The importance of station corrections for local earthquake magnitudes: the example from the seismicity in NEOM (Gulf of Aqaba and northern Red Sea)

The NEOM multi-billion-dollar project on the eastern coast of the Gulf of Aqaba will bring underground infrastructures, new cities, and tourist destinations. This project will dramatically increase the seismic risk associated with active faults in the Gulf of Aqaba and the northern Red Sea. The Gulf of Aqaba, located between northern Saudi Arabia and the Sinai peninsula and formed by the transtension at the southern termination of the Dead Sea Transform, is a 180-km-long fault system that can generate earthquakes of magnitude at least 7.3 (as occurred in 1995). South of the gulf, the fault system connects to the Red Sea rift, where an earthquake of magnitude larger than 5 occurred in 2020. To investigate the regional tectonics and to better understand the associated seismic hazard, we have run a temporary network of 12 broadband seismic stations in the area since 2019.

In this contribution, we present a new local magnitude scale calibrated by using more than 10,000 half-peak-to-peak amplitudes, automatically measured and Wood-Anderson-corrected, from earthquakes recorded by our network from May 2019 to February 2021. We used the amplitudes from the two horizontal components of each station to constrain the constants of the distance-dependent correction term of the local magnitude formula (n, related to the geometrical spreading, and k, related to the attenuation), magnitudes, and station corrections.

We used a least-square regression scheme in two steps to ensure the convergence of the solution and independence of the results from the initial values. In the first step, we only inverted for n, k, and magnitudes. In the second step, we also inverted for station corrections and we used the magnitudes obtained in the first step as initial values for the second step. Conversely to most previous studies, we did not introduce any constraints on the station corrections. We run several regressions in a grid search approach to tackle the trade-off between n and k and find the best solution.

We found that the estimated station corrections, because of the lack of constraints on them, are strongly correlated with the rock properties and topographic attributes. We also compared the frequency-magnitude distributions obtained with our best solution, including the station corrections (case A), the Hutton and Boore (1967) formula (case B), and Hutton and Boore formula with our station corrections (case C). We found that magnitudes for A are lower than for B and C. However, differences in statistical parameters, such as b-values, between A and C are neglectable.

Our work provides NEOM with a reliable and locally calibrated earthquake magnitude scale. This new magnitude scale can also be applied in surrounding regions with similar geological features (e.g., Egypt, Jordan, and Israel). Moreover, this work highlights that estimations of station corrections are critical, and at least, as important as a locally calibrated magnitude scale.