abstracts

Abstracts

2007 AGU Fall Meeting

San Francisco, California, December 10-14, 2007

DI51A-0288
Determination of Rupture Velocities of Deep-focus Earthquakes Using Combination of Teleseismic and Regional Data
Park, S. and J. Mori

Although rupture velocity is an important parameter in understanding the physics of the earthquake source, it is often difficult to determine. For shallow earthquakes, not only seismic waveform data but also geodetic or surface displacement data are frequently available, so that they can be constraints for the estimation of source parameters including rupture velocity. However these closely observed data are not available for deep-focus earthquakes and this fact makes it more difficult to accurately estimate earthquake source parameters. One common method to estimate rupture velocity of deep-focus earthquakes is to use relative locations between the hypocenter and subevents, using time differences between the initiation and later phases in waveforms depending on the direction from the source to stations. This method has many uncertainties especially when it is difficult to pick the arrivals from subevents in the waveforms and when the event has a simple source time function. Another method is with seismic waveform inversions, which also often used for shallow earthquakes. This, however, does not have strong constraints on the fault dimension and rupture velocity because of the trade- off between them. To estimate rupture velocities of deep-focus earthquakes with better resolution, we used a new method, which is combination of teleseismic waveform inversion and forward modeling of regional data. We first carried out teleseismic waveform inversions for rupture velocities of 0.5 - 6 km/s on both nodal planes obtained by Harvard CMT solutions, varying grid size for the change of rupture velocity. Next, forward modeling of regional data was performed with each slip distribution obtained by teleseismic inversions, using empirical Green function method. Then we estimated the rupture velocity when the synthetics calculated by forward modeling best explain the observed regional data. Using this method we were able to estimate the rupture velocities of three deep-focus earthquakes surrounding Japan. The estimated rupture velocities are about 1 - 2 km/s, which are equivalent to 20 - 40 % of shear wave velocity. These values of rupture velocities are quite slow, compared to typical values for shallow earthquakes, which are 70 - 80 % of shear wave velocity.

S52B-04
Comparison of Early Aftershocks for the 2004 Mid-Niigata and 2007 Noto Hanto Earthquakes in Japan
Mori, J, Y. Kano, B. Enescu

We compared the aftershock sequences of the similar 2004 Mid-Niigata (Mw6.6) and 2007 Noto Hanto (Mw6.7) earthquakes in central Japan. Although the two mainshocks had similar size, depth, and focal mechanisms, the numbers of aftershocks were quite different, with the Niigata mainshock producing a much stronger sequence. We examined the continuously recorded data from nearby Hi-Net stations operated by the National Institute for Earth Science and Disaster Prevention (NIED), to identify the early aftershocks following both mainshocks. A 5 hz high-pass filter was chosen to facilitate identification of the high-frequency arrivals from individual aftershocks. We used 6 stations distributed at distances within about 30 km. Aftershocks were identified by looking at large printouts of the continuous records for the six stations and peak amplitudes were measured to calculate the magnitude. The magnitude determination using these high-pass filtered records was calibrated by using a set of 30 earthquakes that were also listed in the catalog of the Japan Meteorological Agency (JMA). We estimate that the completeness level of small aftershocks is about Mj3.5. The event counts show that the aftershock sequences of the two earthquakes were quite similar for about the first 7 minutes. Following that time, the Niigata aftershocks clearly continue at a much higher rate which is about 3 times the rate of the Noto earthquake. The time where the rates diverge corresponds to the occurrence of a Mj6.3 earthquake in the Niigata sequence. This pattern can be seen in both the plots for the Mj>3.5 and Mj>4.0 events. Since there are more earthquakes for the Mj>3.5 data set, the time resolution is better. These results show an enhanced triggering of aftershocks for the Niigata sequence several minutes after the mainshock. The Niigata region is an area of hydrocarbon production with regions of high pressure fluids, and Sibson (2007) proposes that the swarm-like behavior is due to upward discharge of fluids from a deeper overpressured region. Fluid may flow into openings of fault zones caused by strong earthquake shaking, resulting in a reduction of the normal stresses on the faults.

The 2007 Kyoto Prize Workshop in Basic Sciences,

Symposium "From Great Earthquake Seismology to Real Time Seismology"

Kyoto, Japan, November 12, 2007

University-Government Programs in Earthquake Hazards: The Caltech-USGS Collaboration

Mori, J.

