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Event Dating and Spatio-Temporal Hazard Estimation in the AVF

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Event Dating and Spatio-Temporal Hazard Estimation in the AVF Event dating and spatio-temporal hazard estimation in the AVF Mark Bebbington and Shane J. Cronin 1Volcanic Risk Solutions, Massey University, Palmerston North, New Zealand Intraplate field volcanism in northern New Zealand Bay of Auckland Volcanic Field Islands Kaikohe, Whangarei ~10 ma to present Auckland South Auckland Ngatutura ~2.0,1.0 and 0.25ma 100 km Constructing an Age-model for the AVF Data: • Stratigraphy, ~33 vents constrained in at least one direction • Age determinations • C14, ~13 vents • Tephrostratigraphy (Molloy et al), 24+ tephra in 5 cores • Ar-Ar, ~4 vents • Magnetic (Cassidy and others), 5 coeval vents Decreasing reliability reliability DecreasingDecreasing • Thermoluminesence, 2 vents • K-Ar, unreliable due to excess Ar •Relative geomorphology or weathering Also: known vent locations, reasonable volume data But: evidence of multiple phases at single vents Core Data 24+ tephras within cores (Sandiford et al. 2001; Shane and Hoverd 2002; Molloy et al. 2009) Thicknesses in various sites are known and ages are implied from direct dating, tephrochronology or accumulation rates ID Thickness (mm) Age (ka) Pupuke Hopua Orakei Onetopo Pukaki Pupuke Hopua Orakei Onetopo Pukaki AVF24 22 0.8 ±0.1 AVF23 3 10.0 ±0.7 AVF22 1 15.1 ±0.7 AVF21 290 3 20.0 ±0.7 19.7 ±0.7 AVF20 235 3 20.5 ±0.7 19.7 ±0.7 AVF19 1 24.4 ±0.8 AVF18 40 8 0.5 26.6 ±0.8 24.5 ±0.8 24.7 ±0.8 AVF17 5 24.7 ±0.8 AVF16 50 25.5 ±0.8 AVF15 12 26.7 ±0.9 etc. Can apply the reverse of a simple ashfall-attenuation function of Rhoades et al., 2002: rV=−1.851/3 + exp[ (8.67 + 1.13log VT − log ) / 2.38 ] V = volume (km 3); T = thickness (cm); r = distance (km). Validated against 1977 Ukinrek Maars eruption Tephra Attenuation (Pupuke) If A then B then C then not A ... b****r! • Wind-direction (low level) assumed random throughout record • Stratigraphy must be maintained • Assigned tephras must be consistent with reliable dates • Tephras must not “skip-over” sites to be only at distal areas • All tephras must be assigned Results in a likely and feasible ordering of events: Tephra ID / Source Age Error Notes No. Name 1Domain AV2 68300 6400 2 Albert Park Heavily eroded [> St Heliers] 3 St Heliers >>45000 [> 90000] 4 OnetopoBasin Shaneand Sandiford 2003 248000 28000 5 Tank Farm AV1 (Onepoto) 75000 5400 6 Lake Pupuke Cassata et al. 2008 200100 7400 7 Mt Cambria ≈ Mt Victoria 8Mt Victoria AV10 32200 1000 9 North Head AV8(Onepoto / Pupuke) 32700 1000 >MtVictoria 10 Mt Albert AV6 (Onetopo) 34700 1400 >MtRoskill,>30000 etc. Monte Carlo Ordering Algorithm • 39 from 51 (multiple events from Mt Wellington and Rangitoto) events have an age (+error) assigned • Of the remaining 12 events: • Mt Cambria assumed same age distribution as Mt Victoria, and Taylor’s Hill as normal distribution defined by sampled ages of Mt Richmond, Wiri Mtn, Puketutu and Crater Hill • 5 (Mt Roskill, Pukeiti, Otuataua, Styaks Swamp and Little Rangitoto) have upper and lower bounds on age, assuming that the only center younger than Mt Wellington is Rangitoto • 4 have lower bounds only (St Heliers, Pukekiwiriki, Te Pouhawaiki