Incorporating geologic and geodetic data into probabilistic seismic hazard assessment of infrequent large magnitude earthquakes: insights from southern Malawi

Large magnitude (M_W 7-8) earthquakes are known to occur in the East African Rift (EAR). However, low EAR extension rates (1-5 mm/yr) imply that such high-severity events are rare. Furthermore, they may not be adequately characterised by previous EAR seismic hazard studies as these typically only considered its short (<65 years) instrumental record. Using southern Malawi as a case study, we outline how geologic and geodetic data can be combined to incorporate these infrequent events into probabilistic seismic hazard assessment (PSHA). First, we estimate fault slip rates from a systems-based approach constrained by active fault maps, geodetic rift-extension rates, and observations of strain distribution between border and intrabasinal faults in continental rifts . We then combine slip rates with fault geometry and earthquake scaling relationships to assess the likely magnitude and frequency of different rupture scenarios across southern Malawi's segmented faults. This fault source model is then integrated into PSHA by simulating a stochastic event catalogue (e.g. 1 million years or longer), in which earthquakes occur randomly following a time independent Poisson process. Finally, hazard curves and maps are generated from this catalogue through the random selection of multiple ground motion models.

Compared to hazard estimates that consider the instrumental seismic record alone, the annual probability of exceedance for a given level of peak ground acceleration in southern Malawi is potentially doubled . This discrepancy is most pronounced at long vibration periods and low probabilities of exceedance. This result demonstrates that the instrumental record alone does not adequately characterise the hazard of low-probability (recurrence intervals 10³-10⁴ years) M_W 7-8 events in southern Malawi. Significant challenges remain in fully quantifying uncertainty in our fault source data, particularly in the rate and partitioning of rift-extension, and the relative probabilities of different rupture scenarios. Nevertheless, we suggest that incorporating geological and geodetic data improves assessments of southern Malawi's seismic hazard. This approach could be adapted for hazard assessment of infrequent earthquakes in other low strain rate regions with short instrumental records and active fault maps.