Fault Slip History Research Articles

Distinguishing fault offset from climatically modulated channel deflections: Insights from the Pearblossom slip-rate site, Mojave section of the San Andreas fault, California, USA

Fault slip histories are essential for understanding seismic hazard and regional fault system development but fundamentally depend on identifying, dating, and reconstructing displaced markers. Here, we use a case study of the Pearblossom site along the Mojave section of the San Andreas fault in California (USA) to show how pulses of sediment aggradation during wet periods can complicate such reconstructions by producing “imposter offsets”—landforms that develop with an initial deflection that is easily misread as tectonic displacement caused by fault slip. Specifically, we document two channels on the downstream side of the fault: a subtle one that we interpret to have been beheaded and displaced 24–49 m from a source channel on the upstream side of the fault, and a second and more prominent one that we interpret as an imposter offset of 36–88 m. Using optically stimulated luminescence dating, we determine that the source channel incised between 1.44 ± 0.43 ka and 1.27 ± 0.18 ka with a subsequent phase of alluvial fan aggradation at ~0.6 ka, when the channel with the imposter offset formed. Because the pulse of fan deposition coincides temporally with a wet period in Southern California precipitation records, we attribute formation of the imposter offset and the alluvial fan into which it incised to climatically modulated deposition at the site. Comparing precipitation records with charcoal ages compiled from multiple Mojave Desert region locations suggests that other slip-rate sites may be similarly affected. Although climatic effects can complicate slip-rate studies, we show that the morphology and upstream position of the deflected channel can indicate whether a site likely records useful information about fault slip.

Effects of fault roughness on estimating critical slip-weakening distance from fault slip history: A laboratory study

Earthquakes are the dynamic rupture of faults governed by fault weakening processes. Critical slip-weakening distance (Dc) is a crucial source parameter of earthquakes, and the determination of Dc is of great concern to semiologists. However, determining Dc for natural earthquakes is challenging due to the trade-off in inversed source models. To solve this problem, Fukuyama and his coworkers proposed a simple method (denoted as the F&M method) to estimate Dc directly from slip-velocity functions. According to the F&M method, the fault slip at the peak slip velocity (Dc') can be used as an approximation of Dc. However, the feasibility of this method has not been completely resolved. Here, we performed laboratory earthquake rupture experiments to examine the validity of the F&M method. Experiments were conducted on faults with different roughness and stress level. We studied the effect of fault roughness on the validity of the F&M method. Our results show that the increase in fault roughness could complicate the fault weakening process, producing repeated weakening and strengthening phase, which leads to a prominent deviation between Dc' and Dc. Furthermore, we also observed a correlation between the critical slip-weakening distance Dc and the final fault slip D. Such correlation implies the scale-dependent nature of Dc, which is consistent with seismic observations in the field.

Coupling rare earth element analyses and high-resolution topography along fault scarps to investigate past earthquakes: A case study from the Southern Apennines (Italy)

AbstractThe systematic study of faults that have released strong earthquakes in the past is a challenge for seismic hazard assessment. In carbonate landscapes, the use of rare earth element (REE) concentrations on slickensides may aid the reconstruction of fault slip history. We applied this methodology to the Caggiano normal fault (Southern Apennines, Italy), cropping out southeast of the Irpinia 1980 CE earthquake fault (Mw 6.9), which was responsible for both the 1561 CE and partly the 1857 CE Basilicata earthquakes (Mw 6.7 and 7.1). We integrated the REE analysis approach with a high-resolution topographic analysis along 98 serial topographic profiles to measure vertical separations attributable to faulting since the Last Glacial Maximum (LGM). The asymmetric scarp height profiles suggest fault-lateral propagation and along-strike variations in the fault evolution. Our results indicate the occurrence of 7 to 11 earthquakes with variable slip between ~40 cm and ~70 cm within post-LGM times. We estimated the magnitudes of the respective earthquakes, between 5.5 and 7.0, and most commonly between 6.3 and 6.5. The results suggest a recurrence time between 1.6 k.y. and 2.3 k.y. and a slip rate ranging between 0.6 mm/yr and 0.9 mm/yr. This approach may be useful for application to carbonate fault planes in similar tectonic contexts worldwide.

