Geology topics

Frederick Pollitz

Publications and source records attributed to Frederick Pollitz.

At least 37 records · Page 2Linked to original sources

USGS2022

Western U.S. deformation models for the 2023 update to the U.S. National Seismic Hazard Model

This report describes geodetic and geologic information used to constrain deformation models of the 2023 update to the National Seismic Hazard Model (NSHM), a set of deformation models to interpret these data, and their implications for earthquake rates in the western United States. Recent updates provide a much larger data set of Global Positioning System crustal velocities than used in the 2014 NSHM, as well as hundreds of new faults considered as active sources for the 2023 NSHM. These data are interpreted by four geodetic models of deformation that estimate fault slip rates and their uncertainties together with off‐fault moment release rates. Key innovations in the 2023 NSHM relative to past practice include (1) the addition of two new (in addition to two existing) deformation models, (2) the revision and expansion of the geologic slip rate database, (3) accounting for fault creep through development of a creep‐rate model that is employed by the four deformation models, and (4) accounting for time‐dependent earthquake‐cycle effects through development of viscoelastic models of the earthquake cycle along the San Andreas fault and the Cascadia subduction zone. The effort includes development of a geologic deformation model that complements the four geodetic models. The current deformation models provide a new assessment of outstanding discrepancies between geologic and geodetic slip rates, at the same time highlighting the need for both geologic and geodetic slip rates to robustly inform the earthquake rate model.

Seismological Research Letters↗

USGS2022

Viscoelastic fault-based model of crustal deformation for the 2023 update to the U.S. National Seismic Hazard Model

The 2023 update to the National Seismic Hazard (NSHM) model is informed by several deformation models that furnish geodetically estimated fault slip rates. Here I describe a fault‐based model that permits estimation of long‐term slip rates on discrete faults and the distribution of off‐fault moment release. It is based on quantification of the earthquake cycle on a viscoelastic model of the seismogenic upper crust and ductile lower crust and mantle. I apply it to a large dataset of horizontal and vertical Global Positioning System (GPS) interseismic velocities in the western United States, resulting in long‐term slip rates on more than 1000 active faults defined for the NSHM. A reasonable fit to the GPS dataset is achieved with a set of slip rates designed to lie strictly within a priori geologic slip rate bounds. Time‐dependent effects implemented via a “ghost transient” have a profound effect on slip rate estimation and tend to raise calculated slip rates along the northern and southern San Andreas fault by up to several mm/yr.

Seismological Research Letters↗

USGS2022

Kinematic slip model of the July 8, 2021 M6.0 Antelope Valley, California, earthquake

We present a kinematic slip model of the July 8, 2021 Antelope Valley earthquake from a finite-source inversion based on regional seismic waveforms and static offsets from GPS and InSAR. Seismic waveforms are employed at 6s dominant period out to 100 km from the epicenter, and the combined GPS and InSAR datasets cover the near field and far field out to ∼ 100 km and constrain the overall rupture size. The aftershock pattern defines a nearly north-striking, 50◦ east-dipping fault plane. We find a unilateral rupture along this fault plane propagating southward and updip with predominantly normal slip up to ∼ 1.5m. The estimated seismic moment of 8.47 × 10 22 17 Nm is equivalent to Mw 5.92. A finite-source inversion that retains seismic waveforms and GPS static offsets but omits InSAR range changes yields a seismic moment of 1.08 × 10 25 18 Nm (Mw 5.99). Despite vigorous aftershock activity between 10 km and Earth’s surface, coseismic slip is concentrated in the depth interval 7 - 10 km.

California↗

USGS2021

Post-seismic relaxation following the 2019 Ridgecrest, California, earthquake sequence

