Velocity structure of the upper mantle discontinuities beneath North America from waveform inversion of broadband seismic data using a genetic algorithm

We present a velocity structure of the mantle discontinuities beneath the U nited States from a full waveform inversion of broadband seismic data. The data consist of 10 moderate earthquakes in the western United States that w ere recorded on 13 broadband seismometers across the United States operated by the U. S. National Seismic Network. A total of 36 seismograms with dist ance ranging from 1150 to 2700 km were used. A full waveform inversion base d on two-dimensional modeling and one-dimensional inversion methods using a genetic algorithm was adopted. The inversion method is objective, provides error bounds on the model, and does not require any a priori information a bout these discontinuities. We find that the upper discontinuity is similar to 10 km thick, from 395 to 405 km, where the velocity increases from 8.80 to 9.15 km s(-1) with a velocity gradient of 0.035 s(-1). The lower discon tinuity is similar to5 km thick, occurring over a depth range of 650-655 km , where the velocity increases from 10.20 to 10.70 km s(-1). Our results ar e in agreement with previous studies of the lower discontinuity but do not agree for a sharp (< 5 km) upper mantle discontinuity.