The sound of gunshots has become an all-too-familiar and unwelcome occurrence in many communities across the nation. When shots ring out, 911 calls from worried citizens may come from a large area. Unfortunately, even with numerous reports, police are often frustrated in their efforts to silence this gunfire because they cannot pinpoint the location of gunshots rapidly. A U.S. Geological Survey (USGS) scientist recognized that sound waves traveling through the air away from a gunshot are basically similar to the sound waves traveling through the ground away from an earthquake. Scientists then have adapted their methods for quickly finding the exact source of an earthquake to the problem of locating gunshots. Field testing of a gunshot--locating system inspired by earthquake technology began in 1995. After only a few weeks of testing and improving the Software, the system was locating many signals that were clearly associated with gunfire. Automatic weapons fire was the easiest to identify because of the regular time interval between individual shots. The system was more sensitive during the night, when there was less background noise from traffic and other urban activity. By the last spring, the system was undergoing final acceptance trials. Captain Jim Granucci of the Redwood City Police Department stated that 'even before the system was in use, the number of illegal gunshots declined as word of its existence spread'. In this test the gunshot-locating system worked remarkably well, and according to Commander Dominick Peloso of the Menlo Park Police Department, 'This system, when fully developed, holds great promise for assisting police in controlling and reducing violent crime.' When the results of the test were made public, there was an enthusiastic response from San Francisco Bay area residents, who asked local government officials to implement such systems in their communities. Interest was also expressed by private firms and law enforcement agencies both in California and elsewhere in the nation. Robert Showen, founder of a company that is now marketing a gunshot-locating system, said, 'The USGS test demonstrated beyond a doubt that the concept was feasible and could be implemented quickly. Without this test, I would have been hesitant to invest in the development of my product.' The problem of gunfire is difficult to solve because ______.