Search

We present a statistical analysis of focal mechanism orientations for nine California fault zones with the goal of quantifying variations of fault zone heterogeneity at seismogenic depths. The focal mechanism data are generated from first motion polarities for earthquakes in the time period 1983–2004, magnitude range 0–5, and depth range 0–15 km. Only mechanisms with good quality solutions are used. We define fault zones using 20 km wide rectangles and use summations of normalized potency tensors to describe the distribution of double-couple orientations for each fault zone. Focal mechanism heterogeneity is quantified using two measures computed from the tensors that relate to the scatter in orientations and rotational asymmetry or skewness of the distribution. We illustrate the use of these quantities by showing relative differences in the focal mechanism heterogeneity characteristics for different fault zones. These differences are shown to relate to properties of the fault zone surface traces such that increased scatter correlates with fault trace complexity and rotational asymmetry correlates with the dominant fault trace azimuth. These correlations indicate a link between the long-term evolution of a fault zone over many earthquake cycles and its seismic behaviour over a 20 yr time period. Analysis of the partitioning of San Jacinto fault zone focal mechanisms into different faulting styles further indicates that heterogeneity is dominantly controlled by structural properties of the fault zone, rather than time or magnitude related properties of the seismicity. [ABSTRACT FROM AUTHOR]

Using InSAR and hydrogeologic time-series spanning 1995 to 2000, we characterize the elastic storage and surface deformation in the vicinity of the San Bernardino basin, California. The region encompasses a complex major aquifer located at the junction of the San Andreas and San Jacinto faults that supplies groundwater to over 600,000 people. We remove the elastic vertical surface displacement associated with changes in groundwater levels from the InSAR time-series by a least squares inversion. Our method estimates a poroelastic ratio at 60 well sites, which we normalize by basin depth, allowing for comparison of the elastic response of the aquifer skeleton from site to site. Our method also estimates residual vertical displacement rates at each well site, surface displacement not explained by observed trends in groundwater levels. Residual vertical displacement rates reveal patterns of subsidence and uplift across the basin over the 5-yr period from 1995 to 2000. In a narrow zone of observed residual subsidence near the San Jacinto fault, where many normalized poroelastic ratios trend higher than expected, we find permanent compaction of clay-rich strata to be the most likely explanation for 0.5–2.0 mm yr–1 of residual subsidence. This permanent compaction is likely a result of delayed compaction due to previous overdraft conditions and/or a result of the installation of deep production wells during the span of the InSAR time-series. Observations of localized subsidence within stepovers of the San Jacinto fault zone and relative uplift at the range fronts are consistent with current basin development models; however, interseismic strain modelling of the regional faults does not reproduce the surface displacement pattern or magnitude of these observations. [ABSTRACT FROM AUTHOR]

Swarm-like earthquake sequences are commonly observed in a diverse range of geological settings including volcanic and geothermal regions as well as along transform plate boundaries. They typically lack a clear mainshock, cover an unusually large spatial area relative to their total seismic moment release, and fail to decay in time according to standard aftershock scaling laws. Swarms often result from a clear driving phenomenon, such as a magma intrusion, but most lack the necessary geophysical data to constrain their driving process. To identify the mechanisms that cause swarms on strike-slip faults, we use relative earthquake locations to quantify the spatial and temporal characteristics of swarms along Southern California and East Pacific Rise transform faults. Swarms in these regions exhibit distinctive characteristics, including a relatively narrow range of hypocentral migration velocities, on the order of a kilometre per hour. This rate corresponds to the rupture propagation velocity of shallow creep transients that are sometimes observed geodetically in conjunction with swarms, and is significantly faster than the earthquake migration rates typically associated with fluid diffusion. The uniformity of migration rates and low effective stress drops observed here suggest that shallow aseismic creep transients are the primary process driving swarms on strike-slip faults. Moreover, the migration rates are consistent with laboratory values of the rate-state friction parameter b (0.01) as long as the Salton Trough faults fail under hydrostatic conditions. [ABSTRACT FROM AUTHOR]

Geometrical properties of an earthquake population can be described by summation of seismic potency tensors that provide a strain-based description of earthquake focal mechanisms. We apply this method to ∼170 000 potency tensors for southern California earthquakes recorded between January 1984 and June 2003. We compare summed tensors for populations defined by faulting region and earthquake magnitude to investigate the relation between earthquake characteristics, tectonic domains and fault-related length scales. We investigate spatial scales ranging from ∼1–700 km and use the results to identify systematic differences between seismic behaviour for different faults and different regions. Our results show features that are indicative of both scale-invariant and scale-dependent processes. On the largest scale the overall potency tensor summation for southern California earthquakes over ∼20 yr corresponds closely to a double-couple (DC) mechanism with slip direction parallel to relative plate motion. The summed tensors and derived quantities for the different regions show clear persistent variations that are related to the dominant tectonic regime of each region. Significant differences between the non-DC components of the summed tensors, which we relate to fault heterogeneity, indicate systematic differences in deformation associated with earthquake populations from different fault zones or different magnitude ranges. We find an increase of heterogeneity for populations of smaller earthquakes and for regions where faulting deviates strongly from the overall sense of deformation, even when corrected for quality. The results imply an overall organization of earthquake characteristics into domains that are controlled to first order by geometrical properties of the largest faults and the plate motion. Smaller scale characteristics are related to local variations in the orientation, complexity and size of faults. [ABSTRACT FROM AUTHOR]

