Might Recent Quakes in Italy, Mexico Be Relative to Utah?

By Taylor, Scott | Deseret News (Salt Lake City), November 12, 2017 | Go to article overview

Might Recent Quakes in Italy, Mexico Be Relative to Utah?


Taylor, Scott, Deseret News (Salt Lake City)


By Scott Taylor

Deseret News

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 sizes, shapes 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. …

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