Two-stage antiparallel subhorizontal rupture

In Figure 3.3, we show the map views and cross-sections of the time-space evolution of the mainshock rupture resulting from the 3D BP of P waves in high

frequency range of 0.5-2 Hz recorded by the US and EU seismic network. Snapshots of the rupture propagation illuminated by the strongest seismic radiation associated with the largest peaks of the MUSIC pseudospectrum. The maximum amplitude of

pseudospectrum with respect to time (Pseudospectrum Source Time Function, PSTF for short) reflects the temporal rupture evolution and the total duration time of the earthquake, although the MUSIC pseudospectrum is govern by both the relative source strength and the coherency of wavefield. The color-shaded regions corresponding to the source emitters with the pseudospectrum power greater than 95% of its maximum value are used for a crude measure of the uncertainties. The spatial uncertainties are majorly conducted by the smearing effect (Ishii et al., 2007) which usually forms an ellipse with long axis parallel to the ray-to-array path.

The PTSFs of NA and EU arrays reveal the complex temporal rupture history. For NA seismic array, the PSTF (Figure 3.3c) shows two-stage of energy bursts both lasting for about 16 s. The map view of 3D BP image of NA array indicating that two-stage of ruptures propagating in opposite direction. In first 16 s, the rupture stands at the epicenter for ~4 s and then propagates toward N45°E direction for about 30 km.

Later, the second rupture starts at 16 s and turns to S-ward for at least 70 km. If

looking further at the profiles along two opposite ruptures (Figure 3.3b), we notice that the two ruptures propagate horizontally but at different focal depth. The initial

NE-rupture occurred at depth of ~610 km, however, the Southward-rupture is about 20 km deeper than the initial rupture stage. The profile across two rupture, CC’, highlight the vertical aperture between two antiparallel ruptures (Figure 3.3b).

Figure 3.3 High frequency P-wave (0.5~2.0 Hz) 3D BP imaging snapshots for the mainshock.

Map and cross-sections showing the 3D BP imaging results from NA (a)-(c) and EU (d)-(f) networks.

The colored circles (NA) or diamonds (EU) mark the location with maximum pseudospectrum power of each time window with the corresponding elapsed time after the onset of P wave. The symbols are also scaled with the pseudospectrum power. The colored shadows representing the spatial uncertainties are illustrated by the 95% iso-power with the corresponding maximum pseudospectrum power of each time window. The slab contours from slab 1.0 model are shown by gray lines. (b)&(e) The cross-sections

dipping fault plane. (c)&(f) The pseudospectrum power source time function (PTSF) of 3D BP images.

The BP images obtained from EU seismic array as shown in Figure 3.3d suffered much stronger smearing effects than that from US array which result in non-negligible large spatial uncertainties. Considering the ray path of P-wave traveling to EU network, the P waves may be interfered with the subducting slab and the local velocity anomaly of the slab not taken into account for the travel time correction may aggravate the smearing effect of EU BP images. The PSTF (Figure 3.3f) shows the initial strong source rupture amplitude followed by three weak energy bursts. During the first 16 s, the rupture initiates strongly and goes toward ENE-direction for ~ 25 km which is corresponding to the initial strong and first minor energy bursts in PSTF. After 18 s, during the 2^nd and 3^rd minor energy bursts period, the rupture turns to Southeast and propagates for at last 80 km. Taking average depth of BP results for the first 18s and later 20s, we also notice a ~15 km vertical difference between the ENE-ward and SE-ward ruptures. In view of much larger spatial uncertainty of BP images from EU array, we prefer the results revealed by NA array. Even though with notable spatial uncertainty of 3D BP images especially the EU array, two-stage anti-parallel ruptures propagating at different focal depth are observed from both seismic arrays.

The P-wave 3D BP images of NA seismic array reveal that 2013 great Okhotsk deep earthquake involve two stage antiparallel subhorizontal ruptures, the first

NE-ward and the second S-ward deeper ones. The depth difference between two planar ruptures is about 15 ~ 20 km. In order to further clarify the depth difference, we try to diminish the smearing effect by adding the depth phases BP image to P-wave ones in the same filtering frequency band. However, only the pP waves recorded in the NA seismic array is capable to perform BP within the long period of 1~8s. Figure 3.4a&b

show the long period BP results of P- and pP-waves, respectively. Comparing the 3D P-wave BP images in high and low frequency bands (Figure 3.3a & Figure 3.4a), the BP results are roughly consistent with each other showing anti-parallel subhorizontal rupture within two rupture stages. Not surprisingly, the longer period BP images have more severe smearing effects than the short period ones do. As the pP-wave, the 3D BP image also show great uncertainties not only in horizontal but also along the vertical direction, which is also predictable since the depth phases going up to free surface then to distant receivers typically suffer much more interfering and energy decaying from the local velocity structures above the hypocenter than the P-waves do. In Figure 3.4b, we observe consistent initial NE-ward rupture for first 10 s. After that, the 3D BP results become unstable because of the strong smearing effect. Even though, when we simply sum the BP images of P- and pP waves directly with equal weighting, the smearing effects represented by the colored shadow zones are effectively diminished (Figure 3.4). The 3D integrated BP images showing similar spatiotemporal rupture propagation with the high frequency BP results of P-wave in NA array (Figure 3.3a) ascertain the 10~15 km vertical aperture between two-stage anti-parallel ruptures.

Our 3D BP results reveal an en echelon like anti-parallel rupture behavior of 2013 Okhotsk deep earthquake: two stages of ruptures propagating horizontally in opposite direction with a 10~15 aperture of depth (Figure 3.3 & 3.4). Figure 3.5 summarizes the kinematic rupture properties of rupture length and time revealed by 4 BP imaging results from Figure 3.3 & 3.4. The 1^st rupture initiates slowly and propagates NE-ward 30~40 km for 8~12s which leads to average rupture speed of 3.0~3.3 km/s. Later, the 2^nd rupture initiates at the east of the original epicenter with focal depth 10~15 km deeper and propagates in much higher speed of 4.25~4.80 km/s for at least 80 km.

Figure 3.4 Low frequency (0.125~1.0 Hz) 3D BP imaging snapshots for mainshock.

Map and cross-sections showing the 3D BP results of using (a) P-wave (b) pP waves and (c) the

integrated P and pP 3D BP images from NA array. The layout configurations and symbol explanations are the same as in Figure 3.3.

Figure 3.5 3D BP imaging results summaries for the mainshock.

The relation of rupture length respect to time for the (a) 1^st NE-ward rupture and (b) 2^nd S-ward rupture from the BP results of high frequency P wave at NA or EU array, low frequency pP wave at NA array and low frequency integrated BP results of P- and pP-wave at NA array. The number on each line represents the average rupture speed estimation based on each BP result.