The California Institute of Technology (Caltech) and the US Geological Survey (USGS) have worked closely together in southern California for over 30 years. The two organizations jointly operate the large seismograph network for southern California and collaborate on earthquake research. Applications of new seismic instrumentation have traditionally made southern California one of the best sources of earthquake data in the world. Also, there is close coordination in providing regional hazard information about earthquakes. Following strong earthquakes in California, the public and news media look to Caltech/USGS for the latest information.

In 1990, one of the first realtime earthquake information systems was established in southern California by CUBE (Caltech-USGS Broadcast of Earthquakes). This pioneering project combined the current communication technology with the improved seismic network, to provide quick information following earthquakes. The system continued to be developed and improved through interactions between seismologists and information users. For the 1994 Northridge earthquake, which caused extensive damage in the Los Angeles area, the system helped provide valuable information to regional utility companies.

Combining the credibility of an established federal agency with the more flexible resources of a renown private university, has led to this productive partnership in Pasadena. Much of the leadership for the collaboration has been provided by Hiroo Kanamori.

2007 Seismological Society of Japan Fall Meeting

Sendai, October 24-26, 2007

C21-03

Early Aftershocks of the 2004 Mid-Niigata and 2007 Noto Hanto Earthquakes

Mori, J., Y. Kano, B. Enescu

We examined continuously recorded seismograms of the 2004 Mid-Niigata and 2007 Noto Hanto earthquakes to compare the early aftershocks for these two similar earthquakes in central Japan. Although the two mainshocks had similar size, depth, and focal mechanisms, the aftershock levels were quite different, with the Niigata case being much stronger.

We used the continuously recorded data from nearby Hi-Net stations operated by the National Institute for Earth Science and Disaster Prevention (NIED), to identify the early aftershocks following both mainshocks. A 5 hz high-pass filter was chosen to facilitate identification of the high-frequency arrivals from individual aftershocks. We used 6 stations distributed at distances within about 30 km. Aftershocks were identified by looking at large printouts of the continuous records for the six stations and peak amplitudes were measured to calculate the magnitude. The magnitude determination using these high-pass filtered records was calibrated by using a set of 30 earthquakes that were also listed in the catalog of the Japan Meteorological Agency (JMA). We estimate that the completeness level of small aftershocks is about Mj3.5

The cumulative numbers of aftershocks as a function of time for Mj≥3.5 events are shown in the figure below. The event counts show that the aftershock sequences of the two earthquakes were quite similar for about the first 7 minutes. Following that time, the Niigata aftershocks clearly continue at a much higher rate which is about 3 times the rate of the Noto earthquake. The time where the rates diverge corresponds to the occurrence of a Mj6.3 earthquake in the Niigata sequence. This pattern can be seen in both the plots for the Mj≥3.5 and Mj≥4.0 events. Since there are more earthquakes for the Mj≥3.5 data set, the time resolution is better.

These results show an enhanced triggering of aftershocks for the Niigata sequence about 7 minutes after the mainshock. The Niigata region is an area of hydrocarbon production with regions of high pressure fluids, and Sibson (2007) proposes that the swarm-like behavior is due to upward discharge of fluids from a deeper overpressured region. Fluid may flow into openings of fault zones caused by strong earthquake shaking, resulting in a reduction of the normal stresses on the faults.

C21-5

Ultra-micro Seismic Events in the Yamasaki fault Observed Using a Hydrophone Array

Kano, Y., J. Mori, T. Yanagidani, H. Ito, Y. Kuwahara

D31-06

The 2007 Solomon Islands earthquake

Park, S-C. and J. Mori

P2-077

Attenuation Relationships of Characteristic Frequency, Peak Ground Velocity, and Moment Magnitude of Intermediate-depth Earthquakes

Nishitsuji, Y. and J. Mori

P3-015

Attenuation Structure in the Source Region of the Low-frequency Earthquakes in the Bungo Channel and Shikoku area, Japan

Nugraha, A. D. and J. Mori

P3-035

The Foreshock and Aftershock Sequence of the Kuril Islands Earthquakes in 2006 and 2007

