Conjugate Faulting, Lower Mantle Seismicity, and Slab Settling Associated with the 2015 Bonin Islands Deep-Focus Earthquake

Abstract:

Deep-focus earthquakes occur at subduction zones below depths of 300 km and provide insight into how subducting slabs deform over a range of spatial and temporal scales as they descend into the mantle. The mechanisms that allow deep-focus earthquakes to occur at high pressures and temperatures associated with these great depths inside the Earth have been debated since their discovery. Key observations that constrain potential mechanisms include the locations and rupture properties of deep-focus earthquakes with respect to subducting slabs. However, imaging the sources of seismic energy both during and immediately following large deep-focus earthquakes has remained a challenge due to the uncertain environments in which these events occur and complex high frequency waveforms that tend to hide the characteristics of early aftershock sequences. Here we use a novel 4D source imaging approach to show that failure during the 2015 Mw 7.8 Bonin Islands deep-focus earthquake occurred via the creation of conjugate faults that triggered distal aftershocks down to depths of 750 km. This is the first reported group of earthquakes anywhere on Earth that initiate in the lower mantle. A comparison with tomographic models of the region suggest that these lower mantle events and a previously reported group of aftershocks between 350 and 500 km depth are focused at the lower and upper edges of a slab segment that deforms as it penetrates into the lower mantle. It is hypothesized that failure during the mainshock within an inner arc of this deformed slab segment released elastic bending-related stresses and allowed gravitational settling of the slab to occur which resulted in seismicity far from the mainshock rupture planes. Both the observations of coseismic failure on conjugate rupture planes during the main event and aftershocks fully within the lower mantle have profound implications for the physical mechanisms that are permissible to explain the occurrence of deep-focus earthquakes.