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Iceberg Harmonic Tremor Seismometer Data, Antarctica

Summary

Seismometers were placed on a 25 km by 50 km iceberg called C16 in the Ross Sea, Antarctica, to identify the Iceberg Harmonic Tremor (IHT) source mechanism and to understand the relevance of IHT to iceberg calving, drift and break-up. The seismic observations reveal that the IHT signal consists of extended episodes of stick-slip icequakes (typically thousands per hour) generated when the ice-cliff edges of two tabular icebergs rub together during glancing, strike/slip type iceberg collisions (e.g., between C16 and B15A). With the source mechanism revealed, IHT may provide a promising signal useful for the study of iceberg behavior and iceberg-related processes such as climate-induced ice-shelf disintegration.

Here, a single day of seismometer data for a single station on iceberg C16 is provided as an example of "a day in the life of an iceberg" for use by scientists and students wishing to know more about IHT. The station data is from C16 "B" site on C16's northeast corner, and the day is 27 December 2003, a day when B15A struck C16 and caused an episode of tremor that was particularly easy to identify and understand.

This represents only a small fraction of the total data that exist for the seismic program on iceberg C16. The full data are archived at the IRIS data center (where seismic data is commonly archived). This one-day data set is to provide glaciologists with ready access to a good example of IHT that they can use for teaching and for demonstration purposes. Data are available in comma-delimited ASCII format and Matlab native mat files. Data are available via FTP.

Citing These Data

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

MacAyeal, Douglas, Emile Okal, Richard Aster, and Jeremy Bassis. 2008. Iceberg harmonic tremor seismometer data, Antarctica. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N5445JD6.

Overview Table

Category Description
Data format comma-delimited ASCII format, Matlab native mat files
Spatial coverage and resolution Southernmost Latitude:77 South
Northernmost Latitude: 77 South
Westernmost Longitude: 168 East
Easternmost Longitude: 168 East
Temporal coverage and resolution 27 December 2003 to 28 December 2003, 100 Hz sample rate
File naming convention .HHN: 100 hertz sample rate (HH), North (N)
File size 150 megabytes
Parameter(s) seismology, harmonic tremor, iceberg, Antarctica, Ross Ice Shelf
Procedures for obtaining data

Available via FTP


Table of Contents

  1. Contacts and Acknowledgments
  2. Detailed Data Description
  3. Data Access and Tools
  4. Data Acquisition and Processing
  5. References and Related Publications
  6. Document Information

1. Contacts and Acknowledgments

Investigator(s)

Douglas MacAyeal
Department of Geophysical Sciences
University of Chicago
5734 S. Ellis Ave.
HGS 413
Chicago, Illinois 60637 USA

Emile Okal
Department of Earth and Planetary Sciences
Northwestern University
Evanston, Illinois 60208-2150, USA

Richard Aster
Dept of Earth & Environmental Science
New Mexico Tech
Socorro, New Mexico 87801 USA

Jeremy Bassis
Institute for Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California, San Diego
La Jolla, California, 92093-0225 USA

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Acknowledgements

This research was supported by National Science Foundation (NSF) Office of Polar Programs (OPP) grant 0229546 awarded to D. MacAyeal.

2. Detailed Data Description

Format

Space delimited ASCII, one header line. The data for each channel are stored as Albuquerque Seismic Lab (ASL) format in an ascii text file. To read the data, the header gives the station name, channel, start time and sample rate. Following the header (one line only) is the time series.

File Naming Convention

File is named according to the following convention:

.HHN: 100 hertz sample rate (HH), North (N)


HHZ, HHE and HHN are standard seismological variables, they stand for the 100Hz vertical, east and north seismometer channel readings (HH says "100 hz sample rate" and Z, E or N appended on tells the channel, or component of motion.)

File Size

Approximately 150 Megabytes

Spatial Coverage

Southernmost Latitude: 78 South
Northernmost Latitude: 78 South
Westernmost Longitude: 178 West
Easternmost Longitude: 178 West

Temporal Coverage

27 December 2003 to 28 December 2003, 100 Hz sample rate

Parameter or Variable

Parameters include: seismology, harmonic tremor, iceberg, Antarctica, Ross Ice Shelf

Sample Data Record


To read the data, the header gives the station name, channel, start time and sample rate.
Following the header (one line only) is the time series.

