inqury with the lsim command

I am doing the time-history analysis of structure under earthquake motions. The time step of the recorded motion is 0.001. When I use lsim function to conduct the direct integration, and it is noted with the following warning. ---- Warning: The input signal is undersampled. Use a sampling period smaller than 0.00049. > In warning at 26 In ssdata.lsim>LocalCheckSampling at 142 In ssdata.lsim at 43 In lti.lsim at 102 In msup at 92 ----

so how i can solve this problem? reduce the time step? which i believe is not a good option.

[David Zhang]

any suggestions on this warning?