Reassessment of the ORIGIN of the AD 365 Earthquake FROM Geologic and Geochronologic Relationships, Crete, Greece

The 365 AD event occurred on a large high angle S-down normal fault, and is not directly associated with subduction. The Earthquake of AD 365 is one of the largest in the greater Mediterranean, and as such is critically important for a broader assessment of the seismic and tsunamigenic hazards associated with Mediterranean subduction zones. The central Mediterranean contains c. 3350 km of subducting plate boundary including the Hellenic, Calabria, and more slowly converging Apenninic systems. What is remarkable is the near absence of definitively documented large-to-great (≥ M 8) subduction earthquakes in this region, especially given historic records spanning nearly three millennia (e.g., Ambryaseys, 2009). The largest known regional event, the July 21, AD 365 earthquake generated widespread structural damage on Crete (e.g. destruction of Kisamos; Stiros & Papageorgiou, 2001) and a regional tsunami recorded in Alexandria, Egypt (Stanley, 2005; Shaw et al., 2008). The AD 365 earthquake is traditionally modeled as a subduction event; although opinions differ as to whether the rupture occurred on the subduction interface or above it on a steeply-dipping splay fault. We present an alternative view of the source structure for the AD 365 earthquake that is most compatible with available topographic, bathymetric, geologic, and geochronologic datasets from Crete.

Physical evidence for the coseismic surface uplift of Crete is preserved around much of the northwestern to southwestern coastline in the form of an uplifted and tilted Holocene shoreline, or bioerosion notch, first reported by Spratt (1865) and examined by many others (e.g. Thommeret et al., 1981), that exists today between 0 and 9 m asl. The presence of this uplifted shoreline constitutes the critical data set utilized in published interpretations suggesting that the AD 365 earthquake nucleated along the subduction interface or on a steeply-dipping splay thrust. Most attribute the maximum 9 m of Holocene shoreline uplift to a single massive earthquake with an epicenter located several tens of km off the SW point of Crete; although this structure has yet to be recognized in the submarine geology south of Crete. Critical to the thrust source model for the AD 365 earthquake is the assumption that much if not all of the up to 9m of uplift recorded in the geometry of the uplifted Holocene shoreline occurred during a single event, and that the magnitude of coseismic uplift is too large for uplift on the footwall of a normal fault due to standard fault-scaling relationships.

We performed a reassessment of all available radiocarbon dates reported in the literature from calcareous marine invertebrates collected from below the maximum elevation of the uplifted “AD 365” shoreline (n > 50) from the northwest, west, and southwest coast of Crete. Radiocarbon ages were corrected for isotopic fractionation and the marine reservoir effect prior to calibration with the Marine09 calibration curve using Calib 6.0 calibration software (Reimer et al., 2009). We find that sessile organisms (e.g. solitary corals, bryozoa, and vermitids) sampled from 0 to 0.3 m below the maximum elevation of the Holocene bioerosion notch consistently provide 2-sigma calibrated ages that are statistically older than samples from 3 to 4.5 m below the maximum elevation of the bioerosion notch at a given locality. For example, at Phalarsana on the west Coast of Crete (where the prominent Holocene bioerosion notch is at 6.6 m above sea level, we find that the weighted means of the “high” and “low” samples (n = 16) are AD 139 ± 112 and AD 422 ± 52, suggesting that there at least two uplift events accounting for the 6.6 m of uplift. An earlier uplift event at Phalarsana, perhaps during the period 64 to 66 AD, is consistent with the geoarchaeologic and sedimentologic evidence indicating rapid sea level fall and evidence for a tsunami as preserved in deposits associated with the uplifted Roman harbor here (e.g. Dominey-Howes et al., 1998).

Wegmann (2008) documented secular Late Pleistocene rock uplift rates from marine terraces from southwester Crete of 1 to 2 mm/yr. Combining the secular uplift rate with estimates of late Holocene sea level rise (c. 0.25 m) in the eastern Mediterranean since AD 365 reduces the amount of coseismic uplift recorded in the uplifted Holocene shoreline from a maximum of 9 m to 5.5 to 7.2 m.

The western Crete escarpment rises precipitously over 6,000 m from the floor West Cretan trough (WCT; -3600 m) to the summit of the Lefka Ori (2454 m). The 175 km long escarpment extends west from the eastern end of the Mesara graben to its intersection with the Ionian trough. The WCT is the underfilled-offshore continuation of the actively extending Mesara graben (e.g. Becker et al.., 2010), with the southwest coastline of Crete and its uplifted Holocene shoreline embedded in the uplifting footwall of a large S-down normal fault(s).

Available data provides evidence supporting the idea that tsunamigenic large historic earthquakes, including AD 365 event, may have nucleated within the upper plate along high-angle normal faults embedded in a zone of rapid rock uplift and left-lateral transtension and that the AD 365 event was likely not as large, in terms of absolute vertical surface uplift or magnitude as previously reported.