Seismic research at UWA

SEISMICITY IN THE PILBARA REGION

In October 2005, 8 portable seismographs were deployed in the Pilbara region. It is hoped that they will be collecting earthquake data in the region for at least three years. The instruments were supplied by the Reaseach School of Earth Sciences of the ANU, and the data collected is stored on the instrument, and will be collected on subsequent visits.

Three similar instruments were deployed in the area for a different survey in the second half of the year 2000. A small number of near earthquakes occurred during this period, which will be more accurately located as a result of the survey.

Below - location of seismographs in the Pilbara area

( Blue - seismographs installed 2005, Orange- seismographs deployed July - Dec 2000, White - permanent seismographs operated by Geoscience Australia)

ANU seismographs in 2000

Crustal Structure under South West Western Australia

Reseach is currently being undertaken by PhD student Konstantin Galybin,(galybk01@segs.uwa.edu.au) under the supervision of Associate Professor Mike Dentith, into the crustal structure under SW WA. This is being done by studying the travel times of large quarry blasts to various receiver locations.

The South West Seismic Zone (SWSZ) is a major intraplate earthquake province and represents a seismic hazard to Perth, a vibrant city of 1.5 million. A study into structure of the crust, and subsequently the causes of seismicity in southwest of WA, is of major importance as in the last 20 years, as over six thousand earthquakes occurred in the SWSZ. Some of magnitude well over 5 (see Recent WA Eqs section). The famous Meckering, Cadoux and Burakin earthquakes originated in the SWSZ.

Quarry blasts are excellent seismic events for geophysicts to study the structure of the Earth. Quarry blasts can be thought of as small earthquakes, where the precise position and time of the event is known. The blasts are recorded in sections, approximately 120 km long, at different offsets with approximately 5 km spacing between the receivers, and then combined into a complete profile of over 300 km long. A North-South profile stretching from Moora, 150 km north of Perth, to Collie, 150 km south of Perth, has been created using various quarry blasts along the recording line: Moora silica mine (North end); Red Hill, The Lakes, Orange Grove, Gosnells Readymix and Byford roadstone mines (Perth Area); and Collie coal mines (South end). Some small sections are presented below.

The data are collected using portable seismic Solid State Recorders made by the ANU and Geoscience Australia. Recorders are deployed on the surface with the geophones buried in the ground. The data are recorded for 38 hours, at 100 Hz to avoid aliasing of the seismic wave. Data are then loaded into Matlab and processed (filtering, stacking, etc) to create a complete profile (see below).

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Analysis of the first breaks and seismic phases of the blasts, at such long offsets, can be used to determine the deep crustal structure of the Earth. The cross-over for the critically refracted arrivals from the Moho is usually about 120-150 km away from the source, thus allowing for the determination of the depth to the Moho. This study complements similar studies carried out by Mathur (1974 and 1977), but the main objective of it, is to build up a more accurate seismic velocity model for the SWSZ, which can be used in a study of the intraplate stress distribution. There has been some conjecture about the causes of intraplate seismic activity in WA; some of the models proposed for the SWSZ are similar to those of the New Madrid seismic zone in the US, and others similar to the south Indian shield and this study aims to test these hypotheses.

The quarry blasts being used are from the following sites

Byford

Red Hill

Moora

The Lakes