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S. Strauch, H. Stark (TEM imaging), S. Dechert, and M. Sivis
(Raman spectroscopy), as well as James E. Evans and Nigel D. Browning
(high-resolution TEM, Pacific Northwest National Laboratory),
are gratefully acknowledged. This work was partially funded by
the Deutsche Forschungsgemeinschaft (DFG-ZuK 45/1 and
DFG-SFB 1073). M.G. was financially supported by the
German National Academic Foundation. A.M. W. and H.K. Y.
gratefully acknowledge support from the Alexander von
Materials and Methods
Figs. S1 to S8
9 January 2014; accepted 22 May 2014
Supershear rupture in a Mw 6.7
aftershock of the 2013 Sea of
Zhongwen Zhan,1,2 Donald V. Helmberger,2 Hiroo Kanamori,2 Peter M. Shearer1
Earthquake rupture speeds exceeding the shear-wave velocity have been reported for
several shallow strike-slip events. Whether supershear rupture also can occur in deep
earthquakes is unclear, because of their enigmatic faulting mechanism. Using empirical
Green's functions in both regional and teleseismic waveforms, we observed supershear
rupture during the 2013 moment magnitude (Mw) 6.7 deep earthquake beneath the Sea
of Okhotsk, an aftershock of the large deep earthquake (Mw 8.3). The Mw 6.7 event
ruptured downward along a steeply dipping fault plane at an average speed of 8 kilometers
per second, suggesting efficient seismic energy generation. Comparing it to the highly
dissipative 1994 Mw 8.3 Bolivia earthquake, the two events represent end members of deep
earthquakes in terms of energy partitioning and imply that there is more than one rupture
mechanism for deep earthquakes.
Most earthquakes rupture at speeds less than the shear-wave speed (VS). Theory and laboratory experiments indicate that rupture speeds in excess of VS are pos- sible (1–3), and supershear ruptures have
now occasionally been reported for large strike-slip events (mode II), including the 1979 Imperial
Valley (4), 1999 Izmit (5), 2001 Kunlun (6–8), 2002
Denali (7, 9), 2010 Yushu (10), and 2013 Craig (11)
earthquakes. All of these documented occurrences
were shallow earthquakes with a simple fault geo-
metry (12), and mostly with surface breaks, which
is consistent with theoretical studies that the free
surface helps promote supershear rupture (13, 14).
No definitive evidence has yet been obtained for
supershear rupture in deep earthquakes (depth >
300 km) (15). However, the rupture speeds of
these events are difficult to estimate because
of a general absence of near-field observations,
and they appear highly variable. For example,
the rupture speeds of the two largest deep earthquakes observed to date, the 1994 moment magnitude (Mw) 8.3 Bolivia earthquake and the 2013
Mw 8.3 Sea of Okhotsk earthquake (16–18), were
about 0.2 to 0.4 and 0.7 VS, respectively. About
80% of the rupture velocities for deep earthquakes fall between 0.3 and 0.9 VS (19), a greater
range than is seen for shallow earthquakes (15).
The rupture speed may depend on the slab ther-
mal state (20, 21), with ruptures propagating more
slowly in warm slabs than in cold slabs, but seismic
observations have been inconclusive (22, 23). The
one previous example of observed supershear rup-
ture during the 1990 Mw 7.1 Sakhalin deep earth-
quake neglected to take into account waveform
changes from attenuation and the high-velocity
subducted slab (24, 25).
The 24 May 2013 Mw 8.3 Sea of Okhotsk event
(depth, 607 km) was the largest deep earthquake ever recorded (Fig. 1), slightly larger than
the 1994 Bolivia earthquake. On the same day,
an Mw 6.7 earthquake at a depth of 642 km occurred about 300 km southwest of the mainshock
and was recorded by many teleseismic stations
and one regional station (Fig. 1). An extraordinary feature of the Mw 6.7 event was its sharp
teleseismic P waves, which had displacement
pulse widths at most azimuths of 1 to 2 s (Fig. 1).
These are much less than the expected source
duration of 8 s, based on its magnitude and previous studies of scaled durations of deep earthquakes (26, 27). If taken as a rough estimate of
the Mw 6.7 earthquake’s source duration, these
very short teleseismic P-wave durations imply
extremely high stress drops in a range from
157 MPa to 5.856 GPa (17). On the other hand,
the regional station PET (distance ≈ 495 km) on
the Kamchatka Peninsula to the east displayed a
much longer direct P wavetrain of about 5 s (Fig.
1). Because the P wave to the PET station left the
source along an upgoing ray path, instead of the
downgoing rays for the teleseismic stations, this
longer P-wave duration at PET suggests possible
downward rupture directivity during the Mw 6.7
earthquake. However, to test this possibility we
first need to account for possible path effects such
as wave diffractions along the high-velocity slab
in which the earthquake occurred and site effects
at the stations.
We used waveforms from two nearby smaller
earthquakes (Fig. 1; the 24 June 2013 Mw 4.3
204 11 JULY 2014 • VOL 345 ISSUE 6193 sciencemag.org SCIENCE
1Institute of Geophysics and Planetary Physics, Scripps
Institution of Oceanography, University of California, San
Diego, La Jolla, CA 92093–0225, USA. 2Seismological
Laboratory, California Institute of Technology, 1200 East
California Boulevard, Pasadena, CA 91125, USA.
*Corresponding author. E-mail: firstname.lastname@example.org, zwzhan@