The Cadoux Earthquake, 2 June 1979

-

Above - damage to the Recreation complex, Cadoux, June 1979

Cadoux is a small settlement in the WA wheatbelt region, some 180 km to the northeast of Perth. Seismic activity in the region in the Cadoux- Burakin region was relatively low before the major earthquake just east of Cadoux on 2 June 1979. This earthquake had a Richter magnitude (ML) of 6.1 and was the second most damaging (after Meckering) in the history of WA. Damage totalled approximately $3.8 million ( 1979 dollars).

Most tall buildings in Perth swayed during the Cadoux earthquake, but the only damage reported was the loss of mercury from the Rottnest Island lighthouse.

The most noticable effect of the Cadoux earthquake in the Perth metropolitan area was a widespread electrical blackout caused by the tripping of mercury switches in transformers.

There were many aftershocks from this earthquake, and although their frequency has declined with time, aftershocks of the event are apparently still happening.  LEFT - The Cadoux Fault Scarp

The following information has been extracted from  Geol Survey of WA Report 11 (Lewis, Daetwyler, Bunting & Moncrief, 1981)

Fault Statistics - max throw , 1.4 m, max heave, 0.68 m, max lateral displacement, 0.65 m

ACTIVITY PRIOR TO JUNE 1979

The apparently aseismic nature of the Cadoux area prior to 1959 is confirmed by the report of the (Mundaring Geophysical) Observatory that only one event, that of 3 October 1966 (ML 3.6) was located within 25 km of the town between 1959 and 1968. Since 1968 however, activity has been episodic and at times quite intense.

In February 1968 a tremor of ML 4.0 shook the town and this was followed, between 25 March and l8 April, by 20 further tremors, culminating, on 8 April, in a shock of ML 4.4 which caused minor damage. These events, however, were eclipsed by the major earthquake at Meckering in October 1968. Further series of tremors occurred in 1969, 1972 and 1974, but in the intervening years there were only a few small tremors. or none at all.

Since 1968, therefore, Cadoux had been established as an active seismic area, but at a level no greater than that experienced at many other centres within the South West Seismic Zone.

THE CADOUX EARTHQUAKE OF 2 JUNE 1979

FORESHOCKS

In 1978 only one tremor was recorded in the Cadoux area, on 28 October, with a magnitude of ML 3.6. However, between 13 and 15 March 1979, there were four tremors with magnitudes decreasin~ from ML 3.9 to ML 3.1, the first three of which were distinctly felt in the town. In April and May of 1979 there were three further small tremors (Table 2) and at 5.54 a.m. on 2 June a large tremor of ML 5.2 was felt in Cadoux with an intensity of MMVI on the Modified Mercalli Scale (see Gordon and Lewis, 1980, p.218 for details). A further five tremors of ML 3.0-3.8 were recorded in the next six hours. In retrospect, not only the tremors of the morning of 2 June but the whole of the increased seismic activity of early 1979, after an inactive period between 1975 and 1978, can be seen as foreshocks to the major earthquake that followed. The epicentres of the foreshocks are plotted on Figure 5, and it will be noted that they are southeast of Cadoux, in the vicinity of the postulated main fault.

THE PRINCIPAL EARTHQUAKE

 
The major shock of the Cadoux earthquake series occurred almost precisely twelve hours after the large foreshock of 2 June. The seismological details (Gregson and Paull, 1979) are as follows:

Origin time: 2 June 1979, at 09 h 48. 01.1 UT
Magnitude ML 6.2, M 6.4, mb 6.3
Epicentre Lat. 30 49'48"S, Long. 117 09' 00" E Depth 15 km.

 
The position accuracy is approximately +/- 5 km in depth and ~2 km laterally. The earthquake was felt over most southwestern Australia and was recorded worldwide by at least 120 seismological stations. The earthquake was among the largest recorded in Australia, and, after the Meckering Earthquake of ML 6.9, the second largest in the southwest Seismic Zone.

A preliminary isoseismal map of the earthquake prepared by Gregson and Paull (1979) , is presented as Figure 4. The lower limit for major structural damage is the MMVI isoseismal and this encloses a roughtly circular area, 65 km in diameter, around the epicentre. isoseismal at MMIV, the intensity at which almost everyone recognizes seismic shaking, was approximately 775 km in diameter and encloses most of the principal population centres in the south of the State. At Cadoux itself the destructive effects of the earthquake indicate that an intensity of MMIX was reached. A number of intensity ratings in the epicentral region are plotted on Fig 33 and suggest that the higher isoseismals were elliptical and elongated parallel to the fault zone. The highest intensities, MMVIII-IX, were associated with the group of conjugate faults north of Cadoux and the MMVII isoseismal extends nowhere more than a few kilometres from the fault zone. (Lewis et al, 1981)

DAMAGE

Of 25 building of masonry construction inspected in the Cadoux area, 21 suffered more than minor damage. In contrast, of 29 buildings of non-masonry contruction which were inspected, only 4 suffered more than minor damage.

INJURIES

There were no reported injuries from the Cadoux earthquake.
 
