Gisborne Seismic and Tsunami Hazard: Constraints from Marine Terraces at Puatai Beach
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Gisborne seismic and tsunami hazard: constraints from marine terraces at Puatai Beach N.J. Litchfield U.A. Cochran K.R. Berryman K.C. Clark B.G. McFadgen R. Steele GNS Science Consultancy Report 2016/21 June 2016 BIBLIOGRAPHIC REFERENCE Litchfield, N.J.; Cochran, U.A.; Berryman, K.R.; Clark, K.J.; McFadgen, B.G.; Steele, R. 2016. Gisborne seismic and tsunami hazard: constraints from marine terraces at Puatai Beach, GNS Science Report 2016/21. 99 p. N.J. Litchfield, GNS Science, PO Box 30 368, Lower Hutt, New Zealand U.A. Cochran, GNS Science, PO Box 30 368, Lower Hutt, New Zealand K.R. Berryman, GNS Science, PO Box 30 368, Lower Hutt, New Zealand K.J. Clark, GNS Science, PO Box 30 368, Lower Hutt, New Zealand B.G. McFadgen, Victoria University of Wellington, PO Box 600, Wellington, New Zealand R. Steele, Riverside Road, Gisborne, New Zealand © Institute of Geological and Nuclear Sciences Limited, 2016 www.gns.cri.nz ISSN 1177-2425 (Print) ISSN 2350-3424 (Online) ISBN 978-0-947510-22-0 (Print) ISBN 978-0-947510-23-7 (Online) CONTENTS LAYPERSON’S ABSTRACT ................................................................................................ V TECHNICAL ABSTRACT .................................................................................................... VI KEYWORDS ....................................................................................................................... VII 1.0 INTRODUCTION ........................................................................................................ 1 1.1 GEOLOGIC AND GEOMORPHIC SETTING ............................................................... 3 1.2 PREVIOUS WORK ............................................................................................... 5 1.2.1 Ota, Berryman et al. work in the late 1980’s ...................................................... 5 1.2.2 Litchfield, Clark, Berryman unpublished work in the mid 2000’s ........................ 9 2.0 METHODS ................................................................................................................ 17 2.1 PUATAI BEACH TRENCH ................................................................................... 17 2.2 RADIOCARBON DATING .................................................................................... 18 2.3 RTK GNSS SURVEYING .................................................................................. 19 2.4 MICROFOSSIL ANALYSES .................................................................................. 19 3.0 RESULTS ................................................................................................................. 21 3.1 TRENCH STRATIGRAPHY AND MORPHOLOGY ...................................................... 21 3.2 RADIOCARBON AGES ....................................................................................... 31 3.2.1 Upper terrace....................................................................................................31 3.2.2 Middle terrace ...................................................................................................31 3.2.3 Lower terrace....................................................................................................32 4.0 DISCUSSION............................................................................................................ 33 4.1 PUATAI BEACH MARINE TERRACES ................................................................... 33 4.1.1 Terrace altitude and uplift-per-event ................................................................33 4.1.2 Terrace ages and the timing of earthquakes ....................................................34 4.1.3 Comparison with marine terraces at Pakarae River mouth .............................36 4.1.4 Comparison with marine terraces at Waihau Beach ........................................38 4.2 TSUNAMI AND STORM DEPOSITS ....................................................................... 38 4.2.1 Origin of S0–S5 sand units ...............................................................................38 4.2.2 Possible Tsunami Sources ...............................................................................42 4.2.3 Comparison with tsunami deposit and event ages at other sites .....................42 4.3 IMPLICATIONS FOR SEISMIC AND TSUNAMI HAZARD ............................................. 44 4.3.1 Gable End Fault................................................................................................44 4.3.2 Tsunami hazard ................................................................................................45 5.0 CONCLUSIONS ....................................................................................................... 47 6.0 ACKNOWLEDGEMENTS ......................................................................................... 49 7.0 REFERENCES ......................................................................................................... 49 GNS Science Report 2016/21 i FIGURES Figure 1.1 A) Tectonic setting of Gisborne (G) and the study site, Puatai Beach (P). ................................... 1 Figure 1.2 A and B) Puatai Beach and marine terraces, viewed toward the south. ....................................... 4 Figure 1.3 A) Puatai Beach marine terrace map of Ota et al. (1991). B) Marine terrace profile (located in A) of Ota et al. (1991). ................................................................................................ 5 Figure 1.4 Coast-parallel graph from Ota et al. (1991), showing the altitudes and tentative correlations of marine terraces at Pakarae River mouth, Puatai Beach and Waihau Bay ............ 9 Figure 1.5 A) Puatai Beach marine terrace map constructed from the RTK GNSS survey. B) Close- up of the three marine terraces showing selected terrace surface altitudes (in metres above mean sea level), and locations of the auger cores (A1–A4), radiocarbon samples (PB02-06 etc.), coast-normal profiles, and the Puatai Beach trench. ......................................... 11 Figure 1.6 RTK GNSS – surveyed profiles across the Puatai Beach marine terraces (see Figure 1.5B for locations). ..................................................................................................................... 12 Figure 1.7 A) Stratigraphic logs of auger cores and natural exposures into the upper marine terrace. See Figure 1.5B for locations. .................................................................................................... 13 Figure 2.1 Puatai Beach trench near the northern end of Puatai Beach...................................................... 17 Figure 2.2 Puatai Beach trench viewed from the hillslope above the trench. .............................................. 18 Figure 3.1 Bedrock, shore platform and beach deposits beneath the middle terrace, and the buried cliff and surface riser between the middle and upper terraces. .................................................. 21 Figure 3.2 Summary log of Puatai Beach trench. ........................................................................................ 23 Figure 3.3 Beach deposits exposed in the Puatai Beach trench (see Figure 3.1 for locations). A) Gravel beneath the middle terrace, B) Sand and gravel beneath the lower terrace, C) Shelly gravel from which radiocarbon sample 21 was collected. ................................................ 25 Figure 3.4 Colluvium at the landward edge of the middle terrace (see Figure 3.1 for location). .................. 26 Figure 3.5 Tephra lenses preserved on the upper terrace (see Figure 3.1 for locations). ........................... 27 Figure 3.6 Enigmatic lenses of silt and gravel near the middle of the lower terrace which are tentatively interpreted as fluvial channel deposits. ..................................................................... 28 Figure 3.7 Silt and sand layers S0–S5, which may be tsunami deposits ..................................................... 30 Figure 3.8 Probability density functions (PDFs) of the calibrated radiocarbon ages obtained from the Puatai Beach trench ................................................................................................................... 31 Figure 4.1 Probability density functions for the calibrated beach deposit radiocarbon ages from Puatai Beach, Pakarae River Mouth (RM), and Waihau Beach. ................................................ 35 Figure 4.2 Sites of paleotsunami and earthquake event ages which overlap the Puatai Beach tsunami deposit ages and are discussed in the text. .................................................................. 43 TABLES Table 1.1 Puatai Beach marine terrace radiocarbon ages. .......................................................................... 7 Table 4.1 Height differences between successive terraces and the modern shore platform. ..................... 33 Table 4.2 Terrace uplift ages and inter-event times. .................................................................................. 34 Table 4.3 Comparison of marine terrace shoreline angle altitudes and height differences at Puatai Beach and Pakarae River mouth. ............................................................................................... 37 Table 4.4 Characteristics of the S0–S5 sand units preserved in the coverbed sequences of the Puatai Beach marine terraces. ................................................................................................... 39 Table 4.5 Uplift rates calculated from the upper and middle terraces........................................................