Magnetotelluric Imaging of Subsurface Structure in Rahat Volcanic Field, Madinah City (Saudi Arabia)

Abstract

Rahat volcanic field, Saudi Arabia, has three major geohazard events; the historical eruption, the fissure eruption, and seismic swarm. A magnetotelluric survey was carried out to evaluate these hazards. The results revealed four major resistivity zones. The first is a layer of intermediate resistivity (40–250 O m) which is thicker to the south-west and thinner to the eastern edge. This layer is underlain by a resistant (>1000 O m) granitic basement. Intruding into the resistive basement are two near-vertical conductive (<20 O m) structures. One is located west of the historic eruption (1256 AD) center, at a depth of about 15 km. The other intruder is on the southern end of the survey area, at the same depth. Two conductive “channels” trending NW-SE and NE-SW were observed at depths of about 18 km. The NE-SW aligned “channel” runs through the northern intrusive while the NW-SE trending “channel” runs in the middle of the study area and connects both intrusive structures. The conductive intrusive structures and “channels” may be attributed to partial melts stored in the pre-existing structures within the lower crust. Although the youngest known trachytic eruptions from the study area are several hundred thousand years old, recent and ongoing seismicity strongly suggests there may be magmatic activities in the lower and probably upper crusts.