GeoHazards

Article Tsunami Alert Efficiency in the Eastern Mediterranean Sea: The 2 May 2020 Earthquake (Mw6.6) and Near-Field Tsunami South of ()

Gerassimos A. Papadopoulos 1,*, Efthymios Lekkas 2,3, Katerina-Navsika Katsetsiadou 2, Emmanouil Rovythakis 2,4 and Amir Yahav 5 1 International Society for the Prevention & Mitigation of Natural Hazards, 10681 , Greece 2 Department of Geology & Geoenvironment, National & Kapodistrian University of Athens, 15784 Athens, Greece; [email protected] (E.L.); [email protected] (K.-N.K.); [email protected] (E.R.) 3 Earthquake Planning and Protection Organization, 154 51 Neo Psychiko, Greece 4 Fire Brigade Department, Crete, 72200 Ierapetra, Greece 5 National Steering Committee for Earthquake Preparedness, Tel-Aviv 61171, Israel; [email protected] * Correspondence: [email protected]; Tel.: +30-2106007533

 Received: 1 July 2020; Accepted: 27 July 2020; Published: 30 July 2020 

Abstract: The Mediterranean tsunami warning system is based on national monitoring centers (Tsunami Service Providers, TSPs) and operates under the IOC/UNESCO umbrella. For the first time we evaluate in depth the system’s performance for improving its operational effectiveness in conditions of extremely narrow time frames due to the near-field tsunami sources. At time 10 ( 2) ± min from the origin time, to, of the 2 May 2020 (Mw6.6) earthquake in Crete, the Greek, Italian and Turkish TSPs sent alerts to civil protection subscribers. A small tsunami (amplitude ~16 cm a.m.s.l.) of magnitude Mt6.8, arriving at south Crete in ~17 min from to, was documented from tide-gauge records and macroscopic observations. The analysis of the upstream alert messages showed that the tsunami alert efficiency is not adequate, since (1) earthquake parameters of low accuracy were used for the initial message, (2) alerts were issued after some forecasted wave arrival times had expired, (3) alert messages are characterized by various discrepancies and uncertainties. Our calculations showed that the upstream component improves if the alert time is shortened by a few minutes and the initial earthquake parameters are more accurate. Very late procedures were noted in the Greek civil protection downstream component, thus rendering response actions useless. In Israel, a bit more time was available to the authorities for decision making. A drastic improvement of the downstream component is needed to achieve timely alerting for local authorities and communities.

Keywords: near-field tsunamis; Mediterranean warning system; 2020 earthquake and tsunami; upstream alert component; downstream alert component; tsunami alert efficiency; Crete; Greece; Israel

1. Introduction Near-field tsunamis produced in subduction zones arrive at the closest coasts in tenths of minutes or less [1], thus leaving very little time for early warning. About 80% of victims due to tsunamis worldwide are caused within the first 1 h of tsunami propagation [2]. In the eastern Mediterranean, the Hellenic Subduction Zone (HSZ) marks the convergence lithospheric plate boundary between the Nubian (African) plate and the southern margin of the Eurasian plate where the former subducts underneath the later (Figure1a). On 2 May 2020 a strong earthquake with a moment magnitude Mw = 6.6 ruptured to the south of Crete, in the Hellenic Subduction Zone (HSZ) (Figure1a). The earthquake epicenter was located at a distance of ~90 km from the closest coast of Crete, which implies that it was a very near-field source for tsunami generation. According to the alerting protocols adopted within the

GeoHazards 2020, 1, 44–60; doi:10.3390/geohazards1010005 www.mdpi.com/journal/geohazards GeoHazards 2020, 1 45 frame of the North-East Atlantic and Mediterranean Tsunami Warning System (NEAMTWS) of the Intergovernmental Oceanographic Commission (IOC) of UNESCO, the Tsunami Service Providers (TSPs) in the eastern Mediterranean region issued tsunami warning messages within about 10 min from the 2 May 2020 earthquake origin time. In the Mediterranean Sea basin, tsunami waves are generated not only by strong submarine or coastal earthquakes, like the 2 May 2020 one, but also by volcanic eruptions and landslides [3,4]. The most active geodynamic structure in the Mediterranean is the arc and trench system of the HSZ, which produces mega thrust tsunamigenic earthquakes [5,6]. Several pre-historical and historical tsunamis have been reported in the Mediterranean region [3–7]. In around 1600 B.C. a large tsunami was produced during the giant eruption at the volcano of Thera (Santorini) in the back-arc region of the Hellenic Arc. Well-known historical examples are the tsunamis of AD 365 and 1303, which were generated by major earthquakes with estimated magnitudes M~8.0–8.5 and propagated up to remote places in the eastern Mediterranean, such as Alexandria and Haifa (Figure1). In the 20th century two strong and destructive seismic tsunamis were caused in the Messina strait, south Italy, on 28 December 1908, and in the Sea, Greece, on 9 July 1956 [3,4]. Although large tsunamis are rare in the Mediterranean, they remain high impact events. Coastal zones of the region are threatened mainly by very near-field tsunami sources. However, in case of basin-wide tsunamis, coastal zones at distances of ~1 h of tsunami travel time are also at risk. For example, coasts in the Levantine Sea, e.g., Israel, are threatened not only by local tsunamis but also by remote tsunamis which are generated along the HSZ [8,9]. In the last decade or so, several small tsunamis were detected after strong earthquakes of magnitude < 7 thanks to the tide-gauge networks developed after the establishment of the Intergovernmental Coordination Group (ICG)/NEAMTS (see the Discussion section). In the North-East Atlantic and Mediterranean (NEAM) region, a systematic and coordinated tsunami risk mitigation program practically started in the aftermath of the Indian Ocean’s mega tsunami on Boxing Day 2004. However, the need to develop such a program, including the operation of early warning systems, was stressed well before 2004 thanks to relevant EU-supported tsunami research projects [10,11]. In November 2005 the country members of the IOC/UNESCO established the Intergovernmental Coordination Group (ICG) of the NEAMTWS, which is supported by five national monitoring centers, acting as TSPs, and by several technical Working Groups and Task Teams. The monitoring centers started operations gradually from the summer of 2012. However, NEAMTWS provides services only for seismic tsunamis, leaving aside tsunamis produced by other causes, such as volcanic eruptions and landslides. After more than five years of real operational experience within NEAMTWS, it is of interest to evaluate the performance of the system. So far the effort for such an evaluation remains quite limited [9]. On 1 July 2009 an earthquake and tsunami similar to that of 2 May 2020 were observed to the south of Crete [12] but the NEAMTWS was not yet operational. More recently, the system’s operational evaluation was examined [13] after a local tsunami observed in SE Aegean Sea after a Mw = 6.6 earthquake that ruptured the Bodrum- area on 20 July 2017. On the other hand, so far no information is available on the extent to which the tsunami alert messages reached local communities. However, for the local tsunami that followed, the 2 May 2020 earthquake (Mw = 6.6) to the south of Crete in the HSZ, Greece (Figure1), data are available for both the upstream and downstream components of the warning system, and, therefore, this case offers a good opportunity to evaluate the operational performance of both components. To this aim we present the main observations regarding the 2020 earthquake and tsunami, analyze the operational efficiency of the upstream component of the system based on a series of alert messages issued by three TSPs, examine the downstream component focusing on the civil protection response in Greece and in Israel and discus realistic perspectives for improving the tsunami alert efficiency. GeoHazards 2020, 1 46 GeoHazards 2020, 1, x FOR PEER REVIEW 3 of 17

(a) (b)

