Cut-off Depth Of Seismicity Research Articles

New Zealand Fault-Rupture Depth Model v.1.0: A Provisional Estimate of the Maximum Depth of Seismic Rupture on New Zealand’s Active Faults

ABSTRACT We summarize estimates of the maximum rupture depth on New Zealand’s active faults (“New Zealand Fault-Rupture Depth Model v.1.0”), as used in the New Zealand Community Fault Model v1.0 and as a constraint for the latest revision of the New Zealand National Seismic Hazard Model (NZ NSHM 2022). Rupture depth estimates are based on a combination of two separate model approaches (using different methods and datasets). The first approach uses regional seismicity distribution from a relocated earthquake catalog to calculate the 90% seismicity cutoff depth (D90), representing the seismogenic depth limit. This is multiplied by an overshoot factor representing the dynamic propagation of rupture into the conditional stability zone, and accounting for the difference between regional seismicity depths and the frictional properties of a mature fault zone to arrive at a seismic estimate of the maximum rupture depth. The second approach uses surface heat flow and rock type to compute depths that correspond to the thermal limits of frictional instabilities on seismogenic faults. To arrive at a thermally-based maximum rupture depth, these thermal limits are also multiplied by an overshoot factor. Both the models have depth cutoffs at the Moho and/or subducting slabs. Results indicate the maximum rupture depths between 8 (Taupō volcanic zone) and >30 km (e.g., southwest North Island), strongly correlated with regional thermal gradients. The depths derived from the two methods show broad agreement for most of the North Island and some differences in the South Island. A combined model using weighting based on relative uncertainties is derived and validated using constraints from hypocenter and slip model depths from recent well-instrumented earthquakes. We discuss modifications to the maximum rupture depths estimated here that were undertaken for application within the NZ NSHM 2022. Our research demonstrates the utility of combining seismicity cutoff and thermal stability estimates to assess the down-dip dimensions of future earthquake ruptures.

Seismological data versus rheological modelling: Comparisons across the Aegean Region for improving the seismic hazard assessment

Compared analyses of the seismicity cutoff depth and the corresponding BDT (brittle-ductile transition) depth obtained from rheological modelling have been performed for numerous test sites all over the Aegean Region. The major goal of this research is to determine whether the rheological transition could be effectively correlated to the seismological one, so that both information can independently contribute in determining the seismogenic layer thickness and hence in constraining the maximum width of active faults affecting a region. As concerns the seismological data, only relocated events have been considered for the purpose of this paper, included either in seismic sequences associated with a mainshock, or in datasets of background seismicity. In the same areas, we carried out rheological modelling for reconstructing the local strength envelope, focusing our attention on the BDT depth. The systematic comparison at the investigated sites indicates that 90% of the relocated seismicity always occurs within the corresponding rheological transition. Moreover, for the datasets also providing magnitudes, 99% of the total seismic scalar moment is always released above the BDT depth. Such results are verified for different tectonic regimes and geological settings, both extensional, transcurrent and compressional. The achieved results confirm that the BDT depth based on accurate strength envelopes could be considered a reliable indicator for the thickness of the seismogenic layer, thus contributing to constrain the maximum widths for active faults affecting those regions. Based on these constraints, we also applied some empirical relationships to estimate the possible maximum magnitude for seismogenic sources in correspondence of the tested sites, thus potentially improving our seismic hazard assessment analyses for these areas.

Heat flow and geothermal gradients of the Campania region (Southern Italy) and their relationship to volcanism and tectonics

The heat flow and distribution of temperatures at depth in the Campania region were analysed and correlated with the volcanism and tectonics of the area. The temperature data, a part of the inventory of the AGIP, SAFEN and ENEL Companies (Inventario delle Risorse Geotermiche Nazionali), were gathered during drilling campaigns that began in 1940. The Campania region is characterised by the presence of two active and high-risk volcanic districts (the Campi Flegrei, Ischia and Vesuvius) emerging at the western boundary of the Campania Plain structural graben, and by the outcropping of the carbonate basement along the borders of the plain. The thermal anomalies have been correlated to different processes: the rising of the upper mantle (at about 20 km depth), the heat flow mass transport due to advection of hot fluids above magma reservoirs and the pure conductive heat transport of deeper crust. Furthermore, the presence of the carbonate basement has possibly buffered the deeper crustal thermal processes. The data also provided an estimation of the brittle-ductile transition zone that has been compared with the seismicity cut-off depth of the area.

