Evaluation of earthquake hazard for the city of Tiberias

M. Francesca Ferrario1, Rivka Amit2, Oded Katz2

1. University of Insubria, Como, Italy 2. Geological Survey of Israel, Jerusalem, Israel

Prepared for the Steering Committee for Earthquake Readiness in Israel

December, 2014

Ministry of National infrastructures Energy and Water Resources Geological Survey of Israel

1. Introduction

The Dead Sea Transform (DST) is the tectonic boundary between the Arabia and Sinai (Africa) plates,

extending from the Red Sea in the south to the Eurasia Plate in the north. Based on geological observations

and kinematic considerations it is generally accepted that the DST is a continental transform, along which

the principal movement has been a post-Miocene 105 km left-lateral displacement (e.g., Bartov et al, 1980;

Garfunkel, 1981; Joffe & Garfunkel 1987; Smit et al., 2010). The DST is segmented to a few tectonically

active northwards extended major segments, each tens to hundreds kilometres long (Garfunkel, 1981). One

of them is the Jordan Valley Fault (JVF), extending some 110 km between the northern coast of the Dead

Sea and the eastern coast of Sea of Galilee (SOG), with inferred average slip rate of about 5mm/y (Ferry et

al., 2007).

Bounded by the JVF in the east and the Western Marginal Fault in the west (Fig. 1) (Hurwitz et al., 2002),

the SOG is part of a N-S extended pull-apart basin, one of a few located along the Dead Sea Transform

(Garfunkel, 1981; Ben Avraham et al., 1996). Several historical large destructive (M > 6.5) earthquakes are

apparently the result of the JVF rupture (Ben Menahem, 1991; Amiran et al., 1994; Guidoboni et al., 1994;

Guidoboni and Comastri, 2005), attesting for its intense tectonic activity. Likewise, the large destructive

earthquake of 749 AD is attributed to this segment with reported heavy damage to the cities of Tiberias

and Susita (Marco et al., 2003; Yagoda-Biran and Hatzor, 2012) and to the village of Umm El Qanatir

(Wechsler et al., 2009), all situated within the SOG vicinity, at a radial distance of less than 15 km from the

JVF. Moreover, the earthquakes of 343 AD and 1033 AD are also attributed to the JVF segment (Begin,

2005; Ben Menahem, 1991). Recent seismicity along the northern extension of the JVF is instrumentally

recorded (Ven Eck and Hofstetter, 1990), including the 1973, M=4.5 earthquake, that occurred at the

southern shore of SOG (Arieh et al., 1977).

However, the long term prehistorical record of earthquake behavior including determination of

recurrence time intervals of large events is yet missing in this highly populated area of northern Israel.

Driven by this, and by the fact that there is a high number of faults and lineaments displacing Quaternary

sedimentary sequences, and large landslides are evident along the SOG eastern and western margins, we

study the western side of the Sea of Galilee to follow the upper-Pleistocene - Holocene seismic history of

this area. The current study continues previous studies at the western and eastern coasts of the Sea of

Galilee and the vicinity of Tiberias city such as those of Marco et al. (2003), Zion et al. (2004), Salamon et al.

(2009), Katz et al. (2010), Yagoda-Biran et al. (2010). Here we integrate paleoseismic data of Quaternary

fault segment rupturing the surface with slope stability analyses that spatially coincide with the faults. It

includes evaluation of magnitudes and recurrence times of the large earthquakes and determination of

historical peak ground acceleration that is related to the large earthquakes. The results of this study can be

used to enhance seismic hazard estimation, to calibrate, and to update seismic building codes for the SOG

area.

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The study site is located on the west side of the Sea of Galilee, close to Berniki archaeological site (Fig.

1), where dislocations and tilting are present on the theatre walls (Fig. 2), possibly related to palaeo-

earthquakes. Here, the main normal fault of the western side of the SOG (WMF, Western Marginal Fault) is

located at about 200-300 m west of the coastline; we infer the presence of a secondary fault NNE-SSW

directed and crossing the Berniki site. A paleoseismic trench was dug on the presumed fault trace south of

the Berniki archaeological site.

