Philippine Sea Slab Research Articles

東京直下に沈み込む2枚のプレートと首都圏下の特異な地震活動

Recent investigations based on seismic tomography, hypocenter determinations and focal mechanism analyses using dense seismic network data reveal the precise configurations of the Pacific (PAC) and Philippine Sea (PHS) plates subducting beneath the Tokyo metropolitan area. Estimated geometry shows a broad contact zone between the two plates located directly beneath the Kanto plain. The overlap with the PHS plate subducting above it provides the PAC plate with protection from being heated by the hot mantle wedge. Moreover, the fore-arc portion of the PHS plate, before its subduction beneath Kanto, had been cooled by the subduction of the PAC plate from the Izu-Bonin trench. These cause lower-temperature conditions within the two oceanic plates and the upper continental plate beneath the Tokyo metropolitan area. As a result, depth limits of seismic activities within the plates and along their boundaries are anomalously deep. Seismic tomography studies show that the easternmost portion of the PHS slab mantle is serpentinized. The PHS slab may have been torn in two along the western boundary of this serpentinized mantle, with the eastern portion being left behind relative to subduction of the western portion. This is accompanied by the generation of large intraslab earthquakes along the boundary. We need to take these observations into consideration to understand the mechanism generating M7-class earthquakes, which are anticipated to occur in the southern Kanto region with a high probability.

Imaging the subducting slabs and mantle upwelling under the Japan Islands

A high-resolution P-wave tomography of the crust and mantle down to 700km depth beneath the Japan Islands is determined using a large number of high-quality arrival-time data from local earthquakes and teleseismic events simultaneously. The tomography shows that the Philippine Sea slab is subducting aseismically down to 430km depth under southwest Japan, though the seismicity within the slab ends at 180km depth. A low-velocity (low-V) zone in the mantle wedge under Tohoku and Kyushu is found to extend westward from the volcanic front to the backarc under the Japan Sea and East China Sea. Significant low-V anomalies are revealed in the deep portion of the mantle wedge (400–500km depth) above the Pacific slab under southwest Japan, which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the Pacific slab. Low-V anomalies appear at 420–700km depths beneath the Pacific slab under eastern Japan, which may reflect hot mantle upwelling associated with the deep subduction of the Pacific slab and its collapsing down to the lower mantle.

P wave anisotropic tomography of the Nankai subduction zone in Southwest Japan

The active subduction of the young Philippine Sea (PHS) plate and the old Pacific plate has resulted in significant seismic heterogeneity and anisotropy in Southwest (SW) Japan. In this work we determined a detailed 3‐D P wave anisotropic tomography of the crust and upper mantle beneath SW Japan using ∼540,000 P wave arrival times from 5,249 local earthquakes recorded by 1095 stations. The PHS slab is imaged clearly as a high‐velocity (high‐V) anomaly which exhibits considerable lateral variations. Significant low‐velocity (low‐V) anomalies are revealed above and below the PHS slab. The low‐V anomalies above the PHS slab may reflect the upwelling flow in the mantle wedge and the PHS slab dehydration, and they form the source zone of the arc volcanoes in SW Japan. The low‐V zones under the PHS slab may reflect the upwelling flow in the big mantle wedge above the Pacific slab. The anisotropy in the crust and upper mantle is complex. In Kyushu, the P wave fast velocity direction (FVD) is generally trench‐normal in the mantle wedge under the back‐arc, which is consistent with the corner flow driven by the PHS slab subduction. The FVD is trench‐parallel in the subducting PHS slab under Kyushu. We think that the intraslab seismicity is a potential indicator to the slab anisotropy. That is, the PHS slab with seismicity has kept its original fossil anisotropy formed at the mid‐ocean ridge, while the aseismic PHS slab has reproduced the anisotropy according to its current deformation.

