Kilauea's East Rift Zone Research Articles

Induced polarization of volcanic rocks. 5. Imaging the temperature field of shield volcanoes

SUMMARY Kilauea is an active shield volcano located in Hawaiʻi. An induced polarization survey was performed in 2015 at the scale of the caldera. The data were acquired with a 2.5 km cable with 64 electrodes and a spacing of 40 m between the electrodes. A total of 6210 measurements were performed. The apparent chargeability data were inverted using a least square technique to obtain a chargeability tomogram. The normalized chargeability tomogram is obtained by multiplying cell-by-cell the chargeability by the conductivity. Once the conductivity and normalized chargeability tomograms are obtained, they are jointly interpreted using a dynamic Stern layer conduction/polarization model, which explains the low-frequency polarization spectra of volcanic rocks. This conductivity/polarization model is tested here on new laboratory experiments performed on 24 samples from a drill-hole located on the Kilauea East Rift Zone (Hole SOH-2). We show that for Kilauea, the ratio between the normalized chargeability and the conductivity is equal to a dimensionless number R = 0.10 ± 0.02 proving that the conductivity and the normalized chargeability are both controlled by the alteration products of the volcanic rocks with a minor role of magnetite except close to the ground surface. In turn, the degree of alteration is controlled by temperature and therefore normalized chargeability and electrical conductivity can both be used as a non-intrusive temperature sensor. This approach is then applied to the field data. Meaningful temperature tomograms can be produced from both electrical conductivity and normalized chargeability tomograms.

Air quality impacts of the 2018 Mt. Kilauea Volcano eruption in Hawaii: A regional chemical transport model study with satellite-constrained emissions

Abstract Volcanic eruptions could bring a vast amount of sulfur dioxide (SO2) and ash into the air, often imposing substantial impacts on air quality and the ecosystem. Quantifying its impacts, however, is difficult due to the uncertainties in estimating the strength and variations of volcanic emissions. Here we developed and evaluated a new approach to combine satellite SO2 detection and chemical transport modeling to assess the impact of the 2018 Mt. Kilauea eruption on air quality over Hawaii. During the sustained eruption of the Kilauea Volcano in Hawaii's Big Island from May to July 2018, considerable SO2 and PM2.5 enhancements were observed both from the ground and from space. We studied this case using an experimental version of the NOAA National Air Quality Forecast Capability (NAQFC) modeling system. Daily emissions of SO2 and ash were estimated using a combination of SO2 column density retrieved by Ozone Mapping and Profiling Suite (OMPS) Nadir-Mapper (NM) aboard the Suomi-NPP satellite and the NAQFC model with an inverse emission modeling approach. We found that the volcanic SO2 emission rates peaked at 15,000 mol/s from the Kilauea's East Rift zone and Summit. The formation and transport of volcanic smog, or Vog, was highly dependent upon the vertical distribution of the volcanic emission, controlled by the heat flux of emission sources. We conducted four model simulations with various emission settings, and compared them to satellite data (CALIOP, OMPS and VIIRS) and in-situ measurements. All the runs tended to underpredict the peak values of surface SO2 and PM2.5 (particulate matter smaller than 2.5 μm in diameter). The “No Plume Rise” run underestimated the Vog plume rise and downstream transport. Using fixed emission rate or removing the temporal variations (”3-Day Mean”) led to miss peak Vog effects or inconsistent transport pattern compared to the observations. Therefore, the Base simulation with daily-varying emission and plume rise was used to quantify the air quality effects of the Kilauea eruption. We found that the volcanic eruption elevated surface PM2.5 concentration by 30–40 μg/m3 in the southeast part of the Big Island, with peak values up to 300 μg/m3. The Vog effect on trace gases, such as O3, NOx, and non-methane hydrocarbons, were much weaker (

Rupture in the 4 May 2018 MW 6.9 Earthquake Seaward of the Kilauea East Rift Zone Fissure Eruption in Hawaii

AbstractThe 4 May 2018, MW 6.9 thrust‐faulting earthquake occurred a day after the first fissure eruption in Leilani Estates along the East Rift Zone of Kilauea, Hawaii, relatively early in the 2018 eruptive sequence. The earthquake's location and geodetic deformation pattern are similar to those of the larger 1975 Kalapana earthquake (MW 7.7), which is thought to have occurred on the décollement between the island volcanics and the former Pacific seafloor. The 2018 event has a shallow‐dipping thrust plane and shallow epicenter located just offshore. A finite‐fault kinematic slip model is determined by inversion of teleseismic body waves, strong‐ground motion recordings, and coseismic Global Positioning System offsets along the southeastern coast of Hawaii. Inversions indicate a preferred dip of 7°, slip of up to 3.0 m near 5‐km deep offshore of the coast, and low average rupture speed of ~1 km/s. These are consistent with rupture of a weak décollement.