FigureFigure 1. 1.(a)( Seismotectonica) Seismotectonic frame frame of the of the2 May 2 May 2020 2020 earthquake earthquake (Mw = (M 6.6)w = to6.6) the tosouth the southof Crete. of Crete.The NubianThe Nubian lithosphere lithosphere subducts subducts from fromthe Mediterrane the Mediterraneanan underneath underneath the theAegean Aegean Sea Sea at atthe the southern southern EurasianEurasian plate plate margin [12[12].]. ArrowsArrows and and stars stars show show lithospheric lithospheric plate motionsplate motions and earthquake and earthquake epicenters, epicenters,respectively. respectively. The Ptolemy The (PTT),Ptolemy Pliny (PTT), (PLT) Pliny and (P StraboLT) and (STR) Strabo trenches, (STR) as trenches, well as theas well epicenters as the of epicentersthe large historicalof the large earthquakes historical ofearthquakes AD 365 and of 1303, AD are365 also and shown. 1303, are The also tsunami shown. caused The by tsunami the 2020 causedearthquake by the was 2020 recorded earthquake at two was near-field recorded tide-stations at two ne (solidar-field triangles tide-stations in b) as (so welllid astriangles at the Alexandria in b) as wellstation. as at Nothe clearAlexandria wave signal station. was No detected clear wave at the signal Zygi was and detected Haifa stations. at the Zygi (b) Epicenter and Haifa of stations. the 2 May (b2020) Epicenter earthquake of the as2 May determined 2020 earthquake by various as institutes:determined N1, by N2various (= NOA1, institutes: 2), K1, N1, K2 N2 ( =(= KOERINOA1, 1,2), 2), K1,IN K2 (= (=INGV) KOERI,R( 1,= 2),reference IN (= INGV), epicenter R (= by reference GFZ, similar epicenter to the by one GFZ, determined similar to by the USGS); one determined 1,2 stand for byinitial USGS); and 1,2 updated stand for epicenters, initial and respectively. updated epic Acronymsenters, respectively. INGV, NOA, KOERIAcronyms are explainedINGV, NOA, in Section KOERI 2.2 areand explained GFZ in Sectionin section 2.3.1 2.2. The and 2020 GFZ tsunami in 2.3.1. was The recorded 2020 tsunami at the Ierapetrawas recorded and Kasos at the tide-stations Ierapetra and and Kasosreported tide-stations from Arvi, and Kastri reported and Chrysifrom Arvi, islet. TheKast 1ri Julyand 2009 tsunami islet. wasThe reported1 July 2009 from tsunami Arvi, was reportedand Chrysi from islet. Arvi, Solid Myrtos dots and show Chrysi localities. islet. Solid dots show localities. 2. Materials and Methods 2. Materials and Methods 2.1. Method Overview 2.1. Method Overview We briefly introduce the current architecture of the NEAMTWS as well as the operational upstream andWe downstream briefly introduce components the ofcurren the systemt architecture and the of responsibilities the NEAMTWS of the as TSPs. well Theas the large operational earthquake upstreamof 2 May and 2020 downstream and its associated componen tsunamits of the are system described and the on respon the basissibilities of instrumental of the TSPs. records The large and earthquake of 2 May 2020 and its associated tsunami are described on the basis of instrumental macroscopic observations, while the tsunami magnitude, Mt, is calculated. Based on the tsunami recordsforecasts and contained macroscopic in the observations alert messages, while issued the tsunami by three magnitude, TSPs after M thet, is earthquake, calculated. weBased examine on the in tsunami forecasts contained in the alert messages issued by three TSPs after the earthquake, we depth the tsunami alert efficiency for the first time in the NEAMTWS history. Our analysis included examine in depth the tsunami alert efficiency for the first time in the NEAMTWS history. Our both the upstream alert component and the downstream performance of the tsunami response by the analysis included both the upstream alert component and the downstream performance of the authorities in Greece and Israel. This evaluation has been made in light of the extremely narrow early tsunami response by the authorities in Greece and Israel. This evaluation has been made in light of warning time imposed by the near-field nature of the tsunami source. All times referred to are in UTC the extremely narrow early warning time imposed by the near-field nature of the tsunami source. unless otherwise noted. All times referred to are in UTC unless otherwise noted. 2.2. NEAMTWS Operational Architecture 2.2. NEAMTWS Operational Architecture The operational architecture of NEAMTWS is currently based on five TSPs, which are national centersThe chargedoperational with architecture seismic and tsunamiof NEAMTWS monitoring is curr inently France based (CENALT, on five Centre TSPs, d’Alerte which are aux national Tsunamis, centersParis, France),charged Italywith (INGV,seismic Istituto and tsunami Nazionale monitori di Geofisicang in France e Vulcanologia), (CENALT, Greece Centre (NOA, d’Alerte National aux Tsunamis,Observatory Paris, of Athens),France), PortugalItaly (INGV, (IPMA, Istituto Instituto Nazionale Portugu diês Geofisica do Mar e dae Vulcanologia), Atmosfera) and Greece Turkey (NOA,(KOERI, National Kandilli Observatory Observatory of and Athens), Earthquake Portugal Research (IPMA, Institute). Instituto The Português national centersdo Mar CENALT, e da Atmosfera)INGV, NOA and and Turkey KOERI (KOERI, obtained Kandilli international Observat accreditationory and Earthquake within the frameResearch of IOCInstitute)./UNESCO The in nationalSeptember centers 2016 CENALT, while IPMA’s INGV, was NOA obtained and KOERI in December obtained 2019. international No responsibility accreditation areas within have beenthe frameendorsed of IOC/UNESCO by the ICG/NEAMTWS in September but each2016 TSPwhil hase IPMA’s its own competencewas obtained monitoring in December area. 2019. A country No responsibilitymember subscribes areas have to receive been tsunamiendorsed alert by messagesthe ICG/NEAMTWS from one or but more each TSP(s) TSP onhas the its basis own of competencebilateral agreement(s). monitoring area. Within A country the upstream member componentsubscribes to of receive the system, tsunami tsunami alert messages alert messages from one or more TSP(s) on the basis of bilateral agreement(s). Within the upstream component of the system, tsunami alert messages are issued on the basis of two main elements: instrumental records

GeoHazards 2020, 1 47 are issued on the basis of two main elements: instrumental records of the earthquake and of the tsunami, if any, and a Decision Matrix (DM). More information on the NEAMTWS can be found at http://www.ioc-tsunami.org/index.php?option=com_content&view=article&id=10&Itemid=14. The earthquake solution is based on national and regional seismograph networks and provides focal parameters of the seismic event, i.e., origin time, earthquake epicenter, focal depth and magnitude usually within ~2–4 min from the earthquake origin time, to. Networks of tide-gauge stations installed at selected near-shore spots record sea level changes. However, no ocean bottom instruments for operational tsunami purposes are available so far in the Mediterranean Sea. Because of the near-field tsunami character and the absence of ocean bottom instruments, the potential generation of a tsunami initially relies on the DM, i.e., a tool composed by a set of empirical rules indicating the possibility for an earthquake to generate a tsunami depending on the values of the focal parameters. The DM also predicts the threat level based on these parameters. The ICG/NEAMTWS endorsed a common DM form but allows each TSP to decide about the specifications of the DM numerical values adopted. An earlier evaluation of the DM’s performance in the NEAM region can be found in Ref. [14]. In the Mediterranean region, the DMs in use by the TSPs are similar in their general form, although there are some differences. The tsunami threat level at a particular coastal point scales with the earthquake magnitude, M, and the epicentral distance, d. Magnitude M is the best available to a TSP at the time of the tsunami alert issue. Three threat levels are predicted by the DMs in current use: Tsunami Information (5.9 M or 6.0 M 6.4 and d 100 km), Tsunami Advisory (6.0 M 6.4 and d < 100 km ≤ ≤ ≤ ≥ ≤ ≤ or 6.5 M 6.9 and d 100 km or 7.0 M 7.4 and d > 400 km), Tsunami Watch (6.5 M 6.9 and ≤ ≤ ≥ ≤ ≤ ≤ ≤ d < 100 km or 7.0 M 7.4 and 400 km d 100 km or M 7.5). The general features of the threat ≤ ≤ ≥ ≥ ≥ levels are shown in Table1. Tsunami alerts are categorized as local (d < 100 km), regional (100 km d ≤ 400 km) and basin-wide (d > 400 km). The above scheme is valid for the DM in use by INGV and ≤ KOERI. In the DM which is used by NOA, however, the characterization of the tsunami threat level is shifted by a lower magnitude range step. For example, for 5.5 < M 6.0 at local and regional distances, ≤ the threat levels Advisory and Information are predicted, respectively, while in the range 6.0 < M 6.5 ≤ the threat level is Watch for local distance and Advisory for regional distance, and so on.

Table 1. Tsunami threat levels and their features as adopted by the Intergovernmental Coordination Group (ICG)/NEAMTWS.

Threat Level Tsunami Wave Effects on the Coast Tsunami Information No wave expected No tsunami threat Tsunami wave height ranges Currents, bores, recession, damage in Tsunami Advisory between about 0.2 m and 0.5 m harbors, small inundation on beaches and/or tsunami run-up < 1 m Tsunami wave height > 0.5 m Tsunami Watch Coastal inundation and/or tsunami run-up > 1 m

There are four types of tsunami alert messages: Initial message (I), Ongoing message (O), End message (E) and Cancel message (C). The I-message is the very first alert message that a TSP issues after an earthquake which might cause tsunami, according to the DM. Such a message is based on the very first solution of the seismic event available to a TSP. The O-messages are issued when updated earthquake solutions and/or tide-gauge records are available. The E-messages are issued when TSPs decide that no more information of operational interest is available for dissemination. A C-message is issued in case there is need to cancel the alert procedure, e.g., in case of false alarm. At the current stage of the NEAMTWS development, the tsunami messages issued by the TSPs are not public but are transmitted only to prescribed Civil Protection Authorities (CPAs). The initial determination of the earthquake focal parameters is of crucial importance for organizing the content of an I-tsunami message immediately after an earthquake event and according to the DM which is applicable. At the current stage of development, the target time endorsed by the ICG/NEAMTWS to issue an I-message is 10 min. The content of an I-message includes the time GeoHazardsGeoHazards 20202020, 1, ,x1 FOR PEER REVIEW 5 of 17 48 includes the time and number of the message issued, the focal parameters of the earthquake, the threatand level number and of the the respective message issued, area threatened, the focal parameters as well as of the the forecasted earthquake, wave the threatarrival level times and at the prescribedrespective forecast area threatened, points. as well as the forecasted wave arrival times at prescribed forecast points. TheThe tsunami tsunami alert alert messages messages issued issued by bya TSP a TSP are are transmitted transmitted via via e-mail, e-mail, fax fax machines machines and and GTS GTS (Global(Global Telecommunication Telecommunication System) System) to to subscribed subscribed national CPAsCPAs of of the the IOC IOC country country members, members, to otherto otherTSPs, TSPs, to IOCto IOC/UNESCO,/UNESCO, as as well we asll as to to the the ERCC ERCC (Emergency (Emergency Response Response Coordination Coordination Center) Center) of of the theEuropean European Commission Commission in in Brussels Brussels (Figure (Figure2). 2).

