Uncertainty in strain-rate from field measurements of the geometry, rates and kinematics of active normal faults: Implications for seismic hazard assessment

Multiple measurements of the geometry, kinematics and rates of slip across the well-exposed Auletta fault scarp (Campania, Italy) are presented, and we use these in order to investigate: (1) the spatial resolution of field measurements needed to accurately calculate a representative strain-rate for seismic hazard calculations; (2) what aspects of the geometry and kinematics would introduce uncertainty in calculated strain-rate, if those are not measured in the field. Our results show that the magnitude of the post-glacial maximum (15&#177;3 ka) throw gradually decreases towards the tip of the fault, but variations are observed along strike, across areas of structural complexity such as along-strike bends in the fault plane where the fault dip is greater. We find that if such variations are unnoticed, different values of strain-rate would be produced, and hence different values would result in seismic hazard calculations. To demonstrate this, we calculate the strain-rate across the Auletta fault using all our measurements, and subsequently degrade the dataset removing one measurement at a time and recalculating the implied strain-rate at each step. The results show that excluding measurements can alter strain-rate results beyond 1 &#963; uncertainty, thus we suggest caution when using only one measurement of slip-rate along a fault for calculating hazard, as a full understanding of the potential implied errors needs consideration. Furthermore, we investigate the effect of approximating the throw profile along the fault using boxcar and triangular slip distributions; we show that this can underestimate or overestimate the strain-rate, with results in the range of 72&#8211;237% of our most detailed strain-rate calculation. We suggest that improved understanding of the potential implied errors in strain-rate calculations from field structural data should be implemented in seismic hazard calculations.

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Uncertainty in strain-rate from field measurements of the geometry, rates and kinematics of active normal faults: Implications for seismic hazard assessment

Journal of Structural Geology ◽

10.1016/j.jsg.2019.103934 ◽

2020 ◽

Vol 131 ◽

pp. 103934 ◽

Cited By ~ 2

Author(s):

Claudia Sgambato ◽

Joanna P. Faure Walker ◽

Gerald P. Roberts

Keyword(s):

Seismic Hazard ◽

Strain Rate ◽

Hazard Assessment ◽

Seismic Hazard Assessment ◽

Field Measurements ◽

Normal Faults

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Temporal and spatial earthquake clustering revealed through comparison of millennial strain-rates from 36Cl cosmogenic exposure dating and decadal GPS strain-rate

Scientific Reports ◽

10.1038/s41598-021-02131-3 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Francesco Iezzi ◽

Gerald Roberts ◽

Joanna Faure Walker ◽

Ioannis Papanikolaou ◽

Athanassios Ganas ◽

...

Keyword(s):

Seismic Hazard ◽

Strain Rate ◽

Active Faults ◽

Slip Rate ◽

Normal Faults ◽

Fault Activity ◽

Exposure Dating ◽

Extensional Strain ◽

Temporal And Spatial

AbstractTo assess whether continental extension and seismic hazard are spatially-localized on single faults or spread over wide regions containing multiple active faults, we investigated temporal and spatial slip-rate variability over many millennia using in-situ 36Cl cosmogenic exposure dating for active normal faults near Athens, Greece. We study a ~ NNE-SSW transect, sub-parallel to the extensional strain direction, constrained by two permanent GPS stations located at each end of the transect and arranged normal to the fault strikes. We sampled 3 of the 7 seven normal faults that exist between the GPS sites for 36Cl analyses. Results from Bayesian inference of the measured 36Cl data implies that some faults slip relatively-rapidly for a few millennia accompanied by relative quiescence on faults across strike, defining out-of-phase fault activity. Assuming that the decadal strain-rate derived from GPS applies over many millennia, slip on a single fault can accommodate ~ 30–75% of the regional strain-rate for a few millennia. Our results imply that only a fraction of the total number of Holocene active faults slip over timescales of a few millennia, so continental deformation and seismic hazard are localized on specific faults and over a length-scale shorter than the spacing of the present GPS network over this time-scale. Thus, (1) the identification of clustered fault activity is vital for probabilistic seismic hazard assessments, and (2) a combination of dense geodetic observations and palaeoseismology is needed to identify the precise location and width of actively deforming zones over specific time periods.

