1. Deep Earthquakes

2. Depth distribution and classification
Earthquakes generally classified as
Intermediate ( km)
Deep ( km)
Not a single earthquake deeper than 690 km
It is quite possible that intermediate and deep earthquakes have different causes
Cessation of deep earthquakes at 690 km appears to occur at the “660 km discontinuity”
Earthquakes deeper than 660 occur where the discontinuity is depressed

3. Deep Earthquake Distribution with Depth
Frohlich [1989]

4. Deep earthquakes are not frictional sliding
Frictional resistance = μ FN
FN is proportional to ρgh
ρ is fairly constant for different rock types
So frictional resistance exceeds strength of rock for earthquakes deeper than km

5. Proposed intermediate & deep earthquake faulting mechanisms
Dehydration embrittlement
-- pore fluids from dehydration reactions allow brittle failure at high normal stress
Transformational faulting
-- metastable phase transformation forms small lenses of low viscosity stable phase, which coalesce and lead to shear failure
Plastic shear instabilities/thermal runaway
-- formation of a shear zone due to strain and temperature softening, with runaway shear heating leading to failure
-- extreme case would be fault zone melting

6. How to allow frictional slip at high pressure?
High fluid pressure reduces
effective normal stress
Fault motion occurs through
ductile shear, not brittle friction

7. Transformational Faulting
“Anticracks” are observed
in metastable phase transitions
Associated with reductions in
volume
Filled with very fine grained
low-strength transformed
material
Shear failure occurs along the
low strength crack.

8. Shear zones & thermal runaway

9. transformational faulting model
Diagram of the transformational
Faulting model – deep earthquakes
occur in a wedge of metastable
olivine
Evidence against the
transformational faulting
model
The width of the seismic
region is greater than
possible metastable wedge
widths
Seismological evidence
for a metastable wedge is
unclear

10. Intermediate depth earthquakes often show a double seismic zone

11. Intermediate depth eqs result from dehydration?
Hacker et al., 2003

12. Double Seismic Zone depth thermally controlled
 Hot
Cold 
Wei et al., 2017

13. Imaging dehydration in the subducting crust?
Abers et al, 2013

14. Dehydration of uppermost mantle serpentine ?

15. Similarities between Shallow and Deep Earthquakes
Shear failure on planar surfaces (double couple)
Generally similar rupture parameters (velocity, stress drop)
Similar aftershock decay and magnitude statistics
Aftershocks align along mainshock fault planes
Repeating earthquakes and earthquake triggering observed

16. Differences Between Deep and Shallow Earthquakes
Similar magnitude-frequency
relations worldwide
Large earthquakes occur
only where small events
occur
Many aftershocks
Deep earthquakes
Seismic zones show vastly
different magnitude-
frequency relations
Largest earthquakes are
isolated
Fewer aftershocks by 1 to 2
orders of magnitude

17. Deep earthquakes result from shear failure along a plane (double-couple)
Not CLVD
Mechanisms not isotropic
Kawakatsu, 1996
Kuge & Kawakatsu, 1993;
data from Harvard CMT

18. Statistical properties of deep earthquake aftershock sequences
Data from 1994 Tonga (Mw 7.6)
Aftershock sequence
(Wiens & McGuire, 2000)

19. Deep earthquake aftershocks align along the main shock fault plane
Visualization of the
Aftershock sequence of
the 1994 Tonga
deep earthquake
(Wiens et al., 1994)
Ellipsoids represent 95%
confidence regions of
relative locations;
white is mainshock;
green are aftershocks.

20. Magnitude-frequency as a function of slab thermal parameter
Wiens & Gilbert, 1996

21. The largest deep earthquakes occur in aseismic regions
August 19, 2002
Tonga second
mainshock (white),
foreshock (yellow),
aftershocks (green),
and background
seismicity (blue).
From
Tibi, Wiens, Inoue
(2003)

22. Important clues to the mechanics of deep earthquakes
Deep earthquake rupture is sensitive to the slab temperature
Deep earthquakes show both static and dynamic triggering
Events in previously aseismic regions are particularly sensitive to dynamic triggering
Repeating earthquakes recur on the same fault
and show small recurrence times

23. Deep Earthquake rupture is temperature sensitive
Magnitude-Frequency relations
Aftershock productivity
Wiens & Gilbert, 1996
Thermal parameter – the product of the subduction convergence
rate and the age of the lithosphere – inversely proportional to slab
temperature for a cooling halfspace thermal model and cooling
by conduction

24. Deep earthquakes can be dynamically triggered - two examples -
2002 Tonga
1986 Tonga
Both cases involve dynamic triggering in aseismic regions with
time scales of minutes
Suggests that slabs are surrounded by critically stressed regions
where seismic rupture initiates with difficulty
Triggered events do not correspond to passage of seismic waves
and do not define a consistent strain velocity – probably involve
non-linear initiation mechanisms
Tibi, Wiens, & Inoue,2003

25. Repeating Deep Earthquakes
Deep earthquakes show repeated
earthquakes showing nearly
identical waveforms, suggesting
recurrent faulting at the same
location (Wiens & Snider, 2001)
This pair of co-located events
show that even details of the rupture
process may be repeated.

26. Repeated rupture along the same fault
Repeating deep earthquakes
can be located accurately (+ 1 km)
using cross correlation methods
These events define fault-like
features containing events with
identical focal mechanisms
The rupture area of each event
can be estimated from the
duration and scaling relations
These events define repeated
ruptures along the same fault
segment
Repeated rupture on the same fault
may be inconsistent with transformational
faulting, which is a non-repeatable
process
Wiens and Snider, 2001

27. Complicated deformation in deep slabs

28. Does Transformational Faulting Explain deep earthquakes?
Do Metastable olivine wedges exist?

29. Slab seismic zone is too wide for transformational faulting, dehydration embrittlement
Slab Seismicity and aftershocks
1994 Tonga Event
Fault Slip during mainshock
1994 Tonga Event
McGuire et al, 1997
Blue – background seismicity
White - mainshock epicenter
Green - aftershocks
Yellow – foreshock
Red -rupture termination from directivity
Dots – aftershocks
X - rupture initiation
F – foreshock
T – rupture termination

30. Travel time anomalies from metastable and equilibrium slabs
Slab modeling is carried out
for the Tonga slab (Koper at al, 1998),
With data from the Labatts Experiment
Results are inconclusive – effects
of a metastable wedge are subtile
due to diffraction around the low
velocity wedge

31. Waveform effects of a metastable olivine wedge
A metastable wedge
produces large later
phases following
P and S waves due
to guided waves
within the low velocity
wedge.
A systematic search
suggests these later
phases are not
observed
Koper & Wiens, 2000

32. Metastable wedge imaged with receiver functions ?

33. Mechanism of intermediate and deep earthquakes still uncertain
Intermediate depth earthquakes are most likely associated with dehydration reactions (but what about the deep zone of DSZs?)
The most popular models for deep earthquakes are ductile shear zones and transformational faulting
The lack of field observations is very limiting – progress will come from comparison of seismological and experimental results

35. Do slabs contain a metastable olivine wedge?
Forward modeling: takes
advantage of knowledge about
slab temperature, mineralogy,
morphology to perform resolution
tests.
Slab models include:
Temperature structure from
finite difference thermal model
spatial configuration of the slab,
as inferred from seismicity
Phase transformations and
associated velocity changes
Slab models can incorporate a
metastable olivine wedge, defined
by an isotherm
Temperature structure is converted
to a velocity anomaly by assuming
derivative that is found using a
grid-search