Small earthquake swarm detected near Roosevelt, Arizona: Feb. 25, 2022

/ 28 February 2022 / 0 Comments

Beginning February 12 of this year a small swarm of earthquakes rattled the southern side of Theodore Roosevelt Lake. Although earthquakes are commonly not thought to occur in southern Arizona, the area surrounding Roosevelt, Arizona has experienced many historical earthquakes (Cover image - caption below). Beginning in 2008, the Arizona Geological Survey began a statewide effort to monitor earthquakes using 8 broadband seismometers. In 2017, 6 new stations were installed, including one near Roosevelt, Arizona, and two additional stations were added to the seismic network in 2018 (Figure 2). With increasing station coverage, more of Arizona’s smaller earthquakes are being detected and better located. The latest swarm detected near Roosevelt Lake included 3 events (summary table below) with M 2.5 or greater and 10s of M1.0-1.5s that were too small to be located accurately (Figure 2).

Date	Location	Size M	Depth -km
02-12-2022	33.621/-111.118	2.5	5.4
02-15-2022	33.656/-111.106	2.5	9.8
02-25-2022	33.655/-111.128	2.6	5.0

Helicord diagram — Figure 2: TONTO station helicorder showing the most recent earthquake within the larger yellow circle, bottom of image, and small earthquakes circled in yellow. The smaller quakes were within the Md 1.0 to 1.4 range in size.

The Tonto Basin is located within the Transition Zone province of Arizona and lies between the Mazatzal Mountains to the southwest and the Sierra Ancha Mountains to the northeast. The Basin formed from faulting that occurred during the middle and late Miocene, roughly 12 to 5 million years ago (Richard, 1999). There are no known active faults within the Tonto Basin that could be directly related to the earthquakes recorded there. In 2007, 62 earthquakes occurred near the Lake with an average depth of about 9km (Lockridge et al., 2012). Although reservoir filling has been known trigger seismicity due to the increased weight of water and hydrostatic pressure that may lubrication faults, examination of those seismic events did not appear to correlate with fluctuations in reservoir levels. Subsequently, ML 3.1 and 3.4 earthquakes occurred near Roosevelt Lake in June 2010 and September 2014, respectively. Continued monitoring may improve our understanding of the earthquakes in the Tonto Basin, and other parts of Arizona too.

AZGS broadband seismic stations — Figure 3: Statewide broadband seismic network (general location shown as yellow stars). The Arizona Geological Survey operates and maintains 15 stations that contribute to the National Earthquake Information Center's catalogue of earthquakes for local and regional earthquakes.

REFERENCES:
Lockridge, J.S., Fouch, M.J., Arrowsmith, J R., and Linkimer, L., 2012. Analysis of seismicic activity near Theodore Roosevelt Dam, Arizona, during the occupation of the EarthScope/USArray Transportable Array, Seis. Res. Letters, 83 (6): 1014-1022.

Richard, S., 1999. Brief overview of the geology and mineral resources of the Tonto Basin, Gila County, Arizona, Arizona Geological Survey Open-File Report 99-11.

Cover image: Past earthquakes shown as red-orange circles and recent quakes shown as red stars in the area surrounding the Tonto Basin/Theodore Roosevelt Lake, Arizona.

Posted by Jeri Y. Ben-Horin (2/27/2022)