Structural heterogeneity of the midcrust adjacent to the central Alpine Fault, New Zealand: Inferences from seismic tomography and seismicity between Harihari and Ross

AbstractDetermining the rates and distributions of microseismicity near major faults at different points in the seismic cycle is a crucial step toward understanding plate boundary seismogenesis. We analyze data from temporary seismic arrays spanning the central section of the Alpine Fault, New Zealand, using double‐difference seismic tomography. This portion of the fault last ruptured in a large earthquake in 1717 AD and is now late in its typical 330 year cycle of Mw∼8 earthquakes. Seismicity varies systematically with distance from the Alpine Fault: (1) directly beneath the fault trace, earthquakes are sparse and largely confined to the footwall at depths of 4–11 km; (2) at distances of 0–9 km southeast of the trace, seismicity is similarly sparse and shallower than 8 km; (3) at distances of 9–20 km southeast of the fault trace, earthquakes are much more prevalent and shallower than 7 km. Hypocenter lineations here are subparallel to faults mapped near the Main Divide of the Southern Alps, confirming that those faults are active. The region of enhanced seismicity is associated with the highest topography and a high‐velocity tongue doming at 3–5 km depth. The low‐seismicity zone adjacent to the Alpine Fault trace is associated with Vp and Vs values at midcrustal depths about 8 and 6% lower than further southeast. We interpret lateral variations in seismicity rate to reflect patterns of horizontal strain rate superimposed on heterogeneous crustal structure, and the variations in seismicity cutoff depth to be controlled by temperature and permeability structure variations.

Radiography of a normal fault system by 64,000 high‐precision earthquake locations: The 2009 L'Aquila (central Italy) case study

AbstractWe studied the anatomy of the fault system where the 2009 L'Aquila earthquake (MW 6.1) nucleated by means of ~64 k high‐precision earthquake locations spanning 1 year. Data were analyzed by combining an automatic picking procedure for P and S waves, together with cross‐correlation and double‐difference location methods reaching a completeness magnitude for the catalogue equal to 0.7 including 425 clusters of similar earthquakes. The fault system is composed by two major faults: the high‐angle L'Aquila fault and the listric Campotosto fault, both located in the first 10 km of the upper crust. We detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops. We observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays. The width of the L'Aquila fault zone varies along strike from 0.3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution, up to 1.5 km at the fault tips with an increase in the geometrical complexity. These characteristics, similar to damage zone properties of natural faults, underline the key role of aftershocks in fault growth and co‐seismic rupture propagation processes. Additionally, we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological (i.e., creeping) discontinuity explaining how normal faults detach at depth.

Calibration of the Tibetan Plateau Using Regional Seismic Waveforms

We use the recordings from 51 earthquakes produced by a PASSCAL deployment in Tibet to develop a two-layer crustal model for the region. Starting with their ISC locations, we iteratively fit the P-arrival times to relocate the earthquakes and estimate mantle and crustal seismic parameters. An average crustal P velocity of 6.2–6.3 km/s is obtained for a crustal thickness of 65 km while the P velocity of the uppermost mantle is 8.1 km/s. The upper layer of the model is further fine-tuned by obtaining the best synthetic SH waveform match to an observed waveform for a well-located event. Green's functions from this model are then used to estimate the source parameters for those events using a grid search procedure. Average event relocation relative to the ISC locations, excluding two poorly located earthquakes, is 16 km. All but one earthquake are determined by the waveform inversion to be at depths between 5 and 15 km. This is 15 km shallower, on average, than depths reported by the ISC. The shallow seismicity cut-off depth and low crustal velocities suggest high temperatures in the lower crust. Thrust faulting source mechanisms dominate at the margins of the plateau. Within the plateau, at locations with surface elevations less than 5 km, source mechanisms are a mixture of strike-slip and thrust. Most events occurring in the high plateau where elevations are above 5 km show normal faulting. This indicates that a large portion of the plateau is under EW extension.

Mid-crustal electrical conductors and their correlations to seismicity and deformation at Itoigawa-Shizuoka Tectonic Line, Central Japan

An active fault segment at the northern Itoigawa-Shizuoka tectonic line (ISTL), Central Japan, which will potentially cause M8-class intraplate earthquake, was imaged by wide-band magnetotellurics. Three parallel profiles across ISTL revealed along-strike variation of the resistivity structure. Three resistivity models commonly showed the thickening conductors in the upper crust to east of ISTL which imply the heavily folded Miocene sediments with maximum thickness of several kilometers. Thus the upper crustal structure seems two-dimensional throughout the segment. We found mid-crustal conductors, top of which correlate well with the cutoff depth of seismicity. The seismicity clusters mainly in the resistive crust that is underlain by the mid-crustal conductors. This implies the local distribution of fluids below the brittle-ductile boundary and suggests that the fluid migration into resistive zone is triggering earthquakes. However, the distribution of these mid-crustal conductors is not consistent with the strike of ISTL, but rather it is better correlated with the negative dilatation anomaly inferred from GPS. This suggests the weakening of the crust by the existence of fluids.

