Charlotte A. Rowe

Automatic P-Wave Arrival Detection and Picking with Multiscale Wavelet Analysis for Single-Component Recordings

We have developed an automatic P -wave arrival detection and picking algorithm based on the wavelet transform and Akaike information criteria (AIC) picker. Wavelet coefficients at high resolutions show the fine structure of the time series, and those at low resolutions characterize its coarse features. Primary features such as the P -wave arrival are retained over several resolution scales, whereas secondary features such as scattered arrivals decay quickly at lower resolutions. We apply the discrete wavelet transform to single-component seismograms through a series of sliding time windows. In each window the AIC autopicker is applied to the thresholded absolute wavelet coefficients at different scales, and we compare the consistency of those picks to determine whether a P -wave arrival has been detected in the given time window. The arrival time is then determined using the AIC picker on the time window chosen by the wavelet transform. We test our method on regional earthquake data from the Dead Sea Rift region and local earthquake data from the Parkfield, California region. We find that 81% of picks are within 0.2-sec of the corresponding analyst pick for the Dead Sea dataset, and 93% of picks are within 0.1 sec of the analyst pick for the Parkfield dataset. We attribute the lower percentage of agreement for the Dead Sea dataset to the substantially lower signal-to-noise ratio of those data, and the likelihood that some percentage of the analyst picks are in error.

An Automatic, Adaptive Algorithm for Refining Phase Picks in Large Seismic Data Sets

We have developed an adaptive, automatic, correlation- and clustering- based method for greatly reducing the degree of picking inconsistency in large, digital seismic catalogs and for quantifying similarity within, and discriminating among, clusters of disparate waveform families. Innovations in the technique include (1) the use of eigenspectral methods for cross-spectral phase estimation and for providing subsample pick lag error estimates in units of time, as opposed to dimensionless relative scaling of uncertainties; (2) adaptive, cross-coherency-based filtering; and (3) a hierarchical waveform stack correlation method for adjusting mean intercluster pick times without compromising tight intracluster relative pick estimates. To solve the systems of cross-correlation lags we apply an iterative, optimized conjugate gra- dient technique that minimizes an L1-norm misfit. Our repicking technique not only provides robust similarity classification-event discrimination without making a priori assumptions regarding waveform similarity as a function of preliminary hypocenter estimates, but also facilitates high-resolution relocation of seismic sources. Although knowledgeable user input is needed initially to establish run-time parameters, sig- nificant improvement in pick consistency and waveform-based event classification may be obtained by then allowing the programs to operate automatically on the data. The process shows promise for enhancing catalog reliability while at the same time reducing analyst workload, although careful assessment of the automatic results is still important.

Precise relocation of earthquakes following the 15 June 1991 eruption of Mount Pinatubo (Philippines)

[1]xa0The 15 June 1991 climactic eruption of Mount Pinatubo (Philippines) was followed by intense seismicity that remained at a high level for several months. We located 10,839 events recorded between 1 July and mid-December 1991. In contrast to the preeruptive seismicity which was focused in two groups below the summit area, posteruptive events were widely distributed below and around the volcano. The classification of the events indicates the presence of several large multiplets, and the application of relative relocation techniques to the similar events by calculating high-precision delays between traces outlines a number of clear seismogenic structures. We used different methods to confirm the validity of our results; these tests indicate that reliable features can be detected with a small monitoring network. While the main cluster of activity can be attributed to an intrusive process starting from below the 15 June crater, the volcanic origin of the seismic activity in the other areas is more difficult to establish. Away from the summit, relocations define streaks or planes which are oriented predominantly southwest-northeast, with in several cases the presence of northwest-southeast conjugate structures. Most of the composite focal mechanisms that we could determine indicate predominantly strike-slip, right-lateral faulting. Our results indicate that most of the seismicity that occurred after the 15 June eruption is related to the east-west regional compressional stress field related to the subduction. We suggest that the regional stress field induces seismicity along new or preexisting faults in the medium surrounding the volcano where the stress field was locally disturbed by the volcanic eruption.

Relocation of seismicity preceding the 1984 eruption of Mauna Loa Volcano, Hawaii: Delineation of a possible failed rift

Abstract Waveform cross-correlation based repicking of P arrival times and subsequent relocation of 187 earthquakes that occurred near the summit of Mauna Loa, Hawaii, prior to the March 1984 eruption has illuminated a previously obscured structure beneath the northwestern flank. Simultaneous inversion for hypocenters and velocity model parameters using the refined arrival times resulted in well-constrained relative earthquake locations with very low arrival time misfits (average RMS 0.041 s). Pre-eruption seismicity from this time period occurred in two groups: a shallow group located near Mauna Loa’s summit, at depths of 1–3 km, and a deeper group (5–10 km) located 4–6 km northwest of the summit. After relocation, we found that most of the northwest flank earthquakes occurred along a 3-km-long planar feature striking about 60° West of North in a thin band about 1 km thick. There is a temporal migration of epicenters from the northwestern edge of the zone migrating towards the summit. Focal mechanisms in this zone reveal a change in faulting from strike-slip in the southeast to a mix of strike-slip and normal faulting in the northwest. This feature we interpret to be related to a rift zone that was stunted by the buttressing of the adjacent volcanoes Hualalai and Mauna Kea. Previous gravity and magnetic studies provide supporting evidence for the existence of a rift zone. There was only a modest increase in clustering of the summit events. After relocation, hypocenters that were previously located within a diffuse zone (4.5 km) were reduced to a 2.5-km-wide zone. They extend from a depth of 6 km to about sea level, trending toward the summit. The focal mechanisms from the summit events are mix of faulting types without a consistent pattern.