Stress loading history of earthquake faults influenced by fault/shear zone geometry and Coulomb pre-stress

Whether the stress-loading of faults to failure in earthquakes appears to be random or to an extent explainable, given constraints on fault/shear-zone interaction and the build-up and release of stress over many earthquake cycles, is a key question for seismic hazard assessment. Here we investigate earthquake recurrence for a system of 25 active normal faults arranged predominantly along strike from each other, allowing us to isolate the effects of stress-loading due to regional strain versus across- and along-strike fault interaction. We calculate stress changes over 6 centuries due to interseismic loading and 25 > Mw 5.5 earthquakes. Where only one fault exists across strike, stress-loading is dominated by the regional tectonics through slip on underlying shear zones and fault planes have spatially smooth stress with predominantly time-dependent stress increase. Conversely, where faults are stress-loaded by across-strike fault interactions, fault planes have more irregular stress patterns and interaction-influenced stress loading histories. Stress-loading to failure in earthquakes is not the same for all faults and is dependent on the geometry of the fault/shear-zone system

Slip-partitioned surface ruptures for the Mw 7.0 16 April 2016 Kumamoto, Japan, earthquake

An ENE-trending ~30-km-long surface rupture emerged during the Mw = 7.0 16 April 2016 Kumamoto earthquake along the previously mapped Futagawa and northern Hinagu faults. This included a previously unknown 5-km-long fault within the Aso Caldera, central Kyushu. The rupture zone is mostly composed of right-lateral slip sections, with a maximum of 2-m coseismic slip. One of the noteworthy features we observed in the field are ~10-km-long segmented normal fault scarps, dipping to the northwest, along the previously mapped Idenokuchi fault, 1.2–2.0 km south of and subparallel to the Futagawa fault. The maximum amount of coseismic throw on the Idenokuchi fault is ~2 m, which is nearly equivalent to the maximum slip on the strike-slip rupture. The locations and slip motions of the 2016 rupture are also manifested as interferogram fringe offsets in InSAR images. Together with geodetic and seismic inversions of subsurface fault slip, we present a schematic structural model where oblique motion occurred on a northwest-dipping subsurface fault and the slip is partitioned at the surface into strike-slip and normal fault scarps. Our simple dislocation model demonstrates that this bifurcation into pure strike-slip and normal faults likely occurs for optimally oriented failure near the surface. The Kumamoto case, with detailed geological observations and geophysical models, would be the second significant slip-partitioned earthquake around the globe. It provides an important insight into scale- and depth-dependent stress heterogeneity and an implication to a proper estimate of seismic hazard in complex and broad multiple fault strands

Coseismic Throw Variation Across Along-Strike Bends on Active Normal Faults: Implications for Displacement Versus Length Scaling of Earthquake Ruptures

Fault bends, and associated changes in fault dip, play a key role in explaining the scatter in maximum offset versus surface rupture length fault scaling relationships. Detailed field measurements of the fault geometry and magnitude of slip in the 2016-2017 central Italy earthquake sequence, alongside three examples from large historical normal-faulting earthquakes in different tectonic settings, provide multiple examples in which coseismic throw increases across bends in fault strike where dip also increases beyond what is necessary to accommodate a uniform slip vector. Coseismic surface ruptures produced by two mainshocks of the 2016-2017 central Italy earthquake sequence (24th August 2016 Mw 6.0, 30th October 2016 Mw 6.5) cross a ~0.83 km amplitude along-strike bend, and the coseismic throws for both earthquakes increase by a factor of 2-3 where the strike of the fault changes by ~30o and the dip increases by 20-25o. We present similar examples from historical normal faulting earthquakes (1887, Sonora earthquake, Mw 7.5; 1981, Corinth earthquakes, Mw 6.7-6.4;1983, Borah Peak earthquake, Mw 7.3). We demonstrate that it is possible to estimate the expected change in throw across a bend by applying equations that relate strike, dip and slip vector to horizontal strain conservation along a non-planar fault for a single earthquake rupture. The calculated slip enhancement in bends can explain the scatter in maximum displacement (Dmax) versus surface rupture length scaling relationships. If fault bends are un-recognized, they can introduce variation in Dmax that may lead to erroneous inferences of stress drop variability for earthquakes, and maximum earthquake magnitudes derived from vertical offsets in paleoseismic datasets

