Mechanisms of Deep-focus Earthquakes: A brief
foray into deep-earth seismology
Matthew Tarling
I.
Introduction
Earthquakes with a focus depth greater than 300 km are known as deep-focus earthquakes. These
earthquake occur in subducting oceanic lithosphere along a dipping tabular zone of seismicity in the
subducting slab known as the Benioff-Wadati zone (Frohlich, 1989).
II.
Seismic Characteristics
What does a deep-focus earthquake look like?
Deep-focus earthquakes typically create minimal surface waves (Frohlich, 2006). It is because
they are so deep that they are less likely to produce seismic wave motion with energy concentrated
at the surface. The path of deep-focus earthquake seismic waves from focus to recording station goes
through the heterogeneous upper mantle and highly variable crust only once (Frohlich, 2006). Since
the wave path contains a lesser proportion of variable and discontinuous parts of the earth, the body
waves undergo less attenuation and reverberation than seismic waves from shallow earthquakes.
This is what gives a sharp body wave seismic signals for deep earthquakes. Otherwise, deep-focus
earthquakes are very similar to shallow focus earthquakes in terms of seismic signal (Frohlich, 1989).
III.
Comparison with Shallow Earthquakes
How do deep earthquakes compare to shallow ones?
Shallow earthquakes occur as the rapid release of strain energy built up over time in a rock mass
by brittle fracture and frictional slip along a fault plane (Kearey et al., 2009).
Subducted lithosphere at the pressures and temperatures regime deeper than 300 km should not
exhibit brittle behavior. Instead, strain is expected to be accommodated by plastic deformation
(Frohlich, 2006). Due to this fact, determining what mechanism is responsible for deep earthquakes
remains an outstanding problem in the field of deep earth seismology.
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IV.
Focal Mechanisms
The pattern of energy radiation of an earthquake is represented by the moment tensor solution,
which is graphically represented by beachball diagrams.
Different types of focal mechanisms result in different seismic sources :
• An explosive or implosive mechanism produces an isotropic seismic source (Frohlich, 1989).
• Slip on a planar fault surface gives a double-couple source.
• Uniform outward motion in a single plane due to normal shortening gives a compensated linear
vector dipole source (Kearey et al., 2009).
Deep-focus earthquakes seismic signals contain a varying combination of these sources (Dziewonski
& Woodhouse, 1983).
I. Solid-solid phase transitions
The earliest and most proposed focal mechanism for deep-focus earthquakes is an implosion due
to a phase transition of material to a higher density, lower volume phase (Frohlich, 2006). The
olivine-spinel phase transition that occurs at a depth of 410 km is thought to be responsible (Kearey
et al., 2009).
For solid-solid phase transitions to a deep earthquake source, metastable olivine subducting
oceanic lithosphere would undergo a sudden phase transition to spinel structure once past 410 km
depth. This sudden change in volume corresponds to the isotropic seismic source.
The solid-solid phase transitions mechanism by itself has been largely discredited due to the lack
of a significant (> 10 %) isotropic signature in the moment tensor solution of deep-focus earthquakes
(Frohlich, 1989).
II. Transformational faulting or anticrack faulting
By itself, a solid-solid phase transition is not a good hypothesis for the focal mechanism. However,
the phase transition may still play a critical role. Transformational faulting (anticrack faulting)
is the phase transition of olivine to spinel structure in response to shear stress in a finite
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shear zone (Kirby et al., 1991). The transformation happens along a plane of maximum shear stress.
Like the solid-solid phase transitions theory, metastable olivine subducted past the olivine-spinel
transition at 410 km depth is thought to be responsible for the phenomenon (Green Ii & Burnley,
1989). The transformation creates a weak plane of fine grained material. This planes of weakness
can then yield, resulting to an earthquake.
III. Dehydration embrittlement
Dehydration reactions of mineral phases with high weight percent water could a spike in the
pore-pressure of the subducting slab. This pressure reduces the normal stress inside the slab and
allow slip to occur on pre-existing planes weakness at much greater depths than normally possible
(Kearey et al., 2009). The mechanism is unlikely to be significant beyond 350 km depth due to the
fact that the slab is though to be completely dehydrated by the time it reaches 150 to 300 km depth
(5-10 GPa) (Frohlich, 2006).
IV. Stress-induced melting and shear thermal instability
In stress-induced melting, earthquakes happen by a runaway plastic creep event (Frohlich, 2006).
Shear stress in the slab induce plastic creep. Some of this mechanical work produces heat in the
subducting slab. The heat can only leave the slab slowly by conduction and can therefore can build
up (Wiens, 2001). Thermal runaway then occurs, partially melting the slab in zones of maximum
shear stress. This melting builds a positive-feedback loop, since it lowers the viscosity, which in turn
allows more slip and then more heating which creates more melt (Frohlich, 2006).
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References
Dziewonski, A. M. & Woodhouse, J. H. (1983). An experiment in systematic study of global
seismicity: Centroid-moment tensor solutions for 201 moderate and large earthquakes of 1981.
Journal of Geophysical Research: Solid Earth (1978–2012), 88 (B4), 3247–3271.
Frohlich, C. (1989). The nature of deep-focus earthquakes. Annual Review of Earth and Planetary
Sciences, 17 , 227.
Frohlich, C. (2006). Deep earthquakes. Cambridge university press.
Green Ii, H. & Burnley, P. (1989). A new self-organizing mechanism for deep-focus earthquakes.
Kearey, P., Klepeis, K. A., & Vine, F. J. (2009). Global tectonics. John Wiley & Sons.
Kirby, S. H., DuRHAM, W. B., & Stern, L. A. (1991). Mantle phase changes and deep-earthquake
faulting in subducting lithosphere. Science, 252 (5003), 216–225.
Wiens, D. A. (2001). Seismological constraints on the mechanism of deep earthquakes: Temperature
dependence of deep earthquake source properties. Physics of the Earth and Planetary Interiors,
127 (1), 145–163.
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