Carbon dioxide of Pu`u`O`o volcanic plume at Kilauea retrieved by AVIRIS hyperspectral data

A remote sensing approach permits for the first time the derivation of a map of the carbon dioxide concentration in a volcanic plume. The airborne imaging remote sensing overcomes the typical difficulties associated with the ground measurements and permits rapid and large views of the volcanic processes together with the measurements of volatile components exolving from craters. Hyperspectral images in the infrared range (1900–2100 nm), where carbon dioxide absorption lines are present, have been used. These images were acquired during an airborne campaign by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) over the Pu`u` O`o Vent situated at the Kilauea East Rift zone, Hawaii. Using a radiative transfer model to simulate the measured up-welling spectral radiance and by applying the newly developed mapping technique, the carbon dioxide concentration map of the Pu`u` O`o Vent plume were obtained. The carbon dioxide integrated flux rate were calculated and a mean value of 396 ± 138 t d − 1 was obtained. This result is in agreement, within the measurements errors, with those of the ground measurements taken during the airborne campaign.

Comparative velocity structure of active Hawaiian volcanoes from 3-D onshore–offshore seismic tomography

We present a 3-D P-wave velocity model of the combined subaerial and submarine portions of the southeastern part of the Island of Hawaii, based on first-arrival seismic tomography of marine airgun shots recorded by the onland seismic network. Our model shows that high-velocity materials (6.5–7.0 km/s) lie beneath Kilauea's summit, Koae fault zone, and the upper Southwest Rift Zone (SWRZ) and upper and middle East Rift Zone (ERZ), indicative of magma cumulates within the volcanic edifice. A separate high-velocity body of 6.5–6.9 km/s within Kilauea's lower ERZ and upper Puna Ridge suggests a distinct body of magma cumulates, possibly connected to the summit magma cumulates at depth. The two cumulate bodies within Kilauea's ERZ may have undergone separate ductile flow seaward, influencing the submarine morphology of Kilauea's south flank. Low velocities (5.0–6.3 km/s) seaward of Kilauea's Hilina fault zone, and along Mauna Loa's seaward facing Kao'iki fault zone, are attributed to thick piles of volcaniclastic sediments deposited on the submarine flanks. Loihi seamount shows high-velocity anomalies beneath the summit and along the rift zones, similar to the interpreted magma cumulates below Mauna Loa and Kilauea volcanoes, and a low-velocity anomaly beneath the oceanic crust, probably indicative of melt within the upper mantle. Around Kilauea's submarine flank, a high-velocity anomaly beneath the outer bench suggests the presence of an ancient seamount that may obstruct outward spreading of the flank. Mauna Loa's southeast flank is also marked by a large, anomalously high-velocity feature (7.0–7.4 km/s), interpreted to define an inactive, buried volcanic rift zone, which might provide a new explanation for the westward migration of Mauna Loa's current SWRZ and the growth of Kilauea's SWRZ.

Stratigraphy of the Hawai‘i Scientific Drilling Project core (HSDP2): Anatomy of a Hawaiian shield volcano

The Hawai‘i Scientific Drilling Project (HSDP2) successfully drilled ∼3.1 km into the island of Hawai‘i. Drilling started on Mauna Loa volcano, drilling 247 m of subaerial lavas before encountering 832 m of subaerial Mauna Kea lavas, followed by 2019 m of submarine Mauna Kea volcanic and sedimentary units. The 2.85 km stratigraphic record of Mauna Kea volcano spans back to ∼650 ka. Mauna Kea subaerial lavas have high average olivine contents (13 vol.%) and low average vesicle abundances (10 vol.%). Most subaerial Mauna Kea flows are ‘a‘ā (∼63%), whereas the Mauna Loa section contains nearly equal amounts of pāhoehoe and ‘a‘ā (like its current surface). The submarine Mauna Kea section contains an upper, ∼900 m thick, hyaloclastite‐rich section and a lower, ∼1100 m thick, pillow‐lava‐dominated section. These results support a model that Hawaiian volcanoes are built on a pedestal of pillow lavas capped by rapidly quenched, fragmented lava debris. The HSDP2 section is compared here to a 1.7 km deep hole (SOH1) on Kilauea's lower east rift zone. Differences in the sections reflect the proximity to source vents and the lower magma supply to Kilauea's rift zone. Both drill core sections are cut by intrusions, but the higher abundance of intrusions in SOH1 reflects its location within a rift zone, causing more extensive alteration in the SOH1 core. The HSDP2 site recovered a relatively unaltered core well suited for geochemical analyses of the single deepest and most complete borehole ever drilled through a Hawaiian or any other oceanic island volcano.