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Methodology for Earthquake Rupture Rate estimates of fault networks: example for the Western Corinth Rift, Greece

10.5194/nhess-2017-124 ◽

2017 ◽

Cited By ~ 1

Author(s):

Thomas Chartier ◽

Oona Scotti ◽

Hélène Lyon-Caen ◽

Aurélien Boiselet

Keyword(s):

Active Faults ◽

Slip Rate ◽

Seismic Hazard Assessment ◽

System Level ◽

Fault System ◽

Normal Faults ◽

Accurate Estimation ◽

Probabilistic Seismic Hazard Assessment ◽

Earthquake Rupture ◽

Multiple Faults

Abstract. Modelling the seismic potential of active faults is a fundamental step of probabilistic seismic hazard assessment (PSHA). An accurate estimation of the rate of earthquakes on the faults is necessary in order to obtain the probability of exceedance of a given ground motion. Most PSHA studies consider faults as independent structures and neglect the possibility of multiple faults or fault segments rupturing simultaneously (Fault to Fault -FtF- ruptures). The latest Californian model (UCERF-3) takes into account this possibility by considering a system level approach rather than an individual fault level approach using the geological , seismological and geodetical information to invert the earthquake rates. In many places of the world seismological and geodetical information long fault networks are often not well constrained. There is therefore a need to propose a methodology relying only on geological information to compute earthquake rate of the faults in the network. In this methodology, similarly to UCERF-3, a simple distance criteria is used to define FtF ruptures and consider single faults or FtF ruptures as an aleatory uncertainty. Rates of earthquakes on faults are then computed following two constraints: the magnitude frequency distribution (MFD) of earthquakes in the fault system as a whole must follow an imposed shape and the rate of earthquakes on each fault is determined by the specific slip-rate of each segment depending on the possible FtF ruptures. The modelled earthquake rates are then confronted to the available independent data (geodetical, seismological and paleoseismological data) in order to weigh different hypothesis explored in a logic tree. The methodology is tested on the Western Corinth Rift, Greece (WCR) where recent advancements have been made in the understanding of the geological slip rates of the complex network of normal faults which are accommodating the ~15 mm/yr North-South extension. Modelling results show that geological, seismological extension rates and paleoseismological rates of earthquakes cannot be reconciled with only single fault rupture scenarios and require hypothesising a large spectrum of possible FtF rupture sets. Furthermore, in order to fit the imposed regional Gutenberg-Richter MFD target, some of the slip along certain faults needs to be accommodated either with interseismic creep or as post-seismic processes. Furthermore, individual fault’s MFDs differ depending on the position of each fault in the system and the possible FtF ruptures associated with the fault. Finally, a comparison of modelled earthquake rupture rates with those deduced from the regional and local earthquake catalogue statistics and local paleosismological data indicates a better fit with the FtF rupture set constructed with a distance criteria based on a 5 km rather than 3 km, suggesting, a high connectivity of faults in the WCR fault system.

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Active fault databases: building a bridge between earthquake geologists and seismic hazard practitioners, the case of the QAFI v.3 database

Natural Hazards and Earth System Science ◽

10.5194/nhess-17-1447-2017 ◽

2017 ◽

Vol 17 (8) ◽

pp. 1447-1459 ◽

Cited By ~ 5

Author(s):

Julián García-Mayordomo ◽

Raquel Martín-Banda ◽

Juan M. Insua-Arévalo ◽

José A. Álvarez-Gómez ◽

José J. Martínez-Díaz ◽

...

Keyword(s):

Seismic Hazard ◽

Active Fault ◽

Slip Rate ◽

Seismic Hazard Assessment ◽

Recurrence Interval ◽

Maximum Magnitude ◽

Geological Evidence ◽

Seismic Parameters ◽

Standard Quality

Abstract. Active fault databases are a very powerful and useful tool in seismic hazard assessment, particularly when singular faults are considered seismogenic sources. Active fault databases are also a very relevant source of information for earth scientists, earthquake engineers and even teachers or journalists. Hence, active fault databases should be updated and thoroughly reviewed on a regular basis in order to keep a standard quality and uniformed criteria. Desirably, active fault databases should somehow indicate the quality of the geological data and, particularly, the reliability attributed to crucial fault-seismic parameters, such as maximum magnitude and recurrence interval. In this paper we explain how we tackled these issues during the process of updating and reviewing the Quaternary Active Fault Database of Iberia (QAFI) to its current version 3. We devote particular attention to describing the scheme devised for classifying the quality and representativeness of the geological evidence of Quaternary activity and the accuracy of the slip rate estimation in the database. Subsequently, we use this information as input for a straightforward rating of the level of reliability of maximum magnitude and recurrence interval fault seismic parameters. We conclude that QAFI v.3 is a much better database than version 2 either for proper use in seismic hazard applications or as an informative source for non-specialized users. However, we already envision new improvements for a future update.

