Harmonic excitation of mantle Rayleigh waves by the 1991 eruption of Mount Pinatubo, Philippines

An unusually long (at least two hours) seismic wave train having periods of about 230 sec was recorded at many worldwide seismic stations during the major eruption of Mount Pinatubo in the Philippines on June 15, 1991. This wave train exhibits two sharp spectral peaks at 228 and 270 sec. The group velocity, phase velocity, and the particle motion of this wave train indicate that it is a Rayleigh wave. The most probable excitation mechanism is acoustic coupling of atmospheric oscillations that were set off by continuous thermal energy flux from the volcano. The two spectral peaks correspond to the characteristic periods of acoustic and gravity modes of the Earth's atmosphere. The magnitude of the vertical single force equivalent to the acoustic coupling is 1.6×10^(17) dynes over a frequency band of 0.003 to 0.01 Hz. The results suggest the possibility of using acoustically coupled Rayleigh waves for detection, characterization and quantification of volcanic eruptions. Acoustic coupling of the atmosphere and the solid Earth provides a unique seismic source with long duration

Initial rupture of earthquakes in the 1995 Ridgecrest, California Sequence

Close examination of the P waves from earthquakes ranging in size across several orders of magnitude shows that the shape of the initiation of the velocity waveforms is independent of the magnitude of the earthquake. A model in which earthquakes of all sizes have similar rupture initiation can explain the data. This suggests that it is difficult to estimate the eventual size of an earthquake from the initial portion of the waveform. Previously reported curvature seen in the beginning of some velocity waveforms can be largely explained as the effect of anelastic attenuation; thus there is little evidence for a departure from models of simple rupture initiation that grow dynamically from a small region. The results of this study indicate that any “precursory” radiation at seismic frequencies must emanate from a source region no larger than the equivalent of a M0.5 event (i.e. a characteristic length of ∼10 m). The size of the nucleation region for magnitude 0 to 5 earthquakes thus is not resolvable with the standard seismic instrumentation deployed in California

Large-amplitude Moho reflections (SmS) from Landers aftershocks, southern California

Closely spaced aftershocks of the 28 June 1992 Landers earthquake (M_w 7.3) were used to make event record sections that show the transverse components of S and SmS arrivals at a distance of 70 to 170 km. For the data recorded toward the north in the Mojave desert, large SmS phases are observed with amplitudes 2 to 5 times greater than the direct S. For similar distances to the south, the SmS arrival is comparable to or smaller than the S. Comparisons to synthetic seismograms indicate that the large-amplitude SmS phases are produced by the simple crustal structure of the Mojave desert that allows a large Moho reflection. In contrast, the more complex geologic structure to the south partitions the seismic energy into a more complicated set of seismic phases, so that the Moho reflection is diminished in amplitude. The large SmS phases observed in the Mojave enhance the overall ground motions by a factor of 2 to 3. This suggests that when damaging earthquakes occur in other regions of simple crustal structures, Moho reflections will produce amplified strong motions at distance ranges around 100 km depending on the local structure

P-wave picking for earthquake early warning: refinement of a T[pd] method

Detecting P-wave onsets for online processing is an important component for real-time seismology. As earthquake early warning systems around the world come into operation, the importance of reliable P-wave detection has increased, since the accuracy of the earthquake information depends primarily on the quality of the detection. In addition to the accuracy of arrival time determination, the robustness in the presence of noise and the speed of detection are important factors in the methods used for the earthquake early warning. In this paper, we tried to improve the P-wave detection method designed for real-time processing of continuous waveforms. We used the new T[pd] method, and proposed a refinement algorithm to determine the P-wave arrival time. Applying the refinement process substantially decreases the errors of the P-wave arrival time. Using 606 strong motion records of the 2011 Tohoku earthquake sequence to test the refinement methods, the median of the error was decreased from 0.15 to 0.04 s. Only three P-wave arrivals were missed by the best threshold. Our results show that the T[pd] method provides better accuracy for estimating the P-wave arrival time compared to the STA/LTA method. The T[pd] method also shows better performance in detecting the P-wave arrivals of the target earthquakes in the presence of noise and coda of previous earthquakes. The T[pd] method can be computed quickly, so it would be suitable for the implementation in earthquake early warning systems

Localized boundary layer below the mid-Pacific velocity anomaly identified from a PeP precursor

