Historical earthquake simulation using ambient seismic noise in Vrancea (Romania): preliminary results

The Vrancea Seismic Zone (VSZ), located in Romania, at the sharp bend of the southeastern Carpathians, is an anomalous intraplate seismic region releasing the largest strain in continental Europe. The seismicity is concentrated in a high-velocity focal volume down to 200 km, challenging classic earthquake mechanism theories due to its remote location and deep hypocenters in an expectedly ductile lithosphere. The last significant earthquake in Vrancea occurred in 1977 causing destruction to Romanian cities and long-term economic damage to an already struggling developing country. The seismic infrastructure was underdeveloped in Romania at that time and the earthquake was not well-recorded locally. The recent increase in seismic station coverage in Romania now provides the opportunity to systematically study seismogenic processes and apply the most novel processing methods.

We apply the recently developed algorithm of Virtual Earthquake Approach (VEA, Denolle et al., 2013) to reconstruct realistic ground motion records as if the stations operating today recorded historical earthquakes, such as the 1977 event. Predicting accurate ground motion is critical for earthquake hazard analysis, particularly in situations where sedimentary basins trap and amplify seismic waves. We gathered one year of three-component ambient noise data from 44 broadband seismic stations around the VSZ. We then construct the ambient noise Green’s tensor between pairs of stations and add the signatures of a realistic earthquake: double couple mechanism, buried source and a realistic earth model in the epicentral area.

Using the Romanian earthquake catalog (Romplus, www.infp.ro), we selected the last Mw>6.0 earthquakes since 1940 from the area and extracted the moment tensor solutions. Subsequently, we simulate the ground motion generated by these earthquakes recorded by modern seismometers decades after their occurrence. Our new results demonstrate the viability of this innovative method and provide a unique opportunity for more accurate seismic hazard analysis.

Keywords: ambient noise, historical earthquake, virtual earthquake approach.

Reference:

• A. Denolle, E. M. Dunham, G. A. Prieto, G. C. Beroza, Ground motion prediction of realistic earthquake sources using the ambient seismic field, Journal of Geophysical Research: Solid Earth, Vol. 118, 2102–2118, doi:10.1029/2012JB009603, 2013.