Cosmogenic exposure data can be used to calculate time-varying fault slip rates on normal faults with exposed bedrock scarps. However, the method relies on assumptions related to how the scarp is preserved, which should be consistent at multiple locations along the same fault. Previous work commonly relied on cosmogenic data from a single sample locality to determine the slip rate of a fault. Here we show that by applying strict sampling criteria and using geologically informed modelling parameters in a Bayesian-inference Markov chain Monte Carlo method, similar patterns of slip rate changes can be modelled at multiple sites on the same fault. Consequently, cosmogenic data can be used to resolve along-strike fault activity. We present cosmogenic 36Cl concentrations from seven sites on two faults in the Italian Apennines. The average slip rate varies between sites on the Campo Felice Fault (0.84 0.23 to 1.61 0.27 mm yr ^-1), and all sites experienced a period of higher than average slip rate between 0.5 and 2 ky and a period of lower than average slip rate before 3 ky. On the Roccapreturo fault, slip rate in the centre of the fault is 0.550.11 and 0.350.05 mm yr ^-1 at the fault tip near a relay. The estimated time since the last earthquake is the same at each site along the same fault. These results highlight the potential for cosmogenic exposure data to reveal the detailed millennial history of earthquake slip on active normal faults.
Large continental earthquakes necessarily involve failure of multiple faults or segments. But these same critically-stressed systems sometimes fail in drawn-out sequences of smaller earthquakes over days or years instead. These two modes of failure have vastly different implications for seismic hazard and it is not known why fault systems sometimes fail in one mode or the other, or what controls the termination and reinitiation of slip in protracted seismic sequences. A paucity of modern observations of seismic sequences has hampered our understanding to-date, but a series of three Mw>6 earthquakes from August to November 2016 in Central Italy represents a uniquely well-observed example. Here we exploit a wealth of geodetic, seismological and field data to understand the spatio-temporal evolution of the sequence. Our results suggest that intersections between major and subsidiary faults controlled the extent and termination of rupture in each event in the sequence, and that fluid diffusion, channelled along these same fault intersections, may have also determined the timing of rupture reinitiation. This dual control of subsurface structure on the stop-start rupture in seismic sequences may be common; future efforts should focus on investigating its prevalence.
There are two major aims in my thesis. The first is primarily a methodological focus.
Cosmgenic istope dating of bedrock normal fault scarps is being increasingly used
to determine slip rates on normal faults. These slip rates often form the basis of
geodynamic and seismic hazard models, however it has not been determined that it is a
consistent method for measuring slip rate. I aim to test whether it is a robust technique
for determining slip rate by cosmogenic isotope; investigating the reproducibility of the
method at multiple sites along a single fault. This will provide greater confidence in
the method. I have chosen to undertake this study in the central Italian Apennines,
because there it has the highest concentration of existing 36Cl fault scarp studies, which
I can integrate with my studies.
To understand how fault networks behave requires information on their slip rates,
and by determining slip rates on faults that are next to each other, I may gain insight
into how faults are interacting over millennial timescales. Quaternary slip rates on faults
in western Turkey are not constrained in many areas, and it is one of the most rapidly
extending regions on earth. I aim to determine slip rates on some of these normal faults
using cosmogenic isotope dating of limestone fault scarps, and see what information
this can provide on how faults are interacting in the region. I have chosen to work in
the Mugla-Yatagan basin because no Quaternary slip rates have been determined on
the faults which lie close to a major city, and the close proximity of 3 faults may provide
insight into how faults interact over km scales. Finally there are also appropriate sample
sites on each of the major faults, allowing the 36Cl fault scarp dating method to be used.
We provide a database of the coseismic geological surface effects following the Mw 6.5 Norcia earthquake that hit central Italy on 30 October 2016. This was one of the strongest seismic events to occur in Europe in the past thirty years, causing complex surface ruptures over an area of >400 km2. The database originated from the collaboration of several European teams (Open EMERGEO Working Group; about 130 researchers) coordinated by the Istituto Nazionale di Geofisica e Vulcanologia. The observations were collected by performing detailed field surveys in the epicentral region in order to describe the geometry and kinematics of surface faulting, and subsequently of landslides and other secondary coseismic effects. The resulting database consists of homogeneous georeferenced records identifying 7323 observation points, each of which contains 18 numeric and string fields of relevant information. This database will impact future earthquake studies focused on modelling of the seismic processes in active extensional settings, updating probabilistic estimates of slip distribution, and assessing the hazard of surface faulting.