Hawaiian lava flows in the third dimension: Identification and interpretation of pahoehoe and ′a′a distribution in the KP‐1 and SOH‐4 cores

Hawaiian lava flows are classified as pahoehoe or 'a'a by their surface morphology. As surface morphology reflects flow emplacement conditions, the surface distribution of morphologic flow types has been used to study the evolution and eruptive history of basaltic volcanoes. We extend this analysis to the third dimension by determining the distribution of flow types in two deep drill cores, the Scientific Observation Hole‐4 (SOH‐4) core, drilled near Kilauea's East Rift Zone (ERZ), and the pilot hole (Kahi Puka‐1 (KP‐1)) for the Hawaiian Scientific Drilling Project (HSDP), drilled through distal flows from Mauna Loa and Mauna Kea. Flows are classified using both internal structures and groundmass textures, with the latter useful when identification based on mesoscopic flow features (e.g., surface morphology and vesicle content and distribution) is ambiguous. We then examine the temporal distribution of pahoehoe and ′a′a flows in proximal (SOH‐4) and distal (KP‐1) settings. Sequence analysis shows that the two flow types are not randomly distributed in either core but instead are strongly clustered. The proximal SOH‐4 core is dominated by thin pahoehoe flows (∼60% by volume), consistent with the common occurrence of surface‐fed pahoehoe flows in near‐vent settings. The distal KP‐1 core has a high proportion of ′a′a (∼58% by volume), although the proportion of pahoehoe and ′a′a varies dramatically throughout the Mauna Kea sequence. Thick inflated pahoehoe flows dominate when the drill site was near sea level, consistent with the numerous inflated pahoehoe fields on the current coastal plains of Kilauea and Mauna Loa. ′A′a flows are abundant when the site was far above sea level. As slope increases from the coastal plains to Mauna Kea's flank, this correlation may reflect the combined effect of long transport distances and increased slopes on flow emplacement. These results demonstrate that flow type and thickness variations in cores provide valuable information about both vent location and local site environment. Observed variations in flow type within the KP‐1 core raise interesting questions about feedback between volcano evolution and flow morphology and suggest that flow type is an important variable in models of volcano growth and related models for lava flow hazard assessment.

A comparison between subaerial and submarine eruptions at Kilauea Volcano, Hawaii: implications for the thermal viability of lateral feeder dikes

Eruption styles on the subaerial East Rift Zone (ERZ) of Kilauea volcano are reviewed and a classification scheme for the different types of eruption is proposed. The various eruption types are produced by differing thermal and driving pressure behaviour in the feeder dikes. Existing evidence is reviewed and new evidence presented of the types and volumes of eruptions on the Puna Ridge, which is the submarine extension of the ERZ. Eruptions on the Puna Ridge fall into the same five classes as, and are of comparable volume to, those on the subaerial ERZ. Evidence is presented which suggests that feeder dikes for Puna Ridge eruptions are more thermally viable than those feeding subaerial eruptions, and this difference causes long-lived, large-volume eruptions to be more common on the Puna Ridge than on the subaerial ERZ. This systematic variation in thermal viability may be due to increased dike width for Puna Ridge dikes or increased pressure gradients driving magma flow. Lateral dike emplacement is common to many basaltic systems including on other Hawaiian volcanoes, in Iceland and at mid-ocean ridges. The systematic trend inferred for the ERZ of Kilauea implies that in the other systems large-volume eruptions may also be more common at great distances than they are close to the magma centre.