Non-volcanic tremor (NVT) is an extended-duration and non-impulsive seismic signal observed away from volcanic regions. Although NVT has been found along the circum-Pacific subduction zones and the transform plate boundary in California, it is not clear whether NVT occurs in other tectonic environments. NVT is often associated with slow-slip events, and can be triggered instantaneously during the surface waves of teleseismic events. However, the underlying mechanisms of tremor generation remain mysterious. Here we show clear evidence of NVT beneath the Central Range (CR) in Taiwan triggered by the 2001 Mw 7.8 Kunlun earthquake in northern Tibet. Tremor occurs when the Love wave displacement is to the southwest (the lateral motion direction for the CR), suggesting a simple frictional response to the driving force. We hypothesize that tremor occurred on the weak basal detachment fault beneath the CR. Our observations indicate that tremor may exist at a wide range of active tectonic environments. [ABSTRACT FROM AUTHOR]

This paper documents a newly discovered pattern of biological disjunction between NW and SE New Caledonia. The disjunction occurs in 87 (mapped) taxa, including plants, moths and lizards, and correlates spatially with the West Caledonian fault. This fault is controversial; some geologists interpret it as a major structure, others deny that it exists. It may have undergone 150–200 km of lateral movement and it is suggested that this has caused the biological disjunction by pulling populations apart. The disjunction matches similar dextral disjunctions of taxa along transform faults in New Zealand, New Guinea, California and Indonesia. Major biogeographic patterns – whether centres of diversity, boundaries of allopatric taxa or disjunctions – all include taxa with many different degrees of differentiation. Studies using a clock model of evolution will therefore interpret a biogeographic pattern as the result of many disparate events. However, this line of reasoning reaches the untenable conclusion that biogeographic patterns, including normal allopatry, are always caused by chance dispersal, never by vicariance. A more productive approach, avoiding the pitfalls of a fossil-based molecular clock, involves a close examination of molecular clades, comparative biogeography and tectonics. The New Caledonia example documented here shows that this can lead to novel, testable predictions. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158, 470–488. [ABSTRACT FROM AUTHOR]

Deterministic earthquake scenario simulations are playing an increasingly important role in seismic hazard and risk estimation. Our aim is to calculate a substantial number of different finite-source scenarios, embedded in a 3-D structure for a particular fault or fault system, by pre-calculating numerical Green's functions (NGFs). A large seismic fault is divided into subfaults of appropriate size for which synthetic Green's functions at the surface are calculated and stored. Consequently, ground motions from arbitrary kinematic sources can be simulated for the whole fault or parts of it, by superposition. To illustrate this approach and its functionalities we simulate M 7 (up to 0.5 Hz) scenario earthquakes for a simplified model of the Newport-Inglewood (NI) fault in the Los Angeles (LA) Basin. We quantify the variations of surface ground motion (e.g. peak ground velocity PGV and synthetic seismograms) due to source parameters (e.g. hypocentre location and corresponding slip history). The results show a complex behaviour, with dependence of absolute PGV and its variation on asperity location, source directionality and local structure and demonstrate the necessity to combine 3-D structural and finite-source effects to quantify ground motion characteristics and their variations. [ABSTRACT FROM AUTHOR]

A 2-D upper crustal structural profile with seven steeply dipping faults was constructed from the Los Angeles Region Seismic Experiment phase II (LARSE II) seismic refraction data recorded over the north central Transverse ranges and adjacent Mojave desert in southern California. The profile extends to a depth of approximately 4 km. The faults were identified from distinctive features in the seismic first arrival data, while the velocity structure was determined from travel times and refined by forward modelling. The resulting seismic velocity structure is correlative to the geological structure along the profile and is used to help constrain a geological cross-section. Six of the seven faults detected from the seismic data correspond directly to geologically mapped faults in the central Transverse ranges and adjacent Mojave desert. From south to north, they are the Pelona fault, the San Francisquito fault, the Clearwater fault, the San Andreas fault and two faults that cut the Portal ridge. The seventh fault is buried beneath Quaternary deposits of the western Mojave desert and appears to correlate with an additional fault that cuts Portal ridge out of the section line. The southernmost fault (the Pelona fault) separates the younger Vasquez formation in the south from the Pelona schist in the north and is determined to be an intermediate (40°–60°) south-dipping normal fault. This coupled with field observations indicate that the Pelona fault was instrumental in the late Oligocene—early Miocene formation of the Soledad basin, and the structural ascent and exhumation of the Pelona schist along the proto-Sierra Pelona intrabasin ridge. Analogous Neogene extensional deformation is found to have affected the western Mojave desert region, where faults 6 and 7 appear to have had north side down normal displacement and to have been instrumental in forming the western Antelope basin. A 6.0 km s−1 basal layer to our shallow seismic structure is correlated with the Pelona schist south of the San Andreas fault and with the tectonically related Rand schist beneath the western Mojave desert. Granitic and gneissic crystalline rocks, which form regional upper plate complexes along the Vincent thrust above the Pelona schist and along the Rand thrust above the Rand schist, yield consistently low seismic velocities as compared with the expected velocities for the constituent rock types. These anomalous velocities are reconciled by the degraded structural and textural state of the upper plate crystalline rocks that resulted from extreme shearing, brittle fracturing and related retrogressive hydration reactions starting during or shortly after the latest Cretaceous—early Palaeocene underthrusting of the schists. Extension, translation along the San Andreas transform system and compressional faulting with the uplift of the Transverse ranges further accentuated the basement deformations. The integrated result is that the survival of the Precambrian through Mesozoic granitic crust of the region became a rootless and highly attenuated upper crustal layer. [ABSTRACT FROM AUTHOR]

Examines the changes in the tectonic regime from subduction to strike-slip in the Mendocino triple junction. Structure of the San Andreas Fault system; Changes in the asthenospheric mantle; Penetration of strike-slip faults through the entire crust to the upper mantle.

Projects to drill into and recover samples from active faults are progressing in two widely separated locations: off the coast of Japan and in California. [ABSTRACT FROM AUTHOR]