Figure 2. Flow chart of the North-East Atlantic and Mediterranean Tsunami Warning System (NEAMTWS)/Intergovernmental Oceanographic Commission (IOC)/UNESCO operational architecture. Figure 2. Flow chart of the North-East Atlantic and Mediterranean Tsunami Warning System The upstream component of the system includes the steps until the alert messages are sent to National (NEAMTWS)/IntergovernmentalCivil Protection Authorities. TheOceanographi next steps of thec procedureCommission compose (IOC)/UNESCO the downstream component.operational architecture. The upstream component of the system includes the steps until the alert messages are sent2.3. to The National 2020 Earthquake Civil Protection and Tsunami Authorities. Event The next steps of the procedure compose the downstream component. 2.3.1. Observations 2.3. The On2020 2 MayEarthquake 2020, atand 12:51:06.5 Tsunami Event UTC, a shallow strong earthquake ruptured offshore to the south of Crete, at the central segment of the HSZ (Figure1). The initial magnitude of Mw = 6.6 and 2.3.1. Οbservations hypocenter (25.75◦ E/34.27◦ N, h = 10 km) determined by the GEOFON network of the Deutsches GeoForschungsZentrumOn 2 May 2020, at 12:51:06.5 (GFZ) UTC, (https: a //shallowgeofon.gfz-potsdam.de strong earthquake/) have ruptured been consideredoffshore to the as reference south of parametersCrete, at the for central our analysis, segment since of theythe HSZ are consistent (Figure 1). with The the initial determinations magnitude ofof other Mw = centers, 6.6 and e.g., hypocenterUSGS (https: (25.75°//earthquake.usgs.gov E/34.27° N, h = 10/earthquakes km) determined/search by/). the The GEOFON 2 May 2020 network earthquake of the was Deutsches perceptible GeoForschungsZentrumin various places of Crete (GFZ) and (https://geofon.gfz-pots beyond, but no damagedam.de/) was reported. have been considered as reference parametersSoon for after our the analysis, earthquake, since localthey authoritiesare consistent and with eyewitnesses the determinations reported a of tsunami other centers, wave observed e.g., USGSalong (https://earthquake.usgs.gov/earthquakes the coast of the southeastern side of Crete./search/). The tsunami The 2 has May been 2020 documented earthquake in a serieswas of perceptiblepictures andin various video shootingsplaces of Crete taken an byd local beyond, authority but no persons, damage by was the reported. press as well as by amateurs at coastalSoon spotsafter ofthe the earthquake, Ierapetra town, local at theauthorities nearby villages and eyewitnesses of Arvi and Kastrireported as well a tsunami as at the northernwave observedside of Chrysialong the islet coast (Figure of 1theb, Figuresoutheastern3, Figure side4 and of AppendixCrete. TheA ).tsunami From eyewitness has been documented accounts it results in a seriesthat of the pictures tsunami and of video 2 May shootings 2020 was taken similar by to lo thecal oneauthority reported persons, after theby earthquakethe press as ofwell 1 July as by 2009. amateursThe 2009 at tsunamicoastal spots was observed of the Ierapetra at Arvi andtown, Myrtos at the villagesnearby asvillages well as of at Arvi the northand Kastri side of as Chrysi well as islet at the(Figure northern1b) [ 12side]. Noof Chrysi tide-gauges islet (Figures were operating 1b, 3 and in 4 theand areaAppendix). in 2009. From However, eyewitness a video accounts record it that resultsdocumented that the tsunami the strong of water2 May flow 2020 in was the similar harbor to of the Arvi one has reported been collected after the by earthquake us. of 1 July 2009. The 2009 tsunami was observed at Arvi and Myrtos villages as well as at the north side of Chrysi islet (Figure 1b) [12]. No tide-gauges were operating in the area in 2009. However, a video record that documented the strong water flow in the harbor of Arvi has been collected by us.

GeoHazards 2020, 1, x FOR PEER REVIEW 6 of 17 GeoHazards 2020, 1, x FOR PEER REVIEW 6 of 17 From the observational material collected, it is evident that the 2 May 2020 tsunami appeared asFrom a series the of observational relatively short- materialperiod collected, oscillations it is and evident strong that wa theter 2currents. May 2020 The tsunami maximum appeared sea level as arise series and of drop relatively was macroscopically short-period oscillations estimated atand ~25 strong cm at wa Ierapetrater currents. and at The ~30–35 maximum cm in Chrysisea level islet. riseNo and damage drop was was macroscopically reported from estimatedthe tsunami. at ~25 The cm 2020 at Ierapetra tsunami andhas atalso ~30–35 been cm documented in Chrysi islet. on the Notide-gauge damage was records reported of fromthe Ierapetra the tsunami. (NOA-04) The 2020 and tsunami Kasos has(NOA-03) also been stations documented and situated on the at tide-gaugeepicentral records distances of ofthe Δ ~90Ierapetra km and (NOA-04) Δ~180 km, and respectively Kasos (NOA-03) (Figure 1b). stations In Ierapetra and situatedstation, southat epicentralCrete, Greece, distances the of sea Δ ~90water km disturbance and Δ~180 initiatedkm, respectively at ~13:07 (Figure with a 1b).likely In retreat Ierapetra of the station, water. south This is Crete,consistent Greece, with the seaseveral water eyewitness disturbance accounts. initiated A at peak ~13:07 amplitude with a likely of A ~0.16retreat m of was the recorded water. This at 13:16is consistent(Figure with5). Taking several the eyewitness crest-to-trough accounts. values A peak of A amplitude for three sequentialof A~0.16 m oscillations was recorded recorded at 13:16 from (Figure13:12 5).to 13:18,Taking we the obtained crest-to-trough once again values an average of A for maximum three sequential value of oscillations 0.16 m and recordeda wave period from of 13:12T~3.5 to 13:18,min. In we Kasos obtained station, once SE again Aegean an Sea,average Gree maximumce, the record value showed of 0.16 a mfirst and sea a waterwave disturbance,period of T~3.5likely min. water In Kasos rise, station,at ~13:39. SE AAegean peak amplitude Sea, Greece, of the A~0.05 record m showedwas recorded a first seaat 13:54 water (Figure disturbance, 6), while likelythe waterwave periodrise, at was~13:39. found A peak at T~8 amplitude min. In Alexandria of A~0.05 mtide-gau was recordedge station at IDSL-23, 13:54 (Figure situated 6), atwhile Δ~485 thekm wave to periodthe southeast was found of atthe T~8 epicenter min. In Alexandria (Figure 1) tide-gau and operatedge station by IDSL-23, the National situated Institute at Δ~485 for kmOceanography to the southeast and ofFisheries the epicenter (NIOF), (FigureEgypt, a1) we andak tsunamioperated signalby the has National been recorded Institute with for an Oceanographyamplitude of and 0.02–0.03 Fisheries m at(NIOF), time ~14:27 , (Figure a we ak7). tsunamiThe inspection signal ofhas the been tide-gauge recorded record with atan the amplitudestation Zygiof 0.02–0.03 Marina, mCyprus, at time situated ~14:27 (Figureat Δ~670 7). km The to inspectionthe east (Figure of the 1), tide-gauge showed no record clear attsunami the stationGeoHazardssignal. Zygi 2020 Marina,, 1 Cyprus, situated at Δ~670 km to the east (Figure 1), showed no clear tsunami49 signal.

(a) (b) (a) (b) Figure 3. (a) Sea retreat and (b) sea level oscillation by ~25 cm at the Ierapetra port, southeastern FigureFigureCrete, 3. 3. observed((aa)) Sea Sea retreat a few andand minutes (b(b)) sea sea after level level the oscillation oscillation 2 May 2020 by by ~25 earthquake ~25 cm cm at theat the Ierapetra(photo Ierapetra courtesy port, port, southeastern of southeasternMr. Emmanouil Crete, Crete,observedRovythakis). observed a few minutesa few minutes after the after 2 May the 2020 2 May earthquake 2020 earthquake (photo courtesy (photo of Mr.courtesy Emmanouil of Mr. Emmanouil Rovythakis). Rovythakis).