Anomalously high b-values in the South Flank of Kilauea volcano, Hawaii: evidence for the distribution of magma below Kilauea's East rift zone

The pattern of b-value of the frequency–magnitude relation, or mean magnitude, varies little in the Kaoiki-Hilea area of Hawaii, and the b-values are normal, with b=0.8 in the top 10 km and somewhat lower values below that depth. We interpret the Kaoiki-Hilea area as relatively stable, normal Hawaiian crust. In contrast, the b-values beneath Kilauea's South Flank are anomalously high ( b=1.3–1.7) at depths between 4 and 8 km, with the highest values near the East Rift zone, but extending 5–8 km away from the rift. Also, the anomalously high b-values vary along strike, parallel to the rift zone. The highest b-values are observed near Hiiaka and Pauahi craters at the bend in the rift, the next highest are near Makaopuhi and also near Puu Kaliu. The mildest anomalies occur adjacent to the central section of the rift. The locations of the three major and two minor b-value anomalies correspond to places where shallow magma reservoirs have been proposed based on analyses of seismicity, geodetic data and differentiated lava chemistry. The existence of the magma reservoirs is also supported by magnetic anomalies, which may be areas of dike concentration, and self-potential anomalies, which are areas of thermal upwelling above a hot source. The simplest explanation of these anomalously high b-values is that they are due to the presence of active magma bodies beneath the East Rift zone at depths down to 8 km. In other volcanoes, anomalously high b-values correlate with volumes adjacent to active magma chambers. This supports a model of a magma body beneath the East Rift zone, which may widen and thin along strike, and which may reach 8 km depth and extend from Kilauea's summit to a distance of at least 40 km down rift. The anomalously high b-values at the center of the South Flank, several kilometers away from the rift, may be explained by unusually high pore pressure throughout the South Flank, or by anomalously strong heterogeneity due to extensive cracking, or by both phenomena. The major b-value anomalies are located SSE of their parent reservoirs, in the direction of motion of the flank, suggesting that magma reservoirs leave an imprint in the mobile flank. We hypothesize that the extensive cracking may have been acquired when the anomalous parts of the South Flank, now several kilometers distant from the rift zone, were generated at the rift zone near persistent reservoirs. Since their generation, these volumes may have moved seaward, away from the rift, but earthquakes occurring in them still use the preexisting complex crack distribution. Along the decollement plane at 10 km depth, the b-values are exceptionally low ( b=0.5), suggesting faulting in a more homogeneous medium.

Tomographic image of P‐velocity structure beneath Kilauea's East Rift Zone and South Flank: Seismic evidence for a deep magma body

We present first results from the analysis of P‐wave arrival time data recorded from November 11 to December 31, 1999, by a temporary 29‐station network installed across Kilauea Volcano's East Rift Zone (ERZ) and South Flank (SF) on Hawaii, augmented by data from the permanent network of the Hawaiian Volcano Observatory. Starting with the inversion result for a minimum 1D velocity model, we use arrival time data from 135 local earthquakes to invert for the 3D P‐velocity structure. The resulting tomographic image shows evidence for a deep magma body beneath the ERZ just east of its southward bend, and a smaller magma body at about 5 km depth beneath the WNW‐ESE trending segment of the ERZ. We also observe a drastic change in velocities south of the Hilina fault system from velocities around 5.5 km/s in the west to 6.5 km/s to the east.

Magmatic history of the East Rift Zone of Kilauea Volcano, Hawaii based on drill core from SOH 1

Deep drilling has allowed for the first time an examination of most of the shield stage of a Hawaiian volcano when it is centered over the hotspot and most of its volume is produced. We determined the lithologies, ages, geochemical characteristics and accumulation rates of rocks from the continuously cored, ∼1.7km deep Scientific Observation Hole (SOH) 1, which was drilled into Kilauea's East Rift Zone. The uppermost ∼750m of this hole contain relatively unaltered subaerially quenched lavas; the lower portion is a mixture of weakly to moderately altered hyaloclastites and massive basalts intruded by dikes. Unspiked K–Ar dating was attempted on 14 lavas but only four yielded geologically reasonable ages. The oldest age is 351±12ka for a sample from 1541m. Eighty XRF major and trace element analyses of lavas from throughout the section demonstrate that these lavas have the same compositional range as historical Kilauea lavas and that there was no systematic geochemical variation for the volcano over the last 350ka. Kilauea's average yearly magma supply during this period was probably ∼0.05±.01km3, which is identical to its average historical magma supply rate. Thus, Kilauea's overall source and melting conditions have remained remarkably constant for the last 350ka despite its having drifted ∼42km over the Hawaiian plume. These results require that the portion of the plume being melted to produce Kilauea magmas be relatively large and well mixed, which is inconsistent with models that place this source on the margin of a radially zoned plume.

