Yes in ripple-lib the default elliptic curve is set in sjcl-custom/sjcl-secp256k1.js

// Overwrite NIST-P256 with secp256k1 so we're on even footing
sjcl.ecc.curves.c256 = new sjcl.ecc.curve(...)

Then in keypair.js when a new key pair is instantiated its _curve attribute is set to secp256k1.

function KeyPair() {
  this._curve  = sjcl.ecc.curves['c256'];
  this._secret = null;
  this._pubkey = null;
};