Cenozoic slip along the southern Sierra Nevada normal fault, California (USA): A long-lived stable western boundary of the Basin and Range

AbstractThe uplift history of the Sierra Nevada, California, is a topic of long-standing disagreement with much of it centered on the timing and nature of slip along the range-bounding normal fault along the east flank of the southern Sierra Nevada. The history of normal fault slip is important for characterizing the uplift history of the Sierra Nevada, as well as for characterizing the geologic and geodynamic factors that drove, and continue to drive, normal faulting. To address these issues, we completed new structural studies and extensive apatite (U-Th)/He (AHe) thermochronometry on samples collected from three vertical transects in the footwall to the east-dipping southern Sierra Nevada normal fault (SNNF). Our structural studies on bedrock fault planes show that the SNNF is a steeply (~70°) east-dipping normal fault. The new AHe data reveal two elevation-invariant AHe age arrays, indicative of two distinct periods of cooling and exhumation, which we interpret as initiation of normal faulting along the SNNF at ca. 28–27 Ma with a second phase of normal faulting at ca. 17–13 Ma. We argue that beginning in the late Oligocene, the SNNF marked the now long-standing stable western limit, or breakaway zone, of the Basin and Range. Slip along SNNF, and the associated unloading of the footwall, likely resulted in two periods of uplift of Sierra Nevada during the late Cenozoic. Trench retreat, driven by westward motion of the North American plate, along the Farallon–North American subduction zone boundary, as well as the gravitationally unstable northern and southern Basin and Range pushing on the cold Sierra Nevada, likely drove the late Oligoceneaged normal slip along the SNNF and the similar-aged but generally local and minor extension within the Basin and Range. We posit that the thick proto–Basin and Range lithosphere was primed for late Oligocene extension by replacement of the steepening Farallon slab with hot and buoyant asthenosphere. While steepening of the Farallon slab had not yet reached the southern Sierra Nevada by late Oligocene time, we speculate that late Oligocene slip along the SNNF reactivated a late Cretaceous dextral shear zone as the Sierra Nevada block was pulled and pushed westward in response to trench retreat and gravitational potential energy. The dominant middle Miocene normal faultslip history along the SNNF is contemporaneous with high-magnitude slip recorded along range-bounding normal faults across the Basin and Range, including the east-adjacent Inyo and White mountains, indicating that this period of extension was a major regional tectonic event. We infer that a combination of slabdriven trench retreat along the Juan de Fuca–North America subduction zone boundary and clockwise rotation of the southern ancestral Cascade Range superimposed on continental lithosphere preconditioned for extension drove this episode of middle Miocene normal slip along the SNNF and extension to the east across the Basin and Range. Transtensional plate motion along the Pacific–North America plate boundary, and likely a growing slab window, continued to drive extension along the SNNF and the western Basin and Range, but not until ca. 11 Ma when the Mendocino triple junction reached the latitude of our northernmost (U-Th)/He transect.

Revealing the hidden signature of fault slip history in the morphology of degrading scarps

Active faults accommodate tectonic plate motion through different slip modes, some stable and aseismic, others characterized by the occurrence of large earthquakes after long periods of inactivity. Although the slip mode estimation is of primary importance to improve seismic hazard assessment, this parameter inferred today from geodetic observations needs to be better constrained over many seismic cycles. From an analytical formulation developed for analyzing fault scarp formation and degradation in loosely consolidated material, we show that the final topographic shape generated by one earthquake rupture or by creep (i.e., continuous slip) deviates by as much as 10–20%, despite a similar cumulated slip and a constant diffusion coefficient. This result opens up the theoretical possibility of inverting, not only the cumulated slip or averaged slip rate, but also the number of earthquakes and their sizes from scarp morphologies. This approach is all the more relevant as the number of rupture events is limited. Estimating the fault slip history beyond a dozen earthquakes becomes very difficult as the effect of erosion on scarp morphology prevails. Our modeling also highlights the importance of trade-offs between fault slip history and diffusive processes. An identical topographic profile can be obtained either with a stable fault creep associated with rapid erosion, or a single earthquake rupture followed by slow erosion. These inferences, derived from the simplest possible diffusion model, are likely to be even more pronounced in nature.

