Controlling Factors of Seismicity and Geometry in Double Seismic Zones

Abstract

Double seismic zones are ubiquitous features of subduction zones, where seismicity is distributed along two layers separated by a region with significantly less seismic activity. Dehydration embrittlement is thought to be responsible for earthquakes in the subducting crust (upper layer), but the case for it in the lithospheric mantle (lower layer) is less clear. We apply a recently developed relative relocation technique to characterize seismicity in 32 slab segments. The high‐precision hypocentral depths allow us to assign events to either the upper or lower layer and to separately estimate frequency size distributions for each plane. We find consistently larger b values, correlating with slab age, for the upper layer and roughly constant values for the lower. We also show that thermal parameter and plate age are the key controls on double seismic zone geometry. Our results point to a relatively dry lower layer and suggest a fundamentally different mechanism for lithospheric mantle earthquakes.

Seismogenic Necking During Slab Detachment: Evidence From Relocation of Intermediate‐Depth Seismicity in the Alboran Slab

“…Most events are concentrated in what would have been the middle depths of the lithosphere before subduction, with a few scattered at the presubduction bottom (lower lithosphere, western edge) while no seismicity is observed in the presubduction top (crust and uppermost mantle lithosphere, eastern edge) of the slab (Figure 3). This distribution of seismicity is not consistent with the general observations in most subduction zones, where intermediate-depth earthquakes often occur within the subducted crust and uppermost mantle and commonly form double seismic zones (Brudzinski et al, 2007;Florez & Prieto, 2019;Rietbrock & Waldhauser, 2004;Yamasaki & Seno, 2003) with a second seismic band lying in the subducted lower lithospheric mantle. Events do not seem to be homogeneously distributed along the strike within the Alboran slab.…”

Earthquake Depth Phase Extraction With P Wave Autocorrelation Provides Insight Into Mechanisms of Intermediate‐Depth Earthquakes

“…We combine direct and double‐difference relocation and take the average depth as our final depth, given that the difference between them is small (Figure S4). The relocation reveals a well‐defined double seismic zone (Figure a), with a width of about 30 km between the upper and the lower layer, which is consistent with Brudzinski et al () and Florez and Prieto ().…”

“…This would be consistent with some observations, such as water infiltration in the trench outer rise (Ranero et al, ), positive correlation of seismicity rate and fault density (Boneh et al, ), and low velocity in the crust using guild waves (Shiina et al, ). Along with the low Vp/Vs in the lower layer in the double seismic zones, this may indicate that two mechanisms operate on the intermediate‐depth earthquakes (Florez & Prieto, ). In view of uncertainty in both slab interfaces and event locations, this needs further study.…”

“…Recently findings of correlation of seismicity rate and off‐trench faults distribution (Boneh et al, ), deep hydration in the Mariana subduction zone (Cai et al, ), and high Vp/Vs in the lower layer in northern Chile (Bloch et al, ) point to a dehydration embrittlement mechanism for intermediate earthquakes. However, the high value in the upper layer (Florez & Prieto, ) may lead to the dominance of upper layer seismicity in estimating the seismicity rate in the intermediate‐depth range, thus suppressing the contribution of lower layer seismicity which may operate with other mechanisms. Hydration to about 24 km below the Moho in the Mariana subduction zone, as observed by Cai et al (), may not be deep enough to reach the lower layer in the double seismic zone, whose width may reach 40 km (Brudzinski et al, ; Florez & Prieto, ).…”

“…However, the high value in the upper layer (Florez & Prieto, ) may lead to the dominance of upper layer seismicity in estimating the seismicity rate in the intermediate‐depth range, thus suppressing the contribution of lower layer seismicity which may operate with other mechanisms. Hydration to about 24 km below the Moho in the Mariana subduction zone, as observed by Cai et al (), may not be deep enough to reach the lower layer in the double seismic zone, whose width may reach 40 km (Brudzinski et al, ; Florez & Prieto, ). Neither in Japan nor Chile do we find anomalous sP to pP differential times for events in the lower layers of the double seismic zones, which argues against deep penetration of water into the upper mantle and, thus, dehydration embrittlement as the mechanism for the lower of the double seismic zone.…”