scholarly journals Geologic and geodetic constraints on the seismic hazard of Malawi’s active faults: The Malawi Seismogenic Source Database (MSSD)

2021 ◽  
Author(s):  
Jack N. Williams ◽  
Luke N. J. Wedmore ◽  
Åke Fagereng ◽  
Maximilian J. Werner ◽  
Hassan Mdala ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Active Fault ◽  
Active Faults ◽  
Slip Rate ◽  
Slip Rates ◽  
Source Database ◽  
Seismogenic Sources ◽  
Seismogenic Source ◽  
Recurrence Intervals ◽  
Seismic Reflector

Abstract. Active fault data are commonly used in seismic hazard assessments, but there are challenges in deriving the slip rate, geometry, and frequency of earthquakes along active faults. Herein, we present the open-access geospatial Malawi Seismogenic Source Database (MSSD), which describes the seismogenic properties of faults that have formed during East African rifting in Malawi. We first use empirical observations to geometrically classify active faults into section, fault, and multi-fault seismogenic sources. For sources in the North Basin of Lake Malawi, slip rates can be derived from the vertical offset of a seismic reflector that is estimated to be 75 ka based on dated core. Elsewhere, slip rates are constrained from advancing a ‘systems-based’ approach that partitions geodetically-derived rift extension rates in Malawi between seismogenic sources using a priori constraints on regional strain distribution in magma-poor continental rifts. Slip rates are then combined with source geometry and empirical scaling relationships to estimate earthquake magnitudes and recurrence intervals, and their uncertainty is described from the variability of outcomes from a logic tree used in these calculations. We find that for sources in the Lake Malawi’s North Basin, where slip rates can be derived from both the geodetic data and the offset seismic reflector, the slip rate estimates are within error of each other, although those from the offset reflector are higher. Sources in the MSSD are 5–200 km long, which implies that large magnitude (MW 7–8) earthquakes may occur in Malawi. Low slip rates (0.05–2 mm/yr), however, mean that the frequency of such events will be low (recurrence intervals ~103–104 years). The MSSD represents an important resource for investigating Malawi’s increasing seismic risks and provides a framework for incorporating active fault data into seismic hazard assessment in other tectonically active regions.

scholarly journals A systems-based approach to parameterise seismic hazard in regions with little historical or instrumental seismicity: active fault and seismogenic source databases for southern Malawi

Solid Earth ◽  
2021 ◽  
Vol 12 (1) ◽  
pp. 187-217
Author(s):  
Jack N. Williams ◽  
Hassan Mdala ◽  
Åke Fagereng ◽  
Luke N. J. Wedmore ◽  
Juliet Biggs ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Active Fault ◽  
Slip Rate ◽  
Fault Slip ◽  
East African ◽  
Slip Rates ◽  
Fault Slip Rates ◽  
Seismogenic Source ◽  
Recurrence Intervals ◽  
African Rift

Abstract. Seismic hazard is commonly characterised using instrumental seismic records. However, these records are short relative to earthquake repeat times, and extrapolating to estimate seismic hazard can misrepresent the probable location, magnitude, and frequency of future large earthquakes. Although paleoseismology can address this challenge, this approach requires certain geomorphic setting, is resource intensive, and can carry large inherent uncertainties. Here, we outline how fault slip rates and recurrence intervals can be estimated by combining fault geometry, earthquake-scaling relationships, geodetically derived regional strain rates, and geological constraints of regional strain distribution. We apply this approach to southern Malawi, near the southern end of the East African Rift, and where, although no on-fault slip rate measurements exist, there are constraints on strain partitioning between border and intra-basin faults. This has led to the development of the South Malawi Active Fault Database (SMAFD), a geographical database of 23 active fault traces, and the South Malawi Seismogenic Source Database (SMSSD), in which we apply our systems-based approach to estimate earthquake magnitudes and recurrence intervals for the faults compiled in the SMAFD. We estimate earthquake magnitudes of MW 5.4–7.2 for individual fault sections in the SMSSD and MW 5.6–7.8 for whole-fault ruptures. However, low fault slip rates (intermediate estimates ∼ 0.05–0.8 mm/yr) imply long recurrence intervals between events: 102–105 years for border faults and 103–106 years for intra-basin faults. Sensitivity analysis indicates that the large range of these estimates can best be reduced with improved geodetic constraints in southern Malawi. The SMAFD and SMSSD provide a framework for using geological and geodetic information to characterise seismic hazard in regions with few on-fault slip rate measurements, and they could be adapted for use elsewhere in the East African Rift and globally.