Norimatsu, K. and J. Mori

P3-047

Distribution of Dynamic and Static Stress Drops during the 2002 Eastern Tottori Earthquake

Hiura, H. and J. Mori

10th Kyoto University International Symposium, Active Geosphere Science
Bandung, Indonesia, July 26-28, 2007

Temperature Measurements of the Frictional Heat from the 1999 Chi-Chi, Taiwan Earthquake
J. Mori, Y. Kano

Following the 1999 Chi-Chi, Taiwan earthquake (Mw7.6) we made temperature measurements in the vicinity of the Chelungpu fault at 1111 m depth, in a region that had large amounts of slip during the earthquake. These temperature estimates were part of the Taiwan Chelungpu fault Drilling Project (TCDP). The purpose of our observations was to look for a temperature anomaly that would be indicative of the frictional heat generated during the faulting. This was one of the first attempts to measure the heat following a large earthquake, especially where there was large slip (an asperity) of about 6 meters. This addresses a long standing issue in seismology about the level of dynamic friction during large earthquakes. The results of the measurements, which were made 6 years following the earthquake, showed a small symmetric temperature signature of about 0.06 ℃ center on the fault zone. Interpreting this temperature signature as the frictional heat generated at the time of faulting, implies a very small amount of heat and a very low level of dynamic friction. The apparent coefficient of friction is less than 0.1. There are some complicating issues, such as possible flows of fluids and the local changes in heat conductivity, which may affect the interpretation of our results. However, these effects do not change the basic conclusion that the measured heat is very small. Model calculations of fluid flows show that the reasonable fluid flows can translate the position of the peak of the temperature anomaly, but do not significantly reduce the amplitude.

With these low estimates of heat generation during rupture, mechanisms are needed to explain the low levels of dynamic friction. The low temperature appears to rule out fault melting, at least at the shallow depth of 1 km. We make some calculations for thermal pressurization, which reduces normal stress on the fault by the temperature induced expansion of water. However, this mechanism does not seem to strong enough to reduce the friction to the observed values. There are apparently some material related properties that have low values of friction at high slip rates.

The estimate of frictional heat can be combined with the seismic radiated energy and an estimate of fracture energy to obtain the total energy budget for the large earthquake. Fracture energy is estimated using near-field seismograms that are recorded within a few kilometers of the fault. These records clearly show the slip and slip velocity of the fault. The results show a relatively high value of the seismic efficiency, that is that the energy radiated in seismic waves is close to half of the total energy. This value is quite high compared to previous estimates.

24^th General Assembly of the International Union of Geodesy and Geophysics

Perugia, Italy, July 2-13, 2007

The Foreshock Sequence of the 2002 Eastern Tottori Earthquake and Its Effect on Mainshock
Hiura, H. and J. Mori

Radiated energy, static stress drop, seismic moment and radiation efficiency of intermediate-depth earthquakes in the Pacific slab beneath Japan
Nishitsuji, Y. and J. Mori

Japan Geoscience Union Meeting 2007

Makuhari, Chiba, May 19-24, 2007

S143-010
Worldwide Observations for Continuous Tremor and Low-Frequency Events in Subduction Zones
Mori, J. and E. E. Brodsky

The recent observations continuous tremor and low-frequency events associated with subduction zones in Japan, Cascadia, Alaska, New Zealand and other regions of the world, have sparked interest in processes that are occurring near the slab in this region. There is still little known about the physical mechanisms for these events, and even their position relative to the subducting slab is uncertain, because the low amplitude and emergent waveforms hamper precise locations. Some of the notable characteristics of the events include, episodic sequences, triggered occurrences from regional and teleseismic earthquakes, and propagation of events over distances greater than a hundred kilometers. Sometimes there are close associations of the occurrence of the events with slow slip events that are on the plate interface. Since it is difficult to analyze the waveforms of continuous tremor, much of the information is inferred from the discreet low-frequency seismic events, which are assumed to have similar mechanisms to the continuous tremor. From locations of lowfrequency events, their position relative to the slab varies depending on the region. In some places low-frequency events occur close to the slab interface while in other places they are 10 to 20 km above the interface. These events are often associated with fluid flow or pressure changes. This is inferred from the triggering stresses of teleseisms and the propagation across long distances. The long durations of continuous tremor that last for minutes to hours may be due to a fluid pressure change with a feedback mechanism that can continually induce further pressure changes.

