Aftershocks are real, yet few buildings are designed with aftershocks in mind
As the disaster in Turkey-Syria has shown, earthquakes are rarely one and done. Aftershocks can be just as strong, as deadly and yet they are largely overlooked in seismic design. Many seismic device types yield in the main event and leave the structure vulnerable to aftershocks. In this article, we explore the inconvenient truth of aftershocks.
What is an aftershock?
Large earthquakes are often followed by aftershocks, the result of changes in the surrounding crust brought about by the initial shock. Aftershocks are most common immediately after the main quake. As time passes and the fault recovers, they become increasingly rare (Omori's Law).
Typically, aftershocks are smaller than the earthquake as was the case following the magnitude 7.8 Kaikoura earthquake in New Zealand on 14 November 2016.
In some cases, the aftershock can be even bigger than the initial shake. For example, in April 2016, a magnitude 6.5 earthquake hit Kumamoto, Japan. A magnitude 7.3 aftershock followed two days later.
Perhaps the most infamous aftershock in recent memory was the magnitude 6.2 that occurred directly underneath Christchurch some five and a half months after the original M7.1 event further west of the City. It was the aftershock that caused most of the damage and all 181 deaths.
Vulnerability to aftershocks
The amount of damage caused by the aftershock is mostly down to the magnitude, depth and location. Christchurch was so devastating because the aftershock was directly under the city, it was shallow, and it was still relatively large.
When a building is subjected to a sequence of earthquakes, damage resulting from the main event can increase its vulnerability to collapse in subsequent events that occur prior to performing the necessary repairs. This has been demonstrated in recent earthquake sequences in Chi-Chi (1999), Wenchuan (2008), Christchurch (2010-2011), Tohoku (2011) and Central Italy (2016).
Calculating a building’s aftershock vulnerability is extremely complex and a field of active research. For that reason, design methodologies typically ignore aftershock vulnerability and focus on the main event itself.
Structural design methodologies and aftershocks
Building codes in the United States, New Zealand, and elsewhere provide minimum standards to ensure that new buildings are very unlikely to have deadly collapses in large earthquakes. Building codes aim to produce buildings that minimize loss of life; they do not aim to reduce economic impact from damage and downtime.
The Building Code is based on a theoretical design-level earthquake, and the design-level earthquake relates only to the strength of the mainshock. There is no such thing as a design-level aftershock.
Non linear time history analysis (NLTHA) subjects the structural model to a series of historical earthquakes and calculates the impact. Those time series typically include several ground motions in which to test the structure; more rigorous analysis might include 11 such exemplar earthquakes.
ASCE-7 in the US is the most widely used standard for determining design loads for buildings and other structures. ASCE 41 is the standard applied to existing buildings. Both standards refer to design methods like NLTHA as a way to accurately predict the impact of an earthquake on the proposed structure. Since NLTHA does not take into account aftershocks, neither ASCE 7 nor ASCE 41 consider the impact of aftershocks either.
Performance based design (PBD) came about in response to the limitations of building to code, and is more rigorous. PBD involves subjecting a structural model to analytical methods such as NLTHA to understand how the building would respond given a certain type of earthquake. Therefore it also does not consider aftershocks.
In the US, FEMA P-58 is a methodology and recommended procedure to assess the probable seismic performance of individual buildings based on their unique site, structural, nonstructural, and occupancy characteristics. Users of P-58 can even output X number of days until the building can safely be reoccupied. However, it also does not consider the impact of aftershocks.
In conclusion, the standards and advanced methodologies used by the engineering community do not adequately account for the impact of aftershocks only the main event.
Seismic devices and aftershocks
Most seismic protection devices (like buckling restrained braces and conventional friction dampers) are designed to yield. In other words, they act like a fuse. In the earthquake they fail and need to be replaced or repaired.
Practically speaking it takes months to get around and do this work. First, the devices need to be inspected by experts who will assess their condition and recommend repairs and replacements. Then, specialist work crews will be sent in to make the necessary corrective work. Oftentimes this would involve complete replacement of the seismic device.
The buildings are therefore vulnerable until the work is completed. Should an aftershock occur in the interim (and it almost certainly will), the building is left exposed.
One study on buckling restrained brace frames found that exposing them to a seismic sequence (i.e. main shock and aftershocks) has the potential to increase the inter-story drift, residual drift, damage index and global ductility factor.
Therefore, many seismic devices do not provide adequate protection through main event and aftershocks. Seismic design and analysis methods rarely consider the impact should an aftershock occur. Yet most earthquakes are indeed followed by aftershocks.
The latest generation of seismic devices, like the Tectonus RSFJ, are able to absorb the impact of an earthquake then automatically reset themselves. Base isolation too, when done right, can effectively reset the building ready for another shake.
Given what we’ve said about aftershocks, this is a huge advantage because:
- The building is fully protected through aftershocks
- Work crews will not be needed to mend the seismic devices
- Buildings can be reoccupied more quickly
- Economic damages will be minimized.
As research continues into aftershocks and how best to protect buildings from them, and seismic design standards grapple with how to include them in their analysis, it's wise to consider how the structure and seismic device will respond in the event of an earthquake series and not just an isolated main shock event.
Find out more
If you are considering a new build or building upgrade we strongly recommend giving thought to aftershocks. Traditional seismic devices may protect your investment in an earthquake but could leave you exposed in the event of strong aftershocks such as occurred in Christchurch in 2011 or Turkey-Syria in 2023.
Contact us now for a free consultation.