Estimation of Radiated Seismic Energy from Teleseismic Body Waves
Ryota Kiuchi
Earthquake
radiated seismic energy is a fundamental parameter for understanding source
physics. Using teleseismic waveforms, we can estimate the radiated seismic energy
for a wide range of focal mechanisms and tectonic settings. We are especially interested
in studying the apparent stress (rigidity multiplied by the ratio of radiated
energy to seismic moment) of strike-slip earthquakes in the oceanic
lithosphere, for which there are often high reported values (Choy and McGarr,
2002). Estimates of radiated seismic energy from teleseismic P waves can be
unstable, because take-off angles from the source are often close to nodes in
the focal mechanisms, which can cause large variations in the estimated values
of the apparent stress.
For
strike-slip events with where many data are close to nodes in the focal
mechanisms, this is a large and often unstable correction for the radiation
pattern. In most studies, nodal arrivals are not used because of this problem.
We use an improved method which takes into account
uncertainties of values for the strike, dip and rake for nodal arrivals. This
enables us to use more stations for the estimate of radiated energy. Also, we
use station corrections determined from a selected set of well determined
events.
In
this study, we
use P waves for the teleseismic waveform, because of the interference with
other phases for S waves. We use data recorded by teleseismic stations
(epicentral distances of 30 to 90 deg) recorded on
the GSN network and focal mechanisms published by USGS and Global CMT Project.
Our
energy estimate for 171 recent large earthquakes (>Mw 7.0,
between 2000 and September, 2013) are studies for scaling with moment, depth
dependence and focal mechanism differences. We see no
strong dependence with seismic moment. Deeper earthquakes (>70 km depth)
have slightly higher values of apparents stress compared to shallow events. We
find that the strike-slip earthquakes have apparent stress values that are 5 to
6 times higher than dip-slip earthquakes with the oceanic events having almost
same values with continental events. In addition, using our
improved methods, we can estimate the apparent stresses for strike-slip
earthquakes with more reliability, since the error of radiated seismic energies
becomes smaller.