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
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.