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Arizona Seismic Update – January - July 2013

Article Author(s): 

Jeri Young

The Arizona Broadband Seismic Network documented over 50 earthquakes in Arizona from January to July 2013 (Figure 1). The quakes were mostly located in northern Arizona and were at depths ranging from 1.4 to 26 km (0.9 to 16.25 miles depth). Because our resources only permit locating quakes at Duration Magnitude (Md) ~2.0 and larger, the average size of earthquakes recently recorded reflect this threshold and are Md =2.0. In addition, there are several “blind spots” in the state where additional station coverage is necessary to identify and locate Md 2.0s and greater. Some of these blinds spots include north-northeastern Arizona on the Dine (Navajo) Reservation, and in the south-central and southeastern parts of the state. Seismic blind spots also include areas surrounding and including mines, such as the Morenci Mine near Morenci, or Peabody’s Kayenta coal mine near Kayenta.

Figure 1: A) Epicenter locations of Arizona earthquakes from January through July of 2013.

Figure 1: A) Epicenter locations of Arizona earthquakes from January through July of 2013.

Figure 1: B) Plot of depths of earthquakes that occurred from January to the end of July, 2013. The median depth is around 9.6 km (6 miles). Earthquakes deeper than 20 km (12.4) miles are not uncommon on the Colorado Plateau where crustal thickness is the greatest.Figure 1: B) Plot of depths of earthquakes that occurred from January to the end of July, 2013. The median depth is around 9.6 km (6 miles). Earthquakes deeper than 20 km (12.4) miles are not uncommon on the Colorado Plateau where crustal thickness is the greatest.

There are over 200 operational hard rock and industrial mines in Arizona, many of which use blasting technologies to break up rock (Figure 2). Blasting associated with mining activity occurs frequently in Arizona and can be confused for real earthquakes. If a small magnitude earthquake occurs near an active mine, it goes unrecorded because mine blasts and real earthquakes are too similar to differentiate. This keeps the Arizona earthquake catalog free from mine blasts. With additional seismic stations, and resources, these areas could potentially be more accurately monitored for seismic activity.

Figure 2: Locations of active mines in Arizona (black triangles). Approximate footprints of seismic monitoring blind spot areas are shown as red circles (related to mining) and blue squares (related to poor station coverage). Arizona county boundaries (light gray) and major roadways (purple) are shown, too.

Figure 2: Locations of active mines in Arizona (black triangles). Approximate footprints of seismic monitoring blind spot areas are shown as red circles (related to mining) and blue squares (related to poor station coverage). Arizona county boundaries (light gray) and major roadways (purple) are shown, too.

The largest earthquake recorded in Arizona in the last 7 months was an Mw = 3.5 located on the North Rim of Grand Canyon (the magnitude scale for this event is a moment magnitude (Mw) calculation made by the U.S. Geological Survey). Several aftershocks followed the North Rim event, but only Md > 2.0 events were located (Figure 3: Map of North Rim events and faults). The Mw 3.5 event and series of aftershocks (see table below) are interesting because they occurred adjacent to two known faults, the West Kaibab and Sinyala faults. The Sinyala fault has a young geomorphic expression, but has not been mapped as an active Quaternary fault because offsets along the fault are small. The West Kaibab fault system is thought to be an active fault with a low slip rate (< 0.2mm per year). The National Earthquake Information Center computed focal mechanism for the mainshock revealed that movement occurred on either a mostly east-west striking fault, or a more northerly striking fault with a left-lateral normal faulting component (Figure 4). The strike of the Kaibab fault system is generally N-S; however, the mainshock and aftershocks appear to collectively trend parallel to the strike of the Sinyala fault (to the northeast). Given the uncertainties in quake locations due to a lack of station coverage and the fact that quakes smaller than M 6.0 rarely rupture the ground-surface, it is not possible to precisely determine what fault the quakes actually occurred on. The mainshock was felt by several people in northern Arizona with no reports of damage or injuries.

Recorded Earthquakes near M 2.0 or above associated with the North Rim Event of 07/07/13

Date

Time (MST)

Location (Lat/Long)

Size

Depth (km)

07/07/2013

01:38:59

36.456/-112.579

3.5 Mw

5

07/07/2013

01:51:28

36.506/-112.560

2.3 Md

3

07/07/2013

04:06:29

36.524/-112.4265

1.8 Md

20

07/18/2013

23:43:00

36.529/-112.4651

2.0 Md

7

Figure 3: Red circles are the locations of the mainshock and larger aftershocks that occurred during the month of July 2013. A Md 2.0 quake occurred northwest of the mainshock on the same day (07/07/13). The red lines represent mapped, active faults, and the white line trending to the NE-SW is the Sinyala fault. The green triangle is the approximate location of AZGS’ broadband North Rim seismic station. Temblors were centered between Fredonia(shown as yellow teardrop), Jacobs Lake (green teardrop), and Havasupai(turquoise teardrop).

Figure 3: Red circles are the locations of the mainshock and larger aftershocks that occurred during the month of July 2013. A Md 2.0 quake occurred northwest of the mainshock on the same day (07/07/13). The red lines represent mapped, active faults, and the white line trending to the NE-SW is the Sinyala fault. The green triangle is the approximate location of AZGS’ broadband North Rim seismic station. Temblors were centered between Fredonia (shown as yellow teardrop), Jacobs Lake (green teardrop), and Havasupai (turquoise teardrop).

Plane

Strike of fault

Dip

Rake

Nodal Plane 1

106°

65°

-136°

Nodal Plane 2

354°

51°

-33°

Figure 4: The National Earthquake Information Center’s focal mechanism for the Mw 3.5 earthquake. The white areas represent quadrants in which the P-wave first motions are toward the source. The black dot represents the axis of maximum compressional strain and the white dot represents the axis of extensional strain. Nodal planes (shown as lines crossing the image) represent the two possible fault orientations. See the USGS website for further explanation (http://earthquake.usgs.gov/earthquakes/eventpage/terms.php).

Figure 4: The National Earthquake Information Center’s focal mechanism for the Mw 3.5 earthquake. The white areas represent quadrants in which the P-wave first motions are toward the source. The black dot represents the axis of maximum compressional strain and the white dot represents the axis of extensional strain. Nodal planes (shown as lines crossing the image) represent the two possible fault orientations. See the USGS website for further explanation (http://earthquake.usgs.gov/earthquakes/eventpage/terms.php).

In other seismic news, the Arizona Earthquake Information Center (AEIC) at Northern Arizona University has been upgrading the AEIC’s analog network. This network consists of 6 operational analog seismometers that transmit their interpretations of ground movement back to a computer at NAU via radio waves. The Arizona Geological Survey assisted the AEIC in installing a new station north of the town of Williams last June. The new station is called Three Sisters and is the youngest in the analog array.

Research Geologist
Arizona Geological Survey

 

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