Interpretational Analysis of Liquefaction Dike Systems in Christchurch, New Zealand. McKinnon Main 1,2 & Josh Borella 1

Transcription

1 Interpretational Analysis of Liquefaction Dike Systems in Christchurch, New Zealand McKinnon Main 1,2 & Josh Borella 1 1 Department of Geological Sciences, University of Canterbury, New Zealand 2 Earth and Atmospheric Sciences, Cornell University, USA Abstract Analyses of a Sullivan Park trench face provide insight into the behaviour of liquefaction dyke sequences. Trench logging and subsequent interpretational analysis focused on the characteristics of liquefaction dykes in order to consider the potential controls affecting distribution and reactivation. A map of the trench dimensions overlaying aerial photography of the site area following a major lateral spreading event allow lateral spreading features to be identified within the trench. Introduction The liquefaction process tends to occur in water-saturated, loosely compacted sandy sediments which, when subjected to a heavy load or earthquake-induced ground shaking, can experience an overburden stress applied to the pore water pressure (Obermeier 1996). When excess pore water pressure overcomes the sheer strength of the soil, contacting sediment grains separate and the area transitions into a liquefied state (Castro 1975, Seed & Idriss 1982). Recent liquefaction events spawned by the Christchurch Earthquake Sequence (CES) in New Zealand rendered many blocks of neighbourhood homes uninhabitable due to the differential settlement of liquefied sediments that ejected and flowed through lateral spreading cracks causing subsidence and irreversible damage to housing foundations (Earthquake Commission Liquefaction and Vulnerability Study). Liquefaction and lateral spreading cracks ultimately resulted in >NZ$1 billion in damages and government buyouts of residential properties in Red Zone districts. The major liquefaction events of the CES were tied to the 4 September 2010 M w 7.1 Darfield mainshock and three subsequent aftershock events occurring on 22 February 2011 (M L 6.3, 5.8, and 5.9 events within 2h), 13 June 2011 (M L 5.6 and 6.4 within 1h 20min), and 23 December 2011 (M L 5.8 and 6.0 within 1h 20min) (Quigley et al. 2013). Ten distinct

2 liquefaction episodes were recorded at researcher Mark Quigley s own home located in the same Red Zone district as the Sullivan Park study area examined in this paper. Ongoing research in the area has involved trenching across lateral spreading cracks and sand blow events to reveal the subsurface geometry of liquefaction feeder dike systems, and CPT geotechnical testing has been employed to determine the depth of liquefiable strata (Bastin et al. 2013, Quigley et al. 2013). Findings suggest that sand blow dikes were persistently reactivated during successive events across the CES; liquefaction dike system reactivation has also been described at sites around the world including the United States (Saucier 1989, Sims et al. 1995) and Japan (Kuribiyashi et al. 1975). It is then reasonable to hypothesize that pre-ces paleo-liquefaction in Christchurch may have occurred, possibly affecting the liquefaction vent distribution during the CES. Understanding the controls affecting liquefaction vent distribution could potentially aid in mitigating future societal vulnerability and economic loss in the event of earthquake-induced liquefaction. In the Sullivan Park trench both anthropogenic and geologic features, specifically sheep bone waste pits and evidence of paleo-liquefaction were exposed alongside CES liquefaction feeder dikes, and their positions in relation to CES dike distribution were logged and interpreted. Study Area Sullivan Park is located ~125m off the apex of an inside meander bend of the Avon River in Avonside, eastern Christchurch. The water table is shallow (~1-2m) and dependent on tides, while the river is subject to tidal current reversal (Bastin et al. 2013). The area lies primarily on alluvial silt and sand deposits, drained peat swamps and estuaries, sand of stable to semistable dunes, and the underlying Christchurch Formation of marine sands that formed as sea levels transgressed then regressed from a mid-holocene highstand at 6.5ka (Brown et al. 1995). The relatively young, loose, fine-grained alluvial sands and silts are especially susceptible to liquefaction due to their minimal cementation (Elder et al. 1991), a high water table, and their proximity to a low point in elevation at the river bend. A nearby wool scouring factory operating in the mid-late 1800 s dug waste pits consisting of sheep bones and soil that have been exposed by previous Sullivan Park trenching (Bastin et al. 2013). It is likely that there are many waste pits in the area, though the total number is not known.

