What is the expected peak acceleration at my site from a felt earthquake?

Often after a moderate earthquake a client will contact us wondering if their seismic switch, seismic monitor or seismic controller should have ‘closed the valve’ or generated some type of alarm.

LOOK UP THE EXPECTED ACCELERATION: If you know the earthquake magnitude and your location’s distance from the focus or epicenter (either will work for this estimate) you can use the chart below to get an idea of the expected peak acceleration at your site. At each ‘distance’ in the table three (3) values are given based on Donovan’s formula, McGuire’s formula and then the average of the two. We typically just use the average value (we don’t know which Irishman to trust!)

COMPARE IT WITH YOUR INSTRUMENTATION’S SETPOINT: If your seismic system’s acceleration setpoint is greater than the expected acceleration from the table below then it decreases the probability that your system would have tripped. Conversely, if your acceleration setpoint is lower than the expected acceleration then it increases the probability that your system would trip.

TAKE THE RESULTS WITH A ‘GRAIN OF SALT’: The values in the table are only a ROUGH estimate as the actual peak acceleration can differ up to three-fold! There are MANY factors which will impact the actual level of shaking at a given location including: the fault rupture mechanism, fault geometry, fault orientation, regional geology, local soil conditions and more.

The table below is based on formulae developed using acceleration data from California strong motion records (a fragmented geology, we do have our ‘faults’!). On the east coast of North America where the geology is more monolithic, strong motion travels much farther. Thus, the near-field entries in the table below (i.e. within 16 km) would likely be ‘in the ball park’. However, the peak acceleration at greater distances would likely be higher than that shown in the table.

The table is available in higher resolution here: https://eqsafetysys.com/wp-content/uploads/2022/02/shakemap22.pdf