Teleseismic time functions for large, shallow subduction zone earthquakes

Broadband vertical P-wave records are analyzed from 63 of the largest shallow subduction zone earthquakes which have occurred in the circum-Pacific in the last 45 yr. Most of the records studied come from a common instrument, the Pasadena, California, Benioff 1-90 seismometer. Propagation and instrument effects are deconvolved from the P-wave records using a damped least-squares inversion to obtain the teleseismic source time function. The inversion has the additional constraint that the time function be positive everywhere. The period band over which the time functions are considered reliable is from 2.5 to 50 sec. Fourier displacement amplitude spectra computed for each of the 1-90 P-wave trains indicate spectral slopes measured between 2 and 50 sec of ω^−1.0 to ω^−2.25 with an average value of ω^−1.5. These values assume an average attenuation of t^* = 1.0. The seismic moments derived from the P-wave time functions compare well with other published values for earthquakes having moments smaller than 2.5 × 10²⁸ dyne-cm (M_w = 8.2). Because the 1-90 seismometer has little response at very long periods, this technique underestimates the moments of the very largest events. The time functions are characterized using five parameters: (1) spectral slope between 2 and 50 sec; (2) roughness of the time function; (3) multiplicity of sources; (4) pulse widths of individual sources; and (5) overall signal duration. The 63 earthquakes studied come from 15 subduction zones with a wide range in the ages of subducted lithosphere, convergence rates, and maximum size of earthquakes. Comparing the time function parameters with age, rate, and M_w of the subduction zone does not yield obvious global trends. However, most of the subduction zones do behave characteristically and can be grouped accordingly.