# Earthquake Forecasting

## Is the seismic gap theory valid and reliable?

The seismic gap theory says that, if there have been large earthquakes on an active fault's neighboring faults in the past, will that unbroken piece of fault be the most likely location for the next major earthquake? A considerable amount of research has tried to answer this question, and at the present time the answer seems to be "no." One reason might be that the unbroken piece of fault is just stronger than the surrounding faults, and it takes much more tectonic stress to break it.

## Is there any way to predict where the next big earthquake is likely to occur?

Right now, an accurate prediction of the location of the next major earthquake is not possible. However, regions of high probability for the next large earthquake can be identified and mapped. In fact, the Personal Earthquake Forecast tool (under the Tools tab) here on this web site can display the probability that a major earthquake will occur within 50 miles of any location you choose.

## What magnitude earthquakes can OpenHazards forecast?

The Open Hazards method generally computes probabilities for the occurrence of earthquakes in space and time, and for magnitudes typically larger than M > 5.0.  In addition, the methods can be more specifically applied to compute probabilities for other magnitude ranges.  For example Open Hazards methods have been applied to compute the probabilities for great earthquakes having magnitudes M > 8.0 in regions such as Indonesia where these occur.

## How does OpenHazards validate its forecasts?

Open Hazards validates its forecasts using the same types of statistical testing that are used in the weather/climate/financial forecasting communities.  These tests are used to determine resolution, the ability of a forecast to discriminate between alternative outcomes; reliability, whether the predicted frequency of events matches the observed frequency of events; and sharpness, whether events tend to occur at high forecast probabilities, and no events tend to  occur at low forecast probabilities, in contrast to

## How are probability forecasts typically validated?

Forecasts are validated by a process called backtesting as well as a process called monitoring.  In backtesting, data from the past are divided into a training period (prior data) and a testing period (posterior data).  Forecasts are made using prior data to forecast events that occur during the posterior interval.  The accuracy of these forecasts are then scored by a variety of statistical tests.  Forecasts that achieve a pre-determined level of accuracy are considered to be validated at the observed confidence level

## What methods are currently used for earthquake forecasting?

The United States Geological Survey, through its Working Groups on California Earthquake Probabilities, has been developing long  term earthquake forecasts for regions in California since 1988.  These forecasts are based on data describing historic averages of major earthquakes as well as paleoseismic geologic data, obtained from trenching studies on active fault traces, and instrumental data.  The result of these studies are 30 year probabilities for major earthquakes, typically having magnitudes M >

## Are earthquake forecasts currently being made?

Yes.  The official forecast for the state of California is a collaboration between the US Geological Survey, the California Geological Survey, and a large group of scientists from universities and commercial companies.  These forecasts are used to set earthquake insurance rates in California.