searchTwitter来自指定位置的所有推文

I want to generate stats for twitter by location, e.g. number of tweets per city by date. If I use the searchTwitter() function in the twitteR package I can add a geocode location and a since time paramater but I must also specify a search string. Is there any way I can just search for tweets by geocode location and date with no search string? What I am currently using is something like thisI want to generate stats for twitter by locatio