Constraining Paleoseismicity of the Wulashan Piedmont Fault on the Northern Margin of the Ordos Block From Fault Scarp Morphology

Fault scarps preserve important information about past earthquakes on a fault, and thus can be applied to investigate the fault slip histories and rupture patterns. In this study, the morphology of fault scarps was used to constrain the paleoseismicity of the Wulashan Piedmont Fault located on the northern margin of the Ordos Block based on high-resolution LiDAR topography. We constructed the vertical displacement distribution of the fault through measuring the heights of a large number of scarp profiles extracted on different geomorphic surfaces along the fault. Through statistical analysis of the dense collection of vertical displacement dataset, a total of seven paleoseismic events were identified which followed a characteristic slip pattern with an average slip of ∼1.0 m. We further detected slope breaks in the fault scarp morphology to quantify the number of paleoearthquakes that occurred on the scarps, and discriminated at least five individual surface-breaking events. Both the number and slip of paleoearthquakes recognized from the morphology of fault scarps were in good agreement with previous paleoseismic trenching records. Based on the empirical scaling relationship between moment magnitude and rupture parameters, a moment magnitude ofMw6.7–7.5 was determined for the paleoearthquakes occurred on the fault. With the fault slip rate derived by previous studies, we estimated an average recurrence interval of 1.3–1.8 kyr for the paleoseismic events, which is very close to the elapsed time since the most recent earthquake, indicating a high potential seismic hazard on the Wulashan Piedmont Fault.

Plate boundary trench retreat and dextral shear drive intracontinental fault-slip histories: Neogene dextral faulting across the Gabbs Valley and Gillis Ranges, Central Walker Lane, Nevada

Abstract The spatial-temporal evolution of intracontinental faults and the forces that drive their style, orientation, and timing are central to understanding tectonic processes. Intracontinental NW-striking dextral faults in the Gabbs Valley–Gillis Ranges (hereafter referred to as the GVGR), Nevada, define a structural domain known as the eastern Central Walker Lane located east of the western margin of the North American plate. To consider how changes in boundary type along the western margin of the North American plate influenced both the initiation and continued dextral fault slip to the present day in the GVGR, we combine our new detailed geologic mapping, structural studies, and 40Ar/39Ar geochronology with published geologic maps to calculate early to middle Miocene dextral fault-slip rates. In the GVGR, Mesozoic basement is nonconformably overlain by a late Oligocene to Miocene sequence dominated by tuffs, lavas, and sedimentary rocks. These rocks are cut and offset by four primary NW-striking dextral faults, from east to west the Petrified Spring, Benton Spring, Gumdrop Hills, and Agai Pah Hills–Indian Head faults. A range of geologic markers, including tuff- and lava-filled paleovalleys, the southern extent of lava flows, and a normal fault, show average dextral offset magnitudes of 9.6 ± 1.1 km, 7.0 ± 1.7 km, 9.7 ± 1.0 km, and 4.9 ± 1.1 km across the four faults, respectively. Cumulative dextral offset across the GVGR is 31.2 ± 2.3 km. Initiation of slip along the Petrified Spring fault is tightly bracketed between 15.99 ± 0.05 Ma and 15.71 ± 0.03 Ma, whereas slip along the other faults initiated after 24.30 ± 0.05 Ma to 20.14 ± 0.26 Ma. Assuming that slip along all four faults initiated at the same time as the Petrified Spring fault yields calculated dextral fault-slip rates of 0.4 ± 0.1–0.6 ± 0.1 mm/yr, 0.4 ± 0.1–0.5 ± 0.1 mm/yr, 0.6 ± 0.1 mm/yr, and 0.3 ± 0.1 mm/yr on the four faults, respectively. Middle Miocene initiation of dextral fault slip across the GVGR overlaps with the onset of normal slip along range-bounding faults in the western Basin and Range to the north and the northern Eastern California shear zone to the south. Based on this spatial-temporal relationship, we propose that dextral fault slip across the GVGR defines a kinematic link or accommodation zone between the two regions of extension. At the time of initiation of dextral slip across the GVGR, the plate-boundary setting to the west was characterized by subduction of the Farallon plate beneath the North American plate. To account for the middle Miocene onset of extension across the Basin and Range and dextral slip in the GVGR, we hypothesize that middle Miocene trench retreat drove westward motion of the Sierra Nevada and behind it, crustal extension across the Basin and Range and NW-dextral shear within the GVGR. During the Pliocene, the plate boundary to the west changed to NW-dextral shear between the Pacific and North American plates, which drove continued dextral slip along the same faults within the GVGR because they were fortuitously aligned subparallel to plate boundary motion.

