It was only a magnitude 3.5 earthquake that struck the unpopulated North Rim of Grand Canyon early on Sunday morning, but it is evidence of a major issue in assessing seismic hazards in Arizona. We have had a statewide seismic monitoring network for only the past few years. We think we can detect all magnitude 3 earthquakes and most magnitude 2.5 events anywhere in the state for the first time. The small quakes may delineate active faults, including those without surface expression, or that were not previously identified as active.
Dr. Jeri Young, who manages the seven stations that comprise the Arizona Broadband Seismic Network posted a short summary of the North Rim earthquake and concludes that we cannot determine whether it and its aftershocks occurred on the West Kaibab fault, which is a normal
fault considered to be an active fault or on the nearby Sinyala fault. This latter fault
trends NE-SW, which coincides with the trend of quake locations. [Right, red circles mark the North Rim earthquakes. Red lines are known active faults. The white line is the Sinyala fault. The green triangle is the approximate location of the ABSN's North Rim seismic station. Credit, Jeri Young, AZGS]
The Sinlaya fault was previously mapped but there was not enough evidence to demonstrate that is active. If the North Rim earthquakes did occur on the Sinlaya fault, it will confirm that we have a newly confirmed active fault that must be considered in regional seismic hazard assessments. It also reminds us that we have only a very short history of seismic activity across the entire state. How many undiscovered or under-evaluated faults cross the state that will only be found when earthquakes occur on them?
It is important that the state's seismic network continue to monitor small earthquakes in order to help provide a more comprehensive and correct characterization of our seismic hazards. AZGS receives no state or federal support for the ABSN. A one time corporate donation has run out so we are scraping up funds internally to try to keep the machines running and make the interpretations that describe our seismic environment.