Source-Scanning Algorithm Research Articles

Identifying the Rupture Plane of the 2001 Nisqually, Washington, Earthquake

The 2001 Nisqually earthquake occurred within the subducting Juan de Fuca plate. Previous seismic and geodetic studies could not confidently identify its actual fault plane from the two nodal planes. In this study, we apply the recently developed source-scanning algorithm to local seismic waveforms and show unambiguously that the steeply east-dipping plane is the rupture plane. The rupture began near the bottom of the subducting crust and propagated downward into the subducting uppermost mantle. If intraslab earthquakes are assumed to be due to dehydration embrittlement, the source dimension is unlikely to grow any larger because the warm thermal state of the subducting Juan de Fuca plate limits dehydration to a shallow depth below the slab surface. Numerical modeling of the thermal structure indicates that dehydration embrittlement can only take place in the top 10 km of the subducting mantle, implying that the maximum size of an intraslab earthquake in northern Cascadia would be M w∼7 or less.

Rapid identification of earthquake rupture plane using Source-Scanning Algorithm

SUMMARY Accurate identification of an earthquake's rupture plane is critical in many aspects, ranging from scientific, such as seismology and plate tectonics, to societal, such as emergency rescue planning. We modify the recently developed Source-Scanning Algorithm (SSA) to rapidly image the rupture pattern of an earthquake using waveform data recorded at local and regional distances. The method calculates the brightness function of a grid point by summing the observed amplitudes of the P wave envelopes at the corresponding predicted arrival times at all stations. The composite image, consisting of brightness functions of all grid points within a prescribed source volume and time interval, illuminates the locations on the rupture plane where significant seismic energy is emitted. Synthetic tests indicate that the proposed method is robust even under the presence of hypocentral mislocation and origin time errors. Application of this method to the 2003 San Simeon and 2004 Parkfield earthquakes (both occurred in central California) clearly identifies the NW–SE-striking planes as the actual rupture planes, a conclusion consistent with all available evidences. Limited azimuthal distribution of seismic stations will not severely affect the identification result, making this method ideal for offshore earthquake studies. Because the proposed method is able to identify the rupture plane using seismic waveform data only, it is perfectly suited for near-real-time operation and may enable routine report of earthquake rupture planes by local seismic networks in the future.

Spatial‐temporal patterns of seismic tremors in northern Cascadia

We study in detail the two consecutive episodic tremor‐and‐slip (ETS) events that occurred in the northern Cascadia subduction zone during 2003 and 2004. For both sequences, the newly developed Source‐Scanning Algorithm (SSA) is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors. In map view, the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30‐km and 50‐km depth contours of the plate interface. The horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km/d. In cross section, tremors in both sequences span a depth range of over 40 km across the interface, with the majority occurring in the overriding continental crust. In particular, 50–55% of them are located within 2.5 km from the strong seismic reflector bands above the plate interface. The lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences. The source spectra of tremors clearly lack high‐frequency content (>5 Hz) relative to local earthquakes. We propose two possible models to explain the relationship between slip and tremors. The first one regards ETS tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone. In the second model, tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface. Neither model can be dismissed conclusively at this stage.

The Source-Scanning Algorithm: mapping the distribution of seismic sources in time and space

SUMMARY We introduce a new method, which we call the Source-Scanning Algorithm (SSA), for imaging the distribution of seismic sources in both time and space. Using trial locations and origin times, the method calculates the ‘brightness’ function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times. The spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness. The greatest advantages of this method are that: (1) it exploits waveform information (both arrival times and relative amplitudes) without the need to calculate highfrequency synthetic seismograms; and (2) it requires neither pre-assembled phase-picking data nor any a priori assumptions about the source geometry. A series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval. Finally, we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip (ETS) sequence in the northern Cascadia subduction zone.