The shock-acoustic waves generated by earthquakes

We investigate the form and dynamics of shock-acoustic waves generated by e arthquakes. We use the method for detecting and locating the sources of ion ospheric impulsive disturbances, based on using data from a global network of receivers of the GPS navigation system, and require no a priori informat ion about the place and time of the associated effects. The practical imple mentation of the method is illustrated by a case study of earthquake effect s in Turkey (17 August and 12 November 1999), in Southern Sumatra (4 June 2 000), and off the coast of Central America (13 January 2001). It was found that in all instances the time period of the ionospheric response is 180-39 0 s, and the amplitude exceeds, by a factor of two as a minimum, the standa rd deviation of background fluctuations in total electron content in this r ange of periods under quiet and moderate geomagnetic conditions. The elevat ion of the wave vector varies through a range of 20-44 degrees, and the pha se velocity (1100-1300 m/s) approaches the sound velocity at the heights of the ionospheric F-region maximum. The calculated (by neglecting refraction corrections) location of the source roughly corresponds to the earthquake epicenter. Our data are consistent with the present views that shock-acoust ic waves are caused by a piston-like movement of the Earth's surface in the zone of an earthquake epicenter.