Abstract Cosmogenic exposure data can be used to calculate time‐varying fault slip rates on normal faults with exposed bedrock scarps. The method relies on assumptions related to how the scarp is preserved, which should be consistent at multiple locations along the same fault. Previous work commonly relied on cosmogenic data from a single sample locality to determine the slip rate of a fault. Here we show that by applying strict sampling criteria and using geologically informed modeling parameters in a Bayesian‐inference Markov chain Monte Carlo method, similar patterns of slip rate changes can be modeled at multiple sites on the same fault. Consequently, cosmogenic data can be used to resolve along‐strike fault activity. We present cosmogenic 36 Cl concentrations from seven sites on two faults in the Italian Apennines. The average slip rate varies between sites on the Campo Felice Fault (0.84 ± 0.23 to 1.61 ± 0.27 mm yr −1 ), and all sites experienced a period of higher than average slip rate between 0.5 and 2 ka and a period of lower than average slip rate before 3 ka. On the Roccapreturo fault, slip rate in the center of the fault is 0.55 ± 0.11 and 0.35 ± 0.05 mm yr −1 at the fault tip near a relay zone. The estimated time since the last earthquake is the same at each site along the same fault (631 ± 620 years at Campo Felice and 2,603 ± 1,355 years at Roccapreturo). These results highlight the potential for cosmogenic exposure data to reveal the detailed millennial history of earthquake slip on active normal faults.
Abstract The temporal evolution of slip on surface ruptures during an earthquake is important for assessing fault displacement, defining seismic hazard and for predicting ground motion. However, measurements of near-field surface displacement at high temporal resolution are elusive. We present a novel record of near-field co-seismic displacement, measured with 1-second temporal resolution during the 30 th October 2016 M w 6.6 Vettore earthquake (Central Italy), using low-cost Global Navigation Satellite System (GNSS) receivers located in the footwall and hangingwall of the Mt. Vettore - Mt. Bove fault system, close to new surface ruptures. We observe a clear temporal and spatial link between our near-field record and InSAR, far-field GPS data, regional measurements from the Italian Strong Motion and National Seismic networks, and field measurements of surface ruptures. Comparison of these datasets illustrates that the observed surface ruptures are the propagation of slip from depth on a surface rupturing (i.e. capable) fault array, as a direct and immediate response to the 30 th October earthquake. Large near-field displacement ceased within 6–8 seconds of the origin time, implying that shaking induced gravitational processes were not the primary driving mechanism. We demonstrate that low-cost GNSS is an accurate monitoring tool when installed as custom-made, short-baseline networks.
<p>In zones of distributed continental faulting, it is critical to understand how slip is partitioned onto brittle structures over both long-term millennial time scales and shorter-term earthquake cycles. Measuring earthquake slip histories on different timescales is challenging due to earthquake repeat-times being longer or similar to historical earthquake records, and a paucity of data on fault activity covering millennial to Quaternary scales in detail. Cosmogenic isotope analyses from bedrock fault scarps have the potential to bridge the gap, as these datasets track the exposure of fault planes due to earthquakes with millennial resolution. In this presentation, we show new <sup>36</sup>Cl data combined with active fault maps to document the spatial and temporal complexity of extensional faulting in western Turkey.</p><p>Extensional faulting covers an area ~460 x 460 km in western Turkey. The dynamics controlling extension are debated, but there is an overall pattern of anticlockwise rotation superimposed on N-S directed upper-plate extension related to eastern Mediterranean subduction. This has resulted in several major east-west trending extensional grabens along the western coast of Turkey and NE-SW to NW-SE trending conjugate grabens towards the southern coast and central Anatolia. The active fault map of Turkey is well characterised by the MTA (General Directorate of Mineral Research and Exploration), but recent mid-magnitude earthquakes have occurred on some un-characterised fault zones, suggesting that there is further complexity in the trace and locations of active faults. This complexity may indicate recent reactivation of pre-existing structures.</p><p>Most of the major bedrock normal fault scarps are well preserved in carbonate and marble successions distributed across the region. These scarps preserve an excellent record of Late Pleistocene to Holocene earthquake activity, which can be quantified using cosmogenic isotopes that track the exposure of the bedrock fault scarps. <sup>36</sup>Cl accumulates in the fault scarps as the footwall is progressively exhumed by earthquakes and the concentration of <sup>36</sup>Cl measured up the fault plane reflects the rate and patterns of slip. In this presentation, we utilise Bayesian modelling techniques to estimate slip histories based on new cosmogenic data from several faults across western Turkey. Each sampling site is carefully characterised using field mapping and LiDAR to ensure that fault plane exposure is due to slip during earthquakes and not sediment transport processes. We will compare several neighbouring fault zones with variable slip rates to investigate how they interact over multiple earthquake cycles, and put this temporal complexity into the context of spatial complexity, and the resultant challenges for hazard forecasts in western Turkey.</p>
Abstract 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 (24 August 2016 M w 6.0 and 30 October 2016 M w 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 ~28 o and the dip increases by 20–25 o . We present similar examples from historical normal faulting earthquakes (1887, Sonora earthquake, M w 7.5; 1981, Corinth earthquakes, M w 6.7–6.4; and 1983, Borah Peak earthquake, M w 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 nonplanar fault for a single earthquake rupture. The calculated slip enhancement in bends can explain much of the scatter in maximum displacement ( Dmax ) versus surface rupture length scaling relationships. If fault bends are unrecognized, they can introduce variation in Dmax that may lead to erroneous inferences of stress drop variability for earthquakes, and exaggerate maximum earthquake magnitudes derived from vertical offsets in paleoseismic data sets.
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