 

ISOSEISMAL MAP of reported ground shaking from the Cadoux earthquake

 
CADOUX REGION EARTHQUAKES SINCE 1960 FROM CSS CATALOGUE

DATE      TIME  UTC   LAT    LONG   MB
6/1/79   21:53:59.9 -30.744 117.308 4.9 
6/2/79   09:47:58.1 -30.812 117.179 6.1 
6/3/79   07:45:32.9 -30.832 117.253 5.1 
6/7/79   06:45:14.4 -30.769 117.206 4.9 
10/11/79 04:04:07.6 -30.612 117.336 5 
12/10/80 04:35:00.9 -30.594 117.399 4 
4/7/81   20:15:55.8 -30.633 117.543 3.6 
1/22/82  18:02:40.3 -30.793 117.053 3.4 
1/24/82   04:06:18.8 -30.746 117.37 3.5 
2/6/82   15:24:37.8 -30.851 117.357 4.7 
4/15/82  17:50:27.3 -30.782 117.643 3.3
1/26/83  06:16:12.8 -30.656 117.241 5.1 
10/10/85 13:34:27.7 -30.541 117.241 3.4 
11/27/85 23:18:17.1 -30.654 117.392 4.5 
3/7/87   05:38:06.7 -30.739 117.124 4 
5/8/90   18:40:53.2 -30.607 117.324 3.9 
12/11/91 17:59:52.7 -30.257 117.913 3.3 
9/19/00  10:23:14.2 -30.583 117.209 3.1 
9/22/00  06:04:47.0 -30.517 117.132 3.6 
9/25/00  15:50:11.5 -30.606 117.215 3.5 
9/28/01  02:54:53.3 -30.465 117.339 4.7 
9/28/01  03:00:44.0 -30.470 117.300 3 

 
Damage to the Masonic Lodge

BELOW - damage to house of E. Cummins

MORE INFORMATION ON THE CADOUX EARTHQUAKE .

The main Cadoux (ML 6.2) event of 2nd June 1979, occurred at 5:48 p.m. local time. There were several relatively large foreshocks, the most significant of which was ML 5.2, at 5:54 a.m. on 2nd June, 12 hours before the principal event. The isoseismal map is shown below. There were thousands of aftershocks over the next few years, and events still occurring near the Cadoux Fault are still considered to be aftershocks of the 1979 event.
The Cadoux event was accompanied by surface faulting in a roughly NNE-SSW direction,. The faulting was about 15 km long, with sub-faults giving an overall width of about 3 km. Faulting was simpler in the south, with the Robb Fault forming the single longest fault in the complex. In the northern sector, it was much more complicated, with shorter fractures being seen in many diferent azimuths. The maximum observed displacement was 1.5 m. The fau lt was considered to have a strike of 18 deg (i.e. NNE), and a dip of 81 deg to the ESE, but significant variation was observed along the length of the fault. The focal mechanism and depth for the Cadoux earthquake (Fredrich, McCaffrey & Denham,1988) was very similar to that of the Meckering event - a shallow thrust fault, dipping to the east. The easterly dip of the fault was at odds with the apparently westerly dip mapped at the surface on the Robb Fault. Lewis et al, (1981) suggested that the observed west-dipping structure did not continue to great depth, and cut across the more significant easterly-dipping fault, which had no surface expression. Following the Cadoux event, overcoring experiments were carried out in the vicinity to determine stress levels (Denham et al, 1987) . High stress levels were detected - a measurement less than 1 km from the fault gave a level of 4 MPA, while another at a distance of 15 km from the fault gave a level of 20 MPA. They concluded that the crust in the SWSZ is un der east-west compressive stress, which is released by earthquake activity in small areas, rather than along linear fault zones. They also concluded that earthquakes of a similar magnitude to the Cadoux event could well take place in the SWSZ over the n ext 50 years.

Aftershock studies

A plot of epicentres presented by Lewis et al (1981) suggested that practically all Cadoux events (foreshock and aftershocks) were to the east of the surface rupture. A cross section they presented suggested focal dep ths of up to 25 km. However, locations at the time were not good, and the data to be discussed below suggests that their plots are largely in error.

A temporary seismograph was operated at Burakin, approximately 30 km north of Cadoux, between June and August, 1979. Data from this seismograph was described by Dent (1990b). A more detailed study of aftershocks was conducted in 1983, using a network of approximately 20 portable recorders (Dent & Gregson, 1986, Dent, 1988, Dent, 1991), as described earlier in this chapter. This study indicated that most of the aftershocks recorded by the network were less than 4 km deep. Nearly 40 events were located from this survey, all but one of which were to the west of Robb Fault ( or its extension to the north).

Another survey using an array of five portable analogue recorders was conducted from May to Dec, 1987. However, the data obtained was not of high quality, and data from this survey have not yet been published.
Comparing aftershock distribution in the period June- Dec 1979, with the period 1980-1983, using locations from the regional seismograph network, Denham et al, (1987) concluded that stress release, after the first six months, was concentrated at either end of the fracture zone. In the first 6 months, it was more evenly distributed along the (NS) length of the fracture.
The accuracy of earthquake locations near Cadoux has improved since the installation of a Kelunji digital seismograph (CAA) in the area in May 1993 (Figure 2-16). A plot of epicentres for earthquakes near Cadoux, since January 1993, ML > 1.9, is shown in Figure 2-16. This shows a distinct clustering of epicentres to the western side of the Robb Fault, re-enforcing the suggestion of Dent (1991) that the fault plane dips to the west. The seismograph station CAM is also shown on this figure, but it only operated intermittently between 1995 and 1996, and did not signifi cantly contribute to the location of epicentres on Figure 2-16. Epicentres to the north of the Robb Fault seem more randomly distributed, which might be expected considering the confused nature of surface rupturing in that area.