T2-P31

Hydroacoustic Observation of the Great 2011 Tohoku EarthquakeSukyoung Yun1, Won Sang Lee1

1. Korea Polar Research Institute yun@kopri.re.kr

The great 2011 Tohoku Earthquake (Mw 9.0) occurred offshore near

the east coast of Honshu, Japan on 11 March 2011, and strong T-waves

generated by the event were recorded by the Hawaii hydroacoustic

array operated by the International Monitoring System. We examine

the back-azimuths of the signals and spectral contents of the T-waves,

and we compare them with the rupture models estimated from previous

seismic studies. The results show that the complex rupture process

probably caused the scattered back-azimuths and several local peaks.

We also analyzed the T-waves of the Mw 7.6 normal-faulting

aftershock. It showed a unique envelope shape and frequency contents

comparing with those of other thrust-faulting events. These differences

would reflect the different source and excitation mechanisms.

Abstract

Tertiary (T) Waves

Seismically generated acoustic waves

propagate within the SOund Fixing

And Ranging (SOFAR) channel.

[Tolstoy and Ewing, 1950]

(1) Little transmission loss

(2) Higher sensitivity

(3) Well defined sound velocity

structure

(4) Slow velocity

(5) T-wave radiator location

Attenuation = r -2

Attenuation = r -1

Figure 1. Schematic illustration showing

excitation and propagation of T-waves. [from

NOAA website]

PMCC methods

(1) Conditions

- Plane wave : long epicentral distance

- Small aperture between sensors

(2) Calculating Procedure

- Correlation of signals in three sensor

- Calculating relative time differences

(Consistency (tcon) must be very close

to zero

- Computing slowness vector (speed &

azimuth)

Figure 2. Cartoon for describing PMCC

method.

Introduction

Results and Discussion

Main Event

Origin Time : March 11, 2011 at 05:46:24 UTC

Location : 38.297°N, 142.372°E, 29 km(depth)

Magnitude : Mw 9.0

Tsunami with a maximum runup height of 37.88 m at Miyako

Foreshock

Origin Time : March 09, 2011 at 02:45:20 UTC

Location : 38.440°N, 142.840°E, 32 km(depth)

Magnitude : Mw 7.3

Thrust-fault Aftershock

Origin Time : March 11, 2011 at 06:15:40 UTC

Location : 36.281°N, 141.111°E, 42 km(depth)

Magnitude : Mw 7.9

Normal-fault Aftershock

Origin Time : March 11, 2011 at 06:25:50 UTC

Location : 38.06°N, 142.59°E, 18 km(depth)

Magnitude : Mw 7.6

Figure 4. Source time function,

describing the rate of moment

release with time after earthquake

origin. [from USGS website]

Times after the rupture

initiation (s)0 40 60 90 115 125

Arrival times (s) 2107 2233 2151 2302 2284 2275

Time lag (s) 0 126 44 195 177 168

The 2011 Tohoku

Earthquake

Figure 3. Focal mechanisms of the main event, a foreshock, and two

aftershocks. The pink line is boundary of plates, and the black triangles are

IMS hydrophone array in Hawaii. The white lines are great circle paths

between the main event and the stations.

Figure 5.

Mainshock

spatio-

temporal slip

distribution

map. Numbers

with colored

stars denote

time after the

rupture

initiation. [Ide

et al., 2011]

Table 1. Arrival times after origin time of the major slips and their time lags at H11N station. They are estiamted from the

spatio-temporal slip distribution [Ide et al., 2011]

Figure 6. hydrophone (H11N1) data

and cross-correlation analysis. From

the top, azimuth, time series of raw

data, MS envelope, and

spectrogram of the main event. The

back-azimuth values of the major

events show scattered pattern,

which is a different feature from

that of the Great Sumatra-Andaman

Earthquake [Tolstoy and

Bohnenstiehl, 2005]. This would be

due to complex slip distribution of

the fault. However, local peaks of

the waves coincide with spatio-

temporal slip distribution estimated

from broadband seismograms [Ide

et al., 2011].

Figure 7. hydrophone (H11N1) data and cross-correlation analysis results. From the left, the foreshock, the thrust-fault

aftershock, and the normal-fault aftershock. The normal-fault aftershock have bigger high-frequency energy than the thrust

fault aftershock even though its magnitude is smaller. This might be caused by difference of source-depth. The thrust-faults

events have sharper peaks than that of the normal-fault event. These features may imply different T-wave excitation

mechanisms.

This work is supported by KOPRI grant PE13050, PN13050.