Fault Slip and Exhumation History of the Willard Thrust Sheet, Sevier Fold‐Thrust Belt, Utah: Relations to Wedge Propagation, Hinterland Uplift, and Foreland Basin Sedimentation

AbstractZircon (U‐Th)/He (ZHe) and zircon fission track thermochronometric data for 47 samples spanning the areally extensive Willard thrust sheet within the western part of the Sevier fold‐thrust belt record enhanced cooling and exhumation during major thrust slip spanning approximately 125–90 Ma. ZHe and zircon fission track age‐paleodepth patterns along structural transects and age‐distance relations along stratigraphic‐parallel traverses, combined with thermo‐kinematic modeling, constrain the fault slip history, with estimated slip rates of ~1 km/Myr from 125 to 105 Ma, increasing to ~3 km/Myr from 105 to 92 Ma, and then decreasing as major slip was transferred onto eastern thrusts. Exhumation was concentrated during motion up thrust ramps with estimated erosion rates of ~0.1 to 0.3 km/Myr. Local cooling ages of approximately 160–150 Ma may record a period of regional erosion, or alternatively an early phase of limited (<10 km) thrust slip. Propagation of the Sevier wedge front and major thrust slip during the late Early to mid‐Cretaceous were synchronous with increasing subsidence and deposition of thick synorogenic strata in the foreland, crustal thickening in the hinterland, growing igneous activity in the Sierran magmatic arc, and increasing plate convergence rates. Along‐strike, other parts of the Cordilleran retroarc fold‐thrust belt also experienced major shortening during the late Early to mid‐Cretaceous, following a period of earlier Cretaceous quiescence. Late Jurassic shortening was concentrated nearer the arc, and thus mostly in the hinterland at the latitude of northern Utah, related to width and location of the passive‐margin sedimentary wedge relative to the plate margin.

The Role of Near‐Fault Relief Elements in Creating and Maintaining a Strike‐Slip Landscape

AbstractStrike‐slip landscapes are often associated with a suite of characteristic geomorphic features that provide primary evidence for interpreting fault slip histories. Here we explore the role of shutter ridges, areas of relief advected laterally along faults, in generating two classic strike‐slip processes: progressive lateral offset of channels and stream capture. Landscape models and comparative analysis of the Marlborough Fault System, NZ, show that the length of channel offsets observable in a landscape is primarily controlled by the length of shutter ridges. In our simple landscape model, this scale is controlled by the drainage spacing, and therefore by the geometry of the mountain range. In a more complex landscape, this scale may be controlled by lithologic or structural contrasts. We also find that shutter ridge relief inhibits stream capture, especially at slow fault slip rates relative to hillslope erosion rates. In this case, lateral drainage advection enables streams to “outrun” capture.

