From the Mantle to the Upper Crust: Migrating Volcanic Earthquakes After the 2009 Harrat Lunayyir Dike Intrusion, Saudi Arabia

Abstract:

Harrat Lunayyir (HL) is an active volcanic field located ~100 km east of the Red Sea rift on the Arabian shield. The May 2009 dike intrusion episode beneath HL was accompanied by ≥30,000 earthquakes, with a maximum magnitude of M_w 5.7. HL has remained the most seismically active region within the Arabian plate since this dike intrusion episode. This seismicity overwhelmingly occurs in the upper-crust near the initial dike intrusion, but up to 1% of the events can be reliably located in the lower crust and the mantle lithosphere. Seismicity within the mantle lithosphere occurs within a spatially consistent region over nearly a decade of data, with episodic pulses of seismicity occurring in the mantle lithosphere, below the well-determined Moho at 38 km (Blanchette et al. 2018). Occasionally this mantle seismicity (40–50 km depth) is followed by a migration of earthquake hypocenters (and, we infer, magma) through the lower-crust (25–30 km depth), to the southern terminus of the dike at a depth of ~20 km (left figure). The hypocentral locations of the seismicity are stable throughout time and are separated by distinct aseismic zones at depths between the upper and lower crust and between the lower-crust and upper-mantle. The lower-crustal earthquakes are located approximately vertically beneath the upper-crustal (dike) earthquakes, but are laterally offset 17 km from the mantle events, suggesting aseismic lateral migration of magma in the ductile deepest crust. The number of earthquakes occurring beneath Harrat Lunayyir has been slowly decreasing over time (8,473 earthquakes in 2011, 3,142 in 2018). We will present an analysis of the spatio-temporal patterns of this seismicity, its evolution, and a discussion of the types of earthquakes present within the seismic catalog with particular focus on calendar year 2014, for which we have continuous seismic data.