Seismic Swarm S20031009.1: Analysis of Earthquake Activity Near Floriston, California

The seismic swarm designated S20031009.1 occurred approximately 10 km east-southeast of Floriston, California, in the northern Sierra Nevada region. This event sequence began at 19:03 on 8 October 2003 and concluded at 15:02 on 11 October 2003, spanning 67 hours and 59 minutes. During this period, 40 earthquakes were recorded, providing a clear example of swarm-type seismicity in an area influenced by regional extensional tectonics.

The earthquakes exhibited low magnitudes, ranging from 0.0 to 1.6, with the largest event measuring 1.6 on 9 October 2003 at 01:28:48. Depths were predominantly shallow, clustered between 5 km and 12 km, consistent with activity along upper crustal faults. Activity peaked on 9 October, with multiple events occurring in quick succession during midday hours, followed by a gradual decline through 10 and 11 October. This temporal pattern reflects typical swarm behavior, where events occur in clusters without a dominant mainshock-aftershock sequence.

Geologically, the Floriston area lies near the boundary between the Sierra Nevada and the Basin and Range Province. This transition zone features active normal faulting driven by east-west extension, which accommodates regional strain accumulation. The underlying bedrock consists primarily of Mesozoic granitic intrusions and Paleozoic metamorphic rocks, overlain by Quaternary volcanic and sedimentary deposits. Historical seismic records indicate that while isolated earthquakes are not uncommon, swarm events remain infrequent in this specific locale.

Data compiled since 1 January 2000 confirm that only one swarm has been documented in the region, with S20031009.1 representing that occurrence. Such rarity underscores the localized nature of the stress release mechanism at play during this sequence.

References

USGS Earthquake Catalog California Geological Survey Regional Fault Maps SeismoSight Internal Classifications