Wadati-Benioff Zone Seismicity Research Articles

Oceanic intraplate faulting as a pathway for deep hydration of the lithosphere: Perspectives from the Caribbean

AbstractThe recycling of water into the Earth’s mantle via hydrated oceanic lithosphere is believed to have an important role in subduction zone seismicity at intermediate depths. Hydration of oceanic lithosphere has been shown to drive double planes of intermediate-depth, Wadati-Benioff zone seismicity at subduction zones. However, observations from trenches show that pervasive normal faulting causes hydration ~25 km into the lithosphere and can explain neither locations where separations of 25–40 km between Wadati-Benioff zone planes are observed nor the spatial variability of the lower plane in these locations, which suggests that an additional mechanism of hydration exists. We suggest that intraplate deformation of >50-m.y.-old lithosphere, an uncommon and localized process, drives deeper hydration. To test this, we relocated the 25 November 2018 6.0 MW Providencia, Colombia, earthquake mainshock and 575 associated fore- and aftershocks within the interior of the Caribbean oceanic plate and compared these with receiver functions (RF) that sampled the fault at its intersection with the Mohorovičić discontinuity. We examined possible effects of velocity model, initial locations of the earthquakes, and seismicphase arrival uncertainty to identify robust features for comparison with the RF results. We found that the lithosphere ruptured from its surface to a depth of ~40 km along a vertical fault and an intersecting, reactivated normal fault. We also found RF evidence for hydration of the mantle affected by this fault. Deeply penetrating deformation of lithosphere like that we observe in the Providencia region provides fluid pathways necessary to hydrate oceanic lithosphere to depths consistent with the lower plane of Wadati-Benioff zones.

Extending Alaska’s plate boundary: Tectonic tremor generated by Yakutat subduction

Abstract The tectonics of the eastern end of the Alaska-Aleutian subduction zone are complicated by the inclusion of the Yakutat microplate, which is colliding into and subducting beneath continental North America at near-Pacific-plate rates. The interaction among these plates at depth is not well understood, and further east, even less is known about the plate boundary or the source of Wrangell volcanism. The drop-off in Wadati-Benioff zone (WBZ) seismicity could signal the end of the plate boundary, the start of aseismic subduction, or a tear in the downgoing plate. Further compounding the issue is the possible presence of the Wrangell slab, which is faintly outlined by an anemic, eastward-dipping WBZ beneath the Wrangell volcanoes. In this study, I performed a search for tectonic tremor to map slow, plate-boundary slip in south-central Alaska. I identified ∼11,000 tremor epicenters, which continue 85 km east of the inferred Pacific plate edge marked by WBZ seismicity. The tremor zone coincides with the edges of the downgoing Yakutat terrane, and tremors transition from periodic to continuous behavior as they near the aseismic Wrangell slab. I interpret tremor to mark slow, semicontinuous slip occurring at the interface between the Yakutat and North America plates. The slow slip region lengthens the megathrust interface beyond the WBZ and may provide evidence for a connection between the Yakutat slab and the aseismic Wrangell slab.

Teleseismic P‐wave tomography and mantle dynamics beneath Eastern Tibet

We determined a new 3-D P-wave velocity model of the upper mantle beneath eastern Tibet using 112,613 high-quality arrival-time data collected from teleseismic seismograms recorded by a new portable seismic array in Yunnan and permanent networks in southwestern China. Our results provide new insights into the mantle structure and dynamics of eastern Tibet. High-velocity (high-V) anomalies are revealed down to 200 km depth under the Sichuan basin and the Ordos and Alashan blocks. Low-velocity (low-V) anomalies are imaged in the upper mantle under the Kunlun-Qilian and Qinling fold zones, and the Songpan-Ganzi, Qiangtang, Lhasa and Chuan-Dian diamond blocks, suggesting that eastward moving low-V materials are extruded to eastern China after the obstruction by the Sichuan basin, and the Ordos and Alashan blocks. Furthermore, the extent and thickness of these low-V anomalies are correlated with the surface topography, suggesting that the uplift of eastern Tibet could be partially related to these low-V materials having a higher temperature and strong positive buoyancy. In the mantle transition zone (MTZ), broad high-V anomalies are visible from the Burma arc northward to the Kunlun fault and eastward to the Xiaojiang fault, and they are connected upward with the Wadati-Benioff seismic zone. These results suggest that the subducted Indian slab has traveled horizontally for a long distance after it descended into the MTZ, and return corner flow and deep slab dehydration have contributed to forming the low-V anomalies in the big mantle wedge. Our results shed new light on the dynamics of the eastern Tibetan plateau.

