StdLoc

Generic locator plugin for SeisComP.

Description

StdLoc is a SeisComP locator plugin that combines standard location methods and was developed with the focus on local seismicity, although the methods are generic enough to work at different scales.

Plugin

To enable StdLoc the plugin stdloc must be loaded.

How does it work?

The locator can apply a multitude of location methods and it is particularly useful to combine them to achieve better solutions. The methods available are: LeastSquares, GridSearch, OctTree and, more interestingly, two mixed methods: GridSearch+LeastSquares and OctTree+LeastSquares:

• LeastSquares: this is the classic algorithm that solve the linearized problem via iterative Least Square. However an initial location estimate is required and that makes its solutions dependent on a good initial location. For this reason this method should be combined with GridSearch or OctTree, unless it is intended to be used only as a re-locator of an existing solution e.g. in scolv or screloc

• GridSearch: finds the source location by evaluating the hypocenter probability of each grid cell and returning the maximum likelihood hypocenter. The source time is derived from the weighted average of arrival travel times. The downside of this method is that the resolution of the hypocenters depends on the grid size, but dense grids can be very slow to compute,

• GridSearch+LeastSquares: this method works similarly to GridSearch but it performs an additional LeastSquares for each grid cell, using the cell centroid as initial location estimate. This solves the major issues of both LeastSquares and GridSearch: by trying multiple initial location estimates allow LeastSquares to not be dependent on a bad initial location estimate and the grid doesn’t need to be dense, which makes the method faster than GridSearch and with higher resolution. For very local seisicity monitoring it could be used with a single cell only, which corresponds to starting LeastSquares from the same location with every event.

• OctTree: this method follows the NonLinLoc approach. The OctTree search starts similarly to GridSearch by evaluating the hypocenter probability of each grid cell, computed as the probability density at the cell center coordinates times the cell volume. The search then continues by repeatedly fetching the cell with high probability and dividing it in 8 sub-cells. These 8 cells are then inserted in the pool of cells to fetch from at next iteration. The search terminates after either a maximum number of iterations or after reaching a minimum cell size. At that point the maximum likelihood hypocenter is selected.

• OctTree+LeastSquares: the solution can be further improved combining OctTree with the Least Squares algorithm, which can use the OctTree solution as initial location estimate. This allows OctTree to stop after reaching a big cell size (i.e. it is fast) and letting LeastSquares to refine the solution. Knowing that the initial location estimate for LeastSquares is the maximum probability cell of OctTree the solution should not get stuck in a local minima. For example it is possible to define a grid size that covers a whole network and set the OctTree cell minimum size to a couple of kilometers. LeastSquares will then improve the location resolution of that coarse grid.

The algorithms implemented in StdLoc are standard methods described in “Routine Data Processing in Earthquake Seismology” by Jens Havskov and Lars Ottemoller. The OctTree search algorithm is based on NonLibLoc by Antony Lomax.

Why is stdloc suitable for local seismicity?

When dealing with very local seismicity (few kilometers or hundreds of meters) simplifications that are common for regional seismicity have to be removed. In particular the locator should take into consideration:

• station elevation and even negative elevation (e.g. borehole sensors)

• earthquake location can be above a seismic sensor (e.g. borehole sensors)

• possible negative earthquake depth (above surface)

More importantly the travel time tables used by the locator must be able to take into consideration all the above too.

StdLoc is simple, but can deal with all this requirements.

Travel Time Table

StdLoc can be configured with any Travel Time Table type available in SeisComP, however only the homogeneous type is able to take into consideration station elevation, negative source depth and sources happening above stations. For this reason homogeneous should be the preferred choice when working on very local seismicity and especially with borehole sensors.

Module Configuration

StdLoc.profiles

Type: list:string

Defines a list of profiles to make available to the plugin.

Note

StdLoc.profile.$name.* $name is a placeholder for the name to be used and needs to be added to StdLoc.profiles to become active.

StdLoc.profiles = a,b
StdLoc.profile.a.value1 = ...
StdLoc.profile.b.value1 = ...
# c is not active because it has not been added
# to the list of StdLoc.profiles
StdLoc.profile.c.value1 = ...

StdLoc.profile.$name.method

Default: LeastSquares

Type: string

The location method to use: LeastSquares, GridSearch, OctTree, GridSearch+LeastSquares or OctTree+LeastSquares.

StdLoc.profile.$name.tableType

Default: LOCSAT

Type: string

Travel time table format type.

StdLoc.profile.$name.tableModel

Default: iasp91

Type: string

The model to be used. The format depends on tableType.

StdLoc.profile.$name.PSTableOnly

Default: true

Type: boolean

If enabled the arrival travel time information are fetched using ‘P’ and ‘S’ tables only and the user selected phase type is not considered (e.g Pg, Sg, PmP, SmS, P1, S1, etc).

