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Preprints
https://doi.org/10.5194/se-2020-104
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2020-104
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 Jul 2020

14 Jul 2020

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A revised version of this preprint is currently under review for the journal SE.

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

Jack N. Williams1, Hassan Mdala2, Åke Fagereng1, Luke N. J. Wedmore3, Juliet Biggs3, Zuze Dulanya4, Patrick Chindandali5, and Felix Mphepo2 Jack N. Williams et al.
  • 1School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK
  • 2Geological Survey Department, Mzuzu Regional Office, Mzuzu, Malawi
  • 3School of Earth Sciences, University of Bristol, Bristol, UK
  • 4Geography and Earth Sciences Department, University of Malawi, Zomba, Malawi
  • 5Geological Survey Department, Zomba, Malawi

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.

Jack N. Williams et al.

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Jack N. Williams et al.

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Short summary
Earthquake hazard is often characterised using instrumental records. However, this record may not accurately forecast the location and magnitude of future earthquakes as it is short relative to their frequency along geologic faults (100 vs. 1000's of years). Here, we describe an approach to assess this hazard using fault maps and GPS data. By applying this to southern Malawi, we find this region may experience rare (1 in 10 000 years) M > 7 earthquakes that pose a risk to its growing population.
Earthquake hazard is often characterised using instrumental records. However, this record may...
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