Portion Of Fault Research Articles

Geometrical and Frictional Effects on Incomplete Rupture and Shallow Slip Deficit in Ramp‐Flat Structures

AbstractSeveral recent moderate‐magnitude (Mw > 7) earthquakes, such as the 2015 Gorkha, Nepal, ruptured only the deep (>15 km depth) portions of megathrust faults, leaving the updip sections unbroken. Here we investigate the effects of geometrical and frictional variations at depth on the stress accumulation and release in ramp‐flat structures using 2‐D finite element models. Our results show that ramp‐flat structures allow for faster but lower shear stress accumulation with increasing dip of the deep ramp section while increasing frictional strength of the faults allows more stress accumulation. These factors lead to earlier yet smaller failures of the ramp followed by larger and less frequent failures affecting the shallow section. Our models thus suggest that the dynamics of strain reservoirs are related to both the frictional strength and dips of ramp‐flat megathrust structures, and the failure time of the shallow fault section is affected by the stress regime at the deep fault segment.

A unified earthquake catalogue for the Sea of Marmara Region, Turkey, based on automatized phase picking and travel-time inversion: Seismotectonic implications

The Marmara section of the North Anatolian Fault Zone (NAFZ) is late in its seismic cycle and can be expected to produce a magnitude M up to 7.4 earthquake during the next decades in direct vicinity to the 15-million population center Istanbul. This setting translates the seismic hazard into very high risk and makes a thorough understanding of the current seismotectonic setting of this NAFZ section a pressing task. The absence of near-fault stations along the most part of the offshore Marmara section limits the reliability of existing seismicity catalogues for this region. For the first time we combine the different regional permanent networks thereby optimizing azimuthal coverage and present a refined hypocenter catalogue for the Sea of Marmara on this basis. Compared to the original locations, adoption of a refined automated technique to determine precise onset times for the different body waves and an iterative travel-time inversion scheme, lead to substantial improvement of 6812 absolute earthquake locations, particularly in the epicentral distribution. The automated processing is explained in detail. Further optimization is achieved through relative relocation of 4407 earthquakes. Our catalogue covers more than a decade (2006–2016) with a regional moment-magnitude of completeness of Mc = 2.1. The epicentral distribution delineates the Marmara Section, i.e. the northern NAFZ branch, as the seismically most active fault strand. We identify several aseismic fault patches that are interpreted to represent locked parts of the fault. Seismic activity in the past decade predominantly occurs off the main fault on the edges of the aseismic patches, supporting previous studies that the Marmara section of the NAFZ contains both locked and creeping fault portions. Single-event focal mechanisms (2.7 ≤ Mw ≤ 4.5) indicate that currently both strike-slip and normal faulting occur, confirming the transtensional setting of the region.

Complex Fault Geometry and Rupture Dynamics of the MW 6.5, 30 October 2016, Central Italy Earthquake

AbstractWe study the 30 October 2016 Norcia earthquake (MW 6.5) to retrieve the rupture history by jointly inverting seismograms and coseismic Global Positioning System displacements obtained by dense local networks. The adopted fault geometry consists of a main normal fault striking N155° and dipping 47° belonging to the Mt. Vettore‐Mt. Bove fault system (VBFS) and a secondary fault plane striking N210° and dipping 36° to the NW. The coseismic rupture initiated on the VBFS and propagated with similar rupture velocity on both fault planes. Updip from the nucleation point, two main slip patches have been imaged on these fault segments, both characterized by similar peak‐slip values (~3 m) and rupture times (~3 s). After the breakage of the two main slip patches, coseismic rupture further propagated southeastward along the VBFS, rupturing again the same fault portion that slipped during the 24 August earthquake. The retrieved coseismic slip distribution is consistent with the observed surface breakages and the deformation pattern inferred from interferometric synthetic aperture radar measurements. Our results show that three different fault systems were activated during the 30 October earthquake. The composite rupture model inferred in this study provides evidences that also a deep portion of the NNE trending section of the Olevano‐Antrodoco‐Sibillini thrust broke coseismically, implying the kinematic inversion of a thrust ramp. The obtained rupture history indicates that in this sector of the Apennines, compressional structures inherited from past tectonics can alternatively segment boundaries of NW trending active normal faults or break coseismically during moderate‐to‐large magnitude earthquakes.

