To support the World Vision Tsunami Appeal, DMITRE's Mineral Resources Division produced a poster that displayed the seismic record of the Sumatran earthquake as recorded by a local seismic station.
This poster contrasts the strength of the Indian Ocean earthquake with the normal background movements of the Earth's crust. It has tremendous visual impact and is a valuable educational resource and archive of the devastating event.
This full colour poster is 594mm x 841mm (23 1/4in x 33 in) and is printed on high quality paper. A low-resolution A4 example is available for viewing (.pdf file ~ 390kb). The cost of the poster is $10 and as stated, proceeds from the sale will go to the World Vision Tsunami Appeal. To order a copy of the poster, please contact the Customer Information Centre on the ground floor, 101 Grenfell St, Adelaide.
PIRSA Earthquake Poster
GPO Box 1264
Adelaide, SA 5001
Phone orders: +61 8 8463 3000
The poster displays two days of recording, a quiet Christmas day followed by a chaotic Boxing day, with the main magnitude 9.0 earthquake followed by many aftershocks. It was recorded on the Hallett seismograph station, north of Burra. Timing marks are indicated.
Moderately large earthquakes occur virtually every day in some part of the world. On the Christmas day record are two earthquakes in Indonesia, a magnitude 5.4 event in Sulawesi and a magnitude 4.5 event near Java. The Boxing Day earthquake took about 5 minutes to rupture from its starting point (hypocentre) offshore from Sumatra, northwards through the Nicobar and Andaman Islands. This rupture was about 1100km long. From this north-south fracture, with the west side going down and the east side coming up, the tsunami was generated. The main shock arrived at Hallett about 10 minutes after the earthquake began near Sumatra. This recording is from a short period (high frequency) seismograph, and the waves drove the recorder off-scale for about 10 minutes.
A record a number of aftershocks were recorded after the main earthquake. The larger ones are labelled (with magnitudes) on the poster but many, many more can also be seen. Almost every squiggle on the Boxing Day record is an aftershock from the big event (except for one magnitude 5.4 event in the Philippines).
Occasionally a smaller magnitude event, say a magnitude of 5.3, looks larger than a bigger event, say 6.3. Magnitudes are calculated by averaging a number of stations. The swing of the pen is not always a good indicator of magnitude; the duration of vibration is often better. A deeper earthquake is often more clearly recorded by a big swing of the pen.
Vibration continued after the main earthquake for about 3 hours, as waves travel outwards from the rupture, reflecting and refracting from various layers, and travelling at various speeds. For the main shock and the largest aftershock, long period surface waves can be seen well after the P arrival. Even for some of the magnitude 6.2 to 6.5 aftershocks small long period waves can be seen. Long period seismographs measure strong movement for many hours after big events, and very long period seismographs show movement for days.
The seismogram displayed on the poster is recorded on a long sheet of paper wrapped around a drum. The pen travels from left to right on the top line, then left to right on the second line and so on. Each line represents 15 minutes of recording.
Each minute, hour, six hour and 24 intervals are marked by an upward bump.
Universal Time, much like Greenwich Mean Time, is used by seismologists worldwide for convenience.
More information on South Australian recording stations can be found here.
A Combined Effort
Many earthquakes such as this one, are recorded in many countries, spanning many time zones, and the readings from many seismographs are required to work out the most accurate epicentre, depth and time of occurrence, and the sense of movement. This makes seismology an international effort that crosses many boundaries. Combined readings from modern seismographs can be analysed to work out the starting point of the rupture, the speed and direction of its progress, and the approximate amount of movement.