Geotechnical Factors for Assessing Seismic Risk and Building Retrofitting

You hear the seismic magnitude scale announcements on the news immediately following any earthquake: ‘only’ a magnitude 2, a catastrophic magnitude 6. It should be noted that the earthquake magnitude is only one of the factors impacting seismic force and damage. However, when reported on the news, this is the most commonly-cited statistic, other than the devastation that ensues. In reality, the damage sustained by structures will depend much more on the ground acceleration felt and the resiliency of the structural design to handle lateral loads than just its seismic scale value. But what does that really mean? How is it determined?

Earthquake magnitude is a tricky thing to measure. The most direct way to measure the energy released at a specific location is in terms of ground accelerations, which is the main tool that engineers use in building design. The maximum ground acceleration that a structure will feel depends on the magnitude of the earthquake, the direction of the earthquake movement, the geotechnical properties of the soil and rock between the earthquake and building, and the thickness and stiffness of the soil in the upper 50 to 100 feet below the building.

The recent Searles Valley Earthquake broke a 20-year lull in major seismic action in the state of California, and fortunately, relatively little damage occurred. The last quake measuring over 7.0 M was in 1999. The fault responsible has not been formally named, but there are a large number of faults in this particular area of the state. Searles Valley is located at the southeastern end of the Sierra Nevada mountains, in a deep basin known as the Mojave Desert (containing the famous “Death Valley”) and immediately west of the Basin and Range province. For 80 Million years the tectonic subduction of the Pacific Plate below the north American plate has pushed up the Sierra Nevadas and the Rocky Mountains and stretched out and thinned the crust of the intermediate space. To make matters more complex, a volcanic field sprouted up in the thin section of crust containing the Mohave Desert, leading to the formation of the Coso Volcanic field near to the epicenter of the recent earthquake.

The two named, active/potentially active faults closest to the epicenter are the Little Lake and the Coso fault zones. The developed areas closest to the epicenter are the town of Ridgecrest, and the China Lake Naval Air Base. Mapped Alquist-Priolo fault zones traverse both of these areas, indicating that a quake of this nature was expected to occur at some point.

Preliminary data indicated that the fault type was a strike-slip fault with a moment magnitude of 7.1. This type of faulting results in rupture at the ground surface, which can literally tear apart structures that are built across it. In this case, rupture at the ground surface was significant enough to be picked up by satellite imagery and could be visually observed in roadway cracks captured in the below photograph, as the painted street stripes can be seen to have been deflected laterally along the cracking.

What are the Building Risks?

In the case of ground rupture, most buildings are not properly designed to withstand this type of damage. The purpose of the Alquist-Priolo fault zone maps are to prevent people from building directly on the trace of an active fault line. Structures such as un-reinforced masonry (i.e. chimneys), crawl space homes that are not securely anchored to foundations for lateral loading, and buildings with tuck-under parking that do not contain moment frames are particularly sensitive. Many of the older buildings in California have been updated to address these common design weaknesses; however, there are many that are still out of date. If your property has not been properly evaluated or retro-fitted to conform to the latest ASTM E2557 standards, there are things that you can do to assess and remedy potential risks.

Solutions/Evaluations

Beyond seismic zone screening, some lenders require a Probable Maximum Loss evaluation and report for properties with peak ground acceleration of more tan 0.15g based on a 10% probability of exceedance in 50 years. A Probable Maximum Loss (PML) is a study intended to suggest how the property might be affected by ground shaking, although it is not an exact science and cannot accurately determine the performance of the property in a seismic event. As a result, it is important for you to select a level of assessment that satisfies your unique objectives and risk tolerance. A preliminary review of the building design is performed consistent with engineering design criteria, though the verification of design is limited. These studies often rely on interpolation of widely spaced geologic data that is compiled with many assumptions on regional or state maps.

How to Mitigate Risk

For sites that are close to the acceptable PML limits, more precise site-specific studies may be attempted to refine the values. These can be completed by a geologist and/or qualified geotechnical engineer. In looking at such sites, the risk tolerance of the owner should be carefully considered with the understanding that seismic activity can vary beyond the limits predicted by even the most careful studies, and that predicted accelerations can change after purchase, which may result in a change to PML.

In jurisdictions with a history of fault surface-rupture activity, site specific fault studies may be requested in suspected fault areas within or even beyond the Alquist-Priolo fault zone. Such studies involve cutting geologic exploration trenches or the use of closely spaced boring transects to determine if and where fault features are present on the site. These are planned in close coordination with regulatory geologists who will carefully review the studies once complete. In general, a 50-ft offset from estimated fault traces is required, however, this can sometimes be reduced depending on the resilience of the planned new construction, the precision of the data from the fault study and the nature of the activity on the fault line in question.

For all of these assessments and retrofitting options engagement with a qualified geologist, working in collaboration with a geotechnical engineer and/or structural engineer is required for optimizing the best risk management analyses and business solutions.