Imaging Shear Strength Along Subduction Faults

AbstractSubduction faults accumulate stress during long periods of time and release this stress suddenly, during earthquakes, when it reaches a threshold. This threshold, the shear strength, controls the occurrence and magnitude of earthquakes. We consider a 3‐D model to derive an analytical expression for how the shear strength depends on the fault geometry, the convergence obliquity, frictional properties, and the stress field orientation. We then use estimates of these different parameters in Japan to infer the distribution of shear strength along a subduction fault. We show that the 2011 Mw9.0 Tohoku earthquake ruptured a fault portion characterized by unusually small variations in static shear strength. This observation is consistent with the hypothesis that large earthquakes preferentially rupture regions with relatively homogeneous shear strength. With increasing constraints on the different parameters at play, our approach could, in the future, help identify favorable locations for large earthquakes.

Surface creep and slip-behavior segmentation along the northwestern Xianshuihe fault zone of southwestern China determined from decades of fault-crossing short-baseline and short-level surveys

This study examines the ~200-km-long northwestern Xianshuihe fault zone (NWXFZ), southwestern China, using more than three decades of geodetic observations from fault-crossing short-baseline and short-leveling surveys at seven sites. These data enable estimates of creep rates and depths, and examination of the long-term slip behavior. The surface motion of the NWXFZ is dominated by sinistral creep, although sinistral, transverse, and vertical slip components show spatio-temporal variations. Combining these slip variations with data of earthquake rupture, coseismic slip, seismicity, fault geometry, and far-fault movement velocity, and using the velocity-and-state friction theory, our analysis indicates that the surface slip behavior of the NWXFZ is segmented along strike. The 1973 rupture section of this fault zone exhibits spatio-temporally variable slip behavior, showing time-decaying post-1973 afterslip on the northwestern and southeastern parts of the rupture at depths above 5.8−7.0km with average sinistral-creep rates of 1.3 and 3.5mm/yr, respectively, but being relocked in the central part of the rupture. The 1923/1981 rupture section is generally in locking state, with postseismic and interseismic sinistral-creep at 1.1mm/yr on its central part at depths above 2.0–2.8km. The 1893 rupture section has been tightly locked without creep since at least the early 1980s. The thickness of the shallow velocity-strengthening (or creep) layer and the restraining bend geometry of the NWXFZ are the key factors that control spatio-temporal variations in surface creep rates. Two surface-observed locked fault portions are located within two different restraining bends in the NWXFZ, both of which act as compressive asperities and hence have enabled the long-term locking of these portions. Creep along the NWXFZ has also been affected to varying degrees by M6.5−Mw9.2 earthquakes at distances of 50−3800 km from the fault zone. Most of these effects have been removed from our estimates of the long-term average creep rates.

The transtensional offshore portion of the northern San Andreas fault: Fault zone geometry, late Pleistocene to Holocene sediment deposition, shallow deformation patterns, and asymmetric basin growth

