Aseismic Deformation During the 2014 M_w 5.2 Karonga Earthquake, Malawi, From Satellite Interferometry and Earthquake Source Mechanisms

Abstract

Aseismic deformation has been suggested as a mechanism to release the accumulated strain in rifts. However, the fraction and the spatial distribution of the aseismic strain are poorly constrained during amagmatic episodes. Using Sentinel-1 interferograms, we identify the surface deformation associated with the 2014 M w 5.2 Karonga earthquake, Malawi, and perform inversions for fault geometry. We also analyze aftershocks and find a variety of source mechanisms within short timescales. A significant discrepancy in the earthquake depth determined by geodesy (3-6 km) and seismology (11-13 km) exists, although both methods indicate M w 5.2. We propose that the surface deformation is caused by aseismic slip from a shallow depth. This vertical partitioning from seismic to aseismic strain is accommodated by intersecting dilatational faults in the shallow upper crust and sedimentary basin, highlighting the importance of considering aseismic deformation in active tectonics and time-averaged strain patterns, even in rifts with little volcanism. Plain Language Summary During the early stages of continental rifting, some accumulated tectonic energy is released with little or no earthquake activity (i.e., aseismic strain). This is most often observed in places where magma intrusion occurs. However, in rift basins lacking evidence of magma intrusion, how much and where the aseismic strain is released remains largely unknown. We investigate a magnitude 5.2 earthquake that occurred in 2014 near Karonga, Malawi, using both local seismic data and satellite ground deformation data. Our analyses show that although the main earthquake and other aftershocks ruptured at ∼12 km deep, the surface deformation is caused by an aseismic source from a shallower region (3-6 km deep). The aftershocks occur seconds to minutes apart and have different rupture sources, suggesting a highly damaged zone. The depth discrepancy between earthquake locations and the aseismic energy source might be caused by a weak, damaged layer where many faults intersect. Since intersecting faults are common in rift systems, our study suggests that this depth discrepancy might be common as well, indicating a large portion of energy released as aseismic strain in a continental rifting system.

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

“…This is consistent with observed patterns of seismicity (Ebinger et al, 2019;Stevens et al, 2021) and the velocity weakening behaviour of representative basement samples from Malawi in deformation experiments at lower crustal pressures and temperatures (Hellebrekers et al, 2019). However, some shallow (depths <6 km) aseismic deformation was observed in northern Malawi following the 2015 MW 5.2 earthquake (Zheng et al, 2020). This could be addressed by dividing the MSSD recurrence intervals by a representative estimate of Malawi's crust's coupling coefficient (Bird and Liu, 2007).…”

The Malawi Active Fault Database: An Onshore‐Offshore Database for Regional Assessment of Seismic Hazard and Tectonic Evolution

“…A more detailed assessment of the seismogenic properties of faults in Malawi is contained in the Malawi Seismogenic Source Database, which is currently in development (Williams, Wedmore, et al., 2021). Nevertheless, it is clear that given low regional extensional rates (0.5–2 mm/yr; Saria et al., 2014; Stamps et al., 2018; Wedmore et al., 2021), large magnitude earthquakes are rare in Malawi (fault recurrence intervals ∼1,000–20,000 years, Hodge et al., 2015; Shillington et al., 2020; Williams, Mdala, et al., 2021), and the occurrence of these events may be further reduced if faults slip aseismically as was observed following an M W 5.2 earthquake in northern Malawi in 2014 (Zheng et al., 2020).…”

Fault-based Probabilistic Seismic Hazard Analysis in Regions with Low Strain Rates and a Thick Seismogenic Layer: A Case Study from Malawi

“…A physical interpretation of the difference between this Ṁ0 estimate and those from the event catalogs (1.2 − 1.8 × 10 18 Nm/yr, Fig. 6) is that shallow aseismic deformation (Zheng et al, 2020) and off-fault events accommodate the deformation required to prevent space problems that would otherwise arise from normal fault obliquity and narrow fault widths in Malawi.…”

“…With regards to the first scenario, the nucleation of earthquakes throughout Malawi's seismogenic layer (Ebinger et al, 2019;Stevens et al, 2021), and energetic slowly decaying aftershock sequences implies overall highly coupled fault (Ben-Zion, 2008;Gaherty et al, 2019). However, some seismic moment may also be released by postseismic shallow afterslip (depths <5-10 km), as for example was observed following a M W 5.2 earthquake near Karonga in 2014 (Zheng et al, 2020) and in the M W 7.0 Machaze earthquake in Mozambique (Copley, Hollingsworth, & Bergman, 2012;Lloyd, Biggs, & Copley, 2019) .…”