The 2019 Ridgecrest, California, earthquake sequence involved predominantly right‐lateral strike slip on a northwest–southeast‐trending subvertical fault in the 6 July M 7.1 mainshock, preceded by left‐lateral strike slip on a northeast–southwest‐trending subvertical fault in the 4 July M 6.4 foreshock. To characterize the postseismic deformation, we assemble displacements measured by Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar. The geodetic measurements illuminate vigorous postseismic deformation for at least 21 months following the earthquake sequence. The postseismic transient deformation is particularly well constrained from survey‐mode GPS (sGPS) in the epicentral region carried out during the weeks after the mainshock. We interpret these observations with mechanical models including afterslip and viscoelastic relaxation of the lower crust and mantle asthenosphere. During the first 21 months, up to several centimeters of horizontal motions are measured at continuous GPS and sGPS sites, with amplitude that diminishes slowly with distance from the mainshock rupture, suggestive of deeper afterslip or viscoelastic relaxation. We find that although afterslip involving right‐lateral strike slip along the mainshock fault traces and their deeper extensions reach a few decimeters, most postseismic deformation is attributable to viscoelastic relaxation of the lower crust and mantle. Within the Basin and Range crust and mantle, we infer a transient lower crust viscosity several times that of the mantle asthenosphere. The transient mantle asthenosphere viscosity is ∼ 1.3 × 10 17    Pa   s ">

California↗

USGS2021

Seismic and geodetic analysis of rupture characteristics of the 2020 Mw 6.5 Monte Cristo Range, Nevada, earthquake

The largest earthquake since 1954 to strike the state of Nevada, United States, ruptured on 15 May 2020 along the Monte Cristo range of west‐central Nevada. The M w 6.5 event involved predominantly left‐lateral strike‐slip faulting with minor normal components on three aligned east–west‐trending faults that vary in strike by 23°. The kinematic rupture process is determined by joint inversion of Global Navigation Satellite Systems displacements, Interferometric Synthetic Aperture Radar (InSAR) data, regional strong motions, and teleseismic P and SH waves, with the three‐fault geometry being constrained by InSAR surface deformation observations, surface ruptures, and relocated aftershock distributions. The average rupture velocity is 1.5 km/s⁠ , with a peak slip of ∼1.6 m and a ∼20 s rupture duration. The seismic moment is 6.9×10 18 N·m⁠ . Complex surface deformation is observed near the fault junction, with a deep near‐vertical fault and a southeast‐dipping fault at shallow depth on the western segment, along which normal‐faulting aftershocks are observed. There is a shallow slip deficit in the Nevada ruptures, probably due to the immature fault system. The causative faults had not been previously identified and are located near the transition from the Walker Lane belt to the Basin and Range province. The east–west geometry of the system is consistent with the eastward extension of the Mina Deflection of the Walker Lane north of the White Mountains.

California, Nevada↗

USGS2021

Exploring GPS observations of postseismic deformation following the 2012 MW7.8 Haida Gwaii and 2013 MW7.5 Craig, Alaska Earthquakes: Implications for viscoelastic Earth structure

The Queen Charlotte-Fairweather Fault (QC-FF) system off the coast of British Columbia and southeast Alaska is a highly active dextral strike-slip plate boundary that accommodates ∼50 mm/yr of relative motion between the Pacific and North America plates. Nine M W ≥ 6.7 earthquakes have occurred along the QC-FF system since 1910, including a M S (G-R) 8.1 event in 1949. Two recent earthquakes, the October 28, 2012 Haida Gwaii ( M W 7.8) and January 5, 2013 Craig, Alaska ( M W 7.5) events, produced postseismic transient deformation that was recorded in the motions of 25 nearby continuous Global Positioning System (cGPS) stations. Here, we use 5+ yr of cGPS measurements to characterize the underlying mechanisms of postseismic deformation and to constrain the viscosity structure of the upper mantle surrounding the QC-FF. We construct forward models of viscoelastic deformation driven by coseismic stress changes from these two earthquakes and explore a large set of laterally heterogeneous viscosity structures that incorporate a relatively weak back-arc domain; we then evaluate each model based on its fit to the postseismic signals in our cGPS data. In determining best-fit model structures, we additionally incorporate the effects of afterslip following the 2012 event. Our results indicate the occurrence of a combination of temporally decaying afterslip and vigorous viscoelastic relaxation of the mantle asthenosphere. In addition, our best-fit viscosity structure (transient viscosity of 1.4–2.0 × 10 18 Pa s; steady-state viscosity of 10 19 Pa s) is consistent with the range of upper mantle viscosities determined in previous studies of glacial isostatic rebound and postseismic deformation.

