Seismogenic Necking During Slab Detachment: Evidence From Relocation of Intermediate‐Depth Seismicity in the Alboran Slab

Abstract

With few exceptions, intermediate‐depth seismicity is associated with active subduction. In the Alboran slab, which is located just east of the Gibraltar Strait in the westernmost Mediterranean, although evidence suggests subduction is no longer active, tens to hundreds of M < 5.0 earthquakes are observed every year at depths of ~80 km. In this paper, we relocated 58 such events recorded by the PICASSO temporary network using a 3‐D velocity model and implementing a grid‐search approach to minimize the normalized misfit between observed and predicted P and S wave arrival times. Meanwhile, we relocated 908 events recorded by the Spanish national network using the double‐difference method and gave a high‐precision picture of seismicity distribution within the slab. Relocation results reveal five clusters in this region separated by small gaps. Jointly considering relocation results and a 3‐D tomography model indicates one cluster is at a shallower depth above the northern, E‐W oriented arm of the Alboran slab, while the other four are aligned from north to south, parallel to the strike of the slab and near its core. Larger events are generally shallower and more scattered, while the deeper, more clustered events tend to have smaller magnitudes. Clusters further to the north show concentrated seismicity on narrow zones that dip ~45° to the south. We suggest this seismicity is associated with active necking and break‐off of the Alboran slab, which is progressing from north to south. Shear instability is likely to be the failure‐enabling mechanism for the occurrence of these earthquakes.

The Gibraltar slab dynamics and its influence on past and present-day Alboran domain deformation: Insights from thermo-mechanical numerical modelling

“…The westward basal drag of the Alboran lithosphere found in the present study would be consistent with this interpretation (Figure 5). Our study supports that this seismicity is likely related to interplate compression processes rather than to shallow slab necking proposed by Sun and Bezada (2020).…”

“…It is worth noting that intermediatedepth earthquakes occur in an area of relatively cold lithosphere due to the down pull of the slab. This cold geotherm is needed to have brittle deformation instead of ductile aseismic creep, and well agrees with tomographic studies showing that this seismicity concentrates in areas of high seismic velocity (Monna et al, 2013;Villasenor et al, 2015;Sun and Bezada, 2020). Santos-Bueno et al ( 2019) obtained focal mechanism of the intermediate seismicity finding that most of them present reverse faulting solutions more abundant in the southern sector, beneath the central Alboran Sea and Northern Morocco.…”

Two Cenozoic Extensional Phases in Mallorca and Their Bearing on the Geodynamic Evolution of the Western Mediterranean

“…We propose that this topographic uplift and coeval extension could be related to flexural and isostatic rebound after an “unloading” of Mallorca through extensional collapse of its orogenic hinterland and excision of its lithospheric mantle root, perhaps driven by tectonics (slab detachment or edge‐delamination). The latter tectonic mechanism has been proposed by several authors under the Betics setting during the Late Miocene until the Pliocene or Present, driving concomitant topographic uplift and thinning of the South Iberian lithosphere (Capella et al., 2020; Chertova et al., 2014; Duggen et al., 2003; García‐Castellanos & Villaseñor, 2011; Mancilla et al., 2015; Negredo Moreno et al., 2020; Sun & Bezada, 2020). Slab detachment may have initiated in the Serravallian further to the NE, below part of the BP, to later propagate toward the SW.…”

Contributions of Grain Damage, Thermal Weakening, and Necking to Slab Detachment

“…While slab detachment is likely dominated by viscous necking, it has also been invoked to explain deep seismicity in various regions on Earth, such as e.g., the Hindu-Kush (Kufner et al, 2017), the Vrancea region (Mitrofan et al, 2016), or the Alboran slab in the Mediterranean (Sun and Bezada, 2020). The mechanisms resulting in the observed seismicity are still contentious, as brittle failure at this depth seems to be unlikely due to the high pressures and temperatures.…”