Systematics of deep‐focus earthquakes along the Kuril‐Kamchatka Arc and their implications on mantle dynamics

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The classic double seismic zone in the central portion of the Kuril‐Kamchatka arc seems to turn into a regular Wadati‐Benioff zone with downdip compression and extension in the northeastern and southwestern ends of the arc, respectively. To delineate this lateral changeover, we studied 33 large‐ to moderate‐sized (mb≥5.1) earthquakes that occurred at depths shallower than 200 km between 1963 and 1991. These events took place in two regions where the double seismic zone terminates and single seismic zones emerge. Precise earthquake source parameters, including focal depths and mechanisms, are determined from a careful analysis of a mixture of analog and digital P and SH waveforms recorded at teleseismic distances. Our results indicate that the double seismic zone is confined between approximately 46° and 53°N. Northeast of 53°N, intermediate‐depth earthquakes (∼70–200 km) show only downdip compressional focal mechanisms. Furthermore, the Wadati‐Benioff zone there seems to be a continuation of the upper, compressional layer of the double seismic zone. Thus the changeover from a double to a single seismic zone in this case is simply disappearing of the lower, extensional sheet of the double seismic zone. Similarly, southwest of 46°N, intermediate‐depth earthquakes show only downdip extensional mechanisms. The single seismic zone is unlikely to consist of two layers of extensional seismicity. In fact, nowhere along the entire arc did we observe two distinct seismogenic layers with a single state of strain. In other words, these extensional events did not occur in the same seismogenic structure as the upper (compressional) layer of the double seismic zone. However, we cannot ascertain whether the single seismic zone in extension is simply a southwestern extension of the lower (extensional) layer of the double seismic zone. Lateral variation from a single zone under extension in the southwest, to a double seismic zone, and finally to a single zone under compression cannot be accounted for by conventional models of a double seismic zone. On the other hand, the addition of a component of stress transmitted from depths greater than 250 km can explain the observed lateral variation in the state of strain at intermediate depths because the slab between depths of 250–450 km is under extension in the southwestern end of the arc, whereas the rest of the arc is under compression. Vanishing of the lower (extensional) layer of the double seismic zone toward the northeastern end of the arc is interpreted as a result of compression transmitted from below overshadowing extension caused by effects such as unbending. A similar reasoning applies to the southwestern termination of the double seismic zone.

Section: Introductionsupporting

confidence: 74%

The double seismic zone in Kuril‐Kamchatka: The tale of two overlapping single zones

Kao¹,

Chen²

1994

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“…The five moment tensor solutions are very consistent with each other, and in each case the departure from a double-couple source is small. The moment tensor solutions determined by Dziewonski et al [1991] (Figure 1) and Glennon and Chen [1993] are also consistent with these solutions. I observed no systematic trend of seismic moment as a function of frequency, so the source for the short-period seismic energy is equivalent to that for the long-period seismic energy.…”

Section: Source Moment Tensor Estimated Fromsupporting

confidence: 82%

Rapid rupture and complex faulting of the May 12, 1990, Sakhalin deep earthquake: Analysis of regional and teleseismic broadband data

Kuge¹

1994

A large (Mw=7.1) deep earthquake (606 km) occurred beneath Sakhalin Island, about 200 km away from the nearest well‐defined Wadati‐Benioff zone in southwestern Kurile. Moment tensors obtained in various frequency bands using the teleseismic data show that the Sakhalin event produced double‐couple radiation over a very broadband frequency range, and the P waveforms appear simple at most teleseismic stations (Δ=30–90°). On the other hand, many broadband seismometers successfully recorded the seismic waves at regional distances (Δ=3–25°) in Japan, showing two significant arrivals in the P wave seismograms whose time interval is about 3 s. The two P wave arrivals have the same polarity, the same apparent velocity, and similar amplitudes, which suggests that the two arrivals are the result of two sub‐events in the source process of the Sakhalin event. Analysis of the regional and teleseismic waveform data suggests an anomalously rapid slip propagation in the source process of the Sakhalin deep event. The difference in P waveform between the regional and teleseismic stations can be explained by having a long distance (>14–15 km) and short time interval (<1.5–2 s) between the two subevents, which implies that the apparent rupture velocity is larger than the S wave velocity unless the two subevents occurred simultaneously. The observation that the source duration of the Sakhalin event is shorter than those of other deep events of the same size supports the notion of rapid slip‐propagation for the Sakhalin event.

“…As such, the reference signal is a good approximation of the source time function, including the effect of a finite source along that particular azimuth. For SH waveforms, we use a teleseismic SH wave train and/or an appropriately attenuated teleseismic P wave train in deconvolution [e.g., Glennon and Chen, 1993;Zhou et al, 2000]. When the noise level is low, the two reference signals give identical results.…”

Section: Data and Analysismentioning

confidence: 99%

A petrologic anomaly accompanying outboard earthquakes beneath Fiji‐Tonga: Corresponding evidence from broadband P and S waveforms

Brudzinski

Chen

2003

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[1] Using high-resolution, triplicate body waveforms, we investigate strong lateral variations in P and SH wave speeds (V P and V S ) beneath the Tonga subduction zone where cold lithosphere is subducting rapidly at 200 mm yr À1 . Under these conditions, the lack of prominent anomalies of high V P and V S in the lower mantle implies that a large amount of recently subducted material remains above the lower mantle. This inference is also evident from a subhorizontal swath of outboard earthquakes in the transition zone that is up to 1000 km away from the Wadati-Benioff zone. Beneath the back arc, we find a sharp, eastwest trending demarcation north of which outboard earthquakes occur and V P and V S abruptly drop by $3%. Thus the effect of cold temperature to raise V P and V S , as indicated by the presence of outboard earthquakes, must be counteracted by petrologic variations. We interpret that outboard earthquakes occur where impounding of subducted metastable olivine or volatiles reduces seismic wave speeds and triggers deep earthquakes. Considering the density of these candidates and the strong effect of partial melt on V S , volatiles in the form of hydrous phases or melt induced by dehydration are not consistent with observed reductions in V P and V S . Meanwhile, metastable olivine remains a viable candidate. As a cold slab passes below the 410-km discontinuity, olivine becomes metastable and makes the tip of the slab buoyant. Such a mechanism could explain why the leading end of a fast subducting slab may be successively sequestered in the transition zone.INDEX TERMS: 1025 Geochemistry: Composition of the mantle; 7203 Seismology: Body wave propagation; 8120 Tectonophysics: Dynamics of lithosphere and mantle-general; 7218 Seismology: Lithosphere and upper mantle; KEYWORDS: petrologic anomaly, mantle structure, seismic wave speeds, waveform modeling, outboard earthquakes, Fiji-Tonga Citation: Brudzinski, M. R., and W.-P. Chen, A petrologic anomaly accompanying outboard earthquakes beneath Fiji-Tonga: Corresponding evidence from broadband P and S waveforms,