Nikolai M. Shapiro

Towards forecasting volcanic eruptions using seismic noise

Volcanic eruptions are preceded by increased magma pressures, leading to the inflation of volcanic edifices1. Ground deformation resulting from volcano inflation can be revealed by various techniques such as spaceborne radar interferometry2, or by strain- and tiltmeters3. Monitoring this process in real time can provide us with useful information to forecast volcanic eruptions. In some cases, however, volcano inflation can be localized at depth with no measurable effects at the surface, and despite considerable effort4, 5 monitoring changes in volcanic interiors has proven to be difficult. Here we use the properties of ambient seismic noise recorded over an 18-month interval to show that changes in the interior of the Piton de la Fournaise volcano can be monitored continuously by measuring very small relative seismic-velocity perturbations, of the order of 0.05%. Decreases in seismic velocity a few weeks before eruptions suggest pre-eruptive inflation of the volcanic edifice, probably due to increased magma pressure. The ability to record the inflation of volcanic edifices in this fashion should improve our ability to forecast eruptions and their intensity and potential environmental impact.

Broadband ambient noise surface wave tomography across the United States

[1]xa0This study presents surface wave dispersion maps across the contiguous United States determined using seismic ambient noise. Two years of ambient noise data are used from March 2003 through February 2005 observed at 203 broadband seismic stations in the US, southern Canada, and northern Mexico. Cross-correlations are computed between all station-pairs to produce empirical Green functions. At most azimuths across the US, coherent Rayleigh wave signals exist in the empirical Green functions implying that ambient noise in the frequency band of this study (5–100 s period) is sufficiently isotropically distributed in azimuth to yield largely unbiased dispersion measurements. Rayleigh and Love wave group and phase velocity curves are measured together with associated uncertainties determined from the temporal variability of the measurements. A sufficient number of measurements (>2000) is obtained between 8 and 25 s period for Love waves and 8 and 70 s period for Rayleigh waves to produce tomographic dispersion maps. Both phase and group velocity maps are presented in these period bands. Resolution is estimated to be better than 100 km across much of the US from 8–40 s period for Rayleigh waves and 8–20 s period for Love waves, which is unprecedented in a study at this spatial scale. At longer and shorter periods, resolution degrades as the number of coherent signals diminishes. The dispersion maps agree well with each other and with known geological and tectonic features and, in addition, provide new information about structures in the crust and uppermost mantle beneath much of the US.

Surface wave tomography of the western United States from ambient seismic noise: Rayleigh wave group velocity maps

We have applied ambient noise surface wave tomography to data that have emerged continuously from the EarthScope USArray Transportable Array (TA) between October 2004 and January 2007. Estimated Greens functions result by cross-correlating noise records between every station-pair in the network. The 340 stations yield a total of more than 55,000 interstation paths. Within the 5- to 50-s period band, we measure the dispersion characteristics of Rayleigh waves using frequency-time analysis. High-resolution group velocity maps at 8-, 16-, 24-, 30-, and 40-s periods are presented for the western United States. The footprint of the TA encloses a region with a resolution of about the average interstation spacing (∼70 km). Velocity anomalies in the group velocity maps correlate well with the dominant geological features of the western United States. Coherent velocity anomalies are associated with the Sierra Nevada, Peninsular, and Cascade Ranges, Great Valley, Salton Trough, and Columbia basins, the Columbia River flood basalts, the Snake River Plain and Yellowstone, and mantle wedge features associated with the subducting Juan de Fuca plate.

Is ambient noise tomography across ocean basins possible

[1] Based on year-long cross-correlations of broad-band seismic records obtained at sixty-six stations within or adjacent to the Pacific Basin, we show that broad-band ambient noise is observed to propagate coherently between island stations and between island and continent stations. For many station pairs, high signal-to-noise ratio (SNR) fundamental mode Rayleigh wave Green functions are observed, which establishes the physical basis for ambient noise tomography across the Pacific. Similar trends for continental and oceanic stations are observed in the relationship between the ambient noise level at a station and the ‘‘noise coherence distance’’ – the longest distance at which a high SNR cross-correlation signal is observed for a station. Because locally generated noise obscures long distance coherent noise, situating stations at quiet locations on islands is necessary for the success of ambient noise tomography. Local noise poses a particular challenge at atoll sites and, on the basis of analysis of data from station H2O, at ocean bottom sites at periods above 25 sec. Citation: Lin, F.-C., M. H. Ritzwoller, and N. M. Shapiro (2006), Is ambient noise tomography across ocean basins possible?, Geophys. Res. Lett., 33, L14304, doi:10.1029/ 2006GL026610.