scholarly journals A systems-based approach to parameterise seismic hazard in regions with little historical or instrumental seismicity: The South Malawi Active Fault Database

2020 ◽  
Author(s):  
Jack N. Williams ◽  
Hassan Mdala ◽  
Åke Fagereng ◽  
Luke N. J. Wedmore ◽  
Juliet Biggs ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Active Fault ◽  
Slip Rate ◽  
Fault Slip ◽  
Slip Rates ◽  
Scaling Relationships ◽  
Regional Strain ◽  
Earthquake Scaling ◽  
Recurrence Intervals ◽  
Fault Slip Rate

Abstract. Seismic hazard is frequently characterised using instrumental seismic records. However, in regions where the instrumental record is short relative to earthquake repeat times, extrapolating it to estimate seismic hazard can misrepresent the probable location, magnitude, and frequency of future large earthquakes. Although paleoseismology can address this challenge, this approach requires certain geomorphic settings and carries large inherent uncertainties. Here, we outline how fault slip rates and recurrence intervals can be estimated through an approach that combines fault geometry, earthquake-scaling relationships, geodetically derived regional strain rates, and geological constraints of regional strain distribution. We then apply this approach to the southern Malawi Rift where, although no on-fault slip rate measurements exist, there are theoretical and observational constraints on how strain is distributed between border and intrabasinal faults. This has led to the development of the South Malawi Active Fault Database (SMAFD), the first database of its kind in the East African Rift System (EARS) and designed so that the outputs can be easily incorporated into Probabilistic Seismic Hazard Analysis. We estimate earthquake magnitudes of MW 5.4–7.2 for individual fault sections in the SMAFD, and MW 6.0–7.8 for whole fault ruptures. These potentially high magnitudes for continental normal faults reflect southern Malawi's 11–140 km long faults and thick (30–35 km) seismogenic crust. However, low slip rates (intermediate estimates 0.05–0.8 mm/yr) imply long recurrence intervals between events: 102–105 years for border faults and 103–106 years for intrabasinal faults. Sensitivity analysis indicates that the large range of these estimates can be reduced most significantly from an improved understanding of the rate and partitioning of rift-extension in southern Malawi, earthquake scaling relationships, and earthquake rupture scenarios. Hence these are critical areas for future research. The SMAFD provides a framework for using geological and geodetic information to characterize seismic hazard in low strain rate settings with few on-fault slip rate measurements, and could be adapted for use elsewhere in the EARS or globally.

Italy’s Database of Individual Seismogenic Sources (DISS), 20 years on: lessons learned from the construction of a SHA-oriented fault database

2020 ◽  
Author(s):  
Gianluca Valensise ◽  
Roberto Basili ◽  
Pierfrancesco Burrato ◽  
Umberto Fracassi ◽  
Vanja Kastelic ◽  
...  
Keyword(s):  
Regional Scale ◽  
Active Faults ◽  
Lessons Learned ◽  
Early Years ◽  
Geometrical Parameters ◽  
Slip Rates ◽  
Magnitude Distribution ◽  
Seismogenic Sources ◽  
Seismogenic Source ◽  
Geodynamic Models

