Landslide area probability density function statistics to assess historical landslide magnitude and frequency in coastal California

• Two landslide area inventories characterize historical slides in a small CA watershed.
• The landslide area probability density function quantifies slide magnitude-frequency.
• Power functions of the linear portion of the distributions have exponents near − 2.0.

Landslide area probability density function (PDF) statistics elucidate the landslide magnitude–frequency distribution in a small coastal watershed in central California. Detailed mapping into a GIS and compilation of two slide inventories in Walker Creek, Marin County, reflect erosional effects of storms occurring in 1941 and 1998, respectively. We focus on the spatial distribution and area of slides to illustrate that during both periods slides originate mainly from hillslopes underlain by Franciscan Complex units with slopes ranging from ~ 10 to 40%. The majority of slides (~ 70%) have areas ranging between 100 and 1000 m2. The magnitude–frequency distribution of slides represented by the PDF produces a curve that has a linear portion representing the relatively large slides. The power functions of the linear portion of these curves have exponents of about − 2.0 for both inventories. Results suggest that the storm event-based PDF of landslide area developed using historical aerial images is a viable means of quantifying the landslide magnitude–frequency relationship at the scale of a small watershed.