Modeling Fundamentals: Re-Evaluating the Seismic Vulnerability of Australia’s Major Construction Types

July 28, 2020 / 11 min read

Editor's Note: The 2020 update to the Verisk Earthquake Model for Australia provided an opportunity to add support for new risk types and re-evaluate the modeled vulnerability for property in Australia. Cavity double brick masonry and masonry veneer constructions, which constitute the majority of residential buildings in Australia, are used as examples to illustrate this update in light of newly available data from Australia and New Zealand.

This year marks the 31st anniversary of the magnitude 5.6 earthquake that struck near the town of Boolaroo, 15 kilometers west of Newcastle’s central business district and about 111 km north-northeast of Sydney on December 28, 1989. The Newcastle quake caused damage to more than 60,000 buildings over a 9,000-km area and 13 deaths. Many of the buildings damaged by the earthquake were masonry structures that were either unreinforced cavity double brick masonry or masonry veneer (see box). More recently, the Mw 5.2 Kalgoorlie earthquake struck on April 20, 2010, near the city of Kalgoorlie-Boulder, 595 km east-northeast of Perth in the Goldfields-Esperance region of Western Australia. Despite its moderate magnitude, the April 20 earthquake’s shallow focus caused damage to buildings, many of which were unreinforced masonry construction, and was deemed one of the most damaging earthquakes experienced recently in Australia.

Even earthquakes of moderate magnitude are capable of causing extensive property damage and loss of life in Australia, depending on when and where they strike. Given the prevalence of cavity double brick masonry and masonry veneer construction throughout Australia, the Newcastle and Kalgoorlie earthquakes underscore the importance of accurately modeling these construction types. In this article, we re-evaluate and compare the vulnerability of these two building types and discuss how newly available data either confirmed or enhanced our view of vulnerability for these risk types in the updated Verisk Earthquake Model for Australia.

Re-Evaluating Damage from Past Earthquakes

The two prevalent construction types in Australia—cavity double brick masonry (CDBM) and masonry veneer—are both popular for low-rise residential buildings. The exteriors of CDBM and masonry veneer construction look very similar because they both consist of a nonstructural masonry façade, typically made of clay bricks. Their underlying structural systems are different, however, which means that the seismic vulnerability of each of these construction types is different also. The lateral load–carrying capacity of CDBM construction is derived from unreinforced masonry walls that have historically demonstrated very poor performance in past earthquakes. By comparison, the lateral load resistance of masonry veneer buildings is achieved through structurally sheathed timber framed walls, which typically exhibit good performance during earthquake ground shaking.

Even though the last significant earthquake to occur in Australia was the 2010 Kalgoorlie earthquake, two major opportunities to re-evaluate the vulnerability of these two prominent construction types arose in recent years. First, claims submitted after both the 1989 Newcastle and 2010 Kalgoorlie earthquakes were revisited in light of new insights into the ground shaking experienced by the built environment during these events. With the recent revision of the NSHM for Australia in 2018, a more accurate and region-specific approach was adopted for revising the magnitudes of many historical earthquakes. As a result, more realistic ground shaking intensity footprints have been associated with the claims from these two historical events. Thus, it was possible to derive a better relationship between the ground shaking intensity and the damage sustained. Second, similar to Australia, New Zealand has a significant number of masonry veneer buildings and while CDBM is less common there, significant damage to these two types of construction was experienced during the 2010 and 2011 earthquakes that impacted the area around Christchurch. Many damage observations and claims made with respect to the seismic performance of these construction types were recently settled and made available for detailed analysis.

Based on these two recent opportunities to do so, Verisk re-evaluated the vulnerability of CDBM and masonry veneer in preparation for the 2020 release of the updated Australia earthquake model.

Unreinforced masonry damage observations from historical earthquakes that have occurred around the globe are abundant, leading to a highly confident understanding of unreinforced masonry vulnerability. Nevertheless, the 2020 update to the Verisk Earthquake Model for Australia provided us with yet another opportunity to evaluate our view of CDBM, a specific type of unreinforced masonry particularly common in both Australia and New Zealand. Figure 1 shows a comparison of the different views of CDBM vulnerability from various sources. The view of CDBM vulnerability held by Geoscience Australia (GA) prior to their recent update is shown as the red dashed line; this view exhibits higher vulnerability as compared to the view of vulnerability contained within the Verisk Earthquake Model for Australia in Touchstone® 2019. Even though our 2019 view of vulnerability is lower than GA’s prior view, it is in line with damage data obtained from the 1989 Newcastle and 2010 Kalgoorlie earthquakes. Similarly, the CDBM vulnerability contained within the Global Earthquake Model (GEM) and the recently updated view published by GA is in line with these historical damage observations. As a result of our re-evaluation, we determined that the view of CDBM vulnerability did not need to change for the updated Verisk Earthquake Model for Australia in Touchstone 2020.

Figure 1. Cavity Double Brick Masonry Vulnerability.

Verisk took a similar approach in re-evaluating the vulnerability of masonry veneer construction (Figure 2). Again, the prior view of masonry veneer vulnerability published by GA and the Verisk Earthquake Model for Australia in Touchstone 2019 both exhibited similarly high vulnerability for this construction type. A re-analysis of damage data from the 1989 Newcastle earthquake, however, suggested a lower view of vulnerability. As a result, the vulnerability of masonry veneer was lowered for the updated Verisk Earthquake Model for Australia in Touchstone 2020, which is in line with the latest view of masonry veneer published by GA.

