No evidence of slow slip events but anomalous transients in horizontal displacement time series along the Sumatra-Andaman subduction zone

In the decades following the occurrence of the 2004 Sumatra-Andaman earthquake, the epicentral region has been enduring large-scale postseismic deformation as evident from continuous GPS measurements in the Andaman-Nicobar region. Although the contribution of postseismic deformation continues to decrease exponentially, its magnitude is so large that it could mask some low amplitude deformation. The deformation data also exhibit small seasonal variations due to the hydrological cycle. While searching for slow slip events (SSE) in the GPS data from the Andaman region, we observed two unprecedented large-amplitude, multi-week transients in 2020 and then again in 2023 at a continuous GPS site, HBAY in Little Andaman. These transient excursions are not linked with any known earthquakes, volcanic activity, or hydrological processes. The possibility of unmodelled ocean tides or some other regional large-scale anomaly is also ruled out as such transients are absent at other GPS sites in the Andaman-Nicobar region, implying that the source of the transient is local. The possibility of the signal being related to some tectonic source, such as a SSE, is remote as the majority of the movement appears to be recoverable, thus confirming earlier studies on the absence of short-term SSEs in the Sumatra-Andaman region.