Calving localization at Helheim Glacier using multiple local seismic stations

Mei, M. Jeffrey; Holland, David M.; Anandakrishnan, Sridhar; Zheng, Tiantian

doi:https://doi.org/10.5194/tc-11-609-2017

Articles | Volume 11, issue 1

https://doi.org/10.5194/tc-11-609-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/tc-11-609-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 11, issue 1

Research article

|

22 Feb 2017

Research article |

| 22 Feb 2017

Calving localization at Helheim Glacier using multiple local seismic stations

M. Jeffrey Mei, David M. Holland, Sridhar Anandakrishnan, and Tiantian Zheng

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Final revised paper (published on 22 Feb 2017)
Preprint (discussion started on 20 Apr 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review', Anonymous Referee #1, 03 Jun 2016
- AC1: 'Response to reviewer 1', M. Jeffrey Mei, 02 Jul 2016
  - RC4: 'Reply to questions', Anonymous Referee #1, 11 Jul 2016
RC2: 'review of Mei et al.', Jason Amundson, 16 Jun 2016
- AC2: 'Response to review', M. Jeffrey Mei, 05 Jul 2016
RC3: 'Review of Mei et al.', Anja Diez, 29 Jun 2016
- SC1: 'Grid search for all events?', M. Jeffrey Mei, 05 Jul 2016
  - SC2: 'updated plot with gridsearch vs hyperbolas', M. Jeffrey Mei, 05 Jul 2016
- AC3: 'Response', M. Jeffrey Mei, 30 Jul 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by M. Jeffrey Mei on behalf of the Authors (10 Aug 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Oct 2016) by Robert Bingham

RR by Jason Amundson (19 Oct 2016)

Suggestions for revision or reasons for rejection

The authors have addressed my comments from my first review. I now have only a few minor comments for the authors to address.

Page 1

Line 3: It kind of feels like there is a sentence or phrase missing here. Perhaps: “The crossing points of the hyperbolas provide an estimate of the location of the calving event.” If acceptable, then delete the sentence starting with “Using local stations...”

Line 4: Suggest “emergent nature of calving-generated seismograms”. You mean that the seismograms are emergent, and not the calving events themselves (correct?).

Line 15: Although this statement is correct, what is especially unique about tidewater glaciers is that they can undergo very large and rapid changes in volume with minimal climate forcing and that their sensitivity to climate depends on where their termini are located.

Page 2

Line 5: “variations in meltwater sliding”? Do you mean “variations in basal motion due to meltwater input”?

Line 8: Suggest emphasizing that you are referring to terrestrial radar.

Line 12: Delete “(providing seismic data)”

Lines 19-20: This sentence seems out of place and unnecessary.

Lines 21-22: Tsai et al proposed that glacial earthquakes were caused by icebergs pushing off of the glacier terminus.

Page 8

Line 2: Why is that intersection an estimate of the size of the calving event, and not an indication of error in the method?

Line 5: “we have Figure 7” sounds awkward.

Page 12

Line 11: “regional” is misspelled.

Line 29: Density varies very little with depth in glaciers and is generally treated as a constant. Crevasses obviously affect the bulk density, and air bubbles in the ice have a small impact on the density. It seems unlikely to me that seismic wave velocities would change much as a result of variations in ice density.

Page 13

Line 11: Suggest “as the calving events occur” (delete “are”)

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RR by Andreas Köhler (19 Oct 2016)

Suggestions for revision or reasons for rejection

The revised manuscript has improved and most of my previous comments have been addressed. However, I feel that some more revisions are necessary. There are some important issues the authors need to address and clarify, particularly, related to the gridsearch method and the use of camera imagery.

Introduction

The second paragraph needs to be reorganized a bit. The description of the motivation for calving monitoring, the direct calving monitoring methods available, and the use of seismic detection and subsequently localization methods is mixed and therefore a bit hard to follow. I would suggest the following structure:
(1) Motivation
For example, sentence from “There have not been enough ..” (Page 2 line 2) until “... on a long-term basis to capture events.” (line 6) should be moved after “The lack of understanding ...” on Page 1 line 18.
(2) Overview about direct calving monitoring methods and their disadvantages or limits: visual time lapse, satellite, radar, ...
(3) Seismic methods: Here, it would be good to clearly distinguish between event detection (for example STA/LTA) and localization (main focus of this study).
- Page 2 line 20: Rephrase sentence, for example to: “ A common automated calving … is to use triggers based on short-time and long-time averages of the continuous seismic signal (STA/LTA).”
- Page 2 line 1: Sentence unclear. Rephrase maybe to: “To implement and validate detection and localization methods, visual confirmation of calving locations are required for example through terrestrial observations or (for sufficiently large events) satellite imagery.”

