Clusters of Megaearthquakes on Upper Plate Faults Control the Eastern 1

Originally published as: Mouslopoulou, V., Nicol, A., Begg, J. G., Oncken, O., Moreno, M. (2015): Clusters of mega-earthquakes on upper-plate faults control the Eastern Mediterranean hazard. - Geophysical Research Letters, 42, 23, pp. 10282—10289. DOI: http://doi.org/10.1002/2015GL066371 PUBLICATIONS Geophysical Research Letters RESEARCH LETTER Clusters of megaearthquakes on upper plate faults 10.1002/2015GL066371 control the Eastern Mediterranean hazard Key Points: Vasiliki Mouslopoulou1, Andrew Nicol2, John Begg3, Onno Oncken1, and Marcos Moreno1 • Uplift along the Hellenic forearc transient due to earthquake clustering 1German Research Centre for Geosciences, GFZ Helmholtz Centre Potsdam, Germany, 2Department of Geological Sciences, • Earthquakes occur mostly on upper 3 plate faults rather than the plate University of Canterbury, Christchurch, New Zealand, GNS Science, Lower Hutt, New Zealand interface • Seismic hazard in Eastern Mediterranean underestimated Abstract The Hellenic subduction margin in the Eastern Mediterranean has generated devastating historical earthquakes and tsunamis with poorly known recurrence intervals. Here stranded paleoshorelines indicate – Supporting Information: strong uplift transients (0 7 mm/yr) along the island of Crete during the last ~50 kyr due to earthquake • Supporting Information S1 clustering. We identify the highest uplift rates in western Crete since the demise of the Minoan civilization and • Table S1 along the entire island between ~10 and 20 kyr B.P., with the absence of uplifted Late Holocene paleoshorelines in the east being due to seismic quiescence. Numerical models show that uplift along the Hellenic margin is Correspondence to: V. Mouslopoulou, primarily achieved by great earthquakes on major reverse faults in the upper plate with little contribution [email protected] from plate-interface slip. These earthquakes were strongly clustered with recurrence intervals ranging from hundreds to thousands of years and primarily being achieved by fault interactions. Future great earthquakes Citation: will rupture seismically quiet areas in eastern Crete, elevating both seismic and tsunami hazards. Mouslopoulou, V., A. Nicol, J. Begg, O. Oncken, and M. Moreno (2015), Clusters of megaearthquakes on upper plate faults control the Eastern 1. Introduction Mediterranean hazard, Geophys. Res. Lett., 42, 10,282–10,289, doi:10.1002/ Subduction of the African plate along the Hellenic margin in the Eastern Mediterranean is primarily accommo- 2015GL066371. dated by steady aseismic slip of ~35–40 mm/yr on the plate interface [Reilinger et al., 2006] (Figure 1). Despite the high proportion of relative plate motion taken up by aseismic slip (≥80%) [Vernant et al., 2014], devastating great Received 28 SEP 2015 earthquakes (>M8) and associated tsunamis occur in the Hellenic forearc [Guidoboni and Comastri, 1997; Accepted 6 NOV 2015 Accepted article online 30 NOV 2015 Papadopoulos, 2011], with the most widely documented being the A.D. 365 M~8.5 event which uplifted Published online 14 DEC 2015 western Crete by as much as 10 m [Flemming,1978;Pirazzoli et al., 1982, 1996; Stiros,2001;Shaw et al., 2008; Papadopoulos, 2011]. Unlike most subduction systems worldwide [Tichelaar and Ruff,1993],the source of the great A.D. 365 earthquake is thought to be a reverse fault in the overriding plate rather than the subduction thrust [Stiros and Drakos, 2006; Shaw et al., 2008; Papadimitriou and Karakostas, 2008; Stiros, 2010; Shaw and Jackson, 2010]. Independent of whether the 10 m of uplift in western Crete was due entirely to the A.D. 365 earthquake or to a sequence of events between A.D. 0 and 500 [Pirazzoli et al., 1982, 1996; Kelletat,1991;Stiros,2001;Shaw et al., 2008], it is clear that the uplift respon- sible for the presence of Crete is at least partly coseismic. Therefore, uplift of paleoshorelines in coastal Crete constrains the uplift history, providing important information on the timing and magnitude of large prehistoric earthquakes in the Hellenic forearc. These paleoshorelines are spectacularly preserved as platforms or sea notches encrusted in calcareous beachrock (see Figures S1 and S2 in the supporting information) and have been extensively studied [Angelier et al., 1982; Pirazzoli et al., 1982, 1996; Shaw et al., 2008; Gallen et al., 2014; Strobl et al., 2014; Tiberti et al., 2014; Mouslopoulou et al., 2015]. We use the altitudes and radiocarbon ages of these paleoshorelines in combination with sea level curves to analyze, for the first time, uplift rates from the entire length of Crete over the last 50 kyr (Figure 2a). Our analysis highlights strong temporal transients in uplift rates that varied little spatially on Crete prior to the period of Minoan civilization (~2700–1450 years B.C.), despite an interpreted apparent eastward change from seismic to aseismic uplift [Meier et al., 2007; Zachariasse et al., 2008; Becker et al., 2010; Gallen et al., 2014; Strobl et al., 2014]. 