# Extra tips specific to event based calculations¶

Event based calculations differ from classical calculations because they produce visible ruptures, which can be exported and made accessible to the user. In classical calculations, instead, the underlying ruptures only live in memory and are normally not saved in the datastore, nor are exportable. The limitation is fundamentally a technical one: in the case of an event based calculation only a small fraction of the ruptures contained in a source are actually generated, so it is possible to store them. In a classical calculation all ruptures are generated and there are so many millions of them that it is impractical to save them, unless there are very few sites. For this reason they live in memory, they are used to produce the hazard curves and immediately discarded right after. The exception if for the case of few sites, i.e. if the number of sites is less than the parameter max_sites_disagg which by default is 10.

## Sampling of the logic tree¶

There are real life examples of very complex logic trees, like the model for South Africa which features 3,194,799,993,706,229,268,480 branches. In such situations it is impossible to perform a full computation. However, the engine allows to sample the branches of the complete logic tree. More precisely, for each branch sampled from the source model logic tree a branch of the GMPE logic tree is chosen randomly, by taking into account the weights in the GMPE logic tree file.

The details of how the sampling works are documented here:

It should be noticed that even if source model path is sampled several times, the model is parsed and sent to the workers only once. In particular if there is a single source model (like for South America) and number_of_logic_tree_samples =100, we generate effectively 1 source model realization and not 100 equivalent source model realizations, as we did in past (actually in the engine version 1.3). The engine keeps track of how many times a model has been sampled (say Ns) and in the event based case it produce ruptures (with different seeds) by calling the appropriate hazardlib function Ns times. This is done inside the worker nodes. In the classical case, all the ruptures are identical and there are no seeds, so the computation is done only once, in an efficient way.

## Convergency of the GMFs for non-trivial logic trees¶

In theory, the hazard curves produced by an event based calculation should converge to the curves produced by an equivalent classical calculation. In practice, if the parameters number_of_logic_tree_samples and ses_per_logic_tree_path (the product of them is the relevant one) are not large enough they may be different. The engine is able to compare the mean hazard curves and to see how well they converge. This is done automatically if the option mean_hazard_curves = true is set. Here is an example of how to generate and plot the curves for one of our QA tests (a case with bad convergence was chosen on purpose):

$oq engine --run event_based/case_7/job.ini <snip> WARNING:root:Relative difference with the classical mean curves for IMT=SA(0.1): 51% WARNING:root:Relative difference with the classical mean curves for IMT=PGA: 49% <snip>$ oq plot /tmp/cl/hazard.pik /tmp/hazard.pik --sites=0,1,2


The relative difference between the classical and event based curves is computed by computing the relative difference between each point of the curves for each curve, and by taking the maximum, at least for probabilities of exceedence larger than 1% (for low values of the probability the convergency may be bad). For the details I suggest you to look at the code.