Hi Chris. The April 25, 1992, Petrolia earthquake was a 7.1Ms. . The Cascadia subduction zone had been relatively seismically inactive until the onset of the mainshock. This quake was an underthrusting event.

It was followed by two separate and distinct large aftershocks (Ms = 6.6 for both) occurring at 07:41 and 11:41 on April 26, as well as thousands of other small aftershocks.
Within two days of the mainshock, four aftershocks (Md = 4 to 5) occurred within 4 hours of each other that were located offshore and along the Mendocino fault. I don’t know if these four quakes were near the area of the ones we had today.

The main shock on April 25 was exceptional for a couple of reasons: It generated a tsunami, which was well recorded along the West Coast and measurable in the Hawaiian Islands. It also produced distinct uplift (maximum uplift = lm) along a 25-km-long section of the coastline. Ground shaking during the quake reached at least intensity VIII on the Modified Mercalli scale. The April 25 main shock is the largest historic earthquake in this region with an epicenter clearly located on land.

There was also a quake on September 1, 1994 that occurred about 110 miles west of the California coast. Preliminary magnitudes for this earthquake have been estimated from a low of 6.8 (Ml from UC Berkeley) to a high of 7.2 (Mw from the National Earthquake Information Center (NEIC) of the USGS). Both UC Santa Cruz and Harvard obtained Mw=7.0. There was no report of damage from this quake. Take Care…Don in creepy town