StdLoc.profile.$name.usePickUncertainties

Default: false

Type: boolean

Whether to use pick time uncertainties rather than a fixed time error. If true, then the uncertainties are retrieved from each individual picks and a weight associated accordingly to ‘pickUncertaintyClasses’ parameter.

StdLoc.profile.$name.pickUncertaintyClasses

Default: 0.000,0.025,0.050,0.100,0.200,0.400

Type: list:string

Unit: s

Define the pick time uncertainty classes from which the pick weight is computed. This parameter defines a list of uncertainty time intervals: the interval in which a pick time uncertainty falls defines the pick class e.g. a pick wich uncertainty of 0.150 falls in the 4th interval and the pick class is then 4. if pick uncertainties are absent, the lowest class will be used. The pick weight then is computed as: 1 / 2^(class-1)

StdLoc.profile.$name.confLevel

Default: 0.9

Type: double

Confidence level, between 0.5 and 1.0, used in computing the confidence ellipsoid.

StdLoc.profile.$name.enableConfidenceEllipsoid

Default: true

Type: boolean

Compute the confidence ellipsoid for the location. This is optional since if it is not required it can be disabled to save some computation.

Note

StdLoc.profile.$name.GridSearch.* Find the source location by evaluating the hypocenter probability of each grid point and returning the maximum likelihood hypocenter. The source time is derived from the weighted average of the arrival travel times. The solution can be further improved combining it with the Least Squares algorithm, which will then be run for each grid point, using it as initial location estimate. In this case only few sparse points are required.

StdLoc.profile.$name.GridSearch.center

Default: auto,auto,20

Type: list:string

Unit: deg,deg,km

Grid center defined as: latitude,longitude,depth. The special value "auto" can be used and the corresponding latitude, longitude and/or depth will be automatically computed as the average of the arrival station locations.

StdLoc.profile.$name.GridSearch.size

Default: 40,40,30

Type: list:string

Unit: km

Grid size in km defined as: X,Y,Z direction extents around the ‘GridSearch.center’, where X is the longitudinal extent, Y the latitudinal extent and Z the vertical extent.

StdLoc.profile.$name.GridSearch.numPoints

Type: list:string

Grid number of points in the X, Y, Z direction defined as: numX,numY,numZ. The first and last point will lie on the grid boundary unless the number of points is 1 and the point will be in the grid center.

StdLoc.profile.$name.GridSearch.misfitType

Default: L1

Type: string

The type of misfit to use, from which the likelyhood function is derived: L1 or L2 norm. L1 is less sensitive to outliers and so more suitable with automatic picks, L2 is the preferred choice for manual picks.

StdLoc.profile.$name.GridSearch.travelTimeError

Default: 0.25

Type: double

Unit: s

Typical error in seconds for travel-time to one station. This value affects the uncertainty of the location. In OctTree it also influences the probability density computation: a too conservative value increases the number of iterations required by OctTree to converge to a high resolution solution.

Note

StdLoc.profile.$name.OctTree.* Find the source location and time via OctTree search. This method uses the parameters defined in GridSearch, but applies the OctTree search algorithm to the grid. The starting cells of the OctTree search are created by dividing the initial grid in equally sized cells. The number of cells in each direction is ‘GridSearch.numPoints’ - 1, since the grid points becomes the cell vertices. The solution can be further improved combining OctTree with the Least Squares algorithm, which can use the OctTree solution as initial location estimate.

StdLoc.profile.$name.OctTree.maxIterations

Default: 50000

Type: int

Number of iterations after which the search stops. A zero or negative value disable this stopping mechanism.

StdLoc.profile.$name.OctTree.minCellSize

Default: 0.1

Type: double

Unit: km

Minimum cell size to be generate by the OctTree search to stop. A zero or negative value disable this stopping mechanism.

Note

StdLoc.profile.$name.LeastSquares.* Find the source location and time via iterative Least Square. However an initial location estatimate is required. if that is not available then this method should be combined with GridSearch or OctTree.

StdLoc.profile.$name.LeastSquares.depthInit

Default: 20

Type: double

The initial depth estimate, used only when method is ‘LeastSquares’ and no initial location is provided.

StdLoc.profile.$name.LeastSquares.iterations

Default: 20

Type: int

Number of iterations. Each iteration will use the location and time from the previous Least Square solution.

StdLoc.profile.$name.LeastSquares.dampingFactor

Default: 0

Type: double

Damping factor value to be used when solving the equation system. 0 menas no damping factor.

StdLoc.profile.$name.LeastSquares.solverType

Default: LSMR

Type: string

Algorithm to use: either LSMR or LSQR