Nuclear Explosions and Seismology Q: What is the Comprehensive Test Ban Treaty (CTBT)? A: On September 10, 1996, the United Nations General Assembly voted 158-3 to approve a treaty prohibiting all nuclear tests. The Comprehensive Test Ban Treaty has been signed by 130 nations - including the United States. President Clinton Clinton signed the agreement on September 24, 1996. Q: What is the role of seismology in monitoring a CTBT? A: Seismology is one of several fields which plays a role in monitoring the CTBT. Underground nuclear explosions produce seismic waves with unique characteristics which allow the discrimination between explosions and earthquakes. Q: What are the differences between explosions and earthquakes? A: Both earthquakes and nuclear tests can rapidly release a large amount of energy. The energy source for small yield (typically less than 50 kilotons) thermonuclear devices is the splitting of heavy radioactive isotopes. This process produces about 20 million times the energy of each reacting atom in a chemical explosive. The energy source for an earthquake is tectonic strain accumulated by the relative motion of Earth's tectonic plates which is driven by mantle heat flow in the presence of the earth's gravitational field. In a nuclear test, all of the energy is suddenly (within milliseconds) released in the form of heat from a relatively small volume surrounding the thermo- nuclear device. The tremendous heat causes rapid expansion of a spherical cavity, which in turn generates seismic waves. The heat gradually conducts away from the cavity into the surrounding rock. However, rock is a poor conductor of heat so it can take many years for the thermal signature of the thermonuclear explosion to subside and the increase in the surface temperature above the explosion is insignificant. Nuclear tests are also very shallow sources with the depth of burial generally less than a few hundred meters (the depth of burial is typically proportional to the cube root of the expected yield). The estimated yields of the larger Indian and Pakistani tests are approximately 2-40 kilotons. In a large earthquake, the elastic strain energy stored in the Earth's crust is released, within a few seconds to a few tens of seconds, by rupture along a fault and the strain energy is released from a relatively large volume of rock surrounding the fault rupture. For example, the recent (5/30/98 at 06:22:28 UT) magnitude 6.5 earthquake in Afghanistan (37.4 N, 70.0 E), had a source duration of about 5 seconds and an estimated source volume of order 4000 cubic kilometers. This earthquake also had a focal depth of 18 km. The energy release is equivalent to a 2000 kiloton nuclear explosion. Q: Can nuclear explosions cause earthquakes? A: On January 19, 1968, a thermonuclear test, codenamed Faultless, took place in the Central Nevada Supplemental Test Area. The codename turned out to be a poor choice of words because a fresh fault rupture some 1200 meters long was produced. Seismographic records showed that the seismic waves produced by the fault movement were much less energetic than those produced directly by the nuclear explosion. Analysis of local seismic recordings (within a couple of miles) of nuclear tests at the Nevada Test Site shows that some tectonic stress is released simultaneously with the explosion. Analysis of the seismic wavefield generated by the blast shows the source can be characterized as 70-80 percent dilational (explosive-like) and 20-30 percent deviatoric (earthquake-like). The rock in the vicinity of the thermonuclear device is shattered by the passage of the explosions shock wave. This releases the elastic strain energy that was stored in the rock and adds an earthquake-like component to the seismic wavefield. The possibility of large Nevada Test Site nuclear explosions triggering damaging earthquakes in California was publicly raised in 1969. As a test of this possibility, rate of earthquake occurrence in northern California (magnitude 3.5 and larger) and the known times of the six largest thermonuclear tests (1965-1969) were plotted and it was obvious that no peaks in the seismicity occur at the times of the explosions. This is in agreement with theoretical calculations that transient strain from underground thermonuclear explosions is not sufficiently large to trigger fault rupture at distances beyond a few tens of kilometers from the shot point. The Indian and Pakastani test sites are approximately 1000 km from the recent Afghanistan earthquake epicenter. The question that has been asked is whether or not the occurrence of these nuclear tests influenced the occurrence of the large earthquake in Afghanistan. The most direct cause-effect relationship is that the passage of the seismic waves, generated by the thermonuclear explosion, through the epicentral region in Afghanistan somehow triggered the earthquake. For example, following the occurrence of the magnitude 7.3 Landers earthquake in southern California on June 28, 1992, the rate of seismicity in several seismically active regions in the western US, as far as 1250 km from the epicenter, abruptly increased coincident with the passage of the earthquake generated seismic wavefield through each site. The abrupt increases in seismicity occurred primarily in regions of geothermal activity and recent volcanism. The mechanism by which this occurred remains unknown. The Afghanistan earthquake occurred at 06:22:28 UT on May 30, 1998 and the thermonuclear test most closely associated in time occurred at 06:55 UT or after the occurrence of the earthquake. The other nuclear tests occurred 2-20 days before the earthquake. The elastic strains induced in the epicentral region by the passage of the seismic wavefield generated by the largest of the nuclear tests, the May 11 Indian test with an estimated yield of 40 kilotons, is about 100 times smaller than the strains induced by the Earth's semi-diurnal (12 hour) tides that are produced by the gravitational fields of the Moon and the Sun. If small nuclear tests could trigger an earthquake at a distance of 1000 km, equivalent-sized earthquakes, which occur globally at a rate of several per day, would also be expected to trigger earthquakes. No such triggering has been observed. Thus there is no evidence of a causal connection between the nuclear testing and the large earthquake in Afghanistan and it is pure coincidence that they occurred near in time and location. One last point. The largest underground thermonuclear tests conducted by the US were detonated in Amchitka at the western end of the Aleutian Islands and the largest of these was the 5 megaton codename Cannikin test which occurred on November 6, 1971. Cannikin had a body wave magnitude of 6.9 and it did not trigger any earthquakes in the seismically active Aleutian Islands. Suggested reading: "Nuclear Explosions and Earthquake, the Parted Veil", by Bruce A. Bolt, W. H. Freeman and Co., San Francisco, 1976. Q: Can we use alot of explosives to cause small earthquakes in order to prevent having large ones? A: No. Even huge amounts of explosive almost never cause even small earthquakes (see above), and it would take hundreds and thousands of small earthquakes to equal a large one, even if it could be done. In addition, we wouldn't have any control over the size of the earthquake being created if it worked, since small and large earthquakes all start out in exactly the same way. It's just not physically possible.