Νear-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 (Figure 1
a). 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) (Figure 1
a). 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 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
]. The most active geodynamic structure in the Mediterranean is the arc and trench system of the HSZ, which produces mega thrust tsunamigenic earthquakes [5
]. Several pre-historical and historical tsunamis have been reported in the Mediterranean region [3
]. 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 (Figure 1
). 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 south Aegean Sea, Greece, on 9 July 1956 [3
]. 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
]. 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
]. 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-Kos 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 (Figure 1
), 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.
2. Materials and Methods
2.1. Method Overview
We briefly introduce the current architecture of the NEAMTWS as well as the operational upstream and downstream components of the system and the responsibilities of the TSPs. The large earthquake of 2 May 2020 and its associated tsunami are described on the basis of instrumental records and macroscopic observations, while the tsunami magnitude, Mt, is calculated. Based on the tsunami forecasts contained in the alert messages issued by three TSPs after the earthquake, we examine in depth the tsunami alert efficiency for the first time in the NEAMTWS history. Our analysis included both the upstream alert component and the downstream performance of the tsunami response by the authorities in Greece and Israel. This evaluation has been made in light of the extremely narrow early warning time imposed by the near-field nature of the tsunami source. All times referred to are in UTC unless otherwise noted.
2.2. NEAMTWS Operational Architecture
The operational architecture of NEAMTWS is currently based on five TSPs, which are national centers charged with seismic and tsunami monitoring in France (CENALT, Centre d’Alerte aux Tsunamis, Paris, France), Italy (INGV, Istituto Nazionale di Geofisica e Vulcanologia), Greece (NOA, National Observatory of Athens), Portugal (IPMA, Instituto Português do Mar e da Atmosfera) and Turkey (KOERI, Kandilli Observatory and Earthquake Research Institute). The national centers CENALT, INGV, NOA and KOERI obtained international accreditation within the frame of IOC/UNESCO in September 2016 while IPMA’s was obtained in December 2019. No responsibility areas have been endorsed by the ICG/NEAMTWS but each TSP has its own competence monitoring area. A country member subscribes to receive tsunami 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 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 Table 1
. 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
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.
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 and number of the message issued, the focal parameters of the earthquake, the threat level and the respective area threatened, as well as the forecasted wave arrival times at prescribed forecast points.
The tsunami alert messages issued by a TSP are transmitted via e-mail, fax machines and GTS (Global Telecommunication System) to subscribed national CPAs of the IOC country members, to other TSPs, to IOC/UNESCO, as well as to the ERCC (Emergency Response Coordination Center) of the European Commission in Brussels (Figure 2
2.3. The 2020 Earthquake and Tsunami Event
On 2 May 2020, at 12:51:06.5 UTC, a shallow strong earthquake ruptured offshore to the south of Crete, at the central segment of the HSZ (Figure 1
). The initial magnitude of Mw
= 6.6 and hypocenter (25.75° E/34.27° N, h = 10 km) determined by the GEOFON network of the Deutsches GeoForschungsZentrum (GFZ) (https://geofon.gfz-potsdam.de/
) have been considered as reference parameters for our analysis, since they are consistent with the determinations of other centers, e.g., USGS (https://earthquake.usgs.gov/earthquakes/search/
). The 2 May 2020 earthquake was perceptible in various places of Crete and beyond, but no damage was reported.
Soon after the earthquake, local authorities and eyewitnesses reported a tsunami wave observed along the coast of the southeastern side of Crete. The tsunami has been documented in a series of pictures and video shootings taken by local authority persons, by the press as well as by amateurs at coastal spots of the Ierapetra town, at the nearby villages of Arvi and Kastri as well as at the northern side of Chrysi islet (Figure 1
b, Figure 3
, Figure 4
). From eyewitness accounts it results that the tsunami of 2 May 2020 was similar to the one reported after the earthquake 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 1
]. 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.
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 Δ
~90 km and Δ
~180 km, respectively (Figure 1
b). 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 (Figure 5
). 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 (Figure 6
), while the wave period was found at T
~8 min. In Alexandria tide-gauge station IDSL-23, situated at Δ
~485 km to the southeast of the epicenter (Figure 1
) 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 (Figure 7
). The inspection of the tide-gauge record at the station Zygi Marina, Cyprus, situated at Δ
~670 km to the east (Figure 1
), showed no clear tsunami signal.
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.
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
]; Zakynthos (Mw
= 6.8), 25 October 2018 [18
]; 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
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 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 November 2015. Then, the tsunami alert efficiency, F
, for the I-message is significantly improved by a factor of 0.22 to 0.30 (Figure 9
). On the other hand, the improvement of the tsunami alert efficiency, mainly for the O-message, is expected by densifying the tide-gauge network with the installation of additional recording devices in critical coastal spots. For example, the installation of new stations in the Crete area would improve the upstream tsunami alert efficiency for tsunami sources like the ones of 2009 and 2020. In spite of the low resolution of the bathymetry grid, tsunami simulations were performed in the area with synthetic tide-gauge networks, two of them installed at the north side of Chrysi islet (Figure 1
]. Τhοse simulations indicate that the time of the first tsunami record could be shortened by ~3 min. In that case, F
would improve by a factor of 0.14 to 0.24 (Figure 10
Of interest to tsunami service providers are also the operational consequences of the establishment in the Mediterranean region of a well calibrated tsunami magnitude scale, Mt
, since it may provide a new tool for the tsunami wave heights expected in near-field forecasting points as soon as the earthquake magnitude has been determined. Such a prospect was already noted several years ago [28
]. However, the utilization of tsunami height/amplitude offshore near the source and in real time conditions becomes of crucial importance. The reason is that one may take into account the need for noise reduction as well as the dynamic effects of the fault on the sea surface and bottom [18
At the current stage of development of the Mediterranean tsunami warning system, no wave amplitude is forecasted for each of the forecast points. As an alternative, the alert messages indicate the threat level at each point, and this characterization is connected with the wave amplitude range, as shown in Table 1
. In the tsunami alert messages issued on 2 May 2020, the threat levels in general are consistent with the observations, although for a few forecast points the Watch
characterization overestimated the observations, given that no wave exceeding 0.5 m in amplitude was observed. The criteria to determine the point of time to end the tsunami alert procedure is still under discussion 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.
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 (Δ
~180 km), SE Aegean Sea, and in Alexandria (Δ
~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
] and of magnitude 6.87 in the Mt
], which is close to the seismic moment magnitude Mw
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.