Local Fault Geometry Research Articles

Injection-induced fault slip and associated seismicity in the lab: Insights from source mechanisms, local stress states and fault geometry

Probing source mechanisms of natural and induced earthquakes is a powerful tool to unveil associated rupture kinematics. The source processes of failure and slip instability driven by stress loading are affected by fault geometry, but the source ruptures of injection-induced seismicity in relation to fault structures and local stress states remain poorly understood. We have conducted a series of fault reactivation and slip experiments on sandstone samples containing faults with different surface roughness (smooth saw-cut fault and fractured rough fault). We impose progressive fluid injection to induce fault slip, and simultaneously monitor the associated acoustic emission (AE) activity. Using high-resolution AE recordings, we perform full moment tensor inversion of all located AE sources, and investigate the changes of AE source characteristics associated with induced fault slip and their relation to fault roughness. For the complex and rough fault, we observe significant non-double-couple components of AE sources and a high degree of focal mechanism heterogeneity. The temporal changes of AE mechanisms associated with injection-induced fault slip on the smooth fault reveal increasing proportions of double-couple components and decreasing variability of AE focal mechanisms when approaching the onset of slip events. The observed inconsistency between the nodal planes of AE sources and the macroscopic fault plane orientation is attributed to the development of secondary fracture networks surrounding the principal slip surface. We analyze changes in the magnitude-frequency characteristics and source mechanisms of AEs with fault-normal distance, showing that for the smooth (mature) fault, Gutenberg–Richter b-value of on-fault seismicity is lower and focal mechanisms are less heterogeneous, compared to off-fault seismicity. Our results emphasize the important role of roughness-related changes in local fault geometry and associated stress heterogeneity for source mechanisms and rupture kinematics of injection-induced seismicity.

Throw-rate variations within linkage zones during the growth of normal faults: Case studies from the Western Volcanic Zone, Iceland

This work investigates how throw-rates vary within fault bends and sites of fault linkage during the process of normal fault growth. In the Western Volcanic Zone, Iceland, through detailed field mapping and field measurements of fault throws, normal faults are mapped and along-strike throw profiles are constructed in order to understand how the throw-rates relate with the local fault geometry along faults at different stages of linkage. The results show that throw-rates increase within linkage zones and propagating fault bends independently from the stage of maturity of the fault bend. This implies that 1) the relationship between the local fault geometry and the along-strike distribution of throw-rate is driven by the deeper part of the fault, where established fault bends start propagating to the surface; 2) faults grow first by linkage and coalescence of separate faults, and then by accumulation of slip on the resultant fault, in agreement with models of fault growth by linkage and coalescence; 3) incipient fault bends can produce uncertainty associated with palaeoseismological results, if fault bends remain unrecognised. Moreover, this work demonstrates that existing models showing increased co-seismic and throw-rates within fault bends and sites of fault linkage found in continental extensional settings are valid in a geodynamic context of a mid-oceanic rifts.

Two‐Phase Exhumation of the Santa Rosa Mountains: Low‐ and High‐Angle Normal Faulting During Initiation and Evolution of the Southern San Andreas Fault System

AbstractLow‐angle detachment fault systems are important elements of oblique‐divergent plate boundaries, yet the role detachment faulting plays in the development of such boundaries is poorly understood. The West Salton Detachment Fault (WSDF) is a major low‐angle normal fault that formed coeval with localization of the Pacific‐North America plate boundary in the northern Salton Trough, CA. Apatite U‐Th/He thermochronometry (AHe; n = 29 samples) and thermal history modeling of samples from the Santa Rosa Mountains (SRM) reveal that initial exhumation along the WSDF began at circa 8 Ma, exhuming footwall material from depths of >2 to 3 km. An uplifted fossil (Miocene) helium partial retention zone is present in the eastern SRM, while a deeper crustal section has been exhumed along the Pleistocene high‐angle Santa Rosa Fault (SFR) to much higher elevations in the southwest SRM. Detachment‐related vertical exhumation rates in the SRM were ~0.15–0.36 km/Myr, with maximum fault slip rates of ~1.2–3.0 km/Myr. Miocene AHe isochrons across the SRM are consistent with northeast crustal tilting of the SRM block and suggest that the post‐WSDF vertical exhumation rate along the SRF was ~1.3 km/Myr. The timing of extension initiation in the Salton Trough suggests that clockwise rotation of relative plate motions that began at 8 Ma is associated with initiation of the southern San Andreas system. Pleistocene regional tectonic reorganization was contemporaneous with an abrupt transition from low‐ to high‐angle faulting and indicates that local fault geometry may at times exert a fundamental control on rock uplift rates along strike‐slip fault systems.