D203-005
Mud eruption in Java Island: SAR interferometric analysis of PALSAR data and boundary element modeling
Fukushima, Y., J. Mori, M. Hashimoto, Y. Kano

On 29 May 2006, a mixture of mud, steam, gas and water started to erupt at a gas exploration site in eastern Java. More than 0.012 cubic kilometers of mud had been erupted as of November 2006, and the activity is still ongoing. There have been 13 fatalities and more than 11,000 people are being evacuated. It therefore has a social importance to understand the current state of the activity and to forecast the future development of the eruption. A synthetic aperture radar (SAR) PALSAR equipped on the Japanese ALOS satellite launched in January 2006 has captured several images at the eruption site. This study preliminarily analyzes these radar data by 1) measuring the ground subsidence associated with the eruption by SAR interferometry, and 2) modeling the measured data by using a method that consists of a boundary element method and a Monte-Carlo inversion algorithm. From the pair of the SAR images acquired on 19 May 2006 and 4 October 2006, we obtained ellipsoidal fringes roughly in a zone NS 4 km x EW 3 km centered at the eruptive vent (Figure, left. A color cycle of red-blue-yellow corresponds to an increase in the distance between the satellite and the ground. The arrow indicates the horizontal projection of the radar look direction. The incidence angle is approximately 41.5 degrees). Around a roughly circular zone of diameter 1.5 km covered by mud, we observe 7 cycles of color change, indicating that at least 80 cm of displacements away from the satellite have occurred in the four and a half months of observation period. In the modeling, we first assumed a horizontal planar ellipsoidal crack whose longer axis lies in the NS direction. Inverting for the location, lengths of the longer and shorter axes, and overpressure of the source found a model having its center at close to the vent, its longer and shorter axes 2.5 km and 1.5 km, respectively, and its depth at 0.36 km (Figure, middle. The black break curve denotes the source geometry). The sum of the squared residuals was 7 percent of that of the observed data. Next, we assumed a spheroid having its thickness in the vertical direction. This assumption predicted a model having its smallest depth about 0.1 km and greatest depth 1 km. The sum of the squared residuals was 5 percent of that of the observed data. Considering that the eruption started when the drilling reached close to 3 km beneath the ground, materials that triggered the eruption probably come from this depth. On the other hand, our analysis indicates that the deformation source lies at several hundred meters beneath the ground. Our results are consistent with Davies et al. (2007, GSA Today, 17, 4-9) who conclude from their consideration based on geologic data that fluid coming from depth entrains mud at a shallower depth. We plan to employ more detailed SAR interferometric analysis and to consider more realistic model of the ground subsidence mechanisms, in order to forecast the future activity of the eruption and hazard risks.

S143-P009
The foreshock sequence and source process of the 2002 Eastern Tottori Earthquake

Hiura, H. and J. Mori

In order to understand the earthquake nucleation process, it is important to know how foreshocks occur. Dodge et al. (1996) pointed out that foreshock sequences are not compatible with the cascade model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer. Instead, they noted that foreshocks seem to concentrate near structural discontinuities in the fault and may be a product of an aseismic nucleation process. In this study, we investigated the foreshock sequence of the 2002 Mj 5.5 Eastern Tottori Earthquake. This event was about 40 km west of the 2000 Mj 7.3 Western TottoriEarthquake and the focal mechanism was a strike-slip fault with a nearly horizontal P axis striking NW-SE. The mainshock waspreceded by about 30 foreshocks with similar waveforms and the largest foreshock occurred 11 days before the mainshock. We estimated relative locations of the mainshock, similar foreshocks and aftershsocks with cross correlation analysis and HYPO71 (Lee and Valdes, 1985). We found that foreshocks were concentrated 0.5 km SSW and 1.5 km shallower from the mainshock. The aftershock distribution suggested that the fault plane of the mainshock was along N105W, but the foreshocks’ row was aligned in the direction of N125W. This suggests that the foreshocks might not be related to aseismic nucleation process. Next we estimated the relative source time functions (RSTFs) of the mainshock with an empirical Green’s function (EGF) method and projected Landweber deconvolution (Bertero et al, 1997). The largest foreshock (Mj 2.5) was chosen as an EGF and we found two clear peaks in the RSTF’s of stations west of the mainshock. This suggests that the mainshock rupture might to propagate to the west.

