Article,
Subevent location and rupture imaging using iterative backprojection for the 2011 Tohoku Mw 9.0 earthquake
Geophysical Journal International, 190 (2): 1152--1168 (August 2012)
DOI: 10.1111/j.1365-246X.2012.05541.x
Abstract
Knowledge of the rupture speed and spatial-temporal distribution of
energy radiation of earthquakes is important for earthquake physics.
Backprojection of teleseismic waves is commonly used to image the
rupture process of large events. The conventional backprojection
method typically performs temporal and spatial averaging to obtain
reliable rupture features. We present an iterative backprojection
method with subevent signal stripping to determine the distribution
of subevents (large energy bursts) during the earthquake rupture.
We also relocate the subevents initially determined by iterative
backprojection using the traveltime shifts from subevent waveform
cross-correlation, which provides more accurate subevent locations
and source times. A bootstrap approach is used to assess the reliability
of the identified subevents. We apply this method to the Mw 9.0 Tohoku
earthquake in Japan using array data in the United States. We identify
16 reliable subevents in the frequency band 0.2-1 Hz, which mostly
occurred around or west of the hypocentre in the downdip region.
Analysis of Tohoku aftershocks shows that depth phases can often
produce artefacts in the backprojection image, but the position and
timing of our main shock subevents are inconsistent with depth-phase
artefacts. Our results suggest a complicated rupture with a component
of bilateral rupture along strike. The dominant energy radiation
(between 0.2 and 1 Hz) is confined to a region close to the hypocentre
during the first 90 s. A number of subevents occurred around the
hypocentre in the first 90 s, suggesting a low initial rupture speed
and repeated rupture or slip near the hypocentre. The rupture reached
the coastal region about 106 km northwest of the hypocentre at 43
s and the region about 110 km north of the hypocentre at 105 s with
a northward rupture speed \~2.0 km/s at 60-110 s. After 110 s,
a series of subevents occurred about 120-220 km southwest of the
hypocentre, consistent with a 3 km/s along-strike rupture speed.
The abundant high-frequency radiation in the downdip region close
to the coast suggests intermittent rupture probably in the brittle-ductile
transition zone. The lack of high-frequency radiation in the updip
region suggests the rupture near the trench was more continuous,
probably due to more homogeneous frictional properties of the shallow
slab interface. The lack of early aftershocks in the updip region
indicates that most of the accumulated slip in the updip region during
the interseismic period was probably released during the main shock.