January 30, 1997 eruptive event on Kilauea Volcano, Hawaii, as monitored by continuous GPS

A continuous Global Positioning System (GPS) network on Kilauea Volcano captured the most recent fissure eruption in Kilauea's East Rift Zone (ERZ) in unprecedented spatial and temporal detail. The short eruption drained the lava pond at Puèu Oèo, leading to a two month long pause in its on‐going eruption. Models of the GPS data indicate that the intrusion's bottom edge extended to only 2.4 km. Continuous GPS data reveal rift opening 8 hours prior to the eruption. Absence of precursory summit inflation rules out magma storage overpressurization as the eruption's cause. We infer that stresses in the shallow rift created by the continued deep rift dilation and slip on the south flank décollement caused the rift intrusion.

Constructing the upper crust of the Mid‐Atlantic Ridge: A reinterpretation based on the Puna Ridge, Kilauea Volcano

The volcanic morphology of a number of segments of the slow spreading Mid‐Atlantic Ridge (MAR) have been reinterpreted based on our understanding of dike emplacement, dike propagation, and eruption at the East Rift Zone of Kilauea Volcano, Hawaii and its submarine extension, the Puna Ridge. The styles of volcanic eruption at the submarine Puna Ridge are remarkably similar to those of the axial volcanic ridges (AVRs) constructed on the median valley floor of the MAR. We use this observation to relate volcanic processes occurring at Kilauea Volcano to the MAR. We now consider that volcanic features (e.g., seamounts and lava terraces) built on the flanks of the AVRs are secondary features that are fed from lava tubes or channels, not primary features fed directly from an underlying dike. We examine simple models of pipe flow and conclude that lava tubes can transport lava down the flanks of submarine rifts to build all of the volcanic features observed there. In addition, deep water lava tubes are strong enough to withstand the pressures of a few megapascals that the building of a volcanic structure 150 m high at the end of the tube would generate. The volumes of individual volcanic terraces and seamounts on the Puna Ridge and at the MAR are large (0.1–1 km3) and similar to the volumes of lava flows that are broadly distributed at the subaerial East Rift Zone of Kilauea. This striking difference in the volcanic morphology on a scale of 1–2 km (producing terraces and seamounts underwater and low‐relief flows on land) must be related to the enhanced cooling and to the greater mechanical stability of tubes in the submarine environment. We suggest that at the MAR a crustal magma reservoir, most likely located beneath shallow, flat sections of the segment, provides magma to the rift axis through dikes that propagate laterally tens of kilometers. The zone of dike intrusion, at least in the neighborhood of the magma body, is likely narrower than the width resurfaced by flows, yielding a crustal structure that has a rapid vertical transition from lavas to sheeted dikes. At segment ends the zone of dike intrusion is likely to be wider, giving a resulting structure with a more gradual transition from lavas to dikes.

Stabilization of volcanic flanks by dike intrusion: an example from Kilauea

Dike propagation and dilation increases the compression of adjacent rocks. On volcanoes, especially oceanic shields, dikes are accordingly thought to be structurally destabilizing. As compression is incremented, volcanic flanks are driven outward or downslope and thus increase their susceptibility to destructive earthquakes and giant landslides. We show, however, that the 2-m-thick dike emplaced along the east rift zone of Kilauea in 1983 actually stabilized that volcano's flank. Specifically, production of flank earthquakes dropped more than twofold after 1983 as maximum downslope motion slowed to 6 cm·year–1 from approximately 40 cm·year–1 during 1980–1982. As much as 65 cm of deflationary subsidence above Kilauea's summit and upper rift zones accompanied the dike intrusion. According to recent estimates, this deflation corresponds to a reduction in magma-reservoir pressure of approximately 4 MPa, probably about as much as the driving pressure of the 1983 dike. The volume of the dike, approximately 0.10–0.15 km3, is orders of magnitude less than the estimated 200- to 250-km3 volume of Kilauea's reservoir of magma and nearby hot, mushy rock. Thus, deflation of that reservoir reduces the compressional load on the flank over a much larger area than intrusion of the dike adds to it, particularly at the dominant depth of seismicity, 8–9 km. A Coulomb block model for flank motion during intervals between major earthquakes requires the low-angle fault beneath Kilauea's flank to exhibit slip weakening, conducive to earthquake instability. Accordingly, the triggering mechanism of destructive earthquakes, several of which have struck Hawaii during the past 150 years, need not require stresses accumulated by dike intrusions.

