The 1971 Sylmar earthquake in brief
The 1971 Sylmar earthquake struck the San Fernando Valley region of Los Angeles on February 9, 1971. Strong shaking typically lasted 12 to 15 seconds, with the ground motion perceptible for about a minute in many places. This profile explains how we define duration, what controls shaking length, and how this event compares to other Southern California earthquakes.
What is earthquake duration
Earthquake duration is the length of time that strong ground shaking is felt at a given location. It is not the same as the total time energy is released, which can extend longer, but the period during which motions are intense enough to be felt or cause damage. Duration depends on magnitude, distance, local geology, and the fault mechanism. For widely felt earthquakes near cities, strong shaking commonly ranges from a few seconds to 30–60 seconds.
How long did the 1971 Sylmar earthquake last at key locations
At many sites in the San Fernando Valley and nearby Los Angeles areas, the strongest shaking from the 1971 Sylmar earthquake lasted roughly 12 to 15 seconds. The overall perceptible shaking, including weaker waves, continued for about a minute. These values align with reports from the United States Geological Survey and the California Strong Motion Instrumentation Program. The table below summarizes selected measured durations and intensities.
| Metric | Verified Detail | Source Type |
|---|---|---|
| Strong shaking duration | 12 to 15 seconds | CSMIP stations |
| Perceptible shaking | About 60 seconds | USGS report |
| Epicenter location | San Fernando Valley, Los Angeles County | USGS |
| Peak intensity | MMI VII (Very strong) | CSMIP |
| Magnitude | 6.5–6.7 | USGS |
What influences how long shaking lasts
Several factors control how long strong shaking is felt during an earthquake near a city:
- Magnitude: Larger quakes generally produce longer duration shaking.
- Distance from the rupture: Sites closer to the rupture may feel shorter but more intense shaking; farther sites may feel longer but less intense shaking.
- Local geology: Soft soils can trap and amplify waves, extending duration; firm bedrock tends to transmit shorter, sharper motions.
- Fault type and orientation: Strike-slip events like Sylmar tend to produce shorter, sharper pulses compared to some dip-slip events.
Comparison with other Southern California earthquakes
How the 1971 Sylmar earthquake’s duration compares with other events that affected the Los Angeles region:
| Earthquake | Year | Magnitude | Typical strong shaking duration | Notes |
|---|---|---|---|---|
| Northridge | 1994 | 6.7 | 20–40 seconds | Longer duration due to slip at depth and basin amplification |
| San Fernando (Sylmar) | 1971 | 6.5–6.7 | 12–15 seconds | Strong motion recorded on firm ground |
| Landers | 1992 | 7.3 | 30–60 seconds | Large strike-slip event with long surface waves |
Why reported durations can vary
Reported durations for the 1971 Sylmar earthquake differ across studies because of different measurement methods. Some reports focus on strong motion recorded by instruments, while others consider perceptibility by people or damage duration. Instrumental records typically show 10–20 seconds of significant motion; community accounts may describe shaking felt for closer to a minute. Differences in site conditions also affect local observations. This variability is normal for many earthquakes and does not indicate uncertainty about the event itself.
How this affects buildings and infrastructure
Shaking duration influences how much stress structures accumulate. Longer duration shaking can increase demand on mid- and low-rise buildings, especially if motions are near resonance with the building’s natural period. The 1971 Sylmar earthquake caused notable damage to unreinforced masonry, non-ductile concrete, and some freeway structures, partly due to the intensity and the duration of strong motion. Modern design codes now account for duration effects, particularly in regions with significant soft-soil amplification.
Key takeaways
- Strong shaking from the 1971 Sylmar earthquake lasted about 12 to 15 seconds.
- Perceptible shaking continued for roughly 1 minute in most areas.
- Duration depends on magnitude, distance, local geology, and fault type.
- Compared to later events like Northridge, Sylmar produced shorter but intense shaking on firm ground.