S143-P010
The source parameters of intermediate-depth earthquakes in the Pacific slab beneath Japan

Nishitsuji, Y. and J. Mori

Although the studies of source parameters’ relationships for shallow earthquakes have been investigated (e.g. Kanamori et al [1993]), those for intermediate-depth earthquakes are much fewer. Our interests are whether the relations of source parameters depend on the depth and the earthquake size. In this study, we investigated the intermediate-depth earthquakes (Depth:70km-150km, MJ :4.0-6.0) in the Pacific slab beneath Japan along with attenuation properties such as Q-values. We estimated the radiation efficiency, which is considered an important index to express the rupture dynamics. This parameter can be calculated from the radiated energy and the fracture energy. Furthermore, we investigated the relations between the static stress drop and the seismic moment. Our results indicate that the radiation efficiency does not depend on the depth and the earthquake size. The ratios of the radiation energy to the seismic moment are similar to the studies for shallow events reported (e.g. Abercrombie [1995] and Kanamori et al [1993]). These results suggest that the mechanism of the earthquakes does not depend on the depth and its size.

S144- P012

Determining the Location of the Fault Plane for the Nankai Subduction Zone

Enescu B. and J. Mori

We estimate the location of the thrust plane for large earthquakes in the Nankai subduction zone by using high quality locations of small earthquakes. We assume that the small events in the depth range of 10 to 80 km are occurring close to the plate interface or within the subducted oceanic crust a few kilometers below. We are studying the region near the Kii Peninsula, where seismic experiments have already produced a good starting velocity model (Ito et al., 2006). We have collected waveform data at 70 seismic stations, from 450 earthquakes (M larger than 1.7) with hypocenter locations determined by both JMA and NIED. In order to recalculate the hypocenters, we are using P- and S-waves arrival times determined by JMA, as well as high-precision differential travel times from waveform correlations of P-waves. For all possible earthquake pairs with catalog-derived distances of less than 15 km apart, waveforms on vertical component, recorded at common seismic stations, were cross-correlated to obtain differential travel times. The waveforms were filtered to 2-10 Hz and sliced in 2.00 sec. (200 time sample) windows around the P-arrival time determined by NIED. Two waveforms were considered similar if their correlation coefficient equalled or exceeded a threshold of 0.6. We are using the absolute and relative travel time data together with a double-difference tomography approach (Zhang and Thurber, 2003) to obtain an improved 3D velocity structure and better locations for the earthquakes.

S151-005
Rupture Velocities of Large Deep-Focus Earthquakes Surrounding Japan
Park, S. and J. Mori

Rupture velocity is an important parameter in understanding the physics of the earthquake source, and many studies estimated rupture velocities of shallow and deep earthquakes. For shallow earthquakes, not only seismic waveform data but also geodetic or surface displacement data are frequently available, so that they can be constraints for the estimation of source parameters including rupture velocity. These data are not available for deep-focus earthquakes, and lack of closely observed data makes it more difficult to accurately estimate earthquake source parameters. To estimate rupture velocities of deep-focus earthquakes surrounding Japan, we first carried out teleseismic waveform inversions using empirical Green function method, for rupture velocities of 0.5 ?6 km/s on both nodal planes obtained by Harvard CMT solutions. Next, forward modeling of regional data (F-net and Hi-net) was performed with each slip distribution obtained by teleseismic inversions, also using empirical Green function method. Then we estimated the rupture velocity when the synthetics calculated by forward modeling best explain the observed regional data. Both for teleseismic inversions and forward modeling of regional data, we varied grid size for the change of rupture velocity, so that the number of inversion parameters can be constant. Our new method, that varies grid size and uses both teleseismic and regional data, provides better resolution to estimate the rupture velocity. Using this method we were able to estimate the rupture velocities of three deep-focus earthquakes surrounding Japan. The estimated rupture velocities for the three events are about 1 ?2 km/s, which are equivalent to 20 ?40 % of shear wave velocity.
These values of rupture velocity are quite slow, compared to typical values for shallow earthquakes (70 to 80 % of shear wave
velocity).

