Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

the age of the alluvial degradation deposits; while the other two (CP12, CP22) were of Early Holocene age and probably reflect the presence of post-glacial stratigraphy below the Late Holocene gravels. The dates for CP32 (1800 ± 160 yr) and CP41 (1340 ± 190 yr) are in geomorphic order, i.e., the date on the higher terrace T6 is older than that for T2, and this is also true for all Late Holocene dates through the terrace sequence, elucidated below. The mean of the four largest dextral offsets across terrace T2 is 12.0 ± 1.3 m. The corresponding mean vertical offset is 1.6 ± 0.6 m. A single charcoal sample was located in Pit 0 and yielded a calibrated radiocarbon age of 1095–1275 cal yr BP. This age was corroborated by the IRSL age from the same horizon in Pit 0 (CP41). The charcoal date was used to derive minimum dextral and reverse slip rates of 10 ± 2 and 1.3 ± 0.5 mm/yr, respectively (Langridge et al., 2017). These Late Holocene slip rate estimates highlight the NE’ward decrease in strike-slip and dip-slip rates along the AF-NS as motion is partitioned onto faults of the MFS (Fig. 1; Litchfield et al. 2014; Langridge et al. 2017). PA L E O S E I S M I C R E C O R D F R O M T R E N C H E S Trench-1and -3were excavatedacross the fault scarponterraceT4, while Trench-2 was excavated across terrace T2 (Fig. 2). An earlier paleoseismic study discussed by Yetton (2002) involved a U-shaped trench on terrace T1. The stratigraphy of all trenches comprises a fining-upward sequence of gravel, sand and silt overlain by colluvial deposits that yield Late Holocene ages, and with soils formed into them. This suite of units is consistent with alluvial sedimentation and abandonment. The stratigraphy on the upthrown side of the fault includes fine-grained deposits that yield Early Holocene ages. A shortcoming of this site was the lack of organic material and reliance on charcoal to date layers and paleoseismic events. We used OxCal models to characterise event timings. Trench-1 provides evidence for 3-4 paleoearthquake ruptures, defined by 4 radiocarbon dates, summarised on Figure 3A. The youngest date (94 ± 20 yr BP) comes from soily material associated with an unfaulted colluvium. If this date is valid (and used as a maximum age) then the MRE must have occurred since 1725 CE. Similarly, a deformed colluvial unit has charcoal within it having an age of 511 ± 18 yr BP, thus a maximum timing for the penultimate faulting event (PFE) is 1433-1738 CE. A third event is recognised by an unconformity in the cover gravel units. A charcoal date from a flat silty sand below this leads to an Event III timing of 840-1444 CE. A lower unconformity within this sequence has no further age control. Trench-3 was an additional trench excavated in 2023 across the lower of a double-scarp across T4. A single AMS date from macrophyte seeds within a faulted colluvial deposit yielded an age of 1146 ± 19 yr BP. Trench-2 provides evidence for 3-4 paleoearthquake ruptures, summarised on Figure 3B. The youngest colluvium across the scarp is unfaulted. Two charcoal dates from this colluvium and a second faulted colluvium are closely matched, yielding maximum age ranges of 1490-1838 and 1430-1461 CE. for the MRE and