<p>The prototype version of the DISS was launched and published in July 2000. Twenty years later we present an appraisal of how the database started off, how it evolved, and how it served the seismological and engineering communities.</p><p>During the early years of its development we learned that the three fundamental requirements of any SHA-oriented fault database are:</p><p>1) the capacity to represent seismogenic sources in 3D, thus providing a standardized quantitative basis for subsequent SHA calculations and stressing the hierarchy relationships among all existing active faults;</p><p>2) the completeness, i.e. the ability to portray the vast majority of seismogenic sources existing in the region of relevance and to progressively address the emerging lack of knowledge;</p><p>3) the reliability of the geometrical parameters of each seismogenic source and of the relevant slip and strain rates, and the ability to assess the associated uncertainties.</p><p>Given these requirements, we found it hard to build a database around existing studies of individual large faults, which are often carried out for non-SHA purposes; as such they do not necessarily involve a 3D delineation and a hierarchization of the master fault. Furthermore, most published studies concern surface-breaking faults occurring onshore; they are most relevant to surface faulting hazard, but in shaking-oriented SHA they are less crucial than deeper, hidden faults.</p><p>We initially developed the concept of “Individual Seismogenic Source” (ISS), a simplified but geometrically coherent representation of the presumed causative fault of the largest earthquakes of the investigated region. An ISS is based on original observations, seismological/geophysical evidence, and literature data. Since large portions of the Italian territory are characterized by blind or hidden faulting, we developed strategies based on the analysis of geomorphic evidence for cumulative tectonic strain, on the reappraisal of commercial seismic lines and subsurface data, and on geological and geodetic evidence.</p><p>In 2005 we introduced the “Composite Seismogenic Sources” (CSSs): generalized, unsegmented sources designed to increase the database geographic coverage and completeness, based on the same type of information used for the ISSs and on regional-scale synopses of ongoing tectonic strain. Their identification was progressively extended to offshore areas, often scarcely considered in traditional fault mapping. In 2015 we also introduced the 3D definition of the subduction slabs and associated interfaces for the whole Mediterranean region.</p><p>The ISSs are routinely used in engineering applications aimed at investigating the shaking scenario associated with known earthquakes or well-identified quiescent fault segments. In contrast, the CSSs are not assumed to be capable of a specific-size earthquake; as such, they can be used in any standard PSHA procedure after estimating their activity rate and frequency magnitude distribution, based on tectonic slip rates integrated with the record of past earthquakes and GPS-determined strains, or derived from regional-scale geodynamic models.</p><p>DISS also served as a template for developing EDSF,  the European Database of Seismogenic Faults. Over the years, DISS and EDSF have become the basic geological input for PSHA and PTHA, both at Italian scale (MPS04, MPS19, MPTS19) and European scale (ESHM13, ESHM20, NEAMTHM18).</p>

scholarly journals Database of Active Faults in Slovenia: Compiling a New Active Fault Database at the Junction Between the Alps, the Dinarides and the Pannonian Basin Tectonic Domains

2021 ◽  
Vol 9 ◽  
Author(s):  
Jure Atanackov ◽  
Petra Jamšek Rupnik ◽  
Jernej Jež ◽  
Bogomir Celarc ◽  
Matevž Novak ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Input Data ◽  
Active Fault ◽  
Pannonian Basin ◽  
Mass Movement ◽  
Active Faults ◽  
Fault Slip ◽  
Slip Rates ◽  
Fault Slip Rates ◽  
The Alps

We present the compilation of a new database of active faults in Slovenia, aiming at introducing geological data for the first time as input for a new national seismic hazard model. The area at the junction of the Alps, the Dinarides, and the Pannonian Basin is moderately seismically active. About a dozen Mw > 5.5 earthquakes have occurred across the national territory in the last millennium, four of which in the instrumental era. The relative paucity of major earthquakes and low to moderate fault slip rates necessitate the use of geologic input for a more representative assessment of seismic hazard. Active fault identification is complicated by complex regional structural setting due to overprinting of different tectonic phases. Additionally, overall high rates of erosion, denudation and slope mass movement processes with rates up to several orders of magnitude larger than fault slip rates obscure the surface definition of faults and traces of activity, making fault parametrization difficult. The presented database includes active, probably active and potentially active faults with trace lengths >5 km, systematically compiled and cataloged from a vast and highly heterogeneous dataset. Input data was mined from published papers, reports, studies, maps, unpublished reports and data from the Geological Survey of Slovenia archives and dedicated studies. All faults in the database are fully parametrized with spatial, geometric, kinematic and activity data with parameter descriptors including data origin and data quality for full traceability of input data. The input dataset was compiled through an extended questionnaire and a set of criteria into a homogenous database. The final database includes 96 faults with 240 segments and is optimized for maximum compatibility with other current maps of active faults at national and EU levels. It is by far the most detailed and advanced map of active faults in Slovenia.