The theatre of the Berniki archaeological site was built together with the city in 1st century CE and is

located west of Cardo, adjacent to the city centre on the side of Mt. Berenice. There are three blocks of

seats, totalling 6000 places. The lower section of seats was built on the natural slope, while the middle and

the upper ones on barrel vaults. The theatre was rebuilt in the mid 2nd-century CE after the eastern part

sank and was destroyed. The structures were used until the late Byzantine period (6th century CE) and then

abandoned. Following the 749 earthquake, the theatre filled with eroded soil. In the Abbasid period (9th –

10th centuries CE), living quarters were built on the remains and continued to serve as residences until the

city was abandoned in the Fatimid period (late 11th century CE). City abandonment was caused also by two

earthquakes that strike the city in 1033 and 1066; a small number of citizens resettled in the northern part

of the city. The settlement was conquered by Crusaders in 1099 CE and then controlled by Muslim since

1187 CE until the British Mandate period.

Between Berniki and the trench site, another archaeological site is present: here, Roman and Medieval

findings were found buried below some meters of alluvial sediments. Excavations allowed to uncover city

walls and a drainage channel and to reconstruct part of their history: in the Roman period, a bridge on a

barrel vault was constructed opposite the city gate; the pavement elevation was the same as the gate. Both

the bridge and the vault collapsed, probably during the earthquake of 749 CE, and were then rebuilt. As silt

had blocked the streambed, the new bridge was built on a higher level in respect of the gate’s pavement. In

the Abbasid period, houses were built close to the streambed, which narrowed its course. Support walls

were constructed alongside the streambed to regulate the water flow. The gate and the bridge were in use

until the Fatimid period (end of 11th century). Lack of maintenance caused the streambed to fill up with silt.

Alluvial sequences settled during flood events are visible at the site; any earthquake-related feature is

visible, nor on buildings, nor in deposits.

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Figure 1: Location of the study site, on the western coast of the Sea of Galilee,

close to an archaeological theater (Berniki).

3

Figure 2: Berniki archaeological site; details on deformations and tilting on the walls, possibly earthquake-induced, are shown.

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2. Methods

We used plaeoseismological analysis by trenching the faults detected and identified in the field and in

the seismic lines, as well as archeoseismological analysis to study the historical earthquakes occurred in the

studied area.

3. Results

3.1 Paleoseismic Trench

A paleoseismic trench was dug south of the Berniki archaeological site (city gate) based on a high

resolution seismic line (Medvedev, 2008) and a field reconnaissance. It is ca. 2.5 m high, 15 m long and 1 m

wide; the direction is ca. N70°. The trench does not show any significant evidence of faulting; some

deformations are visible, whose trend is similar to the topography. Several archaeological remains were

found (jar fragments, walls). A cavity with human bones was found at the bottom of the trench; however, it

doesn’t seem a primary burial place, but rather a possible drainage structure later used as a burial site.

Findings are dated approximatively at the Islamic period (11th century CE).

Sediments uncovered by the trench are presumably ascribable to a quite recent time interval, on the

order of 1000 years. Therefore, it is not possible to find, if present, surface faulting related to the main

historical earthquakes (749 CE, 1033 CE, 1066 CE); the estimated magnitude for the strongest earthquake

(749 CE) is of ca. 7.0 (Marco et al., 2003), whereas the most recent events were weaker. The threshold

magnitude that can be revealed by means of palaeoseismological investigations is in the order of 6.5,

because these earthquakes are able to cause appreciable effects on the environment, such as ground

ruptures, liquefactions and slope movements (cfr. ESI scale; McCalpin, 2009; Michetti et al., 2005; Michetti

et al., 2007). If the fault trace run below the area investigated by the trench, the observed deformations

can be the most surficial ones and can be seismically-induced; however, obtained results are not

compelling and other trigger mechanism cannot be excluded.

The stratigraphic units were mapped using a datum that was positioned on the southern wall of the

trench and were further described. A photomosaic composition allowed to have a single image covering all

the trench wall (Fig. 3); a manual log in scale 1:50 has been drawn (Fig. 4).

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Figure 3: photomosaic of the trench southern wall.