(a) (b) (a) (b) Figure 4. (a) The small port at the north side of Chrysi islet before the 2 May 2020 earthquake, and (b) sea level drop by ~35 cm and boats stranded as if on dry land after the earthquake (photo courtesy of Mr. George Garefalakis). From the observational material collected, it is evident that the 2 May 2020 tsunami appeared as a series of relatively short-period oscillations and strong water currents. The maximum sea level rise and drop was macroscopically estimated at ~25 cm at Ierapetra and at ~30–35 cm in Chrysi islet. No damage was reported from the tsunami. The 2020 tsunami has also been documented on the tide-gauge records of the Ierapetra (NOA-04) and Kasos (NOA-03) stations and situated at epicentral distances of ∆~90 km and ∆~180 km, respectively (Figure1b). In Ierapetra station, south Crete, Greece, the sea water disturbance initiated at ~13:07 with a likely retreat of the water. This is consistent with several eyewitness accounts. A peak amplitude of A~0.16 m was recorded at 13:16 (Figure5). Taking the crest-to-trough values of A for three sequential oscillations recorded from 13:12 to 13:18, we obtained once again an average maximum value of 0.16 m and a wave period of T~3.5 min. In Kasos station, SE Aegean Sea, Greece, the record showed a first sea water disturbance, likely water rise, at ~13:39. A peak amplitude of A~0.05 m was recorded at 13:54 (Figure6), while the wave period was found at T~8 min. In Alexandria tide-gauge station IDSL-23, situated at ∆~485 km to the southeast of the epicenter (Figure1) and operated by the National Institute for Oceanography and Fisheries (NIOF), Egypt, a weak tsunami signal has been recorded with an amplitude of 0.02–0.03 m at time ~14:27 GeoHazards 2020, 1, x FOR PEER REVIEW 7 of 17

Figure 4. (a) The small port at the north side of Chrysi islet before the 2 May 2020 earthquake, and (b) sea level drop by ~35 cm and boats stranded as if on dry land after the earthquake (photo courtesy of Mr. George Garefalakis).

2.3.2. Tsunami Alert Data Soon after the 2 May 2020 earthquake generation, the TSPs of INGV, KOERI and NOA produced respective I-alert messages based on the very first solutions of the seismic event available to each TSP. O-messages were issued when updated earthquake solutions and tsunami records were obtained. The time line of the I-, O- and E-messages issued are summarized in Table 2. Ierapetra town and nearby villages are the settlements situated closest to the earthquake source and, therefore, they have been the coastal spots with the shortest forecast times.

Table 2. Time line of upstream tsunami alerts issued by the Tsunami Service Providers (TSPs) of the Istituto Nazionale di Geofisica e Vulcanologia (INGV), National Observatory of Athens (NOA) and Kandilli Observatory and Earthquake Research Institute (KOERI) after the earthquake of 2 May 2020. End messages were issued at 17:50 (KOERI), 17:56 (NOA), 17:59 (INGV). Earthquake reference

parameters are from the Deutsches GeoForschungsZentrum (GFZ). Key: to = earthquake origin time, I = Initial message, O = Ongoing message; Mw = moment magnitude, * shows local magnitude (ML),

φoΝ and λoΕ = epicentral coordinates, Δ = epicentral distance, h = focal depth; tA = record time of amplitude A (maximum peak a.m.s.l.) in Ierapetra tide-gauge station. Threat level: W = Watch, A = Advisory. Forecast points: F1=-Karave, F2=Ierapetra, F3=Chora-Sfakion, F4=-Mesochori, np = not provided. All times are in UTC.

Issue o o Δ h A Institutes/Actions Alert Mw φ Ν λ Ε tA Time (km) (km) (m) Earthquake reference parameters GFZ to = 12:51:06 6.6 34.27 25.75 90 10 Upstream alert messages 13:10/F1A 13:12/F2W INGV-I 12:59 6.7 34.12 25.71 105 20 13:12/F3A 6.0 NOA-I 13:02 13:07/F2A 13:12/F1A 33.93 25.72 120 10 GeoHazards 2020, 1 * 50 KOERI-I 13:03 13:07/F2A 13:11/F1A 6.7 34.06 25.67 110 10 INGV-O 13:21 As in INGV-I 6.7 34.12 25.71 105 20 13:16 0.13 (FigureNOA-O7). The inspection13:27 of13:00/F2W the tide-gauge 13:11/F3W record at6.5 the34.52 station 25.72 Zygi Marina, 55 Cyprus, 15 13:16 situated 0.14 at KOERI-O 13:35 W (np) 6.7 34.28 25.74 85 28.7 13:16 0.14 ∆~670 km to the east (Figure1), showed no clear tsunami signal. INGV-O 13:50 As in INGV-I 6.7 34.12 25.71 105 20 13:34 0.15

GeoHazards 2020, 1, x FOR PEER REVIEW 8 of 17 GeoHazards 2020, 1, x FOR PEER REVIEWFigure 5. Tsunami record at Ierapetra NOA-4 station. 8 of 17 Figure 5. Tsunami record at Ierapetra NOA-4 station.

FigureFigure 6. TsunamiTsunami record at Kasos NOA-3 station. Figure 6. Tsunami record at Kasos NOA-3 station.

FigureFigure 7. TsunamiTsunami record at Alexandria station. Figure 7. Tsunami record at Alexandria station. 2.3.2. Tsunami Alert Data 3. Results 3. ResultsSoon after the 2 May 2020 earthquake generation, the TSPs of INGV, KOERI and NOA produced 3.1.respective Tsunami I-alert Size messages based on the very first solutions of the seismic event available to each TSP. 3.1. Tsunami Size O-messagesIn contrast were to issuedthe progress when updatednoted with earthquake the establishment solutions andof gradually tsunami recordsimproving were earthquake obtained. The time line of the I-, O- and E-messages issued are summarized in Table2. Ierapetra town and nearby magnitudeIn contrast scales, to thesuch progress as the scales noted of with local the magnitude, establishment ML [15], of graduallysurface-wave improving magnitude, earthquake Ms [16], magnitudeand moment scales, magnitude, such as theMw scales[17], no of standard local magnitude, tsunami MmagnitudeL [15], surface-wave scales have magnitude, been introduced Ms [16], so andfar. momentHowever, magnitude, several attempts Mw [17], have no standardbeen made tsunami since themagnitude 1940s see scales review have in beenRef. [18],introduced one of theso far.most However, promising several being attempts the method have introduced been made by since Abe the [19]. 1940s For see this review reason, in tsunami Ref. [18], intensity one of thehas mostbeen promisingused for several being theyears method as a proxy introduced of tsunami by Abe size, [19]. yet Forit is thisa useful reason, tool tsunami to estimate intensity the event has beenimpact. used Here, for severalwe estimate years both as a the proxy intensity of tsunami and the si ze,magnitude yet it is ofa useful the 2 May tool 2020to estimate tsunami. the event impact.Earlier Here, tsunami we estimate intensity both scalesthe intensity were of and only the six magnitude grades see of review the 2 May in Ref. 2020 [18]. tsunami. Α new, widely adoptedEarlier 12-grade tsunami tsunami intensity intensity scales were scale, of that only fo llowedsix grades the seetradition review of in the Ref. 12-grade [18]. Α macroseismicnew, widely adoptedintensity 12-grade scales, wastsunami introduced intensity in scale, 2001 that[20]. fo Basedllowed on the the tradition new scale, of the we 12-grade estimated macroseismic a tsunami intensityintensity scales,level of was grade introduced 4 for both inthe 2001 2020 [20]. and 2009Based local on tsunamis.the new scale, we estimated a tsunami intensityA promising level of grade tsunami 4 for magnitudeboth the 2020 scale and is 2009 based local on tsunamis.Pacific Ocean data and uses formula (1) to calculateA promising tsunami tsunami magnitude, magnitude Mt, from scale the is basedmaximum on Pacific wave Ocean heights, data Hand (in uses m), formulaat near-shore (1) to calculatetide-gauge tsunami records magnitude, at epicentral M distances,t, from the Δ (in maximum km) [19]: wave heights, H (in m), at near-shore tide-gauge records at epicentral distances, Δ (in km) [19]: Mt = log10 H+ log Δ+ 5.80 (1) Mt = log10 H+ log Δ+ 5.80 (1) This formula is similar to the one introduced [16] for the calculation of Ms. The constant in (1) wasThis obtained formula by requiringis similar Mtot the= M onew on introduced the average [16] for forthe thecalibration calculation data of set M s[19].. The The constant dependence in (1) wasof M obtainedt on log 10by H requiring was based M ton = M thew on empirical the average relation for the between calibration Mw anddata log set10 [19]. H. Then, The dependencethe tsunami ofamplitude Mt on log scales10 H was with based the event on the size. empirical Mt represents relation not between only the M woveral and llog physical10 H. Then, size theof a tsunami tsunami amplitudebut also the scales seismic with moment the event of thesize. tsunamigenic Mt represents earthquake. not only the overall physical size of a tsunami but also the seismic moment of the tsunamigenic earthquake.