Waveguide effects within the Philippine Sea slab beneath southwest Japan inferred from guided SP converted waves

SUMMARY High-sensitivity, dense seismic observations and numerical simulations revealed a distinct guided SP converted wave of oceanic mantle earthquakes that occurred within the subducting Philippine Sea (PHS) plate and a waveguide effect of the PHS Plate beneath southwest Japan. Based on seismograms, we identify the oceanic mantle earthquakes of the double seismic zone and find a new type of later phase for these earthquakes. The primary features of these later phases are as follows: (1) these phases are observed at the northeastern inland stations of the convex slab axis, (2) the vertical and radial components are dominant and (3) the apparent velocity is approximately 8.0 km s –1 . To examine the origin of these later phases, we simulate a2 -DP-SV seismic wavefield considering the PHS slab geometry. Based on the numerical models, we conclude that the later phases are guided SP converted waves trapped within the oceanic crust, which acts as low-velocity layer. A series of numerical simulations indicates that two factors are necessary to reproduce the observed SP guided waves: (i) a low-velocity channel must exist between stations and sources and (ii) the oceanic crust must only be in contact with the lower crust near the stations.

Crust–mantle boundaries in the Taiwan–Luzon arc-continent collision system determined from local earthquake tomography and 1D models: Implications for the mode of subduction polarity reversal

In order to better understand the mode of subduction polarity reversal in the Taiwan–Luzon arc-continent collision zone, we mapped its crust–mantle boundaries using local earthquake tomography. By contouring surfaces of constant Vp=7.5kms−1, we identified three Moho discontinuities and the plate interface that juxtaposes Eurasian lower crust against mantle lithosphere of the Philippine Sea plate. The plate interface dips to the east under southeastern Taiwan and steepens progressively towards north until it becomes vertical at 23.7°N. From there it continues northward in a vertical orientation, until the limit of the tomographic model inhibited further mapping. For the Moho, additional depth constraints were derived from 1D models using P-wave arrivals of local earthquakes. The Mohos of the Eurasian and Philippine Sea plates are disconnected across the plate interface. Beneath southern Taiwan, the Eurasian Moho dips to the east at 50–60°, following the orientation of the plate boundary and continuous with the Benioff zone. Towards north, the Eurasian Moho steepens into subvertical, again together with the plate boundary. The Philippine Sea plate Moho exhibits a synform-like crustal root, interpreted as the base of the magmatic Luzon arc. Towards the north, this root deepens from 30 to 70km underneath the Ryukyu trench. In northernmost Taiwan, the hinge of the vertically subducting Eurasian slab steps westward out of the thrust belt, leaving the deformation front to the east inactive and giving way to the north-dipping Philippine Sea plate. A subhorizontal Moho at 30–35km depth overlies the north-dipping Philippine Sea slab and is interpreted as a newly formed Moho, established after westward rollback and delamination of the subducting Eurasian slab. In combination, these data support a model of progressive subduction polarity reversal, in which a tear within the Eurasian lithosphere propagated southwestward, deactivating the deformation front.

Water transportation through the Philippine Sea slab subducting beneath the central Kyushu region, Japan, as derived from receiver function analyses

[1] Receiver function analyses are performed to detect seismic velocity discontinuities in the uppermost mantle beneath the Kyushu subduction zone, Japan. The Philippine Sea slab subducting beneath Kyushu is young (26–50 Ma) and steeply dipping (at greater than 30°). We detect a seismic velocity contrast larger than 10% corresponding to the oceanic Moho down to 90 km in depth, implying that the subducting oceanic crust contains more than 3.0 wt.% water down to this depth. We also detect a discontinuity with downward decreasing seismic velocity at depths of 50–80 km, which is parallel to the oceanic Moho and 10 km shallower than it. This fact indicates that there is a sharp discontinuity between the mantle wedge and the hydrous oceanic crust. The existence of such a sharp discontinuity would require a large temperature gradient around the boundary or a permeability barrier at the upper boundary of the slab. We delineate the continental Moho with downward decreasing seismic velocity and the upper boundary of the slab with upward decreasing seismic velocity beneath the forearc region, which implies the existence of serpentinite and/or free fluid which causes high pore pressure in the forearc mantle.

