DOI: 10.5061/dryad.5x69p8dj9
title: Tephra seismites – understanding seismic hazard of hidden faults by analyzing liquefied tephra layers in lakes

description: Assessing seismic hazards in regions with hidden or poorly expressed
 faults is one of the major challenges in paleoseismology today. Here, we
 utilized CT imaging to quantify the dimensions and distribution of
 liquefaction structures in ≤17.5-ka old tephra layers in 18 lakes
 scattered across the poorly expressed Hamilton Basin fault system in
 northern New Zealand. These ‘tephra seismites’, embedded in
 unconsolidated, organic-bearing lake sediment, increase in occurrence and
 dimensions towards known faults and indicate the occurrence of a local
 hidden fault segment. Through incorporating peak ground acceleration
 modeling, we found that the spatial distribution of tephra seismites
 directly relates to the ground shaking induced by near-field fault
 ruptures. We used the variability in tephra seismites within the
 stratigraphic record, and tephrochronology, to better constrain the
 recurrence intervals and magnitudes of paleoearthquakes from both the
 Hamilton Basin and adjacent Hauraki Basin fault systems. Our methodology
 is globally applicable in volcanic and tectonic regions where liquefaction
 structures are preserved amongst (hidden) faults.
description: # Tephra seismites – understanding seismic hazard of hidden faults by
 analyzing liquefied tephra layers in lakes Dataset DOI:
 [10.5061/dryad.5x69p8dj9](https://doi.org/10.5061/dryad.5x69p8dj9) ##
 Description of the data and file structure This Zip file contains CT data
 of sediment cores from the Hamilton lowlands, New Zealand. CT stacks of a
 total of 60 sediment cores from 18 different lakes are included. The
 following data processing steps were undertaken. CT imaging was performed
 using a medical CT scanner on 60 whole-round cores prior to opening (i.e.,
 all lake cores except HL-A2 and HL-A3 from Rotoroa/Hamilton Lake, which
 were not available for CT imaging) to visualize and unambiguously quantify
 tephra layers and soft-sediment deformation structures. The whole-round
 core was positioned in the center of the CT table, and preliminary scan
 results of the tephra layers and tephra seismites could be observed in the
 CT control room (fig. S1AB). A CT stack (sequence of cross-sectional CT
 images) was generated for each core, with each CT image comprising voxels
 with dimensions of 0.625 mm depth and 0.684 mm width and height. The voxel
 values represent the radiodensity in Hounsfield units HU. An HU value of
 -1000 represents the radiodensity of air, and an HU value of represents
 the radiodensity of distilled water at standard temperature and pressure.
 CT image processing was performed using the public domain image processing
 software package ImageJ and its portable distribution Fiji (version 1.53t)
 and included the following steps (62): (a) Reduction of CT data: The raw
 CT stack initially consists of the core barrel (filled with sediment,
 water, and tephra) located longitudinally in the center of the CT table,
 both surrounded by air. To reduce computation time, the CT stack was
 roughly cropped to the data representing a cuboid around the core barrel
 using Fiji's rectangular selection tool. (b) Combination of CT
 stacks: The majority of the sediment cores were short enough to be
 acquired in a single CT scan, but some of the longer cores extended beyond
 the measurement area of the medical CT scanner. For these cores, distinct
 layers were identified in the overview core scan, and CT imaging was then
 performed once from each core end, allowing the distinct layers to be
 recorded in both scans, but in different directions. The reverse-direction
 CT scan was rotated 180 degrees (using Fijis rotation tool) and then
 combined with the first CT scan using the distinct layers as a reference.
 (c) Core rotation and realignment: Cores were sometimes not perfectly
 aligned longitudinally on the CT table. In addition, longer cores could
 not be placed flat on the CT table (due to a shelf space at the base of
 the CT table) and instead were slightly tilted upward at one end of the CT
 table, causing the core to be slightly rotated or bent upward, or both.