GeoHazards 2020, 1 51 villages are the settlements situated closest to the earthquake source and, therefore, they have been the coastal spots with the shortest forecast times.

Table 2. Time line of upstream tsunami alerts issued by the Tsunami Service Providers (TSPs) of the Istituto Nazionale di Geofisica e Vulcanologia (INGV), National Observatory of Athens (NOA) and Kandilli Observatory and Earthquake Research Institute (KOERI) after the earthquake of 2 May 2020. End messages were issued at 17:50 (KOERI), 17:56 (NOA), 17:59 (INGV). Earthquake reference parameters are from the Deutsches GeoForschungsZentrum (GFZ). Key: to = earthquake origin time, I = Initial message, O = Ongoing message; Mw = moment magnitude, * shows local magnitude o o (ML), ϕ N and λ E = epicentral coordinates, ∆ = epicentral distance, h = focal depth; tA = record time of amplitude A (maximum peak a.m.s.l.) in Ierapetra tide-gauge station. Threat level: W = Watch,A = Advisory. Forecast points: F1 = Gavdos-Karave, F2 = Ierapetra, F3 = Chora-Sfakion, F4 = Karpathos-Mesochori, np = not provided. All times are in UTC.

o o Institutes/Actions Issue Time Alert Mw ϕ N λ E ∆ (km) h (km) tA A (m) Earthquake reference parameters

GFZ to = 12:51:06 6.6 34.27 25.75 90 10 Upstream alert messages 13:10/F1A INGV-I 12:59 13:12/F2W 6.7 34.12 25.71 105 20 13:12/F3A 13:07/F2A NOA-I 13:02 6.0 * 33.93 25.72 120 10 13:12/F1A 13:07/F2A KOERI-I 13:03 6.7 34.06 25.67 110 10 13:11/F1A As in INGV-O 13:21 6.7 34.12 25.71 105 20 13:16 0.13 INGV-I 13:00/F2W NOA-O 13:27 6.5 34.52 25.72 55 15 13:16 0.14 13:11/F3W KOERI-O 13:35 W (np) 6.7 34.28 25.74 85 28.7 13:16 0.14 As in INGV-O 13:50 6.7 34.12 25.71 105 20 13:34 0.15 INGV-I

3. Results

3.1. Tsunami Size In contrast to the progress noted with the establishment of gradually improving earthquake magnitude scales, such as the scales of local magnitude, ML [15], surface-wave magnitude, Ms [16], and moment magnitude, Mw [17], no standard tsunami magnitude scales have been introduced so far. However, several attempts have been made since the 1940s see review in Ref. [18], one of the most promising being the method introduced by Abe [19]. For this reason, tsunami intensity has been used for several years as a proxy of tsunami size, yet it is a useful tool to estimate the event impact. Here, we estimate both the intensity and the magnitude of the 2 May 2020 tsunami. Earlier tsunami intensity scales were of only six grades see review in Ref. [18]. A new, widely adopted 12-grade tsunami intensity scale, that followed the tradition of the 12-grade macroseismic intensity scales, was introduced in 2001 [20]. Based on the new scale, we estimated a tsunami intensity level of grade 4 for both the 2020 and 2009 local tsunamis. A promising tsunami magnitude scale is based on Pacific Ocean data and uses formula (1) to calculate tsunami magnitude, Mt, from the maximum wave heights, H (in m), at near-shore tide-gauge records at epicentral distances, ∆ (in km) [19]:

Mt = log10 H + log ∆ + 5.80 (1)

This formula is similar to the one introduced [16] for the calculation of Ms. The constant in (1) was obtained by requiring Mt = Mw on the average for the calibration data set [19]. The dependence of Mt on log10 H was based on the empirical relation between Mw and log10 H. Then, the tsunami amplitude GeoHazards 2020, 1 52

scales with the event size. Mt represents not only the overall physical size of a tsunami but also the seismic moment of the tsunamigenic earthquake. In the Mediterranean Sea basin, the applicability of formula (1) was tested for the first time very recently [18], after the earthquake (Mw = 6.8) of 25 October 2018 that ruptured the western segment of the Hellenic arc. From five tide records it was found that Mt = 7.0, which is a good approximation to Mw = 6.8, calculated from seismic records. The above procedure was applied to the 2 May 2020 tsunami wave. From the tsunami records at Ierapetra (H = 0.16 m; ∆ = 90 km), Kasos (H = 0.05 m; ∆ =180 km) and Alexandria (H = 0.025 m; ∆ = 485 km), we obtained an average Mt = 6.87, which is also close to the seismic moment magnitude Mw = 6.6.

3.2. Upstream Alert

3.2.1. Tsunami Messages The various tsunami messages are organized on the basis of the earthquake focal parameters, on the DM adopted by a TSP as well as on the tsunami records available, if any. The three I-messages issued were based on the adoption of three different initial seismic solutions (Table2), which are characterized by two main differences. First, the magnitude ML = 6.0 used by NOA was significantly underestimated with respect to Mw = 6.7, used by INGV and KOERI. A few minutes later, NOA adopted Mw = 6.5 and used it in its O-message. Second, the initial solutions by both NOA and KOERI put the earthquake epicenter to the south of Ierapetra but at a larger distance with respect to that adopted by INGV (Table2, Figure1b). These two features had important operational consequences, as analyzed later. The INGV I- and O-messages were based on the same earthquake solution (Table2) and forecasted a Watch threat level for Ierapetra and Advisory level for other, more remote forecast points. Since the INGV earthquake solution did not change, the forecast times remained identical with the ones in the I-message. In the two O-messages released by INGV, additional information of operational interest was incorporated, namely the tsunami amplitude A = 0.13 m recorded at Ierapetra station at time 13:16. The first O-message was issued 13 min after the initiation of the wave in the tide record. A second O-message was released by INGV, slightly updating the tide-gauge parameters (Table2). However, this message was of low operational value, for it was issued at a late time and contained practically no new information with regard to the first INGV O-message. For the town of Ierapetra, the NOA I-message indicated a lower threat level with respect to that of INGV, i.e., Advisory against Watch. Later, NOA adopted the revised magnitude Mw = 6.5 and epicentral distance for Ierapetra ∆~55 km, instead of ∆~120 km (Table2). Consequently, with the O-message of NOA, the threat level was upgraded to Watch. However, the consequence of the change in earthquake parameters has been to change most of the travel times forecasted, which have thus become different not only from the INGV forecasts but also with respect to the initial forecasts listed in the NOA I-message. Most of the forecast times in the O-message of NOA shifted earlier from 7 min to 20 min with respect to the times for the same forecast points in the I-message (Figure8). The average shift was found equal to 4.1 ( 4.12) min. Only one forecast time did not change, while only two − ± shifted a bit later by 1 and 2 min. Such differences issued in a short time interval may cause confusion to decision-making authorities. Like NOA, the KOERI TSP initially adopted a seismic source at ∆~110 km to the south of Ierapetra, but with magnitude Mw = 6.7, which is coincident with the one used by INGV. Because of the long epicentral distance, however, the initial threat level was Advisory. The KOERI I-message was also accompanied by three maps as enriched products of the wave forecasting described in the message itself. Although the number of maps could be reduced to two or even to only one, this material is operationally useful, since it illustrates in an understandable way the overall tsunami threat forecasted. It is of importance for the TSPs to collect responses from message subscribers in order to better understand how such enhanced products meet the operational needs of the subscribers and what are the possible improvements proposed. With its O-message, KOERI adopted a revised earthquake GeoHazards 2020, 1 53

GeoHazards 2020, 1, x FOR PEER REVIEW 10 of 17 solution with ∆~85 km (Table2) and, therefore, upgraded the threat level from Advisory to Watch. levelThe from KOERI Advisory O-message to Watch also included. The KOERI parameters O-message from also the Ierapetraincluded tide-gaugeparameters record from consistentthe Ierapetra with tide-gaugethe parameters record inserted consistent by INGVwith the and parameters NOA. inserted by INGV and NOA.

