Improved finite-source inversion through joint measurements of rotational and translational ground motions: a numerical study

Abstract

Abstract. With the prospects of seismic equipment being able to measure rotational ground motions in a wide frequency and amplitude range in the near future, we engage in the question of how this type of ground motion observation can be used to solve the seismic source inverse problem. In this paper, we focus on the question of whether finite-source inversion can benefit from additional observations of rotational motion. Keeping the overall number of traces constant, we compare observations from a surface seismic network with 44 three-component translational sensors (classic seismometers) with those obtained with 22 sixcomponent sensors (with additional three-component rotational motions). Synthetic seismograms are calculated for known finite-source properties. The corresponding inverse problem is posed in a probabilistic way using the Shannon information content to measure how the observations constrain the seismic source properties. We minimize the influence of the source receiver geometry around the fault by statistically analyzing six-component inversions with a random distribution of receivers. Since our previous results are achieved with a regular spacing of the receivers, we try to answer the question of whether the results are dependent on the spatial distribution of the receivers. The results show that with the six-component subnetworks, kinematic source inversions for source properties (such as rupture velocity, rise time, and slip amplitudes) are not only equally successful (even that would be beneficial because of the substantially reduced logistics installing half the sensors) but also statistically inversions for some source properties are almost always improved. This can be attributed to the fact that the (in particular vertical) gradient information is contained in the additional motion components. We compare these effects for strike-slip and normal-faulting type sources and confirm that the increase in inversion quality for kinematic source parameters is even higher for the normal fault. This indicates that the inversion benefits from the additional information provided by the horizontal rotation rates, i.e., information about the vertical displacement gradient.

Seismological Processing of Six Degree-of-Freedom Ground-Motion Data

“…The use of rotational motion measurements was also demonstrated to reduce the non-uniqueness in finite source inverse problems [83,84] and to improve the characterization of the seismic moment tensor [85]. The incorporation of rotational motions into the source inversion schemes could stabilize results with only half the amount of stations required to achieve comparable results using conventional pure translational data.…”

Fiber optic gyroscope For 6-component planetary seismology

“…Chania, Greece 9 -12 October 2018 Reinwald et al, 2016). Some of the aspects of the development (e.g.…”

“…From left to right; -illustration of planetary deformations under toroidal and spheroidal normal modes of vibration principle of single station location of quakes by using seismic surface waves 'Rx' and 'P/S' body waves -illustration of YORP and Yarkovsky effects which are critical to determine NEO orbitsThe technical developments will directly impact the emerging field of 6-Dof Earth seismology. These new sensors are currently demonstrating that they can bring a lot to the society thanks to a better understanding of various aspects of natural hazard like volcanoes and earthquakes or even tsunamis ([17] Igel et al, 2015,[36] …”

Correcting wavefield gradients for the effects of local small-scale heterogeneities

“…Therefore, to assess any ground motion completely, the six components of strain and the three components of rotation are as important as the three components of translation (Aki & Richards 2002). Collocated measurements of motions can be of great benefit for seismology because they allow us to retrieve instantaneous slowness without using travel times (Edme & Yuan 2016;Wassermann et al 2016;Bernauer et al 2012;Fichtner & Igel 2009;Igel et al 2007Igel et al , 2005Mikumo & Aki 1964), to constrain seismic source properties (Donner et al 2017(Donner et al , 2016Reinwald et al 2016;Bernauer et al 2014;Takeo & Ito 1997) and to estimate parameters like propagation direction (Igel et al 2007;Cochard et al 2006) and rupture history (Takeo & Ito 1997). Concurrent measurements of all three kinds of motions also help us remove the coupling that one motion introduces on the other, for example, horizontal rotations on translation (Aki & Richards 2002, p. 604) and vertical rotation on translation (Graizer 2009;Trifunac & Todorovska 2001).…”