Ambient Noise Recorded by a Dense Broadband Seismic Deployment in Western Iberia

Abstract

The West Iberia Lithosphere and Asthenosphere Structure (WILAS) project densely covered Portugal with broadband seismic stations for 2 yrs. Here we provide an overview of the deployment, and we characterize the network ambient noise and its sources. After explaining quality control, which includes the assessment of sensor orientation, we characterize the background noise in the short-period (SP), microseismic, and long-period (LP) bands. We observe daily variations of SP noise associated with anthropogenic activity. Temporary and permanent stations present very similar noise levels at all periods, except at horizontal LPs, where temporary stations record higher noise levels. We find that median noise levels are extremely homogeneous across the network in the microseismic band (3-20 s) but vary widely outside this range. The amplitudes of microseismic noise display a strong seasonal variation. The seasonality is dominated by very-long-period double-frequency microseisms (8 s), probably associated with winter storms. Stacks of ambient noise amplitudes show that some microseismic noise peaks are visible across the whole ground-motion spectrum, from 0.3 to 100 s. Periods of increased microseismic amplitudes generally correlate with ocean conditions offshore of Portugal. Some seismic records display an interesting 12 hr cycle of LP (100-s) noise, which might be related to atmospheric tides. Finally, we use plots of power spectral density versus time to monitor changes in LP instrumental response. The method allows the identification of the exact times at which LP response changes occur, which is required to improve the understanding of this instrumental artifact and to eventually correct data.

Seasonal and Diurnal Variations in Long‐Period Noise at SPREE Stations: The Influence of Soil Characteristics on Shallow Stations’ Performance

“…6-9). A similar modulation of seismic noise was linked to atmospheric tides by Custódio et al (2014). They observed a 12 hr variation in 100 s noise that could be related to the semidiurnal S 1 tide, which dominates surface pressure variations in coastal settings (Ray and Ponte, 2003).…”

Thermal Nature of Mantle Upwellings Below the Ibero‐Western Maghreb Region Inferred From Teleseismic Tomography

“…The inclusion of SKS phases into the S wave traveltime tomography provides additional, steeper, crossing paths down to the MTZ. The ensemble of stations used encompassed 203 stations from the IberArray deployment (Díaz et al, ), 91 from the PICASSO array (Platt et al, ), 20 from the WILAS project (Custódio et al, ), and 10 ocean‐bottom seismometers (OBSs) deployed by the NEAREST experiment (Carrara and NEAREST Team, ). An additional 86 temporary and permanent instruments from another 13 seismic networks were also used.…”

A common deep source for upper-mantle upwellings below the Ibero-western Maghreb region from teleseismic P-wave travel-time tomography

“…Overlapping in space and time with IberArray, 64 broadband seismometers from the PICASSO project operated in Morocco and part of Spain between 2009 and 2013 (Platt et al, 2008). An additional 23 temporary seismic stations operated in mainland Portugal between 2010 and 2012 as part of the WILAS project (Custódio et al, 2014). We further used data from 24 temporary broadband OBSs deployed by the NEAREST experiment between 2007 and 2008 (Carrara and NEAREST Team, 2008).…”