Imaging a fault to learn more about tectonic plate motion and improve understanding of hazards
Imaging Israel’s Dead Sea Fault to Understand How Continents Stretch and Rift
This article is part of the July 2018 issue of the Sound Waves newsletter.
An international team of scientists collaborated last April to image the deep structure of the Dead Sea fault in Israel. Their findings will help show how tectonic-plate motion deforms continental crust and will improve understanding of earthquake hazards and natural resources along the fault.
Like California’s San Andreas fault, the Dead Sea fault is a transform plate boundary, along which two tectonic plates slide past one another. It separates the Arabia plate from the Africa plate (see map). The fault’s path through Israel is marked by a valley approximately 20 kilometers (12 miles) wide, underlain in places by deep subsurface sedimentary basins, giving rise to the name “Dead Sea Rift.”
Many transform plate boundaries are on the seafloor, where they connect segments of mid-ocean spreading ridges. The San Andreas and Dead Sea faults, in contrast, are on continents and cut through the entire lithosphere (the top approximately 100 kilometers [60 miles] of the Earth). Thus, they provide a window into how the continental crust and upper mantle get deformed by the relentless movement of tectonic plates. The Dead Sea fault gives a somewhat clearer view than the San Andreas fault, which is located over an old subduction zone and cuts through rocks with a complex past.
“The geological history and crustal structure of the Dead Sea area are simpler than those in California,” says Zvi Ben-Avraham, professor at Israel’s University of Haifa and co-supervisor of the April experiment. “We can more readily resolve the configuration of the plate boundary in the Dead Sea Rift, and then project it to similar tectonic environments around the globe.”
USGS research geophysicist Nathan Miller, a participant in the experiment, adds: “Sedimentary basins [low areas where sediment accumulates] along the Dead Sea fault serve as compact laboratories where we can examine continental stretching and rifting.”
Investigating this plate boundary has more than academic value: the Bible and historical and archeological records document numerous earthquakes along the Dead Sea fault. The largest to strike Israel in recent history was the magnitude 7.1 Safed earthquake, which occurred on January 1, 1837, killing more than 5,000 people and causing massive damage to cities and villages. Enhanced understanding of the fault will improve assessments of the hazards it poses. A more detailed picture of its structure can also help identify resources like oil and water, which are commonly stored in reservoirs of sedimentary rocks along such plate boundaries.
The recent experiment ran from April 8 to 12, 2018, and focused on the Sea of Galilee in the northern part of the Dead Sea Rift, where the researchers expected the crust to be different from previously explored areas to the south. To image the fault structure here, they used seismic (sound) energy generated by 12 underground explosive shots, 300–400 kilograms (approx. 700–900 pounds) each, along an east-west and a north-south profile (see map above). The sound waves penetrated as deep as 30 kilometers (20 miles) beneath the surface, bending and reflecting as they hit rock layers with different properties. The return signals were recorded by 550 receivers arranged at 200-meter (650 foot) intervals along the two profiles.
The scientists buried most of the receivers a few centimeters below the ground surface. They modified 40 of the receivers to record data in the Sea of Galilee by housing them in water-tight flotations anchored to the lake’s bottom and replacing the geophones with hydrophones (microphones designed to work in water). The April deployment was the first test of this novel modification of land receivers for underwater work. It appears to have succeeded and will likely be used in future seismic data collection in estuaries and lakes.
In addition to imaging the structure of the Dead Sea fault, the receivers recorded nearby mining explosions and local earthquakes during the 2 days of deployment; these data will be included in the analysis. The controlled explosive shots detonated for the study were also used to calibrate local seismic networks and to study the ground’s response to shaking in Israel, Jordan, and the Palestinian Territories.
The participants came from the University of Haifa (UH), Israel; the USGS; and the Geophysical Institute of Israel (GII). The experiment was funded by the Richard Lounsbery Foundation and by the Israel Ministry of Energy and Infrastructure. Equipment and technical support were provided by The Incorporated Research Institutes for Seismology (IRIS) Portable Array Seismic Studies of the Continental Lithosphere (PASSCAL) Instrument Center in Socorro, New Mexico. The experiment was supervised by Uri ten Brink (USGS) and Zvi Ben-Avraham (UH), coordinated by Eldad Levi (GII), and conducted by Nathan Miller (USGS), Lloyd Carothers (IRIS-PASSCAL), Steve Harder (University of Texas at El Paso), students from the UH, and field crews from GII, and the Israel Oceanographic and Limnological Institute (IOLR).
The successful completion of this experiment on a tight schedule required a high degree of coordination among the field crews, drillers, and explosive experts, as well as coordination with police, military, and civil defense authorities. Data are currently being processed and analyzed at the USGS. A copy of the data has been archived with IRIS-DMC.
Preliminary examination of the data shows sound-wave returns from the top 10 kilometers (6 miles) of rock beneath the seafloor, and some reflections from deeper in the crust. In addition to images of rock deformed by faults and folds, some of the data will help the researchers determine the rock composition under the survey profiles. Stay tuned!