We mapped an ~120 km offshore portion of the northern San Andreas fault (SAF) between Point Arena and Point Delgada using closely spaced seismic reflection profiles (1605 km), high-resolution multibeam bathymetry (~1600 km 2 ), and marine magnetic data. This new data set documents SAF location and continuity, associated tectonic geomorphology, shallow stratigraphy, and deformation. Variable deformation patterns in the generally narrow (∼1 km wide) fault zone are largely associated with fault trend and with transtensional and transpressional fault bends. We divide this unique transtensional portion of the offshore SAF into six sections along and adjacent to the SAF based on fault trend, deformation styles, seismic stratigraphy, and seafloor bathymetry. In the southern region of the study area, the SAF includes a 10-km-long zone characterized by two active parallel fault strands. Slip transfer and long-term straightening of the fault trace in this zone are likely leading to transfer of a slice of the Pacific plate to the North American plate. The SAF in the northern region of the survey area passes through two sharp fault bends (∼9°, right stepping, and ∼8°, left stepping), resulting in both an asymmetric lazy Z–shape sedimentary basin (Noyo basin) and an uplifted rocky shoal (Tolo Bank). Seismic stratigraphic sequences and unconformities within the Noyo basin correlate with the previous 4 major Quaternary sea-level lowstands and record basin tilting of ∼0.6°/100 k.y. Migration of the basin depocenter indicates a lateral slip rate on the SAF of 10–19 mm/yr for the past 350 k.y. Data collected west of the SAF on the south flank of Cape Mendocino are inconsistent with the presence of an offshore fault strand that connects the SAF with the Mendocino Triple Junction. Instead, we suggest that the SAF previously mapped onshore at Point Delgada continues onshore northward and transitions to the King Range thrust.

Aseismic slip and seismogenic coupling in the Marmara Sea: What can we learn from onland geodesy?

AbstractEver since the Mw7.4 Izmit earthquake in 1999, evaluation of seismic hazard associated with the last unbroken segments of the North Anatolian fault is capital. A strong controversy remains over whether Marmara fault segments are locked or are releasing strain aseismically. Using a Bayesian approach, we propose a preliminary probabilistic interseismic model constrained by published GPS data sets. The posterior mean model show that Ganos and Cinarcik segments are locked while creep is detected in the central portion of Marmara fault. Our analysis, however, reveals that creeping segments are associated with large model uncertainties, which mainly results from the sparsity of current geodetic observations. We then discuss how the GPS network can be improved to attain more reliable assessment of interseismic slip rates. With this purpose, we implement a network optimization procedure to identify the most favorable distribution of stations measuring strain accumulation in the Marmara Sea.

A novel Lagrangian approach for the stable numerical simulation of fault and fracture mechanics

The simulation of the mechanics of geological faults and fractures is of paramount importance in several applications, such as ensuring the safety of the underground storage of wastes and hydrocarbons or predicting the possible seismicity triggered by the production and injection of subsurface fluids. However, the stable numerical modeling of ground ruptures is still an open issue. The present work introduces a novel formulation based on the use of the Lagrange multipliers to prescribe the constraints on the contact surfaces. The variational formulation is modified in order to take into account the frictional work along the activated fault portion according to the principle of maximum plastic dissipation. The numerical model, developed in the framework of the Finite Element method, provides stable solutions with a fast convergence of the non-linear problem. The stabilizing properties of the proposed model are emphasized with the aid of a realistic numerical example dealing with the generation of ground fractures due to groundwater withdrawal in arid regions. A numerical model is developed for the simulation of fault and fracture mechanics.The model is implemented in the framework of the Finite Element method and with the aid of Lagrange multipliers.The proposed formulation introduces a new contribution due to the frictional work on the portion of activated fault.The resulting algorithm is highly non-linear as the portion of activated fault is itself unknown.The numerical solution is validated against analytical results and proves to be stable also in realistic applications.

Insights into the permeability of polygonal faults from their intersection geometries with Linear Chimneys: a case study from the Lower Congo Basin

Layer-bound arrays of polygonal compaction faults have long been considered as important migration routes for hydrocarbon fluids leaking to the surface across thick shale sequences. A classic example is the deep offshore of the Lower Congo Basin where numerous fluid-venting structures are present above a Pliocene polygonal fault system. In this paper we present a detailed seismic analysis of a newly recognised system of Quaternary-aged Linear Chimneys and their intersection geometries with pre-existing Pliocene-aged polygonal faults (PF). Most (73%) of the 209 chimneys analysed intersect the lower portions of polygonal faults and almost half of these are rooted in strata below the PF interval. This indicates that fluid (in this case gas) migrated vertically, cross-cutting polygonal faults as it ascended through the tier. This is a strong indicator that PFs did not provide viable migration pathways otherwise chimneys would terminate at the upper tip of the fault, which would be the most likely migration exit point. Only twice in the whole system of Linear Venting Systems did this occur. A sub-set of chimneys stems from or above PF planes but these are restricted to either the lower footwall or from the apex area of hanging wall. At best they are evidence of fluids migrating up the lower part of polygonal faults and exiting deep within the tier, then migrating through most of the tier in their own vertical leakage vents. These results provide strong indicators that at least within this part of the Lower Congo Basin polygonal faults were the least effective/favoured migration pathway and that it was more energy-efficient for migrating gas to hydrofracture its fine-grained overburden than to re-open polygonal faults.