A low-angle detachment fault revealed: Three-dimensional images of the S-reflector fault zone along the Galicia passive margin

A new 3-D seismic reflection volume over the Galicia margin continent–ocean transition zone provides an unprecedented view of the prominent S-reflector detachment fault that underlies the outer part of the margin. This volume images the fault's structure from breakaway to termination. The filtered time-structure map of the S-reflector shows coherent corrugations parallel to the expected paleo-extension directions with an average azimuth of 107°. These corrugations maintain their orientations, wavelengths and amplitudes where overlying faults sole into the S-reflector, suggesting that the parts of the detachment fault containing multiple crustal blocks may have slipped as discrete units during its late stages. Another interface above the S-reflector, here named S′, is identified and interpreted as the upper boundary of the fault zone associated with the detachment fault. This layer, named the S-interval, thickens by tens of meters from SE to NW in the direction of transport. Localized thick accumulations also occur near overlying fault intersections, suggesting either non-uniform fault rock production, or redistribution of fault rock during slip. These observations have important implications for understanding how detachment faults form and evolve over time. 3-D seismic reflection imaging has enabled unique insights into fault slip history, fault rock production and redistribution.

Discriminating between natural versus induced seismicity from long-term deformation history of intraplate faults

To assess whether recent seismicity is induced by human activity or is of natural origin, we analyze fault displacements on high-resolution seismic reflection profiles for two regions in the central United States (CUS): the Fort Worth Basin (FWB) of Texas and the northern Mississippi embayment (NME). Since 2009, earthquake activity in the CUS has increased markedly, and numerous publications suggest that this increase is primarily due to induced earthquakes caused by deep-well injection of wastewater, both flowback water from hydrofracturing operations and produced water accompanying hydrocarbon production. Alternatively, some argue that these earthquakes are natural and that the seismicity increase is a normal variation that occurs over millions of years. Our analysis shows that within the NME, faults deform both Quaternary alluvium and underlying sediments dating from Paleozoic through Tertiary, with displacement increasing with geologic unit age, documenting a long history of natural activity. In the FWB, a region of ongoing wastewater injection, basement faults show deformation of the Proterozoic and Paleozoic units, but little or no deformation of younger strata. Specifically, vertical displacements in the post-Pennsylvanian formations, if any, are below the resolution (~15 m) of the seismic data, far less than expected had these faults accumulated deformation over millions of years. Our results support the assertion that recent FWB earthquakes are of induced origin; this conclusion is entirely independent of analyses correlating seismicity and wastewater injection practices. To our knowledge, this is the first study to discriminate natural and induced seismicity using classical structural geology analysis techniques.

Untangling tectonic slip from the potentially misleading effects of landform geometry

We present a new three-dimensional (3D) approach to the analysis of fault scarps using high-resolution elevation models. Advances in topographic measurement techniques [e.g., lidar (light detection and ranging) and photogrammetric techniques] have allowed extensive measurement of single earthquake and cumulative scarps to draw conclusions about along-strike slip variation and fault slip history. The resulting slip distributions are almost always variable and noisy, but the cause is often unclear. We first present the results of sensitivity analysis to demonstrate significant apparent noise due to varying terrain and fault and measurement geometry (topographic slope attitude, fault dip and slip obliquity). We show, with a case study on the Hoshab fault, Pakistan, that oblique slip can have a significant effect on the measured apparent slip. Individual planar geomorphic markers only constrain one component of the full 3D slip vector. We use the variation in apparent offset with marker geom­etry to constrain the slip vector in 3D. Combining multiple offset measurements along strike, we show that determining the slip vector is reduced to a simple linear formulation. We test our method using a terrestrial lidar data set from the ruptures on the Borrego fault from the 2010 El Mayor–Cucapah earthquake (Baja California, Mexico). Combining 22 observations, we estimate a throw of 1.56 ± 0.02 m and a lateral slip of 1.9 ± 0.3 m. The vertical slip estimate agrees well with previous studies, but the lateral slip is significantly smaller. In regions of steep varied topography or with oblique slip, our method will give enhanced slip resolution while standard methods will give biased estimates.

