Contributions to modern quantification of seismic sources

Saloor, Nooshin

doi:https://doi.org/10.21985/n2-saad-z861

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Contributions to modern quantification of seismic sources

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This dissertation addresses problems in the source properties of major earthquakes. It is composed of four largely independent studies. In chapter 2, we explore the possible theoretical origin of the distance-depth correction q(, h) introduced 75 years ago by B. Gutenberg for the computation of the body wave magnitude mb, and still in use today. We synthesize a large dataset of seismograms using a modern model of P-wave velocity and attenuation, and process them through the exact algorithm mandated under present-day seismological practice, to build our own version, qSO, of the correction, and compare it to the original ones, q45 and q56, proposed by B. Gutenberg and C.F. Richter. While we can reproduce some of the large scale variations in their corrections, the origin of the small-scale details which they built into them remains mysterious. We discuss a number of possible sources of bias in the datasets used at the time, and suggest the need for a complete revision of existing mb catalogs. In chapter 3, we study the spectral amplitudes of the first two Earth radial modes excited by the Sea of Okhotsk earthquake of 24 May 2013, the largest deep event ever recorded, in the search for an isotropic component to its source. In contrast to the case of the 1994 Bolivian earthquake, we detect an implosive component equivalent to 3% of the deviatoric moment, but 14% of the lone component exciting the Earth’s radial modes. An implosive component to the source is expected in the model of transformational faulting in which deep earthquake rupture nucleates and grows upon transformation of metastable olivine to ringwoodite in the cold subducting slab. This interpretation is supported by quantitative estimates (0.9- 4m) of the thickness of the transformed shear zone, which scale favorably, relative to earthquake fault length, with the upper end of the range of laboratory results reported for ices, germanates and silicates. In chapter 4, we extend to distances beyond 80 the computation of the energy to moment slowness parameter , by defining a regional empirical correction based on recordings at distant stations for events otherwise routinely studied. In turn, this procedure allows the study of earthquakes in a similar source-station geometry, but for which the only available data are located beyond the original distance threshold, notably in the case of historical earthquakes predating the development of dense networks of short-period seismometers. This methodology is applied to the twin 1947 earthquakes off the Hikurangi coast of New Zealand for which we confirm slowness parameters characteristic of tsunami earthquakes. In addition, we identify as such the large aftershock of 21 July 1934 in the Santa Cruz Islands, which took place in the immediate vicinity of the more recent 2013 shock, which also qualifies as a tsunami earthquake. In that subduction zone, the systematic compilation of for both recent and pre-digital events shows a diversity in slowness correlating with local tectonic regimes controlled by the subduction of fossil structures. In chapter 5, we extend the slowness parameter originally introduced by Newman and Okal (1998) to intermediate and deep earthquakes. We define four depth bins featuring slightly different algorithms for the computation of . We apply this methodology to a global dataset of 598 intermediate and deep earthquakes and find a slight increase with depth in average values of which however all have intersecting one- bands narrower than their counterpart for a reference dataset of 146 shallow earthquakes. Similarly, we find no correlation between values of and focal geometry.

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