Effects of healing on the seismogenic potential of carbonate fault rocks: Experiments on samples from the Longmenshan Fault, Sichuan, China

AbstractFault slip and healing history may crucially affect the fault seismogenic potential in the earthquake nucleation regime. Here we report direct shear friction tests on simulated gouges derived from a carbonate fault breccia, and from a clay/carbonate fault‐core gouge, retrieved from a surface exposure of the Longmenshan Fault Zone (LFZ) which hosted the 2008 Wenchuan earthquake. The experiments were conducted under dry and hydrothermal conditions, at temperatures up to 140°C, at an effective normal stress of 50 MPa, and involved sequential velocity‐stepping (VS), slide‐hold‐slide (SHS), and velocity‐stepping stages. Dry tests performed on breccia‐derived samples showed no dependence of (quasi) steady state friction (μss) on SHS or VS history, and a log linear relation between transient peak healing (Δμpk) and hold time, or classical “Dieterich‐type” healing behavior. By contrast, all experiments conducted under hydrothermal conditions were characterized by “non‐Dieterich” healing behavior. This included (1) an increase in μss upon resliding after a hold period and (2) an increase in friction rate parameter (a − b), after SHS testing. Comparison with previous results suggests that the healing behavior seen in our wet tests may be attributed to solution transfer processes occurring during hold periods. Our findings imply that the shallow portions of faults with carbonate/clay‐rich cores (e.g., the LFZ) can heal much faster than previously recognized, while the upper limit of the seismogenic zone may migrate to deeper levels during interseismic periods. These effects have important implications for understanding the seismic cycle in tectonically active carbonate terrains.

Practical Examples of Failure Analyses with Lock-in Thermography(Detection Technology of Fault Portions with Thermal Emission)

Practical Examples of Failure Analyses with Lock-in Thermography(Detection Technology of Fault Portions with Thermal Emission)

Failure Analysis in Mounting Boards using Lock-in Thermal Emission(Detection Technology of Fault Portions with Thermal Emission)

Failure Analysis in Mounting Boards using Lock-in Thermal Emission(Detection Technology of Fault Portions with Thermal Emission)

Fault Localization Techniques for Electronic Products and Components using Infrared Lock-in Thermography(Detection Technology of Fault Portions with Thermal Emission)

Fault Localization Techniques for Electronic Products and Components using Infrared Lock-in Thermography(Detection Technology of Fault Portions with Thermal Emission)

Thermal Emission Microscopy using Lock-in Technique(Detection Technology of Fault Portions with Thermal Emission)

Thermal Emission Microscopy using Lock-in Technique(Detection Technology of Fault Portions with Thermal Emission)

Clast-cortex aggregates in experimental and natural calcite-bearing fault zones

We investigated the formation mechanisms of rounded clast-cortex aggregates, a composite grain found in the slipping zones of faults hosted in calcite- and clay-rich rocks. The natural aggregates contain a central clast commonly made of host-rock fragments or reworked cataclasite from the slipping zone. The central clasts are surrounded by an outer cortex of calcite or clay grains a few μm or less in size. In laboratory experiments on calcite gouges using two rotary-shear apparatus we investigated the dependence of clast-cortex aggregate formation on the applied slip rate, normal stress, total displacement and ambient humidity. Clast-cortex aggregates formed at all investigated slip rates (100 μm/s to 1 m/s) but only at relatively low normal stresses (≤5 MPa). The aggregates were better developed with increasing displacement (up to 5 m) and did not form in experiments with water-dampened gouges. In the experiments, aggregates formed in low-strain regions within the gouge layers, adjacent to the highest-strain slip zones. We propose that clast-cortex aggregates in calcite-bearing slip zones form in the shallow portions of faults during shearing in relatively dry conditions, but our experiments suggest that they cannot be used as indicators of seismic slip. Formation involves clast rotation due to granular flow accompanied by accretion of fine matrix material possibly facilitated by electrostatic forces.

