Understanding the frequency scale of a spectrogram
I think what you're saying is that you have a linear, time invariant (LTI) system model for both the propagation medium and the response of the measurement instrument itself. Given that you know the (at least approximate) magnitude response of the instrument, you would like to best estimate what the actual signal at the instrument's input looked like.

This is similar to the process of equalization used in communications systems. Since you have some knowledge of the system (in this case characterized by the impulse response $i(t)$ or the frequency response plot that you showed), you can attempt to undo the effects on the signal that are imposed by the measurement instrument. More generally, this type of operation is known as deconvolution: given an output of a convolution and optionally knowledge of one of the inputs to the convolution, one would like to calculate the other input. In this case, one of the convolution inputs is $i(t)$ and the output is the measurements reported by your instrument. 

It appears that you don't know the exact impulse response of the measurement device, only its magnitude response. This isn't enough information in itself to execute the deconvolution, so you'll need to make some assumptions about the phase response of the instrument in order to move forward with that approach. This is a very powerful technique, however; as the Wikipedia article notes, the approach can be used to compensate for the effects of the seismic propagation medium also (since it's also modeled as a convolution with an impulse response), potentially allowing you to gain more insight as to the original seismic event.

Edit: And, as to your questions about how you're interpreting the plot, your understanding seems correct. The system has a highpass characteristic with a flat passband starting at approximately $0.02\ \text{Hz}$, with the magnitude response decreasing by a factor of 10 per decade of frequency below $0.02\ \text{Hz}$. This could be modeled well as a single-pole highpass filter.