Figure 4: preliminary log of the trench southern wall. The description of the stratigraphic units is given below in the text.

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Stratigraphic units are described based on the trench southern wall:

UNIT 1

Brownish silt with some clay and small gravels; gravels up to 7 cm, both angular and rounded, consist of

ca. 10-20% of the unit. The unit is relatively soft and no sorting or stratification is evident; locally, few sub-

horizontal strata are present, but cannot be followed throughout the unit, nor in vertical nor in horizontal

direction. Presence of archaeological fragments (mainly pottery), and bones max 3 cm long and always

broken. Few biological shells dispersed into the unit. Presence of decorated glass.

The unit constitutes the base of the trench from 0 to 7.5 m; walls and archaeological structures are

excavated in this unit. The upper part is enriched in pebbles and marks the boundary with overlying unit 1b.

UNIT 1b

Alternation of 2 to 10 cm thick strata of small gravels (generally 0.5 – 1 cm, max 5 cm) and clayey silt

containing also small gravels. Color varies from dark to light brown. Gravels polygenic, mainly rounded and

horizontally deposited. Harder than unit 1; presence of small pottery fragments (max 2 cm), bones and

shells (grouped in small lenses up to 1 cm thick).

At 11 m, the base of unit 1b is marked by big boulders (up to 40 cm), but possibly related to an

underlying archaeological structure. The upper limit is sharp with unit 2.

Layers generally dip towards the lake (to the E), except at 13-14 m, where a counter-tilting is observed.

The unit is clearly deposited by flowing water with a certain degree of energy; alternations of smaller and

bigger material can be attributed to higher-lower amount of energy.

Deposition environment: fluvial (floods), but probably quite distant from the main channel (the one

crossing Berko archaeological park?).

UNIT 2

Dark brown to reddish silty clay, rare gravels up to 5 cm, almost all basalts. Harder than the other units,

max thickness of 25-30 cm. At 11-12 m presence of charcoals. The unit is not continuous along the entire

wall but it’s separated in 4-5 parts, due to the presence of archaeological structures or “ditches” filled with

unit 3 (at 7 and 10 m). After 12 m the unit is not present.

Presence of pedons, low energy deposition environment, columnar setting. Lower limit sharp, upper

limit erosional.

The base of Unit 2 always dips toward the lake (E), with an inclination almost constant around 12-14°, as

reported on the table below; lower values are recorded close to the termination of the Unit.

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Table 1: Inclination of the base of Unit 2.

Meter 6.50 7 7.50 8 8.50 9 11 11.50

Inclination (°) 12 12 19 15 10 7 13 6

UNIT 3

Light brown to grey silty fine sand with gravels and boulders up to 30 cm long (ca 30-40%). Polygenic

gravels, both sub-angular and sub-rounded. The unit is rich in archaeological remains, bones up to 10 cm

thick, building rocks and roots. Soft to loose consistence. Any bedding nor stratification. The unit lies in

unconformity above Unit 2 or Unit 1b (if Unit 2 is not present). Lower limit erosional; the unit fills some

ditches and locally truncates strata of unit 1b (clearly visible on the S wall at 13-14 m).

Ruins of archaeological structures and buildings.

UNIT 4

Brown, loose, sandy silt with angular gravels up to 10 cm. Thickness of 20-40 cm, almost constant.

Colluvium, weathered but not enough to develop a soil.

ARCHAEOLOGICAL STRUCTURE

Several archaeological structures has been found (walls, pipes, graves). A tentative age of 11th century is

assigned to the structure visible between 7,5 and 10 meters. Some anomalies in the stratification are

clearly related to these structures (e.g., disappearance of Unit 2 at ca. 3-5 m).

DEFORMATION FEATURES

Some minor deformation structures were observed and mapped, mainly back-tilting (Unit 1b) and joints.

Layers generally dip to the east with an inclination of few degrees; between 12 and 14 meters a clear

change in this trend is visible on both the trench walls (see Table 2). Values up to 9° to the west are

recorded. In the southern wall, the axis is located at about 13 m, whereas in the northern wall it is at about

12-12,5 meters.

