Seismicity in 2019 in the Great Southern region of Western Australia

Abstract

Much of the seismicity in southwest WA occurs in localised groups of recurring earthquakes, or clusters. Four potential cluster sites have been identified in the Great Southern Region of WA in the period January to November 2019. A group near Newdegate contained the largest event of the year (ML 4.3), and it is suggested these events belong to an earthquake group first identified in 2016. The three other groups identified are near Denmark, Gnowangerup and Ongerup. Relocations of many of the events using the VIC5A earth model suggests tighter groupings for the events than the published Geoscience Australia locations. The Gnowangerup and Ongerup groups are also close spatially, and there may be a causal link between the groups. The region has been categorised as a region of relatively low seismicity, and the higher numbers of events in 2019 may be related to the occurrence of two ML 5 event in Lake Muir in late 2018. .

Full text

Australian Earthquake Engineering Society 2019 conference, 30 Nov – 01 Dec, Newcastle.
Seismicity in 2019 in the Great Southern region of Western Australia
Vic Dent1 & David Love 2
1. University Associate, Curtin University, Perth, W.A.; Honorary Research Associate, The UWA Institute of
Agriculture, UWA Perth: Email: vic_dent@yahoo.com
2. Seismological Association of Australia, Adelaide, South Australia; Email: david@earthquake.net.au
Abstract
Much of the seismicity in southwest WA occurs in localised groups of recurring earthquakes, or clusters.
Four potential cluster sites have been identified in the Great Southern Region of WA in the period January
to November 2019. A group near Newdegate contained the largest event of the year (ML 4.3), and it is
suggested these events belong to an earthquake group first identified in 2016. The three other groups
identified are near Denmark, Gnowangerup and Ongerup. Relocations of many of the events using the
VIC5A earth model suggests tighter groupings for the events than the published Geoscience Australia
locations. The Gnowangerup and Ongerup groups are also close spatially, and there may be a causal link
between the groups. The region has been categorised as a region of relatively low seismicity, and the higher
numbers of events in 2019 may be related to the occurrence of two ML 5 event in Lake Muir in late 2018.
.
1 Introduction
The Great Southern region of Western Australia (GSR) is an important agricultural area, in the southern part of
the WA wheatbelt. The principal towns are Albany and Katanning. It also constitutes the southern region of the
southwest seismic zone (SWSZ), defined by Doyle (1971), and in the vicinity of “seismic zone 1” of Gaull &
Leiba (1987). This region is considered generally less seismically active than the region to the north (Gaull &
Leiba’s “zone 2”), which includes the Meckering and Cadoux regions. However two ML 5 events were recorded
Figure 1 Events in The Great Southern region of WA, ML ≥ 1.8, Jan. 2017- Nov. 2019

Australian Earthquake Engineering Society 2019 conference, 30 Nov – 01 Dec, Newcastle.
near Lake Muir (Figure 1) in September and November 2018, and about 800 (mostly ML < 1.5) aftershocks were
recorded by Geoscience Australia (GA) up to February 2019 (Clark et al., 2019).
GA’s Neotectonic Features Database shows four probable neotectonic fault scarps in the region, of which the best
defined is a 36 km long scarp south-west of Dumbleyung (Figure 1). Clark (2010) suggests the most recent
movement on this scarp was about 8,000 years ago. Other scarps in the region are near Katanning, Tambellup and
Nyabing.
Much of the seismicity in the SWSZ occurs in localised clusters. Dent (2016) has suggested there are about 60
cluster locations which have been active in the SWSZ over the last ~10 years. They have been labelled according
to the zone they occur in, and the zones are based on latitude. Zones A to F are between 30 and 33 degrees south.
Zone G is 33-34 degrees south, and zone H is 34 to 35 degrees south. Five cluster locations have so far been
identified in zone G and one in zone H, as shown in Table 1, and plotted on Figure 1. These locations are the
assumed centres of reasonably well-defined groups of earthquakes in the past 6 years, and it is suggested that the
locations may also be the focus of future activity.
The 2019 seismicity of the GSR is shown in Figure 1. It shows 7 events of ML 3.0 and above. Four probable
cluster groupings evident in this plot will be
described in more detail below. They are (1)
near Newdegate, (2) north of Denmark, (3)
northeast of Gnowangerup, and (4) north of
Ongerup. Other smaller cluster groups could
also be present. Many of the 2019 events in the
GSR have been relocated. The relocations use
the VIC5A earth model, and generally include
some new data from the “Public Seismic
Network” or PSN (Dent, 2013). A number of
PSN stations have operated near Gnowangerup
since 2008, and GNOT (in the town), opened
January 2017, has contributed data to many of
the relocations. A “seismometers in schools”
seismograph (Balfour et al., 2013) at Albany
(AUALB), installed in 2012, also contributes
some useful data.
Figure 2 Events near Newdegate and their relocations
Table 1 Cluster locations previously identified in the Great Southern Region
ID
location
Latitude
Longitude
Largest event
Previous ID*
G1
S of Dumbleyung
-33.46
117.66
ML 2.9 (July 2013)
S
G2
N of Kojonup
-33.69
117.03
ML 3.5 (Oct 2011)
Theta
G3
SE of Newdegate
-34.23
119.16
ML 4.3 (July 2019)
Tau
G4
West of Nyabing
-33.50
117.92
ML 2.9 (Dec 2015)
Sigma
G5
NW of Gnowangerup
-33.84
117.94
ML 2.4 (Apr 2013)
H1
N of Rocky Gully
-34.35
117.05
ML 2.9 (June 2013)
T
* . Indicates ID used in Dent (2016)

