The 2018 Lake Muir Earthquakes

Paul Somerville, Risk Frontiers.

A second earthquake with magnitude larger than 5 occurred today (November 9, 2018) near Lake Muir in southwestern Western Australia, and Geoscience Australia assigned it a magnitude of 5.4.  This earthquake, shown by the large red dot in Figure 1, occurred about 10 km southeast of the magnitude 5.7 earthquake that occurred on 17 September 2018.   The aftershocks of today’s earthquake, shown by small red dots in Figure 1, are located at the southern end of the aftershock zone of the September 17 event, shown by the yellow dots. Today’s earthquake was preceded by a series of foreshocks that occurred overnight, and was felt between Albany and Perth. The shaking intensities of the two earthquakes are shown in Figures 2 and 3.

Figure 1. Locations of earthquakes near Lake Muir. Source: Geoscience Australia
Figure 2. Shakemap of the 9 November Lake Muir earthquake. Source: USGS
Figure 3. Shakemap of the 17 September Lake Muir earthquake. Source: USGS

The orientation of the fault plane on which the 17 September earthquake occurred is shown on a Wulf net projection in Figure 4. This shows that the earthquake had a thrust mechanism on a fault plane striking east-northeast. The InSAR (interferometric Synthetic Aperture Radar) map in Figure 5 shows that the west side moved up and the east side moved down on a plane dipping down to the west-northwest.  The change in elevation along this dip direction is shown at the upper left of Figure 5.

Figure 4. Focal mechanism of the 17 September 2018 earthquake. Source: USGS
Figure 5. Uplift on the west side and subsidence on the east side of the fault that caused the 17 Sept 2018 earthquake. The uplift and subsidence profile along the line A-B as shown in the inset at top left.

The two earthquakes are shown by the yellow stars on a map of historical earthquake epicentres in the Southwest Western Australia Seismic Zone (SWWASZ) in Figure 6.  The contours show annual probability of events of magnitude 5 and above from QuakeAUS6. The two earthquakes occurred on the southwestern edge of the SWWASZ.

Figure 6. Locations of the Lake Muir earthquakes (yellow stars) on the southwestern edge of the Southwest Western Australia Seismic Zone. Source: QuakeAUS6

As described in our Briefing Note 373,  we have recently released our new probabilistic earthquake loss model for Australia, QuakeAUS 6.0. The updated model, developed by Dr Valentina Koschatzky with input from Risk Frontiers’ Chief Geoscientist, Dr Paul Somerville, incorporates Geoscience Australia’s recent revision of the Australian Earthquake Catalogue (Allen et al., 2017), which has more than halved the rate of earthquakes exceeding 4.5 in magnitude.  The main features of the new model are:

  • New Distributed Earthquake Source Model (based on RF analysis of the new GA catalogue – 2018)
  • Inclusion of an Active Fault Model
  • Updated Soil Classification (McPherson 2017)
  • Updated Soil Amplification Model (Campbell & Bozorgnia 2014)
  • Updated Variable Resolution Grid (VRG), Exposure & Market Portfolio
    (Gnaf_201802 + Nexis 9)