Post-seismic slip of the 2011 Tohoku-Oki earthquake from GPS observations: implications for depth-dependent properties of subduction megathrusts

Here we inverted the GPS data to infer the coseismic slip of the Tohoku-Oki earthquake and the time-dependent afterslip distribution in the 4 months following the main shock. The Tohoku-Oki earthquake showed an unexpected magnitude and a characteristic depth-dependent differentiation of seismic energy radiation. In this context the estimation and comparison of the distribution of the fault portions that slip coseismically and post-seismically contribute to a better understanding of the variation of frictional characteristics of the plate interface. The inferred coseismic slip extends in a relatively compact region located updip from the hypocentre and reaches its highest value (about 60 m) near the trench. Afterslip occurs mostly outside the coseismic rupture and is distributed in two main modal centres. It reaches its largest values in an area located downdip of the coseismic slip and extends to a depth of 80 km. In the depth range between 30 and 50 km afterslip overlaps the portion of the fault that experienced historical moderate earthquakes, high-frequency seismic radiation and thrust-type aftershocks. The behaviour of this area can be explained by a rheologically heterogeneous region made of a ductile fault matrix interspersed with compact brittle asperities. On the contrary, the region beneath 50–60 km depth is probably characterized by a fully velocity strengthening behaviour. Southern afterslip, located off-Chiba Prefecture, is probably related to the Mw 7.9 Ibaraki-Oki aftershock. The northward extension of the afterslip stops at a latitude of about 40°N, just south of the off-Aomori region. This may be related to three large events occurred in this area during the last century and the consequent strong coupling or complete depletion of the accumulated strain that characterize this region.

Paleoseismology and tectonic geomorphology of the Pallatanga fault (Central Ecuador), a major structure of the South-American crust

The Pallatanga fault (PF) is a prominent NNE-SSW strike–slip fault crossing Central Ecuador. This structure is suspected to have hosted large earthquakes, including the 1797 Riobamba event which caused severe destructions to buildings and a heavy death toll (more than 12,000 people), as well as widespread secondary effects like landsliding, liquefaction and surface cracking. The scope of this study is to evaluate the seismic history of the fault through a paleoseismological approach. This work also aims at improving the seismotectonic map of this part of the Andes through a new mapping campaign and, finally, aims at improving the seismic hazard assessment.We show that the PF continues to the north of the previously mapped fault portion in the Western Cordillera (Rumipamba–Pallatanga portion) into the Inter-Andean Valley (Riobamba basin). Field evidences of faulting are numerous, ranging from a clear geomorphological signature to fault plane outcrops. Along the western side of the Riobamba basin, the strike–slip component seems predominant along several fault portions, with a typical landscape assemblage (dextral offsets of valleys, fluvial terrace risers and generation of linear pressure ridges). In the core of the inter-Andean valley, the main fault portion exhibits a vertical component along the c. 100m-high cumulative scarp. The presence of such an active fault bounding the western suburbs of Riobamba drastically increases the seismic risk for this densely inhabited and vulnerable city. To the east (Peltetec Massif, Cordillera Real), the continuation of the Pallatanga fault is suspected, but not definitely proved yet.Based on the analysis of three trenches, we state that the Rumipamba–Pallatanga section of the PF experienced 4 (maybe 5) Holocene to Historical strong events (Mw>7). The coseismic behavior of the fault is deduced from the occurrence of several colluvial wedges and layers associated with the fault activity and interbedded within the organic black soil sequence. According to a series of 14C datings, we document that these events occurred during the last 6500years. The clear deformation of the shallowest layer (14C: 1633AD) is most likely associated with the 1797 Riobamba earthquake. After retrodeforming one of the 3 trenches, we estimate coseismic vertical throws (0.70 to 0.90m). Because of bad outcrop conditions, we could not determine the horizontal component of slip and we used the slip vector determined in a previous work with a tectonic geomorphology study.Assuming this slip vector, we obtain total coseismic offsets between 3.5 and 4.5m, indicative of earthquake magnitudes around c. Mw 7.5. The estimated recurrence time intervals range between 1300 and 3000years, indicating an average slip rate of c. 2.5mm/a for the Rumipamba–Pallatanga section of the fault.

