The controls on earthquake ground motion in foreland-basin settings: The effects of basin and source geometry

Summary Rapid urban growth has led to large population densities in foreland basin regions, and therefore a rapid increase in the number of people exposed to hazard from earthquakes in the adjacent mountain ranges. It is well known that earthquake-induced ground shaking is amplified in sedimentary basins. However, questions remain regarding the main controls on this effect. It is, therefore, crucial to identify the main controls on earthquake shaking in foreland basins as a step towards mitigating the earthquake risk posed to these regions. We model seismic-wave propagation from range-front thrust-faulting earthquakes in a foreland-basin setting. The basin geometry (depth and width) and source characteristics (fault dip and source-to-basin distance) were varied, and the resultant ground motion was calculated. We find that the source depth determines the amount of near-source ground shaking and the basin structure controls the propagation of this energy into the foreland basin. Of particular importance is the relative length scales of the basin depth and dominant seismic wavelength (controlled by the source characteristics), as this controls the amount of dispersion of surface-wave energy, and so the amplitude and duration of ground motion. The maximum ground motions occur when the basin depth matches the dominant wavelength set by the source. Basins that are shallow compared with the dominant wavelength result in low-amplitude and long-duration dispersed waveforms. However, the basin structure has a smaller effect on the ground shaking than the source depth and geometry, highlighting the need for understanding the depth distribution and dip angles of earthquakes when assessing earthquake hazard in foreland-basin settings.

Geophysical basin characterization using seismic noise H/V spectral ratio and gravity data (Vallès Basin NE-Spain).

<p>Vallès Basin (NE-Spain) is a neogene basin with mainly granitic bedrock and delimited at NW by one normal fault (Vallès fault). This basin presents significant geothermal anomalies reflected with surficial hot thermal waters. Previous studies carried out in the 70s, to define its energy resource potential using one single geophysical technique, were not enough to clearly interpret the subsoil structure and many uncertainties remain still unsolved.</p><p>The aim of this work is to combine two different geophysical techniques for collaborative interpretation of the Vallès Basin structure in order to reduce the uncertainties: 2D gravity profiles and seismic noise H/V spectral ratio measurements distributed in the whole basin area. 2D gravity profiles provide subsurface structural information and basement depth from density models obtained after modelling and inversion processes; whereas the seismic noise H/V spectral ratio technique determines the soil fundamental frequency, which helps to locate the boundary between soft sediments and hard rock materials using empirical equations. Therefore, bedrock geometry and infill sediments structure can be estimated, which is crucial to understand ongoing processes related to the surface geothermal evidences.</p><p>The work methodology consists of combining both geophysical methods comparing the density models obtained from Bouguer anomaly in the 2D gravity profiles with the soft soil-hard rock contact surface obtained from the seismic noise H/V spectral measurement. The co-validation between them is carried out overlapping these two individual geophysical results and complementing models between them to obtain the best fit. Despite using different geophysical techniques to reduce ambiguities, a final discussion about lithology of sediments, geometry of basement and location of main faults is always needed. In this case, two equally probable models are proposed to interpret the Vallès Basin structure. One of them presents a shallow basin with granitic basement below. The other one presents a deeper basin, the granitic bedrock is located at 2000 m depth, with conglomerate deposits near the main fault. In both cases, the obtained models detect the Vallès fault as a sub-vertical fault which slightly diminishes its slope from 1200-1400 m in depth.</p><p>These new results in the Vallès Basin provide valuable information for geothermal purposes, but should be completed with more geophysical data to assure the geological model. As a future work, the gravity data will be extended at the whole basin in order to create a 3D geological model. To accomplish this objective, it will be fundamental to construct a very dense mesh of gravity points (good resolution) which affords a plausible hypothesis about the basin geological structure.</p>