The 3-D surface deformation, coseismic fault slip and after-slip of the 2010 Mw6.9 Yushu earthquake, Tibet, China

Using SAR interferometry on C band Envisat descending track and L band ALOS ascending track SAR images, respectively, we firstly obtain two coseismic deformation fields and one postseismic deformation of the 2010 Yushu earthquake, Tibet, China. In the meanwhile, we also obtain the azimuthal coseismic deformation of the Yushu event by Multi Aperture Interferometry (MAI) technique. With the 3 components of one-dimensional coseismic InSAR measurements, we resolve the complete 3-dimensional deformation of the 2010 Yushu event, which shows conformity and complexity to left lateral slip mechanism. The horizontal deformation is basically consistent with a sinistral slip event; whereas the vertical displacement does show certain level of complexity, which we argue is indicative of local fault geometry variation. Based on the InSAR data and elastic dislocation assumption, we invert for coseismic fault slip and early after-slip of the Yushu event. Our inversion results show major coseismic left lateral strike slip with only minor thrust component. The after-slip model fills most of the slip gaps left by the coseismic fault slip and finds a complementary slip distribution to the coseismic fault slip, which is a good indicator that future earthquake potential on the Yushu segment has been significantly reduced.

Localized and distributed creep along the southern San Andreas Fault

AbstractWe investigate the spatial pattern of surface creep and off‐fault deformation along the southern segment of the San Andreas Fault using a combination of multiple interferometric synthetic aperture radar viewing geometries and survey‐mode GPS occupations of a dense array crossing the fault. Radar observations from Envisat during the period 2003–2010 were used to separate the pattern of horizontal and vertical motion, providing a high‐resolution image of uplift and shallow creep along the fault trace. The data reveal pervasive shallow creep along the southernmost 50 km of the fault. Creep is localized on a well‐defined fault trace only in the Mecca Hills and Durmid Hill areas, while elsewhere creep appears to be distributed over a 1–2 km wide zone surrounding the fault. The degree of strain localization is correlated with variations in the local fault strike. Using a two‐dimensional boundary element model, we show that stresses resulting from slip on a curved fault can promote or inhibit inelastic failure within the fault zone in a pattern matching the observations. The occurrence of shallow, localized interseismic fault creep within mature fault zones may thus be partly controlled by the local fault geometry and normal stress, with implications for models of fault zone evolution, shallow coseismic slip deficit, and geologic estimates of long‐term slip rates.

A 2D Pseudodynamic Rupture Model Generator for Earthquakes on Geometrically Complex Faults

Abstract Geologic observations indicate that faults are fractally rough surfaces, with deviations from planarity at all length scales. Fault roughness introduces complexity in the rupture process and resulting ground motion. We present a 2D kinematic rupture generator that emulates the strong dependence of earthquake source parameters on local fault geometry observed in dynamic models of ruptures on nonplanar faults. This pseudodynamic model is based on a statistical analysis of ensembles of 2D plane strain rupture simulations on fractally rough faults with rate‐weakening friction and off‐fault viscoplasticity. We observe strong anticorrelation of roughness‐induced fluctuations in final slip, rupture velocity, and peak slip velocity with the local fault slope for right‐lateral strike‐slip ruptures. Spatial variability in these source parameters excites high‐frequency seismic waves that are consistent with observed strong‐motion records. Although accurate modeling of this high‐frequency motion is critical to seismic‐hazard analysis, dynamic rupture simulations are currently too computationally inefficient to be of practical use in such applications. We find that the seismic waves excited by the pseudodynamic model have similar intensity and spectral content to the corresponding dynamic model. Although the method has been developed in 2D, we envision that a similar approach could be taken for the 3D problem, provided that computational resources are available to generate an ensemble set of 3D dynamic rupture simulations. The resulting methodology is expected to find future application in efficient earthquake simulations that accurately quantify high‐frequency ground motion.

Hot/Cold Spots in Italian Macroseismic Data

The site effect is usually associated with local geological conditions, which increase or decrease the level of shaking compared with standard attenuation relations. We made an attempt to see in the macroseismic data of Italy some other effects, namely, hot/cold spots in the terminology of Olsen (2000), which are related to local fault geometry rather than to soil conditions. We give a list of towns and villages liable to amplify (+) or to reduce (-) the level of shaking in comparison with the nearby settlements. Relief and soil conditions cannot always account for the anomalous sites. Further, there are sites where both (+) and (-) effects are observed depending on the earthquake. The opposite effects can be generated by events from the same seismotectonic zone and along the same direction to the site. Anomalous sites may group themselves into clusters of different scales. All isolated anomalous patterns presented in this paper can be used in hazard analysis, in particular, for the modeling and testing of seismic effects.