Spatial migration of temporal earthquake clusters driven by the transfer of differential stress between neighbouring fault/shear-zone structures

Uncertainty concerning the processes responsible for slip-rate fluctuations associated with temporal clustering of surface faulting earthquakes is a fundamental, unresolved issue in tectonics, because strain-rates accommodated by fault/shear-zone structures are the key to understanding the viscosity structure of the crust and seismic hazard. We constrain the timing and amplitude of slip-rate fluctuations that occurred on three active normal faults in central Italy over a time period of 20–30 kyrs, using in situ 36Cl cosmogenic dating of fault planes. We identify five periods of rapid slip on individual faults lasting a few millennia, separated time periods of up to 10 millennia with low or zero slip-rate. The rapid slip pulses migrated across the strike between the faults in two waves from SW to NE. We replicate this migration with a model where rapid slip induces changes in differential stress that drive changes in strain-rate on viscous shear zones that drive slip-rate variability on overlying brittle faults. Earthquakes increase the differential stress and strain-rate on underlying shear zones, which in turn accumulate strain, re-loading stress onto the overlying brittle fault. This positive feedback produces high strain-rate episodes containing several large magnitude surface faulting earthquakes (earthquake clusters), but also reduce the differential stress on the viscous portions of neighbouring fault/shear-zones slowing the occurrence of large-magnitude surface faulting earthquakes (earthquake anticlusters). Shear-zones on faults experiencing anticlusters continue to accumulate viscous strain at a lowered rate, and eventually this loads the overlying brittle fault to failure, initiating a period of rapid slip through the positive feedback process described above, and inducing lowered strain-rates onto neighbouring fault/shear-zones. We show that these patterns of differential stress change can replicate the measured earthquake clustering implied by the 36Cl data. The stress changes are related to the fault geometry in terms of distance and azimuth from the slipping structure, implying that (a) strain-rate and viscosity fluctuations for studies of continental rheology, and (b) slip-rates for seismic hazard purposes are to an extent predictable given knowledge of the fault system geometry

Three‐dimensional structure, ground rupture hazards, and static stress models for complex non‐planar thrust faults in the Ventura basin, southern California

To investigate the subsurface geometry of a recently discovered, seismically‐active fault in the Ventura basin, southern California, USA, we present a series of cross sections and a new three‐dimensional fault model across the Southern San Cayetano fault (SSCF) based on integration of surface data with petroleum industry well‐log data. Additionally, the fault model for the SSCF, along with models of other regional faults extracted from the Southern California Earthquake Center three‐dimensional Community Fault Model, are incorporated in static Coulomb stress modeling to investigate static Coulomb stress transfer between thrust faults with complex geometry and to further our understanding of stress transfer in the Ventura basin. The results of the subsurface well investigation provide evidence for a low‐angle SSCF that dips ~15° north and connects with the western section of the San Cayetano fault around 1.5–3.5 km depth. We interpret the results of static Coulomb stress models to partly explain contrasting geomorphic expression between different sections of the San Cayetano fault and a potential mismatch in timings between large‐magnitude uplift events suggested by paleoseismic studies on the Pitas Point, Ventura, and San Cayetano faults. In addition to new insights into the structure and potential rupture hazard of a recently discovered active reverse fault in a highly populated area of southern California, this study provides a simple method to model static Coulomb stress transfer on complex geometry faults in fold and thrust belts

Coulomb pre-stress and fault bends are ignored yet vital factors for earthquake triggering and hazard

Successive locations of individual large earthquakes (Mw>5.5) over years to centuries can be difficult to explain with simple Coulomb Stress Transfer (CST) because it is common for seismicity to circumvent nearest-neighbour along-strike faults where coseismic CST is greatest. We demonstrate that Coulomb pre-stress (the cumulative CST from multiple earthquakes and interseismic loading on non-planar faults) may explain this, evidenced by study of a 667-year historical record of earthquakes in central Italy. Heterogeneity in Coulomb pre-stresses across the fault system is >±50 bars, whereas coseismic CST is <±2 bars, so the latter will rarely overwhelm the former, explaining why historical earthquakes rarely rupture nearest neighbor faults. However, earthquakes do tend to occur where the cumulative coseismic and interseismic CST is positive, although there are notable examples where earthquake propagate across negatively stressed portions of faults. Hence Coulomb pre-stress calculated for non-planar faults is an ignored yet vital factor for earthquake triggering

