Special Features of the Engine

There are a few less frequently used features of the engine that are not documented in the general user’s manual, since their usage is quite specific. They are documented here.

Sensitivity analysis

Running a sensitivity analysis study means to run multiple calculations by changing a parameter and to study how the results change. For instance, it is interesting to study the random seed dependency when running a calculation using sampling of the logic tree, or it is interesting to study the impact of the truncation level on the PoEs. The engine offers a special syntax to run a sensitivity analysis with respect to one (or even more than one) parameter; you can find examples in the demos, see for instance the MultiPointClassicalPSHA demo or the EventBasedDamage demo. It is enough to write in the job.ini a dictionary of lists like the following:

sensitivity_analysis = {"random_seed": [100, 200, 300]}
sensitivity_analysis = {'truncation_level': [2, 3]}

The first example with run 3 calculations, the second 2 calculations. The calculations will be sequential unless you specify the --many flag in oq engine --run --many job.ini. The descriptions of the spawned calculation will be extended to include the parameter, so you could have descriptions as follows:

Multipoint demo {'truncation_level': 2}
Multipoint demo {'truncation_level': 3}

NB: from version 3.17 the engine is also able to run sensitivity analysis on file parameters. For instance if you want to run a classical_risk calculation starting from three different hazard inputs you can write:

sensitivity_analysis = {
  "hazard_curves_file": ["hazard1.csv", "hazard2.csv", "hazard3.csv"]}

Ruptures in CSV format

Since engine v3.10 there is a way to serialize ruptures in CSV format. The command to give is:

$ oq extract "ruptures?min_mag=<mag>" <calc_id>`

For instance, assuming there is an event based calculation with ID 42, we can extract the ruptures in the datastore with magnitude larger than 6 with oq extract "ruptures?min_mag=6" 42: this will generate a CSV file. Then it is possible to run multi-rupture scenario calculations starting from that file by simply setting rupture_model_file = ruptures-min_mag=6_42.csv in the job.ini file. The format is provisional and may change in the future, but it will stay a CSV with JSON fields. Here is an example for a planar rupture, i.e. a rupture generated by a point source:

#,,,,,,,,,,"trts=['Active Shallow Crust']"
seed,mag,rake,lon,lat,dep,multiplicity,trt,kind,mesh,extra
24,5.050000E+00,0.000000E+00,0.08456,0.15503,5.000000E+00,1,Active Shallow Crust,ParametricProbabilisticRupture PlanarSurface,"[[[[0.08456, 0.08456, 0.08456, 0.08456]], [[0.13861, 0.17145, 0.13861, 0.17145]], [[3.17413, 3.17413, 6.82587, 6.82587]]]]","{""occurrence_rate"": 4e-05}"

The format is meant to support all kind of ruptures, including ruptures generated by simple and complex fault sources, characteristic sources, nonparametric sources and new kind of sources that could be introduced in the engine in the future. The header will be the same for all kind of ruptures that will be stored in the same CSV. Here is description of the fields as they are named now (engine v3.11):

seed

the random seed used to compute the GMFs generated by the rupture

mag

the magnitude of the rupture

rake

the rake angle of the rupture surface in degrees

lon

the longitude of the hypocenter in degrees

lat

the latitude of the hypocenter in degrees

dep

the depth of the hypocenter in km

multiplicity

the number of occurrences of the rupture (i.e. number of events)

trt

the tectonic region type of the rupture; must be consistent with the trts listed in the pre-header of the file

kind

a space-separated string listing the rupture class and the surface class used in the engine

mesh

3 times nested list with lon, lat, dep of the points of the discretized rupture geometry for each underlying surface

extra

extra parameters of the rupture as a JSON dictionary, for instance the rupture occurrence rate

Notice that using a CSV file generated with an old version of the engine is inherently risky: for instance if we changed the ParametricProbabilisticRupture class or the PlanarSurface classes in an incompatible way with the past, then a scenario calculation starting with the CSV would give different results in the new version of the engine. We never changed the rupture classes or the surface classes, but we changed the seed algorithm often, and that too would cause different numbers to be generated (hopefully, statistically consistent). A bug fix or change of logic in the calculator can also change the numbers across engine versions.

The minimum_distance parameter

GMPEs often have a prescribed range of validity. In particular they may give unexpected results for points too close to ruptures. To avoid this problem the engine recognizes a minimum_distance parameter: if it is set, then for distances below the specified minimum distance, the GMPEs return the ground-motion value at the minimum distance. This avoids producing extremely large (and physically unrealistic) ground-motion values at small distances. The minimum distance is somewhat heuristic. It may be useful to experiment with different values of the minimum_distance, to see how the hazard and risk change.

The max_sites_disagg

There is a parameter in the job.ini called max_sites_disagg, with a default value of 10. This parameter controls the maximum number of sites on which it is possible to run a disaggregation. If you need to run a disaggregation on a large number of sites you will have to increase that parameter. Notice that there are technical limits: trying to disaggregate 100 sites will likely succeed, trying to disaggregate 100,000 sites will most likely cause your system to go out of memory or out of disk space, and the calculation will be terribly slow. If you have a really large number of sites to disaggregate, you will have to split the calculation and it will be challenging to complete all the subcalculations.

The parameter max_sites_disagg is extremely important not only for disaggregation, but also for classical calculations. Depending on its value and then number of sites (N) your calculation can be in the few sites regime or the many sites regime.

In the few sites regime (N <= max_sites_disagg) the engine stores information for each rupture in the model (in particular the distances for each site) and therefore uses more disk space. The problem is mitigated since the engine uses a relatively aggressive strategy to collapse ruptures, but that requires more RAM available.

In the many sites regime (N > max_sites_disagg) the engine does not store rupture information (otherwise it would immediately run out of disk space, since typical hazard models have tens of millions of ruptures) and uses a much less aggressive strategy to collapse ruptures, which has the advantage of requiring less RAM.

Equivalent Epicenter Distance Approximation

The equivalent epicenter distance approximation (reqv for short) was introduced in engine 3.2 to enable the comparison of the OpenQuake engine with time-honored Fortran codes using the same approximation.

You can enable it in the engine by adding a [reqv] section to the job.ini, like in our example in openquake/qa_tests_data/logictree/case_02/job.ini:

reqv_hdf5 = {'active shallow crust': 'lookup_asc.hdf5',
             'stable shallow crust': 'lookup_sta.hdf5'}

For each tectonic region type to which the approximation should be applied, the user must provide a lookup table in .hdf5 format containing arrays mags of shape M, repi of shape N and reqv of shape (M, N).

The examples in openquake/qa_tests_data/classical/case_2 will give you the exact format required. M is the number of magnitudes (in the examples there are 26 magnitudes ranging from 6.05 to 8.55) and N is the number of epicenter distances (in the examples ranging from 1 km to 1000 km).

Depending on the tectonic region type and rupture magnitude, the engine converts the epicentral distance repi into an equivalent distance by looking at the lookup table and use it to determine the rjb and rrup distances, instead of the regular routines. This means that within this approximation ruptures are treated as pointwise and not rectangular as the engine usually does.

Notice that the equivalent epicenter distance approximation only applies to ruptures coming from PointSources/AreaSources/MultiPointSources, fault sources are untouched.