Geoarchaeology and paleoseismology blend to define the Fucino active normal fault slip history, central Italy

We first describe the late Holocene slip history of one of the major segments of the Fucino active normal fault, in central Italy, by combining geoarchaeological investigations with paleoseismological trenching. The Fucino fault system released a Mw 7 earthquake in 1915 (with many other events with decimetre and/or metre-size palaeoseismic slip events in the past), that is the strongest seismic shock occurred in this portion of the Italian territory over at least the past millennium. We dug trenches across the investigated tectonic structure; then, the sedimentary sequence and its relation with the exposed fault planes have been analysed “vertically”, as typically made in paleoseismological investigations, but also “horizontally”, by deepening the excavations “step-by-step” while digging, i.e. performing archaeological-type stratigraphic excavations. Such a procedure permitted the recognition of different displacement events of the fault, and the progressive surveying of different cultural levels, since the Neolithic Period, interposed with or cut into natural levels. The reconstruction of the interplay between human occupation of the site and the local geomorphic evolution – framed by the late Holocene climatic changes – permitted us to gain reliable chronological data for constraining the fault slip history in the last 5500 yr. Our analyses also confirmed that the investigated structure activated during the 1915 earthquake. Four previous displacement events were recognised: a first event, prior to the 1915 one, occurred slightly after the Roman Period (probably during the 5th–6th century AD); two preceding events occurred between the Late Neolithic and the Roman period, the older of the two during the late Neolithic, while the later during the Late Bronze Age-Early Iron Age. The oldest event predates the Neolithic Period. No evidence of a Late Middle Ages faulting event found by others researchers along another branch of the Fucino fault was found in our trenches. From a methodological viewpoint, the results of our study mark the effectiveness of adopting joint geoarchaeological/paleoseismological approach in terms of chronological constraints for active faulting studies in such contexts where long human occupation took place, where the natural and “human” events rhythmically interplay.

Source rupture processes of the foreshock and mainshock in the 2016 Kumamoto earthquake sequence estimated from the kinematic waveform inversion of strong motion data

The 2016 Kumamoto earthquake sequence started with an MJMA 6.5 foreshock occurring along the northern part of the Hinagu fault, central Kyushu, Japan, and the MJMA 7.3 mainshock occurred just 28 h after the foreshock. We analyzed the source rupture processes of the foreshock and mainshock by using the kinematic waveform inversion technique on strong motion data. The foreshock was characterized by right-lateral strike-slip occurring on a nearly vertical fault plane along the northern part of the Hinagu fault, and it had two large-slip areas: one near the hypocenter and another at a shallow depth. The rupture of the mainshock started from the deep portion of a northwest-dipping fault plane along the northern part of the Hinagu fault, then continued to transfer to the Futagawa fault. Most of the significant slip occurred on the Futagawa fault, and the shallow portion of the Hinagu fault also had a relatively large slip. The slip amount on the shallowest subfaults along the Futagawa fault was approximately 1–4 m, which is consistent with the emergence of surface breaks associated with this earthquake. Right-lateral strike-slip dominated on the fault segment along the Hinagu fault, but normal-slip components were estimated to make a significant contribution on the fault segment along the Futagawa fault. The large fault-parallel displacements recorded at two near-fault strong motion stations coincided with the spatiotemporal pattern of the fault slip history during the mainshock. The spatial relationship between the rupture areas of the foreshock and mainshock implies a complex fault structure in this region.

First direct observation of coseismic slip and seafloor rupture along a submarine normal fault and implications for fault slip history

Properly assessing the extent and magnitude of fault ruptures associated with large earthquakes is critical for understanding fault behavior and associated hazard. Submarine faults can trigger tsunamis, whose characteristics are defined by the geometry of seafloor displacement, studied primarily through indirect observations (e.g., seismic event parameters, seismic profiles, shipboard bathymetry, coring) rather than direct ones. Using deep-sea vehicles, we identify for the first time a marker of coseismic slip on a submarine fault plane along the Roseau Fault (Lesser Antilles), and measure its vertical displacement of ∼0.9 m in situ. We also map recent fissuring and faulting of sediments on the hangingwall, along ∼3 km of rupture in close proximity to the fault's base, and document the reactivation of erosion and sedimentation within and downslope of the scarp. These deformation structures were caused by the 2004 Mw 6.3 Les Saintes earthquake, which triggered a subsequent tsunami. Their characterization informs estimates of earthquake recurrence on this fault and provides new constraints on the geometry of fault rupture, which is both shorter and displays locally larger coseismic displacements than available model predictions that lack field constraints. This methodology of detailed field observations coupled with near-bottom geophysical surveying can be readily applied to numerous submarine fault systems, and should prove useful in evaluating seismic and tsunamigenic hazard in all geodynamic contexts.