Low-frequency earthquakes and tomography in western Japan: Insight into fluid and magmatic activity

Low-frequency (LF) microearthquakes are detected in the lower crust and uppermost mantle beneath western Japan, which may reflect fluid and magmatic activity in the subduction zone. In this work we combine seismic tomography and the LF earthquakes to study the crustal and upper-mantle structure and fluid and magmatic activity in western Japan. High-resolution tomographic images under this region are determined using high-quality arrival-time data. In Kyushu the subducting Philippine Sea slab is detected clearly as a high-velocity zone, and the arc-magma related low-velocity (low- V) anomalies under the active arc and back-arc volcanoes are imaged clearly. Prominent low- V zones are visible in the crust and uppermost mantle beneath the Quaternary volcanoes along the Japan Sea coast, and LF events occur actively under some of those volcanoes. LF events are also detected in the source areas of the 1995 Kobe earthquake (M 7.2) and the 2000 Tottori earthquake (M 7.3), supporting the hypothesis that the large crustal earthquakes were triggered by the crustal fluids. These results indicate that fluid and magmatic activities exist widely in western Japan, which are caused by dehydration of the subducting Philippine Sea slab and corner flow in the mantle wedge.

Stress regime in the Philippine Sea slab beneath Kanto, Japan

[1] We determine the focal mechanisms of earthquakes within the Philippine Sea slab beneath the Tokyo metropolitan area, and perform stress tensor inversions to investigate the detailed stress field within the slab. The results show a characteristic spatial variation in earthquake-generating stress. Slab stress in northeastern part of the PHS slab is characterized by down-dip tension (DDT), except for the uppermost tip of the seismic portion of the slab where down-dip compression (DDC) stress is dominant. We interpret that DDT is caused by the net slab pull and DDC is attributable to local resistance to subduction at the tip of the slab. In southwestern part of the PHS slab, σ1 and σ3 are generally rotated oblique to the dip of the slab, suggesting that earthquakes occur under stress conditions of neither DDC nor DDT. The rotations in σ1 and σ3 may be related to stress accumulation by the slip deficit along the asperity of the 1923 Kanto earthquake (M7.9).

Deep structure and origin of active volcanoes in China

We synthesize significant recent results on the deep structure and origin of the active volcanoes in mainland China. Magmatism in the western Pacific arc and back-arc areas is caused by dehydration of the subducting slab and by corner flow in the mantle wedge, whereas the intraplate magmatism in China has different origins. The active volcanoes in Northeast China (such as the Changbai and Wudalianchi) are caused by hot upwelling in the big mantle wedge (BMW) above the stagnant slab in the mantle transition zone and deep slab dehydration as well. The Tengchong volcano in Southwest China is caused by a similar process in the BMW above the subducting Burma microplate (or Indian plate). The Hainan volcano in southernmost China is a hotspot fed by a lower-mantle plume which may be associated with the Pacific and Philippine Sea slabs’ deep subduction in the east and the Indian slab’s deep subduction in the west down to the lower mantle. The stagnant slab finally collapses down to the bottom of the mantle, which can trigger the upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and may cause the slab–plume interactions.

Cause of M ∼ 7 intraslab earthquakes beneath the Tokyo metropolitan area, Japan: Possible evidence for a vertical tear at the easternmost portion of the Philippine Sea slab

We perform travel‐time tomography beneath Kanto, Japan, to obtain detailed structures of the wedge‐shaped serpentinized mantle previously observed in the Philippine Sea slab. The results suggest that the western boundary of the serpentinized mantle (serpentine boundary) is subvertical and P‐ and S‐wave velocities vary by 15%–20% across it over the short distance of ∼10 km. Two intraslab earthquakes, in 1921 (M7.0) and 1987 (east off Chiba earthquake; M6.7), are inferred to have occurred along the serpentine boundary, accompanied by right‐lateral movement, based on analyses of focal mechanisms and aftershock distribution. A subvertical earthquake cluster penetrating the entire Philippine Sea slab is observed along the serpentine boundary, and four earthquakes in the cluster have strike‐slip focal mechanisms similar to that of the 1987 earthquake. Focal mechanisms obtained for past large earthquakes and present‐day microearthquakes suggest a concentration of right‐lateral deformation along the mechanically weak serpentine boundary. The Philippine Sea slab may have been torn in two along this boundary, with the eastern portion being left behind relative to subduction of the western portion. Assuming that one of the large aftershocks (M7.1) of the 1923 Kanto earthquake (M7.9), which occurred off the Boso Peninsula, ruptured the same asperity as did the 1987 earthquake, the slip deficit accumulated along the serpentine boundary during the 64 year interval is consistent with the fault slip of the 1987 earthquake. Interaction between the seismic slip along the plate interface and the serpentine boundary can explain the series of M7‐class earthquakes before and after the 1923 Kanto earthquake.