2. Spatial and Temporal Uplift Patterns on Crete We show that uplift rates over the last ~50 kyr were transitory and that, prior to about 3000 years B.C., did not vary from west to east along Crete (Figures 2b and 2c). We achieve this by calculating the uplift and uplift rate ©2015. American Geophysical Union. for 108 paleoshorelines at 31 coastal sites on Crete which are today elevated by up to 23 m above sea level All Rights Reserved. (Figure 2a and Table S1). MOUSLOPOULOU ET AL. PALEOEARTHQUAKE CLUSTERS OFFSHORE CRETE 10,282 Geophysical Research Letters 10.1002/2015GL066371 Figure 1. The Hellenic subduction margin and the main forearc faults offshore Crete. (a) Stars indicate the epicenters of the A.D. 365 (west) and A.D. 1303 (east) earthquakes in offshore Crete. Yellow arrows indicate movement vectors from permanent GPS stations [Mouslopoulou et al., 2014]. The northward extent of the accretionary prism is indicated by yellow dashed line. Bathymetry from GEBCO. G = Gavdos Island, WF = Western Fault, GF = Gavdos Fault, EF1 = Eastern Fault 1, EF2 = Eastern Fault 2. (b) Cross sections of the western (a-a′) and eastern (b-b′) Crete (modified from Shaw and Jackson [2010]). The plate interface is indicated by the black line, and the weakly locked portion of the interface is highlighted by yellow polygon. Vertical scale changes with depth. Uplift and uplift rates (Figures 2b and 2c and Table S1) were calculated using the altitude and radiocarbon age of paleoshorelines together with the sea level curves. All radiocarbon ages (Table S1) are recalibrated using the software WinsCal v.6.0 (see online supporting information) and range from ~0.5 to 46 calibrated kyr B.P. We use global sea level curves of Siddall et al. [2003] for pre-Holocene paleoshorelines and the Mediterranean sea level curve of Antonioli et al. [2007] for post ~3000 years B.C. data. The Antonioli et al. [2007] curve predicts sea level rise at a uniform rate of ~1 m/1000 years since 2000 years B.C. and a total of 6–7 m since ~3000 years B.C., which is consistent with the drowning of many archeological sites throughout the Mediterranean [Antonioli et al., 2007; Auriemma and Solinas, 2009; Mouslopoulou et al., 2015], and with the reverse age order (i.e., youngest shorelines are highest) of 0–3000 years B.C. paleoshorelines at the majority of sites in western Crete [Pirazzoli et al., 1996; Mouslopoulou et al., 2015] (Figure S1a and Table S1). Sensitivity tests suggest that the first-order uplift estimates presented here are independent of which sea level curves were employed and, in particular, the rate of Late Holocene sea level rise of the Mediterranean. The age-altitude relationships of paleoshorelines and their associated stratigraphy are different in the west and east of Crete (Table S1 and Figure S1). To the west, paleoshorelines up to 8 m above present sea level mainly range in age up to 3000 years B.C., while higher marine platforms are 20–40 kyr and increase in age with altitude [Mouslopoulou et al., 2015] (Table S1 and Figure S1a). These ages contrast with eastern parts of Crete where no Late Holocene paleoshorelines have been recorded and the ages of marine platforms MOUSLOPOULOU ET AL. PALEOEARTHQUAKE CLUSTERS OFFSHORE CRETE 10,283 Geophysical Research Letters 10.1002/2015GL066371 Figure 2. Locality map of paleoshoreline uplift and uplift rate data along Crete during the last 50 kyr. (a) Letters show loca- tions of the individual study sites presented in Table S1. Localities are color coded according to the interval over which uplift rates were measured in Figure 2c (i.e., white-filled circles 0–5 kyr, grey-filled circles 0–30 kyr, orange-filled circles 0–40 kyr, and black-filled circles 0–50 kyr). (b) Total uplift on western and eastern Crete during the last 50 kyr. (c) Uplift rates along Crete for the last 50 kyr averaged over the color-coded periods of time. See Figure 2a for localities. Color-coded best fit lines indicate uniform, but transitory, uplift rate along Crete and across the inferred boundary between seismic and aseismic uplift. Slight eastward tilt on the 0–30 kyr best fit line reveals the impact of the A.D. 365 earthquake. Ge = Georgioupoli, Th = Thodorou, N = Nopigia, KA = Kalyviani, B = Balos, FH = Falasarna-Harbour, FAS = Falasarna, FS = Falasarna-South, Ch = Chrysoskalittsa, El = Elafonisi, P = Paleochora, R = Roumeli, L = Lissos, S = Sougia, Ma = Marmara, Sf = Sfakia, A = Amoudi, Pr = Preveli, K = Kommos, Fe = Ferma, G = Goudouras, X = Xerokampos, Z = Zakros, It = Itanos, M = Mochlos, T = Tholos, and AO = Ancient-Olous. For details see Table S1. within 10 m of sea level range from ~20 to 45 kyr (Figure S1b), again increasing in age with altitude [Mouslopoulou et al., 2015]. Changes in the ages and altitudes of paleoshorelines from west to east are mainly due to Late Holocene coseismic uplift in the western parts of the island which, if backstripped or removed from the uplift patterns, lowers all paleoshorelines of 20–40 kyr age to within 20 m of the present sea level [Mouslopoulou et al., 2015].

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