the distribution and amount of slip deficit accumulated during interseismic periods. Therefore mapping creep patterns on a fault plane is an important component in the assessment of the seismic hazard. Combining observations of surface creep rate and the distribution of micro-seismicity, with modeling results derived from a visco-elastic finiteelement model driven by far field plate motions, we have analyzed the slip deficit that can be accumulated on the Hayward Fault. Our results show that the interaction of the fault with the surrounding lithosphere leads to a smooth transition of the creep rate from locked to fully creeping areas and implies significant slip deficit accumulation not only in fully locked zones but also in adjacent low friction areas. In order to link seismic potential to the rate at which moment accumulates on the fault plane, we need to understand the patterns and distribution of creep over time. As might be expected, the microseismicity observed on the fault produces only a negligible percentage of the seismic moment dissipated on the Hayward fault, whereas aseismic creep releases about 25% of the moment accumulating on the fault. The distribution of creep on the fault can change throughout the earthquake cycle, in particular after major seismic events. Although at present the post-seismic transients have mostly decayed, the pattern of accumulated moment is significantly different when these transients are included

Imaging Patterns of Fault Creep: Implications for Earthquakes on the Hayward Fault

The slip deficit that accumulates on a fault constrains the potential slip (and moment) for subsequent earthquakes on the fault. Fault creep will reduce the rate at which this slip deficit accumulates, at least on those patches of the fault that undergo such aseismic slip. Mapping the spatial and temporal patterns of such creep then becomes an important component of assessments of earthquake potential on a fault such as the Hayward. In regions where faults are creeping at or near the surface, the pattern of surface deformation can be used to constrain fault creep in the upper few kilometers. Determination of the patterns of creep on deeper sections of a fault is poorly constrained by near-fault surface observations. The inclusion of micro-seismicity in analyzing patterns of fault creep adds an additional constraint on locations of locked and creeping patches on the fault. A second consideration in assessing the accumulation of slip deficit is the potential for timedependent creep behavior – particularly driven by post-earthquake viscous relaxation. We are investigating the potential role of such transient behavior in both biasing observations of creep rate, and also modifying the rate at which slip deficit accumulates. Including the effects of a simulated 1868-like earthquake in our models of Hayward Fault creep indicate a significant variations in both the spatial pattern and rate of fault creep during the first 50-100 years after the earthquake

Numerical Modeling of Strike-Slip Creeping Faults and Implications for the Hayward Fault, California

The seismic potential of creeping faults such as the Hayward fault (San Francisco Bay Area, CA) depends on the rate at which moment (slip deficit) accumulates on the fault plane. Thus, it is important to evaluate how the creep rate observed at the surface is related to the slip on the fault plane. The surface creep rate (SCR) depends on the geometry of locked and free portions of the fault and on the interaction between the fault zone and the surrounding lithosphere. Using a viscoelastic finite element model, we investigate how fault zone geometries and physical characteristics such as frictionless or locked patches affect the observed surface creep when the system is driven by far field plate motions. These results have been applied to creep observations of the Hayward fault. This analysis differs from most previous fault creeping models in that the fault in our model is loaded by a distributed viscous flow induced by far field velocity boundary conditions instead of imposed slip beneath the major faults of the region. The far field velocity boundary conditions simulate the relative motion of the stable Pacific plate respect to the Rigid Sierra Nevada block, leaving the rheology, fault geometry, and mechanics (locked or free to creep patches), to determinate the patterns of fault creep. Our model results show that the fault geometry (e.g. length and depth of creeping) and the local rheology influence the surface creep rate (SCR) and the slip on the fault plane. In particular, we show that the viscoelastic layer beneath the elastic seismogenic zone plays a fundamental role in loading the fault. Additionally, the coupling with the surrounding lithosphere results in a smooth transition from regions free to creep to locked patches