S229-P001
Source Region of the Low-frequency Earthquakes revealed by Tomographic Imaging
Nugraha, A. and J. Mori

We used local P and S phases to determine the three-dimensional P-wave, S-wave and Vp/Vs structure for the region of Bungo channel and western Shikoku in Japan. The velocity model clearly images the high velocity subducting slab and we can see the spatial relation to the low-frequency earthquakes. The low-frequency earthquakes were occurring close to the plate interface in the western Shikoku. However, in adjacent area to the west on the Bungo channel area indicating that low-frequency earthquakes are not occurring on the plate interface, but instead in a region of high Vp/Vs that is located above the subducting plate.

T154-P003
Methods for analysis of high resolution temperature logs in deep boreholes

Kano, Y., O. Matsubayashi, H. Ito, J. Mori

We have made an effort to detect temperature signatures along faults that recently ruptured in large earthquakes. Highresolution temperature logs are essential not only for detecting the heat produced by fault rupture but also for the detailed study of thermal structures. We carried out temperature measurements in a borehole that penetrates the Chelungpu fault, Taiwan, using a high-resolution borehole temperature instrument. The instrument consists of quartz oscillator temperature detectors that can measure temperature with a resolution of 0.001 K. One problem with the instrument is the large response delay caused by the thermal inertia of the case of the instrument, which may make observed temperature logs different from the true temperature structure. For example, a sharp, or peaky, temperature anomaly becomes broad. We can avoid this instrument response by moving the instrument extremely slowly, however this is sometimes not practical because of the limitations of the winch speed. Alternatively, we need to develop methods to calibrate the measured high-resolution temperature logs. We examined two methods to calibrate the response of the instrument. One method is to fit the transient temperature response to a step temperature change by a model that characterizes the thermal time constant (Conaway, 1977). The other method is to calculate a transfer function, or impulse response function for the instrument, using a reference thermometer that measures the temperature without the effects of thermal inertia. To test the methods for calibration, we carried out temperature measurements in a 350-m-deep borehole in Atotsugawa drilled by NIED. We obtained high-resolution temperature logs using the quartz thermometer along with small-size reference thermometers whose instrument response is negligibly small. We assume that the temperature logs measured by smallsize reference thermometers represent the true temperature structure. We imposed temperature changes by abruptly changing\ the depth of the instrument. The quartz thermometer recorded the temperature transients caused by the step-like temperature changes. A single time constant of 5 minutes basically fits the response of the instrument for the sudden temperature changes. Detailed examination revealed that the time constant at the time of the step-like-changes is smaller than 5 minutes. Also, there is a portion with a longer time constant. Multiple time constants are necessary to model the instrument response that consists of components with different thermal inertia. We also calculated the transfer function between the temperature logs obtained by the temperature instrument and by the small-size reference thermometers, using the same temperature transient used to obtain the time constant. The temperature logs modified by the calculated transfer function fit well to the logs obtained by the small-size reference thermometers.

Acknowledgement: We thank K. Omura (NIED) for providing us an opportunity to use the borehole.

Y239-P006

Measurement of the land subsidence associated with a mud eruption in the Java Island and the eruption model