Australian Earthquake Engineering Society 2019 conference, 30 Nov – 01 Dec, Newcastle.
2.1 Southeast of Newdegate
Dent (2016) defined a cluster location in the vicinity of Newdegate (G3) , based on a group of events in June
2015. The largest event in this group was ML 2.8.
The largest event in the area in 2019 was an ML 4.3 event on 20 June (Figure 2). There are five other events near
this event (four on 20 June and a small event on 16 Jan). The relocation of four of the events (Table 2) moves
them by up to 20 km, and closer to cluster centre G3. The ML 4.3 event moves to within 2 km of G3.
In 2018 six events occurred in the area of the 2019 activity (3 in July 2018, including the largest, ML 2.8). It
seems likely that the 2018 and 2019 events represent continuing activity at this cluster centre, but the ML 4.3 of
July 2019 is clearly the largest event in at least four years of activity.
2.2 North of Denmark
There are six events in this group, between 01 Aug
and 13 Sept 2019 (Figure 3 and Table 2). The largest
event (ML 3.8) was the initial event, and the following
four occurred over the following 5 weeks. The GA
location of the main event seems remote from the next
five, but it has a high RMS of residuals (Table 2). GA
assigned a depth of 10 km to all events. Three of the
events in the group have been relocated (using the
VIC5A model), and it seems to bring the main event
closer to the following events, and moves it about 10
km north. A common location is suggested at 34.67oS,
117.48oE The largest event has a depth of 8 km and
the RMS of residuals is low, suggesting it is a good
location. These were the first events recorded in the
Denmark area for at least 20 years.
Figure 3 Events near Denmark, and their
relocations. Event numbers refer to Table 2.
Table 2 Locations/relocations of events near Newdegate and Denmark, 2019
Ref
#
Date/Time
GA location
ML
Phases
/stns
Rms
(sec)
Region
Relocation
reloc
Depth
(km)
Phases
/stns
Rms
(sec)
Lat
Lon
Lat
Lon
16/1 1404
-33.14
119.22
1.9
8/5
0.28
Newdegate
-33.14
119.20
9.9
10/8
0.10
20/6 0655
-33.24
119.16
2.3
7/4
0.40
Newdegate
20/6 1430
-33.10
119.33
4.3
27/19
1.04
Newdegate
-33.21
119.19
9.5
10/7
0.18
20/6 1441
-33.19
119.23
2.5
12/7
1.12
Newdegate
-33.26
119.14
2.3
11/7
0.23
20/6 1511
-33.07
119.07
1.9
5/3
0.75
Newdegate
-33.25
119.12
5.9
5/3
0.19
20/6 1547
-33.20
119.17
2.1
7/4
0.78
Newdegate
1
01/8 1649
-34.70
117.56
3.8
19/27
1.10
Denmark
-34.67
117.48
4.1
9/7
0.14
2
06/8 2006
-34.76
117.42
2.3
4/8
0.69
Denmark
3
23/8 2151
-34.75
117.43
2.2
6/10
0.47
Denmark
4
31/8 0556
-34.76
117.43
2.8
8/13
0.58
Denmark
-34.69
117.47
8.5
9/5
0.13
5
09/9 0853
-34.74
117.39
2.5
7/10
0.52
Denmark
-34.65
117.47
2.0
5/3
0.20
6
13/9 1232
-34.78
117.42
2.1
5/10
0.78
Denmark
Ref # - refers to Figure 3
########
22/2
1403
22/2
1543
22/2
1543
1-Mar
3/3 2206
2206??
2218
11/3
0006
21/4

