Iceberg Harmonic Tremor, seismometer data

Iceberg harmonic tremor (IHT) emanating from tabular icebergs in the Southern Ocean and around the coast of Antarctica is observed as seismoacoustic signals on islands in the equatorial Pacific, as hydro-acoustic signals in the Indian Ocean, and in local and regional Antarctic seismograms. To identify the IHT source mechanism and to understand the relevance of IHT to iceberg calving, drift and break-up, we deployed seismometers on a 25 km by 50 km iceberg called C16 in the Ross Sea, Antarctica. 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.

The seismometer instrument was a Guralp 40T digitized by a Quanterra Q330 on loan from the PASSCAL instrument center. Data were collected on 3 channels (east, north and vertical) at a sample rate of 100 Hz. 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.

An example of Matlab code used to read the data is provided as follows:

% 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:

ground velocity = [Amplitude(counts) * A/D conversion factor(V/counts)] / [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
sRe (radians/s) sIm (radians/s)
0.0000 0.0000
0.0000 0.0000