# 1995 Neftegorsk earthquake

The 1995 Neftegorsk earthquake occurred on 28 May at 1:04 local time in northern Sakhalin Island in the Russian Far East. It was the most destructive earthquake known within the modern borders of Russia, with a magnitude of M$_w$7.1 and maximum Mercalli intensity of IX (Violent) that devastated the town of Neftegorsk. Many buildings collapsed, and 1,989 of its 3,977 citizens were killed, with another 750 injured. Infrastructure was catastrophically damaged, leading to Neftegorsk becoming a ghost town. Surface effects from the earthquake were widespread, with many geological features changing or developing. Due to its location along a poorly understood plate boundary, the earthquake received considerable attention from scientists, and dozens of research papers have been written about it.

## Tectonic setting

Sakhalin lies along the destructive plate boundary zone between the Amur microplate (part of the Eurasian plate system) and the Okhotsk microplate (part of the North American plate system). The Sakhalin-Hokkaido fault is the main plate boundary in Sakhalin with a slip rate of 4 mm (0.16 in)/yr, and runs along the island. Unlike most of the plate boundary, in North Sakhalin faulting is predominantly strike-slip and not reverse.
In the area near the earthquake, three faults take up the brunt of the slip: the Gyrgylanye-Dagy, Piltun-Goromai, and Upper Piltun (also known as the Verkhnii Pil’tun or Gyrgylan’i–Ossoy) faults. The Piltun-Goromai fault runs along the eastern coast of Sakhalin for 40–90 km (25–56 mi) in a roughly north-south alignment with a slip rate of 3–5 mm (0.12–0.20 in)/yr. Along the Piltun-Goromai fault, strong earthquakes recur every 2,300–5,000 years.
The Upper Piltun fault—which ruptured in this earthquake—branches west of the Piltun-Goromai fault and runs south-south-west until it reaches the Gyrgylanye-Dagy fault. It has a strike of 15 degrees and a dip of 60-70 degrees to the west. Three large events in the past 1,800 years have been identified along the Upper Piltun fault. A magnitude 7.8 earthquake occurred on the fault 2,600 years ago, and another large event occurred 4,000 years ago. Recurrence interval of large earthquakes on the Upper Piltun fault is estimated to be 400 years. When geodetic data was collected on the fault between 1941 and 1970, the fault was likely locked, and accumulating stress. The lack of previously recorded seismicity also implies that the fault was locked.
The Gyrgylanye-Dagy fault is the westernmost of the three, running parallel to the Piltun-Goromai fault. The slip rate is estimated to be at least 1 mm (0.039 in)/yr. Prior to this earthquake, North Sakhalin had been relatively aseismic, with the largest earthquake in the area being the M$_w$ 5.8 Nogliki earthquake of 1964.

## Earthquake

The earthquake struck on May 28 at 1:04 local time in northern Sakhalin in the Russian Far East near the town of Neftegorsk. This M$_w$ 7.1 earthquake had a maximum Mercalli intensity of IX (Violent), with ESI intensity reaching XI. The earthquake was also reported to be M$_s$ 7.2–7.6 and M$_{uk}$ 7.8, depending on the source. The earthquake struck on the right-lateral strike-slip Upper Piltun fault. The earthquake was felt widely across the island, and nearby portions of mainland Asia experienced shaking as well. Sabo and Tungor experienced MSK VII (Very strong) shaking, while Okha and Nogliki felt MSK VI (Strong). On mainland Asia, Lazarev felt MSK IV (Largely observed) shaking. There were over 1,000 aftershocks recorded in the first month and a half, with none exceeding M$_s$ 5. The aftershocks spanned a length of 60–80 km (37–50 mi) along the Upper Piltun fault.
The shock ruptured for 35–46 km (22–29 mi) at a strike of 15 degrees, with an average slip of 3.6–3.9 m (12–13 ft). The depth of faulting extended to 10–20 km (6.2–12.4 mi) deep. Three separate segments of fault ruptured, with the northern section slipping the most. Horizontal (right-lateral strike slip) slip peaked at 8.1 m (27 ft), with the minor vertical (reverse) component peaking at 1.5–1.7 m (4 ft 11 in – 5 ft 7 in). The stress drop (how much less stress the fault is under after the earthquake) as a result of this quake was 4–11 MPa, a value typical of an intraplate earthquake. The large peak slip (8.1 m (27 ft)) and the lack of magnitude 5 or larger aftershocks suggest that the mainshock released a significant majority of the accumulated strain on the fault.

