Crustal Imaging

Mainland Portugal has experienced several large earthquakes both offshore and onshore. Examples of significant events in the past include the great Lisbon earthquake in 1755 (M ~8.5-8.7) that generated a devastating tsunami, the 1858 M ~7.1 event in Setúbal, the Benavente earthquake of M ~6.0 in 1909, and the tsunamigenic M8.3 event in the Gloria Fault in 1941. To better characterize seismicity that pose a potential risk to this region, interpret tectonics, and predict strong ground motion, accurate models of regional Earth structure are required. The lack of a region-wide high quality broadband (BB) seismic network in the past however, has hindered a quantitative characterization of the Earth structure in Portugal. Since 2006, the permanent Portuguese seismic network has greatly expanded, which resulted in a continuously accumulating large volume of high quality BB data. Within the scope of the FCT-funded project AQUAREL (Accurate QUAntification of Regional Earthquakes and earth structure: application to western Iberia) project, we utilized this new dataset to measure Rayleigh wave ellipticity from teleseismic events to build one of the first countrywide seismic shear wave crustal structure model beneath Portugal.

Portugal experiences a significant earthquake every few decades. The smaller earthquakes, however, are frequent and they illuminate the locations of faults. The precision of earthquake locations has been improving thanks to an expanding station network.

The above figures show seismicity in the period 1961 - 2013 (left) and the seismic station distribution used in the study (right).

Rayleigh wave ellipticity is the amplitude ratio of Hilbert-transformed radial component to the vertical component measured in the time window that contains the fundamental mode Rayleigh wave. In ideal conditions, the waveforms between these two components are identical. This above figure is for an example measurement made at a central period (T) of 100 s. The ellipticity curve is constructed by making these measurements at a series of periods.

Right: The best-fitting shear velocity model (black line) inverted from the ellipticity curve using a nonlinear Monte Carlo method. Model uncertainties are obtained from an ensemble of models (green lines) whose misfit is within 20% of the minimum misfit. In total, we sample 4000+ models per station.

The above procedure is applied to all stations, from which a map of shear velocity model is constructed. We parameterized the velocity profile as four uniform layers and the following results are obtained from interpolation.

These shear velocity models can be scaled to P wave velocity models using Brocher's (2005) scaling relationships. The scaled P wave velocity models are shown below.

Shear wave velocities inverted from Rayleigh wave ellipticities are lowest in the sedimentary basins and highest in metamorphic belts. These ellipticities are strongly sensitive to the uppermost ~ 5 km of the crust, which is also the region most important for seismic hazard analysis. We observe an anomalously low velocity region in Central Iberian Zone (CIZ), which we interpreted to arise from a previously undetected extensional episode of the crust. A reduction in Moho depth in the same region lends support to this hypothesis.

Attanayake, J., Ana M.G. Ferreira, Andrea Berbellini, Andrea Morelli, 2017. Crustal structure beneath Portugal from teleseismic Rayleigh Wave Ellipticity, Tectonophysics, https://doi.org/10.1016/j.tecto.2017.06.001