A Closer Look at Masonry Veneer Vulnerability

A masonry veneer structure is a building that has been erected with an inner structural wall and an outer nonstructural masonry wall that can be made of clay bricks, concrete masonry units, or natural or synthetic stone facing. Masonry veneer (specifically clay brick veneer) is often used as cladding for low-rise residential buildings where the veneer is either adhered directly to the underlying structural system or is separated from the inner structural wall by a gap and anchored in place using clips or ties. This gap can be hollow or it can be filled with solid foam board to improve the insulative properties of the exterior walls.

There are many advantages to building with masonry veneer. First, when masonry veneer is used in conjunction with a light wood frame structural system, the exterior aesthetic of masonry can be achieved with a much lighter overall building. Second, while brick masonry is not inherently waterproof, the separation gap can be used to direct water that has seeped through the masonry down to the foundation of the building where it can be properly drained away from the structure using flashing. Third, because the exterior masonry veneer is nonstructural, once the interior structural system is completed, work can begin on both the internal and external finishing of the building, allowing construction times to be significantly reduced while work is being performed simultaneously.

Despite the numerous advantages of constructing with masonry veneer, this structural system can pose some challenges when it comes to earthquake vulnerability. Forces that act on a building during an earthquake are directly proportional to the building’s mass. The overall weight of a masonry veneer structure is lighter than that of a purely unreinforced masonry building, despite the fact that the masonry exterior adds a significant amount of mass to the underlying wood frame structure, so a masonry veneer building will experience greater forces than a purely light wood frame structure. Furthermore, because the veneer is typically unreinforced, the structural integrity of the veneer is heavily dependent on the clips or ties used to anchor the wall back to the underlying structural system. In past earthquakes, it has been observed that over time, these ties can rust and deteriorate, making them ineffective at anchoring the wall back to the structure. As a result, significant damage can occur to the masonry veneer during an earthquake. So, while a masonry veneer building will be safer and most likely less vulnerable than a purely unreinforced masonry building, such as cavity double brick masonry, the added weight and reliance on clips or ties makes this construction type more vulnerable than a purely wood frame building.

Building Code Updates and the Impact on Vulnerability

A significant amount of attention has been paid to the updated view of the seismic hazard in Australia contained within the 2018 National Seismic Hazard Model (NSHM). The updated NSHM portrays a much lower view of the seismic hazard as compared to previous versions of the NSHM for Australia. While it is natural to wonder whether this significant reduction in seismic hazard will have an effect on the way buildings are constructed in Australia, it is important to clarify that a change in the NSHM does not necessarily mean a reduction in the building code requirements. An updated NSHM must first be accepted by the engineering community within the country of interest for it to be adopted as the hazard map that governs the seismic design requirements in the building code. For example, the 2012 update to the NSHM for Australia was not adopted for inclusion in the Australian building code; the hazard map in place prior to the 2012 NSHM update was preferred and remained in the building code. So this reduction in the 2018 NSHM for Australia does not automatically trigger a reduction in the seismic design requirements and, in turn, an expected increase in building vulnerability.

The seismic design requirements in Australia were recently updated, however, which has necessitated the inclusion of an additional age band in the 2020 update to the Verisk Australia earthquake model. In 2018 the minimum seismic design factor was doubled from 0.04 to 0.08. While this change does not have an appreciable impact on the vulnerability in some of the areas of higher seismic hazard in Australia, it has resulted in an increase to the lateral strength requirements for some of the lower hazard areas in Australia. Figure 3 shows the spatial and temporal variation in vulnerability as derived from the evolution of the building code requirements. The regions in warmer orange and red colors denote locations of lower hazard, weaker seismic design provisions, and subsequently higher vulnerability. The regions in yellow and green denote areas of higher hazard, stricter seismic design requirements, and therefore lower vulnerability. Between the 2009-2018 and post-2018 age bands, the locations of Pre-Code level assignment have disappeared as a result of the increase in the minimum seismic design factor. The impact of this change to the building code and the inclusion of this additional age band within the updated Verisk Earthquake Model for Australia is that buildings constructed after 2018 in areas of lower seismicity will generally exhibit lower vulnerability than their older counterparts built in those same areas.

Figure 3. Updated spatial and temporal variation in vulnerability modeled within the updated Verisk Earthquake Model for Australia. (Source: Verisk)

Managing Australia Earthquake Risk

Although Australia is located in a seismically stable tectonic region away from the edges of the Indo-Australian plate, several damaging earthquakes have occurred in the past century. Coincidentally, many of these seismic events have struck in some of the most heavily populated areas. Even small to moderate earthquakes can cause severe damage when they happen so close to city centers, as was the case for the 1954 Adelaide and 1989 Newcastle earthquakes. Although these damaging earthquakes are infrequent and may not contribute much to the AAL, their impact can be significant at the 200-year return period—the return period typically used for purchasing reinsurance in Australia.

The findings of the NSHA18 project were so significant that they prompted Verisk to trigger an update of our Australia earthquake model, released this month. Together, the updated Australia earthquake model’s revised view of the seismic hazard and updated vulnerability modeling capabilities provide a better match to historical losses, ensuring that companies operating in this market can make an appropriate assessment of their earthquake risk. By incorporating the highest level of detail and undergoing meticulous scrutiny during development, the Verisk Earthquake Model for Australia provides the most comprehensive view of earthquake hazard and vulnerability and remains a robust and powerful tool.