A Geodetic Strain Rate Model for the East African Rift System

TLDR

Sub‑Saharan Africa contains the East African Rift System, an active divergent boundary between the Nubian and Somalian plates where strain is largely accommodated along the edges of three sub‑plates. The authors aim to develop an improved geodetic strain‑rate field for Sub‑Saharan Africa that incorporates an expanded velocity dataset, seismicity‑guided deformation zones, and updated block‑rotation constraints to better constrain tectonic deformation and seismic‑hazard assessment. SSA‑GSRM v.1.0 covers 22°–55.5° E and –52°–20° N on a 0.25° × 0.2° grid, using rigid block rotations as constraints and fitting bicubic Bessel splines to a new geodetic velocity solution to derive strain rates, velocities, and vorticity from the interpolated velocity‑gradient tensor field. Compared with the Global Geodetic Strain‑Rate model v2.1, SSA‑GSRM v.1.0 reveals previously unresolved spatial heterogeneities that identify zones of elevated seismic risk.

Abstract

Here we describe the new Sub-Saharan Africa Geodetic Strain Rate Model v.1.0 (SSA-GSRM v.1.0), which provides fundamental constraints on long-term tectonic deformation in the region and an improved seismic hazards assessment in Sub-Saharan Africa. Sub-Saharan Africa encompasses the East African Rift System, the active divergent plate boundary between the Nubian and Somalian plates, where strain is largely accommodated along the boundaries of three subplates. We develop an improved geodetic strain rate field for sub-Saharan Africa that incorporates 1) an expanded geodetic velocity field, 2) redefined regions of deforming zones guided by seismicity distribution, and 3) updated constraints on block rotations. SSA-GSRM v.1.0 spans longitudes 22° to 55.5° and latitudes -52° to 20° with 0.25° (longitude) by 0.2° (latitude) spacing. For plates/sub-plates, we assign rigid block rotations as constraints on the strain rate calculation that is determined by fitting bicubic Bessel splines to a new geodetic velocity solution for an interpolated velocity gradient tensor field. We derive strain rates, velocities, and vorticity rates from the velocity gradient tensor field. A comparison with the Global Geodetic Strain Rate model v2.1 reveals regions of previously unresolved spatial heterogeneities in geodetic strain rate distribution, which indicates zones of elevated seismic risk.