Keywords

1 Introduction

Historical seismicity is seismicity for which evidence can be found in the historical record of humankind. For pre-instrumental earthquakes, however, this evidence is preeminent. Their existence, interpretation and use are of major interest to seismic hazard analysis, and are playing a key role in defining and litigating the seismic design of many critical facilities in last time. The historical data, consisting of written evidence of the effects of earthquakes on humans, their civilization and the environment, has been used to determine earthquake location and size, the strength of ground motions and its frequency of occurrence in most parts of the world.

Historical records differ greatly in the length, completeness and quality of the earthquake histories they portray. As we go back in time, the minimum size earthquake mentioned in any given history will usually increase. Typically, only those events which led to disastrous consequences are mentioned in ancient records. Additional information about these events would have been of great benefit to present-day seismologists and engineers. Of practical interest is the estimate of the time duration of the earthquake. These details provide the analyst with much information that allows an improved estimation of seismic hazard from future great earthquake in the area.

Seismic activity in Romania is confined to the crust, except the Vrancea zone, where earthquakes with focal depth down to 200 km occur. Figure 1 displays the spatial distribution of earthquake epicentres in Romania and surrounding areas considering also magnitudes and focal depths. The statistical analysis of the earthquake catalogue compiled in the frame of the BIGSEES Project (2017) gives at a mean return interval of 100 years a magnitude Mw 7.5 for Vrancea intermediate-depth seismogenic zone and much lower magnitudes for crustal seismogenic zones: Mw 5.7 for Banat and Danubian zones (located in South-West of Romania) and Mw 5.6 for a very active zone in Central Romania (Fagaras-Campulung).

Fig. 1
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Seismicity and seimogenic zones contributing to seismic hazard of Romania. Datasource BIGSEES Project 2017

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The strongest event in the documented history and originating in Vrancea area occured on October 26, 1802, having a 7.9 magnitude (Mw) and a depth of 150 km, according to ROMPLUS catalogue (NIEP 2017). It was felt over a large area in Europe: in the Balkans up to Constantinopolis (Istanbul), up to Moscow and Sankt-Petersburg in the North (North-East) and to Mediterranean Sea in the South-West, as stated by Rogozea et al. (2016) and references therein. Actually, a peculiarity of earthquakes occurring at intermediate depths (70–200 km) in Vrancea zone is the quasi-elliptical shape of the macroseismic field with the major axe oriented along NE–SW direction; events with magnitudes higher then 6 can be felt in the whole Balkan area up to Turkey (in the South) and to Poland (in the North).

During the last century in Vrancea seismogenic zone 4 strong earthquakes (Mw > 7) occured at intermediate depths: October 6, 1908 (Mw 7.1) at 125 km depth, November 10, 1940 (Mw 7.7) at 150 km depth, March 4, 1977 (Mw 7.4) at 94 km and August 31, 1986 (Mw 7.1) at 131 km. In spite of being the most destructive event in the previous century, the 1977 earthquake had numerous precedents in Romania. The historical record here extends to Roman times and includes similar intermediate—depth earthquakes with magnitudes of 7 or greater about 2–3 times per century. Over the past several centuries, earthquakes with likely magnitudes of 7 or greater caused damage in Bucharest in 1681, 1738, 1802, 1829, 1838 and more recently in 1940, 1977.

A major part of the information for determining the design basis earthquakes to important structure like nuclear power plants, bridges etc. is a complete set of historical earthquake data. Therefore, it was necessary that the available historical records be collected, extending as far back in time as possible. Most of these historical records will naturally be of descriptive nature, including such information as the number of houses damaged or destroyed, the behaviour of population etc.; from such information a measure of the intensity scale value of each earthquake in modern macroseismic intensity scale values may be determined.

2 New Intensity Data by Using Complementary Data from Churches and Monasteries

In estimating intensity scale values for historical earthquakes, account should be taken of the fact that information obtained from old chronicles, from churches, monasteries and individuals who may have experienced the event shows a tendency to exaggerate the importance of damage and other phenomena associated with earthquakes There are many data on the historical earthquakes. Many writers and journalists along time made many descriptions on earthquake effects on churches, monasteries etc. Authors collected, centralised and analysed also information from literature. For example, many descriptions of damages and macroseismic effects induced by the earthquake of 1802 (MW = 7.9, h = 150 km) are from Mihail Sadoveanu. In the novel “Jderi Brothers” he stated that in many places from Bucharest around Bucharest Cathedral the earth unbind, went out oil, water and sand and many large slits with transversal dimensions between 50 and 150 cm occurred. Considering the effects on the natural environment and taking into account the characteristics of the monasteries (building material, size of constructive elements etc.) values of macroseismic intensity (intensity data points, IDP) were associated to the recorded damages. Some examples are discussed below.