A reinterpretation of seismicity associated with the January 1983 dike intrusion at Kilauea Volcano, Hawaii

In January 1983, a dike intrusion/fissure eruption generated a swarm of 375 magnitude 1 to 3 earthquakes along a 16‐km segment of Kilauea's Middle East Rift Zone. We searched the Hawaiian Volcano Observatory catalog for multiplets of similar events from this region from 1980 through 1985 and obtained precise relative locations by waveform cross correlation. Over 150 of the intrusion earthquakes could be grouped into 14 multiplets of five or more events with sufficient similarity for accurate relocation. Some multiplets were active for only a few minutes during the downrift migration phase of the seismic swarm, consistent with generation near the propagating dike tip, while others were active for several days. The two multiplets nearest the origin of the seismic swarm include events from the preceding days and months. Most multiplets span only 50 to 100 m following relocation, are located at about 3 to 4 km depth, and appear to deepen downrift. The catalog depths of those earthquakes in multiplets and those not in multiplets are similar, suggesting that most of the recorded seismicity may have come from a very limited depth interval despite the fact that the dike breached the surface. By analogy with a mechanical model used to explain a similar clustering of background seismicity in the Upper East Rift in 1991, we infer that the earthquakes are generated in regions of high stress concentration immediately above Kilauea's deforming deep rift body. This conclusion is consistent with the depth of the top of the deep rift body inferred from geodetic data and with numerical calculations suggesting that a significant ambient differential stress is required for dikes to produce earthquakes larger than magnitude 1.

A dynamic balance between magma supply and eruption rate at Kilauea volcano, Hawaii

The dynamic balance between magma supply and vent output at Kilauea volcano is used to estimate both the volume of magma stored within Kilauea volcano and its magma supply rate. Throughout most of 1991 a linear decline in volume flux from the Kupaianaha vent on Kilauea's east rift zone was associated with a parabolic variation in the elevation of Kilauea's summit as vent output initially exceeded then lagged behind the magma supply to the volcano. The correspondence between summit elevation and tilt established with over 30 years of data provided daily estimates of summit elevation in terms of summit tilt. The minimum in the parabolic variation in summit tilt and elevation (or zero elevation change) occurs when the magma supply to the reservoir from below the volcano equals the magma output from the reservoir to the surface, so that the magma supply rate is given by vent flux on that day. The measurements of vent flux and tilt establish that the magma supply rate to Kilauea volcano on June 19, 1991, was 217,000±10,000 m3/d (or 0.079±0.004 km3/yr). This is close to the average eruptive rate of 0.08 km3/yr between 1958 and 1984. In addition, the predictable response of summit elevation and tilt to each east rift zone eruption near Puu Oo since 1983 shows that summit deformation is also a measure of magma reservoir pressure. Given this, the correlation between the elevation of the Puu Oo lava lake (4 km uprift of Kupaianaha and 18 km from the summit) and summit tilt provides an estimate for magma pressure changes corresponding to summit tilt changes. The ratio of the change in volume to the change in reservoir pressure (dV/dP) during vent activity may be determined by dividing the ratio of volume erupted to change in summit tilt (dV/dtilt) by the ratio of pressure change to change in summit tilt (dP/dtilt). This measure of dV/dP, when combined with laboratory measurements of the bulk modulus of tholeitic melt, provides an estimate of 240±50 km3 for the volume of Kilauea's magma reservoir. This estimate is much larger than traditional estimates but consistent with seismic tomographic imaging and geophysical modeling of Kilauea's magma system.

Rapid emplacement of a mid-ocean ridge lava flow on the East Pacific Rise at 9° 46′–51′N

In April, 1991, during a submersible diving expedition to the East Pacific Rise (EPR) crest at 9°46′–51′N, a new volcanic eruption on the sea floor was discovered. Here, we report results from numerical modeling of that eruption, which indicate that ∼ 4 × 106 −6 × 106 m3 of lava was emplaced in ∼ 1–2 h, with an average eruption rate of ∼ 103–106 m3 s−1 — comparable to rates observed in Hawaii at Kilauea's East Rift Zone. If the rapid emplacement of the 1991 EPR lava and its short eruption duration are typical of volcanic events at fast-spreading mid-ocean ridge crests, these characteristics have broad implications for our ability to detect, monitor and understand the evolution of magmatic and volcanic processes within the axial zone.