Alaska, British Columbia↗

USGS2021

Coseismic fault slip and afterslip associated with the M5.7 March 18, 2020 Magna, Utah, earthquake

The 2020 Magna, Utah, earthquake produced observable crustal deformation over a ∼ 100 km 2 area around the southeast margin of Great Salt Lake, but it did not produce any surface rupture. To obtain a detailed picture of the fault slip, we combine strong motion seismic waveforms with GPS static oﬀsets and Interferometric Synthetic Aperture Radar (InSAR) observations to obtain kinematic and static slip models of the event. We sample the regional seismic waveﬁeld with 3-component records from 68 stations of the University of Utah Seismograph Stations network. We ﬁnd that coseismic slip and afterslip, with predominantly normal slip, distributed on a shallowly west-dipping plane, possibly augmented by afterslip on a steeply northeast-dipping plane, best ﬁts the joint dataset. The west-dipping plane locates near previously inferred sources of interseismic creep at depth. Hence the earthquake may have occurred on the downdip ex-tension of the Wasatch fault and activated further slip (afterslip) at shallow depth east of the hypocenter. This inferred afterslip may have driven the vigorous aftershock activity that was concentrated east of the hypocenter.

Utah↗

USGS2020

Rupture process of the M6.5 Stanley, Idaho, earthquake inferred from seismic waveform and geodetic data

The 2020 M 6.5 Stanley, Idaho, earthquake produced rupture in the north of the active Sawtooth fault in the northern basin and range at depth, without any observable surface rupture. Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data yield several millimeters of static offsets out to ∼100 km from the rupture and up to ∼0.1 m of near‐field crustal deformation. We combine the GPS and InSAR data with long‐period regional seismic waveforms to derive models of kinematic slip and afterslip. We find that the coseismic rupture is complex, likely involving up to 2 m combined left‐lateral strike slip and normal slip on a previously unidentified ∼south‐southeast‐striking fault. This slip is predominantly left‐lateral strike slip, different from the dominant east‐northeast–west‐northwest normal faulting of the region. At least one ∼northeast‐trending fault, likely associated with the Trans‐Challis fault system, is inferred to have accommodated a few decimeters of right‐lateral afterslip, consistent with vigorous aftershock activity at depth along northeast‐trending lineations.

Idaho↗

USGS2020

Kinematics of fault slip associated with the July 4-6 2019 Ridgecrest, Californai earthquakes sequence

The 2019 Ridgecrest, California, earthquake sequence produced observable crustal deformation over much of central and southern California, as well as surface rupture over several tens of kilometers. To obtain a detailed picture of the fault slip involved in the 4 July M 6.4 foreshock and 6 July M 7.1 mainshock, we combine strong‐motion seismic waveforms with crustal deformation observations to obtain kinematic and static slip models of both events. We sample the regional seismic wavefield for both the foreshock and mainshock with three‐component records from 31 stations of the California Integrated Seismic Network. The deformation observations include Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and borehole strainmeter recordings of the dynamic strain field. These data collectively constrain the kinematic coseismic slip distributions of the events, with measurements variously observing coseismic slip from one event (e.g., seismic waveforms, kinematic solutions from continuous GPS, and strainmeter time series) or coseismic slip from both events combined (InSAR). We find that the foreshock ruptured two separate faults, one with left‐lateral strike slip on a northeast–southwest‐trending fault and the other with right‐lateral strike slip on an orthogonal fault, with unilateral rupture propagation along both. The mainshock ruptured a series of northwest–southeast‐trending faults with right‐lateral strike slip concentrated in the uppermost 6 km with exceptionally low‐rupture velocity averaging 1.0 – 1.5    km / s ">

California↗

USGS2020

Rapid geodetic observations of spatiotemporally varying postseismic deformation following the Ridgecrest earthquake sequence: The U.S. Geological Survey response

