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Tomography of Crustal Seismic Attenuation in Switzerland and Surrounding Regions: A New Input for the Next Generation of Seismic Hazard Models

Description: 

We present a 3D attenuation model of the upper crust for the Swiss Alps. The 3D inversions derive the quality factor Q (1/attenuation) using path attenuation t∗ observations for > 4500 earthquakes recorded on permanent and temporary stations for the period 2002-2022. We followed a procedure in which a series of inversions are performed on decreasing size grids. Re-gridding is done using a nearest-neighbor approach. This method allows to obtain a reasonable Q model everywhere despite the spatially varying data distribution and to resolve deeper parts of the model thanks to including longer distances phases. The resulting Qs and Qp models show large-scale features in the upper crust, which are consistent with a recently improved high-resolution velocity models of the same region and serve to refine the interpretations of crustal structures from Vp and Vp/Vs. For example, at depths ranging between 2.0-6.5 km, low Q is imaged along the Rhone valley in the Valais region in southwest Switzerland. This region locates in the transition zone between the Central and Western Alps and represents one of the most hazardous areas within the Alpine Arc, which hosts the presently seismically most active fault zones. As the attenuation of fractured rock volume is enhanced by fluids, the low Q values observed in this area may relate to distributed microfractures that produce greater fracture connectivity and permeability in a relatively higher strain-rate zone. This is also confirmed by low-velocity anomalies imaged by recent high-resolution tomography efforts aiming to image the fault-zone structures in this region. Thus, in combination with recently developed Vp and Vs velocity models, the developed 3D attenuation models not only advance our understanding of seismotectonic processes in Switzerland, but will also provide additional constraints in terms of composition and physical properties of the uppermost crust of the Swiss Alps as well as crucial input for next generation seismic hazard models of Switzerland. This new acquired information can be used for a more realistic prediction of earthquake related ground motions.

Session: Crustal Imaging of High Seismic Hazard Regions

Type: Oral

Date: 4/20/2023

Presentation Time: 10:30 AM (local time)

Presenting Author: Federica Lanza

Student Presenter: No

Invited Presentation: 

Authors