The Matata Earthquake Sequence of 1977, Bay of Plenty, New Zealand

<p>An ML. 5.4 earthquake and an associated sequence of smaller earthquakes, including foreshocks, were well recorded in 1977 by a network of 10 seismographs set for a microearthquake survey in the Bay of Plenty region, which is transitional between back-are spreading regions of the Havre Trough and the continental North Island. Upper crustal aftershock origins clustered and migrated within an area 7 km by 15 km elongated east-west. The aftershocks were relatively swarm-like, producing a b- value of 1.29 [plus or minus] 0.13, and were apparently of long sequence duration, with decay coefficient p = 0.67 [plus or minus] 0.03. A northeast-trending rupture fitted for the mainshock, originating close to where foreshocks were centred, and passing between tight concentrations of later aftershock activity to either side. Teleseismic waveforms, in addition to providing a 10.5 km estimate of focal depth, helped to constrain the solution of focal mechanism for the mainshock. The preferred solution is for mainly right-lateral slip on a northeast striking plane but with a normal component. The slip trend parallels the front of recent volcanism. Mechanisms for related events range from normal to strike slip, on parallel and intersecting planes, and are indicative of the complexity of geological structure where north-trending faults of the North Island shear belt meet with the front of recent volcanism. as well as of a prevailing traction across the volcanic front. The volcanic region is characterised by a low Poisson's ratio, suggested by the Wadati method to be v= 0.19 [plus or minus] 0.01 in contrast to v =0.27 [plus or minus] 0.01 for the greywacke region to the southeast; this difference is attributed to contrasting rock types and other conditions either side of the volcanic front. The multiplicity of earthquake sequences in the volcanic region indicates a high degree of heterogeneous structure. A low stress drop of 2.8 MPa inferred for the Matata mainshock suggests that the faulting occurs on pre-existing planes. Off-fault aftershocks occurred where the failure stress increased as a result of the mainshock rupture. A concurrent sequence of earthquakes originating near 50km depth indicated thrusting on the lithospheric plate interface underlying the North Island; thrusting on the interface apparently extends to about 70km depth, where the plates become decoupled. Oblique plate convergence and stick-slip motion on the weakly coupled interface provides the regional dextral shear component observed in the volcanic region for the Matata mainshock. An extensional component is therefore a necessary addition for the observed normal component of faulting, which predominated for the 1987 Edgecumbe mainshock. Wave mode conversions inferred for subcrustal earthquakes and the Matata sequence mainshock indicate that the Moho shallows from 28.5 km to 22 km northwestwards across the volcanic front, suggesting that new crust in the Bay of Plenty region is being created over a wide region rather than by active rifting along a sharp margin.</p>

The Matata Earthquake Sequence of 1977, Bay of Plenty, New Zealand

<p>An ML. 5.4 earthquake and an associated sequence of smaller earthquakes, including foreshocks, were well recorded in 1977 by a network of 10 seismographs set for a microearthquake survey in the Bay of Plenty region, which is transitional between back-are spreading regions of the Havre Trough and the continental North Island. Upper crustal aftershock origins clustered and migrated within an area 7 km by 15 km elongated east-west. The aftershocks were relatively swarm-like, producing a b- value of 1.29 [plus or minus] 0.13, and were apparently of long sequence duration, with decay coefficient p = 0.67 [plus or minus] 0.03. A northeast-trending rupture fitted for the mainshock, originating close to where foreshocks were centred, and passing between tight concentrations of later aftershock activity to either side. Teleseismic waveforms, in addition to providing a 10.5 km estimate of focal depth, helped to constrain the solution of focal mechanism for the mainshock. The preferred solution is for mainly right-lateral slip on a northeast striking plane but with a normal component. The slip trend parallels the front of recent volcanism. Mechanisms for related events range from normal to strike slip, on parallel and intersecting planes, and are indicative of the complexity of geological structure where north-trending faults of the North Island shear belt meet with the front of recent volcanism. as well as of a prevailing traction across the volcanic front. The volcanic region is characterised by a low Poisson's ratio, suggested by the Wadati method to be v= 0.19 [plus or minus] 0.01 in contrast to v =0.27 [plus or minus] 0.01 for the greywacke region to the southeast; this difference is attributed to contrasting rock types and other conditions either side of the volcanic front. The multiplicity of earthquake sequences in the volcanic region indicates a high degree of heterogeneous structure. A low stress drop of 2.8 MPa inferred for the Matata mainshock suggests that the faulting occurs on pre-existing planes. Off-fault aftershocks occurred where the failure stress increased as a result of the mainshock rupture. A concurrent sequence of earthquakes originating near 50km depth indicated thrusting on the lithospheric plate interface underlying the North Island; thrusting on the interface apparently extends to about 70km depth, where the plates become decoupled. Oblique plate convergence and stick-slip motion on the weakly coupled interface provides the regional dextral shear component observed in the volcanic region for the Matata mainshock. An extensional component is therefore a necessary addition for the observed normal component of faulting, which predominated for the 1987 Edgecumbe mainshock. Wave mode conversions inferred for subcrustal earthquakes and the Matata sequence mainshock indicate that the Moho shallows from 28.5 km to 22 km northwestwards across the volcanic front, suggesting that new crust in the Bay of Plenty region is being created over a wide region rather than by active rifting along a sharp margin.</p>