Australian Earthquake Engineering Society 2019 conference, 30 Nov – 01 Dec, Newcastle.
2.3 Northeast of Gnowangerup
There are 22 events in this group (Figures 4, 5 and 6, & Table 3), most of which have been relocated. Relocations
which have GNOT data can be expected to be more accurate. Most events are between February and March 2019,
although the first event was 31 Dec 2018. There was one magnitude 3 event, on 11 March 2019 (ML 3.5).
The GA locations are quite scattered – over about 40 km., but the relocations (Figures 4 and 5) suggest a tighter
grouping. Relocation of the main event (ML 3.5) moves it about 10 km westwards, and closer to other events in
the group. A possible northwest trend to the events could be suggested, but with a probable epicentral
uncertainties of at least +/-5 km, this trend is speculative. Note that trends can be introduced by poor azimuthal
distribution of the recording seismographs.
A common location for the group is suggested at 33.87oS, 118.10oE, close to the relocated position of the main
event.
Figure 4 Events near Gnowangerup and Ongerup, and their relocations. Event numbers near Ongerup
refer to Table 3.

Australian Earthquake Engineering Society 2019 conference, 30 Nov – 01 Dec, Newcastle.
2.4 North of Ongerup There are six events in this group (Figures 4 and Table 3). The activity appearing to
begin in November 2018 with an ML 2.7 event, and continuing until at least August 2019. The largest event was
ML 3.6 on 13 January 2019, and there were two other ML 3 events. Relocations bring them closer together. The
probable location seems to be about 4 km northwest of Ongerup, the suggested coordinates being 33.96oS,
118.47oE.
Note that the Ongerup events are “swarm-like” (ie, the largest event is not at the beginning). Note also that there
were more ML 3 events than ML 2 events. They are also relatively ‘spread-out” over 2019.
Figure 5 Events near Gnowangerup (enlarged)