Page 2 line 22: interaction with the sea surface

Page 2 line 16: Murray et al. 2015b: In that study basal crevassing is part of a model describing the calving of tabular icebergs. The seismic signal (glacier earthquake) is not directly related to the crevassing in that study or at least it is not discussed there. I think the mechanism described in Murray 2015a is more relevant here.

Page 3 line 1: Rephrase, for example: “Another method to locate calving events, known as beamforming, uses the seismic signals recorded on several array stations to determine the time delay associated with a backazimuth that aligns the signals coherently (Koubova, 2015, Köhler et al. 2015).”

Data

Describe camera imagery data and indicate camera position in Fig 1.
The seismic detection method is not explained clearly, i.e., how is the catalog of 11 events obtained (STA/LTA? Visual inspection?). How complete is it?

Hyperbolic method

Page 5 line 5: remove “describe the hyperbolic method and” and change to “... we apply the hyperbolic method ...”

It might be helpful to mention that in order to draw a hyperbola, the parameter “b” needs to be determined, which is b=sqrt(c^2 -a^2) where 2c is the (known) distance between F1 and F2.

The gridseach method is not explained sufficiently. It is just briefly described in the introduction, but the method section would be the proper place for it. I am not sure if I understand the processing flow completely. Please correct me and clarify in the text:

(1) Minimize residuum (dt – dt_measured) with dt = f(x,y,veff)=ds1/veff – ds2/veff, with ds1/2 being the distance between (x,y) and seismometer 1 and 2. If we would plot the residuum map over x/y, we would see that the error is minimal along hyperbolas. So the error is evaluated over all station pairs to obtain a distinct minimum: sum(dt – dt_measured). This is done for each calving event and the mean of all obtained velocities veff is computed. Is this correct?
(2) Draw hyperbolas using veff and get intersection area
(3) Repeat gridsearch with fixed veff

Why is step 3 done? Wouldn't it be more accurate to use directly the best location (x,y) found in the previous gridsearch? What is the benefit of drawing hyperbolas instead of simply using the gridsearch results? I am not sure if the intersection areas can be used as estimates for the calved areas (page 8 line 2). It seems to be more an estimate of the location uncertainty. Please clarify.

Identifying signal lags

“1.44 standard deviations of all gradients”: Please define what “all gradients” means. Do you mean all gradients measured within a time window around the event? How long is it?

Page 7 line 4: The particle motion plots show Rayleigh waves, but also suggest a Love or SH wave contribution on the transverse component. So the conclusion that the signals are dominated by Rayleigh waves is too strong. I would rather say it is dominated by surface waves.

Change “particle plots” to “particle motion plots” (also in Fig 5).

Page 7 Line 12: What is the reason for using these velocity limits (and not a wider range)?

Page 7 Line 13: This is the first time the “11 identified calving events” are mentioned. Are they identified from images or from the seismic data (visually or STA/LTA)? See comment in data section.

Discussion

How complete is the calving catalog of 11 events? How does the ice loss corresponding to these events compare to the frontal retreat?

I would be very helpful to make more use of the images of the calving front which are mentioned several times. Maybe a few examples can be shown. Most important, can the images be used to estimate the locations and sizes of the 11 calving events? If so, how do these locations compare to the seismic locations? Maybe indicate these locations in Fig 7.

Brune model and fault size: As I wrote in my previous comments, I would be careful to interpret the resulting fault sizes too much. There might be some contribution of fracturing to the seismic signal, but faulting is probably not the dominant source mechanism here. I think it is worth to be discussed (as the authors do), but I believe it should be concluded more clearly that the dominant mechanism might for example be the one discussed by Murray et al. 2015a. As I stated earlier, in Murray et al 2015b basal crevassing is described as the initial part of the calving process, but the glacier earthquake generated is not directly related to crevassing. In this context, it would be interesting to investigate if there is an earlier part of the seismic calving signal that could be related to crevassing. Brune's source model could then be applied only to that part. Do you see anything like this in your calving signals?

Page 12 line 18: Could this also be due to ice/glacier anisotropy?

Page 13 line 3: Please explain what you mean with “not fully automatic”

page 13 line 6: remove “are” and “this mean that”

page 13 line 8: please explain “is not robust”. What alternatives would you suggest to determine time lags? Please also mention the envelope cross-correlation (you did this in the review response, but this is also relevant in the manuscript).

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ED: Publish subject to minor revisions (Editor review) (07 Dec 2016) by Robert Bingham

AR by M. Jeffrey Mei on behalf of the Authors (15 Dec 2016) Author's response Manuscript

ED: Publish subject to technical corrections (26 Jan 2017) by Robert Bingham

AR by M. Jeffrey Mei on behalf of the Authors (02 Feb 2017) Manuscript

Short summary

We determine a method to locate calving at Helheim Glacier. By using local seismometers, we are...