Post-diking deformation in Harrat Lunayyir (Saudi Arabia) from InSAR

<p>Magmatic intrusions often produce ground deformation that can be studied by geodetic techniques. In the past two decades, many volcanic dike and sill emplacements (sometimes associated to eruptions) in different tectonic settings have been analyzed through InSAR. However, in only a few cases, the post-intrusive behavior has been studied. Here we analyze the post-diking deformation in Harrat Lunayyir, which is a mononogenetic volcanic field located in western Saudi Arabia on the eastern margin of the Red Sea Rift.</p><p>Between April and July 2009, an intensive seismic swarm occurred in the area with many earthquakes above magnitude 4 and the largest earthquake of M<sub>w</sub> 5.7. InSAR data showed that the earthquake swarm was triggered by the emplacement of a dike intrusion that stopped only ~1 km below the surface. Dike length was estimated to be ~7 km and with a maximum opening 4 m. Above the intrusion, a ~10 km long and ~5 km wide graben formed during the activity with up to 1 m of fault slip on the border faults. In the post diking phase up to present, micro-sesmicity (0<Ml<3.5) has been continuously registered in the graben region gradually fading out either in terms of earthquake rate and energy release.</p><p>In February 2017, a new seismic swarm occurred ~60 km north of Harrat Lunayyir and another swarm started in October 2018, about 30 km southwest of the volcanic field. Both swarms are still ongoing with a few events per week and M<sub>l</sub><3.5. By using Sentinel-1 images, acquired during the period 2015-2019, we derived deformation rate maps for the entire Harrat Lunayyir volcanic field. No ground deformation was detected at the locations of the recent seismic swarms, and a thorough analysis of seismic signals excludes the swarms were caused by new magmatic intrusions. However, within the Harrat Lunayyir graben region, we noticed a steady and long-lasting subsidence of ~1 mm/yr. During the 2015-2019 period, the total seismic moment release would only be able to accommodate less than 0.1 mm of the observed subsidence and thus the current post-diking deformation is mainly aseismic.</p><p>In order to reconstruct the entire post-diking deformation history in Harrat Lunayyir we also analyze older available SAR images (Envisat, ALOS, TerraSAR-X, TanDEM-X). Our preliminary results show that the subsidence rate in the graben area was faster just after the intrusion (few cm in two months) but then rapidly decayed as well as the seismicity. We are now investigating different processes that can cause post-diking deformation, such as residual opening of the dike, post-diking settlement of faults and fractures, release of gases into fractures, cooling of the dike, and post-diking viscoelastic relaxation. Modelling of the deformation source will contribute to the understanding on which of these post-diking processes might be the dominant one in Harrat Lunayyir.</p>

Thermal mapping of the 2009 dyke emplacement at Harrat Lunayyir, Saudi Arabia

The 2009 seismic episode at Harrat Lunayyir signalled a renewed geohazard and resulted in a regional dyke that propagated to a very shallow depth (a few hundred metres) below the surface. Since then, there has been an extensive research debate over the potential links between the volcanic/intrusive activity and tectonic processes, particularly because the earthquake swarm and dyke propagation did not eventually result in an eruption. The current study seeks to estimate the relative changes in surface temperatures as a means for detecting an impending dyke-fed eruption or, alternatively, dyke arrest at a shallow depth. An analysis of thermal (infrared) data with a focus on the spatial distribution of land surface temperatures over a longer period of observation may help reveal the link between volcanic activity and dyke propagation. Here, the land surface temperature changes in the centre of Harrat Lunayyir were recorded when the 2009 dyke was propagating toward the surface. The spatial distribution of the land surface temperatures in the area indicated the segmentation of the dyke and suggested the segments were arrested at somewhat different depths below the surface.