Assessing infrequent large earthquakes using geomorphology and geodesy: the Malawi Rift

Abstract

In regions with large, mature fault systems, a characteristic earthquake model may be more appropriate for modelling earthquake occurrence than extrapolating from a short history of small, instrumentally observed earthquakes using the Gutenberg-Richter scaling law. We illustrate how the geomorphology and geodesy of the Malawi Rift, a region with large seismogenic thicknesses, long fault scarps, and slow strain rates, can be used to assess hazard probability levels for large infrequent earthquakes. We estimate potential earthquake size using fault length and recurrence intervals from plate motion velocities and generate a synthetic catalogue of events. Since it is not possible to determine from the geomorphological information if a future rupture will be continuous (7.4 B M W B 8.3 with recurrence intervals of 1,000-4,300 years) or segmented (6.7 B M W B 7.7 with 300-1,900 years), we consider both alternatives separately and also produce a mixed catalogue. We carry out a probabilistic seismic hazard assessment to produce regional-and site-specific hazard estimates. At all return periods and vibration periods, inclusion of fault-derived parameters increases the predicted spectral acceleration above the level predicted from the instrumental catalogue alone, with the most significant changes being in close proximity to the fault systems and the effect being more significant at longer vibration periods. Importantly, the results indicate that standard probabilistic seismic hazard analysis (PSHA) methods using short instrumental records alone tend to underestimate the seismic hazard, especially for the most damaging, extreme magnitude events. For many developing countries in Africa and elsewhere, which are experiencing

Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift

“…The long border faults within thick strong crust may be capable of producing M w ∼ 8 earthquakes (Jackson and Blenkinsop, 1997;Hodge et al, 2015). As demonstrated by the Karonga earthquake sequence (Biggs et al, 2010;Fagereng, (Farr et al, 2007) and lake depth from Lyons et al (2011).…”

“…Finally, and very importantly, the rifting process is associated with significant seismic and volcanic hazards for the densely populated rift valley (e.g., Fontijn et al, 2011;Hodge et al, 2015). The long border faults within thick strong crust may be capable of producing M w ∼ 8 earthquakes (Jackson and Blenkinsop, 1997;Hodge et al, 2015).…”

“…The long border faults within thick strong crust may be capable of producing M w ∼ 8 earthquakes (Jackson and Blenkinsop, 1997;Hodge et al, 2015). As demonstrated by the Karonga earthquake sequence (Biggs et al, 2010; Fagereng, et al, 2015), active intrabasin faults also pose a hazard to people living near the lakeshore; such faults could potentially be associated with near-field tsunamis.…”

Seismic Risk Assessment of Urban and Rural Settlements around Lake Malawi

“…Conventionally, seismic hazard assessment was done by considering an available instrumental earthquake catalog only (Midzi et al, 1999), which is short in duration in comparison with average recurrence periods of the geological faults (e.g., 50 years versus thousands of years). Given the seismic potential of well-matured geological faults around Lake Malawi, Hodge et al (2015) assessed regional seismic hazard in Malawi by incorporating geological and geomorphological information. In their study, possibility of having Mw7.0-8.0 earthquakes was formally included in probabilistic seismic hazard analysis by exploring different hypotheses regarding different rupture scenarios of the major faults and by evaluating impact of including large earthquakes.…”

Epistemic uncertainties and natural hazard risk assessment. 1. A review of different natural hazard areas

“…the East African Rift) have extended the modelling basis for regional seismicity beyond historical and instrumental earthquake catalogues by using information from mapped geological faults and geodeticallydetermined rates of strain accumulation (e.g. Hodge et al, 2015). It is noteworthy that while such PSHA assessments remain significantly uncertain, they may be better able to capture potential extreme (surprise) events.…”