EGU2020-12092
https://doi.org/10.5194/egusphere-egu2020-12092
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Near-field directionality of earthquake strong ground motions measured by displaced geological objects

Tamarah King1, Mark Quigley1, and Dan Clark2
Tamarah King et al.
  • 1University of Melbourne, School of Earth Sciences, Australia
  • 2Geoscience Australia, Canberra, Australia

Coseismically displaced rock fragments (chips) in the near-field (less than 5 km) of the 2016 moment magnitude (MW) 6.1 Petermann earthquake (Australia) preserve directionality of strong ground motions. Displacement data from 1437 chips collected over an area of 100 km2 along and across the Petermann surface rupture is interpreted to record combinations of co-seismic directed permanent ground displacements associated with elastic rebound (fling) and transient  ground shaking, with intensities of motion increasing with proximity to the surface rupture. The observations provide a proxy test for available models for directionality of near-field reverse fault strong ground motions in the absence of instrumental data. This study provides a dense proxy record of strong ground motions at less than 5 km distance from a surface rupturing reverse earthquake, and may help test models of near-field dynamic and static pulse-like strong ground motion for dip-slip earthquakes.

How to cite: King, T., Quigley, M., and Clark, D.: Near-field directionality of earthquake strong ground motions measured by displaced geological objects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12092, https://doi.org/10.5194/egusphere-egu2020-12092, 2020

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Presentation version 1 – uploaded on 03 May 2020
  • CC1: Comment on EGU2020-12092, Christoph Grützner, 04 May 2020

    Thanks for the great poster and the amazing outcrop pictures.
    Did you find that only small chips were displaced or did you also encounter big blocks? Is there a kind of size limit? I would think that if you have >1 g (?) PHA, any loose block could "jump".

     

    Thanks!

    • AC1: Reply to CC1, Tamarah King, 04 May 2020

      Thanks for you question Christoph! 

      About 80% of the chips in my database are less than ~20cm in length (generally < 5 cm wide). We did encounter 2 or 3 big blocks ~0.5-1m wide (e.g. photo below). However, because we have so few examples of large rocks displaced, I am cautious of stating that these are definitely coseismic. In comparison we see many smaller rocks displaced, particularly with proximity to the surface rutpure, and have clear evidence of their recentness, so I am much more confident in the small rock data. 

      We don't use this data to estimate ground motions, which was previously done in the Californian studies, because literature* for modelling the toppling of statues, etc, suggests that there are complex interactions between a base and pillar (in this case bedrock and the rock fragment) that may dampen or intensify the input energy of strong ground motions. Based on that literature, these rocks may not require >1g to move, but we also cannot say that there *wasn't* >1g. The bedrock/rock/site lcoation interactions are complex on an individual scale, so we have not attempted to model them to estimate absolute PGAs, etc. Rather we just look at the directionality and relative intensity, as best we can estimate that given the complexities of the geology. I think we'd need lots more local data to estimate PGAs, but if anybody reading this has suggestions for extracting PGA data from these rocks I'm open to hearing them! 

      Personally I think the main limiting factor on what moved is the individual outcrop characteristics, rather than ground motions necessarily. In general, there are few large blocks of rock on these surface outcrops because the granite/mylonite predominately weathers through exfoliation sheets, so there are abundant <20cm x 5 cm rock fragments sitting on, semi-attached, or attached to the bedrock. The earthquake can only move what is available, which is a limitation to all past and future displaced rock studies


      *Literature that informed this:

      Bolt, B.A., and Hansen, R.A., 1977, The upthrow of objects in earthquakes: Bulletin of the Seismological Society of America, v. 67, p. 1415–1427.

      Psycharis, I.N., and Jennings, P.C., 1985, Upthrow of objects due to horizontal impulse excitation: Bulletin of the Seismological Society of America, v. 75, p. 543–561.

      • CC3: Reply to AC1, Christoph Grützner, 04 May 2020

        Awesome, thanks for the very thorough answer.
        I see that there are a lot of limitations, but it's great that there are settings in which it works with a large number of samples. Thanks for pointing out all the potential problems, very helpful for any future attempts.

        • AC3: Reply to CC3, Tamarah King, 04 May 2020

          hahaha I am perhaps overly cautious! I am very excited by the observations and I think they show super cool things, but after years of mulling over all the data and discussing the ideas, I think there are many complexities! The Petermann scenario was great because the outcrops were so well distributed with respect to the fault, and without risk of erosion destroying the evidence. But I'm very intrigued to compare with the work from Ridgecrest, as a strike-slip setting without bedrock complications (e.g. rocks displaced from a soft sediment base). Plus, they have near-field instrumentation! That will be a good test to see if this method holds up, and can be useful elsewhere 

  • CC2: Comment on EGU2020-12092, Zoe Mildon, 04 May 2020

    Hi Tamarah, 

    I missed the live chat session this morning for your presentation. How long after the earthquake did you observed the flipped chips? I wonder if there are any applications for looking at historical earthquakes using this approach, but I guess after a certain period of time it will be too difficult with surface processes to determine where a rock chip has moved from. 

    I did field work post-2016 Italy earthquakes in October and November, and we observed a number of boulders that had either been shaken free from cliffs and some that had 'jumped' because of the shaking. I haven't been back since to see if these features are still obvious or not.

    Thanks, Zoe Mildon (Uni of Plymouth)

    • AC2: Reply to CC2, Tamarah King, 04 May 2020

      Hi Zoe, thanks for your comment!

      I would be very cautious about any historical data because of the difficulty in attributing it to coseismic displacement, I was already cautious of the data just a few weeks after the earthquake! Our first field observations were 8 days after the mainshock, we collected data in the central region of the fault during that field trip. We went back about 16 months after the mainshock and collected more data on the far NW and SE of the rupture, and more through the central area. 

      The benefit of the rocks being offset from bedrock, and bedrock being in the Australian desert, is that the 'chips' left really clear 'shadows' on the bedrock because they had been sitting in the same location for many hundreds/thousands of years, so we could still put them back to the original location after 16 months (as opposed to if they had been offset from soil sockets, which would presumably be destroyed by erosion). Even after 16 months though, I assigned higher uncertainties to a lot of that data, because I couldn't be so certain that the rocks moved coseismically. The benefit of our study site is that nobody lives out there anymore, people no longer walk across the landscape, and it's a desert so there's minimal change from geomorphic and erosive forces. But still, we can't completely discount that there could be some  displacement due to kangaroos, camels, lightening, hot/cold changes, and people. We don't think those factors are significantly contributing to our dataset for a variety of reasons, but those reasons are generally time-sensitive. So, I don't think there's much potential to confidently determine directionality effects from historic data, particularly if there are people living close by! But I do think very large movements such as your Italian boulders may offer insights into earthquake intensity, using things like the ESI-07 scale (environmental seismic intensity). Or if they fit into the category of research into 'fragile geological features'. Just, not so small scale as what we looked at!