Fukushima, Y., J. Mori, M. Hashimoto, Y. Kano

On 29 May 2006, a mixture of mud, steam, gas and water started to erupt at a gas exploration site in eastern Java. More than 0.012 cubic kilometers of mud had been erupted as of November 2006, and the activity is still ongoing. There have been 13 fatalities and more than 11,000 people are being evacuated. It therefore has a social importance to understand the current state of the activity and to forecast the future development of the eruption. A synthetic aperture radar (SAR) PALSAR equipped on the Japanese ALOS satellite launched in January 2006 has captured several images at the eruption site. From the pair of the SAR images acquired on 19 May 2006 and 4 October 2006, we obtained ellipsoidal fringes roughly in a zone NS 4 km x EW 3 km centered at the eruptive vent. Around a roughly circular zone of diameter 1.5 km covered by mud, we observe 7 cycles of color change, indicating that at least 80 cm of displacements away from the satellite have occurred in the four and a half months of observation period. In the modeling, we first assumed a horizontal planar ellipsoidal crack whose longer axis lies in the NS direction. Inverting for the location, lengths of the longer and shorter axes, and overpressure of the source found a model having its center at close to the vent, its longer and shorter axes 2.5 km and 1.5 km, respectively, and its depth at 0.36 km The sum of the squared residuals was 7 percent of that of the observed data. Next, we assumed a spheroid having its thickness in the vertical direction. This assumption predicted a model having its smallest depth about 0.1 km and greatest depth 1 km. The sum of the squared residuals was 5 percent of that of the observed data. Considering that the eruption started when the drilling reached close to 3 km beneath the ground, materials that triggered the eruption probably come from this depth. On the other hand, our analysis indicates that the deformation source lies at several hundred meters beneath the ground. Our results are consistent with Davies et al. (2007, GSA Today, 17, 4-9) who conclude from their consideration based on geologic data that fluid coming from depth entrains mud at a shallower depth. Our boundary element models, however, predict that the subsurface volume decrease is in the same order as the erupted mud volume. This suggests that the possibility of a caldera collapse, proposed by Davies et al. (2007), is low. We plan to employ more detailed SAR interferometric analysis and to consider more realistic model of the ground subsidence mechanisms, in order to forecast the future activity of the eruption and hazard risks.

2007 DPRI Annual Meeting
Kyoto, March 5-6, 2007

Invited talk

The July 17, 2006 West Java Earthquake and Tsunami

Mori, J.

A tsunami earthquake (Mw = 7.7) occurred south of Java on July 17, 2006. The event produced relatively low levels of high-frequency radiation and local felt reports indicated only weak shaking in Java. There was no ground motion damage from the earthquake but there was extensive damage and loss of life from the tsunami along 250 km of the south coasts of Western and Central Java. An inspection of the area a few days after the earthquake showed extensive damage to wooden and unreinforced masonry buildings that were located within several hundred meters of the coast. Since there was no tsunami warning system in place, efforts to escape the large waves depended on the reaction of people to the earthquake shaking, which was only weakly felt in the coastal areas. This experience emphasizes the need for adequate tsunami warning systems for regions around the Indian Ocean.

International Geological Workshop on Sidoarjo Mud Volcano
Jakarta, Indonesia, Jan. 20-21, 2007

Some Geophysical Observations and Interpretations of LUSI (Keynote)
Mori, J., Y. Fukushima, Y. Kano, M. Hashimoto

The mud volcano that is erupting near Sidorajo provides a fascinating chance to study processes related to processes related to fluid movements in the crust and eruptive mechanisms. It is unfortunate that this event has disastrous consequences for the local residents, but we hope that a better understanding of LUSI can be used help minimize the damage in Sidarajo, as well as mitigate effects from future mud volcano eruptions.

InSAR data

We used InSAR data was used to study the surface deformation data caused by the mud volcano. The difference in images taken in May and October 2006 shows that there is subsidence across a region of several kilometers that is centered on the vent area. From modeling of this pattern, we estimate an elliptical source with dimensions of 1.8 x 1.5 km at a depth of 300 to 400 meters. These results suggest that much of the volume of the erupted material is coming from relatively shallow depth, less than 500m. This seems inconsistent with temperature and well log data which indicate a greater depth for the source. It is possible that the original source of the hot water is coming from several kilometers depth, but much of the volume of material is from shallower depth. We also plan to look at inSAR data for the time period prior to the eruption to see if there are any signs of a preparatory inflation.

Earthquake Triggering ?

There are intriguing questions whether the triggering of the mud volcano is related to the occurrence of a regional earthquake. It difficult to evaluate this triggering effect, however there have been instances of mud volcanoes eruptions being initiated by shaking from earthquakes, such as the Niikappu earthquake in Japan. Although, usually the ground motions are more severe than for the case of the shaking in Sidorajo from the May 27 Yogyakarta earthquake. It is notable that there were flow rate responses to the earthquake in other nearby wells.

Possibility of Crater Collapse ?

In moderate to large volcanic eruptions, sometimes there is collapse of the crater region, because of the large amount of expelled material. Given the rather shallow source depth from the InSAR data and the current rates of eruption, it seems unlikely that there would be a very large collapse. Large differential stresses would be necessary to first form a coherent ring fracture on which a collapse could take place. However, the possibility should be investigated and it is important to have continuous monitoring of the deformation, along with detection of small earthquakes to watch for signs that may indicate a collapse.