Broadband source spectrum, seismic energy, and stress drop of the 1989 MacQuarie Ridge Earthquake

I compute the broadband source spectrum at periods from 1 to 50 seconds using teleseismic P body waves of the May 23, 1989 Macquarie Ridge earthquake (M w = 8.1) recorded by the GDSN, GEOSCOPE, and IDA networks. The average source spectrum is obtained by windowing, tapering, and Fourier‐transforming P waves, removing from the spectra the effects of attenuation, geometrical spreading, and radiation pattern, and averaging logarithmically over the stations. The source spectrum for the strike‐slip Macquarie Ridge earthquake is higher than an average source spectrum of seven recent large earthquakes (scaled to be comparable to a M w = 8.1 earthquake) by a factor of 2 to 3 at periods of 1 to 20 seconds. These other earthquakes were underthrusting events in subduction zones. Using Haskell's formulation assuming a point source with no directivity, I estimate the seismically radiated energy from the source spectrum by integrating the square of the source spectrum in velocity and scaling the result. The seismic energy thus estimated for the Macquarie Ridge earthquake is 3 to 8 ×10 23 ergs. An Orowan stress drop can be obtained from the seismic energy and moment. The Orowan stress drop for the Macquarie Ridge earthquake is about 20 to 50 bars, much higher than similarly determined stress drops of other recent large earthquakes. There is a correlation between the Orowan stress drops and time since the last earthquake of comparable or larger magnitude for seven recent large earthquakes. This correlation suggests that a healing process operates that may control the mechanical strength of the fault and is important on time scales of tens to hundreds of years.