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Sources are representations of earthquakes that occur with some rate. They can be well defined faults with a specific geometry, or point sources derived from historic earthquake catalogs for which planar pseudo-fault representations may be used. In either case, there may be multiple earthquake sizes associated with a given source.
### Outline
* [Source File Structure](#source-file-structure)
- [Sample File Structure](#sample-file-structure)
* [Common Source Elements](#common-source-elements)
- [Magnitude-Frequency Distributions (MFDs)](#magnitude-frequency-distributions-mfds)
- [Rupture-Scaling Relations](#rupture-scaling-relations)
### Source File Structure
An earthquake source model, or [HazardModel](http://usgs.github.io/nshmp-haz/javadoc/index.html?gov/usgs/earthquake/nshmp/eq/model/HazardModel.html), is specified using XML files grouped in folders by [SourceType](http://usgs.github.io/nshmp-haz/javadoc/index.html?gov/usgs/earthquake/nshmp/eq/model/SourceType.html). The currently supported source types are:
```
[ Area, Cluster, Fault, Grid, Interface, Slab, System ]
```
One level of nested folders is permitted to facilitate logical 'source groups' and the use of different ground motion models ([GMMs](ground-motion-models) or [model initialization](configuration) settings. Source files, except those for [Fault System Sources](source-types#system-sources), and nested 'source group' folders may be given any name. Ground motions are computed using the models defined in the `gmm.xml` file in each source directory. If a 'source group' does not contain `gmm.xml`, one is required in the parent 'source type' folder. See file structure example below.
#### Sample File Structure:
```
ModelDirectory/
├─ config.json
├─ Fault/ <─── source type
│ ├─ faultSources1.xml
│ ├─ faultSources2.xml
│ ├─ ...
│ ├─ gmm.xml
│ │
│ └─ FaultGroup/ <─── source group
│ ├─ otherFaultSources.xml
│ └─ gmm.xml
│
├─ Grid/
│ ├─ gridSources.xml
│ ├─ ...
│ ├─ gmm.xml <─── uses parent GMMs ─┐
│ │ │
│ ├─ GridGroup1/ │
│ │ └─ otherGridSources.xml ──────────┘
│ │
│ └─ GridGroup2/
│ ├─ otherGridSources.xml
│ └─ gmm.xml
└─ ...
```
### Common Source Elements
#### Magnitude-Frequency Distributions (MFDs)
A source requires a description of the sizes and rates of all earthquakes it is capable of generating, otherwise known as a magnitude-frequency distribution. For conciseness, all source files may supply default MFDs so that individual sources need only specify those attributes that are different from the default. Standard [MFD types](http://usgs.github.io/nshmp-haz/javadoc/index.html?gov/usgs/earthquake/nshmp/mfd/MfdType.html):
* Single
* [Gutenberg-Richter](http://en.wikipedia.org/wiki/Gutenberg–Richter_law)
* [Tapered Gutenberg-Richter](http://scec.ess.ucla.edu/~ykagan/moms_index.html)
* Incremental
XML examples:
```XML
<!-- Single magnitude MFD
- Used to represent the rupture of a specific size event
on a fault; historically this may have been referred to
as a 'characteristic' rupture.
- Can float over or fill source. -->
<IncrementalMfd type="SINGLE" weight="0.25" id="NAME"
rate="1.0" floats="[true | false]" m="6.5" />
<!-- Gutenberg-Richter MFD
- Used to represent a range of evenly discretized magnitude
events with systematically varying rates.
- Always floats when used for a fault source; never floats
for grid sources.
- 'a' is the incremental log10(number of M=0 events). -->
<IncrementalMfd type="GR" weight="0.25" id="NAME"
a="1.0" b="1.0" dMag="0.1" mMin="5.0" mMax="7.0" />
<!-- Tapered Gutenberg-Richter MFD
- Same as Gutenberg-Richter, above, but with an exponential
taper applied to the cumulative number of events with
seismic moment greater than M.
- Only used for grid sources. -->
<IncrementalMfd type="GR_TAPER" weight="0.25" id="NAME"
a="1.0" b="1.0" dMag="0.1" mCut="6.5" mMin="5.0" mMax="7.0" />
<!-- Incremental MFD
- General purpose MFD that consists of varying magnitudes
and rates.
- Can float over or fill source. -->
<IncrementalMfd type="INCR" weight="0.25" id="NAME"
mags="[5.05, 5.15, ...]" rates="[1.0e-2, 0.9e-2, ...]"
floats="[true | false]" />
```
#### Rupture Scaling Relations
Fault-based sources commonly require a model that describes the geometry of partial (or floating) ruptures. Likewise, grid sources require a model of fault-length or -area when building pseudo-faults or performing point source distance corrections that consider magnitude-dependent rupture sizes with unkown strike. Such models are composed of published magnitude-length or -area relations and restrictions on aspect-ratio and/or maximum rupture width that are independent of the published model.
See the [API Docs](http://usgs.github.io/nshmp-haz/javadoc/index.html?gov/usgs/earthquake/nshmp/eq/fault/surface/RuptureScaling.html) for details on specific model combinations and implementations. A rupture scaling model is always sepcified as an attribute of `<SourceProperties />`; see examples in next section.
### Next: [Source Types](source-types)