 The CT stacks were reoriented using Fiji's rotation tool. The
 curvature of the cores was removed using the open-source Fiji stack
 alignment plugin. (d) Removal of voxels outside the sediment core: After
 the core was properly rotated and aligned, the data representing the core
 barrel, CT table, and air were removed using Fiji's circular
 selection tool. The remaining CT stack represented only the inside of the
 core barrel (i.e., sediment, water, and tephra) in a cylindrical shape.
 (e) Contrast and brightness enhancement was applied to highlight the
 different sedimentary units in the core, mainly the organic lake sediment,
 tephra layers, and soft-sediment deformation structures. The raw CT images
 that make up a CT stack use a standard linear transfer function between
 radiodensity and a grey scale value. Typically, black represents the
 lowest HU value (air) and white represents the highest HU value in the
 core. The sedimentary units in the core have relatively similar densities
 compared to the surrounding air, so the core appears white in the raw CT
 images (i.e., very little change in pixel values within the core) (first
 panel of fig. S1C). The contrast of CT images was enhanced using
 Fiji's enhance contrast tool. Voxel values were normalized to the
 highest value of the entire CT stack, with 0.35% of all voxels saturated
 (i.e., assigned a maximum value) (second panel of fig. S1C). A custom
 transfer function (mpl-viridis) was applied to the CT images to better
 highlight small density changes within tephra layers (third panel of fig.
 S1C). The custom transfer function assigns dark blue/purple to HUmin,
 green to 0.5HUmax, and light yellow to HUmax. (f) Substacking:
 Stratigraphic units (i.e., tephra layers of known origin and age, organic
 lake sediment, pre-lake alluvial sediment) were identified for each
 sediment core using information from the core description. Once the
 stratigraphic framework was established, substacks were created for the
 relevant tephra layers (Rk, Rr, Oa-8, Wh, Mm, Op, Rm, Ma, Tu, Tn, UnitQ,
 Wo, Mp, Tp). In total, 346 tephra layers were visualized (fig. S2). (g)
 Visualization of CT substacks: Each substack, representing one or more
 tephra layers, was visualized in three ways (fig. S1D): (1) Longitudinal
 CT slices were generated through the center of the core. Typically, three
 longitudinal CT slices were generated at angles of 0, 120, and 240 degrees
 (where 0 degrees was defined as parallel to the cut core surface) to
 facilitate a systematic analysis of the vertical and horizontal extent of
 tephra layers and tephra seismites. Longitudinal CT slices were best
 suited to analyze the thickness of tephra layers as well as the maximum
 vertical length of soft-sediment deformation structures. (2) A series of
 cross-sectional CT slices was generated for each CT stack containing
 tephra seismites to better visualize the changes in thickness and
 morphology of individual soft-sediment deformation structures with depth.
 Cross-sectional CT slices were used to assess the thickness and
 interconnectivity of individual soft-sediment deformation structures. (3)
 Three-dimensional CT volumes were generated from each CT stack using
 Fiji's volume viewer plugin. The organic lake sediments above and
 below the tephra layers were digitally removed by applying an -function
 based on X-ray attenuation, which was significantly different between the
 two constituents (i.e., organic lake sediments versus tephra) and allowed
 images to be exported showing only the tephra deposit. Standardized images
 were generated for each CT volume, including three images orthogonal to
 the core taken at angles of 0, 120, and 240 degrees and two oblique images
 (one from above and one from below). CT volume images, in which the
 low-density organic lake sediment had been filtered out, provided the best
 tool for studying the three-dimensional morphology of the tephra seismite
 and tracking individual soft-sediment deformation structures. A
 compilation of all CT substack visualizations is provided in data S1. ###
 Files and variables #### File: CT_data.zip **Description:** Zip folder
 contains 19 folders, one for each lake for which CT scans were obtained.
 Each of the 19 folders on hierarchy level 1 contains a number of
 subfolders (hierarchy level 2) that show individual CT scans of cores.
 Hierarchy level 3 shows screenshots of the processed CT data for each
 scan. These screenshots show the CT data from different perspectives
 (e.g., longitudinal and cross-sectional). Some of the Scans contain videos
 of CT scans. Hierarchy level 4 (always called IS) contains the processed
 CT slices as .tiff