An Israeli website is reporting that Tehran may have tested a nuclear device over the weekend. Israel Insider is basing its claim on the comments of an Iranian nuclear scientist, who said that a seismic event in the southern part of his country, reported on Saturday night, was associated with a nuclear test.
According to the U.S. Geological Service (USGS), a magnitude 5.0 earthquake was reported near the Persian Gulf coast around midnight (Iranian time) on 25 October. The tremor was centered near the town of Korig, north of the Strait of Hormuz. An earthquake measuring 4.8 on the Richter Scale was reported in the same area on 21 October; the Iranian source claims that earlier tremor was associated with another nuclear test.
At this point, there is little evidence to corroborate the report. The coastal region where the quakes occurred is hundreds of miles from Iranian range complexes where a nuclear weapon might eventually be tested. Additionally, the referenced site is easily monitored by U.S. intelligence assets, including WC-135 CONSTANT PHOENIX aircraft. So far, there's been no report of a recent WC-135 deployment to the Persian Gulf region.
Additionally, if Iran had successfully conducted a nuclear test, there would have been an announcement from Tehran. Iran's acquisition of nuclear weapons is a "game changer" in the Middle East, and Tehran would have little reason to conceal the news. Indeed, the announcement might actually lessen the chances of an Israeli attack, since a test would indicate that Iran had mastered all stages of nuclear development. It would also mean that prospects for successfully derailing the program had all but vanished.
There is also reason to believe that Iranian preparations for a test would be detected by western intelligence agencies. An underground nuclear blast is (typically) preceded by extensive tunneling work; the deployment of test vans and other monitoring equipment; the installation of required cabling, and heightened security at the site--measures that are often detected by satellites and SIGINT assets, among other sources.
And, quite often, the U.S. leaks these discoveries to the press, to let our adversaries know that we're aware of their activities. To date, there have been no western press reports about Iran conducting a test--let alone, preparing for one.
While the odds are admittedly slim, the possibility of an Iranian nuclear test cannot be totally ruled out. In recent years, there has been extensive tunneling along the Persian Gulf coast. While much of the work has been associated with upgrades in Tehran's coastal defenses, it would be relatively easy to bore an extra shaft or two, with an eye towards future nuclear testing. Iran could also (potentially) conceal other preparations for the test, drawing upon its expertise in denial and deception techniques.
It's also worth noting that North Korea is playing a key role in supporting Tehran's nuclear ambitions. The North Koreans are even more adept at deception than their Iranian counterparts, and they learned valuable lessons during their own nuclear test in 2006. With assistance from Pyongyang, Iran would have a better chance at hiding an underground nuclear blast.
But not all traces of a nuclear test can be successfully concealed. Particulate and gaseous debris from a subterranean blast make their way into the atmosphere, and are readily detected by the WC-135 and other sensors. The absence of such discoveries (so far) suggests that claims of an Iranian "test" are nothing more than fiction--at least, for now.