There's a reason that you don't hear seismologists claiming that Hawks fans caused a 1.2-magnitude earthquake. That's because a significantly different type of seismic activity produces this fan-generated shaking.
In a "normal" earthquake, a huge chunk of rock cracks quite suddenly, and energy is released. The waves travel outward from the origin and produce shaking, sometimes quite far from the source (see How Earthquakes Work for a more detailed explanation). When seismologists gauge those natural earthquakes, they're essentially looking to measure the maximum shaking and the distance of the shaking from the source to determine how strong the earthquake is.
Of course, when it's a stadium full of fans jumping and stomping and generally flailing about without any kind of synchronization, there's no one "measurable" source to speak of. Also, keep in mind that the seismometers are located around the stadium, so the shaking and vibrating surround the equipment. Normally, you'd be measuring an earthquake by how hard the shaking is at the seismometer, considering the distance the wave traveled, and that would tell you how strong the earthquake was at the source. In this case you're simply measuring how hard the shaking is at the seismometer, period. However, the Pacific Northwest Seismic Network does make a pretty general statement that these Beast Quakes are roughly equivalent to a very small, natural earthquake [source: PNSN].
The cool thing is that they can use the fan-caused quakes to study earthquake science. Studying the shaking and vibration at the stadium can help them to understand the activity at the source of a quake, and provides a unique opportunity to test equipment and train earthquake scientists and staff.
So are the Seahawks fans going to cause the city to finally experience The Big One? Almost without a doubt, no. As seismologists point out, these little quakes are very small potatoes compared to "even a moderate earthquake on a nearby fault" [source: PNSN].
To learn a lot more about earthquakes -- both natural and man-made -- check out the next page for more seismological info than you could shake a stick at.