Table 2: Measurements of the inclination of some layers between 12 and 14 meters (> 0°: dip to the E; < 0°: dip to the W).

Meter Inclination (°) – S wall Inclination (°) – N wall

12 6; 8; 7; 7; 10; 5 6; 7; 9; 3

12.5 4; 7; 4; 4; 5 1; 4; 2; 7

13 0; 0; -6; 0 1; -1; -2; 0

13.5 0; -3; -4; 0 -6; -6; -4; -9

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Even if it is not possible to rule out that the deformations are due to the archaeological structures, this

hypothesis is not the most convincing. Different mechanisms for explaining the observed deformations are

suggested (Fig. 5); they are related to the activity of the main fault or to secondary structures.

Figure 5: Possible mechanisms for explaining the observed back-tilting in the downthrown block. Sketch not to scale.

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3.2 Archeoseismology: the Berniki theater

North of the trench site, and on the northern extent of the fault segment excavated in the trench, a

recent excavated Roman theater reveal deformation features that might be earthquake related (Fig. 6).

Figure 6: General view of the Berniki theatre (looking NE). The letters show locations of sites described below.

Site A: Tilting on a 1.60 m high wall; the wall direction is N50°; fractures and tilting are located in a 5 m

long zone, up to 15 cm of spacing between the two sides, later cemented.

Figure 7: Deformation observed at the Berniki theatre (Site A).

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Site B: Fractures and tilting on a 1.70 m high wall; 6 m long zone, up to 25 cm spacing,

10 cm left displacement.

Figure 8: Deformation observed at the Berniki theatre (Site B).

Site F: Same wall as B but 7 m to the E, here 1.60 m high; fracture in a 2 m long zone,

up to 10 cm spacing.

Site D: Tilting in a 4 m high wall, spacing up to 2-3 cm, less clear than previous evidences.

Site E: Tilting in a 2 m high wall, feature similar to D, no spacing nor lateral displacement.

Site G: Fracture and tilting on a 80 cm high wall; wall direction: N150°, spacing ca. 1 cm.

Figure 9: Deformation observed at the Berniki theatre (Site G).

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Site H: Vertical fracture on a 1 m high wall, no spacing nor lateral displacement.

Site I: Fracture on a ca. 4 m high wall, no spacing nor lateral displacement.

Site J: Fracture on a 1 m high wall, spacing max 3 cm, no lateral displacement.

Site K: Right displacement of 15 cm on two N35° oriented walls, 80 cm high, part of a rectangular

structure 3x4 m. Some tilting to the E seems visible on the N wall, but not on the S one (pictures 586-587).

Figure 10: Deformation observed at the Berniki theatre (Site K).

Site L: Fractures and tilting on a 1 m long zone, 1 m high wall, spacing up to 10 cm.

Figure 11: Deformation observed at the Berniki theatre (Site L).

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Features A,B,C,D,L,G,K are all aligned along a N140° direction, as shown on the picture below. The dashed

red line represents the alignment of the observed damages.

Figure 12: Alignment of the observed deformation at the Berniki theatre.

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3.3 Archeoseismology: the Mt. Berenice church

The church remains at Mt. Berenice show two different building techniques (different building-stone

sizes, cement between building-stones). Several fractures are visible; some of them seem a result of

building decay or different techniques, or are related to part of the building added later or to weaker

structural elements (e.g. arches). Some other fractures, indeed, can be explained only with the occurrence

of an external event (e.g. earthquake, landslide, geotechnical failure).

A preliminary work for locating possible earthquake related deformation features is presented below.

Figure 13: Preliminary sketch of the Berniki church with observed deformations.

The points showing evidences of damages/deformations/tilting, described in the following, are also shown

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Site A: This is probably the clearest evidence of an external event. Fracture on a 1 m high wall; the

fracture covers all the wall’s height. Open fracture, 3-4 cm between the 2 sides, ca. 2 cm of left

displacement. The W side is vertical, the E side shows an arcuate (concave N-ward) style.

Figure 14: Deformation observed at the Berniki church (Site A).

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Site B: Fracture on a 70 cm high wall; the prosecution is visible for ca. 50 cm in the superior part of the

wall; no lateral displacement.

