Yan Y. Kagan & David D. Jackson Dept. Earth and Space Sciences, UCLA, Los Angeles, CA , Statistical earthquake focal mechanism forecasts

Outline of the Talk Kagan & Jackson (JGR, 1994) forecasted focal mechanisms and their uncertainty. Because of spatial averaging the method does not work properly in polar areas. Difference in azimuth (bearing) must be accounted for in polar regions. Now we produce a high-resolution forecast for the whole Earth. We evaluate its skill and source complexity. GEM and CSEP planning to test forecasts. Kagan & Jackson, Statistical earthquake focal mechanism forecasts, submitted to GJI. Preprint --

World seismicity: 1976 – 2012 (GCMT)

Forecast: Long-term earthquake rate based on GCMT catalog 1977-present. 0.1 x 0.1 degree, Magnitude M>=5.8

Kagan & Jackson, GJI, 2000.

Focal mechanism forecast

CONCLUSIONS 1. The 0.1 degree global forecasts of magnitude 5.8 and larger earthquakes can be meaningfully compared and tested against regional forecasts. Our global ones have the advantage of independence from boundary effects and selection bias. 2. Uncertainties of predicted focal mechanisms depend strongly on the abundance and consistency of nearby focal mechanisms. The uncertainties can be quantified, but we do not yet have a likelihood measure for comparing predictions with observations. 3. Observed focal mechanisms are similar to the forecasted ones. They are not completely independent, but the dependence is weak. A planned prospective test will be definitive.

END Thank you

Abstract The new whole Earth focal mechanism forecast, based on the GCMT catalog, has been created. In the present forecast, the sum of normalized seismic moment tensors within 1000 km radius is calculated and the P- and T-axes for the focal mechanism are evaluated on the basis of the sum. Simultaneously we calculate an average rotation angle between the forecasted mechanism and all the surrounding mechanisms. This average angle shows tectonic complexity of a region and indicates the accuracy of the prediction. The method was originally proposed by Kagan and Jackson (1994, JGR). Recent interest by CSEP and GEM has motivated some improvements, particularly to extend the previous forecast to polar and near- polar regions. The major problem in extending the forecast is the focal mechanism calculation on a spherical surface. In the previous forecast as our average focal mechanism was computed, it was assumed that longitude lines are approximately parallel within 1000 km radius. This is largely accurate in the equatorial and near- equatorial areas. However, when one approaches the 75 degree latitude, the longitude lines are no longer parallel: the bearing (azimuthal) difference at points separated by 1000 km reach about 35 degrees. In most situations a forecast point where we calculate an average focal mechanism is surrounded by earthquakes, so a bias should not be strong due to the difference effect cancellation. But if we move into polar regions, the bearing difference could approach 180 degrees.

Abstract (cont.) In a modified program focal mechanisms have been projected on a plane tangent to a sphere at a forecast point. New longitude axes which are parallel in the tangent plane are corrected for the bearing difference. A comparison with the old 75S-75N forecast shows that in equatorial regions the forecasted focal mechanisms are almost the same, and the difference in the forecasted focal mechanisms rotation angle is close to zero. However, though the forecasted focal mechanisms are similar, closer to the 75 latitude degree, the difference in the rotation angle is large (around a factor 1.5 in some places). The Gamma- index was calculated for the average focal mechanism moment. A non-zero Index indicates that earthquake focal mechanisms around the forecast point have different orientations. Thus deformation complexity displays itself in the average rotation angle and in the Index. However, sometimes the rotation angle is close to zero, whereas the Index is large, testifying to a large CLVD presence. Both new 0.5x0.5and 0.1x0.1 degree forecasts are posted at

Jackson, D. D., and Y. Y. Kagan, Testable earthquake forecasts for 1999, Seism. Res. Lett., 70, Combined long- and short-term forecast for north- and south- western Pacific area