On April 13, 1923 at 15:31 UTC, an earthquake occurred off the northern coast of the Kamchatka Peninsula in the USSR, present-day Russia. The earthquake had a surface-wave magnitude () of 6.8–7.3 and an estimated moment magnitude () of 7.0–8.2. This event came just two months after a slightly larger earthquake with an epicenter struck south of the April event. Both earthquakes were tsunamigenic although the latter generated wave heights far exceeding that of the one in February. After two foreshocks of "moderate force", the main event caused considerable damage. Most of the 36 casualties were the result of the tsunami inundation rather than the earthquake.
Tectonic setting
The earthquake occurred off the Kamchatka Peninsula's east coast, which runs parallel to the Kuril-Kamchatka Trench, the area where the Pacific and Okhotsk Sea plates converge. Being older and therefore denser, the Pacific subducts beneath the Kamchatka Peninsula, which sits on the Okhotsk Sea Plate. These two plates meet along a convergent boundary, marked by the trench. The subduction zone is seismogenic and produces Kamchatka earthquakes, which occasionally generate tsunamis; some of these megathrust earthquakes are very strong (such as the 1952 magnitude 9.0 earthquake, the 5th largest ever recorded).
Earthquake
The April earthquake was part of a sequence of megathrust earthquakes on the Kamchatka Peninsula, which began in February. On February 3, a magnitude 8.4 earthquake, whose hypocenter was 15 km deep, resulted in extreme shaking assigned XI (Extreme) on the Modified Mercalli intensity scale and caused a tsunami with run-ups of 6 meters. It was followed by a magnitude 7.4 aftershock the same month.
The International Seismological Centre placed the April earthquake magnitude at 6.8 or 7.0 , while the NGDC and some older studies placed it at 7.3–7.4 . Beno Gutenberg and Charles Richter assigned the event a magnitude of 7.2 . A 2004 reevaluation of earthquakes in the region revised the earthquake magnitude to 8.2 , based on analyzing the associated tsunami. It was not considered an aftershock of the February earthquake because it ruptured another segment of the subduction zone.
Location
The epicenter coordinates of this earthquake differ across sources and journals. The International Seismological Summary placed this at 55.7°N, 162.5°E, while the ISC-GEM Catalogue lists this event as having an epicenter at 56.36° N 162.70° E. Meanwhile, Beno Gutenberg and Charles Richter suggested the epicenter location to be 56.5° N, 16.5° E. In a 2017 study, Salaree and Okal argued it was located 110 km to the north, at 57.35° N, 162.91° E, which is northwest of Bering Island. In another study by Bourgeois and Pinegina published in 2018, the source area of the April earthquake is north of the February rupture but southwest of Bering Island and in . On the other hand, E.R. Engdahl's relocation places the epicenter coordinates at 56.56°N, 163.03°E, inside the Kamchatskiy Peninsula and close to the ISC-GEM coordinates.
Characteristic
The earthquake rupture displayed features analogous to that of a tsunami earthquake, which generates locally destructive tsunamis with higher run-up heights. A notable feature that makes such events unique is the release of seismic energy which happens at long periods. The release of long-period seismic energy in turn results in moderately strong to no shaking along the coast during these earthquakes. Damage patterns of tsunami earthquakes solely from shaking have been mostly smaller than expected for its size. As a result, local communities usually would have little to no advanced natural warning for when a tsunami hit. Tsunami earthquakes have slower than usual rupture velocities, which could be as slow as 1 km/s. The rupture propagates up-dip along with the shallow subduction interface towards the trench, generating tsunamis. The tsunamis generated by these earthquakes are significantly disproportionate for its surface wave magnitude as a consequence.
Tsunami
Salaree and Okal suggested that apart from the slow rupture velocity, an earthquake-triggered submarine landslide is thought to have generated the unusually high local tsunami run-ups. The presence of subaerial landslides near elevated shorelines after the earthquake was also evidence of landslide-triggering tsunamis. The landslide tsunami model also satisfactorily reproduced the tsunami height distribution. A survey of the affected region discovered high run-up heights at 20–30 meters. The tsunami affected a stretch of coastline from Cape Shubert to Cape Kamchatskiy of the Kamchatka Bay.