Download Dear Professor Chapman, With best regards, Jeff Gu and Coauthors

Dear
Professor
Chapman,
First,
thank
you
for
your
help
throughout
the
submission
and
review
process.
We
have
taken
the
reviews
seriously
and
hope
our
revised
manuscript
shows
a
token
of
our
collective
appreciations.
We
also
thank
the
two
reviewers
for
their
careful
reading
of
the
manuscript,
as
well
as
for
their
insightful
suggestions.
We
have
made
every
effort
to
address
the
reviews
(hence
a
longer‐than‐expected
revision
process).
All
the
points
raised
by
the
reviewers
have
been
answered
with
utmost
care.
We
have
made
the
following
key
changes
to
improve
the
overall
quality
of
the
paper,
1. We
add
and
discuss
3
additional
figures
per
reviewer’s
requests.
Figure
1
has
also
been
modified
to
include
a
small
inset
to
show
the
relative
geographic
location
of
the
array.
2. In
view
of
the
imbalance
of
the
3
topics
(receiver
function,
seismic
tomography,
and
anisotropy)
where
topic
2
had
much
less
coverage
(see
review),
we
have
1)
Switched
the
order
of
presentation
where
receiver
function
and
anisotropy
are
discussed
before
tomography
(which
is
relatively
new).
2.
Add
an
additional
figure
in
the
tomography
section
and
provide
more
details
on
the
method,
results
and
discussions.
All
of
the
above
have
been
performed
with
great
care
and
now
all
three
sections
are
equally
well
covered
and
useful
for
the
readers.
3. Corrected
the
phrase
“Western
Canadian
Sedimentary
Basin”
to
“Western
Canada
Sedimentary
Basin”.
This
includes
a
minor
change
to
the
title
of
the
paper
for
the
sake
of
correctness.
4. The
paper
has
been
proofread
and
very
minor
textual
change
have
been
made
to
weed
out
small
errors/inconsistencies.
The
manuscript
has
improved
significantly
in
our
view,
thanks
to
suggestions
by
you
and
the
reviewers.
Hopefully
it
is
satisfactory.
Again,
we
want
to
express
our
sincere
appreciations
to
you
for
your
speedy
handling
of
this
manuscript.
Attached
to
this
letter
is
a
point­by­point
response
to
the
reviews.
With
best
regards,
Jeff
Gu
and
Co­authors
Point­by­Point
response
to
the
reviews:
Review
1:
(1)
This is a concise, well-written paper that describes the recently installed CRANE
seismic network and presents some preliminary analyses of the data (receiver functions,
ambient noise, and shear wave splitting). The paper provides an excellent documentation
of the CRANE network and a good first-pass analysis of the data. Of course, the analyses
presented here (and their interpretation) are very preliminary, but the authors are very
upfront about this. Overall this paper provides a very good overview of the CRANE
dataset and some preliminary findings. I have a few suggestions for very minor revision,
detailed below, but in my opinion this paper will make a good contribution to SRL.”
We thank the reviewer for the constructive comments.
(2) It is probably worthwhile to mention what type of Trillium sensors are used in the
deployment in the description of the array specs.
We agree, on page 4, section 2, paragraph 2, we have now specified
“Trillium 240 and 120 seismometers”.
(3) The discussion of different tectonic domain names (Buffalo Head, Wabamum,
Thorsby, etc. at end of section 3.1 and elsewhere in the paper) is a little confusing for
readers who are not terribly familiar with the region. Would it be possible to add these
names to a map in one of the figures?
We agree. Now, a new figure (Figure 5) has been added to show the tectonic
regimes. It has been referenced in the main text and figure caption is added.
(4) In the description of the data preprocessing steps for the receiver function analysis –
what epicentral distance range was used?
We agree. Now page 8, paragraph 2, line 2 reads,
“All earthquake-station pairs are restricted to the epicenter distance range of 30-90
deg and the distribution of source-receiver paths shows a dominant northwestsoutheast orientation (Figure 6b).”
(5) Regarding the shear wave splitting measurements: please clarify in the text how
many SKS measurements were made (on average) at each station. Are the measurements
on the map in Figure 10 average measurements or single measurements? Also, it wasn’t
clear to me how the double-layer anisotropy at stations EDM and JOF was inferred. Was
this from the backazimuthal variation of splitting parameters? Was two-layer anisotropy
explicitly modeled? This could be clarified.
We agree with both comments.
---Regarding the number of events and single-vs-multiple layer anisotropy, page 12,
paragraph 2, 2nd last sentence now reads,
“The majority of the stations are constrained by more than 10 earthquakes, with the
glaring exception of HYLO where only one earthquake is available due to a late
deployment schedule. Both single- and two-layer anisotropy are considered for each
station based on waveform correlations.”
We also added the phrase “for multiple earthquakes” after “cross-convolution
method” in the prior sentence to be more specific.
--- Regarding azimuthal coverage, we have decided to add Figure 6 to the
manuscript. This figure shows the event-station locations (a) and azimuth
distribution (b). While this is not specific to SKS splitting, but it is equally
informative for receiver functions and SKS measurements. All relevant elements
(references, captions) have been added to the main text.
(6) Figure 1 – I would suggest adding an inset to this map showing the location of the
study area in the broader geographic and/or tectonic context of North America (for any
readers whose Canada geography is lousy!).
