Sunday, November 15, 2015

Earthquakes threaten Arizona (and that's not all)

(Photo: Bryan MacFarlane/Arizona Geological Survey)
by Lee Allison:

Geologist: Arizona can (and has) fallen victim to plenty of geologic hazards. Here's how we're trying to stop them from causing major damage.

Arizona is subject to a wide array of geologic hazards.

That includes flooding, debris flows or mudslides, landslides, gradual land sinking (called subsidence) and earth fissures, swelling and collapsing soils, sinkholes and -- yes -- earthquakes.

Menace 1: Earthquakes

On Nov. 1, three earthquakes near Black Canyon City with magnitudes of 3.2, 4.0, and 4.1 rattled central Arizona -- home to more than 4.5 million people, or roughly three-quarters of the state’s population.

Because Arizona has far fewer earthquakes than California, there is a widespread sense that they are not a problem here. Yet, in 2014, hundreds of earthquakes were detected in Arizona. Most were too small to be felt or occurred in remote areas. The largest events of the year were a magnitude 5.3 earthquake near Duncan in far southeastern Arizona and a magnitude 4.7 quake in Oak Creek Canyon between Sedona and Flagstaff.

These earthquakes were felt widely in Arizona with dozens of aftershocks. The Duncan earthquake could have caused substantial damage had it occurred in a more densely populated area.

Earthquakes with magnitudes around 6.0 occurred in multiple locations in northern Arizona in the early 1900s. These remind us of the possibility for larger, more damaging earthquakes to occur. Yuma, Prescott and Flagstaff are particularly prone to ground shaking from earthquakes. We calculate that the Lake Mary fault near Flagstaff could generate a magnitude 7.0 earthquake, which would release roughly 30,000 times as much energy as the largest of the Black Canyon City quakes.

Earthquakes outside Arizona also have the potential to affect us. The magnitude 7.5 Sonoran Mexico earthquake of 1887, which occurred just south of the Arizona-New Mexico border, demonstrates that large earthquakes occur in this region. Should an event like this happen today, it would cause substantial  damage and injuries throughout southern Arizona.

A magnitude 6.9 earthquake in the Gulf of California in July 2009 caused high-rise buildings to shake from south of Tucson to Phoenix. A 7.2 M earthquake in Baja California, Mexico was felt more widely in Arizona.

Cascading events associated with large earthquakes may pose significant risk to Arizonans, including infrastructure collapse, dam failure, fire, liquefaction (when the strength of soil fails), seiche (waves in enclosed bodies of water), landslides and rock falls.

Menace 2: Landslides

Earthquakes, however, are just one of our geologic hazards that generate risk to life and property.  Landslides, post-fire debris flows and floods are much more common hazards. Between 1966 and 2011, natural hazards in Arizona have cost $99.7 million (inflation not counted) in state funds and $458.1 million in federal funds.

The 2008 Easter weekend landslide that closed State Route 87 between Phoenix and Payson for six days reactivated a small part of a massive ancient landslide complex in the hills above the highway that had not been previously identified as a potential hazard. Reconnaissance mapping after the event found a series of large, potentially dangerous landslides that may pose continued threats to the highway.

More than $18 million was spent on this one relatively small event, with much more work needed to monitor and mitigate the hazard. The Bitter Springs landslide of February 2013 destroyed a section of U.S. 89 south of Page. Costs to build buttresses and reconstruct the highway reached more than $35 million, plus $28 million to pave Navajo Route 20 as an alternate route.

This also caused substantial negative economic impacts to residents, travelers and the local communities due to the extended highway closure.

Menace 3: Wildfires

Severe wildfires denude mountain sides and damage soils leading to a greatly increased risk of floods and debris flows. The seven largest historical fires in Arizona have all happened since 2002, burning nearly two million acres and putting numerous communities, roads and infrastructure at risk; half of the acres burned were in 2011.

The threat of prolonged drought persists in Arizona, greatly increasing the chance of large wildfires.

Menace 4: Floods

The July and August 2010 floods in Flagstaff caught residents and county officials unprepared. The Schultz fire in June 2010 denuded steep slopes above the Timberline and Doney Park subdivisions so that 25-year rains produced a thousand-year flood/erosion event involving debris flows on the mountain and broad-sheet flooding on the alluvial fan on which homes were built.

Numerous areas of Arizona could be subject to similar flood conditions in the aftermath of wildfires.

Floods in Havasu Creek in August 2008 required emergency rescue of 600 residents and campers from the popular tourist locale, which made national and international news. Subsequent floods in 2010, while not as devastating, further damaged trails and facilities, wiped out the economic base of the tribe and put hundreds of tourists at risk.

Menace 5: Fissures

We have seen earth fissures, caused by land subsidence, swallow a home’s yard overnight following heavy rains. While not as sudden and dramatic as an earthquake or landslide, land subsidence impacts more than 3,000 square miles in southern Arizona. Its economic impact from gradient changes impacting canal and drainage systems, irrigation systems and roads, for example, is a serious long-term hazard that is not adequately appreciated.

So, what are we doing, and what more can we do?

Governments have traditionally responded to natural disasters in hopes of saving lives and protecting property. But increasingly, we need to focus our efforts to identify and reduce the potential for catastrophes before they occur.

Solution 1: Surveying

The Arizona Geological Survey is undertaking the first-ever statewide inventory of landslides. More than 4,400 landslides have been identified so far. We recently mapped all known earth fissures in Maricopa, Cochise, Pima and Pinal Counties, and the online interactive map is used thousands of times a year by home buyers and builders, developers, and local planners to make informed investment decisions.

We are continually updating this mapping as new fissures develop and existing fissures lengthen.

Solution 2: Monitoring

Our seismic monitoring network can detect a magnitude 3.0 earthquake anywhere in the state, something not possible until eight years ago. Detecting small earthquakes gives a better idea of where larger earthquakes may occur.

The Black Canyon City earthquakes were a good wake-up call. No one was injured and damage appears to be slight. Many of our family, friends, neighbors and co-workers who thought that Arizona doesn’t have earthquakes may now be a bit more attuned to the fact that we live in very dynamic geologic environment.

We have embedded a state geologist with U.S. Forest Service Burned Area Emergency Response teams to identify areas most vulnerable to flooding and debris flows after a wildfire

These are important steps, but more can be done to reduce the risks to Arizonans from geologic hazards.

Solution 3: Better construction

More work is needed to develop efficacious construction strategies in hazard-prone areas. Issues to be addressed include determining whether specific setbacks are appropriate and, if so, how extensive they should be, and what mitigation techniques can be applied. Knowing that we are building in landslide areas, for example, alerts us to divert runoff from seeping into these zones and triggering movement.

Floodplain management is done at the local government level, but geologic information should be used to assess potential flood hazards on alluvial fans, in mountain canyons where debris flows are a possibility, and along natural channels where the potential exists for bank erosion during floods.

We have only to look around us at the dramatic landscapes of Arizona to appreciate the power of natural events that throw up mountains and volcanoes, carve immense canyons and constantly sculpt the land beneath our feet.

The geologic forces that created Arizona also create hazards and risks for us. Recent earthquakes are just one reminder of that.
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