Liquefaction Hazard Mapping Liquefaction Vulnerability Mapping for a Given Return Period versus Return Period Mapping for a Given Severity of Liquefaction Vulnerability Liq Hazard Mapping and a new approach that my colleagues Sjoerd, Matt and I have been working on. V. Lacrosse, S. van Ballegooy, M. Ogden

Overview Common practice Liquefaction assessment parameters New methodology Liquefaction Vulnerability Mapping Present the median or mean liquefaction vulnerability for a given level of earthquake shaking Liquefaction Return Period Mapping Determine the level of earthquake shaking required to attain a given level of liquefaction vulnerability Future work This new methodology can be broken down into 2 types of mapping

Common Practice Commonly, when asked to create liq hazard map, we take all geotechnical investigations in an area, run them through a liq ass, calculating a liq vul parameter at each point for a given level of shaking. Sometimes, we take it a step further and interpolate the results. [interpolation over large areas v high density] Example focuses on area in eastern Chch but concept can be applied anywhere.

Liquefaction Assessment Parameters Liquefaction Triggering Method Boulanger & Idriss (2014) Liquefaction Calculated Deformation Method Zhang et al. (2002) Liquefaction Vulnerability Parameter Liquefaction Severity Number Probability of Liquefaction (CRR Curve) PL = 50% Before I explain what the new methodology looks like, here’s a summary of the key liq ass parameters which we are using… Geotechnical Investigations CPT (from NZGD) Magnitude 6

New Methodology

Similar Expected Ground Performance Areas GEOMORPHOLOGY ELEVATION DEPTH TO GROUNDWATER Rather than charging in and immediately running a liq ass over your geotechnical investigations, this methodology forces you to go back to basics. SUB-SURFACE GEOLOGY

Liquefaction Vulnerability Functions LSN Create liq vul fuctions by grouping all geotechnical investigations within an area and running a liq ass. By running these for a range if levels of shaking, we are able to create liq vul functions. Grey line, red line, yellow line… The key thing with this methodology is that it can be applied regardless of the amount of geotechnical information available. The SEPA become more refined the more data you have. Example here is for a CPT-rich area but we have undertaken this exercise in Wellington where we have inferred the qc1ncs from the geological soil profiles and created liq vul functions that way instead. We just find they have greater uncertainty wrapped around them. We can then take a snapshot at a given level of shaking, in this case 0.3g and present the median LSN, 15th, 85th or a range….loss modelling v design PGA (g)

Liquefaction Vulnerability Functions This is what the functions look like for each of the 6 areas. The map in the middle represents the median LSN for each area at 0.3g. Useful high level map but doesn’t capture spatial variability within an area. Area C – greatest scatter (i.e. high spatial variability) Area D – less data but consistent expected performance (i.e. less spatial variability) If we take a snapshot for each area at 0.3g and plot the distribution of LSN, this is what we get.

Liquefaction Vulnerability Functions Frequency Density C – high spatial variability D – low spatial variability LSN

Liquefaction Vulnerability Mapping As mentioned before, this map doesn’t capture spatial variability. However, there is a way this can be done – 100m x 100m grid and sample LSN values from the distributions. This represents x1 realisation… Visually capture liq vul and spatial variability/ Area C – lots of investigations req Area D – few of investigations req

Liquefaction Return Period Mapping LSN Another way to map liq – “how much shaking is required to attain a certain level of vulnerability?” Rather than taking a snapshot at a certain level of shaking, draw a horizontal line and take a snapshot at a certain level of liq vul. This tells us how much shaking is required to attain LSN=10. PGA (g)

Liquefaction Return Period Mapping Here is what it looks like for each of the 6 areas. Area C – Area D – More aligned with how other hazards are mapped + digestible for non-tech audience. For example…flood maps…

Liquefaction Return Period Mapping ~25-200 yr ~25-100 yr >1000 yr ~50-500 yr We could do a similar thing and convert PGA to a return period so that the public can understand how big a shake they would need before their property was subjected to consequential liquefaction damage. Area C – Area D – ~25-100 yr ~500-1000 yr

Future Work Main focus – spatial variability Despite spatial variability in ground conditions, land generally performs homogeneously Case history in areas of similar expected performance Over-prediction in spatial variability? Should standard deviation be reduced? Develop methodology in areas with less data Create a potential liquefaction hazard map for all of New Zealand There are lots of different directions in which we could take this work. My colleagues and I would like to focus on….

Conclusions Methodology is applicable regardless of number of investigations Emphasis on consideration of geomorphology, geology, elevation, groundwater depth By creating areas of similar expected ground performance, can account for spatial variability and uncertainty There are different ways of presenting liquefaction vulnerability Liquefaction return period map more aligned with other hazards

Co-authors Sjoerd van Ballegooy (T+T) Matt Ogden (T+T) Acknowledgements Co-authors Sjoerd van Ballegooy (T+T) Matt Ogden (T+T)