Mechanisms and Implications of Deep Earthquakes

Abstract

Deep earthquakes behave like shallow earthquakes but must have fundamentally different physical processes. Their rupture behaviors, magnitude-frequency statistics, and aftershocks are diverse and imperfectly dependent on various factors, such as slab temperature, depth, and magnitude. The three leading mechanisms for deep earthquakes (i.e., transformational faulting, dehydration embrittlement, and thermal runaway) can each explain portions of the observations but have potentially fundamental difficulties explaining the rest. This situation calls for more serious consideration of hypotheses that involve more than one mechanism. For example, deep earthquakes may initiate by one mechanism, but the ruptures may propagate via another mechanism once triggered. To make further progress, it is critical to evaluate the hypotheses, both single- or dual-mechanism, under conditions as close to those of real slabs as possible to make accurate and specific predictions that are testable using seismic or other geophysical observations. Any new understanding of deep earthquakes promises new constraints on subduction zone structure and dynamics. ▪ Deep earthquakes display the complex structure and dynamics of subduction zones in terms of geometry, stress state, rheology, hydration, and phase changes. ▪ Phase transformation, dehydration, and thermal runaway are the leading mechanisms for deep earthquakes, but all have major gaps or fundamental difficulties. ▪ Deep earthquakes may involve dual-mechanism processes, as hinted at by the diverse rupture and statistic properties and the break of self-similarity. ▪ Further progresses would benefit from specific and testable predictions that consider realistic slab conditions with insights from geodynamics, petrology, and mineral physics.

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

“…Considering the rate of deformation and the process through which strain accumulates in the slab more explicitly may also foster reevaluation of seismic observations related to rupture behavior (moment release, rupture time, b values, aftershock occurrence, and stress drop), and how these are related to different triggering mechanisms. For example, considering the detailed orientations of stress and strain in bending regions in conjunction with a physical model of the failure mechanism may help to explain the preference for near-horizontal failure planes for deep earthquakes ( 48 ) and the large depths (near 660 km) for the largest deep earthquakes ( 11 ). In addition, it is possible that the observed correlation of b values with thermal parameter ( 15 ) may also be related to the strain rate of the slab.…”

“…However, compared with shallow events, deep earthquakes tend to have shorter rupture duration for a given earthquake size, exhibit more rupture complexity (i.e., multiple subevents), have a larger range in stress drop (1 to 100 s MPa) and rupture velocities [0.2 to 0.9 times shear velocity, with some earthquakes exhibiting supershear rupture velocity; see references in (11)], and exhibit depth-dependent aftershock productivity (e.g., almost absent at intermediate depths but present deeper than 550 km). In addition, some very deep events have very low radiation efficiency (<0.1), which has been interpreted to indicate melting during the rupture process (13).…”

“…Despite the differences in pressure-temperature conditions and triggering mechanisms, seismological observations of deep earthquakes suggests that these events are similar to shallow events in several ways. Deep earthquakes have (mostly) double-couple mechanisms indicating that they occur as a shear failure, aftershock sequences follow a standard Omori law for aftershock decay, energy/moment ratios are similar to shallow events, and long-range triggering of deep earthquakes has been observed [for comprehensive reviews, see ( 9 – 11 )]. Analysis of source-time functions also shows remarkable similarity between shallow and deep earthquakes when depth-dependent differences in rigidity are taken into account ( 12 ).…”

Tonga Slab Morphology and Stress Variations Controlled by a Relic Slab: Implications for Deep Earthquakes in the Tonga‐Fiji Region

“…Shallow seismic rupture is produced by brittle-frictional processes. However, increasing temperature and pressure with depth promotes ductile flow which inhibits sliding (Houston, 2015;Zhan, 2020). Nevertheless, different mechanisms have been advanced for the nucleation of deep focus earthquakes, including a metastable olivine phase transition in the cold core of a slab (…”

Hydrous SiO2 in subducted oceanic crust and water transport to the core-mantle boundary

“…Further, boron-, oxygen, carbon and nitrogen isotopes of diamonds and their inclusions derived from the base of the transition zone and uppermost lower mantle indicate a direct role for aqueous fluids associated with seawater serpentinization near the surface and eventual dehydration of slab lithosphere at the base of the transition zone 44,45 . A link between dehydration of slab mantle in cooler subducting slabs in the transition zone and deep focus earthquakes has been postulated 46 , and given its propensity for hydration, stishovite could play a role in whichever of the competing mechanism leads to deep seismicity 47 . The process depicted in Fig.…”