Type of faulting and orientation of stress and strain as a function of space and time in Kilauea's south flank, Hawaii

Earthquake focal mechanisms of events occurring between 1972 and 1992 in the south flank of Kilauea volcano, Hawaii, are used to infer the state of stress and strain as a function of time and space. We have determined 870 fault plane solutions from P wave first motion polarities for events with magnitudes ML ≥ 2.5 and depth ranging between 6 and 12 km. Faulting is characterized by a mixture of decollement, reverse, and normal faults. Most large earthquakes with magnitude M < 7 slip on reverse faults striking NE at 40° and dipping SE between 60° and 70°. In Hawaii, the earthquakes with M > 7 rupture the decollement plane, since it is the only surface large enough to generate magnitude 7 or larger earthquakes. The percentage of reverse faulting events is high compared to the decollement and normal faulting mechanisms for the period 1972–1983. The percentage of decollement type focal mechanisms becomes dominant after 1983. This pattern of faulting activity suggests that pressure was building up within Kilauea's rift zone prior to the 1983 Puu'Oo eruption. Overall, a single stress orientation with the maximum compressive stress oriented SE perpendicular to the rift and dipping at 45° is compatible with the coeval existence of decollement, reverse, and normal faults. However, in a crustal volume east of longitude 155°10′W, we find a change of the orientation of σ1 from nearly horizontal to plunging 45° SE occurring in 1979. This stress rotation suggests magma movements within the aseismic part of Kilauea's east rift zone. The strain and stress orientations are coaxial in the south flank except within the volume where the stress rotation is observed. We observe a change in the relationship between stress and strain directions caused either by the shifting of seismic activity from reverse faults to decollements, while stress stays constant, or by a rotation of stress, while strain remains constant. Assuming that the model of a noncohesive Coulomb wedge is appropriate for Kilauea's south flank, we find that high pore pressures are prevalent along the decollement and within the wedge for a coefficient of friction equal to 0.85.

96/01730 Fluid-inclusion evidence for past temperature fluctuations in the Kilauea East Rift Zone geothermal area, Hawaii

96/01730 Fluid-inclusion evidence for past temperature fluctuations in the Kilauea East Rift Zone geothermal area, Hawaii

Structure of the mobile south flank of Kilauea Volcano, Hawaii

How do huge landslides occur in Hawaii? We can begin to address this question by examining an incipient landslide structure on the mobile south flank of Kilauea volcano. Between 1970 and 1989 this part of the south flank moved in a southeast direction (seaward by 4 m on its eastern end and by 10 m on its western end), parallel to the western boundary of a large oifshore topographic bench. We show that abrupt decreases in cumulative flank movement correspond to abrupt decreases in hypocenter concentration. Most of the earthquakes occur in a 7‐ to 10‐km‐deep band beneath the south flank, and relative relocation of a subset of these events shows that they are derived from a single horizontal plane which appears to be a decollement. Focal mechanisms for this deep seismicity are dominated by thrust fault mechanisms, with the overlying block moving seaward. A comparison of south flank bathymetry with decollement structures on other volcanoes suggests that the distal end of this decollement coincides with the distal end of the offshore topographic bench (30–50 km from shore and 5 km below sea level). The fault system forming the Hilina pali is connected with this decollement, so that slip on the Hilina pali's normal faults coincides with slip of the decollement and with reverse faulting of the distal end of the bench. (This interpretation is supported by studies of the events associated with the M7.2 Kalapana earthquake in 1975.) When the Hilina pali is connected to the outer bench with a decollement inferred from seismicity, a wedge 8‐km thick adjacent to Kilauea's magma system and 1–2 km thick along the edge of the outer bench (an area over 3000 km2) is defined. The thickness adjacent to the magma system decreases eastward along Kilauea's East Rift Zone and southwestward from Kilauea caldera, whereas the thickness of the lip of the outer bench decreases only eastward. Geologic observations and geophysical studies of the subaerial flank reveal an extensive magma system beneath the Koae fault system as well as beneath Kilauea caldera and its East Rift Zone. We infer that the boundary of the proximal end of the mobile flank is the southern boundary of this magma system and that this boundary coincides with the southern boundaries of both the Koae fault system and Kilauea's East Rift Zone.