After the strong earthquake of 1620 (Mw = 7.5; h = 150 km), Vasile Lupu, prince of Moldova, started Putna (MO/1) monastery rehabilitation, in fact its recovery. The restoration of the Church at Putna Monastery was completed in the year 1662. The large earthquake of 1738 (MW = 7.7; h = 130 km) seriously damaged the church, the walls of the enclosure and towers of the four corners.

Stavropoleos Monastery (MO/76) (44.431789 latitude; 26.098869 longitude; altitude = 79 m) in downtown Bucharest (the word “stavropoleos”, name in Greek translates as “cross town”) was build up in 1724. Another period in the building of the Stavropoleos church (1730–1733) extended the altar, built the side apses in stone columns and carved porch. The monastery was strongly affected by the earthquake 1802 (MW = 7.9; h = 150 km). The effects were devastating. Another earthquake took place on January 23, 1838. French consul in Bucharest, noted that “disasters caused in this city of this event are huge and cannot be evaluated”. The structure of the church and inn was so severely affected that on November 15, 1840, the Department of Internal Affairs is forced to submit an address in which require demolition of the tower Stavropoleos. In 1940, when the church was restored after an absence of decades, the monastery was less affected by the 7.7 magnitude event.

Bogdana Monastery (MO/104) (47.842480 latitude; 25.917498 longitude; altitude = 372 m) is located in the heart of historic Bukovina, in Rădăuţi, on one of the roads leading to Putna Monastery. Here in Rădăuţi, Dragoş Vodă has established one of the royal residences and founder Bogdan I, the first voivode of Moldavia is buried here in wooden church in 1367. Moldova have information to historian Ioan Neculce in 31 mai/June 11, 1738 (MW = 7.7; h = 110 km) … have made a great earthquake, the Golia monastery from Iaşi fell and many houses and many other monasteries in Focşani; disaster followed by a “great plague in Iasi”, at Bacău overturned Răchitoasa Monastery (Corfus I., (1967)—“Notes old”, Iaşi, Junimea), Solomon church was destroyed, monastery Bogdana (Rădăuţi) collapsed; also in some places the ground cracked and “went out water and sand”.

Comana Monastery, located in the homonymous village on the meadow of Neajlov river, is mentioned in a charter issued by the chancellery of Vlad Ţepeş on September 27, 1461. In 1588, Radu Şerban, the local nobleman, built a new monastery on the old foundation. Between 1699 and 1701, governor Şerban Cantacuzino started the works of restoration of the monastery. In August 1709, hard times came for Comana monastery. The state of the monastery is sporadically recorded in some writings: “…because of the earthquakes, like that of 1701 (Mw = 7.1; h = 150 km), and because of lack of care, the monastery is in danger of ruining” (Ghenadie Enăceanu notes in quoted work Catagraphy from 1834, quoted by Maria Valeria Picu). This reflects a sad reality, as at Comana Monastery there were: “a chapel in the wall of monastery and the four towers totally broken down, the walls of the surrounding courtyard were totally broken and cracked” (Archives of the state Bucharest. Ministry of Public Instruction Fund, Wallachia, File 4/1828, f.3). In 1854, after the earthquake of 1838 (Mw = 7.5), “demolition and rebuilding” of the monastery is done, as Cezar Bolliac says. The earthquake of October 26, 1802 (Mw = 7.9; h = 150 km) caused great damages to the whole construction, and in 1854 a new restoration began, conducted by architect J. Schlatter, after the earthquake of 1838 (Mw = 7.5). The church of monastery was retrofitted after the earthquakes of 1977 (Mw = 7.4; h = 94 km) and 1986 (Mw = 7.1; h = 131.4 km).

3 Maps with Maxim Macroseismic Intensities Observed Along Time in Romania

Newly collected data regarding seismic damages of 115 monasteries has been synthesized and analyzed through the engineering point of view, considering materials and dimensions of the buildings, towers etc. Some of the resulting intensities presented in Table 1 are completing the known macroseismic field of each event, and data may be used to construct the isoseismic map of the maximum possible Vrancea earthquake. Figures 2, 3, 4, 5, 6 and 7 show the intensities evaluated from the analysis of the damage records at the specific sites.

