A Rayleigh wave back-projection method applied to the 2011 Tohoku
earthquake
Geophysical Research Letters, 39 (2):
L02302+ (Jan 18, 2012)DOI: 10.1029/2011GL050183
Abstract
We study the rupture process of the 2011 Tohoku megathrust by analyzing
384 regional strong-motion records using a novel back-projection
method for Rayleigh waves with periods between 13 and 100 s. The
proposed approach is based on isolating the signal at the selected
period with a continuous wavelet transform, and generating the stack
using arrival times predicted from detailed fundamental mode Rayleigh
wave group velocity maps. We verify the method by back-projecting
synthetic time series representing a point source off the coast of
Tohoku, which we generate with a 3D finite difference method and
a mesh based on the Japan Integrated Velocity Structure Model. Application
of the method to K-NET/KiK-net records of the Mw 9.1 Tohoku earthquake
reveals several Rayleigh wave emitters, which we attribute to different
stages of rupture. Stage 1 is characterized by slow rupture down-dip
from the hypocenter. The onset of stage 2 is marked by energetic
Rayleigh waves emitted from the region between the JMA hypocenter
and the trench within 60 s after hypocentral time. During stage 3
the rupture propagates bilaterally towards the north and south at
rupture velocities between 3 and 3.5 km/s, reaching Iwate-oki 65
s and Ibaraki-oki 105 s after nucleation. In contrast to short-period
back-projections from teleseismic P-waves, which place radiation
sources below the Honshu coastline, Rayleigh wave emitters identified
from our long-period back-projection are located 50-100 km west of
the trench. This result supports the interpretation of frequency-dependent
seismic wave radiation as suggested in previous studies.