Earthquakes in Jerusalem: Geological Insights From the Bible

Earthquakes in Jerusalem: Geological Insights From the Bible

https://www.israeltoday.co.il/read/earthquakes-in-jerusalem-whens-the-next-big-one/

From biblical references to modern-day monitoring, earthquakes have left their mark on Jerusalem for centuries. A top Israeli expert surveys the history of Holy Land tremors and tells us what we can expect today.

By Prof. Amotz Agnon

Jerusalem earthquakes have left written marks long before the recording of detailed history, with the first one in the mid-8th century BC. This earthquake hit Jerusalem during the reign of King Uzziah and earned several references in the Bible. One particular reference (to be reviewed below) is by the prophet Zechariah, whose words convey a surprising geological insight. Some documented earthquakes merely created panic to such an extent as to be recorded, some toppled houses, and a few devastated massive edifices accompanied by heavy death tolls.

Tremblers in the Holy City

A score of documented earthquakes have hit Jerusalem during the last couple millennia. The latest serious earthquake, the Jericho earthquake, devastated numerous houses in in the city on July 11, 1927. Almost four centuries earlier, an earthquake of similar magnitude shook Jerusalem’s defensive walls five years after having been rebuilt by order of the Ottoman Sultan Suleiman the Magnificent.

Hundreds of earthquakes every year

Earthquakes are routinely monitored in Israel, hundreds of which occur annually, mostly below the detection limit of human senses. Dozens of earthquakes can be felt every year, and include those with an epicenter in the East Mediterranean, which occur due to a plummeting of the submarine plate under Anatolia (the Cypriot Arc) and under the Aegean (the Hellenic Arc). The rest are mostly from the Red Sea and the fracture branching northward, namely the Dead Sea Fault. This fault together with the Red Sea was referred to as the Syrian-African Rift until the advent of the theory of Plate Tectonics, some six decades ago.

Israel is an earthquake zone

Minor shocks are felt around the Kinneret (Sea of Galilee) nearly every year, and are recorded by the instruments of the Geological Survey of Israel. Two earlier and larger bursts of activity were felt and recorded in the Kinneret during the last decade. This minor activity serves to remind us that Israel is an earthquake zone.

Even so, tectonic activity in this earthquake zone is moderate relative to countries straddling faster moving faults (such as New Zealand, California, Turkey) and those positioned above plunging plates (such as Chile, Japan, Greece). Local building codes include provisions for earthquake hazard, though these are not always updated as fast as our scientific knowledge grows.

Explaining Israel’s fault lines

The Dead Sea fault is where the Arabian Plate, moving northward at a speed of a few centimeters per year, meets the Levant/Sinai Block that moves at a slower speed. The difference in speed as measured with GPS is half a centimeter per year. So a station in Amman, Jordan moves about half a centimeter faster than one positioned in Jerusalem. This is sufficient to load the system with elastic energy to be periodically released with devastating earthquakes.

The main strand of the southern Dead Sea fault comprises the Gulf of Aqaba, the Arava Valley (Wadi Araba) and the Jordan Valley (including the basins hosting the Dead Sea, Sea of Galilee and the Hula lakes).

The prophet Zechariah (14:4, 5) warned that God would set His feet east of Jerusalem, on a mountain that would split in two, with the eastern half moving north and the western moving south. This type of motion, prophesied to clog the streams, conforms to the plate slip across the Dead Sea fault that modern geology uncovered; the geological insight has been subsequently corroborated by GPS measurements, seismological analysis, and the novel sub-disciplines of paleoseismology and archaeoseismology.

The prophet further compared the geomorphological changes with those during the mid-8th century earthquake, two centuries before his time. Some scholars have suggested that only an earthquake of magnitude circa 8 could cause geomorphological changes that humans could detect and discern from them the relative motion of mountains.

A secondary branch of the Dead Sea fault splays off from the Jordan Valley to Haifa Bay. This fault, known under a variety of names (Carmel, Yagur, Fari’a/Glibo’a, Tirza), accommodates the relative motion of two adjacent sub-blocks: Jezreel Valley, Galilee and southern Lebanon versus the rest of the Levant/Sinai Block, slipping to the south-east at a speed of 0.1 centimeters per year. While this speed is lower than the speed across the main strand (by a factor of 4-5), the risk posed by the Carmel fault is perhaps higher: the fault runs through the heavily populated and industrialized Haifa Bay, where the population of the port city of Haifa exceeds a million people.

An earthquake of magnitude 5.3 shook Haifa on August 24, 1984, with an epicenter about 20 km south-east of the city, rupturing a buried strand of the fault.

The Carmel fault generated a large and devastating earthquake in 363 CE, a year of political and religious turmoil in the Roman Empire. Julian the Apostate tried to return the empire to paganism, four decades after Justin had established Christianity as the empire’s religion. Julian allegedly allowed the Jews to rebuild the Temple in Jerusalem, a city they had been prohibited to enter for the previous three centuries. A most detailed letter ascribed to the Bishop of Jerusalem reported widespread damage across the country. Inspection of the damaged area (augmented by archaeological excavations) reveals two separate regions of devastation: one cut by the Carmel fault, and one by the southern Arava/Araba Valley. This suggests that two separate earthquakes were amalgamated into one in the Bishop’s report.