Figure 8. Difference D = f (I) – f (O) of forecast times in the NOA I- and O-messages vs. f for the same Figure 8. Difference D = f (I) – f (O) of forecast times in the NOA I- and O-messages vs. f for the same forecast points. Nearly all D-values are negative, implying that f times in O-message are systematically forecastshorter aspoints. compared Nearly to f alltimes D- invalues I-message. are negative, implying that f times in O-message are systematically shorter as compared to f times in I-message. It is noteworthy that none of the O-messages issued by the three TSPs included macroscopic informationIt is noteworthy about the that tsunami none that of the was O-messages already observed issued in by the the coastal three zone TSPs of southeastincluded Crete.macroscopic There is informationno doubt that about because the tsunami of the extreme that was press already for time observed the TSPs in werethe coastal unable zone to collect, of southeast cross-check Crete. and Thereverify is macroscopic no doubt that information because thatof the might extreme have beenpress incorporated for time the in theirTSPs O-messages.were unable Nevertheless, to collect, cross-checkit is of operational and verify interest macroscopic to investigate information if this kind ofthat information, might have when been available incorporated in real time in and their after O-messages.cross-checking Nevertheless, for reliability, it couldis of operational be operationally interest useful to forinvestigate inclusion if in this the messageskind of information, regardless of whenwhether available tide-gauge in real records time and are after available cross-checking or not. for reliability, could be operationally useful for inclusion in the messages regardless of whether tide-gauge records are available or not. 3.2.2. Tsunami Alert Efficiency 3.2.2. Tsunami Alert Efficiency According to the standard operational procedures, the I-messages were sent by the three TSPs to theirAccording subscribers to withinthe standard an average operational time τ = procedures10 ( 2) min, the from I-messagesto (Table2 ).were Time sentτ is by equal the three to the TSPs target ± totime their for subscribers issuing the within I-message an average adopted time by τ the = 10 ICG (± /2)NEAMTWS min from t ato (Table the current 2). Time stage τ is of equal the system’s to the targetdevelopment. time for issuing In the I-messages, the I-message the timesadopted forecasted by the forICG/NEAMTWS the first wave arrivalsat the current at the closest stage forecastof the system’spoints in development. south Crete fitIn well the enoughI-messages, thefirst the wavetimes onsetforecasted at Ierapetra for the station. first wave The threearrivals I-messages at the closestwere sentforecast to their points subscribers in south within Crete 5 fit to 9well min enou beforegh the the first first wave wave arrival onset at at Ierapetra Ierapetra town. station. This The time threelag isI-messages extremely short,were sent since to it their leaves subscribers only a few wi minutesthin 5 forto 9 operational min before decisions the first andwave actions arrival in at the Ierapetradownstream town. component This time oflag the is system.extremely short, since it leaves only a few minutes for operational decisionsThe and issue actions time of in O-messages the downstream ranged component between 30of andthe system. 44 min from to and between 11 and 33 min afterThe the issue shortest time forecast of O-messages times (Table ranged2). Thebetween last time 30 and lag leaves44 min nofrom room to and for ebetweenffective operational11 and 33 minactions. after Thethe threeshortest E-messages forecast weretimes issued (Table within 2). The a narrow last time time lag window leaves ofno ~9 room min andfor abouteffective five operationalhours after actions.to. The three E-messages were issued within a narrow time window of ~9 min and about Fromfive hours the previous after to. analysis, it results that there are four critical times in the upstream component of theFrom warning the previous system inanalysis, very near-field it results conditions, that there such are asfour the critical ones of times the 2 Mayin the 2020 upstream tsunami. componentSuppose that of thet is thewarning issue timesystem of anin alertvery messagenear-field and conditions,f is the forecast such as time the for ones the of first the wave 2 May arrival 2020 at tsunami.a given forecastSuppose point. that t Then, is the the issue four time critical of an times alert are message the issue and times f is of the the forecast I- and O-messages, time for thet( Ifirst) and wavet(O) ,arrival and the at respective a given forecast forecast point. times, Then,f (I) and thef (fourO), forcritical a given times forecast are the point. issue Since timesf countsof the I- from andt o, O-messages,of great operational t(I) and interestt(O), and is the the respective remaining forecast time from times,t to f ,f( i.e.,I) and the f di(Off),erences for a givenf (I)-t (forecastI) and f ( Opoint.)-t(O ). SinceThe soonerf counts the from alert messageto, of great is issued operational the larger interest the remaining is the remaining forecast time. time If from the O-message t to f, i.e., inserts the differencesdifferent earthquake f(I)-t(I) and focal f(O parameters)-t(O). The withsooner respect the alert to those message introduced is issued in the the I-message, larger the then remaining time f (O ) forecast time. If the O-message inserts different earthquake focal parameters with respect to those introduced in the I-message, then time f(O) may change at least for some, if not for all, of the forecast points, as already noted regarding the NOA messages after the 2 May 2020 earthquake.

GeoHazards 2020, 1 54

GeoHazardsmay change 2020 at, 1, leastx FOR for PEER some, REVIEW if not for all, of the forecast points, as already noted regarding the11 NOA of 17 messages after the 2 May 2020 earthquake. WeWe introduce the concept of tsunami alert eefficiency,fficiency, FF,, asas F = (f − t)/f (2) F = (f t)/f (2) The parameter F is an indicator of the forecast −time percentage remaining after time t. As an instance,The F parameter = 0.5 meansF isthat an only indicator half of of the the forecast forecast time time remains percentage for a remainingparticular forecast after time pointt. As after an aninstance, alert issue.F = 0.5 Theoretically, means that onlyF = 1 half if the of theI-message forecast is time issued remains simultaneously for a particular with forecastthe earthquake point after at timean alert to. F issue. takes Theoretically,not only positiveF = but1 if also theI-message negative values, is issued and simultaneously this really happens with theif t earthquake> f, i.e., when at thetime alertto. F messagetakes not is only issued positive after buta forecast also negative time has values, expired. and thisWe reallycompared happens the parameter if t > f, i.e., F when for the the forecastalert message times is inserted issued after in athe forecast alert timemessages has expired. issued Weby comparedthe threethe TSPs parameter after theF for 2 theMay forecast 2020 earthquake.times inserted In inthe the INGV’s alert messages I- and O-messages, issued by the the three forecast TSPs times after thefor 2each May forecast 2020 earthquake. point remained In the theINGV’s same I- because and O-messages, the same earthquake the forecast focal times parame for eachters forecast were adopted point remained in the two the messages. same because On the the contrary,same earthquake the earthquake focal parameters parameters were adopted adopted in in NOA’s the two O-message messages. Onare thedifferent contrary, from the the earthquake ones in theparameters I-message. adopted For KOERI, in NOA’s only O-messagethe I-message are contained different fromforecast the times. onesin the I-message. For KOERI, onlyTsunami the I-message alert containedefficiency forecastcurves obtained times. for the I-messages of the three TSPs (Figure 9) are quite Tsunamisimilar but alert have efficiency an apparent curves obtained shift be forcause the the I-messages messages of thewere three issued TSPs at (Figure times9 )t are= 8 quite min (INGV),similar butt = 11 have min an (NOA) apparent and t shift = 12 becausemin (KOERI) the messages after to. However, were issued a difference at times int =the8 alert min issue (INGV), of justt = 11a minfew (NOA)minutes and returnst = 12 mina considerable (KOERI) after differento. However,ce in the a dialertfference efficiency in the alertparameter. issue ofIn just the a forecastsfew minutes of NOA returns and aKOERI, considerable for time di fff uperence to ~20 in min the alertthe parameter efficiency F parameter. takes values In less the than forecasts 0.45 or of evenNOA less and than KOERI, 0.3. forHowever, time f up F tois ~20~0.6 minin the the forecasts parameter of FINGV,takes valuesimplying less better than 0.45alert or efficiency. even less Muchthan 0.3. worse However, is the alertF is ~0.6efficiency in the obtained forecasts for of INGV,both the implying INGV’s better and NOA’s alert effi O-messages.ciency. Much For worse time isf upthe to alert ~30 e ffimin,ciency the obtainedparameter for F bothis negative, the INGV’s which and means NOA’s that O-messages. for this time For range time thef up O-messages to ~30 min, werethe parameter issued afterF is the negative, forecast whichtimes for means several that fore forcast this points time range had already the O-messages expired. wereAgain, issued however, after thethe overall forecast alert times efficiency for several of INGV forecast is better points than had alreadythe one expired.of NOA due Again, to a however, time difference the overall of 6 alertmin ineffi theciency issue of of INGV the O-messages. is better than There the one is no of doubt NOA duethat tothe a timeefficiency difference would of be 6 min worst in thefor issuethe KOERI of the O-messageO-messages. if Thereforecast is no times doubt were that inserted, the efficiency since would the message be worst was for theissued KOERI 8 min O-message after the if forecastone by NOA.times were inserted, since the message was issued 8 min after the one by NOA.

Figure 9. Tsunami efficiency curves and respective equations for the I-messages of INGV (1), NOA (2) Figure 9. Tsunami efficiency curves and respective equations for the I-messages of INGV (1), NOA and KOERI (3); f is forecast time; F is tsunami efficiency alert parameter determined from formula (2) (2)in text;and rKOERIis correlation (3); f coeis forecastfficient. IMPRtime; F (4) is is tsunami the expected efficiency improvement alert parameter if the I-message determined was issuedfrom formula (2) in text; r is correlation coefficient. IMPR (4) is the expected improvement if the I-message only 5 min from the earthquake origin time to = 0 (see Discussion). was issued only 5 min from the earthquake origin time to = 0 (see Discussion).

