What causes ringing effect when using corner filter?
Filtering is always a convolution of the signal. What you are doing in the frequency domain will also be manifested in the time domain. A sharp edge in frequency domain will create a long convolution in time domain. 

Ormsby (or trapezoidal) filters cause strong ringing if the slope is steep due to the Gibbs effect. To reduce the ringing, make the slopes more gentle (eg 6, 12, 45, 60) or try to taper the filter slopes. 

However, it appears that you are using a strong AGC. Check if the ringing is actually a problem before smoothening the filter. 

Some of the ringings will also disappear if you intend to F/K filter the direct waves.