RMS recently hosted a two-day workshop for the United States Geological Survey (USGS) on March 7-8, based at our Newark headquarters in California. The aim of the workshop was to both discuss and receive feedback from the scientific community on the interim update to the ongoing 2018 USGS National Seismic Hazard Map Project (NSHMP).
RMS works actively with the USGS and the community and has previously hosted USGS workshops to help facilitate scientific discussions and to improve understanding of earthquake hazard. The workshop was well attended by over 150 people from academia, the scientific community, and industry. A summary of the workshop can be found here.
The latest update to USGS NSHMP was in 2014, with updates typically occurring every six years. The next full update is anticipated for 2020. So, you might ask yourself, why is the USGS planning an interim update within such a short period of time? The main reason for this update is the release of the Next Generation Attenuation-East (NGA-East) ground motion and amplification models applicable to Central and Eastern North America (CENA).
The objective of NGA-East, a multi-disciplinary research project coordinated by the Pacific Earthquake Engineering Research Center (PEER) at the University of California, Berkeley, is very similar to NGA-West2. It incorporates a larger set of well-recorded strong ground motion data, unprecedented in size for CENA and introduces a new ground motion characterization model with a set of new ground motion prediction equations (GMPEs) and amplification models.
These models are derived from different analytical and statistical approaches than NGA-West2 models due to the well-accepted lack of ground motion data in CENA. The preliminary results from the workshop revealed that there are slight reductions in hazard, thus loss, close to large seismic sources such as in New Madrid and Charleston areas and an increase elsewhere for sites on rock conditions.
Consideration of Basin Models for the First Time
The updates are not just limited to CENA. Another important decision is the first-ever incorporation of the sedimentary basins into the 2018 USGS NSHMP for the Western U.S., i.e. Los Angeles, Seattle, San Francisco, and Salt Lake. This complex modeling would translate into increased losses in general, especially larger for the deeper sedimentary depths within the basin boundaries. The seismic waves generated by earthquakes propagating within the crust from the seismic source are significantly amplified when they enter the basin structure, due to depth effects coupled with each basin’s unique, geologic characteristics, frequency content and duration of the ground motions.
A great example would be the Mexico City basin which amplified the rock ground motions tenfold during the 1985 Mw 8.0 Michoacan Earthquake mostly due to the unique geology of the Mexico City lake basin. The frequency content of the ground motions and the building structures played an important role in the observed damage patterns between the 1985 and 2017 Mexico earthquakes.
The good news is that within the Version 17 RMS® North America Earthquake Models released in April 2017, RMS has modeled the most studied, deeper basins for the Greater Los Angeles and Seattle areas in the Western U.S. Since this was a forward-modeling decision which is now considered by the USGS, the underlying model decisions and assumptions made for Version 17 basins might be different from the 2018 USGS NSHMP. Keeping this in mind, RMS expects manageable changes in the loss results for the existing basins and some increase in loss for the relatively shallower San Francisco and Salt Lake basins.
Unless sedimentary basins have been explicitly modeled in other catastrophe vendor models, large loss increases are to be expected due to this update, so we feel it is a good thing that RMS has proactively considered the inclusion of primary sedimentary basins in Version 17.
So, what are the next steps? RMS is currently studying the impact of the 2018 USGS NSHMP updates with an understanding that the 2020 USGS NSHMP updates will be more extensive than the 2018 updates. USGS plans to release the preliminary hazard maps for public review by the end of June 2018. RMS will review the potential loss change impacts due to the update once they are available. Stay tuned…