Owing to distinct features of seismic velocity and seismicity distributions identified in our tomography results, we can reconsider the causes of abnormally strong mantle seismicity in the BN. We can also observe that the Benioff seismicity at the depth of 130 km forms a continuous lineament of earthquakes on both sides of the BN. This may indicate that the BN seismicity occurs in the middle part of the slab and is not associated with the interaction of different plates, as suggested by some authors 3,17,19,21. The intensity of the BN seismic process is by more than an order of magnitude stronger than the background slab seismicity. The coincidence in space of the BN cluster with the slab-related seismicity shows that the subducting plate might be involved in the origin of this anomalous seismicity zone, which is the major difference distinguishing the BN from the cases of Vrancea 26 and Pamir-Hindu Kush 25,29.
Our interpretation of the obtained velocity and seismicity distributions in the BN area is schematically presented in Fig. 6. We propose that the observed high-velocity anomaly within the BN represents the final stage of delamination, when a drip of higher density material was detached from the lithosphere and falling down to the subducting slab. The delamination scenario in similar settings was proposed in 30 and numerically simulated for the case of Central Andes 31,32. It has been shown that in the case of regional crustal shortening, some part of the lower crust may appear at large depths, which may cause transformation of mafic rocks to high-density eclogites. When reaching a certain critical mass, the eclogite body may cause gravitational instability and trigger the detachment of the mantle part of the lithosphere. As a result, the lithosphere and the lower-crustal material form the high-density drip, which is descending at a relatively high rate (up to 1 m per year 32). This scenario was proposed for the cases of Vrancea26 and Pamir-Hindu Kush25. In NWSA, delamination has been proposed as mechanism related to the Paipa-Iza volcanic complex 19,21,33, located north of Caldas tear and about 100km south of the BN area.
We think that the same mechanism of delamination might be valid for the case of the BN. The distinct feature of the BN from other cases is that here the interaction of the falling drip with the rigid slab may affect the anomalous stress field in the contact zone that in turn may greatly intensify the seismic process. As we can observe in Fig. 6, the high-velocity anomaly at 130–160 km depth may represent the delaminated body entered to the slab. The exceptionally high values of seismic velocities might be explained by compaction of the material due to high pressure caused by the collision of the rapidly fallen delaminated body with the rigid slab. Above the high-velocity BN body, we observe a thin high-velocity anomaly of lower intensity that connects the BN with the bottom of the crust. It may represent a tail of the downgoing flow following the main delaminated drip, similarly as we can observe when we pour out a portion of viscous honey.
An important problem relates to the physical mechanisms responsible for intermediate-depth earthquakes. In the mantle at depths of more than 100 km, in the absence of fluid or melt, high lithostatic pressures make the usual dry frictional failure mechanism unlikely34,35. However, in subduction zones, some hydrated minerals, such as serpentine, undergo phase changes to anhydrous forms, releasing fluids in the process 36,37. These fluids raise pore pressure, which in turn lower the effective pressure to values that permit brittle failure and earthquake occurrence38,39. As the subducting oceanic plate is rich in water, the intermediate-depth seismic activity connected with dehydration reaction is a common mechanism taking place in all subduction zones. For example, in our study we observe a clear belt of the regular Benioff seismicity at 130 km depth, which indicates zones of the dehydration reactions in the slab at this depth.
At the same time, it is very rare when the intermediate-depth seismicity occurs below continents. Although, the continental collision is often associated with descent of large lithosphere masses, in most cases it is not associated with mantle seismicity. In normal conditions, the lower crust and mantle lithosphere that deepen due to collisional processes do not contain sufficient amounts of water and therefore cannot initiate the seismogenic process. For the exceptional cases of Vrancea or Hindu Kush, the seismicity might be associated with the presence of remnant oceanic lithosphere. In both cases, the geological history shows that the phase of continental collision was preceded by closure of paleo oceanic basins, hence the hypothesis on sinking the remnant oceanic lithosphere in these areas is plausible.
For the area of the BN, the situation is completely different, as it is located inside a well-developed continent, where there was no trace of any relict oceanic basins. One of the mechanisms of seismicity generation in the mantle could be thermal shear instability, as was proposed for the BN 17,27,37,40,41, as well as for the Hindu-Kush nest 42.
As an alternative scenario, we propose that the main cause of such a strong seismicity in the BN is the interaction of the “dry” delaminated drip with “wet” subducting slab. The strong impact of the rapidly fallen delaminated body with the slab may intensify the processes of dehydration. One possible mechanism of intensification could be the fact that the delaminated body entered inside the subducting plate and triggered dehydration reactions in layers located inside the slab, which produced much more water than during a regular subduction process, when mostly near-surface layers are involved.
The differentiation between two distinct seismicity clusters called UBN and LBN, as presented in Fig. 6, allows us to reconcile two mechanisms responsible for intermediate-depth seismicity, fluid-induced embrittlement and delamination. The former has been proposed for the BN by several authors 3,9,16,17,19, 27,37,43, and considered as the main mechanism to trigger seismicity at mid-depth 44. We interpret dehydration embrittlement as the seismic activity of the clustering observed at 130 km, which includes the UBN. As for the LBN, given that the Vs values are high, thus implying a low content of fluids, we can rule out dehydration inside the delaminated body; instead, we propose intensified dehydration in the slab as the driving mechanism of seismicity.