<p>The two most widespread geodynamic signals in Iceland are caused by glacio isostatic adjustment (GIA; up to 4.5 cm/yr vertical motion) and tectonic plate spreading (approximately 1.9 cm/yr horizontal motion). GPS measurements of crustal deformation started in Iceland in 1986 and annually tens to hundreds of benchmarks are re-measured. Many of these surveys are on local scales, but the ISNET campaigns in 1993, 2004, and 2016 are the only island-wide efforts. Continuous GPS (cGPS) measurements started in 1995 and now over 100 cGPS stations are running. The cGPS allows for excellent quantification of seasonal variations in position with amplitude up to several cm closest to the glaciers, driven mainly by seasonal snowload. Frequent observations also help to observe temporal changes in uplift rates and correlate to glacier mass balance. In recent years InSAR has been applied to obtain both local signals (e.g., due to glacial surges) and island-wide estimates of GIA and plate motion. However, InSAR does not work under the glaciers where we expect the largest uplift. Regular GPS measurements at several nunataks on Vatnaj&#246;kull started in 2008 and provide the only intra-glacier GIA observations in Iceland. Going further backwards in time is a challenge and relies on local levelling where relative uplift rates can be compared to current relative uplift rates to infer the temporal evolution.</p><p>During 1993-2004 the average observed uplift rates reached at most around 2 cm/yr and were likely at its lowest in the early 1990s, lower than during 1959-1991. During 2004-2010 the uplift rates increased on average by 70% compared to the previous time period. A thin layer of ash from the 2010 Eyjafjallaj&#246;kull eruption enhanced the melting rates and is clearly seen as enhanced uplift rates during 2010-2012. Until 2014 the uplift rates remained high. In 2014 the average uplift rates lowered by around 20%. Comparable changes are observed in the horizontal deformation field. Overall, recent changes in GIA broadly follow changes in climate and mass balance. The first part of the 90s was cold and glaciers in Iceland were overall in equilibrium or gaining a bit of mass. After 1995 the glaciers started losing considerable mass every year. From 2011 the mass loss decreased; in 2015 there was a net mass gain, and in 2017 and 2018 the mass balance was close to equilibrium. The highly variable deformation rates call for a re-evaluation of the current GIA models, working towards a time-dependent response that can be applied to regional deformation studies.</p>
GPS measurements of regional deformation in Southern California: Some constraints on performance