THE Middle East has long been a region beset with tension, if not outright warfare. It is ironic, therefore, that a series of underwater explosions set off in the Dead Sea last November may, with the assistance of Lawrence Livermore seismologists, help to reduce tensions in the area and spur cooperative ventures on geophysical-related issues.|
Conducted by the Geophysical Institute of Israel, the explosions were cofunded by Israel and the U.S. Defense Threat Reduction Agency. The main goal was to improve monitoring of the Comprehensive Test Ban Treaty (CTBT) by calibrating Israel's two International Monitoring System (IMS) seismic stations as well as its national system of seismic monitors. Because the tests were announced well ahead of time, other Middle East nations were afforded the opportunity to calibrate their own national seismic stations and any IMS stations on their territories. The explosions will help scientists to pinpoint the location of suspicious seismic events in the area and distinguish them from other sources of seismic signals.
According to Livermore seismologist Keith Nakanishi, detecting, locating, and identifying a clandestine nuclear test poses a particular challenge in the Middle East. International stations are few and far between in the area. Also, a large number of earthquakes and mining explosions generate thousands of seismic signals annually, some quite similar to the signals that would be generated by a small underground nuclear blast.
Additional "ground truth" for the area is sorely needed, Nakanishi says. Ground truth includes seismic data from well-documented earthquakes, mine explosions, or explosions carried out for calibration purposes. Carefully gathered data from these events improve the knowledge of how regional-specific features in the Earth's crust and upper mantle affect the travel times, amplitudes, and frequencies of weak seismic signals. Such data are particularly important to accurately determine the location and origin time of the seismic sources.
Building a Knowledge Base
Experiments Were Well Characterized|
To be particularly useful, seismic calibration tests must have well-defined locations and origin times. For the Dead Sea tests, these parameters were well determined, says Nakanishi, who attended planning meetings in Israel that focused on such requirements. The location of each test was known to an accuracy of 20 meters, the depth was established to within an accuracy of 5 meters, and the time was determined to an accuracy better than 20 milliseconds.
The explosions were recorded by the Geophysical Institute of Israel and its network of seismic stations, including two IMS stations located in the southern and northern areas of the country. The events were also recorded by a group of more than 30 smaller stations that form Israel's national seismic network and by a few temporary stations Israel installed on the Dead Sea shores. Seismic stations in neighboring countries such as Jordan, Egypt, and Saudi Arabia reportedly also recorded the tests. The Geophysical Institute distributed data electronically to interested parties, including Nakanishi and his colleagues, within a few days.
The Livermore team is analyzing the Dead Sea data and using the results to refine the DOE's knowledge base for the area. For their part, Israel, Jordan, and other Middle East nations are using the data to strengthen their own national means to identify the magnitude and location of any clandestine nuclear blasts and future earthquakes and to better distinguish between the two.
Nakanishi predicts that the explosions will prove as valuable for earthquake monitoring as for CTBT monitoring. "The area is riddled with faults and has a long history of earthquakes dating to Biblical times," he says. The most dangerous fault is the Dead Sea Rift Valley fault that stretches from Syria through Israel and into East Africa, with one fault branch underlying Haifa, Israel. In 1995, an earthquake of magnitude 7.1 on the Richter scale occurred on the fault in the Gulf of Aqaba in the Red Sea near the Israeli city of Eilat.
Well-calibrated seismic networks will allow scientists to better locate the origin of future earthquakes. "By knowing what fault caused the earthquake, we'll know what to expect in terms of aftershocks," Nakanishi explains. He notes that seismic safety has become a larger concern in the area following the strong 1999 temblors in nearby Turkey.
Inernational Meeting to Focus on Tests
Key Words: Comprehensive Test Ban Treaty (CTBT), Dead Sea, Defense Threat Reduction Agency, Gulf of Aqaba, knowledge base, Middle East and North Africa (MENA), seismic monitoring, United Nations Educational, Scientific, and Cultural Organization (UNESCO).
For further information contact Keith Nakanishi (925) 422-3923 (firstname.lastname@example.org).