science
.sciencemag.org/content/
369/6510/1510
/suppl/DC1
Supp
lementary
Material
s
for
Seismic
o
cean
t
hermometry
Wenbo
Wu
*
,
Zhongwen
Zhan
,
Shirui
Peng
,
Sidao
Ni
,
Jörn
Callies
*
Corresponding author.
Email: wenbow@caltech.edu
Published
18 September
2
0
20
,
Science
369
,
1510
(20
20
)
DOI:
10.1126/science.
abb9519
This PDF file
includes:
Materials and Methods
Figs. S1 to S
16
Table
S1
Reference
s
Methods and materials
The principle of seismic ocean thermometry
Seismic ocean thermometry is based on the same principle as conventional acoustic tomogra-
phy (
15
): measuring the travel times of sound waves to infer the average temperature of the
ocean traversed by these waves. It is currently impossible to predict the absolute travel time
of
T
waves with sufficient accuracy, however, because uncertainties in the timing and location
of the source as well as in the structure of the solid earth are too great. The origin times and
locations of earthquakes are typically derived using
P
- and
S
-wave travel times to a set of ref-
erence stations. Due to uncertainties in the solid-earth structure, earthquakes can be mislocated
by tens of kilometers and have timing errors of a few seconds, especially if the station coverage
is sparse. These uncertainties in source properties translate into uncertainties of a few seconds
in the arrival time, which dominates over the sought-after signal arising from changes in the
ocean temperature (Fig. 3).
We circumvent this problem by measuring relative
T
-wave travel times using repeating
earthquakes (Fig. S1). The generation and propagation of seismic waves is sufficiently simi-
lar between repeaters that source and structure uncertainties cancel out and the change in travel
time is dominated by changes in the ocean temperature. We search for repeating earthquakes
based on the waveform similarity of the
P
wave received at a reference station (PSI in Fig. S1)
and of the
T
wave received at the
T
-wave station (DGAR in Fig. S1). Because these waveforms
are highly sensitive to the source locations, a high waveform cross correlation (CC) coefficient
indicates overlapping earthquake locations (
32
). The relative origin time is simultaneously mea-
sured by the CC maximization at the reference station, which is then used to align the
T
waves
and infer the travel time change
D
t
0
(Fig. S1). The structure of the solid earth does not affect
this travel time change as long as changes in the solid earth between the events are negligible.
2
As discussed below, this is generally expected to be a very good assumption, and the travel time
change
D
t
0
can indeed be attributed primarily to changes in the ocean temperature.
Sound speed dependence on temperature, salinity, and pressure
The speed of sound in seawater increases with increasing temperature, salinity, and pressure.
Throughout this work, we calculate the sound speed using the Gibbs Seawater Toolbox (
42
).
A typical speed of sound is
c
=
1
.
5 kms