Tephra seismites—Understanding seismic hazard of hidden faults by analyzing liquefied tephra layers in lakes

Assessing seismic hazards in regions with hidden or poorly expressed faults is one of the major challenges in paleoseismology today. Here, we used computed tomography imaging to quantify the dimensions and distribution of liquefaction structures in ≤17.5-thousand-year-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 toward 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 among (hidden) faults.