The link between earthquakes and structural geology; the role of elapsed time, 3D geometry and stress transfer in the central Apennines, Italy

The role of fault geometry, Coulomb stress and elapsed time is investigated herein to determine whether earthquakes are clustered or not in the central Apennines, Italy. Two earthquake sequences are analysed to determine the relative importance of Coulomb stress transfer and elapsed time. The importance of fault geometry when modelling the Coulomb stress transfer is demonstrated. The earthquake sequences of interest can be partially explained by a combination of Coulomb stress transfer and elapsed time, thus demonstrating that earthquakes in the central Apennines are non-stochastic in nature. Considering the variations in fault geometry over short (hundreds of metres) length scale has been used to demonstrate that the surface bedrock fault scarps are active. Over longer scales (kilometres) the geometry of the faults is shown to affect the pattern of Coulomb stress transferred during earthquakes. A novel methodology outlined in this thesis is used to model Coulomb stress changes throughout the historical record onto faults with strike-variable geometry. It is shown that the Coulomb stress transfer is likely to have played a role in two earthquake se- quences of interest, the 1703 - 1706 A.D. and 2016 - 2017 A.D. sequences. However Coulomb stress transfer cannot fully explain these sequences. The elapsed time on faults of interest to these sequences is considered, and it is shown that faults with longer elapsed time rupture preferentially over faults with shorter elapsed time in both se- quences. When considered together, fault geometry, Coulomb stress and elapsed time considered together can explain the progression of the 2016 - 2017 A.D. earthquake sequence. The 1703 - 1706 A.D. sequence can be explained in a similar manner, how- ever the results are less conclusive due to a lack of elapsed time data. Elapsed time cannot be considered alone, without information about the mean recurrence intervals on faults of interest. The results presented herein have implications for estimations of seismic hazard

Fault slip-rates and Coulomb stress interactions in the intersection zone of the Hope, Kelly and Alpine Faults, South Island, New Zealand

Plate boundary faulting in New Zealand's South Island involves transfer of ∼50% of slip from the largest fault (Alpine Fault) onto the Hope-Kelly Fault system through a structurally complex fault intersection zone. The slip-rate contributions of faults within the Hope-Kelly system and possible role of static stresses in facilitating slip transfer are explored in this study. Lidar-based geomorphic and fault mapping combined with luminescence dating of fault-proximal sedimentary deposits constrain post-last glacial slip-rates on the Hope and Kelly faults. Dextral slip-rates on the central Hope Fault (12–15 mm/yr) decrease westward on the Taramakau section from 5.6 (+2.1/−0.7) mm/yr to 1.7 (+1.0/−0.5) mm/yr. Dextral slip-rates on the Kelly Fault range from 6.2 (+2.7/−1.0) mm/yr to 2.0 (+2.5/−0.7) mm/yr to 6.2 (+7.8/−1.4) mm/yr. Proposed causes of slip-rate spatial variations include (i) complex slip localization and transfer across the deformation zone, (ii) undocumented slip on obscured or unrecognized faults, and (iii) possible transience in slip behaviours. Paleoseismic trenching and radiocarbon ages constrain timing of most recent surface rupture on the western Hope Fault to ca. 1680–1840 CE, with a preferred age of ca. 1800–1840 CE. Coulomb fault stress modelling indicates central Alpine Fault ruptures impart positive stress changes on Hope-Kelly receiver faults >5–10 bars, while Northern Alpine Fault earthquakes reduce Coulomb stresses on Hope-Kelly receiver faults, and vice versa. These results suggest central Alpine Fault earthquakes may propagate onto or trigger ruptures of Hope-Kelly Faults, but Hope-Kelly ruptures reduce stress on the northern Alpine Fault, possibly making ruptures of that fault less likely. This system of stress perturbations provides a mechanism for slip transfer from the central Alpine Fault onto the Hope Fault system