Table 2 q_EM at different altitudes

B_z range for most earthquakes (nT)

5 to 15

− 10 to 0

− 5 to 5

− 5 to − 3

− 1.5 to − 0.5

q_EM for B_z

3.6

2.7

2.1

4.7

3.3

B_z horizontal gradient range for most earthquakes (nT/km)

0.55–0.65

0.225–0.275

0.09–0.11

0.015–0.025

0.007–0.009

q_EM for B_z horizontal gradient

4.1

3.8

4.2

2.7

3.2