Temporal variations of magma composition, eruption style and rate at Fuji Volcano, Japan

Mt. Fuji is an active basaltic volcano near the Tokyo metropolitan area; future eruptions could thus have serious nationwide impacts. To better understand recent volcanism at Fuji Volcano, we here clarify temporal variations of eruption rate and magma composition since 5.6 ka based on time-series volumetric and geochemical data of eruptive products in a new stratigraphic sequence. Volcanic activity during the studied period consisted of (i) the emission of many lava flows that formed a new volcanic edifice between 5.6 and 3.45 ka, (ii) a period dominated by explosive events both at the summit and on the flanks of the volcano between 3.45 and 2.25 ka, and (iii) a period dominated by flank fissure eruptions since 2.25 ka. The eruption rate (dense-rock equivalent, DRE) was 3.5 km3 DRE/kyr during the edifice-building period, decreased to 0.8 km3 DRE/kyr during the explosive period, and then increased to 2.0 km3 DRE/kyr since 1.5 ka. Erupted magmas were dominantly basaltic and geochemically similar through time, except for increased Sr contents and decreased Ca/Sr ratios during the explosive period. Similarly, the geochemical properties of the parental magmas did not change greatly, although the Sr contents and Ca/Sr ratios of magmas erupted during the explosive period reflect the delayed fractionation of plagioclase due to the increased water contents of the parental magmas at that time.

Unusual Perforations in Phlogopite Crystals from Caldara di Manziana (Italy) Caused by Sulphuric Acid Generated by Microbial Oxidation of H2S Emanations

Phlogopite flakes strewn on the soil of Caldara di Manziana (Italy) display multiple minute perforations. The site is a caldera linked to recent volcanism (90 ka to 0.8 Ma) with present emanations of CO2 (~150 t d−1) and H2S (~2.55 t d−1). Stereomicroscopy and SEM–EDX observation of the phlogopite crystals shows holes and depressions <200 µm to 2 mm across. They are circular, pseudo-hexagonal, or irregular. Within the depressions, there are deposits of phlogopite alteration products consistent with a sulphuric acid attack, showing loss of Mg and K. Some are thin and homogeneous; others are thick, irregular, and chemically heterogeneous, including plates, flakes, tubes of Fe-beidellite or Fe-bearing halloysite, silica, Fe oxides, and gypsum. Areas of phlogopite surface are also altered. Sulphuric acid is produced from the H2S gas by the mediation of sulphur-oxidizing bacteria. Pseudo-hexagonal perforations are interpreted to result from slow acid attack with dissolution controlled by phlogopite crystal symmetry. Some depressions developed surrounding films of pseudo-hexagonal shape, interpreted as jarosite crystallizing radially outwards from the depressions. This style of acid attack is possibly promoted by local high humidity and precipitation that generate long-lived water droplets and films on mineral surfaces where sulphuric acid is active for prolonged times.