3.3. Downstream Alert

3.3.1. Greece In Greece, as soon as the national Operational Center for Civil Protection (OCCP) receives a tsunami message from NOA, TSP initiates the response mechanism by forwarding the message to sectorial emergency centers, e.g., Fire Brigade, Police etc., including the Operational Center for Port

GeoHazards 2020, 1 55

3.3. Downstream Alert

3.3.1. Greece In Greece, as soon as the national Operational Center for Civil Protection (OCCP) receives a tsunami message from NOA, TSP initiates the response mechanism by forwarding the message to sectorial emergency centers, e.g., Fire Brigade, Police etc., including the Operational Center for Port Authorities (OCPA). The duty of the sectorial emergency centers is to forward the message further downward. Here, we highlight the actions undertaken in the Greek downstream component of the system after the 2 May 2020 earthquake. Following the message transmission flow explained above (Figure2), the OCCP transmitted NOA’s message at OCPA, which forwarded it to various Port Authorities in Crete and elsewhere in southern Greece. We have no complete picture of the actions undertaken by all the Port Authorities that received the alert. However, it has been possible to track details of the message flow from the central level up to the Port Authorities of and Rethymnon, two of the most important coastal cities in Crete (Figure1b). The Chania Port Authority released an emergency signal directed to local authorities and to the local maritime community in its competence area at 15:38. The emergency, however, was not disseminated to the general population. According to the INGV’s I- and O-messages, the forecasted first wave arrival at Chania was 13:41, which is close to the 13:45 forecasted by the I-message of KOERI. The I- and O-messages of NOA, however, forecasted arrival times at 14:12 and 14:14, respectively. This time difference is considerably high from a geophysical and operational point of view, and may reflect important differences in the tsunami propagation models and possibly in the bathymetry files which are in use by the three TSPs. Whatever the possible explanation, this time difference introduces an additional discrepancy between the tsunami messages issued by different monitoring centers and increases the uncertainties in the operational tsunami forecasting. A similar procedure was followed for setting up emergency in the nearby city of Rethymnon. The tsunami arrival times forecasted with the NOA’s I- and O-messages have been 13:50 and 13:46, respectively, and are consistent with the ones forecasted by INGV (13:44) and KOERI (13:48). In this case, the emergency signal was released by the Port Authority of Rethymnon at 17:35. The procedures of emergency at Chania and Rethymnon cities are judged as being extremely late for an effective operational action. On the other hand, however, it is encouraging that the civil protection mechanism was mobilized and responded at the local level even with considerable delay. This is promising for a better performance in the future, given that no relevant action was undertaken by local authorities in past events, e.g., in response to the 1.5 m-high tsunami initiated by the 20 July 2017 earthquake (Mw = 6.6) in the SE Aegean Sea [13,21]. It has been documented that local people in Ierapetra and Arvi approached the beach to observe the sea level changes after the earthquake. Although people gathering happened in no massive scale, one may assume that it would be more massive in other circumstances, which may include (i) a higher level of seismic intensity felt in Crete, if either the earthquake source was closer to the island or the magnitude was higher, (2) earthquake incidence in peak summer season, and (3) the absence of COVID-19 social distancing restrictions. Soon after, however, local authorities in Ierapetra decided to keep people away from the beach, although this did not happen in Arvi.

3.3.2. Israel According to the instructions by the Israeli National Doctrine for Tsunami Preparedness and Response [22,23], in case of an immediate potential tsunami threat, the Tsunami Watch Team (TWT), i.e., the national monitoring center, should approach the Lighthouse Team, which is the senior decision-making team responsible to decide whether or not to alert the country. The release of a national tsunami alert from the Lighthouse Team implies a series of emergency actions along the coast: the evacuation of hundreds of thousands of people who live or work in the area or are enjoying GeoHazards 2020, 1 56 their vacation, the shutdown of Israel’s coastal power plants, setting up an emergency status and the evacuation of seaports, as well as the shutdown and evacuation of industry. Therefore, for Israel, a tsunami alert is a dramatic event, which implies a dramatic decision. Regarding the earthquake of 2 May 2020, the Israeli coastline was at an epicentral distance of ~840 km. The I-type messages issued by the three TSPs were received a few minutes after time to, e.g., the NOA I-message was received at 13:02 UTC (16:02 local time), i.e., 11 min after to. The alert level was Information and, therefore, the operational decision was to continue monitoring the event but to do nothing about the alert. O-messages released by INGV and KOERI did not change the alert level status for Israel. However, the NOA O-message, received by the TWT at 13:27, was a “game changer”, because it put Israel at the Advisory alert level and forecasted that the first potential tsunami wave arriving at 14:33 would hit Israel’s coast near the city of Haifa, a settlement of 284,000 residents and of major seaport and large petrochemical industry. The message arrived 36 min after to, allowing only 1 h and 3 min before the first forecasted tsunami wave arrival. From operational point of view, it is a short time to decide whether to alert or not and to issue an effective early warning. About 43 min later, i.e., 23 min before the potential hit, the final decision of TWT was to “close” the event after taking into account that only a minor wave had actually been recorded. Otherwise, the time interval of only 23 min would be too short for the Lighthouse Team to make such an important decision, in view of the intrinsic uncertainties involved in the tsunami messages and the high probability to produce a false alarm with many negative economic and social consequences. However, if a higher wave amplitude had been recorded, it would certainly have been preferable to transfer the information to the Lighthouse Team, leaving them to make a decision which could save lives. In any case, the time left to Israeli authorities for decision making was certainly longer with respect to the extremely short time available to the Greek authorities for responding effectively to the tsunami alert.

4. Discussion The findings of our examination should be seen under the light of the extremely limited time available for an efficient tsunami alert for both the upstream and downstream components of the Mediterranean tsunami early warning system. Although the initial tsunami alert messages were issued by three TSPs with a difference of 1 to 4 min from each other, such a difference is important from an operational point of view. Differences in the alert levels and in the forecast times are due to the use of different DMs, tsunami modeling tools and earthquake parameters and possibly to different bathymetry databases. The main issue with the Mediterranean Sea warning system, however, remains the very near-field character of the tsunami sources. In the eastern basin, several tsunamis were generated after strong earthquakes in the past. However, the accurate record of the tsunami wave arrival times has not been possible due to the lack of appropriate recording means. Tide-gauge networks were deployed after the establishment of the NEAMTWS. On the other hand, the use of video records for tsunami documentation is increasingly popular. Several local tsunamis were generated after strong earthquakes (Mw > 6) and recorded thanks to several recording means. In all cases, the first tsunami arrivals were noted in ~30 min or less: Crete (Mw = 6.5), 1 July 2009 [12]; Lesvos (Mw = 6.3), 12 June 2017 [24]; Bodrum-Kos (Mw = 6.6), 20 July 2017 [13,21,24]; Zakynthos (Mw = 6.8), 25 October 2018 [18,25]; Crete (Mw = 6.6), 2 May 2020 (present study). Such moderate tsunamigenic earthquakes are quite challenging to test and calibrate the capabilities of the tsunami early warning system [12]. Nevertheless, questionnaire surveys among local populations in Greece and Italy showed that people’s tsunami risk perception is still low [26,27]. The need to minimize the alert time in both the upstream and downstream components of the tsunami early warning system in the Mediterranean is very urgent. As for the downstream component, it is up to Greece’s central and local civil protection authorities to significantly improve the organization of their emergency plans and to find out the optimum technological and operational features for the on-time transmission of early warning messages to local target communities. Such an effort certainly should include and emphasize actions aiming to improve the public’s tsunami awareness. It is GeoHazards 2020, 1 57

disappointing that after a strong submarine earthquake people gather along the seashore to observe the sea level changes, as happened after the 2 May 2020 event. To further examine the possible improvement of the upstream component efficiency, we considered it absolutely realistic to set up at 5 min the target time to issue I-messages of alert. This has been achieved at least once, e.g., by the NOA TSP after the earthquake (Mw = 6.5) in the Ionian Sea on 17 GeoHazardsNovember 2020 2015., 1, x FOR Then, PEER the REVIEW tsunami alert efficiency, F, for the I-message is significantly improved14 of by 17 a factor of 0.22 to 0.30 (Figure9). On the other hand, the improvement of the tsunami alert e fficiency, themainly installation for the of O-message, additionalis recording expected devices by densifying in critical the coastal tide-gauge spots. network For example, with the the installation installation of ofadditional new stations recording in the devicesCrete area in criticalwould coastalimprove spots. the upstream For example, tsunami the installation alert efficiency of new for stations tsunami in sourcesthe Crete like area the would ones improveof 2009 theand upstream 2020. In tsunamispite of alertthe elowfficiency resolution for tsunami of the sources bathymetry like the grid, ones tsunamiof 2009 simulations and 2020. In were spite performed of the low in resolution the area wi ofth the synthetic bathymetry tide-gauge grid, tsunami networks, simulations two of them were installedperformed at the in thenorth area side with of syntheticChrysi islet tide-gauge (Figure 1b) networks, [12]. Τhο twose simulations of them installed indicate at that the norththe time side of of theChrysi first islettsunami (Figure record1b) [could12]. Those be sh simulationsortened by ~3 indicate min. In that that the case, time F ofwould the first improve tsunami by record a factor could of 0.14be shortenedto 0.24 (Figure by ~3 10). min. In that case, F would improve by a factor of 0.14 to 0.24 (Figure 10).

