# Logic trees FAQ¶

Logic trees are documented in the OpenQuake manual (section “Defining Logic Trees”). Here we will discuss some example for users already familiar with the concept and we will answer the following frequently asked questions:

• what is the meaning of the branch_path column in the “Realizations” output?
• given a realization, how do I extract the corresponding source model, modifications parameters and GMPEs?

As a concrete example we will consider the demo LogicTreeCase2ClassicalPSHA in the engine distribution; the source model logic tree file has the following structure branchset -> branches:

bs1[sourceModel] ->      b11[source_model.xml]
bs2[abGRAbsolute] ->     b21[4.6 1.1], b22[4.5 1.0], b23[4.4 0.9]
bs3[abGRAbsolute] ->     b31[3.3 1.0], b32[3.2 0.9], b33[3.1 0.8]
bs4[maxMagGRAbsolute] -> b41[7.0], b42[7.3], b43[7.6]
bs5[maxMagGRAbsolute] -> b51[7.5], b52[7.8], b53[8.0]


while the gsim logic tree file has the following structure:

bs1[Active Shallow Crust] ->     b11[BooreAtkinson2008], b12[ChiouYoungs2008]
bs2[Stable Continental Crust] -> b21[ToroEtAl2002], b22[Campbell2003]


Since the demo is using full enumeration there are 3**4 * 2**2 = 324 realizations in total.

After running the calculations you will see an output called “Realizations”. If you export it, you will get a CSV file with the following structure:

#,,"generated_by='OpenQuake engine 3.13..."
rlz_id,branch_path,weight
0,AAAAA~AA,3.0740926e-03
1,AAAAA~AB,3.0740926e-03
...
322,ACCCC~BA,3.1111853e-03
323,ACCCC~BB,3.1111853e-03


For each realization there is a branch_path string which is split in two parts separated by a tilde. The left part describe the branches of the source model logic tree and the right part the branches of the gmpe logic tree. In past versions of the engine the branch path was using directly the branch IDs, so it was easy to assess the correspondence between each realization and the associated branches.

Unfortunately, we had to remove that direct correspondence in engine 3.11. The reason is that engine is used in situations where the logic tree has billions of billions of billions … of billions potential realizations, with hundreds of branchsets. If you have 100 branchsets and the branch IDs are 10 characters long, each branch path will be 1000 characters long and impossible to display. The compact representation requires only 1-character per branchset instead. It is possible to pass from the compact representation to the original branch IDs by using the command oq show branches:

$oq show branches | branch_id | abbrev | uvalue | |-----------+--------+---------------------| | b11 | A0 | source_model.xml | | b21 | A1 | 4.60000 1.10000 | | b22 | B1 | 4.50000 1.00000 | | b23 | C1 | 4.40000 0.90000 | | b31 | A2 | 3.30000 1.00000 | | b32 | B2 | 3.20000 0.90000 | | b33 | C2 | 3.10000 0.80000 | | b41 | A3 | 7.00000 | | b42 | B3 | 7.30000 | | b43 | C3 | 7.60000 | | b51 | A4 | 7.50000 | | b52 | B4 | 7.80000 | | b53 | C4 | 8.00000 | | b11 | A0 | [BooreAtkinson2008] | | b12 | B0 | [ChiouYoungs2008] | | b21 | A1 | [ToroEtAl2002] | | b22 | B1 | [Campbell2003] |  The first character of the abbrev specifies the branch number (“A” means the first branch, “B” the second, etc) while the other characters are the branch set number starting from zero. The format works up to 64 branches per branchset, and the characters used in the abbreviations are in the following order: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+-  For instance the realization #322 has the following branch path in compact form: ACCCC~BA  which will expand to the following abbreviations (considering that fist “A” corresponds to the branchset 0, the first “C” to branchset 1, the second “C” to branchset 2, the third “C” to branchset 3, the fourth “C” to branchset 4, “B” to branchset 0 of the GMPE logic tree and the last “A” to branchset 1 of the GMPE logic tree): A0 C1 C2 C3 C4 ~ B0 A1  and then, using the correspondence table abbrev->uvalue, to: "source_model.xml" "4.4 0.9" "3.1 0.8" "7.6" "8.0" ~ "[ChiouYoungs2008]" "[ToroEtAl2002]"  For convenience, the engine provides a simple command to display the content of a realization, given the realization number, thus answering the FAQ: $ oq show rlz:322
| uncertainty_type         | uvalue            |
|--------------------------+-------------------|
| sourceModel              | source_model.xml  |
| abGRAbsolute             | 4.40000 0.90000   |
| abGRAbsolute             | 3.10000 0.80000   |
| maxMagGRAbsolute         | 7.60000           |
| maxMagGRAbsolute         | 8.00000           |
| Active Shallow Crust     | [ChiouYoungs2008] |
| Stable Continental Crust | [ToroEtAl2002]    |


NB: the commands oq show branches and oq show rlz are new in engine 3.13: they may change in the future and the string representation of the branch path may change too. It has already changed twice in engine 3.11 and engine 3.12. You cannot rely on it across engine versions.