EARTHQUAKES AFFECT SIZE, NOT FREQUENCY, OF DEBRIS FLOWS IN SAN GABRIEL MOUNTAINS, CA

The San Gabriel Mountains are infamous for debris flows, earthquakes, and fire. These events are revealed in exquisite detail in over 1 km of trenches originally excavated for a San Andreas fault paleoseismic record at a site 2.5 km NE of the town of Wrightwood, CA (elevation 1900 m). In addition to >30 earthquakes, the stratigraphic record contains 140 individual debris flow layers separated by dark, organic-rich, meadow deposits. Over 200 ¹⁴ C dates on plant remains from the organic layers provide high-resolution age control for this 6000 year record. The debris flows debouch from a 1 km² catchment and are deposited across a ~0.1 km² debris fan. The flows have a minimum runout of ~300 meters and the most voluminous enter Swarthout Creek, which flows down the axial valley formed by the San Andreas fault. The debris-flow layers are laterally continuous and spread across the fan rather than becoming channelized by levee systems. To date, I have examined the stratigraphic record for relationships between the debris flow and the 14 earthquake record from 3000 to 1500 B.C. Debris-flow layers succeeding earthquake horizons are equal or greater than the average layer thickness, and are deposited 10-30 years after the earthquake, suggesting that earthquakes increase the size of the debris flows but sufficiently large storms are required to mobilize the deposits, and these storms occur frequently. In this 1500-year period, debris flow recurrence shows two modes- a common ~25 year return interval punctuated by pauses during which only one debris flow occurs in ~145 years. At first order, these variations are not correlated with variations in the earthquake frequency, indicating that storm frequency plays a stronger role than earthquakes in routing sediment to the fluvial system. Future work will examine the variation of climate and fire frequency with the debris flow frequency and size.