日本列島下のスラブの三次元構造と地震活動

A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. We introduce a precise configuration of the Philippine Sea and Pacific plates subducting beneath the Japanese Islands, which was recently obtained by seismic tomographic imaging, precise earthquake hypocenter determinations, and focal mechanism studies. Seismic tomographic studies show that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the area north of the Izu Peninsula. The estimated geometry of the subducted Pacific and Philippine Sea slabs shows a broad contact zone between the two slabs located directly beneath the Kanto plain. It further shows the wavy configuration of the Philippine Sea slab subducting beneath the entire region of southwestern Japan. Contact between the Philippine Sea plate and the Pacific plate causes anomalously deep interplate and intraslab earthquake activity in Kanto. Moreover, the interplate coupling coefficient estimated from repeating earthquake data shows a distinct change across the northeastern edge of this slab contact zone, suggesting that the overlying plate controls large-scale interplate coupling. High-resolution studies of spatial variations of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittlement hypothesis for the generation of intraslab earthquakes.

Tomography of the westernmost Ryukyu subduction zone and the serpentinization of the fore‐arc mantle

Three‐dimensional tomographic images of the subducting Philippine Sea slab and the fore‐arc mantle beneath NE Taiwan and the westernmost Ryukyu were generated in this study. More than 5600 events recorded simultaneously by seismic networks in Taiwan and Japan were relocated for the inversion for variations in VP, VS, and VP/VS. Analysis of the trade‐off between the data variance reduction and model variance helps to determine the appropriate strictness for regularization to avoid either overinterpretatioin or underinterpretation of data. The regularization parameters were also chosen to ensure suppression of artificial VP/VS anomalies. The subducting slab is characterized by high VP, high VS, and intermediate to low VP/VS. Notable in the mantle wedge is the high VP/VS anomalies that abut the surface of the subducting slab at depths of 30–80 km. The previously identified positive VP/VS channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study. We convert VS and VP/VS to perturbations of temperature and serpentinization of the mantle. The slab is cooled by 200°C–400°C relative to the mantle, in accord with the estimates from theoretical modeling of subduction. The serpentinization reaches ∼15%, or 2% water content, at 50 km depth in the fore‐arc mantle. We interpret the peak serpentinization as hydrated by the water released from the basalt‐eclogite metamorphic reaction in the oceanic crust of the subducting Philippine Sea Plate. The spatial limitation of the present seismic networks in this region with respect to subduction zone events hinders a full description of the pattern of melting in much of the mantle wedge. Resolution tests of the tomographic inversion provide a basis to guide our interpretation to better resolved regions.

Structural heterogeneities in the crust and upper mantle beneath Taiwan

High-resolution 3-D P-wave velocity structure at depths of 0–400 km was determined to investigate the structural heterogeneities and their implications for mountain building, plate collision and tectonics beneath Taiwan by simultaneously inverting a large number of arrival-time data from local and teleseismic earthquakes. In the joint seismic tomography, 227,258 absolute arrival times from 8982 local and regional earthquakes and 18,869 relative arrival-time data collected from 3179 teleseismic events were used. Our tomographic images provide direct geophysical evidence for a tectonic model proposed by previous studies and revealed some new features of structural heterogeneity related to the subducted Eurasian lithosphere. The seismic images show that the Philippine Sea slab descends northwestward from the Ryukyu Trench down to a depth of 200 km, showing agreement with the previous seismic, geochemical and geophysical studies. Low-velocity anomalies beneath the Kueishan volcano are revealed at depths of 0–65 km in North Taiwan, indicating its close relationship to the subducting process of the Philippine Sea slab. The Eurasian lithosphere is generally imaged as a high-velocity zone with a thickness of 65–80 km and it has subducted down to a depth of 300 km under South Taiwan, whilst it is not visible beneath North Taiwan. Low-velocity anomalies above the subducted continental lithosphere are observed at depths of 0–200 km, which might reflect, at least part of the subducted continental crust. Our tomographic results indicate that the plate convergence of the Eurasian plate varies from subduction beneath South Taiwan to collision with the Philippine Sea slab under North Taiwan. These features of the structural heterogeneities in the crust and upper mantle suggest that the mountain building process in the central part of Taiwan, arc magmatism in northeast offshore region and seismotectonics are mainly attributed to the subduction-collision configuration at the boundary between the Eurasian continental lithosphere and the subducting oceanic lithosphere of the Philippine Sea slab.

