Aseismic Deformation During the 2014 Mw 5.2 Karonga Earthquake, Malawi, From Satellite Interferometry and Earthquake Source Mechanisms. Issue 22 (23rd November 2020)

• Record Type: Journal Article
• Authors: Zheng, Whyjay; Oliva, Sarah Jaye; Ebinger, Cynthia; Pritchard, Matthew E.
• Is Part Of: Geophysical research letters. Volume 47:Issue 22(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-23
• Subjects: InSAR -- seismicity -- rift system -- aftershocks -- aseismic deformation -- vertical strain partitioning; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL090930
• Languages: English
• ISSNs: 0094-8276

Abstract

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. Key Points: InSAR implies deformation from a shallow depth (3–6 km), while seismicity suggests a deeper origin (11–13 km) Aftershock source mechanisms suggest fault intersection along steep east and west dipping fault planes Fractured zone might separate shallow aseismic and deeper seismic strain beneath sedimentary basin