Australian Earthquake Engineering Society 2019 conference, 30 Nov – 01 Dec, Newcastle.
2.5 Other historical activity in the area of Figure 4
Cluster location G5 was named in Dent (2017) on the basis of an ML 2.4 event in April 2014, and about 20 small
events (max ~ ML 1.5) recorded by the seismograph GNOC in August 2016. It is about 10 km west of the 2019
activity, and its estimated uncertainty is about +/- 5 km.
Table 3 Locations/ relocations, of events near Gnowangerup and Ongerup, 2019
Ref
#
Date/time
GA location*
ga
Mag
Region
relocation
reloc
dep
Rms
Lat.
Long.
ML
Latitude
Longitude
Km
sec
1
04/11/18 1348
33.96
118.54
2.7
Ongerup
-33.983
118.582
15.3
.144
31/12/18 2336
33.86
118.06
2.1
-33.859
118.117
11.4
.110
2
13/01/19 1943
33.96
118.52
3.6
Ongerup
-33.950
118.498
14.8
.144
28/01/19 0647
33.89
118.08
2.2
-33.834
118.050
4.1
.172
Closest stn LM01
22/02/19 1403
33.87
118.23
2.5
-33.876
118.081
7.2
.252
22/02/19 1543
33.78
118.14
2.5
-33.801
118.092
7.7
.367
26/02/19 1238
33.74
118.19
2.2
-33.867
118.121
2.6
.09
No stns to sth
01/03/19 1246
33.86
118.25
2.6
-33.880
118.088
4.4
.18
03/03/19 1926
33.77
118.08
2.2
-33.847
118.056
11.3
.37
03/03/19 2053
33.61
118.04
2.3
-33.848
118.100
9.3
.212
big move south
03/03/19 2206
33.67
118.11
2.4
-33.846
118.127
0.8
.09
big move south
03/03/19 2236
33.76
118.09
2.0
-33.829
118.115
2.2
.11
03/03/19 2218
33.72
118.11
2.3
-33.847
118.090
4.3
.16
3
6/03/19 0659
33.92
118.55
3.2
Ongerup
-33.949
118.448
2.9
.49
06/03/19 0745
33.73
118.11
2.4
11/03/19 0006
33.83
118.21
3.5
Gnowang.
-33.864
118.117
9.5
.094
21/03/19 1854
33.87
118.16
2.5
-33.882
118.122
5.8
.189
13/04/19 1618
33.94
118.17
2.5
-33.904
118.150
7.3
.255
20/04/19 1032
33.90
118.12
2.5
-33.910
118.141
10.9
.096
20/04/19 1123
33.87
118.07
2.3
-33.871
118.081
9.6
.126
20/04/19 1254
33.89
118.12
2.3
-33.889
118.115
11.7
.094
21/04/19 1544
33.83
118.01
2.1
-33.860
118.087
9.5
.191
21/04/19 1613
33.87
118.09
2.0
-33.862
118.072
10.6
.134
4
28/8/19 0204
33.94
118.44
3.1
Ongerup
-33.955
118.465
15.2
.06
.059
03/08/19 1841
33.88
118.27
2.2
5
30/08/19 0054
33.94
118.39
2.8
Ongerup
-33.938
118.451
7.4
.118
06/09/19 0734
33.69
117.93
2.5
-33.703
117.983
10.2
.227
6
07/11/19 0124
34.00
118.51
2.3
Ongerup
########
22/2
1403
22/2
1543
22/2
1543
1-Mar

Australian Earthquake Engineering Society 2019 conference, 30 Nov – 01 Dec, Newcastle.
A cluster of small events (largest ML 2.3) was recorded by a temporary network around the seismograph ONG1
between November 1990 and March 1991 (Dent, 2008). Another cluster of small events (largest ML 2.3) occurred
in the vicinity of the seismograph GNOW between November 2007 and March 2008 (Dent, 2008). Only two of
these events were recorded by the GA network, but many hundreds of smaller ones were felt at the location of
GNOW.
3 Discussion
Note that, besides being relatively close spatially (ie, about 30 km apart), the Gnowangerup and Ongerup groups
are roughly contemporaneous, suggesting a connection between the two groups. Contemporaneous activity in two
neighbouring cluster groups has been noted before in the SWSZ, eg. activity at clusters west of Beacon and north
of Koorda in 2012, and north of Hyden and north of Holt Rock in 2013 (Dent & Collins, 2018).
The Newdegate sequence demonstrates clusters can be active over several years at least. As these events are
“recent” (all in 2019), it is likely that more events near these locations may be noted in 2020 and probably
onwards. Better locations of future events may allow refinement of the suggested “common locations”.
As noted earlier, the relocations presented here were made using the VIC5A earth model. The depths computed,
as indicated in Tables 2 and 3, seem on average to be a little greater than 10 km. This is a significant change to
earlier studies, where the earth model used was WA2 (with higher seismic velocities), and the average focal depth
was below 5 km. A focal depth of 10km is the adopted value for GA earthquake locations in most of WA, and this
suggests that the WA2 model may not be appropriate for use in this region.
Figure 5 Time vs Magnitude plot for events near Gnowangerup and Ongerup

Australian Earthquake Engineering Society 2019 conference, 30 Nov – 01 Dec, Newcastle.
4 Conclusions
Four earthquake cluster groups active in the GSR in 2019 have been identified, assisted by relocations which
generally use additional data. Possible source locations for the groups are suggested. The temporal and spatial
closeness of the Ongerup and Gnowangerup groups suggests a connection between the groups, but its nature is
unknown. The VIC5A earth model produces epicentres with deeper focal depths than the WA2 model, and of the
models available at present, may be the best one to use in the region.
5 Acknowledgements
Thanks to Clive Collins for his meticulous edits and comments on the report.
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