arizona seismicity summer 2009
an aisn update
MIMI DIAZ, ARIZONA GEOLOGICAL SURVEY
The Arizona Integrated Seismic Network (AISN) has had a busy few months since its inception, both in terms of logistics and in seismic activity. The summer kicked off with a magnitude 3.1 earthquake on May 9th about 10 miles north of Black Canyon City (about 45 minutes north of Phoenix off I-17). While the Modefied Mercalli intensity (MMI) for the quake was fairly minor, about a IV, people reported feeling the earthquake and even going so far as to call in to television stations. The last known seismic activity in this area occurred in the 1970s, with earthquakes of equal or smaller in magnitude than the May 9th event. Interestingly, there has yet to be an active fault system identified that could be responsible for these events. Seismometers in all three partner networks across the state picked up the 9 May event.
Arizona got a somewhat bigger shaking on August 3 when the M6.9 Gulf of California (GCA) earthquake occurred in the ocean floor near Baja California. The earthquake effects were most noticed by people in high-rise buildings as the top floors swayed during the event. People in Phoenix (about 500 miles north of the epicenter) reported seeing suspended objects swinging and experiencing vertigo and motion sickness. One building downtown even evacuated in response to the shaking.
With the seismic data rolling in, the new network is proving to be a good investment. The high-quality data, combined with increased data density and an enhanced Arizona catalogue of seismic activity, will allow us to refine and develop seismic hazard models for the state.
The GCA event highlights the importance of acquiring and deploying new, enhanced tools and techniques to more accurately delineate actual seismic risk impacting Arizona, even if the source originates outside our borders. Buildings swaying in downtown Phoenix caught a lot of people’s attention, and caught them by surprise. The Phoenix metro area experienced an average Modified Mercalli Intensity (MMI) of I-II for the 2009 GCA event. (In 1887, an M7.2 earthquake on the Pitaycachi fault near Cochise County was felt strongly in Phoenix-- the reported MMI was V to VI ; in Tucson the MMI value was VII.)
The data recorded by the seismometers allow scientists to identify important parameters and patterns of Arizona seismicity, which in turn are the basis for seismic hazard modeling, interpretation, and analyses.
The logistics of running a network is challenging and expenses mount quickly; needs include specialized hardware and software to handle the data, trained personnel to maintain and service the stations, and associated administrative costs.
The primary goal for AISN is to refine the seismic hazard analysis for the state, and to coordinate with other responsible agencies on data circulation in the form of updated hazard mitigation plans. AZGS is coordinating with the Arizona Division of Emergency Management and fourteen Arizona counties to this end.