Hypothesis: male researchers ask more questions because: a) they need to show off; b) they are more socially isolated and need interactions (not joking); c) women have a distaste for preening.
Intervention for a) - take a few minutes at the beginning of each seminar to let audience members share any relevant work they've previously been engaged in. Perhaps start seminars 5 minutes early for any who want to do this.
Intervention for b) and c) - ask seminar attendees to pre-submit one- or two- sentence questions to the speaker, so that he/she can choose to address them during the talk, if he/she wants do.
Both interventions could have the knock-on effect of encouraging a more personalized engagement between the speaker and the audience.