Seismicity and state of stress in the central and southern Peruvian flat slab

Abstract We have determined the Wadati–Benioff Zone seismicity and state of stress of the subducting Nazca slab beneath central and southern Peru using data from three recently deployed local seismic networks. Our relocated hypocenters are consistent with a flat slab geometry that is shallowest near the Nazca Ridge, and changes from steep to normal without tearing to the south. These locations also indicate numerous abrupt along-strike changes in seismicity, most notably an absence of seismicity along the projected location of subducting Nazca Ridge. This stands in stark contrast to the very high seismicity observed along the Juan Fernandez ridge beneath central Chile where, a similar flat slab geometry is observed. We interpret this as indicative of an absence of water in the mantle beneath the overthickened crust of the Nazca Ridge. This may provide important new constraints on the conditions required to produce intermediate depth seismicity. Our focal mechanisms and stress tensor inversions indicate dominantly down-dip extension, consistent with slab pull, with minor variations that are likely due to the variable slab geometry and stress from adjacent regions. We observe significantly greater variability in the P-axis orientations and maximum compressive stress directions. The along strike change in the orientation of maximum compressive stress is likely related to slab bending and unbending south of the Nazca Ridge.

Modeling of the post-seismic slip of the 2003 Tokachi-oki earthquake M 8 off Hokkaido: Constraints from volumetric strain

A Sacks-Evertson borehole volumetric strainmeter (SE strainmeter) at a site located 105 km from the epicenter of the mainshock recorded a clear slow strain event following the 2003 M w 8.0 Tokachi-oki earthquake (September 25, 2003, 19:50:06 UTC). This consisted of an episode of contraction for 4 days followed by expansion for 23 days. GPS sites in southeastern Hokkaido also recorded displacement changes during the same time interval. We use quasi-static calculations to generate synthetic waveforms for the measured quantities. All the data are satisfied by a propagating line source 2-stage model of slow reverse slip, uniform amplitude of 50 cm, with rupture propagation velocities of constant 9 cm/s (first stage) and exponentially decreasing from 3 to 0.7 cm/s (second stage). This post-seismic slip event is taken to be coplanar with the main shock rupture on the upper plane of the double Wadati-Benioff seismic zone (DSZ), and largely overlaps the seismic rupture. Regular earthquakes release only about 30% of the plate motion in this section of the subduction zone; post-seismic slip appears to account for at least some of the deficit.

Studies of Wadati-Benioff Zone Seismicity of the Anchorage, Alaska, Region

We have relocated over 8600 earthquakes occurring between 1964 and 1999 at depths >25 km within ∼100 km of Anchorage, Alaska, using the double- difference technique. The relocated seismicity reveals important details of the subducting Pacific plate and Yakutat blocks including clustering of seismicity within the Pacific plate mantle, northeast- and northwest-striking near-vertical faults within the Pacific plate crust, and a region of intense deformation associated with the southwestern edge of the Yakutat block. Stress analyses from the direct inversion of first- motion data and from inversion of focal mechanisms for 713 events indicates a rotation of the maximum compressive stress direction from a trench parallel direction (northeast) in the southwestern portion of the study area to an east–west direction near the southwestern edge of the Yakutat block located just north of Anchorage. Regions of the Pacific plate mantle that produced M w >6.4 earthquakes in 1934 and 1954 continue to be seismically active.

Subduction-zone structure and magmatic processes beneath Costa Rica constrained by local earthquake tomography and petrological modelling

A high-quality data set of 3790 earthquakes were simultaneously inverted for hypocentre locations and 3-D P-wave velocities in Costa Rica. Tests with synthetic data and resolution estimates derived from the resolution matrix indicate that the velocity model is well constrained in central Costa Rica to a depth of 70 km; northwestern and southeastern Costa Rica are less well resolved owing to a lack of seismic stations and seismicity. Maximum H2O content and seismic wave speeds of mid-ocean ridge basalt and harzburgite were calculated for metamorphic phase transformations relevant to subduction. Both the 3-D P-wave velocity structure and petrological modelling indicate the existence of low-velocity hydrous oceanic crust in the subducting Cocos Plate beneath central Costa Rica. Intermediate-depth seismicity correlates well with the predicted locations of hydrous metamorphic rocks, suggesting that dehydration plays a key role in generating intermediate-depth earthquakes beneath Costa Rica. Wadati–Benioff zone seismicity beneath central Costa Rica shows a remarkable decrease in maximum depth toward southeastern Costa Rica. The presence of asthenosphere beneath southeastern Costa Rica, which entered through a proposed slab window, may explain the shallowing of seismicity due to increased temperatures and associated shallowing of dehydration of the slab. Tomographic images further constrain the existence of deeply subducted seamounts beneath central Costa Rica. Large, low P-wave velocity areas within the lower crust are imaged beneath the southeasternmost volcanoes in central Costa Rica. These low velocities may represent anomalously hot material or even melt associated with active volcanism in central Costa Rica. Tomographic images and petrological modelling indicate the existence of a shallow, possibly hydrated mantle wedge beneath central Costa Rica.