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Bayesian earthquake dating and seismic hazard assessment using chlorine-36 measurements (BED v1)

Geoscientific Model Development ◽

10.5194/gmd-11-4383-2018 ◽

2018 ◽

Vol 11 (11) ◽

pp. 4383-4397 ◽

Cited By ~ 1

Author(s):

Joakim Beck ◽

Sören Wolfers ◽

Gerald P. Roberts

Keyword(s):

Inverse Problem ◽

Seismic Hazard ◽

Hazard Assessment ◽

Ad Hoc ◽

Seismic Hazard Assessment ◽

Fault Scarp ◽

Physical Models ◽

Monte Carlo Algorithm ◽

Slip Rates ◽

Chlorine 36

Abstract. Over the past 20 years, analyzing the abundance of the isotope chlorine-36 (36Cl) has emerged as a popular tool for geologic dating. In particular, it has been observed that 36Cl measurements along a fault plane can be used to study the timings of past ground displacements during earthquakes, which in turn can be used to improve existing seismic hazard assessment. This approach requires accurate simulations of 36Cl accumulation for a set of fault-scarp rock samples, which are progressively exhumed during earthquakes, in order to infer displacement histories from 36Cl measurements. While the physical models underlying such simulations have continuously been improved, the inverse problem of recovering displacement histories from 36Cl measurements is still mostly solved on an ad hoc basis. The current work resolves this situation by providing a MATLAB implementation of a fast, automatic, and flexible Bayesian Markov-chain Monte Carlo algorithm for the inverse problem, and provides a validation of the 36Cl approach to inference of earthquakes from the demise of the Last Glacial Maximum until present. To demonstrate its performance, we apply our algorithm to a synthetic case to verify identifiability, and to the Fiamignano and Frattura faults in the Italian Apennines in order to infer their earthquake displacement histories and to provide seismic hazard assessments. The results suggest high variability in slip rates for both faults, and large displacements on the Fiamignano fault at times when the Colosseum and other ancient buildings in Rome were damaged.

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Across-strike variations of fault slip-rates constrained using in-situ cosmogenic 36Cl concentrations.

10.5194/egusphere-egu2020-756 ◽

2020 ◽

Author(s):

Francesco Iezzi ◽

Gerald Roberts ◽

Joanna Faure Walker ◽

Ioannis Papanikolaou ◽

Athanassios Ganas ◽

...

Keyword(s):

Seismic Hazard ◽

Strain Rate ◽

Active Faults ◽

Slip Rate ◽

High Temporal Resolution ◽

Strain Rates ◽

Time Intervals ◽

The Holocene ◽

Regional Strain

It is important to constrain the spatial distribution of strain-rate in deforming continental material because this underpins calculations of continental rheology and seismic hazard. To do so, it is becoming increasingly common to use combinations of GPS and historical and instrumental seismicity data to constrain regional strain-rate fields. However, GPS geodetic sites, whether permanent or campaign stations, tend to be widely-spaced relative to the spacing of active faults with known Holocene offsets. At the same time, the interpretation of seismicity data can be difficult due to lack of historical seismicity in cases where local fault recurrence intervals are longer than the historical record. This causes uncertainty on how regional strain-rates are partitioned in time and space, and hence with uncertainty regarding calculations of continental rheology and seismic hazard. To overcome this issue, we have gained high temporal resolution slip-rate histories for three parallel faults using in situ 36Cl cosmogenic dating of the exposure of three parallel normal fault planes that have been progressively exhumed by earthquakes. We study the region around Athens, central Greece, where there also exists a relatively-dense GPS network and extensive records of instrumental and historical earthquakes. This allows to compare regional, decadal strain-rates measured with GPS geodesy with strain-rates across the faults implied by slip since ~40,000 years BP. We show that faults have all had episodic behaviour during the Holocene, with alternating earthquake clusters and periods of quiescence through time. Despite the fact that all three faults have been active in the Holocene, each fault slips in discrete time intervals lasting a few millennia, so that only one fault accommodates strain at any time. We show that magnitudes of strain-rates during the high slip-rate episodes are comparable with the regional strain-rates measured with GPS (fault strain-rates are 50-100% of the value of GPS regional strain-rate). Thus, if the GPS-derived strain-rate applies over longer time intervals, it appears that single faults dominate the strain-accumulation at any given time, with crustal deformation and seismic hazard localised within a distributed network of faults.&#160;

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Discrete changes in fault free-face roughness: constraining past earthquakes characteristics

10.5194/egusphere-egu2020-4032 ◽

2020 ◽

Author(s):

Olaf Zielke ◽

Lucilla Benedetti ◽

P. Martin Mai ◽

Magali Rizza ◽

Jules Fleury ◽

...