Site C: Vertical fracture on a 2.40 m high wall; the fracture is visible only in the lower 1.60 m. Spacing

between the 2 sides ca. 1 cm, no lateral displacement. The wall for its entire length seems built with 2

different techniques: lower part with much more cement and smaller boulders. The fracture ends ca. at the

boundary between the 2. A, B and C are aligned along the N150° direction.

Figure 15: Deformation observed at the Berniki church (Site C).

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Site D: Here the features (vertical fractures) seems related to different building phases

(presence/absence cement).

Site E and F: Tilting and dislocation of 2 walls (loss of verticality), but related to weaker structural

elements (arches).

Site G: Single stone located at the base of a door, fractured in 2 blocks.

Site H: Series of sub-vertical fractures, in correspondence to a corner in the wall; up to 4-5 cm between

the 2 sides. Possible 1-2 cm of right-lateral displacement. In one point, the fracture broke a 20 cm long

single stone.

Site I: Fractures in a 2.5 m high wall; several stones broken, up to 1 cm between the 2 sides, no lateral

displacement.

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4. Summary

We found in the study area natural as well as archeological deformation features that might be

earthquake related. The fieldwork allowed mapping several different features on at least 3 sites (trench,

Berniki theater and church on Mt. Berenice); these data constitute a base sufficient for constraining at least

part of the fault trace. The integration between stratigraphy, paleoseismology and archaeology can lead to

a more accurate estimate of the geological features and related risks (e.g., Galadini et al., 2006).

The figure below represents a map of ancient Tiberias (modified after Hirschfeld & Gutfeld, 2008) with

indication of the inferred lineaments and trench position. Red lines were drawn after the field survey of

damages in Berniki church and theater. The western line is less constrained than the eastern one because

damages in the church are less diffuse and evident than in the theater.

Concerning the eastern inferred fault, our data reveal that the line is perfectly aligned with the

mountain front to the NW and with the “House of the Bronzes”, too. At this last site, archaeological

excavations were executed and several remains were analyzed.

The excavations highlight some information useful for assessing the earthquake risk in the area. Quoting

from the archaeological report: “Towards the end of the Fatimid period the residential structures were

destroyed and the area was completely abandoned. From the large piles of rubble covering the courtyards

of the houses and their rooms, we believe that the destruction was caused by an earthquake. Another

indication of this is the fissures in the walls of the water tower and other structures”.

The prosecution of the fault trace towards the south is less constrained and it is possible to infer

different interpretation, shown with the dotted yellow and green lines. For more conclusive results some

further work needs to be done, including for example aerial photo interpretation, analysis of the river

network and of slope instabilities.

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Figure 16: Map of ancient Tiberias (modified after Hirschfeld & Gutfeld, 2008) with indication of the inferred lineaments and trench position.

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תקציר

עיר טבריה. במסגרת העבודה נחפרה תעלת אזור ההעבודה המוצגת עוסקת בהערכת הסיכון הסיסמי ב באזור זה. סיסמיים במספר אתרים-ים ארכאורוכן נערכו סק עירעל העתק מדרום ל בדיקה פלאוסיסמית

, מספר מאות ברניקיבאזור האתר הארכיאולוגי של ,דרום -כללי צפון בכיוון התעלה נחפרה לרוחב העתקבסקר סייסמי בהפרדה גבוהה זוהה להעתק שנבדק יש ביטוי בנוף ובנוסף . מטרים מערבית לחוף הכנרת

יחסית טקטונית עד פני השטח. לפיכך נראה שהייתה פעילות ומגיע נראה שהוא מעתיק יחידות צעירות ושנחפרה )אורך נחשף בתעלההסדימנט שלמרות זאת, . אשר גרמה להעתקת פני השטח צעירה על ההעתק

והסטות במקומותסימני העתקה למעט סידוק אנכי הציגלא מטרים( 5.1מטר ועומקה עד 51 -התעלה כשנה 5111 -כלל ממצאים ארכיאולוגים שקבעו מסגרת זמן של כ שנחשף קטנות של יחידות קרקע. הסדימנט

לסלע באתרו. הלפני ההווה. התעלה לא הגיע

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