5A: Rethinking Seismic Engineering Along the Wasatch Front

Brady Cox, PH.D. & Mohsen Zaker Esteghamati, PH.D., P.E. | Chapter Five: Utah's Future

Magna Main Street after earthquake, march 18, 2020 | earthquake.utah.gov

TAKEAWAY

Current code-based seismic designs along the Wasatch Front may underestimate demands from a realistic magnitude 7.0 earthquake by more than 25%, warranting consideration of measures beyond minimum code requirements.

Terms to Know

Probabilistic: Approach using likelihoods and uncertainty to predict possible outcomes.

Deterministic: Approach assuming fixed conditions to predict a single, definite outcome.

Current building codes require seismic design forces to be the lesser of two values: one from probabilistic seismic hazard analysis (PSHA) and one from deterministic seismic hazard analysis (DSHA). PSHA evaluates the likelihood of many possible earthquake scenarios over a specified time interval, providing calculations to balance safety objectives with economic considerations. DSHA, by contrast, identifies a single, large, but historically realistic earthquake that would be expected at a site. PSHA values are always lower than DSHA values along the Wasatch Front. Along the Wasatch Fault this single earthquake is approximately a magnitude 7.0 event. This magnitude is based on our best estimates from large earthquakes that have occurred about every 1,300 years along the Wasatch Fault. The last earthquake of this magnitude occurred approximately 1,400 years ago near Salt Lake City, and more than 2,500 years ago near Brigham City. So, data suggest the Wasatch Front is "overdue" for a magnitude 7.0 earthquake.

The difference between PSHA- and DSHA-based values is significant along the Wasatch Front (refer to Figure 5.A.1). For example, in Salt Lake City, DSHA design forces for a two-story building built on shallow, weathered rock (i.e., Site Class B subsurface conditions) near the Utah State Capitol are about 26% higher than those from PSHA. This means buildings designed only to PSHA-based codes could face much greater demands during a real magnitude 7.0 event, raising the risk of severe damage or collapse.

The Utah Earthquake Engineering Center (UEEC) is studying whether extra safety measures—designing beyond code minimums—may be needed to ensure buildings can withstand a magnitude 7.0 earthquake shaking. Preliminary results show that costs associated with designing to higher DSHA seismic forces are likely quite low for many buildings. These efforts aim to ensure Utah is more resilient to a future major earthquake.

The last earthquake of this magnitude occurred approximately 1,400 years ago near Salt Lake City, and more than 2,500 years ago near Brigham City. So, data suggest the Wasatch Front is "overdue" for a magnitude 7.0 earthquake.

Figure 5.A.1 Seismic Design Forces from DSHA and PSHA

A line graph compares seismic design forces (spectral acceleration, Sa, in g) versus building period (seconds) for PSHA-based and DSHA-based estimates along the Wasatch Front. The y-axis ranges from 0 to 1.25 g in 0.25 g intervals, and the x-axis ranges from 0 to 2 seconds in 0.5 second intervals. Two curves are shown: a blue line labeled "Current Code (PSHA-based)" and a green line labeled "More Realistic (DSHA-based)." Both curves rise sharply near zero period, peak between 0 and 0.5 seconds, and then steadily decline. The PSHA-based (blue) curve reaches a maximum spectral acceleration of approximately 0.93 g, while the DSHA-based (green) curve peaks higher, at about 1.17 g—roughly 26 percent greater. The graph illustrates that DSHA produces higher design forces, particularly for short-period structures such as a two-story building on shallow weathered rock in Salt Lake City.