  On 14th September 2015, a transient gravitational wave (GW150914) was
detected by the two LIGO detectors at Hanford and Livingston from the
coalescence of a binary black hole system located at a distance of about 400
Mpc. We point out that GW150914 experienced a Shapiro delay due to the
gravitational potential of the mass distribution along the line of sight of
about 1800 days. Also, the near-simultaneous arrival of gravitons over a
frequency range of about 100 Hz within a 0.2 second window allows us to
constrain any violations of Shapiro delay and Einstein's equivalence principle
between the gravitons at different frequencies. From the calculated Shapiro
delay and the observed duration of the signal, frequency-dependent violations
of the equivalence principle for gravitons are constrained to an accuracy of
$\mathcal{O}(10^{-9})$
