Search engines measure the level at which they trust Web sites. This is something I discussed recently in my Stone Temple blog post "Trust and Link Building."
I discussed a couple aspects of this, such as the fewer clicks it takes to go from a highly trusted seed page to your page the better. Then there's the reverse notion, which is the number of clicks takes to travel from a page on your site to a known spam page (the further the better).
To significantly expand on the concept, the next stage in understanding search engine trust is learning that trust is topic-specific. So if a site is trusted by the search engines as a source of info on used cars, that doesn't mean that it will be trusted as a source of info on outdoor tool sheds.
However, it's likely to be much more tightly constrained than that. The used cars site may not be trusted for the sales of new cars either.
Why Should Trust be Topic-Specific?
Becoming an expert is hard, as is becoming a credible leader in a given market space. The search engines really want to provide users with the best possible experiences, and this means that they're looking for unique and differentiated content from thought leaders in each space (or the organization with the best product, the best service, and so forth).
A Web site may have a reputation for being the best place to buy used cars, but that doesn't mean it's also the best source of new cars. Just as trust of consumers in a market is earned over time, you need to do the same with the search engines.
For example, the University of California's home page has a PageRank of 10. While PageRank doesn't correspond on a one-to-one basis with trust, any page with a PR10 according the Google Toolbar is a highly trusted page on a highly trusted site.
But why is it trusted? For several areas related to education. Even more narrowly, it's highly trusted as a source of information about the University of California.
So what happens when you get access to a professor's page or a student's personal page and manage to stuff it with all types of ads for Viagra, Cialis, and Levitra? Are you suddenly going to the dominating player for selling ED drugs? Not likely.
What Happens When You Launch a New Site Section?
If there's a close relationship between your new site section and the historical trusted aspect of the site, you'll likely pick up some traffic quite quickly. However, sites stall a bit after that. They get a little taste of the good traffic for their new section, but then it stops growing.
Over a period of time, it will remain frozen, but then if you're doing the right things (developing quality content, link building), you may see a jump in traffic. My own conjecture is that a combination of quality inbound links and time raises the trust level of the new site section.
Once you cross a trust threshold, you enable a new period of growth until you hit the next threshold. Then the cycle repeats. I've seen this behavior several times now during the development and promotion of new sections of content on existing sites.
How Can You Speed Things Along?
We already mentioned the two most important things above. Developing quality content was one of them.
While search engine crawlers can't measure content quality in a direct sense, they can understand the relevance and depth of a Web page, provided you put enough text out there for them to chew on. Also, if a new site section is really thin on content, you can send negative signals to the search engines.
The other thing you need to do? Our old friend, link building. At least some of the signals for evaluating trust are based on link analysis. Getting high quality links from high quality sites will help you establish that trust.
The above is a sandbox scenario, but applied to new content section on an existing site, it operates much the same way. You benefit from the inherent trust of the existing domain, but still need to prove it to the search engines by getting new links to the new section itself.