The header is: C16B HHN 03 361 00 00 24 78 100.00 8625661 1.265E8 50 GSNVBB1

C16B 
HHN
03
361
00
00
24
78
100.00
8625661
1.265E8
50
GSNVBB1
-3824
-3635
-3419
-3357
-3276
-2820
-2962
-3278
-3139
-3511
-2867
-2250
-2546
-2109
-1675
-1475
-1473
-1644
-1721
-1288
-549
-1664
-2427
-1904
-1989
-1542
-1688
-2573
-2981
-2766
-2390
-2344
-2636
-2967
-3054
-3403
-3044
-3337
-3841
-3787
-4369
-4047
-3824
-3554
-3182
-3576
-3598
-3571
-3988
-4460
-4433
-4359
-4330
-4396
-4703
-4560
-4374
-4674
-4773
-4774
-4801
-4530
-4624
-4768
-5056
-5371
-5156
-5146
-4910
-4564
-4624
-4673
-4900
-4953
-4774
-4755
-4705
-4670


3. Data Access and Tools

Data Access

Available via FTP

Volume

Approximately 150 Megabytes (MB)

Software and Tools

An example of Matlab code used to read the data is provided below.

Please visit The Incorporated Research Institutions for Seismology (IRIS) for more information.


% Read ASL files:
clear all fname='/Users/doug/Sea_ice_2006_data/XV_C17_11070346xx.HHZ';
fid=fopen(fname);
"fseek(fid, 0, 'eof');"
filesize = ftell(fid)
fclose(fid);
fid=fopen(fname);

header.informalname='Sea Ice Runway';

%%
% text=fgetl(fid)
% break
%
%
"text=textscan(fid,'%s',1);"
Name=text{:};
"text=textscan(fid,'%s',1);"
Channel=text{:};
"text=textscan(fid,'%n',1);"
year=2000+text{:};
"text=textscan(fid,'%n',1);"
Julianday=text{:};
"text=textscan(fid,'%n',1);"
hour=text{:};
"text=textscan(fid,'%n',1);"
minute=text{:};
"second=textscan(fid,'%n',1);"
second=second{:};
"secondfrac=textscan(fid,'%n',1);"
secondfrac=secondfrac{:};
second=second+secondfrac/100;

"text=textscan(fid,'%n',1);"
sample_rate=text{1};
sample_rate_units='Hz';

"N_samples=textscan(fid,'%n',1);"
N_samples=N_samples{1};

"mysterynumber=textscan(fid,'%f',1);"
mysterynumber=mysterynumber{1};

"mysterynumber2=textscan(fid,'%f',1);"
mysterynumber2=mysterynumber2{1};

"finaltitle=textscan(fid,'%s',1);"
finaltitle=finaltitle{:}; "[cal_month,cal_day] = julian2calendar(Julianday,year);"

time_start=datenum([year cal_month cal_day hour minute second]);
sample_period=1/sample_rate;
time_end=time_start+(N_samples-1)*sample_period/(24*60*60);

"disp(['Reading station ' Name{:} ', channel ' Channel{:} ', ' num2str(N_samples) ' records.'])"
disp(['Time span: ' datestr(time_start) ' to ' datestr(time_end) '.'])

%%

"data=textscan(fid,'%n',N_samples);"
data=data{:};

Please refer to the PASSCAL website for information on how to turn the data (raw instrument counts, units of each channel are ""counts"") into ground displacement.
"To convert the ""counts"" (raw instrument data provided in the files here) to ground velocity, use this formula:"

The appropriate conversion would be:

Amplitude(counts) * A/D conversion factor(V/counts)
ground velocity = ----------------------------------
Preamp gain * Sensitivity (V/m/s)

Sensor parameters for the Guralp 40T are:
Guralp CMG-40T Response

Natural Period
30 seconds (0.03333 Hz)

Sensitivity
800 V/m/s

Normalization Factor
1.0 for a gain of 1.0

Poles: 2
sRe (radians/s) sIm (radians/s)
-0.1481 0.1481
-0.1481 -0.1481
Zeroes: 2
Re (radians/s) sIm (radians/s)
0 0
0 0

Related Data Collections

4. Data Acquisition and Processing

Sensor or Instrument Description

A three-component Guralp 40T seismometer digitized by a Quanterra Q330

Data Acquisition Methods

Data were collected on 3 channels (east, north and vertical) at a sample rate of 100 Hz. The seismometer was placed in a shallow snow pit on iceberg C16 near its northeast corner. Operation was powered by a solar-charged 12-volt battery.

5. References and Related Publications

MacAyeal, D. R., E. A. Okal, R. C. Aster, and J. N. Bassis (2008), Seismic and hydroacoustic tremor generated by colliding icebergs, J. Geophys. Res., 113, F03011, doi:10.1029/2008JF001005.

6. Document Information

Acronyms

The following acronyms are used in this document.

Acronyms and Abbreviations
Acronym Description
FTP File Transfer Protocol
NSIDC National Snow and Ice Data Center
MB Megabytes
ASL Albuquerque Seismic Lab (ASL) format
URL Uniform Resource Locator

Document Creation Date

October 2008

Document URL

http://nsidc.org/data/docs/agdc/nsidc0349_macayeal/index.html