Rheological implications of the strong seismic reflector in the Kakkonda geothermal field, Japan

We re-processed the seismic reflection survey data of the Kakkonda geothermal field. The pre-stack migration delineates a strong and continuous reflector between 1800- and 2800-m depth, below which formations are not reflective. Earthquake data exhibit seismicity in the upper crust. The lower boundary of seismogenic layer is interpreted as the brittle–ductile transition. The thermal structure is thought to be the major factor controlling its depth. We compared the strong reflector with the thermal and rheological structure from drillholes. The depth of the reflector corresponds to the top of the highly–very highly fractured zone observed from formation microscanner imagery (FMI) logging in the Miocene formations. The density of fracture in the Kakkonda granite is very low, suggesting that granite corresponds to the nonreflective zone. The temperature–depth profile of well WD-1a shows that the temperature at the highly–very highly fractured zone is about 350 °C. This corresponds to a hydrothermal convection zone filled with two-phase geothermal fluid. The cut-off depth of seismicity that indicates the brittle–ductile transition lies at the isotherm of 300–350 °C near the reflector. We conclude that the strong seismic reflector is a strong contrast in acoustic impedance at the top of the fractured layer. The fractured layer could be a decoupling plane caused by different tectonic behaviors between the upper brittle and the lower ductile layers or a dehydration front by thermal diffusion. The similarity between the strong reflector and K-horizon, the strong reflector, found in southern Tuscany, Italy suggests that the P-wave reflector at the top of highly fractured zone at the brittle–ductile transition be common in areas with magmatic activity.

RETRACTED: Is the plastic flow uniformly distributed below the seismogenic region?

We compared the cutoff depth of seismicity in and around the Nojima fault broken by the 1995 Kobe earthquake occurring in intraplate Japan with the brittle–ductile transition depth of the widely accepted strength profile model of the crust. We successfully determined the temperature profile from borehole measurements, since almost the same geothermal gradients were observed at two boreholes located about 4 km apart from each other, and the thermal conductivity and heat production were also measured by taking numerous core samples. We found that the cutoff depth was much deeper than the transition depth under the assumption that wet granite is deformed at a strain rate of 3×10−15 s−1. This small strain rate implies, however, that plastic flow is uniformly distributed below the seismogenic region. When the strain rate is assumed to be greater than 10−13 s−1, the cutoff depth can be attributed to the transition depth. This suggests that deformation is localized in a narrow fault zone below the seismogenic region, even in the intraplate region.

Is the plastic flow uniformly distributed below the seismogenic region?

The cut-off depth of seismicity in and around the Nojima fault broken by the 1995 Kobe earthquake occurring in the intraplate Japan was compared with a brittle-ductile transition depth of the widely-accepted strength profile model of the crust. It was found that the cut-off depth is much deeper than the transition depth under the assumption that wet granite is deformed at a strain rate of 10−15/s. Such a small strain rate implies that the plastic flow is uniformly distributed below the seismogenic region. When the strain rate is assumed to be greater than 10−13, the cut-off depth can be attributed to the transition depth. This suggests that deformation is localized in a narrow fault zone below the seismogenic region even in the intraplate region.

Seismogenic layer, reflective lower crust, surface heat flow and large inland earthquakes

Regional variations in the cutoff depth of seismicity in southwest Japan are derived from more than 60,000 well-determined earthquakes. The thickness of the seismogenic layer is closely related to the strength of the crust and accordingly to the occurrence of large inland earthquakes, since the seismic–aseismic boundary is thought to be related to the brittle–ductile boundary in the crust. Large inland earthquakes are likely to occur in the area where the cutoff depth of seismicity changes abruptly. It is therefore important to survey the regional variations in this depth to evaluate the potential of large inland earthquakes. The cutoff depth of seismicity roughly coincides with a temperature range of 300–400°C in the crust. In addition, distinct S-wave reflectors as well as a reflective lower crust have generally been observed in some areas in Japan. The depths of the reflector and the top of the reflective lower crust lie several kilometers below the cutoff depth of seismicity and they seem to coincide with a temperature range of about 300–450°C. Therefore, in those areas where no earthquakes occur but a reflective layer exists, the cutoff depth can be mapped from a survey of reflectivity in the crust. The heterogeneity of the crust is very important for understanding the nature of the crust where large earthquakes are frequent. This paper proposes a model of crust inhomogeneity based on observation results.

