Recent earthquakes highlight subsurface conditions that amplify building damage

New York — The earthquakes last week in Japan and Ecuador are curious in their timing, but are completely unrelated, according to seismic expert Scott M. Adan, Ph.D., P.E., S.E., SECB of CBRE Valuation & Advisory Services, a nationally recognized expert in earthquake and seismic analysis with more than 23 years of structural engineering experience. However, they do highlight how subsurface conditions affect the level of structural damage that occurs.

The earthquake in Japan, an M=7.2, occurred on a strike-slip fault in the Eurasia plate, while the Ecuador M=7.8 was a classic subduction zone event that occurred on a boundary where the Nazca plate is sliding under the South American plate. In other words, the Japan quake is similar to what would happen in California along the San Andres and the Ecuador quake is what would happen in the Pacific Northwest along the Cascadia subduction zone. Both have completely unrelated tectonic features. Dr. Lucy Jones, noted former seismologist with the US Geological Survey, was quoted as saying that the timing of these two events is purely coincidental. 

With respect to the Ecuador earthquake, it is notable that most of the damage and casualties have occurred in Guayaquil, a city located almost 200 miles from the epicenter, according to Adan. This city is founded on the Guayas River delta. Therefore, it is likely that the silt and sandy alluvial sediments that make up the delta amplified the earthquake motions (similar to what could happen in Sacramento, Calif.).

Based on Adan’s observations from available photos, the damage appears concentrated to a certain building type known as "non-ductile reinforced concrete moment-frame." The associated damage is a result of insufficient reinforcing ties, stirrups, lap splices and other factors including frame strong-beam/weak-column configurations. This is a common building type in many Central and South American countries. It was also quite common in the western United States prior to the early 1970s. In Los Angeles, many of these same older concrete buildings are now being targeted for mitigation under the city’s recent seismic retrofit ordinance.

In the case of the Japan earthquake, based on similar photographic observations, the damage appears concentrated to building types known as adobe and rammed-earth structures.  It is notable that many of the traditional Japanese homes were constructed decades ago, using rammed-earth techniques. Over the years, these homes were expanded to include a second level. In many of the photos of the damage, the roof covers a collapsed pile of dirt and rubble where the home was previously standing. The damage occurs because the older earthen walls have no ductile reinforcement, leaving them weak and brittle.

The photos also reveal what appear to be many collapsed soft, weak or open front structures. This vulnerability occurs when, for example, parking is provided directly under the building’s second floor (i.e. "tuck under parking").

There are lessons learned from every earthquake that affects a large population center. In these two events, the lessons will be emerging in the days and weeks to come.

Adan is Director of Seismic Assessment Services at CBRE, in its Valuation & Advisory Services group, a leading global provider of integrated construction risk management and due diligence services. He has been named to the Earthquake Engineering Research Institute’s (EERI) Concrete Coalition Advisory Committee, is a member of the American Institute of Steel Construction (AISC) and the Structural Engineer’s Association of Northern California (SEAONC). His honors include an Award of Merit by SEAONC and a President’s Award for Outstanding Service by the Structural Engineer’s Association of Washington.

His research shows how recent earthquakes across the globe have brought into harsh light the poor seismic performance of non-ductile concrete buildings.