taup-js

Seismic travel-time calculator for JavaScript and TypeScript, implementing the TauP method with the IASP91, AK135, and PREM reference Earth models.

Installation

npm install taup-js

Quick start

import { TauPTime } from "taup-js";

const taup = new TauPTime(); // defaults to IASP91

const arrivals = taup.calculate(
  10,    // source depth (km)
  60,    // epicentral distance (degrees)
  ["P", "S", "PcP"]
);

for (const a of arrivals) {
  console.log(`${a.phase}: ${a.time.toFixed(1)} s`);
}
// P:   594.3 s
// PcP: 681.2 s
// S:   978.6 s

API

new TauPTime(options?)

Option	Type	Default	Description
model	"iasp91" | "ak135" | "prem" | VelocityModelData	"iasp91"	Velocity model to use

.calculate(sourceDepth, distanceDeg, phases?)

Returns Arrival[] sorted by arrival time.

• sourceDepth — source depth in km (0 = surface)
• distanceDeg — epicentral distance in degrees (0–360)
• phases — array of phase names (default: ["P", "S"])

.calculateAll(sourceDepth, distanceDeg)

Calculates all supported phases. Equivalent to calling .calculate() with TauPTime.supportedPhases().

.calculatePhase(phaseName, sourceDepth, distanceDeg)

Calculates a single named phase. Returns Arrival | null if the phase does not arrive at that distance.

.velocityAt(depth, waveType)

Returns the P or S velocity (km/s) at a given depth in the current model.

taup.velocityAt(35, "P");  // velocity just below the Moho

Static methods

TauPTime.supportedPhases()  // string[]
TauPTime.supportedModels()  // ["iasp91", "ak135", "prem"]

Arrival object

interface Arrival {
  phase: string;          // e.g. "P", "PKP"
  time: number;           // travel time in seconds
  rayParam: number;       // ray parameter in s/km
  takeoffAngle: number;   // angle at source (degrees from vertical)
  incidentAngle: number;  // angle at receiver (degrees from vertical)
  distanceDeg: number;    // actual computed distance in degrees
  sourceDepth: number;    // source depth in km
  piercePoints?: PiercePoint[];
}

Supported phases

Phase	Description
P, S	Direct P and S waves through the mantle
PP, SS	Surface-reflected P and S
PcP, ScS, ScP	Reflections off the core-mantle boundary
PKP, PKIKP	P wave through the outer/inner core
PKiKP	P wave reflected off the inner core boundary
SKS, SKKS	S–P conversion through the outer core
pP, sS	Depth phases (upgoing leg from source)
Pdiff, Sdiff	Waves diffracted along the CMB (~97°–170°)

Velocity models

import { TauPTime } from "taup-js";

const ak = new TauPTime({ model: "ak135" });
const prem = new TauPTime({ model: "prem" });

Custom velocity model

Pass a VelocityModelData object directly:

import { TauPTime, VelocityModelData } from "taup-js";

const model: VelocityModelData = {
  name: "simple",
  earthRadius: 6371,
  mohoDepth: 35,
  cmbDepth: 2889,
  icbDepth: 5154,
  layers: [
    { topDepth: 0,  botDepth: 35,   topVp: 6.0, botVp: 6.0, topVs: 3.5, botVs: 3.5, topDensity: 2.7, botDensity: 2.7 },
    { topDepth: 35, botDepth: 2889, topVp: 8.0, botVp: 13.7, topVs: 4.5, botVs: 7.3, topDensity: 3.3, botDensity: 5.6 },
    // ...
  ],
};

const taup = new TauPTime({ model });

Each layer uses linear interpolation of Vp, Vs, and density between topDepth and botDepth.

Method

Travel times are computed using the flat-Earth tau-p method:

• Velocities are transformed to a flat-Earth equivalent (v_flat = v · R/r)
• Layer integrals are solved with exact analytical antiderivatives, avoiding numerical issues near turning depths
• Ray parameter inversion uses a 300-point grid scan followed by bisection, which correctly handles triplications and shadow zones
• All results represent first arrivals on each phase branch