Figure 10. As in Figure9 for the O-messages of INGV (1) and NOA (2), IMPR (3) is the expected Figure 10. As in Figure 9 for the O-messages of INGV (1) and NOA (2), IMPR (3) is the expected improvement if the issue time of O-message is shortened by ~3 min thanks to the hypothetical operation improvement if the issue time of O-message is shortened by ~3 min thanks to the hypothetical of a tide station at Chrysi islet. operation of a tide station at Chrysi islet. Of interest to tsunami service providers are also the operational consequences of the establishment Of interest to tsunami service providers are also the operational consequences of the in the Mediterranean region of a well calibrated tsunami magnitude scale, Mt, since it may provide establishmenta new tool for in thethe tsunamiMediterranean wave heightsregion of expected a well calibrated in near-field tsunami forecasting magnitude points scale, as M soont, since as theit mayearthquake provide magnitude a new tool has for been the tsunami determined. wave Such heights a prospect expected was in already near-field noted forecasting several years points ago [as28 ]. soonHowever, as the earthquake the utilization magnitude of tsunami has been height determ/amplitudeined. Such offshore a prospect near the was source already and noted in real several time yearsconditions ago [28]. becomes However, of crucial the utilization importance. of tsunami The reason height/amplitude is that one may offshore take into near account the source the need and for innoise real time reduction conditions as well becomes as the dynamic of crucial eff importance.ects of the fault The on reason the sea is surfacethat one and may bottom take into [18 ].account the needAt thefor currentnoise reduction stage of developmentas well as the of dyna the Mediterraneanmic effects of tsunamithe fault warning on the sea system, surface no waveand bottomamplitude [18]. is forecasted for each of the forecast points. As an alternative, the alert messages indicate theAt threat the levelcurrent at eachstage point, of development and this characterization of the Mediterranean is connected tsunami with warning the wave system, amplitude no wave range, amplitudeas shown is in forecasted Table1. In for the each tsunami of the alert forecast messages poin issuedts. As an on alternative, 2 May 2020, the the alert threat messages levels in indicate general theare threat consistent level at with each the point, observations, and this characterization although for a fewis connected forecast pointswith the the waveWatch amplitudecharacterization range, asoverestimated shown in Table the 1. observations,In the tsunami given alert messages that no wave issued exceeding on 2 May 0.5 2020, m inthe amplitude threat levels was in observed. general areThe consistent criteria to with determine the observations, the point of al timethough to end for the a few tsunami forecast alert points procedure the Watch is still characterization under discussion overestimatedwithin the ICG the/NEAMTWS. observations, However, given that practically no wave speaking, exceeding the 0.5 TSPs m in usually amplitude allow was enough observed. time to Theensure criteria that to the determine tsunami threat the point has gone of time before to theyend issuethe tsunami E-messages. alert Thisprocedure was also is thestill case under on 2 discussionMay 2020. within the ICG/NEAMTWS. However, practically speaking, the TSPs usually allow enough time to ensure that the tsunami threat has gone before they issue E-messages. This was also the case on 2 May 2020.

5. Conclusions The Mediterranean Sea basin is characterized by very near-field tsunamis, with the shortest wave travel times being, as a rule, of less than 30 min. After the strong earthquake (Mw = 6.6) on 2 May 2020 in Crete, Greece, a small, non-damaging tsunami was macroscopically observed in the town of Ierapetra and in nearby villages on the southeastern coast of Crete. The wave was also recorded by tide-gauge stations in Ierapetra at SE Crete (epicentral distance Δ~90 km), in Kasos

GeoHazards 2020, 1 58

5. Conclusions The Mediterranean Sea basin is characterized by very near-field tsunamis, with the shortest wave travel times being, as a rule, of less than 30 min. After the strong earthquake (Mw = 6.6) on 2 May 2020 in Crete, Greece, a small, non-damaging tsunami was macroscopically observed in the town of Ierapetra and in nearby villages on the southeastern coast of Crete. The wave was also recorded by tide-gauge stations in Ierapetra at SE Crete (epicentral distance ∆~90 km), in Kasos (∆~180 km), SE Aegean Sea, and in Alexandria (∆~485 km), Egypt. The largest wave amplitude of 0.16 m a.m.s.l. was recorded at Ierapetra station. We estimated that the tsunami was of intensity 4 in the 12-grade tsunami intensity scale [20,29] and of magnitude 6.87 in the Mt scale [19], which is close to the seismic moment magnitude Mw = 6.6. An in-depth analysis of the tsunami early warning issued by three TSP monitoring centers that support the upstream component of the warning system in the eastern Mediterranean showed that, at the current stage of development, the system does not provide timely and efficient alerts. The earthquake origin time was 12:51 UTC, while the first alert was issued at 12:59 and the first wave arrival was recorded at 13:07. The tsunami alert efficiency, F, i.e., the percentage of the remaining forecast time after the issue time of the initial alert message, was less than 0.6, or even less than 0.3 for several forecast points. The situation is worst as regards the issue time of the first ongoing alert message (13:21), since F becomes negative for several forecast points. The reason is that the message was issued after the forecast time had expired for those points. Based on previous tsunami wave simulations in the same area [12], we judged that the tsunami alert efficiency could be improved if the issue time of alert messages is shortened by a few minutes, which is an absolutely realistic goal. Inconsistencies between the initial earthquake parameters adopted by the three TSPs caused important operational consequences. We showed that the initial earthquake parameters adopted are of crucial operational importance and that the adoption of revised parameters in an ongoing message is seismologically acceptable but creates serious operational problems. Therefore, TSPs should drastically improve the accuracy of the initial magnitude and location of the earthquake event, to avoid revising ongoing messages within a very short time. In the downstream component of the warning system, the response of the national and local Greek civil protection authorities was too slow and practically rendered useless from an operational point of view. However, even the delayed response is encouraging, since in previous events no response was noted at the local level. The authorities should drastically improve their response system and intensify tsunami awareness programs for the general public. In Israel, the authorities had a bit more time to decide that there was no need to set up a tsunami emergency for the coastal zone of the country. In any case, the event was a challenging test of the Mediterranean tsunami warning system in real conditions.

Author Contributions: Conceptualization and methodology, G.A.P.; software, G.A.P., K.-N.K.; validation, K.-N.K. and E.R.; resources and data curation, G.A.P., K.-N.K., E.R., A.Y.; writing—original draft preparation, G.A.P., A.Y.; writing—review and editing, E.L., K.-N.K., E.R., A.Y.; supervision, G.A.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The tsunami tide-gauge records at stations NOA-03 and NOA-04 (Greece) and NIOF-IDSL-23 (Egypt) were collected from the database of the Space, Security and Migration Directorate-JRC Ispra, EU (https://webcritech.jrc.ec.europa.eu/SeaLevelsDb/Home). Thanks are due to Sergiu Dov Rosen, Haifa, Israel, for a useful discussion regarding the tsunami records. The recommendations of three anonymous reviewers aiming to improve the initial paper submission were greatly appreciated. Conflicts of Interest: The authors declare no conflicts of interest.

Appendix A Video shootings showing the 2 May 2020 tsunami at Ierapetra coast.

https://www.facebook.com/maria.roumeliotakiskyvalou/videos/pcb.2799497790335638/ • 2799492307002853/?type=3&theater; GeoHazards 2020, 1 59

https://www.facebook.com/maria.roumeliotakiskyvalou/videos/pcb.2799497790335638/ • 2799492307002853/?type=3&theater; https://www.facebook.com/maria.roumeliotakiskyvalou/videos/pcb.2799497790335638/ • 2799496940335723/?type=3&theater. Video shootings documenting the 2 May 2020 tsunami at Arvi coast. https://www.youtube.com/watch?v=9Qobru4$\times$4WQ&feature=youtu.be; • https://www.facebook.com/100001618268477/videos/3088854047845174/; • https://www.youtube.com/watch?v=ePnn4g64LeM&feature=youtu.be; • https://www.youtube.com/watch?time_continue=1&v=0JrdCBKssn8&feature=emb_logo. • References

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