Earthquake environmental effects produced by the Mw 6.1, 20th May 2016 Petermann earthquake, Australia
Tamarah R. King1, Mark C. Quigley, 1Dan Clark2 (1School of Earth Sciences, University of Melbourne; 2Geoscience Australia)
In May 2016 I was ~ 4 months into my PhD and preparing to leave for field-work in remote central Australia. Six days before my planned departure, the Mw 6.1 Petermann earthquake occurred just 300 km west of my planned field site. My partner and I drove the 2,200 km from Melbourne in three days, picked up Mark Quigley at the Yulara (Uluru/Ayers Rock) airport, and drove the final 150 km of single lane dirt track to the epicentre location.
At that point, we didn’t know if there was a surface rupture. But as we neared the epicentre, we noticed rock outcrops with clear fresh damage, and cracks along the road increasing in frequency. We camped on the approximate epicentre location and (based on the focal mechanism) knew the fault would be east of us (dipping SW), or west of us (dipping NE).
Mark and I took a gamble and walked east towards a previously mapped fault (the Woodroffe Thrust), noting with excitement all the cracks in the desert sands. An hour or so into the walk and we weren’t seeing many cracks, but we continued in hopes of finding a rupture. That day we walked 20kms in the wrong direction. Turns out, the surface rupture was just 6 km west of our camp. And those cracks told us all we needed to know, if only we’d paid attention!
Below: A video showing the GPS logs from that ~20 km walk. Note that soon after leaving camp, we stop taking GPS points because there is no observable damage. As we near the location of the previously mapped bedrock fault, we actively note there are no cracks. By the time we reach the road (and the car) we know the fault will be to the west, and sure enough we find ample evidence of rupture by the end of the day.
That night we got rained out, and had to evacuate the area because of flooding. Mark flew home and we waited for the flood waters to recede.
Turns out, the desert is actually very wet
Upon arriving back to camp, we immediately hiked 7 km west and found the rupture! Noting an increase in cracking, damaged trees, and overturned rocks as we approached. For the next few days, we hiked in and out of the rupture area, noting and photographing environmental damage as we went.
The rupture where we first located it, not particularly impressive, but still, a fault rupture!
I thought these observations were taking up precious time that could have been spent mapping the rupture. But, when I got back to the office and opened my data points on a map, a clear asymmetry became apparent, which formed the backbone of this journal article.
Key points of the paper:
- (1) Environmental damage increases as you get closer to the surface rupture of a reverse fault, not as you get closer to the epicentre. This is because the fault, the thing generating the shaking energy, is closest to the surface at that location. This is intuitively obvious, but previous uses of the environmental intensity scale highlight its effectiveness in finding the epicentre, not the fault. (Of course, this is exacerbated by shallow ruptures on shallowly dipping faults)
- (2) Environmental damage is far greater on the hanging-wall. This is because the distance between the fault and the surface is shorter on the hanging-wall of a reverse fault. Again, this is a known phenomena and intuitively obvious, but it is still useful to quantify the intensity of that asymmetry. In the case of the Petermann earthquake, the difference was incredibly stark with the foot-wall appearing relatively untouched, while the hanging-wall appeared shaken to bits!
- (3) Even in this remote desert environment, where environmental change happens incredibly slowly, we cannot expect environmental evidence of this earthquake to persist in the landscape long enough to identify old earthquakes (i.e. for paleoseismology). Just a year following this earthquake, we had trouble identifying if vegetation and outcrop damage was coseismic, or just background degradation. We argue that though the damage itself may persist for hundreds or thousands of years, our ability to identify it as earthquake-related diminishes within just a decade or so.
Photos of environmental damage following the Petermann earthquake. All photos were taken on the hanging-wall. Few photos of damage exist for the foot-wall, because damage was difficult to find on the foot-wall (which is, after all, the point of this paper!)