Early electromagnetic waves from earthquake rupturing: II. validation and numerical experiments

We validate the branch-cut integration (BCI) technique presented in the companion paper. The numerical result shows that the early electromagnetic (EM) signal calculated by the BCI method is in good agreement with that in the full waveform calculated by the real-axis integration method. We further find that to calculate the early EM signal only the integrals along the vertical branch cuts that are around the k 0 and kem branch points are needed, whereas neither the integrals along the vertical branch cuts around the Pf(P), S and Ps branch points nor the residues of the poles are necessary. We conduct numerical experiments to analyse the early EM signal generated by an earthquake in a porous half-space, including its component analysis, sensitivities to the conductivity, viscosity and recording depth, and radiation pattern. The component analysis shows that the total early EM (em Total) signal is not a single wave but a combination of three kinds of EM waves, namely, the direct em d wave radiated from the source, the reflected em r waves converted from the em d wave, the direct P and S waves at the free surface and the critically refracted EM0 waves which are also converted from the em d wave, the direct P and S waves at the free surface. Three pulses, namely, the em d-, P- and S-converted pulses are identified in the em Total signal according to their different arrival times. The em d-converted pulse arrives immediately after the occurrence of the earthquake and it is a combination of the em d wave, the em r and EM0 waves converted from the em d wave at the free surface. The P-converted (S-converted, respectively) pulse owns an arrival time approximately equal to that spent by the P wave (S wave, respectively) travelling from the hypocentre to the epicentre, and it is a combination of the em r and EM0 waves converted from the P wave (S wave, respectively) at the free surface. The P-converted pulse is usually weaker than the S- and em d-converted pulses in the electrogram and is always invisible in the magnetogram. Our simulation of an M6 earthquake shows that the electric field of the early EM signal at a receiver with an epicentral distance of 100 km is on the order of 0.1 μVm−1 for a low conductivity on the order of 10− 5 S m− 1, whereas it is on the order of 0.0001μVm−1 for a high conductivity on the order of 10− 2 S m− 1. The magnetic field of the early EM signal keeps on the order of 0.1 pT for either a low or high conductivity. The insensitivity of t