Keyword(s):

Surface Roughness ◽

Active Tectonics ◽

Slip Rate ◽

Fault Scarp ◽

Rock Properties ◽

Normal Faults ◽

Earthquake Recurrence ◽

Face Surface ◽

Free Face ◽

Face Morphology

A driving motivator in many active tectonics studies is to learn more about the recurrence large and potentially destructive earthquakes, providing the means to assess the respective fault&#8217;s future seismic behavior. Doing so requires long records of earthquake recurrence. The lack of sufficiently long instrumental seismic records (that would be best suited for this task) has led to the development of other approaches that may constrain the recurrence of surface rupturing earthquakes along individual faults. These approaches take different forms, depending on the specific tectonic and geographic conditions of an investigated region.For example, around the Mediterranean Sea, we frequently find bedrock scarps along normal faults. Assuming that bedrock (i.e., fault free-face) exposure is caused by the occurrence of sub-sequent large earthquakes, we may measure certain rock properties to constrain the time and size of past earthquakes as well as the fault&#8217;s geologic slip-rate. A now-classic example in this regard is the measurement of 36Cl concentrations along exposed fault scarps in limestones.For the presented study, we looked at another property of the exposed fault free-face, namely its morphologic roughness. We aim to identify whether fault free-face roughness contains information to constrain earthquake occurrence and fault slip-rates following the assumption that &#160;sub-sequent exposure to the elements and sub-areal erosional conditions may leave a signal in how rough (or smooth) the fault free-face is (assuming a somewhat uniform pre-exposure roughness). Here, we present observations of fault free-face surface roughness for the Mt. Vettore fault (last ruptured in 2016) and the Rocca Preturo fault (The underlying models of fault free-face morphology were generated using the Structure-from-Motion approach and a large suite of unregistered optical images.). Employing different metrics to quantify morphologic roughness, we were indeed able to observe a) an increase in surface roughness with fault scarp height (i.e., longer exposure to sub-areal erosion causes higher roughness), and b) distinct (rather than gradual) changes in surface roughness, suggesting a correlation to individual exposure events such as earthquakes. Hence, fault free-face morphology of bedrock faults may serve as an additional metric to reconstruct earthquake recurrence patterns.

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25,000 Years Long Seismic Cycle in a Slow Deforming Continental Region of Mongolia

10.21203/rs.3.rs-310793/v1 ◽

2021 ◽

Author(s):

Laurent Bollinger ◽

Yann Klinger ◽

Steven Forman ◽

Odonbaatar Chimed ◽

Amgalan Bayasgalan ◽

...

Keyword(s):

Seismic Hazard ◽

Active Faults ◽

Ground Surface ◽

Slip Rate ◽

Seismic Hazard Assessment ◽

Return Time ◽

Fault Behavior ◽

Seismogenic Structures ◽

Cumulative Deformation ◽

Large Earthquakes

Abstract The spatial distribution of large earthquakes in Slowly Deforming Continental Regions (SDCR) is poorly documented and, thus, has often been deemed to be random. Unlike in high strain regions, where seismic activity concentrates cyclically along major active faults, earthquakes in SDCR may seem to occur more erratically in space and time. This questions classical fault behavior models, posing paramount issues for seismic hazard assessment. Here, we investigate the M7, 1967, Mogod earthquake in Mongolia, a region recognized as a SDCR. Despite the absence of visible cumulative deformation at the ground surface, we found evidence for at least 3 surface rupturing earthquakes during the last 50,000 years, associated to a slip-rate of 0,06 ± 0,01 mm/yr. These results show that in SDCR, like in faster deforming regions, deformation localizes on specific structures. However, the excessive length of return time for large earthquakes along these structures makes it more difficult to recognize earthquake series, and could conversely lead to the misconception that in SDCR earthquakes would be randomly located. Thus, our result emphasizes the need for systematic appraisal of the potential seismogenic structures in SDCR in order to lower the uncertainties associated with the seismogenic sources in seismic hazard models.

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