Tag Archives: Seismic Assessment

Probable Maximum Loss Reports

If the big one comes, how much damage will your loan portfolio sustain?  A building with significant damage runs a high risk of falling into foreclosure.  If a lender is active in a seismically active state such as California, they may want to get a handle on their seismic risk by ordering Probable Maximum Loss Reports.  

The Probable Maximum Loss Report predicts the amount of damage a building will sustain when the 475-year earthquake comes.  Just like we can estimate how large a 100-year flood will be, we can estimate the magnitude of a 100-year earthquake—and a 475-year earthquake.   We choose this non-round number because the 475-year event has a 10% chance of occurring in the next 50 years.

A PML Report expresses the seismic damage as a percentage intended to represent the expect damage to the building divided by the replacement cost of a building.   For example, if a building that costs $10 million to build and has a 10% PML, then when the 475-year event occurs we are predicting that the building will experience $1 million in damage.  

Most lenders treat the PML as a sort of pass/fail.   Any building with a PML lower than 20% is seen as an acceptable risk and buildings with PMLs over 20% have seismic risks that require mitigation.   Typical mitigation takes the form of either earthquake insurance (expensive) or seismic retrofit (usually expensive).   

The PML has long been a somewhat controversial product for mortgage bankers and borrowers, as too often they have seen two engineers return two significantly different PML numbers for the same property.   Historic use of the inconsistently defined term PML has left much confusion over what has been the measure of risk in the past and what is the comparable measure under ASTM terminology.  This is because the methods employed to calculate the PMLs by engineers have varied widely. Recently, ASTM has updated their original PML Standard with ASTM 2026-07 and published a new standard aimed directly at lenders, ASTM 2557-07 and these new standards have gone a long way toward creating consistently.

The ASTM Standards is more of a toolbox than a strict scope of work.   ASTM 2026-07 is a very flexible standard; this standard is a tool box that literally offers 768 different ways to do a PML.   For a banker, PMLs that are calculated differently, and cannot be compared to each other, create unwanted inconsistency in their underwriting process. 

To fix the 768-types-of-PMLs problem, a banker must specify which method they need.  Here is how to order a PML: ASTM 2557 recommends that the PML is reported as the Scenario Expected Limit, Design Basis Earthquake (DBE), 475-Year-Event and I recommend adding: Level 1 Building Damageability Assessment, Level 1 Building Stability Assessment, Level 1 Site Stability Assessment, and Calculated by the Thiel Zsutty Method.    Wow…that is a mouthful.

Insist that your engineers follow these tips and you will find that your PMLs are more transparent, understandable, and consistent with other finance industry PMLs. 


1.       Report one number, define the PML as the SELDBE.   Offering PMLs as both the Scenario Expected Limit (SEL) and the Scenario Upper Limit (SUL) is too confusing.  Accept the recommendation of ASTM E2557 and require your engineer to report the PML as the SEL only. 


2.       Require that the engineers use the Thiel Zsutty Method to calculate the PML.  This is the most commonly used method and is more transparent than other calculations (the importance of transparency is discussed below).  While the ASTM Standards do not specify a method of calculating the PML, if you allow one engineer on your panel to use Thiel Zsutty and another to use their own proprietary methods, then you will receive inconsistent results.


3.       Show the math.   Simply giving a high PML result without demonstrating how it was derived makes conducting a peer review futile.  How can anyone discuss or refute a computation that is absent?  Peer reviewable work is a fundamental hallmark of the engineering profession, and requiring engineers to show their work should be standard.


4.       Explain the “b” value.  The most controversial variable in the Thiel Zsutty Method is clearly the Building Vulnerablity Parameter, or the “b” value.   The engineer should explain how the “b” value was chosen.  The determination of a building’s damageability factor, b, starts with a table look-up and then must be carefully adjusted to specific earthquake damageability characteristics of the building that the engineer encounters in the field.  Absent this discussion, the report suffers from the fatal flaw of being inscrutable.


5.       Require the work to be done and signed by a registered engineer.  Structural assessment of buildings is at the heart of engineering work.  Only registered engineers possess the requisite certification, knowledge and skill for performing PMLs. 

Bankers have long been frustrated by the lack of consistency and transparency in PMLs.   If bankers instruct the engineers very precisely, the PML products delivered by the engineering community will feel less like supposition and more like science.


By:          Joseph P. Derhake, PE

Partner Engineering and Science

Phone: 800-419-4923