OSL dating of offset streams across the Altyn Tagh Fault: Channel deflection, loess deposition and implication for the slip rate

Two deflected streams flowing across the Altyn Tagh Fault (ATF) were investigated at the Aksay segment, NW China. Due to strike of the ATF, stream channels are deflected and elongated by adding new sections along the fault line. After subsequent water erosion and fluvial refreshment, banks of these deflected streams are overlain by aeolian loess. Geomorphological features suggest that stream deflections along the fault were developed as a result of faulting activities and displacements were progressively accumulated from multiple faulting cycles. Each faulting cycle is composed of faulting event triggered elongation of channels, creation of stream banks or risers and subsequent aeolian loess deposition on the banks. A model is proposed to illustrate the relationship between fault movement and loess deposition, which is supported by OSL dating of the loess on the offset stream channels. Our results demonstrated that: (1) loess deposits along deflected streams can be used to trace fault slip history, given specific geomorphic assumptions discussed in the text; and (2) based on stream channel offsets and loess ages, the Holocene slip rate of the ATF is estimated to be 11±2mm/year along the Aksay segment.

Impact of lithospheric flexure on the evolution of shallow faults in the Timor foreland system

Re-evaluation of structural evolution on the Laminaria High (Timor Sea, Australia), shows that lithosphere flexure, associated with the Tertiary collision between the Australian north-west margin and the Banda volcanic arc, is an important mechanism for Neogene fault development and the reactivation of Jurassic structures. 2D elastic models of the lithosphere flexure show that increasing flexural extension is likely to affect the Laminaria High from the Late Miocene onwards. The evaluation of the Neogene fault slip history, based on instantaneous throw estimates, suggests an initiation of faulting during the Late Miocene when the Laminaria High entered the flexed area (forebulge). Increasing fault growth is recorded during the Pliocene and correlates with the Laminaria High reaching the hinge of the forebulge structure. Maximum episodes of fault growth are recorded between the Late Pliocene and Early Pleistocene when the Laminaria High was located near the forebulge hinge or in the slope where curvature and flexural stress are likely to be maximal. NNW extension associated with plate flexure allowed the oblique extensional reactivation of buried reservoir-bounding Jurassic structures. The orientation of the Jurassic structures relative to the flexural extensional front appears as a key factor controlling the type of segment linkage between the reactivated Jurassic faults and the newly developed Neogene faults. Observations from the Laminaria High suggest that with an angle > 20° between σ Hmax and strike orientation of the Jurassic faults, soft-linkage is likely to occur between Jurassic and Neogene segments. An angle < 20° is more likely to trigger hard-linkages between these segments. The development of continuous fault zones results in a higher up-fault migration risk for hydrocarbon leaking from fault-bounded reservoirs.

Deriving fault-slip histories to test for secular variation in slip, with examples from the Kunlun and Awatere faults

Although offset and age data from displaced landforms are essential for identifying earthquake clusters and thus testing whether faults slip at uniform or secularly varying rates, it is not clear how the uncertainties in such measurements should be propagated so as to yield a robust fault-slip history (i.e., record of fault displacement over time). Here we develop a Monte Carlo approach for estimating the distribution of geologically reasonable fault-slip histories that fit the offset and age data from a population of dated and displaced landforms. The model assumes that the landforms share common faulting histories, the offset and age constraints are correct, and the fault has not reversed shear sense. Analysis of the model results yields both a precise average slip rate, in the case where a linear fit is applied to the data, and a best-fit fault-slip history, in the case where the linear constraint is removed. The method can be used to test for secular variation in slip because the uncertainty on this best-fit history is quantified. By applying the method to previously published morphochronologic data from faulted late Quaternary terrace risers along the Kunlun fault in China and the Awatere fault in New Zealand, we have assessed the extent to which our modeled average slip rates match previously reported values and the data support previous interpretations of uniform slip rate. The Kunlun data set yields average slip rates of 8.7+3.6/−2.1mm/yr and 5.1+1.6/−1.2mm/yr (68.27% confidence), for the central and eastern reaches of the fault, respectively, both of which match previously published slip rates. Our analysis further indicates that these fault reaches have both slipped uniformly over the latest Quaternary. In contrast, analysis of data from the Saxton River site along the Awatere fault reveals a mid-Holocene deceleration in slip rate from 6.2+1.6/−1.4mm/yr to 2.8+1.0/−0.6mm/yr. This result contradicts previous interpretations of uniform slip along the Awatere fault. The Monte Carlo method we present here for quantifying fault-slip histories using the offset and age data from a population of faulted landforms provides an important tool for distinguishing temporally uniform from secularly varying fault slip.

