Teleseismic residual study of the Lassen Volcanic National Park region in California

Berge, Patricia; Monfort, Mary

To investigate the crust and upper mantle structure under Lassen Peak and the surrounding parts of the Cascade Range in northern California, the U.S. Geological Survey conducted several seismic experiments in 1980. An experiment utilizing the teleseismic compress ion al-wave travel time residual method (e.g. Steeples and lyer, 1976; Ellsworth and Koyanagi , 1977; Reasenberg et al ., 1980; lyer et al., 1981) is described here. Data collected concurrently at the same stations, to study both local seismicity and the attenuation structure of the Lassen region, has been presented elsewhere and will not be discussed here (Monfort, 1980; Monfort, 1982). In this report, we present and discuss the travel time residuals and qualitatively describe the P-velocity structure that produced these residuals. A three-dimensional inversion of the residuals to model the velocities beneath the array quantitatively is in progress and will be presented in a later paper. The teleseismic P-wave residual method has been used successfully in many different regions to model low-velocity anomalies associated with magna bodies (Reasenberg et al., 1980; Robinson and lyer, 1981; lyer et al ., 1981). The long history of mafic and silicic volcanism, historic eruptions of magna at Cinder Cone (Figure 1) and at Lassen Peak (Williams, 1932; Clynne, 1984), a well-developed hydrothermal system (Muffler et al., 1982), and seismicity studies (Klein, 1979; Walter et al ., 1984) suggest that a detectable magma chamber may be present beneath the Lassen volcanic center. Estimates from geochemical and gravity data (Eichelberger, 1979; Clynne, 1984; Clynne, 1985) predict that such a chamber may be 10-20 km deep. Teleseisms are events at a distance, delta, of more than 25° from a station. Rays from a teleseism have subvertical incidence angles (typically 15-30°) at a station (Achauer et al., 1986). If the array dimensions are small compared to the raypath length, the rays from an event to an array of distant stations are subparallel. The incidence angles and relative traveltimes at the array are only weakly dependent on structure near the source and along most of the raypath (Aki et al., 1977). Relative travel time residuals across the array therefore arise mainly from compress ion al-wave velocity anomalies in the crust and upper mantle directly beneath the array. Inversions of these traveltime residuals can resolve velocity anomalies to a depth roughly equal to the array length, 35-60 km in this case. The lateral resolution for this type of experiment is limited by the wavelength of teleseisms, about 5-10 km, so that bodies with lateral dimensions smaller than 5 km can not be resolved (Robinson and lyer, 1981). For eleven weeks in the summer and early fall of 1980, 14 of the U.S. Geological Survey's portable "five-day recorder" seismic instruments (Criley and Eaton, 1978) and six permanent network stations (Eaton, 1977) continuously recorded in a 35- x 60-km array centered on Lassen Peak (Figure 1, Table 1). Sixty-five large (m5 > 4.6) teleseisms were recorded (Table 2). Two events came from the northeast, 17 from the southeast, 31 from the southwest, and 15 from the northwest (Figure 2) The PKIKP phase of one of the northwest events was recorded and timed; the P phase of all other events was recorded and timed.

Teleseismic residual study of the Lassen Volcanic National Park region in California

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