Introduction
A magnitude 5.3 earthquake occurred near Duncan, AZ at approximately 10 pm on June 28th, 2014, (Figure 1). The earthquake shaking was strong and caused moderate damage in the Duncan area; it was felt throughout southeastern Arizona (Figure 2) and was recorded by seismometers around the globe. The earthquake began about 7 km (4.4 miles) below the surface, and we have found no evidence that the earthquake ruptured the earth’s surface. Fairly minor damage was reported in Duncan and Safford; cracks developed in concrete structures and at least one home foundation, several trailer homes were displaced, and glassware flew out of cupboards. This was the largest earthquake to occur in southeastern Arizona – southwestern New Mexico in 75 years, and it serves as a reminder that Arizona does indeed have earthquakes and earthquake hazards.
Aftershocks
In the days immediately following the main shock, the Arizona Broadband Seismic Network (ABSN), operated by the AZGS, recorded 156 aftershocks in the M 1.6 to M 3.6 range (Table 1); however, the locations of the smaller events are uncertain because they were not recorded by many seismic stations. Since June 28th, thirty aftershocks ranging from M 2.6 to M4.1 have been reported by the U.S. Geological Survey (Figure 2); all of the earthquakes occurred at depths of approximately 5 to10 km (3 to 6 miles). The largest aftershock, an estimated M 4.1, occurred on July 11th. Some residents felt strong shaking while light damage was reported.
|
DATE |
Md 0-1.9 |
Md 2.0-2.9 |
Md 3.0-3.9 |
TOTAL |
|
6/29/2014 |
21 |
40 |
6 |
67 |
|
6/30/2014 |
35 |
20 |
1 |
56 |
|
7/1/2014 |
20 |
12 |
1 |
33 |
Table 1: Date, size ranges, number and TOTAL recorded aftershocks following the Mw = 5.3 earthquake near the town of Duncan. Md stands for duration-magnitude, a simpler but less accurate estimate of earthquake size.
Historical Seismicity of SE AZ?
A fair number of Quaternary faults (active in the past 2 million years or less) are currently known to exist in southeastern Arizona, and the area has experienced historic earthquakes. An earthquake sequence that occurred in 1938-39 in the New Mexico – Arizona border region is particularly interesting, as it included an M 5.5 earthquake and numerous sizable aftershocks (Table 2). Seismic instrumentation in the United States was very limited at that time, so the earthquakes are likely not located accurately and the record of aftershocks is far from complete.
Nonetheless, there clearly was elevated earthquake activity in the region after the main event, including three M 4.5 earthquakes in the Clifton and Duncan areas. Much more recently, a burst of earthquake activity ranging from M 2.5-4.1 occurred in 2010 in the mountains north of Clifton-Morenci.
Size and location of historical earthquake events (i.e., events that predate deployed seismometers) are often estimated by using historical accounts from people that felt the earthquakes, thus creating a bias towards locating events closer to more populated areas. In addition, mine blasting and subsequent rock collapse is a common occurrence in the Safford/Clifton/Morenci area and can be difficult to distinguish from real earthquakes; therefore, many tremors in the last several decades have been culled from the official earthquake record in an effort to make sure that the record only contains natural earthquakes. Increased earthquake monitoring in the area could improve the size and locations of the events, and help differentiate the natural earthquakes from mining activities. This effort could provide a more accurate seismicity rate for the region and in turn, can help us better estimate seismic hazards.
|
MAGNITUDE |
DATE |
General LOCATION |
|
5.5 |
09-17-1938 |
NE of Buckhorn, NM. |
|
4.5 |
09-19-1938 |
SE of Glenwood, NM. |
|
4.5 |
09-29-1938 |
Clifton, AZ. |
|
4.5 |
06-03-1939 |
Franklin, AZ. |
|
5.3 |
06-28-2014 |
Duncan, AZ. |
Table 2. Magnitude, date and location of seismic events of the 1938-1939 earthquake sequence of eastern Arizona.
June-July 2014 Duncan, AZ Earthquakes and faults
Interestingly, the mainshock and aftershock sequence occurred along two distinct faults. The M 5.3 event was estimated by the USGS to have occurred on a fault that had experienced pulling or tensional forces (Figure 3), while the M 4.1 was estimated to have occurred on a fault with sliding or translational forces (Figure 4). The direction of slip on a fault, and the fault’s orientation in space is referred to as a focal mechanism. The beach ball-like images, or focal mechanisms, are visual representations of the potential faults that generated the M 5.3 and 4.1, respectively (calculated by the U.S. Geological Survey). You can see that they clearly show two distinct patterns. These patterns are related to where the primary wave first motions are away from (shaded) and toward (unshaded) the source. The dots show the axes of maximum compressional (black) and maximum extensional (white) strain that resulted from the earthquakes.
The focal mechanism for the M 5.3 event indicates NW-SE extension, while the focal mechanism for the M 4.1 event indicates more NS-oriented extension. Because the faults did not break the ground surface during the earthquakes, we cannot tell which of the gray, curved lines on the focal mechanisms represent the actual faults. Additional geologic mapping in the area of the earthquakes may reveal a pattern of regional fault orientations, providing insight into which fault orientations are likely related to the larger, recent earthquakes.
Temporary Seismic Network
Following the mainshock, the Arizona Geological Survey deployed 6 temporary seismometers to more accurately locate the aftershocks in the region of the Mw (moment magnitude) 5.3 earthquake (Figures 1 and 3). The station locations were based on proximity to the mainshock, land ownership and vehicle accessibility (Fig. 5). The temporary seismic stations were not all deployed simultaneously, but as instruments became available (Fig. 6). The stations have recorded hundreds of earthquakes, whose magnitudes and locations are currently being analyzed. Because there were so many events, and the seismometers were recording at a high resolution (250 samples per second), there are volumes of data and analysis will take months.
Accurate aftershock locations may elucidate which fault plane solutions shown in the focal mechanisms may be viable, as well as help us determine where the faults are located. This information can be used to gain understanding on how faults in the area interact, as well as the current crustal strain in the region.
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