Midcrustal, Long-period Earthquakes beneath Northern California Volcanic Areas

Abstract

Long-period (LP) volcanic earthquakes are commonly associated with magma migration and magmatic interactions with hydrothermal systems in the shallow crust. Swarms of shallow (depths < 5 km) LP earthquakes serve as one of the more robust seismic precursors to volcanic eruptions (Chouet, 1996). LP earthquakes also occur at mid- to lower crustal depths beneath young volcanic systems (Hasegawa et al. , 1991; Koyanagi et al. , 1987; White, 1996). In this paper, we document the occurrence of LP earthquakes at midcrustal focal depths beneath each of the major volcanic centers in northern California that have produced late-Quaternary to recent eruptions. These volcanic centers include the Mount Shasta-Medicine Lake complex, Lassen Peak, Clear Lake, and the Mammoth Mountain-Mono Craters complex (Figure 1). In addition, we find a small cluster of LP's beneath the central Sierra Nevada 30 km west-southwest of Mammoth Mountain. Seismograms of LP earthquakes differ from the much more common broadband signature of brittle-failure (BF) earthquakes in that they are relatively deficient in energy at frequencies above 5 Hz and have an extended, ringing coda (Figure 1). Their onsets are sometimes impulsive with relatively clear P and S phases but are more commonly rather emergent. They have comblike spectra with dominant peaks generally from 1–5 Hz and often (but not always) in a harmonic relation to one another. The LP earthquakes described in this paper were all detected and located using stations within the Northern California Seismic Network (NCSN). They have focal depths ranging from 10-35 km and are systematically located near or below the base of the seismogenic crust as defined by the maximum focal depth of nearby and much more numerous BF earthquakes. The LP earthquakes are small, with amplitude magnitudes of M ∼2.5 or less. Their dominant spectral peaks are typically in the range …