The point source gridding approximation

WARNING: the point source gridding approximation is used only in classical calculations, not in event based calculations!

Most hazard calculations are dominated by distributed seismicity, i.e. area sources and multipoint sources that for the engine are just regular point sources. In such situations the parameter governing the performance is the grid spacing: a calculation with a grid spacing of 50 km produces 25 times less ruptures and it is expected to be 25 times faster than a calculation with a grid spacing of 10 km.

The point source gridding approximation is a smart way of raising the grid spacing without losing too much precision and without losing too much performance.

The idea is two use two kind of point sources: the original ones and a set of “effective” ones (instances of the class CollapsedPointSource) that essentially are the original sources averaged on a larger grid, determined by the parameter ps_grid_spacing.

The plot below should give the idea, the points being the original sources and the squares with ~25 sources each being associated to the collapsed sources:

_images/gridding.png

For distant sites it is possible to use the large grid (i.e. the CollapsePointSources) without losing much precision, while for close points the original sources must be used.

The engine uses the parameter pointsource_distance to determine when to use the original sources and when to use the collapsed sources.

If the maximum_distance has a value of 500 km and the pointsource_distance a value of 50 km, then (50/500)^2 = 1% of the sites will be close and 99% of the sites will be far. Therefore you will able to use the collapsed sources for 99% percent of the sites and a huge speedup is to big expected.

Application: making the Canada model 26x faster

In order to give a concrete example, I ran the Canada 2015 model on 7 cities by using the following site_model.csv file:

custom_site_id lon lat vs30 z1pt0 z2pt5
montre -73 45 368 393.6006 1.391181
calgar -114 51 451 290.6857 1.102391
ottawa -75 45 246 492.3983 2.205382
edmont -113 53 372 389.0669 1.374081
toront -79 43 291 465.5151 1.819785
winnip -97 50 229 499.7842 2.393656
vancou -123 49 600 125.8340 0.795259

Notice that we are using a custom_site_id field to identify the cities. This is possible only in engine versions >= 3.13, where custom_site_id has been extended to accept strings of at most 6 characters, while before only integers were accepted (we could have used a zip code instead).

If no special approximations are used, the calculation is extremely slow, since the model is extremely large. On the the GEM cluster (320 cores) it takes over 2 hours to process the 7 cities. The dominating operation, as of engine 3.13, is “computing mean_std” which takes, in total, 925,777 seconds split across the 320 cores, i.e. around 48 minutes per core. This is way too much and it would make impossible to run the full model with ~138,000 sites. An analysis shows that the calculation time is totally dominated by the point sources. Moreover, the engine prints a warning saying that I should use the pointsource_distance approximation. Let’s do so, i.e. let us set

pointsource_distance = 50

in the job.ini file. That alone triples the speed of the engine, and the calculation times in “computing mean_std” goes down to 324,241 seconds, i.e. 16 minutes per core, in average. An analysis of the hazard curves shows that there is practically no difference between the original curves and the ones computed with the approximation on:

$ oq compare hcurves PGA <first_calc_id> <second_calc_id>
There are no differences within the tolerances atol=0.001, rtol=0%, sids=[0 1 2 3 4 5 6]

However, this is not enough. We are still too slow to run the full model in a reasonable amount of time. Enters the point source gridding. By setting

ps_grid_space=50

we can spectacularly reduce the calculation time to 35,974s, down by nearly an order of magnitude! This time oq compare hcurves produces some differences on the last city but they are minor and not affecting the hazard maps:

$ oq compare hmaps PGA <first_calc_id> <third_calc_id>
There are no differences within the tolerances atol=0.001, rtol=0%, sids=[0 1 2 3 4 5 6]

The following table collects the results:

operation calc_time approx speedup
computing mean_std 925_777 no approx 1x
computing mean_std 324_241 pointsource_distance 3x
computing mean_std 35_974 ps_grid_spacing 26x

It should be noticed that if you have 130,000 sites it is likely that there will be a few sites where the point source gridding approximation gives results quite different for the exact results. The commands oq compare allows you to figure out which are the problematic sites, where they are and how big is the difference from the exact results.

You should take into account that even the “exact” results have uncertainties due to all kind of reasons, so even a large difference can be quite acceptable. In particular if the hazard is very low you can ignore any difference since it will have no impact on the risk.

Points with low hazard are expected to have large differences, this is why by default oq compare use an absolute tolerance of 0.001g, but you can raise that to 0.01g or more. You can also give a relative tolerance of 10% or more. Internally oq compare calls the function numpy.allclose see https://numpy.org/doc/stable/reference/generated/numpy.allclose.html for a description of how the tolerances work.

By increasing the pointsource_distance parameter and decreasing the ps_grid_spacing parameter one can make the approximation as precise as wanted, at the expense of a larger runtime.

NB: the fact that the Canada model with 7 cities can be made 26 times faster does not mean that the same speedup apply when you consider 130,000+ sites. A test with ps_grid_spacing=pointsource_distance=50 km gives a speedup of 7 times, which is still very significant.