The 2019 Ridgecrest earthquake sequence in Southern California featured a magnitude 7.1 mainshock, the largest earthquake to strike the state in two decades. Understanding the duration of the intense ground movement provides insight into the forces structures must withstand. Seismologists measure this shaking duration by analyzing the arrival and decay of specific seismic waves, which offers a technical answer that differs from the subjective experience of those who felt it.
The Context of the 2019 Ridgecrest Earthquake Sequence
The magnitude 7.1 earthquake occurred on July 5, 2019, following a magnitude 6.4 foreshock just 34 hours earlier on July 4th. This sequence took place in the Eastern California Shear Zone, a region of distributed faulting located in the Mojave Desert, northeast of Ridgecrest and southwest of Searles Valley.
Both major quakes involved shallow strike-slip faulting, where ground blocks moved horizontally past one another. The M7.1 mainshock was significantly larger than the M6.4 event, extending the rupture along a fault system approximately 50 kilometers long. The M7.1 fault movement was primarily right-lateral, occurring along the Paxton Ranch Fault Zone, resulting in a complex, multi-fault rupture pattern defining the entire sequence.
Measuring the Shaking: Duration of the 7.1 Mainshock
Earthquake duration is not a single, fixed number, as it depends on the intensity of shaking and the observer’s location. For the M7.1 Ridgecrest event, the most intense, damaging ground motion near the epicenter is estimated to have lasted for 10 to 20 seconds. This period represents the time of strong ground motion that poses the greatest risk to built infrastructure.
Seismologists define earthquake duration by tracking the arrival of different types of seismic waves. Shaking begins with the primary, or P-wave, a compressional wave often felt as a quick jolt. The destructive shaking then follows with the slower secondary, or S-wave, which causes violent side-to-side and up-and-down motion. The measured duration of strong shaking is the time between the arrival of the S-wave and the point when ground motion intensity decays below a certain threshold.
The felt duration for people farther away from the epicenter can be much longer, as surface waves continue to travel and cause perceptible swaying. Reports from residents suggested the shaking lasted for up to 30 seconds or more, which includes the entire sequence of waves. The duration of the strong, concentrated energy release from the fault rupture itself is the shorter, tens-of-seconds figure, which is the metric most relevant to earthquake engineering and building performance.
The Relationship Between Duration and Earthquake Energy
The duration of strong shaking is closely tied to the size of the fault rupture and the total energy released. Larger magnitude events, such as the M7.1, involve longer fault segments slipping over a greater area, resulting in a longer duration of energy release. The M7.1 Ridgecrest event radiated more than three times the amount of seismic energy compared to the M6.7 Northridge earthquake.
A longer duration of strong shaking is a significant factor in the destructive potential of an earthquake. Sustained shaking subjects structures to cumulative damage, meaning the building is pushed and pulled repeatedly, increasing the stress on structural components. This effect is often more destructive than a brief, high-intensity burst of shaking.
Local geology also plays a role in how long the ground motion lasts. Communities like Trona, built on soft lakebed sediments, experienced amplified and prolonged shaking compared to those on bedrock. This extended exposure contributed to liquefaction and higher levels of damage in soft-sediment areas. The total energy released and the time over which it is released determine the ultimate impact of a large seismic event.