Recent geological excavations in the Wadi Araba fault, the southern on-land segment of the Dead Sea fault, confirmed the southern rupture to be limited to the southern Arava. Hence the Carmel fault is the culprit for the widespread devastation in the north, an area incidentally limited to the western side of the Dead Sea fault. The scarcity of damage reports and ruins east of the fault, in a then-populated province, rules out the Jordan Valley segment as the causative fault. Serious damage from the 363 CE earthquake reached south as far as Ashkelon (a hundred kilometers away) covering all central Israel, and north to the Galilee.

Israel’s biggest earthquake to date

Dating of damage at cave deposits has revealed signs of prehistorical earthquakes, likely stronger than all the historical ones in Israel and its environs, with magnitudes extending to 8. This is similar in size to the famous 1906 San Francisco earthquake. Maximum magnitude estimates for the largest historic earthquakes range between 7.5 and 7.8.

In 1995, a magnitude 7.2 that ruptured in the Gulf of Aqaba was felt in most of Israel and caused minor damage in Eilat (the southernmost city of Israel), some 80 km north of the epicenter. By comparison, the 1927 Jericho earthquake mentioned above reached a magnitude of 6.2 and shook much of the Holy Land.

The next ‘Big One’

Scientists do not have an accepted method for predicting the time of a big event, but some considerations allow for educated guesses. I will take you now on a brief tour of what science has learned in the last three decades owing to the fantastic opportunities offered by the Dead Sea and the uniquely long and detailed human history and archaeology of its surroundings.

A team led by Israeli scientists has established the connection between ground shaking and disruption of lake-bed sediments in the Dead Sea Basin. Here some sediments appear as alternating dark-white millimeter-thick beds (aka laminas). During droughts the brine precipitates thick sequences of rock salt. During humid periods, torrents in the rainy seasons send dark mud into the lake, whereas summer evaporation causes precipitation of lime. Hence each pair of laminas represent a year in the history of the lake, and lamina-pairs are considered annual pages in the book recording the environmental history of the country.

Key proxies to climate changes include the percentages of chemically-precipitated lime and rock-salt during intervals represented by any sedimentary accumulation in the Dead Sea. These proxies are coupled with surveys of dated fossil shorelines enabling a rather detailed history of lake level elevation. This elevation history correlates with the history of earthquakes in a somewhat surprising manner.

Strong ground-shaking events obliterate the alternating laminas of dark mud and white lime, but at the same time they add invaluable information on the timing and extent of earthquake damage. Sub-meter-scale chaotic layers that preserve folded lamina sequences and torn laminas have been found juxtaposed to lake-bottom faults, and elsewhere their isotopic dates were correlated with historical earthquakes. One of the key findings of this study (that started almost three decades ago) is that earthquakes come in clusters, which challenges simplistic predictions.

Clusters have been known for earthquakes around the world for some time. The unique finding in the Dead Sea record is that such clusters may last tens of millennia. A recent publication by an international team analyzed a 460-meter-long core pulled from the bottom of the lake (under 300 meters of hypersaline water) and established an irregular and clustered behavior over 220 millennia. A possible key to clustering may lie in the complex interactions between rocks and fluids contained in their pores. Considerations of such pore fluids and the pressure they sustain brings us to consider the ever-changing levels of the lake that has been filling the Dead Sea Basin.

Comparing histories of lake-levels and earthquakes tests the correlation between these phenomena. Correlation between elevations of lake levels and recurrence intervals of earthquakes are to be expected both on an empirical basis and from mechanical considerations. The empirical aspect arises from over half a century of observations on artificial reservoirs. Shortly after the damming of many reservoirs in previously quiescent regions across the world, alarming earthquake activity has been reported, with some potentially damaging earthquakes (with a magnitude 6.4 in India being the largest). The mechanical aspect is based on the universal observation that rock strength diminishes as a linear function of the pressure sustained by pore fluids. The aquifer underneath the lake, often fractured in relevant regions, may respond quickly to lake-level changes: the strength of rocks diminishes with the rising of the water level. It follows that earthquake recurrence intervals should diminish with lake-level elevation. A recent study of an Israeli team working on the environmental history of the Dead Sea has confirmed such a correlation for levels and earthquakes during the last two millennia.

Assuming that the uncertainties in the historical observations obey certain statistical patterns, the pore-pressure model predicts that, at its current low lake-level, two centuries should pass until the next destructive earthquake will erupt beneath the Dead Sea Basin. However, we should not disregard the hazard to Jerusalem and its environs. Regardless of lake elevation, large earthquakes might nucleate on the Jordan and Arava (Wadi Araba) fault segments, adjacent to the Dead Sea, and propagate into it. Such a scenario cannot be ruled out by the pore-pressure model from the present data set.

Professor Amotz Agnon teaches geology and geophysics at the Fredy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem, where he co-founded Neev’s Center for Geoinfomatics.