One magnitude, many intensities
USGS defines magnitude as a single size for the earthquake, and intensity as the shaking at each place. Intensity changes with distance from the rupture, the direction the rupture traveled, and the geology under the site. The Modified Mercalli Intensity scale is the one used in the United States. It is written in Roman numerals. A moderate earthquake nearby can produce stronger intensity than a larger earthquake that is deeper or farther away.
Depth changes the shaking
The same USGS page compares the 1994 M6.7 Northridge earthquake, at about 3–11 miles depth, with the 2001 M6.8 Nisqually earthquake, at about 30–36 miles. Northridge’s shaking was more intense and covered a wider area even though the magnitude was slightly smaller, because the source was shallower. Bedrock shakes less than soft mud at the same distance, which that page illustrates with three records from the 1989 M6.9 Loma Prieta earthquake.
How this site buckets depth
The one-year USGS snapshot behind the homepage (13 September 2025 through 12 September 2026 UTC) counts M4 and larger earthquakes in three depth buckets: 0–70 km, 11,820 events; 70–300 km, 3,385 events; 300 km or deeper, 1,063 events. None were missing a depth. Those buckets match the usual shallow, intermediate, and deep split. They describe the catalog window. They do not say that deep earthquakes are harmless. A deep earthquake can still be felt, and the Nisqually example was damaging even though it was deeper than Northridge.
Cascadia’s deep earthquakes
USGS Fact Sheet 2025-3050 says typical deep intraslab earthquakes in the Pacific Northwest occur at 25–40 miles (40–60 km), and names Olympia 1949 (M6.8), Seattle 1965 (M6.5), and Nisqually 2001 (M6.8). That is a narrower band than the global 0–70 km bucket. Read the Cascadia page for the megathrust, which is a different source again.