A magnetometer can be used to predict as well as give evidence of a HAARP created earthquake. A magnetometer measures disturbances in the magnetic field in Earth's upper atmosphere. It is not a seismometer which measure motions of the ground. The magnetometer doesn't measure seismic activity it measures and records electromagnetic frequencies in the Earth's atmosphere.
HAARP's phased array antennas beam radio wave frequencies into the atmosphere. A radio wave is essentially an electromagnetic frequency, as is solar radiation. The scientists at the HAARP institute found that a 2.5 Hz radio frequency is the signature frequency of an earthquake. HAARP beams that earthquake frequency into the ionosphere and the ionosphere reflects it back to Earth - penetrating as deeply as several kilometers into the ground, depending on the geological makeup and subsurface water conditions in a targeted area.. By beaming the frequency at a specific trajectory HAARP can trigger an earthquake any place on Earth. A short burst isn't enough to disturb solid matter (the Earth crust) so they keep beaming the 2.5 Hz earthquake frequency for hours or days - until the desired effect is achieved.
HAARP was broadcasting the 2.5 Hz frequency from just before midnight on March 8, 2011 and continued to broadcast the frequency for the entire days of March 9, 2011 and March 10, 2011. The 2.5 Hz frequency continued to be broadcasted and recorded by the magnetometer for another 10 hours the day of the Japan 9.0 magnitude earthquake.
You will notice on the USGS website that the Japan earthquake didn't occur on the Ring of Fire Fault line. The epicenter was some 100 km west of the fault. The earthquake epicenter occurred on solid ground.