Fault slip in the 1997 Manyi, Tibet earthquake from linear elastic modelling of InSAR displacements

SummaryThe Mw 7.6 1997 Manyi earthquake occurred in an area of central northern Tibet where sparse vegetation coverage and a lack of human habitation provide excellent conditions for Interferometric Synthetic Aperture Radar (InSAR) studies. We use coseismic pairs of radar images acquired by the ESA ERS-2 satellite to construct interferograms of the surface displacement field due to the earthquake. The location and extent of the coseismic fault rupture are mapped using a combination of optical satellite imagery, high-resolution digital topography, interferometric correlation and azimuth offset measurements; in so doing, we are able to relate prominent geomorphic features in the fault zone to bends in the fault.Using elastic dislocation models consistent with this mapped fault trace, we then test a range of fault geometries and slip conditions to find the combination which best explains the InSAR displacements. Our favoured model contains a reversal in fault dip, approximately halfway along its length, occurring at the location of a restraining bend. Slip on this model fault is heterogeneous, with two areas of peak slip of 7 m or greater, and components of dip-slip displacement which vary significantly along-strike. The success of this model in fitting the data implies that an observed asymmetry in the coseismic interferograms can be explained in terms of the local fault geometry, rather than by using non-linear elastic rheologies as suggested by earlier authors.

Fault slip rates from three‐dimensional models of the Los Angeles metropolitan area, California

We present results from the first mechanical model of active tectonics in the Los Angeles region to use non‐planar, geologically representative fault surfaces compiled by the Southern California Earthquake Center Community Fault Model. The fault slip rates from our three‐dimensional model match well the available geologic slip rates. Discrepancies in reverse slip along the Upper Elysian Park fault and strike‐slip along the Raymond fault may reflect imprecise knowledge of local fault geometry. Discrepancy in the average dip slip rate along the Palos Verdes fault reveals variations in dip slip along that surface; model predictions at the location of the geological investigation have good match to geologic data. The validated model is used to estimate dip and strike slip rates for 26 active faults in the Los Angeles metropolitan region, many of which are otherwise unconstrained by geologic evidence.

Near‐field transpressive deformation along the San Andreas fault zone in southern California, based on exhumation constrained by (U‐Th)/He dating

Low‐temperature thermochronometry reveals that a narrow crustal sliver trapped within strands of the San Andreas fault zone in southern California has experienced recent, rapid exhumation. Eight apatite (U‐Th)/He ages from a 1‐km‐relief section along Yucaipa Ridge in the San Bernardino Mountains range from 1.4 to 1.7 Ma. The minimal change in age with elevation implies exhumation of ∼5–7 mm yr−1, sustained for at least several hundred thousand years. Three titanite helium ages from the ridge are much older, ranging from 57 to 82 Ma. These show a steep gradient with elevation, representing either an exhumed, partial retention zone or slow cooling through much of the Tertiary. These data imply that a total exhumation of ∼3 to 6 km has occurred since 1.8 Ma. It is uncertain whether this exhumation terminated as early as 1 Ma or has continued up to the present at a decelerated rate. We surmise that this exhumation represents rock uplift in the absence of major surface uplift, in that it kept pace with tectonic uplift as the narrow fault block maintained steady state relief. The record of sedimentation in adjacent basins is consistent with the implied magnitude of erosion. Such rapid, large‐magnitude exhumation within the strands of the San Andreas fault zone is important for models of transpressional tectonics. It is consistent with a strain partitioning model which predicts that pure shear dominated fault zones experience significant vertical strain. However, it is inconsistent with a stress‐partitioning model which predicts that fault zone weakness limits pure shear deformation to the borderlands of the master strike‐slip fault. In addition, a concentration of secondary contraction within the fault zone may require modification of coupling models between strong upper mantle and brittle upper crust via the weak lower crust. These models predict that transpressional deformation will either be uniformly distributed across the plate boundary or be limited to the far‐field borderlands, rather than concentrated in the near field. Alternatively, the exhumation of Yucaipa Ridge may have been driven by the nearby restraining bend in the San Andreas fault at San Gorgonio Pass, in which case it represents local fault geometry rather than accommodation of far‐field plate motion.