and Orakei Basin • Albert Park has no usable stratigraphy, and is assumed > St Heliers • Monte Carlo construction, 1000 simulations: • 39 (+Mt Cambria, Taylor’s Hill) sampled from known normal distribution • If stratigraphy violated – sampling repeated • Remaining 10 assigned ages from uniform distribution between known bounds (using max age of field = max{270ka, age of Onepoto Basin}) Age/Ordering Results Resulting ordering/age model with errors and possible alternatives All but 1 event reordered compared to Allen and Smith, 1994 Name Mean Age Age Error (ka) Order (AS94) MinOrder MaxOrder (ka) Onepoto Basin 246.8 28.8 4 1 8 Albert Park 227.6 41.2 2 1 7 Lake Pupuke 200.2 7.3 6 1 8 Early Pukekiwiriki 195.1 44.2 16 1 8 events St Heliers 182.3 53.0 3 2 8 Orakei Basin 178.5 55.8 45 1 7 Te Pouhawaiki 152.0 70.0 40 1 33 Anywhere Little Rangitoto 92.3 57.7 44 2 46 events Pukaki 83.4 5.5 29 7 11 Tank Farm 75.2 5.5 5 6 12 Intermediate Domain 69.3 5.3 1 7 12 Mt St John 54.7 4.5 42 9 14 events Maungataketake 41.4 0.4 18 11 15 One Tree Hill 37.4 0.7 37 12 17 Motukorea 36.1 0.9 48 13 20 Mt Albert 35.2 0.9 10 14 21 Flare-up Kohuora 35.1 1.0 23 13 21 events Crater Hill 34.1 0.3 24 15 22 Hopua Basin 33.6 0.4 39 18 23 Puketutu 33.0 0.5 26 19 25 North Head 32.9 0.4 9 19 26 Hampton Park 32.4 4.8 34 12 41 etc. Event (re-)ordering , Spatio-temporal Structure? , • Monte Carlo ordering, 1000 repetitions: no evidence of spatial clustering between sequential events; no evidence of correlation between interevent time and distance Allen & Smith (1994) ordering All pairs 95% Confidence Limits Conclusion: Independent temporal and MonteCarlo spatial terms justified in model ordering Time-Varying Eruption Rate , Monte Carlo ordering, 1000 repetitions, kernel smoothing Purchas Hill Mt Wellington 1&2 Exponential (memoryless) MonteCarlo 34 Events Pukaki Tank Farm excess of both short and long reposes Domain • ‘self-exciting’ model Mt St John Pupuke MonteCarlo - - Model – Present Hazard? 7 Events Spatial Structure, Second Order Analysis , • Monte Carlo ordering, 1000 repetitions, azimuths of sequential pairs shows a tendency to AVOID alignments Spatial Likelihood • Kernel estimate of spatial probability, (Connor and Connor 2009) • Ruaumoko location not relatively unlikely ☺ Next Paper ... • New core from Pukaki (Zawalna-Geer, et al.) • New dates • Update tephra volume estimates • Geochemical correlations • ... New Core from Pukaki (Zawalna-Geer et al.) 10 ‘new’ tephra , Mt Richmond, Robertson Hill, Maungataketake, Kohuora should be in Pukaki Tephra (mm) Approx Age Stratigraphy Possible Match Assigned Age 1 22ka < AV16 Mt Smart 21.4ka 11 31ka >AV11, < AV9 Mt Richmond 29.9ka 11 35ka >AV6, < AV4 Kohuora 35.1ka 16 37ka >AV4 Robertson Hill? 27.2ka 0.2 39ka Otara Hill?? 25.9ka 4 41ka Maungataketake 41.4ka 154 56ka Earlier phase of Crater Hill??? 49 63ka ???? 20 80ka ????? 109 83ka Pukaki? 83.4ka • We may need to `unpick’ the model to some extent • BUT .... Tephra Thicknesses in Pukaki Cores Tephra Thickness (mm) , REMOVE: Onepoto, Pupuke, Pukekiwiriki, Orakei (TOO OLD) Three Kings (=AVF12) Mt Mangere, Mt Wellington, Rangitoto (TOO YOUNG) • There appears to be an excess of VERY thick tephra in Pukaki • Thicknesses are approx 5-40 times what they should be.
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