Earthquake Swarm along the Washington Fault Zone, Arizona
Northern’ Arizona’s Washington Fault Zone (WFZ) has been reminding us of its potential for earthquake hazards over the past 3 weeks. Since the later part of December, 20 measureable earthquakes have occurred along the western side of the fault zone, near the AZ-UT border, south of St. George, UT. The WFZ is an active Quaternary normal fault system with mostly west-facing scarps and is made up of three main segments, in all totaling nearly 100km in length. The recent quakes have been occurring near the “Mokaac” and “Northern” segments of the fault zone. The largest quakes measured M3.1 and M3.0, with two events in the mid-M2.0 range and the remainder of events in the M1.0 range (Table 1). Depths of the earthquakes are relatively shallow, with all occurring at less than 11 km (7 miles) depth (Table 1). Errors associated with depth estimates are large since their calculations are dependent on coarse crustal models that assign how fast seismic waves move as a function of depth. None of these events are large enough to generate surface ruptures; surface rupturing is typically associated with earthquakes that are M6 or greater.
Figure 1: Map showing location of earthquake swarm, shown as yellow, white and orange circles that scale with size. The two white circles in the southern portion of the swarm represent the M3.1 and M3.0 quakes. The blue arrows point to the Washington Fault Zone, shown as multiple red lines.
Table 1. Northern Arizona earthquake swarm data.
According to a study completed by the Utah Geological Survey in 2011-2012, the northern Washington fault has had at least two ground-ruptures within the last 7,000 years or so, with the last quake occurring about 1,000 years ago (Lund et. al., 2015). Several fault systems have strong surface expression and have been active in the past 150,000 years: Grand Wash Fault, Hurricane Fault and many subsidiary faults, and the Sevier-Toroweap Fault (Figure 2). These long faults that have large bedrock scarps but spotty evidence of Quaternary activity are likely more active than we realize - it is just that Quaternary deposits are very limited along these fault zones.
Seismic monitoring across the state, especially in the northern part of the state has allowed the Arizona Geological Survey to monitor seismic activity in the areas near these faults, among other active faults throughout the state. It is important to note that the number of small earthquakes recorded in this swarm were detected due to the efforts of three regional seismic networks: Arizona Broadband Seismic Network, University of Utah Seismic Stations, and the Nevada Seismological Lab. This part of the state lies within the overlapping monitoring efforts for these three networks. In other parts of Arizona, small to moderate earthquakes can be detected and located fairly accurately now due to the network of seismic stations in the Arizona Broadband Seismic Network.
Figure 2: Map showing major fault zones in northwestern Arizona: WFZ -Washington Fault Zone, HF - Hurricane Fault, STF - Sevier-Toroweap Fault, and GWF - Grand Wash Fault. There are many other active faults in northern Arizona, some of which are labeled on this figure with green, yellow and purple lines. Yellow lines indicate faults with known activity within the last 10,000 years, green lines indicate known activity within the last 750,000 years and purple lines indicate faults with known activity within the last 2.5 million years.
Lund, W.R., 2015, Introduction to the Washington fault zone, in Lund, W.R., editor, 2015, Surficial geologic mapping and paleoseismic investigations of the Washington fault zone, Washington County, Utah, and Mohave County, Arizona—Paleoseismology of Utah, Volume 27: Utah Geological Survey Miscellaneous Publication 15-6, p. 1–8, CD.
Jeri Y. Ben-Horin, Arizona Geological Survey, University of Arizona
January 16, 2019