Temporal Changes In Seismic Velocity Research Articles

Fractional seismic velocity change related to magma intrusions during earthquake swarms in the eastern Izu peninsula, central Japan

We investigated temporal changes in seismic velocity associated with active seismic swarms and crustal deformations that occurred in 2006 and 2009 in the eastern Izu peninsula of Japan. To detect changes in seismic velocity during and after these swarm episodes, we used a passive image interferometry technique, by estimating the phase shift of auto‐correlation functions of continuous seismic waveforms in a frequency band of 1–3 Hz. During the 2006 and 2009 swarm episodes, a significant velocity decrease (>0.3%) was detected at the station nearest the swarm area. This velocity decrease occurred when the volumetric strain change at the surface due to magma intrusion was larger than 10−6. We could not, however, find a clear correlation between the velocity decrease and the peak ground acceleration caused by moderate earthquakes (M ∼ 5) that occurred during the swarms. We also found that during the interseismic period the velocity decrease gradually recovered, and appears to be proportional to displacement at the surface measured by GPS stations. These results suggest that the change in the subsurface velocity structure in the eastern Izu peninsula strongly reflects strain changes caused by magma intrusion into the crust.

Transmitting seismic station monitors fault zone at depth

Imaging subsurface structure and monitoring related temporal variations are two of the main tasks for modern seismology. With the accumulation of seismic data, clearer and clearer pictures of the Earth's interior are being achieved. However, precise monitoring of subtle subsurface changes associated with tectonic events remains tricky and is subject to high‐performance repeatable seismic sources. Although many observations have been reported on temporal seismic velocity changes in fault zones and volcanic areas, velocity change measurements based on passive sources, such as earthquakes and ambient seismic noise, are limited in resolution by the repeatability and by the spatial and temporal distribution of natural events.

Monitoring volcanoes using seismic noise correlations

In this article, we summarize some recent results of measurements of temporal changes of active volcanoes using seismic noise cross-correlations. We first present a novel approach to estimate volcano interior temporal seismic velocity changes. The proposed method allows to measure very small velocity changes (≈ 0.1%) with a time resolution as small as one day. The application of that method to Piton de la Fournaise Volcano (La Réunion Island) shows velocity decreases preceding eruptions. Moreover, velocity changes from noise cross-correlations over 10 years allow to detect transient velocity changes that could be due to long-lasting intrusions of magma without eruptive activity or to pressure buildup associated to the replenishing of the magma reservoir. We also present preliminary results of noise cross-correlation waveform perturbation associated with the occurrence of dike injection and volcanic eruption. We show that such an analysis leads us to locate the areas of dike injection and eruptive fissures at Piton de la Fournaise Volcano. These recent results suggest that monitoring volcanoes using seismic noise correlations should improve our ability to forecast eruptions, their intensity and thus potential environmental impact.

B01 地震波干渉法でみる岩手山周辺地殻構造の時間変化(火山の地震,口頭発表)

B01 地震波干渉法でみる岩手山周辺地殻構造の時間変化(火山の地震,口頭発表)

Passive image interferometry and seasonal variations of seismic velocities at Merapi Volcano, Indonesia

We propose passive image interferometry as a technique for seismology that allows to continuously monitor small temporal changes of seismic velocities in the subsurface. The technique is independent of sources in the classical sense and requires just one or two permanent seismic stations. We retrieve the Green's functions that we use for interferometry from ambient seismic noise. Applying passive image interferometry to data from Merapi volcano we show that velocity variations can be measured with an accuracy of 0.1% with a temporal resolution of a single day. At Mt. Merapi the velocity variations show a strong seasonal influence and we present a depth dependent hydrological model that describes our observations solely based on precipitation.

Temporal changes in seismic velocity of the crust around Iwate volcano, Japan, as inferred from analyses of repeated active seismic experiment data from 1998 to 2003

AbstractWe have examined temporal changes in seismic velocity of the crust through repeated active seismic experiments at Iwate volcano, Japan, where a significant volcanic activity and an M6.1 earthquake were observed in 1998. We apply a cross spectrum moving window technique to seismic data recorded at eight stations for the six explosions detonated from1998 to 2003. The seismic velocity at the frequency range of 3-9 Hz decreased by about 1% during the three months including the occurrence of M6.1 earthquake. The seismic velocity gradually increased, and about one third of the decrease was recovered by 2002. Then, the seismic velocity decreased again in 2003. Spatio-temporal changes in the volumetric strains predicted from the M6.1 fault mechanism and the volcanic pressure sources are well correlated with the seismic velocity changes observed in 1998. However, the predicted stress fields are not completely matched with the observed velocity changes from 1998 to 2003. This inconsistency may be due to unknown regional tectonic stress field and/or localized stress fields induced by volcanic pressure sources. It should be noted that the observed velocity changes indicate frequency dependent characteristics although the mechanism is not yet understood.

Constraints on temporal variations in velocity near Anza, California, from analysis of similar event pairs

AbstractSimilar earthquake pairs recorded by the Anza Seismic Network in southern California are used as repeatable sources to place an upper limit on temporal changes in seismic velocity which occurred in the vicinity of the Anza seismic gap in the last 9 yr. Relative arrival times for each pair of events are found using a cross-correlation method and relative locations are calculated to verify that the pairs have nearly identical hypocenters. The time separation between events in these pairs varies from less than a day to almost 7 yr. The long-term changes in seismic travel times, as measured from the pairs with the longest time separation, are not significantly greater than the noise level estimated from the short-time-separation event pairs. Almost all P-wave paths show less than 0.06% (0.007 sec) change in travel time and all S-wave paths have less than 0.03% (0.004 sec) change. Sensitivity tests place an upper bound on travel-time changes that could be compensated by hypocenter mislocation at 0.2%. There is no evidence that localized stress accumulation causes measurable changes in seismic velocity in the Anza region.

Quarry blast sources and earthquake prediction: The Parkfield, California, earthquake of June 28, 1966

Quarry blasts provide an excellent source of waves with which to study temporal variations of seismic travel times. Using sources from several active quarries we found no time changes at Gold Hill (GHC), California from 7 months before the 1966 Parkfield mainshock to at least 13 months following the event. GHC is within 0.5 km of the zone of ground breakage associated with the earthquake. This event was one of the largest to occur in central California since 1906. The negative result of our experiment does not rule out the existence of an anomaly prior to the event, rather it emphasizes the difficulty of detecting such anomalies without a fortuitous combination of sources and stations. The routine monitoring of temporal changes in seismic velocity along the San Andreas fault may require not only dense networks of high quality seismic stations, but also sources whose spatial location and origin time can be controlled.

Crustal structure and temporal velocity change in southern California

About twenty blasts are used to determine crustal structure and to monitor temporal seismic velocity changes in southern California. The shot time is determined up to 10 msec by using a disposable pick-up placed directly on the explosive. About 17 permanent stations and 20 temporary stations are used for the recordings. With a fast paper speed (typically 1 cm/sec) and the WWVB radio signals superposed on the seismic trace, absolute timing accuracy of up to 10 msec is achieved. A representative structure thus determined consists of a 4 km thick 5.5 km/sec layer underlain successively by 23.4 km thick 6.3 km/sec layer, 5.0 km thick 6.8 km/sec layer and 7.8 km/sec half space. The details of the lower crust are somewhat uncertain. This structure can explain the travel time data, corrected for the station and source elevations and for the station delays, to ±0.15 sec. Small but systematic temporal velocity changes up to 3% have been found for some of the profiles. If the effect of the migration of the shot point is small enough, these changes are larger than experimental errors and represent real temporal change in the material property between the shot point and the stations.