The U.S. Geological Survey’s geodetic response to the 4–5 July 2019 (Pacific time) Ridgecrest earthquake sequence comprised primarily the installation and/or reoccupation of Global Navigation Satellite System (GNSS) monumentation. Our response focused primarily on the United States’ Navy’s China Lake Naval Air Weapons Station base (NAWSCL). This focus was because much of the surface rupture occurred on the NAWSCL and because of NAWSCL access restrictions only permitting Federal and State of California personnel. In total, we measured or are still measuring at 24 sites, 14 of which were on the NAWSCL and, as of this writing, operational. The majority of sites were set up as continuous stations logging at either 1 sample per second or 1 sample per 15 s. Two stations were recording a 200 m cross‐rupture aperture starting ∼10 hr after the M 6.4 event, and they recorded the coseismic displacements of the M 7.1. Approximately, 1 hr after the M 7.1 event, two new stations were recording a ∼200 m cross‐rupture aperture of the surface rupture. In the days following, we established the rest of the stations ranging to a distance of ∼15 km from the M 7.1 principal rupture trace. The lack of differential displacement across the M 6.4 rupture during the M 7.1 event suggests that it did not reactivate the M 6.4 plane. The lack of differential cross‐fault displacement for both events suggests that rapid shallow afterslip did not occur at those two locations. The postseismic time series from these stations shows centimeters of horizontal displacement over periods of a few months. They record a mixture of fault‐parallel and fault‐normal displacements that, in conjunction with analysis of more spatially complete Interferometric Synthetic Aperture Radar displacement fields, suggest that both poroelastic and afterslip phenomena occur along the M 6.4 and 7.1 rupture planes. Using preliminary data from these and other regional stations, we also explore the Ridgecrest sequence’s effect on regional GNSS time series and the differentiation of long‐term postseismic motions and secular deformation rates. We find that redefining a common‐mode noise filter using different GNSS stations that are assumed to be unaffected by the earthquakes results in small but systematic differences in the regional velocity field estimate.

Seismological Research Letters↗

USGS2020

Coseismic and post-seismic gravity disturbance induced by seismic sources using a 2.5-D spectral element method

I present a prescription for computing free-air coseismic and post-seismic gravity changes induced by seismic sources in a viscoelastic earth model. I assume a spherical earth geometry and a 2.5-D calculation, that is, 3-D motions that satisfy the equations of quasi-static equilibrium on a 2-D viscoelastic structure. The prescription permits application to regional gravity computations where a 2-D structure adequately represents the structural heterogeneity. I use a hybrid approach where deformation is computed on a discretized domain and the resulting density perturbations are expanded with spherical harmonics to produce the free-air gravity field. Starting with a solution to the equations of quasi-static displacements in the Laplace transform domain for a given dislocation source, I solve Poisson’s equation using Lagrangian interpolation on spectral element nodes to compute the required deformation quantities that contribute to free-air gravity. A numerical inverse Laplace transform then yields time domain results. This methodology is tested with analytic solutions on a spherically stratified viscoelastic structure, then applied to evaluate the effect of a descending slab of relatively high viscosity on post-seismic gravity in a megathrust faulting setting.

Geophysical Journal International↗

USGS2019

Coseismic slip and early afterslip of the M6.0 August 24, 2014 South Napa, California, earthquake

We employ strong motion seismograms and static offsets from the Global Positioning System, Interferometric Synthetic Aperture Radar, and other measurements in order to derive a coseismic slip and afterslip model of the M6.0 24 August 2014 South Napa earthquake. This earthquake ruptured an ∼13‐km‐long portion of the West Napa fault with predominantly right‐lateral strike slip. In the kinematic seismic slip inversions, we couple the coseismic slip and afterslip distributions by requiring both distributions to involve right‐lateral strike slip with positive amplitude, with the net static slip being the sum of the two. We consider several candidate fault geometries: a first involving two steeply east dipping fault planes that reach Earth's surface at the western surface trace (STW), where most surface rupture was observed, a second involving a steeply west dipping plane that also reaches Earth's surface at the STW, and a third involving a combination of two variably west dipping planes constrained to pass through the locus of postseismic seismicity located ∼1 km west of the STW. The data are best fit using the model of two east dipping fault planes, with coseismic slip up to ∼1.2 m on a dominant shallow asperity about 10 km north of the hypocenter and on deeper asperities on the southern part of the rupture. Afterslip up to 1 m is concentrated along the southern part of the rupture at depths 5 km, consistent with surface observations of afterslip. Seismic moments associated with coseismic slip and afterslip are 1.13×10 18 N m (Mw 6.00) and 3.64×10 17 N m, respectively.

California↗

USGS2019

Induced seismicity reduces seismic hazard?