Seismotectonics beneath the Tokyo metropolitan area, Japan: Effect of slab‐slab contact and overlap on seismicity

We first determine the configuration of the upper surface of the Pacific (PAC) slab beneath Kanto, Japan, from the distribution of interplate earthquakes relocated by an appropriate 1‐D velocity model. Then, traveltime tomography is carried out to estimate three‐dimensional seismic velocity structures around Kanto using 735,520 P wave and 444,049 S wave arrival times from 6508 local earthquakes. The obtained results suggest that the Philippine Sea (PHS) slab is subducting to depths of 130–140 km without a gap, even to the northwest of the Izu collision zone. We subsequently define the lateral extent of the contact zone between the bottom of the PHS slab and the upper surface of the PAC slab (PHS‐PAC interface) and reveal that the slab contact zone underlies a wider area beneath Kanto in harmony with the Kanto plain. The downdip limit of interplate (thrust‐type) earthquakes on the PAC slab is deepened by ∼30 km locally under the slab contact zone. This deepening is probably caused by a lower‐temperature environment in the PAC slab, resulting from the overlap with the PHS slab subducting above and consequent thermal shielding by the PHS slab from the hot mantle wedge. We detect an extremely low‐velocity anomaly in the easternmost portion of the PHS slab, which is probably attributable to serpentinization of mantle peridotite. Interplate earthquakes are almost absent along the PHS‐PAC interface overlain by the serpentinized mantle in the PHS slab, suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine.

Modification of the Network-MT method and its first application in imaging the deep conductivity structure beneath the Kii Peninsula, southwestern Japan

Abstract The Network-Magnetotelluric (NMT) method is well-suited for investigating deep and large-scale conductivity structure; however, application of the method is strongly dependent on the availability of telecommunication facilities (specifically, metallic transmission cables). To overcome the problem posed by the progressive replacement of metallic transmission cables with fiber cables, we developed a modified NMT (modified NMT) method consisting of purpose-built electrodes, making use of local metallic telecommunication lines, without a transmission cable. We first applied this modified NMT method over the Kii Peninsula, southwestern Japan, undertaking two-dimensional conductivity modeling along a transect across the central part of the peninsula. The model is characterized by a large (∼20 km wide and depths of 10–60 km) and highly conductive (< 10 Ω m) zone in the central part of the peninsula between the Conrad discontinuity and the upper surface of the Philippine Sea slab. This zone contains the hypocenters of many deep low-frequency tremors but regular earthquakes are rare. The zone also corresponds to a high-V p /V s area. The presence of fluid in the zone plays a key role in the absence of regular earthquakes, occurrence of deep low-frequency tremors, and elevated V p /V s values, as well as enhancing conductivity.

Seismic interferometry of teleseicmicS-wave coda for retrieval of body waves: an application to the Philippine Sea slab underneath the Japanese Islands

SUMMARY The reconstruction of surface waves from the cross-correlation of random wavefields has recently been extensively inspected by theoretical and experimental approaches. However, the reconstruction of body waves has not been extensively studied. In this study, we present a method for extracting body waves, that is, direct P and S waves, and reflected waves from the Philippine Sea slab. We use the cross-correlation of the wavefield generated by the teleseismic S waves observed by Hi-net tiltmeters with a passband of 0.07–0.5 Hz. To enhance the contribution of the S coda, we reduced the source-time function and the deterministic phases sS, ScS and SS, and the surface wave in S coda. The records of the processed S coda observed at every station-pair are cross-correlated for each teleseismic event. The cross-correlation functions (CCFs) of different earthquakes are stacked to retrieve wavefields propagating between the station-pairs. As a result, direct P, S and the reflected waves could be extracted when the stacked CCFs are aligned as a function of distance of separation between two stations. The gradients of the traveltime curves for the direct P and S waves are approximately 5–7 and 3k m s −1 , respectively. These velocities correspond to the seismic velocities of the crust or the P-wave velocity of the uppermost mantle. In order to enhance the reflected waves, we searched for the reflection points by assuming that the later phases in the CCFs are SS reflections and map the amplitudes onto the depth sections. As a result, the negative phases dipping to the north can be traced right below the region of hypocentre distribution. These negative phases probably correspond to the oceanic Moho within the Philippine Sea slab. We also show that the oceanic Moho can also be traced by assuming PP reflections. These results indicate that the CCFs plausibly contain information regarding both P and S waves propagating between the two receivers, and are capable of detecting reflected phases in addition to the direct waves.

