Final answer:
When the sum of earthquake moments over a time interval is less than the plate-margin moment, it means that the energy released by the earthquakes during that time period is smaller than the total energy accumulated along the plate boundaries. This can happen when there is a longer interval between earthquakes, allowing more stress to build up in the crust. When the accumulated stress is finally released, it results in a larger earthquake event.
Step-by-step explanation:
When the sum of earthquake moments over a time interval is less than the plate-margin moment, it means that the energy released by the earthquakes during that time period is smaller than the total energy accumulated along the plate boundaries. This can happen when there is a longer interval between earthquakes, allowing more stress to build up in the crust. When the accumulated stress is finally released, it results in a larger earthquake event.
An example of this is the San Andreas Fault in California. The central part of the fault slips every 25 years, moving about 1 meter each time. However, the average interval between major earthquakes in the Los Angeles region is about 150 years, with an average motion of about 7 meters.
So, if the sum of earthquake moments is less than the plate-margin moment, it indicates that there is still significant stress and energy stored along the plate boundaries, which could be released in a future, potentially larger earthquake event.