Deep slab seismicity limited by rate of deformation in the transition zone

Abstract

Deep earthquakes within subducting tectonic plates (slabs) are enigmatic because they appear similar to shallow earthquakes but must occur by a different mechanism. Previous attempts to explain the depth distribution of deep earthquakes in terms of the temperature at which possible triggering mechanisms are viable, fail to explain the spatial variability in seismicity. In addition to thermal constraints, proposed failure mechanisms for deep earthquakes all require that sufficient strain accumulates in the slab at a relatively high stress. Here, I show that simulations of subduction with nonlinear rheology and compositionally dependent phase transitions exhibit strongly variable strain rates in space and time, which is similar to observed seismicity. Therefore, in addition to temperature, variations in strain rate may explain why there are large gaps in deep seismicity (low strain rate), and variable peaks in seismicity (bending regions), and, possibly, why there is an abrupt cessation of seismicity below 660 km.

Slab Transport of Fluids to Deep Focus Earthquake Depths—Thermal Modeling Constraints and Evidence From Diamonds

“…There is significant evidence from seismic tomography (Li et al., 2008) and dynamical modeling (Billen, 2008, 2020) that slabs tend to bend, thicken, and stagnate in the transition zone as they approach the higher viscosity and slower convection of the lower mantle (Mao & Zhong, 2018). Variations in strain rate caused by slab deformation at these depths may play an important contributing role to the detailed spatial distribution of deep seismicity, independent of triggering mechanism (Billen, 2020). Slower slab movement means that warming is inevitable as the ∼1,000°C mantle of the cool slab interior at 15 GPa (Figure 7) sits in an ambient mantle of ∼1,500°C.…”

“…We suggest the answer lies in the dynamics of slab subduction and the potential for slabs to stall and heat up in the transition zone (Bina, 1997). There is significant evidence from seismic tomography (Li et al, 2008) and dynamical modeling (Billen, 2008(Billen, , 2020 that slabs tend to bend, thicken, and stagnate in the transition zone as they approach the higher viscosity and slower convection of the lower mantle (Mao & Zhong, 2018). Variations in strain rate caused by slab deformation at these depths may play an important contributing role to the detailed spatial distribution of deep seismicity, independent of triggering mechanism (Billen, 2020).…”

The Seismicity of Indonesia and Tectonic Implications

“…A probable mechanism that governs this decrease is metamorphic dehydration within subducting slabs due to increasing temperature and pressure with depth prior to entering the MTZ (Zhan, 2020); (c) Seismic gaps are observed for all subregions at depths ≥70 km, implying regional variability in the conditions necessary for deep earthquakes to occur, such as ambient mantle temperature (Houston, 2015); (d) The deepest events in Indonesia, ≥670 km depth, only occur in the Banda Sea‐Timor and Celebes Sea‐Philippines regions; (e) Seismicity rates deeper than 300 km decrease westward until no seismicity is evident beneath NW Sumatra, with the exception of West Papua; (f) Elevated seismicity between 100 and 200 km is only present in the Banda Sea‐Timor region, implying a distinct process or boundary not evident in other subregions; And (g) the most frequent MTZ seismicity is located in the Banda Sea‐Timor and Celebes Sea‐Philippines regions, where seismicity peaks at 410 km and variably between 520 and 660 km imply that the 410, 520, and 660 km discontinuities may play a significant role in increasing upper mantle seismicity. While there are several factors that contribute to intraslab seismicity, phase transitions at these boundaries increase viscous resistance of the mantle to the motion of subducting slabs (Billen, 2020; Zhan, 2020). Increased mantle resistance contributes to increased strain rates and therefore elevated intraslab seismicity as slabs pass through the MTZ (Billen, 2020; Zhan, 2020).…”

Seismic Strain Rate and Flexure at the Hawaiian Islands Constrain the Frictional Coefficient

“…The key advancement of the present study is to take the magnitude of seismic events into account by inferring the seismic strain rate and using it in combination with flexure to constrain both frictional sliding and low-temperature plasticity more precisely. The seismic strain rate has been previously estimated for continental (England & Molnar, 1997;Holt et al, 1995;Jackson & McKenzie, 1988;Masson et al, 2005), oceanic (Gordon, 2000;Wiens & Stein, 1983) and subducted lithospheres (Billen, 2020), but the present study represents the first to investigate an oceanic region with a large number of earthquakes and significant lithospheric deformation. In the following Section 2, we present observations of flexure and infer the seismic strain rate.…”