Table 1 Macroseismic intensities values (MSK64 scale) evaluated using historical data from churches and monasteries
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Fig. 2
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IDP’s evaluated for Vrancea earthquake of June 12, 1701: MW = 7.1 h = 150 km

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Fig. 3
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IDP’s evaluated for Vrancea earthquake on June 11, 1738, MW = 7.7

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Fig. 4
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New IDPs for Vrancea earthquake on October 26, 1802. MW = 7.9

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Fig. 5
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IDP evaluated for Vrancea earthquake on January 23, 1838. MW = 7.5

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Fig. 6
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IDP’s evaluated Vrancea earthquake on November 10, 1940. MW = 7.7

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Fig. 7
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Re-evaluated IDPs from Vrancea earthquake on March 4, 1977. MW = 7.4

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The estimated intensity values for the 1738, 1802 and 1940 events are consistent with map of the macroseismic intensities expected in the case of a Vrancea “maximum credible” earthquake (Marmureanu et al. 2011) where the shape of isoseismal surfaces are elongated much more towards NE than to the SW. The sites with maximum intensities observed in the last century (IX and X MSK) are shifted towards East with respect of the epicentre area, inside or at the edges of the Focsani depression (a foredeep basin formed in front of the East Carpathians, where sediments are reaching even 14 km depth).

In Fig. 8 is presented the observed macroseismic intensity map of Romania by using newly compiled information about the damages experienced by 115 churches and monasteries after strong intermediate-depth earthquakes (Mw ≥ 6.9) occurred in 1471, 1516, 1590, 1620, 1701, 1738, 1802, 1838, 1940, 1977, 1986 and 1990. The new information is compiled together with relevant macroseismic data published by Constantin et al. (2011), Kronrod et al. (2013), Marmureanu (2016) and Rogozea et al. (2014, 2016). For each seismic event were computed the mean values of the intensity data points available within the administrative units, as defined nowadays. For the map presented in Fig. 8 only the maximum value among the earthquake means is reported in each land administrative unit.

Fig. 8
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Maximum observed macroseismic intensities due to Vrancea intermediate-depth earthquakes. New intensity data points using complementary data from churches and monasteries where averaged within each land administrative unit for each earthquake considered in this study

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Many attempts have been made to correlate the observed intensity with recorded ground motion parameters, as well as the hazard assessment for future events, which require a regression relationship between intensity and strong-ground-motion parameters. Enescu et al. (2001) elaborated an estimation method of the seismic hazard generated by strong and deep Vrancea earthquakes considering only the strong earthquakes produced in the area after 1940 (Table 2). It was observed that all epicentres are situated on a line oriented NE–SW and the corresponding maximum macroseimic intensities I0 occur on a line which is parallel to the epicentre’s line at distance of about 23 km.

Table 2 Strong Vrancea earthquakes (MGR > 6.5) after 1935
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4 Concluding Remarks

Strong, intermediate-depth (70–200 km) earthquakes in the Vrancea area (Romania) occur irregularly, but not infrequently. They have caused a high toll of casualties and extensive damage over the last several centuries. The three strongest earthquakes (M > 7) that occurred in Vrancea during the last century (1940, MW = 7.7 and h = 150 km; 1977, MW = 7.4 and h = 94 km; 1986, MW = 7.1 and h = 131.4 km) caused large damage over a wide European territory, including distant, long period elements of the built environment. The quake of March 4, 1977, MW = 7.4 caused significant damage in Romania & Bulgaria and was felt up to Central Europe.

This study is of interest to seismologists and other earth scientists who want to gain an understanding of how historical information is used in estimating earthquake hazard along time of centuries in Romania, engineers who want to know more about the seismic input to be used in design and risk calculations, decision makers and other public officials who want to make decisions, particularly that related to the seating and evaluation of nuclear plants etc. Typically, only those events which led to disastrous consequences are mentioned in historical records, however it is not to be neglected that smaller earthquakes can lead to a more insightful seismic hazard evaluation.

In estimating intensity scale values for historical earthquakes, account should be taken of the fact that information obtained from old chronicles, from churches, monasteries and individuals who may have experienced the event shows a tendency to exaggerate along time the importance of damage and other phenomena associated with earthquakes.

The results on seismic hazard obtained in BIGSEES Project provide a very solid background for the upcoming update of the EN 1998-1 National Annex.