Earthquakes caused by human activities have been observed for decades. Often these are related to industrial activities pumping fluids into deep geologic formations, like with wastewater disposal. The simplest theory connecting these processes to earthquakes is straightforward: injection leads to fluid pressure changes that either reduce the strength of preexisting faults or generate new faults. In practice, the conditions that lead to induced earthquakes are not always clear in ways that can be generalized. Kao et al. (2018, https://doi.org/10.1029/2018GL079288) show how the distribution of induced earthquakes in Western Canada relate to natural rates of deformation in the crust. Using these new results, they discuss an intriguing paradox: induced seismicity can cause short-term increases in the seismic hazard that are followed by a period of reduced seismic hazard. Such hazard-reducing scenarios are plausible but hinge upon simplifying assumptions about how the crust stores and releases strain energy in the form of earthquakes.

Geophysical Research Letters↗

USGS2019

Fault slip associated with the 2 September 2017 M 5.3 Sulphur Peak, Idaho, earthquake and aftershock sequence

The 2 September 2017 M 5.3 Sulphur Peak, Idaho, earthquake is one of the largest earthquakes in southern Idaho since the 1983 M 6.9 Borah Peak earthquake. It was followed by a vigorous aftershock sequence for nearly two weeks that included five events above M 4.5. The coseismic and early postseismic deformation was measured with both Interferometric Synthetic Aperture Radar and Global Positioning System (GPS), yielding up to 3 cm subsidence southwest of the mainshock epicenter and horizontal motions of ∼ 1    mm "> ∼ 1 mm at sites ∼ 40    km "> ∼ 40 km east and west of the epicenter. We derive dislocation models of the net slip during the ∼ 14 ‐ day "> ∼ 14 ‐ day s warm from Sentinel 1A interferograms and GPS offsets, allowing for both fault‐zone collapse and normal faulting to account for the observed geodetic motions. Slip inversions yield several decimeters of normal slip on one or more normal faults near the mainshock hypocenter. Distributed normal slip on a moderately (55°) east‐dipping fault, normal slip on one or more shallowly west‐dipping faults, or a combination thereof explain the data equally well and are difficult to distinguish from one another on the basis of geodetic data alone. Previously mapped regional Sevier‐age thrust structures and later normal faults dip westward, suggesting that the sequence reactivated one or more ancient thrust structures with normal slip. If a moderately east‐dipping fault accommodated substantial slip, it would imply a nascent fault structure that cuts across the reactivated ancient thrust structures. The inferred geodetic moment of 3.02 – 4.39 × 10 17    N · m "> 3.02 – 4.39 × 10 17 N ⋅ m ( ⁠ M w "> M w 5.62–5.73) greatly exceeds the 1.15 × 10 17    N · m "> 1.15×10 17 N·m ( ⁠ M w "> M w 5.34) seismic moment of the 2 September mainshock, showing that most of the moment release occurred during the aftershock sequence. Up to ∼ 0.2    m "> ∼ 0.2 m of fault‐zone collapse may have occurred on a shallow west‐dipping fault, suggesting possible large‐scale expulsion of fluids from the fault zone at depth.

Idaho↗

USGS2019

Sea level rise in the Samoan Islands escalated by viscoelastic relaxation after the 2009 Samoa‐Tonga earthquake

The Samoan islands are an archipelago hosting a quarter million people mostly residing in three major islands, Savai'i and Upolu (Samoa), and Tutuila (American Samoa). The islands have experienced sea level rise by 2–3 mm/year during the last half century. The rate, however, has dramatically increased following the M w 8.1 Samoa‐Tonga earthquake doublet (megathrust + normal faulting) in September 2009. Since the earthquake, we found large‐scale gravity increase (0.5 μ Gal/year) around the islands and ongoing subsidence (8–16 mm/year) of the islands from our analysis of Gravity Recovery And Climate Experiment gravity and GPS displacement data. The postseismic horizontal displacement is faster in Samoa, while the postseismic subsidence rate is considerably larger in American Samoa. The analysis of local tide gauge records and satellite altimeter data also identified that the relative sea level rise becomes faster by 7–9 mm/year in American Samoa than Samoa. A simple viscoelastic model with a Maxwell viscosity of 2–3 × 10 18 Pa s for the asthenosphere explained postseismic deformation at nearby GPS sites as well as Gravity Recovery And Climate Experiment gravity change. It is found that the constructive interference of viscoelastic relaxation from both megathrust and normal faulting has intensified the postseismic subsidence at American Samoa, causing ~5 times faster sea level rise than the global average. Our model indicates that this trend is likely to continue for decades and result in sea level rise of 30–40 cm, which is independent of and in addition to anticipated climate‐related sea level rise. It will worsen coastal flooding on the islands leading to regular nuisance flooding.