3 Methods Two subsurface trenches were excavated perpendicular to the axis of CES lateral spreading cracks and sand blow vents which could be seen by aerial photographs taken the day after the February 2011 major shaking and liquefaction event (Figure 1). The trench was 14m long at a depth of 1.25m which did not cross the water table at the time of excavation. The trench walls were cleaned using hand scrapers, a 0.5m grid was laid with string and nails, and lines were scraped in the trench face to delineate feature boundaries for the trench logger. The trench walls were photographed and the log was later digitized using ArcMap Sediment structures, textures, and compositions were described on site. Close examination of dike features occurred during logging, and further interpretation using photographs and log data sought to characterize the distribution and crosscutting relationships of both singular and reactivated dyke sequences. Results and Discussion Sullivan Park trench #5 revealed alluvial sand and silt deposits interrupted by six subvertical planar dike features and two anthropogenic waste pits ~4m wide. Dike systems ranged from 3-50cm in diameter, consisting of medium-fine grain sands and some (<<1%) gravels in the larger feeder dikes. The three largest dike ejecta (between 0-1m, 3-4m, and 12-13m, respectively) correlate with the position of three lateral spreading cracks evident in aerial photographs (See Figures 1 and 2). In all dike systems, extremely thin ~1mm silt curtains lined the walls of the dike ejecta, and their identification facilitated interpretation of the sequences of reactivated dike systems. Bioturbated soils were present in many of the dikes, and their origin is considered on a case to case basis in the following discussion and interpretation of three of the exposed dike features m Dike Sequence The dike sequence expression lying within the fourth lateral meter of the trench grid (Figure 2, 3) was ~30cm wide, initially narrowing slightly from the base then widening back out as it reached towards the surface. The dike activated through the right corner of the leftmost waste pit, ejecting through and around bioturbated pit fill and sheep bones. A stalled portion of the dike is marked by a fining upward sequence of compacted sand to silt that appears to have collided and stalled upon collision with lingering pit fill. Alternatively, this fining sequence

4 may have occurred during a vent reactivation in a later shaking event, but silt curtains delineating a second dike wall were not observed to continue down to the surface of the trench, leading us to prefer the a single-sequence interpretation. Near the surface of the dike, a small offshoot of ejecta seems to have changed direction to descend back down towards the subsurface at a ~45 pitch. This small downward-pitching liquefaction feature is unique for the trench face studied. 7-8m Dike Sequence The feeder dike system within the eighth meter if the trench grid (Figure 2, 4) had prominent silt contacts marking 3 activation events. A ~7cm wide dike can be traced from the trench floor to the topsoil, but a larger ~15cm wide x 35cm tall ejecta sequence has no obvious feeder source or near-surface expression. A small and narrow dike feature terminating ~20cm above the trench floor has unique characteristics to the trench face, namely strongly bioturbated margins causing deterioration to the silt curtain as well as oxidation or mottling affecting surrounding sediments. Mottles precipitate in pore space when the water table recedes over the course of seasonal cycles. These factors suggest that that the dike may have formed prior to the CES as a paleo-liquefaction feature; evidence for paleo-liquefaction dikes has also been identified and interpreted in previously dug Sullivan Park trenches (Bastin et al. 2013). A likely fourth ejecta sequence may be inferred by a well-defined silt curtain running through the center of the dike system, but its expression could not be completely traced down to the trench surface m Dike Sequence This dike sequence was ~50cm across at the base, ejecting through the right corner of an anthropogenic waste pit. At least three liquefaction events are likely to have occurred based on crosscutting relationships interpreted during logging and photograph analysis (Figure 2, 5). We suggest that the largest evident component of the dike system named event II (See Fig. 5) picked up clasts of waste pit soil and ejected them upwards. The large lower clast appears to have stalled after colliding with embedded alluvial sediments, and below it a third liquefaction event stalled and formed a fining upward sequence near, but not quite upon contact with the bottom of an ejected pit fill clast (EPF1) (Fig. 5a). The upper clast EPF3 is strange in that it is clearly pit fill because of the presence of a sheep bone, yet its size is too large to have been ejected on the exposed trench plane. We interpret that the ejected clast originated somewhere along the perpendicular axes of the lateral spreading crack near the

