The three largest events were the M7.2 thrust mainshock that struck near the unincorporated community of Petrolia midday on April 25 and two primary strike-slip aftershocks measuring 6.5 and 6.6 that followed early the next morning.

As the largest earthquake in California since the 1989 Loma Prieta event, the mainshock caused a non-destructive tsunami that quickly reached the coast, and eventually Alaska and Hawaii several hours later.

The Mendocino triple junction (strike-slip/strike-slip/trench) formed 29–30 mya at 31° N (west of present-day Baja California) when the Pacific-Farallon spreading center initially approached the subduction zone off the coast of western North America.

While the focal mechanism indicated slip on a thrust fault striking N.10°W with a shallow dip of 13° to the east-northeast, the rupture most likely propagated to the west, based on the mainshock location at the southeastern boundary of the aftershock zone.

Investigation of more than 1,200 surveys from the North Coast area led to the assignment of an intensity rating of IX (Violent) on the Modified Mercalli scale for the region near Petrolia.

Of the several thousand aftershocks in the sequence, none were found to have occurred on the Mendocino fracture zone, but numerous events were located on the eastward projection of that fault.

In Rio Dell, across the Eel River from Scotia, glass store fronts along the main street were shattered and numerous buildings slipped into a culvert along Monument Road.

In Petrolia (the small community closest to the epicenter) the general store (combined with a post office and gas station) was destroyed by fire, and in Fortuna, damage totaled $4 million.

A large fire was triggered following the first aftershock at a shopping center in Scotia that destroyed four businesses, with the resulting damage at that site alone estimated at $15 million, and was the largest individual financial misfortune of the sequence of earthquakes.

Some were mere seconds while others lasted for hours, but the hydroelectric plant's performance at Ruth Reservoir was deemed acceptable, and power that was not generated locally was unaffected.

A post-earthquake lab test of the seismometer and an inspection of the photographically enlarged accelerogram revealed that the limit was hit twice, with a maximum deflection of 31 mm, as the needle bounced off the unit's mass.

[13] The sequence of earthquakes caused widespread landslides from the coast to east of Scotia and from the northern extent of the Eel River basin near Thompson Hill to south of Petrolia.

[14]The mainshock generated a small tsunami that was recorded by the National Oceanic and Atmospheric Administration's sea level gauge stations on the coasts of northern California, Oregon, and Hawaii.

The location of the islands lay on a great circle route that is also perpendicular to the region of (presumed) uplifted land at the coast near Cape Mendocino and any energy distributed would be the strongest in that direction.

No tsunami was detected at Johnston Atoll, 5,050 kilometers (3,140 mi) from Cape Mendocino in the north Pacific Ocean, but bottom pressure recorders registered a maximum amplitude of .4 cm (0.16 in) in 4,000 meters (13,000 ft) of water in the Gulf of Alaska, with 3.75 hours of travel time.

While the waves generated by the earthquake were limited, the event demonstrated the rapid onset of tsunami hazards, giving little time for coastal residents to prepare.