## Impact

Due to the previous lack of strong earthquakes in the area, Neftegorsk was hit hard as it was unprepared. During the earthquake, 17 five-story residential buildings collapsed, leading to the deaths of 1,989 and 750 more injured. These buildings housed a majority of the town, and 90% of the fatalities were from the collapses. These buildings were only designed to withstand MSK VI (Strong) rather than the IX (Destructive) – X (Devastating) experienced in reality. The 650 residents who lived in the shorter brick buildings—which didn't collapse—survived the earthquake. In nearby Okha, balconies fell off of buildings and furniture broke. This earthquake was the most destructive earthquake ever recorded within modern day Russian borders.
The shock instantly cut off power, water, and telephone lines. The quake severely damaged infrastructure, with "destruction of buildings, bridges, railways and roads, breakage of oil and gas pipelines, electric and communication lines". The administration building, a building to generate heat, and some shops were also destroyed. The destroyed railway had deformation up to 300 m (980 ft) long, with locations close to the rupture even experiencing wave-like bends. Two oil pipelines were badly damaged in the area, and they resultantly began leaking oil which seeped into small lakes. Telegraph poles, railways beds, and even a locomotive at the railway station all shifted to the east-southeast. The Belgian Centre for Research on the Epidemiology of Disasters' EM-DAT database places the total damage at $64.1 million, while the United States' National Geophysical Data Center assesses the damage at $300 million.

### Geological effects

The earthquake caused significant surface changes, including landslides, new islands, surface fractures, liquefaction, and mud volcanoes. Landslides damaged roads, and were observed on almost every coastal slope near the rupture area. The largest landslide was 50 km$^3$ (12 cu mi) and blocked the Cadylanye River for a bit. Mud volcanoes cropped up both near the fault and dozens of kilometers away, with the best example occurring 25 km (16 mi) from the source. The mud volcanoes were varied, with almost every single type of volcano structure represented on the mud volcanoes formed after the earthquake. Surface ruptures were widespread. In the Piltun Bay, several new small, sandy islands formed. Liquefaction was observed outside the epicentral region along the seashore.

## Response

Immediately after the earthquake struck, Russian rescue teams from the surrounding areas prepared to deliver aid. Eighteen planes and helicopters were prepared; however, some were unable to deliver their aid due to heavy fog preventing landings. Since the temperatures in Sakhalin were bitterly cold, the cargo included blankets along with food. The Russian government released 30 billion rubles to use on rescue and aid. Authorities were able to secure basic necessities for survivors, with food, water, and shelter being taken care of. The largest transport planes faced challenges in delivering their cargo to Neftegorsk as they could not land at the nearby airport as the planes were too big. Field hospitals were set up and hundreds of injured victims were medevaced to larger nearby cities like Khabarovsk.

## Aftermath

After the rescue operations had been carried out and things had settled down, Moscow decided not to rebuild Neftegorsk. Residents longed to get away from the horror they experienced during the earthquake, so few had interest in remaining. When offered the chance to move elsewhere on Sakhalin island by government officials, the former residents only wanted compensation rather than to remain on the island. After the earthquake, the town completely emptied out and became a ghost town. Due to the unprecedented and unpredicted seismicity in the area, the building code and seismic risk maps were soon updated to reflect the earthquake and its impacts.

## Scientific interest

This event occurred on the poorly defined plate boundary between the Amur microplate and the Okhotsk microplate in an area where no large earthquakes were previously known. Due to this, many scientific studies were conducted to help understand the earthquake and the tectonic background of the area it struck in. A total of 26 research papers have been published on the earthquake itself. Despite many seismic sensors being removed from the island just before the earthquake due to a lack of funding, the earthquake still provided valuable insight into understanding and resolving the insufficiently understood plate boundary.