FRAME – a Fast Response Aftershock network for the Moroccan Earthquake (Mw 6.9, 08.09.2023)

Abstract

The 2023 Mw 6.9 Al Haouz Earthquake in the Moroccan High Atlas mountains is the most recent example of a destructive event in an intraplate setting under the absence of a causative plate boundary. Their large inter-event times in such seismotectonic regimes hinders the study of single focus regions and apparently underestimates their larger seismic hazard potential, leaving local communities unaware of the scale of their exposure to these risks. One major shortcoming in the immediate scientific analysis of this unprecedented earthquake is the lack of (publicly) available seismic waveform recordings in the source area.An immediate impact is displayed in the source location uncertainty, including a high variability in focal depth estimations. Associating the earthquake origin to a causative fault remains puzzling, bearing in mind the deeper-than-average focal depth of around 32 km. The complex tectonic history of the Atlas mountain chains is depicted in a plethora of active, reactivated and abandoned faults. In addition, prior studies noticed a lithospheric anomaly underneath the High Atlas mountain range, that lacks a classical mountain root. Thus, this event raise questions on the thermo-elastic parameters at depth in order to support a large seismogenic thickness of the crust.In the immediate aftermath of the Al Haouz, we initiated a rapid-response task force for setting up a temporal seismic network in the High Atlas. Within two weeks, six autonomous seismological stations have been installed between 20 and 80 km from the estimated epicenter. Due to the large degree of destruction, problematic access and absence of major infrastructure in the epicentral region, we relied on autarkic and robust sensors. As a novelty, the stations consisted of SmartSolo® three component 5 Hz geophone sensors. These industrial-level instruments are light-weight and easy to deploy in any terrain. In a compact casing, the passive sensors host digitizer, data storage, GPS and battery life-time of up to 30 days.Here, we present the first data of this 2 months temporary network. Based on preliminary analysis, we could obtain more than 1000 events recorded on all stations of the network during the 54 recording days. The largest recorded events had an assigned magnitude of ML 4.5. Considering single stations detections as well, we estimate more than 10,000 earthquake detections overall. This catalog will greatly expand the scientific insight into the mechanisms of this exceptional earthquake in the near-future.Further, through the use of simple geophone sensors this study presents a proof of concept for rapid response installations of temporary aftershock network. These sensor types strongly outperform the fully elaborated counterparts in installation speed, transportability and external requirements for potential installation sites without large drawbacks in data quality.