scholarly journals Active Fault Systems in the Inner Northwest Apennines, Italy: A Reappraisal One Century after the 1920 Mw ~6.5 Fivizzano Earthquake

Geosciences ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 139
Author(s):  
Giancarlo Molli ◽  
Isabelle Manighetti ◽  
Rick Bennett ◽  
Jacques Malavieille ◽  
Enrico Serpelloni ◽  
...  
Keyword(s):  
Large Scale ◽  
Active Fault ◽  
Active Faults ◽  
Dense Network ◽  
Earthquake Fault ◽  
Slip Rates ◽  
Fault Systems ◽  
Seismological Data ◽  
Seismogenic Faults

Based on the review of the available stratigraphic, tectonic, morphological, geodetic, and seismological data, along with new structural observations, we present a reappraisal of the potential seismogenic faults and fault systems in the inner northwest Apennines, Italy, which was the site, one century ago, of the devastating Mw ~6.5, 1920 Fivizzano earthquake. Our updated fault catalog provides the fault locations, as well as the description of their architecture, large-scale segmentation, cumulative displacements, evidence for recent to present activity, and long-term slip rates. Our work documents that a dense network of active faults, and thus potential earthquake fault sources, exists in the region. We discuss the seismogenic potential of these faults, and propose a general tectonic scenario that might account for their development.

scholarly journals Tectonic Geomorphology and Paleoseismology of the Sharkhai fault: a new source of seismic hazard for Ulaanbaatar (Mongolia)

2021 ◽  
Author(s):  
Abeer Al-Ashkar ◽  
Antoine Schlupp ◽  
Matthieu Ferry ◽  
Ulziibat Munkhuu
Keyword(s):  
Seismic Hazard ◽  
Scaling Laws ◽  
Active Faults ◽  
Seismic Hazard Assessment ◽  
Capital City ◽  
Tectonic Geomorphology ◽  
Time Interval ◽  
Coseismic Slip ◽  
Slip Rates ◽  
Large Earthquakes

Abstract. We present new constraints from tectonic geomorphology and paleoseismology along the newly discovered Sharkhai fault near the capital city of Mongolia. Detailed observations from high resolution Pleiades satellite images and field investigations allowed us to map the fault in detail, describe its geometry and segmentation, characterize its kinematics, and document its recent activity and seismic behavior (cumulative displacements and paleoseismicity). The Sharkhai fault displays a surface length of ~40 km with a slightly arcuate geometry, and a strike ranging from N42° E to N72° E. It affects numerous drainages that show left-lateral cumulative displacements reaching 57 m. Paleoseismic investigations document the faulting and deposition record for the last ~3000 yr and reveal that the penultimate earthquake (PE) occurred between 1515 ± 90 BC and 945 ± 110 BC and the most recent event (MRE) occurred after 860 ± 85 AD. The resulting time interval of 2080 ± 470 years is the first constraint on the Sharkhai fault for large earthquakes. On the basis of our mapping of the surface rupture and the resulting segmentation analysis, we propose two possible scenarios for large earthquakes with likely magnitudes between 6.4 ± 0.2 and 7.1 ± 0.2. Furthermore, we apply scaling laws to infer coseismic slip values and derive preliminary estimates of long-term slip rates between 0.2 ± 0.2 and 1.0 ± 0.5 mm/y. Finally, we propose that these original observations and results from a newly discovered fault should be taken into account for the seismic hazard assessment for the city of Ulaanbaatar and help build a comprehensive model of active faults in that region.

scholarly journals Integrating faults and past earthquakes into a probabilistic seismic hazard model for peninsular Italy

