LANDSCAPE DISTURBANCE HISTORY IN WESTLAND, NEW ZEALAND: THE IMPORTANCE OF INFREQUENT EARTHQUAKES ALONG THE ALPINE FAULT A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University by Andrew Wells Lincoln University 1998 ii ABSTRACT Abstract of a thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy LANDSCAPE DISTURBANCE HISTORY IN WESTLAND, NEW ZEALAND: THE IMPORTANCE OF INFREQUENT EARTHQUAKES ALONG THE ALPINE FAULT by Andrew Wells This thesis investigates landscape disturbance history in Westland since 1350 AD. Specifically, I test the hypothesis that large-magnitude regional episodes of natural disturbance have periodically devastated portions of the landscape and forest, and that these were caused by infrequent earthquakes along the Alpine Fault. Forest stand history reconstruction was used to determine the timing and extent of erosion and sedimentation events that initiated new forest cohorts in a 1412 ha study area in the Karangarua River catchment, south Westland. Over 85 % of the study area was disturbed sufficiently by erosion/sedimentation since 1350 AD to initiate new forest cohorts. During this time four episodes of catchment-wide disturbance impacted the study area, and these took place about 1825 AD ± 5 years (Ruera episode), 1715 AD ± 5 years (Sparkling episode), 1615 AD ± 5 years (McTaggart episode), and 1445 AD ± 15 years (Junction episode). The three most recent episodes disturbed 10 %, 35-40 % and 32-50 % respectively of the study area. The Junction episode disturbed at least 6 % of the study area, but elimination of evidence by more recent disturbances prevented an upper limit being defined. The three earliest episodes correspond to the date-ranges for three Alpine Fault earthquakes from geological data, and are the only episodes of disturbance within each date-range. An earthquake cause is also consistent with features of the disturbance record: large portions of the study area were disturbed, disturbance occurred on all types 'of landforms, and terrace surfaces were abandoned upstream of the Alpine Fault. On this basis erosion/ sedimentation induced by Alpine Fault earthquakes has disturbed 14-20 % of the land '-. iii surface in the study area per century. Stonns and other non-seismic erosional processes have disturbed 3-4 % per century. To examine the importance of the Alpine Fault earthquakes to forest disturbance throughout Westland, I collated all available data on conifer stand age structures in the region and identified dates of disturbance events from 55 even-aged cohorts of trees. Three region-wide episodes of forest disturbance since 1350 AD were found in this sample, and these matched the three Alpine Fault earthquake-caused episodes found in the Karangarua. Forest disturbance at these times was widespread across Westland over at least 200 km from Paring a to Hokitika, and originated from both tree fall and erosion processes. This disturbance history can explain the long-observed regional conifer forest pattern in Westland, of a predominance of similar-sized stands of trees and a relative lack of small­ sized (young) stands. The many similar-sized stands are a consequence of synchronous forest disturbance and re-establishment accompanying the infrequent Alpine Fault earthquakes, while the dominance of mature stands of trees and relative lack of young small-sized trees in stands is explained by the long lapsed time since the last Alpine Fault earthquake (c. 280 years). I applied the landscape disturbance history infonnation to the existing geological data to reconstruct the paleoseismicity of the Alpine Fault since 1350 AD. Best estimates for the timing of the most recent three rupture events from these data are 1715 AD ± 5 years, 1615 AD ± 5 years and 1445 AD ± 15 years. Earthquake recurrence intervals were variable, ranging from about 100 years to at least 280 years (the lapsed time since the last event). All three events caused forest and geomorphic disturbance over at least a 200 km section of Fault between the Karangarua and Hokitika Rivers, and were probably single rupture events. Suppressions in cross dated tree-ring chronologies in the western South Island suggest that the last rupture occurred in 1717 AD, and extended as a single rupture from Haupiri to Fiordland, a distance along the Fault of 375 km. Keywords: forest disturbance, Alpine Fault, erosion history, earthquakes, cohort, terraces, Westland, conifer forest, tree-rings, paleoseismicity. iv CONTENTS PAGE Abstract ii Contents IV List of tables ix List of figures x GENERAL INTRODUCTION 1 Review of disturbance history infonnation in Westland 4 i) Evidence for disturbance in the forest pattern 4 ii) Evidence from geomorphic features and erosion studies 6 iii) Direct