Transition from oblique subduction to collision: Earthquakes in the southernmost Ryukyu arc‐Taiwan region

Tectonic characteristics of the region between Taiwan and the southernmost Ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5.5≤mb≤6.6. Five major seismogenic structures can be delineated: the Collision Seismic Zone (CSZ), the Interface Seismic Zone (ISZ), the Wadati‐Benioff Seismic Zone (WBSZ), the Lateral Compression Seismic Zone (LCSZ), and the Okinawa Seismic Zone (OSZ). In the CSZ, located along the east coast of Taiwan and offshore, earthquake focal mechanisms show horizontal P axes distributed in two directions, 287°±10° and 333°±16°, possibly reflecting a strain partition associated with the relative plate convergence between the Eurasia plate and the Philippine Sea plate. The corresponding seismic strain tensor indicates a maximum compressive strain rate of 1.2×10−7 yr−1 along 293° and a comparable extension in vertical direction, presumably resulted from plate collision in the region. The geometry of the ISZ, which is distorted significantly at its westernmost end, can be approximated by a north dipping plane that is gradually pushed northward with increasing dip. The seismogenic portion of the interface spans a short depth range from ∼10 km to ∼35 km. A clear pattern of earthquake slip partition is observed; the average slip vector residual is as large as 35°. Seismic strain patterns within the subducted Philippine Sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at ∼270 km, different from the pattern of strain segmentation observed for the rest of the Ryukyu arc where the northern and southern portions are dominated by downdip extension and compression, respectively. Owing to the large convergence obliquity, the slab is descending at a rate significantly slower near Taiwan than in the southern Ryukyu. Thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab's negative buoyancy. A number of thrust or oblique strike‐slip earthquakes between ISZ and WBSZ show a consistent pattern of lateral compression with P axes oriented roughly parallel to the local strike of the trench‐arc system. They are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere. Shallow normal‐faulting earthquakes show successive rotation of T axes from approximately N‐S in the Okinawa trough to approximately E‐W in northeast Taiwan, possibly as a result of interaction between the extension from the opening of the Okinawa trough and the compression from collision. One normal event (January 18, 1991, mb=5.9) occurred in the Central Range with T axis roughly parallel to the structural trend of Taiwan, implying that the nature of the orogeny in Taiwan has changed from “thin‐skinned” deformation to lithospheric collision involving the whole crust and uppermost mantle.

Rheological control of Wadati‐Benioff zone seismicity

Intermediate and deep focus earthquakes in Wadati‐Benioff zones are thought to occur in the cold interiors of downgoing slabs which are significantly stronger than the warmer mantle. Given that earthquakes in oceanic lithosphere appear restricted by an isotherm, and hence a given value of lithospheric strength, we investigate whether a similar formulation is useful for subducting plates. Strength in downgoing slabs should be affected by both pressure and temperature, an effect previously treated using a depth‐dependent limiting temperature for seismicity [Wortel, 1982]. We find this limiting temperature implies that a possible limiting strength increases strongly with depth, unless either the temperatures were too low or the activation volume too large. Comparison of the analytic model used by Wortel with numerical thermal models appears to exclude the first possibility. We explore the second possibility by using the numerical thermal model to compute strength contours for flow law constants reported from laboratory experiments, and find that the expected pressure strengthening is large enough that the slab should have considerable strength well below the deepest seismicity. We conclude that if laboratory results are applicable to these conditions, either a strongly depth‐dependent limiting strength exists or factors in addition to strength control the distribution of subduction zone earthquakes.

Transition from double to single Wadati‐Benioff seismic zone in the Shumagin Islands, Alaska

A discontinuity exists in the moderate‐depth seismicity located by the Shumagin seismic network in the eastern Aleutian subduction zone. The boundary is nearly perpendicular to the arc and has a double planed Wadati‐Benioff zone to the west and a single planed zone to the east. The transition does not appear to be an artifact of mislocations, as shown by three‐dimensional ray trace modeling and examination of phases of representative earthquakes. Current theory on the presence or absence of double seismic zones suggests a change in stress across the boundary. If so, the east and west regions may either have different aseismic to seismic slip ratios, or be in different stages of the earthquake cycle.

Transition from double to single Wadati-Benioff seismic zone in the Shumagin Islands, Alaska : Hudnut, K.W. and J.J. Taber, 1987. Geophys. Res. Letts, 14(2):143–146

Transition from double to single Wadati-Benioff seismic zone in the Shumagin Islands, Alaska : Hudnut, K.W. and J.J. Taber, 1987. Geophys. Res. Letts, 14(2):143–146