2017 ◽  
Vol 17 (11) ◽  
pp. 2017-2039 ◽  
Author(s):  
Alessandro Valentini ◽  
Francesco Visini ◽  
Bruno Pace
Keyword(s):  
Seismic Hazard ◽  
Seismic Activity ◽  
Grid Point ◽  
Active Faults ◽  
Gaussian Model ◽  
Probabilistic Seismic Hazard ◽  
Maximum Magnitude ◽  
Slip Rates ◽  
Earthquake Sources ◽  
The Impact

Abstract. Italy is one of the most seismically active countries in Europe. Moderate to strong earthquakes, with magnitudes of up to ∼ 7, have been historically recorded for many active faults. Currently, probabilistic seismic hazard assessments in Italy are mainly based on area source models, in which seismicity is modelled using a number of seismotectonic zones and the occurrence of earthquakes is assumed uniform. However, in the past decade, efforts have increasingly been directed towards using fault sources in seismic hazard models to obtain more detailed and potentially more realistic patterns of ground motion. In our model, we used two categories of earthquake sources. The first involves active faults, and using geological slip rates to quantify the seismic activity rate. We produced an inventory of all fault sources with details of their geometric, kinematic, and energetic properties. The associated parameters were used to compute the total seismic moment rate of each fault. We evaluated the magnitude–frequency distribution (MFD) of each fault source using two models: a characteristic Gaussian model centred at the maximum magnitude and a truncated Gutenberg–Richter model. The second earthquake source category involves grid-point seismicity, with a fixed-radius smoothed approach and a historical catalogue were used to evaluate seismic activity. Under the assumption that deformation is concentrated along faults, we combined the MFD derived from the geometry and slip rates of active faults with the MFD from the spatially smoothed earthquake sources and assumed that the smoothed seismic activity in the vicinity of an active fault gradually decreases by a fault-size-driven factor. Additionally, we computed horizontal peak ground acceleration (PGA) maps for return periods of 475 and 2475 years. Although the ranges and gross spatial distributions of the expected accelerations obtained here are comparable to those obtained through methods involving seismic catalogues and classical zonation models, the spatial pattern of the hazard maps obtained with our model is far more detailed. Our model is characterized by areas that are more hazardous and that correspond to mapped active faults, while previous models yield expected accelerations that are almost uniformly distributed across large regions. In addition, we conducted sensitivity tests to determine the impact on the hazard results of the earthquake rates derived from two MFD models for faults and to determine the relative contributions of faults versus distributed seismic activity. We believe that our model represents advancements in terms of the input data (quantity and quality) and methodology used in the field of fault-based regional seismic hazard modelling in Italy.

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

2017 ◽  
Vol 17 (8) ◽  
pp. 1447-1459 ◽  
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.

scholarly journals Time independent seismic hazard analysis of Greece deduced from Bayesian statistics

2003 ◽  
Vol 3 (1/2) ◽  
pp. 129-134 ◽  
Author(s):  
T. M. Tsapanos ◽  
G. A. Papadopoulos ◽  
O. Ch. Galanis
Keyword(s):  
Seismic Hazard ◽  
Bayesian Statistics ◽  
Hazard Analysis ◽  
Earthquake Occurrence ◽  
Time Intervals ◽  
Seismogenic Sources ◽  
Earthquake Resistant Design ◽  
Seismogenic Source ◽  
Earthquake Resistant ◽  
Of Greece

Abstract. A Bayesian statistics approach is applied in the seismogenic sources of Greece and the surrounding area in order to assess seismic hazard, assuming that the earthquake occurrence follows the Poisson process. The Bayesian approach applied supplies the probability that a certain cut-off magnitude of Ms = 6.0 will be exceeded in time intervals of 10, 20 and 75 years. We also produced graphs which present the different seismic hazard in the seismogenic sources examined in terms of varying probability which is useful for engineering and civil protection purposes, allowing the designation of priority sources for earthquake-resistant design. It is shown that within the above time intervals the seismogenic source (4) called Igoumenitsa (in NW Greece and west Albania) has the highest probability to experience an earthquake with magnitude M > 6.0. High probabilities are found also for Ochrida (source 22), Samos (source 53) and Chios (source 56).

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

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

<p>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 <sup>36</sup>Cl 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.</p><p> </p>

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