We agree, though we think the reviewer is being modest here. A map inset has been
added to figure 1 to show the relative location of the study region within North
America. It actually improves the overall look of that figure!
(7) Figure 4 – the caption to this figure mentions the high-frequency energy from a
regional earthquake visible on some of the records. Of course, the other striking feature
in most of these records is the very well-recorded microseism with a period of ~8 sec or
so – might be worth mentioning this feature in the caption?
We agree, that is an astute observation. We have modified the caption of this figure
and added to the end,
“Also visible on these records is a persistent ~8-sec microseismic signal.”
(8) Finally, the paper seems to be missing an abstract (at least in the version I have).
Was this an omission?
This may have been a problem with the adobe reader on the reviewer’s site.
Review #2:
The purpose of the paper is to provide an overview of the array and some of the results
from the deployment. Although the results are preliminary, I find they are rather
interesting and important, provided that these are first-hand observations for this part of
North America. The paper is clearly written and well organized, however there are
certain places that need clarification and improvement from a technique point of view. I
therefore suggest its publication for the journal with minor revisions.
Again, we appreciate the kind words and constructive comments.
(1) In Abstract three teleseismic techniques are mentioned, however strictly speaking the
ambient noise part doesn’t carry as much weight as the other two. For example, Fig. 8
provides merely data observations. What tectonic implications can we gain from this?
We agree with reviewer. To remedy this, we have added the following sentences
before the last sentence in the abstract,
“Finally, our preliminary inversions using ambient seismic noise indicate 0.8+
km peak-to-peak phase velocity variations throughout the crust. The upper
crust beneath the Alberta Basin is dominated by low Rayleigh-wave phase
velocities. A lower-than-expected correlation between seismic velocities and
tectonic domain boundaries suggests significant tectonic overprinting in the
southern Western Canada Sedimentary Basin.”
---- Furthermore, we have switched the order of presentation for sections 3.3 and
3.4. Now shear wave splitting is discussed earlier (which is more mature), while
significantly improved discussions are provided for the noise correlation
tomography as ongoing efforts. Please check section 3.4 which we wrote as well
as we could to raise it to the level of the other two sections while keeping an open
mind on specific features. A new plot (see Figure 14) is added to show the
tomographic results.
(2) Section 3.2. This is a tectonically very interesting region and the crustal thickness
estimates from receiver functions there are the direct observation of the transition from
the more tectonically active Cordilleran orogen to the stable core part of the craton.
These are important estimates, however I feel several improvements could be used.
1. Why the PREM model instead of a better regional model is used here? How much
different the thickness would change if a different model is applied?
We have calculated the difference between PREM and a “perturbed” regional
model by increasing the contrast between mantle P and crustal S around Moho
using Dziewonski and Gilbert (1976). The result shows about 2.6 km movement in
crustal depths. This is a conservative estimate that introduces a net change of 2
km/sec in velocity contrast across the boundary, and this level of depth uncertainty
will not affect the key results (contrast between Rockies at 55-60 km depth and the
plains at ~40 km). The following sentences have been added to the main text to
discuss this little experiment,
“We convert these time-domain receiver functions to depth based on PREM
(Dziewonski and Anderson, 1981), which provides a first-order approximation for
the depths of crust and mantle reflectors in the absence of more accepted regional P
or S velocity models. If one assumes a travel time uncertainty of 1 sec and an
epicentral distance of 60-deg, a net velocity perturbation of 2 km/sec (1 km/sec
increase in mantle P velocity and 1 km/sec decrease in lower-crustal S velocity
relative to PREM) will displace the Moho interface by ~2.6 km using the
perturbation theory introduced by Dziewonski and Gilbert (1976).”
2. It seems the crust thickness is read from depth-migrated traces. If it’s true the
estimated thickness is less accurate than the H-k method by Zhu and Kanamori, which is
popularly used by many others.
We agree. However, in view that this study is really trying to get a first-order
analysis of Moho, with the specific goal of surveying all reflective structures in the
crust and mantle, the depth should be satisfactory as long as PREM velocities are
not too several km/sec away from the truth (see comments above). The consistency
of our measurements with other regional exploration-based or teleseismic results is
also reassuring. That being said, an independent paper focusing on receiver
function imaging (with much more details) will take this suggestion into careful
consideration in the near future.
3. It would be easier for readers if the terrane names (Loverna Block; Locombe domain;
etc) are labels on Fig.1 or Fig. 6
We agree. This echoes the comments from review #1. First, Figure 1 has added a
map inset. More importantly, a new figure (Figure 5) is fully devoted to the domain
names and boundaries.
4. What is the event distribution used in the receiver function and SKS splitting stacks?
This is important especially for the SKS stacking if back-azimuthal dependent
measurements are observed. How are the stacks formed?
We agree. Again, this echoes comments from Review #1. We have added a new plot
(now Figure 6) that shows the earthquake-station pairs and the general distribution
of back-azimuths.
5. More details of the two-layer anisotropy at stations EDM and JOF (Fig. 10) could be
elaborated.
We agree. Page 14 paragraph 2 is specifically aimed at addressing this issue, but a
concluding statement would definitely help the readership. We have added to page
14, paragraph 2 as the second last sentence,
“In other words, shear deformation base of the lithosphere (~200 km) and disrupted
flow at shallower depths could both be present, hence producing complex, multilayered anisotropy in this region.”