ASSESSING THE EARTHQUAKE GROUND SHAKING HAZARD ALONG THE WASATCH FRONT, UTAH - A LONG AND WINDY ROAD

Approximately 80 percent of Utah's 2.7 million people live along the Wasatch Front and adjacent to the Wasatch fault zone, the most significant source of earthquakes and seismic hazard within the State and the Intermountain west. The 343-km-long westward-dipping Wasatch fault consists of several segments that probably rupture independently of each other. The central segments, where the greatest population is concentrated, have ruptured repeatedly in large moment magnitude 7 and greater earthquakes. The mean Holocene recurrence intervals between characteristic large earthquakes along the central segments range from about 1,000 to 1,500 years. The elapsed times since the last large events along the Brigham City and Salt Lake City segments are 2,130 and 1,230 years, respectively, compared to their mean recurrence intervals of about 1,300 years. Hence the Salt Lake City segment is approaching its mean recurrence interval and the Brigham City segment has significantly exceeded its interval.

In traditional probabilistic assessments of ground shaking hazard, only time-independent estimates of the hazard are made, i.e., the elapsed time since the last large earthquake on a fault is not considered. The recurrence interval data along the central segments of the Wasatch fault are now robust enough that time-dependent hazard can now be estimated with some reliability. Statistical analysis of paleoseismic data for the past 6 ky from five of the central segments suggest overall periodic occurrence of large surface-faulting earthquakes with a COV of 0.4. However, the uncertainties in event ages, correlations, and recurrence intervals along a given segment still can result in large uncertainties in the hazard estimates particularly for some segments. Time-dependent estimates indicate, not surprisingly, a higher hazard, compared to Poisson estimates for the Brigham City segment and to a lesser extent, for the Salt Lake City segment. Other significant issues that impact the ground shaking hazard along the Wasatch Front are the increased ground motions on the hanging wall, the amplification due to near-surface alluvial sediments in the adjacent basins, and long-period amplification due to the basin geometry.