Mantle earthquakes in recently thinned Neoproterozoic lithosphere: Harrat Lunayyir, Saudi Arabia

Abstract

We use earthquake geothermometry, measured heat flow, and structural constraints from P-wave receiver functions to model the thermal evolution of the lithosphere beneath Harrat Lunayyir. We suggest that the lithosphere thinned to its present 60-km thickness in a second stage of lithospheric thinning at 15-12 Ma following initial Red Sea extension at~27 Ma. Harrat Lunayyir is an active volcanic field located in the Arabian Shield > 150 km east of the Red Sea rift axis. In the lithospheric mantle beneath Harrat Lunayyir we locate 64 high-frequency earthquakes at depths of 42-48 km, all with mL ≤ 2.5. These brittle-failure earthquakes must have nucleated at relatively low temperatures, based upon global maximum nucleation depths and temperaturedependent-deformation experimental results. Therefore, the mantle earthquakes show that the upper-mantle lithosphere is not in thermal equilibrium with the shallow (60 km) underlying asthenosphere. Our thermal modeling indicates that the lithosphere beneath Harrat Lunayyir thinned to its current 60-km thickness at 12 ± 2 Ma, as constrained by thermal modeling of: (1) surface heat-flow; (2) the depth to the mid-crustal brittle-ductile transition, and (3) the depth to the upper-mantle brittle-ductile transition.

Seismic Hazard and Site Suitability Evaluation Based on Multicriteria Decision Analysis

“…The epicenter of this activity was located about 120 km southwest of Yanbu' city. The earthquake was followed by a number of aftershocks with a maximum magnitude of 4.2 [64], [65] and ground-shaking intensity of level V on the MMI scale [15], [22], [59], [65]. Spatial distribution of the main events affecting the mapped area is shown in Figure 2 [66].…”

“…It is characterized by dense clustering of earthquakes, with the maximum-recorded magnitude being from the 2009 Al-Ays event. Major tectonic features in this zone are aligned in an NW-SE direction, almost parallel to the Red Sea trend [64], [79]. Maximum faults in this zone follow an NW trend and are interpreted to form a part of the Najd Fault System [80].…”

“…The second zone (Z 2 ) is characterized by a few scattered seismic events without any clear trend, with the maximum reported magnitude being that of the 2009 Badr event. Both Z 1 and Z 2 are linked to intra-plate seismic sources influenced by Red Sea tectonics [64], [79].…”

“…In the case of the first two zones (Z 1 and Z 2 ), the origin of peculiar seismicity is not yet properly known. Consequently, it is still unclear whether the seismicity in these zones is caused by magma intrusion through extensional faulting, tectonic processes, or temporal coupling/mechanical interaction between tectonic and magmatic processes [64]. Dense-clustering of earthquakes in the first and fourth seismic zones (See Figure 3(b)) is attributed to the accumulation of stresses as a result of intra-plate and rifting-related tectonic activity, respectively [54], [58], [59].…”