Inverting geodetic time series with a principal component analysis‐based inversion method

The Global Positioning System (GPS) system now makes it possible to monitor deformation of the Earth's surface along plate boundaries with unprecedented accuracy. In theory, the spatiotemporal evolution of slip on the plate boundary at depth, associated with either seismic or aseismic slip, can be inferred from these measurements through some inversion procedure based on the theory of dislocations in an elastic half‐space. We describe and test a principal component analysis‐based inversion method (PCAIM), an inversion strategy that relies on principal component analysis of the surface displacement time series. We prove that the fault slip history can be recovered from the inversion of each principal component. Because PCAIM does not require externally imposed temporal filtering, it can deal with any kind of time variation of fault slip. We test the approach by applying the technique to synthetic geodetic time series to show that a complicated slip history combining coseismic, postseismic, and nonstationary interseismic slip can be retrieved from this approach. PCAIM produces slip models comparable to those obtained from standard inversion techniques with less computational complexity. We also compare an afterslip model derived from the PCAIM inversion of postseismic displacements following the 2005 8.6 Nias earthquake with another solution obtained from the extended network inversion filter (ENIF). We introduce several extensions of the algorithm to allow statistically rigorous integration of multiple data sources (e.g., both GPS and interferometric synthetic aperture radar time series) over multiple timescales. PCAIM can be generalized to any linear inversion algorithm.

Kilauea slow slip events: Identification, source inversions, and relation to seismicity

Several slow slip events beneath the south flank of Kilauea Volcano, Hawaii, have been inferred from transient displacements in daily GPS positions. To search for smaller events that may be close to the noise level in the GPS time series, we compare displacement fields on Kilauea's south flank with displacement patterns in previously identified slow slip events. Matching displacement patterns are found for several new candidate events, although displacements are much smaller than previously identified events. One of the candidates, 29 May 2000, is coincident with a microearthquake swarm, as are all of the previously identified slow slip events. The microearthquakes follow the onset of slow slip, implying that they are triggered by stress changes during slip. The new slow slip event brings the total number of events on Kilauea, between 1997 and 2007, to eight, the smallest having MW = 5.3, and the largest having MW = 6.0. While the recurrence time between the four largest events is 2.11 ± 0.01 years, the repeat time for all eight events is 0.9 ± 0.6 years. We invert for the fault geometry and distribution of slip during the slow slip events. The optimal source depths of 5 km, assuming uniform slip dislocations in an elastic half‐space, are considerably shallower than the accompanying swarm earthquakes (6.5–8.5 km), which would place the earthquakes in a zone of decreased Coulomb stress. Inversions including the effects of topography and layered elastic structure in the forward models favor depths comparable to microearthquake depths, such that the earthquakes are located in a region of increased Coulomb stress. We also invert for time‐dependent fault slip directly from the 30 s GPS phase observations, constraining the source to the optimal uniform slip geometry. On the basis of these inversions, the larger events last between 1.5–2.2 days. The data are unable to resolve migration of slip along the fault. The temporal pattern of accompanying microearthquakes is consistent with the fault slip history assuming a seismicity rate theory based on rate and state‐friction, making the swarm earthquakes coshocks and aftershocks of the slow slip events.