Dense record sections from deep earthquakes in Fiji and Argentina recorded on hundreds of short-period stations in California at distances of 81° to 85° are used to investigate the detailed P wave velocity structure above the core-mantle boundary (CMB). In the Fiji data a secondary phase arriving 2 to 4 s after the direct P is identified as a precursor to PcP. This phase provides good evidence for a reflection off the top of a thin low-velocity layer above the CMB. Comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5%–10% compared to the overlying mantle. A record section from an Argentina event does not show the PcP precursor, indicating that the low-velocity layer is not a global feature. This thin low-velocity layer is in the same place as a much larger S wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the CMB under the mid-Pacific

Stress drops and radiated energies of aftershocks of the 1994 Northridge, California, earthquake

We study stress levels and radiated energy to infer the rupture characteristics and scaling relationships of aftershocks and other southern California earthquakes. We use empirical Green functions to obtain source time functions for 47 of the larger (M ≥ 4.0) aftershocks of the 1994 Northridge, California earthquake (M6.7). We estimate static and dynamic stress drops from the source time functions and compare them to well-calibrated estimates of the radiated energy. Our measurements of radiated energy are relatively low compared to the static stress drops, indicating that the static and dynamic stress drops are of similar magnitude. This is confirmed by our direct estimates of the dynamic stress drops. Combining our results for the Northridge aftershocks with data from other southern California earthquakes appears to show an increase in the ratio of radiated energy to moment, with increasing moment. There is no corresponding increase in the static stress drop. This systematic change in earthquake scaling from smaller to larger (M3 to M7) earthquakes suggests differences in rupture properties that may be attributed to differences of dynamic friction or stress levels on the faults

The 3 December 1988, Pasadena earthquake (M_L = 4.9) recorded with the very broadband system in Pasadena

Since 1 December 1987, a very broadband seismographic system has been in operation at the Kresge Laboratory of the California Institute of Technology. This system consists of the Streckeisen-1 very broadband sensor (Wielandt and Streckeisen, 1982), the Kinemetrics FBA-23 triaxial force balance accelerometer and a Quanterra data-logger with a 24-bit (for Streckeisen-1) and a 16-bit (for FBA-23) digitizer. The details of the data logger are described in Steim (1986). The overall dynamic range of this system is about 200 db. This system was constructed as a joint project between the California Institute of Technology, the U.S. Geological Survey, the University of Southern California and the International Research Institution for Seismology (IRIS), and is an element of the IRIS global network as well as the TERRAscope network of California Institute of Technology. A brief description of the system is given by Given et al. (1989)

Excitation of atmospheric oscillations by volcanic eruptions

We investigated the mechanism of atmospheric oscillations with periods of about 300 s which were observed for the 1991 Pinatubo and the 1982 El Chichón eruptions. Two distinct spectral peaks, at T = 270 and 230 s for the Pinatubo eruption and at T = 195 and 266 s for the El Chichón eruptions, have been reported. We found similar oscillations for the 1980 Mount St. Helens and the 1883 Krakatoa eruptions. To explain these observations, we investigated excitation problems for two types of idealized sources, “mass injection” and “energy injection” sources, placed in an isothermal atmosphere. In general, two modes of oscillations, “acoustic” and “gravity” modes, can be excited. For realistic atmospheric parameters, the acoustic and gravity modes have a period of 275 and 304 s, respectively. For a realistic time history of eruption, atmospheric oscillations with an amplitude of 50 to 100 Pa (0.5 to 1 mbar) can be excited by an energy injection source with a total energy of 10^17 J. This result is consistent with the observations and provides a physical basis for interpretation of atmospheric oscillations excited by volcanic eruptions

Global Positioning System Resurvey of Southern California Seismic Network Stations

Systematic errors in travel-time data from local earthquakes can sometimes be traced to inaccuracies in the published seismic station coordinates. This prompted a resurvey of the stations of the Caltech/USGS Southern California Seismic Network (SCSN) using the Global Positioning System (GPS). We surveyed 241 stations of the SCSN using Trimble and Ashtech dual-frequency GPS receivers and calculated positions accurate to 3 m using differential positioning from carrier phase measurements. Twelve percent of the stations that were surveyed were found to be mislocated by more than 500 m. Stations of the TERRAscope and USC networks were also surveyed, as well as a network of portable seismic stations deployed shortly after the 1992 Joshua Tree and Landers earthquakes. The new coordinates and the offsets from the old coordinates are given below. The new coordinates are being used in SCSN locations as of 1 January 1994