Extent 7.2 Tremor SOUTH OF SAND POINT
On July 15, 2023, at 10:48 p.m. Alaska time, a magnitude 7.2 earthquake struck the offshore Alaska Peninsula region, nearly three years after the magnitude 7.8 Simeonof Earthquake occurred in the same region. The 7.2 event was located 50 miles south of Sand Point (around 100 miles southeast of the M7.8 event) at a depth of approximately 20 miles. A tsunami warning was issued following the earthquake but was downscaled about an hour later to an advisory level and finally cancelled shortly before 1:00 a.m. Tsunami waves with a maximum height of 0.5 ft. were observed in King Cove and Sand Point. Ground shaking was reported felt in many communities on the Alaska Peninsula and eastern Aleutian Islands, with intensity as high as V, moderate.
The July 15 earthquake occurred within the M7.8 aftershock zone. While the M7.8 aftershock activity had greatly diminished since its peak in the summer of 2022, the Earthquake Center was still observing elevated levels of seismic activity within the M7.8 aftershock zone in 2023. Therefore, the M7.2 earthquake can be recognized as a recent delayed aftershock of the M7.8 earthquake. Its source mechanism is similar to the Simeonof event and indicates fault rupture along the Aleutian megathrust. We anticipate that the M7.2 earthquake will generate its own aftershock sequence, similar to other moderate-sized earthquakes in the region. Thus far, the largest aftershock, an M5.0, occurred three minutes after the mainshock.
Another significant earthquake, magnitude 8.2, occurred on July 29, 2022 northeast of the Simeonof Earthquake epicenter. The M8.2 rupture propagated to the northeast, away from the M7.8 rupture zone.
The M7.2 earthquake on July 15, 2023, is another major event within a span of three years that ruptured the subduction zone interface from the Shumagin Islands in the southwest to Kodiak Island in the northeast. The Shumagin Island region had been identified as a seismic gap, an area where no significant earthquakes had occurred recently, prior to these three large earthquakes. The recent sequence of events partially filled this gap.