Double difference relocation of local earthquakes in the Nepal Himalaya

Interseismic strain across the Himalaya is associated with intense microseismic activity. In this study we analyze in detail this seismicity to explore in more details how it relates to Himalayan tectonics. We use the Double Difference Relocation Method to relocate local earthquakes recorded by National Seismological Centre in the period between 1995 and 2003. We also determined fault plane solutions for 10 earthquakes based on waveforms modeling and first P-motion data to complement the existing dataset of focal mechanisms. The results depict a narrow belt of intense seismic activity, at depth between 10 and 20km, which can be traced all along the topographic front of the Higher Himalaya in Nepal. This zone coincides with the zone of interseismic strain build up at the down dip end of the locked portion of the Main Himalayan Thrust fault. These earthquakes probably activate minor faults within the Himalayan wedge, in a volume which experiences stress build up in the interseismic period. The intense seismic activity generally cease abruptly as the elevation of topography gets higher than 3500 m. This elevation also coincides with a change of tectonic regime. Earthquakes within the seismicity belt at front of the Higher Himalaya are of thrust type indicating N-S shortening. Where the topographic elevation is higher than 3500 m focal mechanisms indicate E-W extension. The effect of topography on the regional stress field thus provides a simple explanation for the distribution of focal mechanisms and the seismicity cut-off at the 3500m elevation contour line.

Are the frictional properties of creeping faults persistent? Evidence from rapid afterslip following the 2011 Tohoku‐oki earthquake

Geophysical observations and numerical studies have shown that creeping portions of faults have persistent rate‐strengthening frictional properties and can act as barriers to earthquake rupture propagation. On the basis of GPS data following the 2011 MW 9.0 Tohoku‐oki earthquake in Japan, we find that the evolution of afterslip and postseismic shear stress on the plate interface is inconsistent with persistent rate‐strengthening frictional properties but is consistent with slip‐rate‐dependent frictional properties that exhibit less rate‐strengthening with increasing slip rate. Such slip‐rate‐dependent frictional properties tend to prevent creeping regions from acting as barriers to rupture propagation and therefore could be an important factor in determining the spatial extent of individual earthquakes.

Slip localization on the southern Alpine Fault, New Zealand

AbstractResults of a detailed field study of the southern onshore portion of New Zealand's Alpine Fault reveal that for 75 km along‐strike, dextral‐normal slip on this long‐lived structure is highly localized in phyllosilicate‐rich fault core gouges and along their contact with more competent rocks. At three localities (Martyr River, McKenzie Creek, and Hokuri Creek), we document complete cross sections through the fault. New40Ar/39Ar dates on mylonites, combined with microstructural and mechanical data on phyllosilicate‐rich fault core gouges show that modern slip is localized onto a single, steeply dipping 1 to 12 m‐thick fault core composed of impermeable (k = 10−20to 10−22 m2), frictionally weak (μs = 0.12–0.37), velocity‐strengthening, illite‐chlorite, and saponite‐chlorite‐lizardite fault gouges. Fault core materials are (1) comparable to those of other major weak‐cored faults (e.g., San Andreas Fault) and (2) most compatible with fault creep, despite paleoseismic evidence of quasiperiodic large magnitude earthquakes (Mw > 7) on this portion of the Alpine Fault. We conclude that frictional properties of gouges at the surface do not characterize the overall seismogenic behavior of the southern Alpine Fault.