Radiated Energy of Shallow Earthquakes
Marine Denolle
UC San Diego
- Date & Time
- Location
- Building 3, Room 3240 (main USGS conference room)
- Summary
Radiation of seismic waves during large earthquakes is the most central element of earthquake hazard. Measures of radiated energy ER and estimates of stress drop Ds are signature of earthquake dynamic processes. While most earthquakes of M > 5 are recorded on the global seismic network, seismograms are altered by wave propagation through the Earth. We use a Green's function that captures the path effects (attenuation and geometrical spreading) estimated from a simplified Earth model. P-wave arrivals are seen most cleanly at angular angular distances between 30 and 90 degrees, where the effect of the upper mantle discontinuities and core diffraction are minimized. At these distances, the takeoff angles of the P waves are relatively steep. The source pulse width of moderate-to-large shallow earthquakes is larger than arrival time difference between the direct body waves P and the depth phases (pP, sP) and destructive interferences alter the shape of the recovered source spectra and bias energy measurements. They introduce a higher apparent corner frequency and decrease the amplitude at low frequencies. We predict the shape of the spectrum, thereby solving for the focal depth and source spectrum, and construct a correction for measures of radiated energy ER. We use Madariaga (1976)'s approach to estimate Ds from corner frequencies. We focus on thrust earthquakes of magnitude 5.5 and greater shallower than 50 km. We find globally a robust and positive scaling of scaled radiated energy (ER/M0) and of stress drop that varies along strike of major subduction zones. We then look at finite source effects using the Nepal, 2015, earthquake sequence.
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