geological evidence for Alpine Fault ruptures 7 Background to this study 8 Review of forest response to disturbance events 10 Study objectives 11 SECTION 1. FOREST DISTURBANCE AND EROSION HISTORY IN THE KARANGARUA CATCHMENT, SOUTH WESTLAND, SINCE 1350 AD. INTRODUCTION 15 METHODS 17 The study area 17 1) The Welcome Flat study site 19 2) The lower Karangarua study site 23 Rationale for using tree population age structures to determine erosion history 25 Sampling methods 28 Identification of pulses in tree recruitment reflecting disturbance events 32 v Dates of cohort recruitment and erosion/sedimentation events 33 Areas affected by disturbance events 33 Reconstruction of catchment disturbance history 34 Disturbances recorded in tree growth patterns 35 RESULTS 37 1. Types of land surface disturbance and stand structure in the Karangarua catchment 37 i) Surface descriptions 42 ii) Detection of past disturbance events 42 Totally new surfaces 42 Partially modified surfaces 45 2. Temporal patterns of forest establishment and disturbance events in the Karangarua 50 i) Current distribution of forest ages 50 ii) Terrace sequences in the Karangarua and Copland Rivers 55 3. Reconstruction of disturbance episodes in the last 650 years 60 The Ruera episode 60 The Sparkling episode 64 The McTaggart episode 70 The Junction episode 76 Other disturbance events in the study area 80 4. Comparisons between disturbance episodes 83 5. Errors in surface age estimates 86 1. Variation in time for colonisation and growth to sampling height of the first tree 86 2. Failure to sample the first colonising tree 89 3. Error in estimating tree age from increment cores 89 6. Tree-ring growth releases and suppressions: evidence for the episodes in disrupted growth of surviving trees 91 7. Probable synchrony of events in episodes 95 8. Actual dates of episodes 96 9. Causes of the disturbance episodes 97 DISCUSSION 101 Overall disturbance regime 101 Comparison with other studies of disturbance histories in Westland 103 vi Comparison of Sparkling and McTaggart episodes with impacts from historical earthquakes 105 Forest and soil residence times 109 Alpine Fault earthquake events as a sediment supply mechanism 110 Limitations in the detection of disturbance events 111 Cohort recruitment intervals 113 Ecological consequences of the disturbance regime 114 SECTION 2. THE IMPORTANCE OF INFREQUENT, EARTHQUAKE­ INDUCED FOREST DISTURBANCE THROUGHOUT THE WESTLAND REGION. INTRODUCTION 116 Review of current information on forest disturbance in Westland 117 THE STUDY AREA 120 METHODS 122 Identification of pulses in tree recruitment reflecting disturbance events 122 Dates of cohort recruitment and disturbance events 124 Examination of regional forest disturbance patterns 125 RESULTS 126 1. General distribution of forest ages in Westland 126 2. Regional consistency and comparison with earthquake history and disturbance in the Karangarua catchment 129 3. Overall disturbance regime in central-south Westland 133 4. Storm- versus earthquake-induced forest disturbance . 135 DISCUSSION 136 Implications for regional forest dynamics 136 Dynamics of lowland rimu-dominated forest 139 Disturbance regime in non-conifer forests in Westland 141 Ratalkamahi forest dieback 142 vii Other studies around the world 144 Limitations of the data 145 Regional ecological consequences 147 SECTION 3. PALEOSEISMICITY OF THE ALPINE FAULT SINCE 1350 AD: A SYNTHESIS OF PRESENT INFORMATION. INTRODUCTION 148 1. REVIEW OF PREVIOUS WORK ON P ALEOSEISMICITY OF THE ALPINE FAULT 152 i) Investigations prior to 1997 152 ii) Investigations since 1997 153 2. SUMMARY OF REGIONAL FOREST DISTURBANCE mSTORY FROM SECTIONS 1 AND 2 158 Basis for applying forest history to paleoseismic investigations 158 1) Evidence of past Alpine Fault earthquakes recorded in episodes of forest disturbance and landscape erosion dated by trees 160 2) Terrace sequences upstream of the Alpine Fault in the Karangarua and Wanganui Rivers 162 3) Tree growth releases and suppressions coinciding with episodes of forest disturbance 163 Why are the forest disturbance episodes caused by the Alpine Fault earthquakes and not storms? 165 3. EARTHQUAKE mSTORY ALONG THE ALPINE FAULT SINCE 1350 AD, BASED ON THE COMBINED GEOLOGICAL AND FOREST EVIDENCE 166 a) Timing of the earthquakes 166 b) Extent of the earthquakes 169 c) Summary of paleoseismic history 173 d) Recurrence intervals 174 e) Comparison with previous estimates of Alpine Fault earthquake history 176 f) Uplift rates east of the Alpine Fault 177 viii 4. TOAROHA RIVER EVENT: FURTHER INVESTIGATION USING TREE- RING CHRONOLOGIES 181 The chronologies in western South Island 182 Unusual growth at the time of the Toaroha River event 184 A possible