Stress drop behavior of the earthquakes associated with the September 19, 2021, La Palma volcanic eruption in Cumbre Vieja, Canary Islands (Spain)

The stress drop of an earthquake is a key source parameter directly related to fault rupture dynamics. In the context of volcanic eruptions, such as the 2021 Cumbre Vieja event on La Palma Island, analyzing associated seismicity provides insights into variations in shear velocity, fault dimension, or effective normal stress. The local magnitudes (M_L) of the recorded volcano-tectonic events ranged from 0.3 to 4.1. To simulate the source properties, we used a theoretical point source model based on seismic wave response in a simplified medium. A spectral method was applied to 712 events with $M_L\ge 2.5$, selected from a total of 8488 events that occurred immediately before and during the eruption process. The estimated seismic moments ranged from 1.36 × 10¹² to 1.92 × 10¹⁵ Nm, and the corresponding stress drop values were in the range of 0.30–48.38 MPa. Two main seismic clusters were identified, one shallow located at the crust–mantle boundary at approximately 14 km depth and the other deep at 32–39 km depth. No significant depth dependence was observed in the stress drop values. The earthquakes associated with this volcanic eruption exhibited non-self-similarity, a behavior reported here for the first time in the context of volcanic earthquakes. The use of a normalized stress drop helped delineate the open conduit from the magma reservoir in the mantle in its way to the magma chamber also delineated with this parameter. Additionally, seismicity in the upper crust shows high normalized stress drop values at places where the magma ascent changes in direction.