P‐wave tomographic images in the Central Betics‐Alborán Sea (South Spain) using local earthquakes: Contribution for a continental collision

To investigate the relationship between shallow and intermediate‐depth earthquakes and the crust and upper mantle structure beneath the Betic‐Cordillera and Alborán sea region, we have applied seismic tomography to 2569 P‐wave arrival times from 367 microearthquakes recorded by both permanent and temporary seismic stations deployed in the region. These earthquakes occurred in the depth range of 0 to 110 km beneath the Central Betic Cordilleras and Alborán sea. Our results have revealed significant structural heterogeneities in the crust and upper mantle beneath the study area. In the upper crust there is a high‐velocity anomaly at Sierra Gorda (western boundary of the Granada basin) penetrating to 15 km depth, which is in good agreement with the aeromagnetic anomaly. In the Granada basin, a low‐velocity anomaly is located in the middle crust, which coincides with a previously detected greater cutoff depth of seismicity, and are considered to be associated with a highly fractured zone generated by an intracrustal detachment. The northern boundary of the Alborán sea (Málaga coast) is well imaged as low velocities from 50 to 90 km depths. This low‐velocity zone in the upper mantle is associated with the intermediate‐depth seismicity, which outline a section of continental crust related to the collision/subduction beneath the northern part of the Alborán sea‐southern part of the Central Betics.

A lithospheric cross-section through the Swiss Alps-II. Constraints on the mechanical structure of a continent-continent collision zone

SUMMARY The calculation of strength profiles along the European Geotraverse (EGT) through the Swiss Alps yields constraints on the large-scale vertical and lateral mechanical structure through the Alpine continent-continent collision zone. Strength profiles are evaluated for different assumptions on petrological stratification and strain rate and are based on temperature-depth profiles derived from transient thermo-kinematic modelling of the Neoalpine orogeny. The main contribution to the total strength results from the mantle lithosphere, which is strongly controlled by temperature. In contrast, the crustal contribution is mainly determined by variations in petrological stratification. A direct correlation between surface heat flow and the total strength of the crust, the mantle lithosphere and the entire lithosphere (crust and mantle lithosphere) is not observed. Our results demonstrate that in tectonically active areas a transient thermal model, along with detailed knowledge of the deep structure and petrology, is necessary to evaluate lithospheric strength envelopes. Inside the collision zone, strain rate has a strong control on the bottom of the mechanically strong crust, whereas outside the collision zone the effect is less significant. The cut-off depth of seismicity along the profile, which correlates largely with the bottom of the mechanically strong crust, deviates from the 300-400°C isotherm. The inferred effective elastic thickness for the Molasse Basin north of the Alps is in agreement with flexural modelling results, whereas for the Southern Alps the predictions deviate.

特集 火山活動と地殻応力場 地溝幅が火山付近で狭くなる現象について：雲仙‐島原地溝の場合

The central Kyushu is a north-south spreading region characterized by east-west trending grabens and abundant active volcanoes. Available data indicate that the width of graben is 11-12 km apart from eruption centers of Unzen volcano, whereas it narrows down to about 7 km dose to the eruption centers. Focal mechanisms of earthquakes suggest that north-south tensional axis is not altered by the presence of volcanic conduit. Width of graben in general is pos tively correlated with thickness of lithosphere, and the width of graben is narrowed locally in accordance with the thinning of lithosphere beneath active volcanoes by the presence of magma chamber in middle crust. Evidences supporting this idea include shallow cutoff depth of seismicity and mid-crust low velocity area beneath the active volcano, shallow Curie temperature distribution, and high oxygen isotopic composition of the volcanic rocks.

Cutoff depth of seismicity and large earthquakes near active volcanoes in Japan

Abstract Cutoff depths of seismicity in the earth's crust change from place to place over a range of 5–25 km. These variations are closely related to the values of the surface heat flow. In order to examine regional variations in greater detail, available cross sections of well-determined focal depths have been collected for five areas located near active volcanoes, where the thermal structure is considered to change abruptly. The seismic-aseismic boundary is located at depths of about 5–8 km immediately below the craters, with increasing depth towards the flanks of the volcanoes. The boundary is found at depths of 10–15 km at distances of about 10 km from the center of the volcanoes. Further, an examination was performed of the magnitudes of earthquakes versus the distances from the craters for seven volcanoes, near which large earthquakes ( M ≥ 6) occurred during the past 30 years. All of the large earthquakes were found to have occurred at distances of approximately 10 km or greater from the center of the volcanoes. Moreover, the distances between active volcanoes and earthquakes were calculated for all large intra-plate earthquakes in Japan that have occurred during the period 1885–1990. It was found that all the large earthquakes occurred at distances of more than 10 km from active volcanoes. These facts suggest that the seismogenic layer near the center of an active volcano is thin and not strong enough to accumulate sufficient stress for the occurrence of large earthquakes.

