Searching for Brittle-Failure Earthquakes in the Mantle Lithosphere beneath Harrat Rahat, Saudi Arabia

Harrat Rahat is a historically-active volcanic field on the western Arabian Shield, spanning the region between and potentially threatening the holy cities of Medina and Mecca in Saudi Arabia. In 1999, a sparse seismic network recorded a swarm of 500 earthquakes, including 8-37 events reported from below Moho depths (based on Moho depths between 35-40 km). However, there was likely significant uncertainty in these depth determinations. A recent study (Blanchette et al., 2018, EPSL, 497, 92) of a nearby also-active volcanic field, Harrat Lunayyir, used network data to locate numerous high-frequency, brittle-failure earthquakes unequivocally in the shallow mantle of the Arabian Shield. These earthquakes likely represent continuing low-volume magma intrusion as dykes into the shallow mantle and continue for at least five years after the last confirmed upper-crustal intrusion in 2009.

In this study, we apply a new technique for earthquake detection known as Fingerprint And Similarity Thresholding (FAST) recently developed at Stanford University (Yoon et al., 2015, Sci. Adv., 1, e1501057) to existing seismic datasets from both Harrat Rahat and Harrat Lunayyir. The FAST algorithm is a computationally efficient, generally applicable and highly sensitive method for detecting seismic events in large sets of data via a process of characterizing each seismic signal as a "fingerprint" - a compressed yet unique binary representation of each waveform - and then cross correlating all fingerprints across the dataset. We are currently using FAST to analyze additional continuous seismic data from 16 three-component broadband seismometers surrounding Harrat Rahat and 19 similar detectors surrounding Harrat Lunayyir. Our goal is to verify the existence of mantle earthquakes beneath Harrat Rahat and enlarge the database of mantle earthquakes beneath Harrat Lunayyir. The analysis of additional earthquakes will allow for a better understanding of hypocenter geometries, focal mechanisms and source physics.