Configuration of the subducting Philippine Sea slab in the eastern part of southwestern Japan with seismic array and Hi-net data

It is important to know the shape of a subducting slab in order to understand the mechanisms of inter-plate earthquakes and the process of subduction. Seismicity data and converted phases have been used to detect plate boundaries. The configuration of the Philippine Sea slab has been obtained at the western part of southwestern Japan. At the eastern part of southwestern Japan, however, the configuration of the Philippine Sea slab has not yet been confirmed. A spatially high-density seismic network makes it possible to detect the boundaries of the Philippine Sea slab. We used a spatially high-density temporal seismic array in the area. The configuration of the Philippine Sea plate is obtained at the eastern part of southwestern Japan using the temporal seismic array and permanent seismic network data and comparing the seismic structure obtained from the results of refraction surveys. The configuration of the Philippine Sea plate obtained by this study does not bend sharply compared to previous models obtained from receiver function analyses. We delineated the upper boundary of the slab to a depth of about 45 km. The weak image of the boundary, which corresponds to the upper mantle reflector beneath the source area of the 2000 Western Tottori earthquake, was detected using the spatially dense array.

Slab segmentation revealed by anisotropicP‐wave tomography

[1] Seismic anisotropy is a useful indicator for identifying the physical and chemical condition of the Earth's interior, such as stress and flow fields, and in situ constituent minerals. Using traveltime tomography, we examined three-dimensional anisotropic P-wave velocity structure of the Kii Peninsula, southwest Japan, where source regions of megathrust earthquakes along the Nankai Trough are presumed to exist. The tomography revealed that the Philippine Sea slab beneath the peninsula is segmented, i.e., a high-velocity slab with E–W anisotropy obtained during seafloor spreading is broken by an anomalous low-velocity region with N–S anisotropy. The anomaly lies along the segmentation boundary of two previous rupture zones, the 1944 Tonankai and 1946 Nankai earthquakes, and can be explained by the occurrence of a fracture zone with N–S oriented fractures and fault planes revealed by wide-angle seismic data. It is likely that this anomalous region will form a clear segmentation boundary during future earthquakes.

Correction to “Three‐dimensional seismic velocity structure and configuration of the Philippine Sea slab in southwestern Japan estimated by double‐difference tomography”

Correction to “Three‐dimensional seismic velocity structure and configuration of the Philippine Sea slab in southwestern Japan estimated by double‐difference tomography”

Seismic evidence for thermally‐controlled dehydration reaction in subducting oceanic crust

We perform travel‐time tomography to estimate detailed seismic velocity structures in the crust of the Pacific slab from northeastern (NE) Japan to the Kanto district, Japan, and reveal that the depth extent of the low‐velocity (hydrated) oceanic crust varies along the arc. The low‐velocity oceanic crust is subducting to depths of 120–150 km beneath Kanto, which is 40–70 km deeper compared to NE Japan. Such deeper preservation of the low‐velocity oceanic crust beneath Kanto can be explained by lower‐temperature conditions in the Pacific slab as a result of the subduction of the Philippine Sea slab immediately above it. These observations suggest that dehydration reactions accompanied by large velocity changes are controlled principally by temperatures, not by pressures. We also find spatial correspondence between intensive seismicity in the oceanic crust and the disappearance depth of the low‐velocity oceanic crust, suggesting that breakdown of hydrous minerals triggers earthquakes in the oceanic crust.