1 Ambient Seismic Noise D. E McNamara 1 , R. P. Buland 1 , R. I. Boaz 2 , B. Weertman 3 and T. Ahern 3 1 U.S. Geological Survey Golden CO, 2 Boaz Consultancy, 3 Incorporated Institutions for Seismology, Data Management Center manuscript in preparation: SRL short note Aug. 2005: PREPRINT v3.0 Correspondence to: Daniel E. McNamara USGS 1711 Illinois St. Golden, CO 80401 (303) 273-8550 VOICE (303) 273-8600 FAX mcnamara@usgs.gov Abstract For earthquake-monitoring and detection purposes it is important to know the ambient seismic nose levels experienced by a network of seismic stations. In this paper, we present global ambient noise levels using 159 worldwide broadband stations within the Global Seismographic Network (GSN) and the Advanced National Seismic System (ANSS). To determine ambient noise conditions, we analyze the distribution of seismic noise as a function of period and determine the highest probability power levels. While it may be scientifically interesting to determine the absolute quietest noise levels achieved by a network, we find that these low noise levels are generally very low probability occurrences (1-3%) and do not closely track significantly higher probability (20-30%) “ambient” noise conditions determined by the mode of the power distribution. In fact, statistical mode noise levels are as much as 25dB above the minimum at higher frequencies (1-10Hz) significant to earthquake monitoring.