Journal of Geophysical Research↗

USGS2019

Surface imaging functions for elastic reverse time migration

Reverse time migration is often used to interpret acoustic or three‐component seismic recordings by creating an image of subsurface seismic reflectors. Here I describe elastic reverse time migration imaging functions that are cast as waveform misfit sensitivity kernels of contrasts in material parameters across hypothetical seismic discontinuities, that is, specular reflectors. The proposed “surface” imaging functions are theoretically applicable to either reflected or converted waves in order to estimate the location and reflectivity of these discontinuities. The surface imaging functions, as well as volumetric sensitivity kernels that target point diffractors, are tested on sets of synthetic surface array recordings that sample the 3‐D seismic wavefield on simple 2‐D structures generated using a 2.5‐D spectral element method. These tests illustrate that in contrast with the volumetric sensitivity kernels, the reflectivity is generally dominated by a high‐amplitude peak that coincides with input locations of discontinuities. Passive recordings of microseismicity, shot gathers, or a combination thereof can be potentially interpreted with the new surface imaging functions to yield useful reflectivity images.

Journal of Geophysical Research Solid Earth↗

USGS2019

Lithosphere and shallow asthenosphere rheology from observations of post-earthquake relaxation

In tectonically active regions, post-earthquake motions are generally shaped by a combination of continued fault slippage (afterslip) on a timescale of days to months and viscoelastic relaxation of the lower crust and upper mantle on a timescale of days to years. Transient crustal motions have been observed following numerous magnitude >~7 earthquakes in various tectonic settings: continental rift zones (Basin and Range), continental plate boundary zones (San Andreas fault corridor; Alaska; Turkey), subduction zones (Japan, Chile, Sumatra), ongoing continental collision zones (Arabia; Tibet), and mid-ocean rifting zones (Iceland). When afterslip can be discriminated from viscoelastic relaxation and when temporal coverage of the postseismic measurements is broad (i.e., geodetic surveys of at least several years duration are available), a wide spectrum of relaxation timescales are usually identified. Current temporal resolution and modeling approaches (e.g., Burgers body analog) allow identification of transient (Kelvin) and steady-state (Maxwell) viscosities that are operable in the short-term and long-term, respectively. I compile results from 40 studies of post-earthquake motions, augmented by ten studies of contemporary surface loading or unloading, that illuminate current estimates of transient and steady-state viscosity of the lower crust and/or uppermost mantle. Lower crust viscosity estimates range from ~ 10 18 to 10 21 Pa s, with most estimates near the upper end except in areas of overthickened crust. Mantle lithosphere and asthenosphere viscosity estimates are particularly abundant and yield a picture of transient viscosity ranging from ~10 16 to 10 19 Pa s and steady-state viscosity ranging from ~10 18 to 10 21 Pa s. To first order, both transient and steady-state viscosities are well correlated with regional heat flow, and steady-state viscosities are comparable with temperature and strain-rate dependent rock viscosities from laboratory-based flow laws.

Physics of Earth and Planetary Interiors↗

USGS2017

Viscoelastic lower crust and mantle relaxation following the 14–16 April 2016 Kumamoto, Japan, earthquake sequence

The 2016 Kumamoto, Japan, earthquake sequence, culminating in the M w =7.0 16 April 2016 main shock, occurred within an active tectonic belt of central Kyushu. GPS data from GEONET reveal transient crustal motions from several millimeters per year up to ∼3 cm/yr during the first 8.5 months following the sequence. The spatial pattern of horizontal postseismic motions is shaped by both shallow afterslip and viscoelastic relaxation of the lower crust and upper mantle. We construct a suite of 2-D regional viscoelastic structures in order to derive an optimal joint afterslip and viscoelastic relaxation model using forward modeling of the viscoelastic relaxation. We find that afterslip dominates the postseismic relaxation in the near field (within 30 km of the main shock epicenter), while viscoelastic relaxation dominates at greater distance. The viscoelastic modeling strongly favors a very weak lower crust below a ∼65 km wide zone coinciding with the Beppu-Shimabara graben and the locus of central Kyushu volcanism. Inferred uppermost mantle viscosity is relatively low beneath southern Kyushu, consistent with independent inferences of a hydrated mantle wedge within the Nankai trough fore -arc.

Kumamoto↗