5 plane of the exposed trench. The potential for this type of directional motion during dike ejection had been largely ignored during trench logging, and was only considered later during interpretive analysis. However, the 2-3m dike exposure (Figure 2) manifested 0.5m above the trench floor also reveals ejecta that likely originated on a plane ~perpendicular to the trench face, as dike sands could only be found to continue downwards when a hand scraper was carved into the trench face. Conclusions The features exposed and interpreted in the trench display characteristics that have been described in prior research; liquefaction dikes are prone to reactivation across multiple shaking events within an earthquake sequence, and liquefaction tends recur in the same area as evidenced by paleo-liquefaction features. Within these dikes, we believe non-liquefied clasts were comprised of both down-dropped topsoil and upward-ejecting pit fill. Stalled ejecta sequences tend to create a fining-upwards sequence of sand to silt; understanding the controls that cause stalled sequences could be further researched and discussed. In the localized area, anthropogenic pit fill may be a control on vent distribution: the discontinuity between pit fill and alluvial depositional sediments likely marks a weakened contact between two different grain types. Given that the two largest lateral spreading cracks in the trench erupted through and along this discontinuity in sediment, we believe it should be considered as a controlling factor affecting the expression of lateral spreading cracks in the area Acknowledgments Thanks to Luc Charbonneau for his part in the trench logging process. Thanks to Sarah Bastin and Mark Quigley for their help in describing and identifying liquefaction features during trench logging. Thanks to Sarah Bastin and Maddy Niles for providing GPS data for the mapping of the trench site, and thanks to Grayson Carlisle for coining the term silt curtain. Finally, thanks to Darren Gravely and Max Borella for their organizational roles in the Frontiers Abroad program

6 References Bastin, S.H., Quigley, M.C., Bassett, K., Green, R.A., Characterisation of modern and paleo-liquefaction features in eastern Christchurch following the Canterbury earthquake sequence. Proc, 19 th NZGS Geotechnical Symposium. Ed. Cy Chin, Queenstown. Brown, L.J., Beetham, R.D., Paterson, B.R., and Weeber, J.H., 1995, Geology of Christchurch,New Zealand. Environmental & Engineering Geoscience, v. 1, p Castro, Gonzalo., Liquefaction and Cyclic Mobility of Saturated Sands. Journal of the Geotechnical Engineering Division (1975): Web. Kuribiyashi, E. and Tasuoka, F., Brief review of liquefaction during earthquakes in Japan. Soils and Foundations, Japanese Society of Soil Mechanics and Foundation Engineering, Vol. 15, No. 4; Obermeier, S.F.,1996.Use of liquefaction-induced features for paleoseismic analysis - An overview of how seismic liquefaction features can be distinguished from other features and how their regional distribution and properties of source sediment can be used to infer the location and strength of Holocene paleo-earthquakes. Engineering Geology, Vol. 44, pp Quigley, Mark, Sarah Bastin, and Brendon Bradley, Recurrent liquefaction in Christchurch, New Zealand, during the Canterbury earthquake sequence. Geology. 41.4: Web. Saucier, R.T., Evidence for episodic sand blow activity during the New Madrid (Missouri) earthquake series. Geology, 17(2); Seed, H.B. & Idriss, I.M., Ground Motions and Soil Liquefaction During Earthquakes. Monograph Series, Earthquake Engineering Research Institute, Berkeley, California. Sims, J.D., Garvin, C.D., Recurrent liquefaction induced by the 1989 Loma Prieta earthquake and aftershocks: implications for paleoseismic studies. Bulletin- Seismological Society of America, Vol. 85.1, pp Tonkin & Taylor Ltd,. New Zealand. Earthquake Commission. Liquefaction Vulnerability Study Web. <

7 **Note: Figure 2 takes a full page so Figure 3 is provided first Figure 1: Sullivan Park aerial photograph taken 23 Feb showing extensive lateral spreading cracks throughout the park. GPS data of trenches excavated and logged in February 2014 shows the trench sight bisecting multiple spreading cracks to reveal their subsurface geometries. 196 Figure 3: a) Dike sands (ES) eject (ES), change direction (DE), and stall (FS) traveling through pit fill (PF) and alluvial sediments (AS). b) A photograph of the stalled FS sequence does not show a silt trace continuing down to the trench floor.

8 Figure 2: Trench log of Sullivan Park Trench #5 West Wall

9 203 Figure 4: a) Interpreted sequencing of liquefaction dikes ejected through alluvial sediments (AS) include Paleoliquefaction (PL, event I), event II displaying no trench floor or surface expression, and event III. The silt curtain labelled? may mark a fourth event, but it does not continue down to the trench floor. b) Zoomed photograph of PL, showing some mottled sediments. c) Zoomed photograph of prominent silt curtains outlining walls of dike III Figure 5: a) 12-13m dike sequence interpretation. Ejected pit fill (EPF) clasts appear to have been been carried by event II, with EPF1 stalling after collision with embedded alluvial sediment (ES). b) A third event (III) appears to stall and fine upward below EPF1.