QuakeIndex

Fault · Alaska

Middle Ground Shoal anticline (Cook Inlet folds)

43 kmlong (27 mi), as mapped
1–5 mm/yrhow fast it slips
< 1.6 million yearssince it last broke the surface
44quakes of M2.5+ within 10 km since 2016

In plain English

The two sides move 1 to 5 mm a year — 10 to 50 cm (4 to 20 inches) every century.

It has moved within the last 1.6 million years.

The largest earthquake recorded near it was M 6.7 in 1934 (6 km WNW of Nikiski, Alaska) — about 380,000 times the energy of an M3.0, the size people start to feel. It released about 180 kilotons of TNT.

This week vs. its last ten years

0/100

quiet

0 quakes of M1.5+ within 5 km of the trace in the last 7 days — busier than 0% of weeks since 2016. A normal week: 0–1.

Computed from the USGS catalogue · a description of the past, not a forecast · method

Denali faultCastle Mountain faultGranite Point anticline (Cook Inlet folds) M 6.7 · 1934 · 6 km WNW of Nikiski, Alaska M 6.4 · 1941 · 19 km SSE of Tyonek, Alaska
The Middle Ground Shoal anticline (Cook Inlet folds) as mapped at the surface (glowing), with the other mapped faults nearby, 44 of the M2.5+ earthquakes within 10 km of it since 2016 (dots), and the epicentres of the large historic earthquakes recorded near it (rings). Traces: USGS Quaternary Fault and Fold Database. Historic epicentres are estimates.

Every year since 2016

Earthquakes of M2.5+ with an epicentre within 10 km of the mapped trace. Near is not the same as on: some will have slipped on neighbouring faults.

The big ones recorded near it

  1. M 6.7 6 km WNW of Nikiski, Alaska
    June 18, 1934 · epicentre 7 km from the trace
  2. M 6.4 19 km SSE of Tyonek, Alaska
    July 30, 1941 · epicentre 21 km from the trace

Before 1973 from the USGS ComCat historical catalogue (within 25 km; epicentres of old earthquakes are estimates). Since 2016, M4+ within 10 km.

Most recent near it

  1. M 2.5 16 km W of Salamatof, Alaska
    2026-09-08 · 4 km from the trace
  2. M 2.5 7 km WNW of Nikiski, Alaska
    2026-09-04 · 6 km from the trace
  3. M 2.5 15 km NNW of Nikiski, Alaska
    2026-08-30 · 5 km from the trace
  4. M 2.7 21 km SSW of Tyonek, Alaska
    2026-08-17 · 6 km from the trace
  5. M 3.4 20 km W of Salamatof, Alaska
    2026-08-02 · 7 km from the trace
  6. M 2.5 12 km WSW of Salamatof, Alaska
    2026-05-21 · 3 km from the trace

Neighbouring faults

Fault trace, slip rate and age: USGS Quaternary Fault and Fold Database (public domain), simplified for the web. Earthquakes: USGS ComCat. QuakeIndex does not forecast earthquakes; a fault's history describes the past. For early warning use ShakeAlert.