The geometry of Aleutian subduction: Three‐dimensional kinematic flow model

We construct a steady state kinematic model of slab flow along the Aleutian Arc assuming the slab's geometry is known. The slab geometry is constrained by local network data where the slab is seismically active and by P wave residual‐sphere analysis of 11 earthquakes and one nuclear explosion where it is aseismic. The slab is approximated by a thin Newtonian sheet subducting in a less viscous and incompressible mantle. Particles enter the trench at known rates of relative plate motion and are required to subduct along the predefined slab geometry. For discrete, but small (0.1 m.y.) time steps, we determine the speed and direction of each slab particle that minimizes dissipation power as the spherical shell (oceanic lithosphere) deforms to obtain the observed slab shape. Because the dip of the slab in the central Aleutians is steep relative to the radius of curvature of the trench, the slab must stretch along arc to obtain its present configuration. Our kinematic model represents one internally consistent flow that results in the observed geometry. Furthermore, it provides a lower bound on the total amount of internal in‐plane deformation required by the shape and an estimate of stream lines and of the orientation and spatial distribution of the deformation rate. The predicted in‐plane strain rate field is dominated by along arc extension, reaching values as large as 10−15s−1 in the central Aleutians and decaying by an order of magnitude to the east and west. The slab accumulates an in‐plane strain of 10% by the time it reaches the depth of the seismicity cutoff. Along‐arc variations in both the magnitude and orientation of calculated strain rates are consistent with the spatial distribution of seismic moment release and source mechanism orientations of intermediate‐depth earthquakes, suggesting that in order to understand intermediate‐focus earthquakes one must consider along‐arc deformation. Three‐dimensional analyses of subduction in a spherical geometry, such as the present study, are required to model along‐arc effects. Relative plate motions along the trench vary from nearly normal in the eastern Aleutians to transform farther west. Coincident with this change, the maximum depth of seismicity within the descending slab decreases from 250 to 50 km and active andesitic volcanism ceases. The kinematic flow model predicts that the slab subducted obliquely along the central portion of the Aleutian Arc is transported laterally westward producing cold slab material to a depth of at least 300 km along the entire arc after only 15 m.y. of subduction. The lack of volcanic activity in the western Aleutians is apparently related to the lateral transport of the slab, perhaps indicating that the slab is dehydrated and incapable of producing the volatiles necessary for arc magmatism. The along‐arc variation in the maximum depth of seismicity can be explained thermally. We construct temperature estimates within the slab using a quasi three‐dimensional finite difference scheme, including entrained mantle flow, stream lines from the kinematic model and the time history of the age of the subducting lithosphere as a function of position along the trench. The theoretical temperature at the seismicity cutoff depth is remarkably close to a constant fraction of the depth‐dependent mantle solidus along the entire arc. This observation is consistent with the hypothesis that seismicity terminates because of the initiation of high‐temperature, steady state creep and provides an explanation for the lack of intermediate‐focus earthquakes in the western Aleutians even though slab material exists there.

Regional variations of the cutoff depth of seismicity in the crust and their relation to heat flow and large inland-earthquakes.

More than 8, 000 earthquakes have been relocated to derive regional variations of the seismic-aseismic boundary in the mid-crust of the northern Kinki district of Japan. The boundary depths are estimated as 13-15 and 18-20 km in the southwestern and northeastern parts of the study area, respectively. The relationship between the seismic cutoff depth and the cause of large intraplate earthquakes is studied, making use of the present observations and other available data of seismicity and surface heat-flow, on the basis of the brittle-ductile transition of rock deformation. The regional variations in the cutoff depth of seismicity appear to be well correlated with the thermal structure of the crust. The cutoff depths in various heat-flow provinces in Japan and other countries are found to be inversely proportional to the surface heat-flow values, with the depths roughly corresponding to isotherms of 200-400°C. The shape of the depth-frequency distribution of earthquakes calculated from high-quality data is quite similar to that of the shear resistance calculated using a simple brittle-ductile transition model. Large intraplate earthquakes appear to originate at the peak or just below the peak in the depth-frequency distribution, which also corresponds to the deepest portion of the seismogenic layer. Furthermore, in the source area of large earthquakes, rupture seems to start where sharp changes occur in the cutoff depth of seismicity. Thus, the seismic-aseismic boundary is closely related to large intraplate earthquakes and, in turn, to the tectonics of island arcs.