PROVO — Kevin Franke has visited a half-dozen major earthquake sites worldwide in as many years, studying recovery and rebuilding in the devastated areas and projecting how ground waves, soils and structures are intertwined when it comes to a quake’s impact.
The Brigham Young University civil and environmental engineering professor hopes improved engineering codes and standards, based on understanding a quake’s hazards, in turn help increase safety and save lives.
“Even the type of soil that we construct on can play a role,” said Franke, who sports a “DIRTMAN” Idaho vanity license plate in his campus office. The substance and depth of soil, a quake’s epicenter and magnitude and the size and shape of affected structures all can factor into the degree of potential damage.
He points to data from the Sept. 19 central Mexico quake — the deadliest so far of 2017 — that resulted in 370 deaths and more than 6,000 injured in the states of Puebla and Morelos and the Greater Mexico City area. The 7.1 magnitude earthquake hit just two hours after a morning drill done annually on the anniversary of the massive 1985 Mexico City earthquake — that 8.0 magnitude event resulted in some 5,000 deaths and major destruction in and around Mexico’s capital city.
Franke was part of a National Science Foundation-funded Geotechnical Extreme Events Reconnaissance mission to Mexico, with specialists using drones to help map the results and to help create 3-D computer models projecting “structure from motion” damage — or why some buildings were impacted more than others.
Those models and projections can ultimately be helpful when a quake hits close to home. And home it can hit, since Franke and an estimated 2.1 million people reside along the Wasatch Front, which straddles the Wasatch Fault, one of the world’s most active normal fault lines stretching from southern Idaho to central Utah.
The seismic waves generated by earthquakes come in a variety of frequencies, which in turn can be amplified by soil conditions and then resonate into different buildings because of their size, shape or structural materials.
And the worst damages come when, as Franke said, “the building is in tune in the surface and the soils,” or when all elements coincide in “perfect storm” conditions — a certain wave amplified by a certain soil and most adversely affecting a certain type of building.
Soil that is thick, soft and clay-based tends to amplify low-frequency seismic waves and increase motions at the surface, with the slow, rolling and shaking moves adversely affecting taller buildings.
Thin, stiff soil over bedrock amplifies high-frequency waves, resulting in vigorous ground vibrations and extensive damage to shorter buildings and residential houses. And bedrock itself doesn’t amplify or prolong ground motions or change the motion as much, resulting in predictable shaking.
Meanwhile, quake impact relates to a building’s shape and size, with buildings that are more square or rectangular impacted less than irregular-shaped buildings. Taller buildings tend to respond by swaying back and forth, while shorter structures are jarred from side to side in an earthquake.
Franke likens the taller and shorter buildings and their respective responses to flexible reeds and shorter, stiffer sticks.
Building materials factor in the equation as well, with wood and steel structures more flexible or “ductile” than the more rigid brick, concrete and masonry materials, which transfer ground motions into the structures.
The 1985 Mexico City quake produced waves amplified by deeper, softer soil, which in turn felled taller buildings — those 12 stories or taller. Meanwhile, the 2017 central Mexico quake — with its epicenter actually closer to Mexico City than its ’85 predecessor — affected shorter buildings, those that were four to eight stories high.
Damage from the 2017 quake was kept to what Franke describes as a very narrow band of the western part of Mexico City that follows the ancient lakebed underneath the metro area. “The soil there was in tune,” he said. “If the soil would have been thicker or thinner, it wouldn’t have resonated.”
Contrast that with the region of last fall’s central Italy quake, which Franke also visited. That 6.2 magnitude quake resulted in 299 deaths and nearly 400 injured in one of Italy’s most seismically active areas. However, it occurred in a mountainous area and damaged the medieval and pre-medieval villages, much of which featured mostly unreinforced masonry structures of brick, concrete or cement constructed on bedrock and little soil.
Besides benefitting from research and results from similar recent earthquakes in New Zealand, Japan and the like, Franke says Utah can draw upon the Mexico and Italy events when projecting what will happen in Utah because of the similarities.
Those include fault lines in the mountains (similar to central Italy) and the valleys’ lakebed soils (Utah’s Lake Bonneville being similar to Mexico City’s ancient lakebed).
However, one difference is Utah’s proximity of mountain-lining faults located next to neighboring lakebed soils differs; the epicenters of the 1985 and 2017 quakes were 115 to 225 miles away from Mexico City.
“Here in Utah, the fault will rupture right beneath our feet,” said Franke, adding “Mother Nature has yet to clean house here along the Wasatch Front.”
He notes there are two public approaches to earthquakes. One is more passive, being “let’s react to what the earthquake does,” while the more proactive approach is “we know it’s coming; we don’t know when, but let’s look for weaknesses and address them now.”
Quake timelines show the Wasatch fault is way overdue for a major event, and while Utahns should be mindful and preparing, “experts have long been pointing out the deficiencies,” he said.
Challenges include the majority of the Wasatch Front’s population and structures bottlenecked into one single corridor, the gravity-fed delivery canals of culinary water crisscrossing the fault lines, the likelihood of underground gas and utility lines becoming casualties from the quake-induced liquefaction of soils and a substantial exacerbation of landslides and rockfalls in the mountains from the tremors.
Add in the fact that Utah’s Salt Lake and Utah valleys will produce a worsening basin effect — waves rippling back and forth like a pool of water as well as bouncing around the edges like sound waves on the edge of the bell.
Franke acknowledges reviews and modifications that have been done on a number of larger, more central structures throughout the Wasatch Front. But Utah still has one of the highest percentages of unreinforced masonry structures per capita in the United States.Comment on this story
“One question is how we are doing with our regular buildings — the residential homes, the small commercial buildings and even some of the larger ones,” he said,
A closer look is needed at Wasatch Front structures built prior to the mid-1970s, a time which construction quality improved and seismic codes began to be strengthened because of lessons learned from preceding California quakes.
He cited Mexico City and the 1985 and 2017 quakes as resulting in BYU improvements from one event to the next, thanks to better construction and better codes.
“Building codes do work, when properly implemented and followed,” Franke said.