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

Influence of Fault System Geometry and Slip Rates on the Relative Role of Coseismic and Interseismic Stresses on Earthquake Triggering and Recurrence Variability

We model Coulomb stress transfer (CST) due to 30 strong earthquakes occurring on normal faults since 1509 CE in Calabria, Italy, including the influence of interseismic loading, and compare the results to existing studies of stress interaction from the Central and Southern Apennines, Italy. The three normal fault systems have different geometries and long‐term slip‐rates. We investigate the extent to which stress transfer can influence the occurrence of future earthquakes and what factors may govern the variability in earthquake recurrence in different fault systems. The Calabrian, Central Apennines, and Southern Apennines fault systems have 91%, 73%, and 70% of faults with mean positive cumulative CST in the time considered; this is due to fewer faults across strike, more across strike stress reductions, and greater along‐strike spacing in the three regions respectively. In regions with close along strike spacing or few faults across strike, such as Calabria and Southern Apennines, the stress loading history is mostly dominated by interseismic loading and most faults are positively stressed before an earthquake occur on them (96% of all faults that ruptured in Calabria; 94% of faults in Southern Apennines), and some of the strongest earthquakes occur on faults with the highest mean cumulative stress of all faults prior to the earthquake. In the Central Apennines, where across strike interactions are the predominant process, 79% of earthquakes occur on faults positively stressed. The results highlight that fault system geometry plays a central role in characterizing the stress evolution associated with earthquake recurrence

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

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

A Shallow Earthquake Swarm Close to Hydrocarbon Activities: Discriminating between Natural and Induced Causes for the 2018–2019 Surrey, United Kingdom, Earthquake Sequence

Earthquakes induced by subsurface industrial activities are a globally emotive issue, with a growing catalog of induced earthquake sequences. However, attempts at discriminating between natural and induced causes, particularly for anomalously shallow seismicity, can be challenging. An earthquake swarm during 2018–2019 in southeast England with a maximum magnitude of ML 3.2 received great public and media attention because of its proximity to operating oilfields. It is therefore vital and timely to provide a detailed characterization of the earthquake sequence at present, and to decide based on current evidence, whether the earthquakes were likely natural or induced. We detected 168 low-magnitude earthquakes and computed detailed source parameters of these events. Most earthquakes occurred at a shallow depth of 2.3 km, >1 km deeper than the geological formations targeted by the oilfields, and laterally >3 km away from the drill sites. We combine the east–west-trending cluster of the seismicity with 2D seismic reflection profiles to find the causative fault system for the earthquakes. A b-value close to unity and strike-slip faulting mechanisms are consistent with tectonic reactivation along a pre-existing fault. Overall, we find no indicators in the earthquake parameters that would strongly suggest an induced source. Nor do we find any clear trends between seismicity and drilling activities based on operational logs provided by the operators. Injected volumes are near zero and monthly production amounts are many orders of magnitude smaller than other reported cases of extraction-induced seismicity. On balance, and based on the available evidence, we find it currently unlikely that nearby industrial activities induced the seismic swarm. Most likely, the Surrey earthquakes offer a uniquely detailed insight into shallow seismicity within sedimentary basins. Nevertheless, self-reporting of injection and production times and volumes by operators, and the lack of easily and publicly available oilfield operational data continues to be a point of concern for local residents

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