Nucleation

Nucleation forms candidate events from picks. Catalyst offers four methods, each on its own tab in the Pick processing → Nucleation panel. All of them

• work on the unassociated picks, or only on the selected picks if picks are selected,

• ignore picks that are hidden by the pick filters,

• create one preliminary seed origin per detected event.

A seed origin is only a starting point. Locate the seeds afterwards, usually with Smart-locate all (see Analyst rules and Smart-locate), and remove duplicates with Merge origins (see Refining origins).

The number of origins nucleation may create is limited by maxOrigins in catalyst.cfg.

The Nucleation panel with the DBSCAN page open. Each method is a page of the panel; Nucleate runs the open one. The DBSCAN parameters are described below.

DBSCAN

DBSCAN [5] groups picks that are close to each other in time and space. The distance between two picks is measured in seconds as the vector sum of

• their pick-time difference and

• the distance between their stations divided by a velocity.

A pick with at least Min size picks, itself included, within Max dist starts a cluster. Clusters grow through neighbouring picks. Each cluster that spans at least Min sta different stations becomes a seed origin.

The picks are played back in time order. At each pick only the picks not older than Window take part in the search, like maxPickDelay in scanloc. The window must be at least as long as the pick-time span of one event.

Parameter	Description
Phase	Picks to cluster: P, S, P+S or Is (infrasound). All other picks are ignored. In P+S mode a mixed P–S pair uses the S velocity and the larger of the two maximum distances.
Max dist P / S / Is [s]	Largest distance between two neighbouring picks (DBSCAN eps), per phase.
Vel P / S / Is [km/s]	Velocity that converts station distance into time. Roughly 6–7 for P, 3.5–4 for S and 0.34 for infrasound.
Min size	Minimum number of picks in a cluster.
Min sta	Minimum number of different stations in a cluster. A cluster from only one or two stations cannot be located, however many picks it has. 0 switches the check off.
Window [s]	Part of the pick buffer used for the search. 0 = 180 s.
Use 3D distances	Include the station elevation in the station distance.
Dist pctl [%]	Percentile used by Auto dist. 100 = maximum.
Pg max [km]	Only picks within this distance are used for the apparent-velocity fit, so that one line is not fitted across the Pg/Pn crossover. 0 = all picks.

Choosing the parameters from trusted events

Select a few well-located origins in the origin table first, then use the buttons next to the fields:

• Auto vel. to set the velocity to the median apparent velocity of the selected origins, fitted from their used picks of the chosen phase,

• Auto dist. to set Max dist with the k-distance method: the chosen percentile of each member pick’s distance to its (Min size − 1)-th nearest neighbour,

• Auto next to Window to set the window from the largest pick-time span of the selected origins, with a margin.

Fit V on the Move-out tab of the active origin sets the velocity from a single origin.

Back projection

Back projection searches a grid of trial sources. For each grid node it computes the origin time implied by each pick and counts how many picks agree on a common origin time within t0 tol. Nodes with enough agreeing picks become detections.

Several phases can be used together (joint search): a pick then supports a node if any of the selected phases explains it. Picks that carry a measured back-azimuth, for example from arrays, only support nodes within Az tol of their bearing.

For station sets wider than about 400 km the search uses spherical geometry.

Parameter	Description
Grid [km]	Spacing of the search grid. Finer is slower. The seed only needs to be good enough for the locator to converge.
Depth [km]	Depth assumed during the search. 0 suits infrasound and shallow events.
Max range [km]	Largest source–station distance, and the margin the grid adds around the stations.
Min size	Minimum number of picks for a detection.
Phases	P, S and Is. Tick several for a joint search.
t0 tol [s]	How closely picks must agree on the origin time.
Az tol [deg]	Back-azimuth tolerance for picks that carry one.
Max az gap [deg]	Reject detections with a larger station azimuthal gap. 360 = off.
EDT stacking	Score each node with an EDT stack instead of counting picks (see below).
EDT σ [s]	Differential-time uncertainty for EDT stacking.

EDT stacking

With EDT stacking each node is scored by an Equal Differential Time stack [6]. Every pick pair adds a Gaussian of its differential-time residual. The differential time does not depend on the origin time, so the stack is robust against a wrong seed time, and a bad pick, which disagrees with all its pairs, cannot build up score. It is slower than counting, but often cleaner in swarms.

EDT σ controls the width of the Gaussian. Too small finds only near-perfect events; too large merges nearby sources. Auto σ estimates it from the origins currently shown in the origin table: the robust (MAD) spread of their travel-time residuals, scaled by √2 for a pick pair. Filter the origin table to trusted origins first.

GaMMA-lite

GaMMA-lite is a simplified implementation of the GaMMA phase association method [14]. Picks are first grouped by time with a coarse window. Within each group, an epicentre grid search and the origin-time spread separate the events.

Parameter	Description
Grid [km]	Epicentre search grid spacing.
Coarse window [s]	Time window for the coarse grouping of picks.
OT tol [s]	Origin-time gap that separates two events within a group.
Processing window [s]	Sliding window for large data sets. 0 = one pass over all picks. Must exceed the pick-time span of the longest event; 90–120 s for a local network.
Min size	Minimum number of picks for an event.
Depth [km]	Initial source depth.
Max range [km]	Largest source–station distance.
Max OT spread [s]	Reject a cluster whose best origin-time spread is larger.

Grid search (scautoloc)

The grid-search nucleator uses the grid of scautoloc. For each node of the grid it projects the picks back to the node and looks for enough stations that agree on an origin time.

Parameter	Description
Grid file	Path to the scautoloc grid.conf file. Required.
Station file	Optional scautoloc station.conf file with per-station distance limits.
Min. stations	Lowest number of stations that must agree at a node. A node’s own minimum applies if it is higher.
Max. distance [deg]	Largest station distance.
Agreement offset [s]	Additive term of the arrival-time agreement window.
Projected window [s]	Rough projected-time window per node; scautoloc uses 50 s.
Max. radius factor	Reserved. Set to 0 to ignore it.
Processing window [s]	Trim each node’s pick buffer to this span. 0 = no trimming, the safe setting.

Choosing a method

• DBSCAN is fast and needs no velocity model beyond one velocity per phase. It works best for local networks and for picks from a compact source region, such as swarms.

• Back projection handles sparse and irregular networks, infrasound and array picks with back-azimuths, and large regions.

• GaMMA-lite separates events that overlap in time.

• Grid search reproduces the nucleation of scautoloc and reuses its grid configuration.