Shallow Structure of the Eagle Rock and Raymond Faults in Arroyo Seco, Los Angeles County, California

To understand the location, dip, and possible structural connection of the Eagle Rock and Raymond faults in Pasadena and South Pasadena, California, we acquired and analyzed high-resolution seismic reflection and refraction data, as well as gravity observations, along the floor of the Arroyo Seco. The studies were conducted to aid in understanding the seismic hazards of these faults in this urban setting. Seismic reflection and refraction data, including both P-wave and S-wave records, were collected along two profiles, a 1.2-km-long northern profile crossing the Eagle Rock fault, and a 450-m-long southern profile crossing the Raymond fault. Seismic sources were Betsy-Seisgun shots, accelerated weight drops, and repeated sledge-hammer impacts, which were recorded on multi-channel seismograph systems connected to vertical- and horizontal-component geophones spaced at a 5-m interval. Gravity data were collected along a single ~3-km-long profile coincident with and extending beyond and between the two seismic profiles, with stations spaced every 25-m near the fault traces and at greater intervals farther from the fault traces. We carefully accounted for the gravity effects of the adjacent concrete drainage channel and of the walls of the arroyo, to generate gravity anomalies that reflect sub-surface density contrasts across the Eagle Rock and Raymond faults. Seismic reflection image quality is compromised by the highly-deformed Miocene strata offset by these faults. However, reflection and especially refraction results indicate that both the Eagle Rock and Raymond faults consist of multiple, steeply-north-dipping fault strands. P- and S-wave seismic tomography results of the uppermost 50-100 m yield velocity variations that can be converted to probable density variations, and thus be included in the gravity anomaly analysis. The gravity anomalies predicted from the velocity variations account for less than one-third of the anomalies observed across the faults, indicating that physical property contrasts extend significantly deeper than was imaged by the refraction tomography seismic survey. At a broader scale, we modeled aeromagnetic anomalies along a 40-km-long north-south profile across the Eagle Rock, Raymond, and neighboring faults. The clearest aeromagnetic anomaly is associated with the Eagle Rock fault, where our modeling constrains its dip to be ~60° to the north in at least the uppermost 5 km of the crust.