Monitoring of Salt Diapir-Related Land Deformation in an Urban Setting

Abstract:

Salt diapirs are rarely preserved on the surface and seldom considered as environmental hazards. The Jazan city diapir (JZD) is located in southwest Saudi Arabia along the Red Sea coastal plain and outcrops in the middle of a rapidly growing port city. The intrusion of the diapir (~ 2 km²) into the overlying caprock continues to cause uneven surfaces, compromised building foundations and infrastructure, dissolution sinkholes, and limits the expansion of the city along the coastline. Using integrated Interferometric Synthetic Aperture Radar (InSAR) datasets (Envisat: 2003–2009; Sentinel-1: 2014–2018), terrestrial gravity surveys (this study grid spacing: 1 km; Saudi Aramco study station spacing: 300-400 m; line spacing: 2-3 km), and passive seismic data, we assessed the distribution, nature, and magnitude of the ongoing land deformation caused by the rising diapir. The extracted radar-based deformation velocities were used to identify current and potential areas of concern for future city expansion. Field, drilling, GPS, and optical remote sensing data were used to validate and calibrate the radar-based velocities. Our findings include: (1) the JZD outcrop and bordering areas have been undergoing substantial uplift (up to 4.7 mm/yr at the center of the diapir), whereas the surrounding sabkhas immediately to the east are witnessing subsidence (up to −7.5 mm/yr); (2) a low relative Bouguer anomaly was observed over the JZD with the steepest gradient along its eastern side indicating a steep flank; (3) strong and clear horizontal/vertical (H/V) spectral ratio and high frequency (5–10 Hz) over the JZD outcrop and areas proximal to its western margin but not directly to the east (1.5-3 HZ); (4) drilling confirmed presence of a shallow (4 m) salt bedrock layer west of the JZD, and the absence of this layer to its east over the sabkhas (up to depths of 60 m); (5) the presence of rising diapirs proximal to the shoreline and their apparent absence inland could be related to their preservation in areas affected by sea water intrusion and their dissolution in contact with freshwater aquifers inland. Future expansion of the city should be focused to the east of the diapir and further inland to avoid future building and infrastructure damage.