We describe seismic monitoring and modeling of an active volcano for the purpose of predicting the magma transport process in real time. We selected the Piton de la Fournaise, which has been monitored by an observatory since the early 1980s, and is active, with 28 eruptions from 1980 until August 1992. It erupted on March 9, 1998, after an unusually long quiet period. The eruption lasted until September 21, 1998. Before the March eruption, we completed the initial construction of a model of a magma system based primarily on long‐period (LP) events and coda localization. We found that LP events are associated with the lateral movement of magma to the rift zone and not with the vertical movement to the summit. We also found that the source of 2 Hz LP events is located below that of 1 Hz LP events. The coda localization is a newly discovered phenomenon that helped to locate a magma body. This model was used for interpreting the incoming information from the precursory seismic crisis and the eruption tremor during the 6 months of the eruption. The simultaneous monitoring and modeling of an active volcano led us to a quantitative simulation of the eruption process, computing excess pressure and flow rate in a system of reservoirs connected by channels.
The dependence of the seismic source spectrum on earthquake magnitude was determined with improved accuracy in the frequency range from 1 to 25 Hz using coda waves from local earthquakes. Separation of the source effects from the effects of scattering and attenuation was achieved by the method of Aki and Chouet (1975) assuming that the earth is a randomly heterogeneous medium with a uniform statistical property. Scaling laws constructed for seismic regions in Japan, California, and Hawaii show marked variations which may be attributed to the differences in the scale length of inhomogeneity and strength of the earth's crust among these regions. In the magnitude range from 1 to about 5, the stress drop increases in most areas from roughly 1 bar to about 1 kbar. The exception is Hawaii where the stress drop is very low and almost constant. The dependence of the stress drop on earthquake magnitude reflects the heterogeneous material properties in the seismic zone and is explained by a fault plane with barriers which may be skipped, unbroken, when the tectonic stress is low.
abstract Using P-wave residuals for teleseismic events observed at the Montana Large Aperture Seismic Array (LASA), we have determined the three-dimensional seismic structure of the lithosphere under the array to a depth of 140 km. The root-mean-square velocity fluctuation was found to be at least 3.2 per cent which may be compared to estimate of ca. 2 per cent based on the Chernov random medium theory. The solutions are given by both the generalized inverse and stochastic inverse methods in order to demonstrate the relative merit of different inversion techniques. The most conspicuous feature of the lithosphere under LASA is a low-velocity anomaly in the central and northeast part of the array siting area with the N60°E trend and persisting from the upper crust to depths greater than 100 km. We interpret this low-velocity anomaly as a zone of weakness caused by faulting and shearing associated with the building of the Rocky Mountains.
This paper describes a new method for calculating strong motion records for a given seismic region on the basis of the laws of physics using information on the tectonics and physical properties of the earthquake fault. Our method is based on a earthquake model, called a «barrier model», which is characterized by five source parameters: fault length, width, maximum slip, rupture velocity, and barrier interval. The first three parameters may be constrained from plate tectonics, and the fourth parameter is roughly a constant. The most important parameter controlling the earthquake strong motion is the last parameter, «barrier interval». There are three methods to estimate the barrier interval for a given seismic region: 1) surface measurement of slip across fault breaks, 2) model fitting with observed near and far-field seismograms, and 3) scaling law data for small earthquakes in the region. The barrier intervals were estimated for a dozen earthquakes and four seismic regions by the above three methods. Our preliminary results for California suggest that the barrier interval may be determined if the maximum slip is given. The relation between the barrier interval and maximum slip varies from one seismic region to another. For example, the interval appears to be unusually long for Kilauea, Hawaii, which may explain why only scattered evidence of strong ground shaking was observed in the epicentral area of the Island of Hawaii earthquake of November 29, 1975. The stress drop associated with an individual fault segment estimated from the barrier interval and maximum slip lies between 100 and 1000 bars. These values are about one order of magnitude greater than those estimated earlier by the use of crack models without barriers. Thus, the barrier model can resolve, at least partially, the well known discrepancy between the stress-drops measured in the laboratory and those estimated for earthquakes.
abstract The Parkfield, California, earthquake of 1966 produced the first recording of ground motion in the immediate vicinity of an earthquake fault. The simplicity of displacement pulse observed at station 2 aroused great interest among seismologists working in the area of earthquake source mechanism. Since then, numerous theoretical works on simulating strong motion have been carried out, starting with the use of a simple kinematic model assuming a uniform slip over a fault plane. The assumption of uniform slip had to be modified to explain the strong motion observed during the Parkfield earthquake of 1966, the Borrego Mountain earthquake of 1968, the San Fernando earthquake of 1971, the Imperial Valley earthquake of 1979, the El Asnam earthquake of 1980, and others. Through these studies, the simulation technique has been advanced to include a more realistic medium. In the beginning, most methods used Green9s function for an unbounded homogeneous medium. Then, the free surface effect, the effect of a sedimentary layer as well as laterally heterogeneous basin structure have been included in the simulation. The above simulation studies were, however, restricted to relatively low-frequency waves represented in a displacement or velocity seismogram. In order to predict an acceleration seismogram dominated by high-frequency waves, hybrid models have been proposed in which gross features of rupture propagation are specified deterministically, but the details of the process are described by a stochastic model specified by a small number of parameters. The application of these models to several California earthquakes revealed encouraging results that some of the key parameters of the model are stable among earthquakes. Mathematical modeling techniques will play an important role in the prediction of strong motion for a great earthquake in California as well as for major earthquakes outside California, based on the records of moderate earthquakes already acquired in California.
Recently, Bouchon (1979 a ) reinterpreted strong motion seismograms obtained during the Parkfield earthquake of 1966 using a new method applicable to a finite propagating dislocation source in a layered medium. His results and other pertinent data, interpreted in terms of the barrier model of Das and Aki (1977), suggest that the rupture may be stopped by a barrier with the specific fracture energy of about 10 9 erg cm −2 . Using the formulas of Ida (1973 a ), we estimated parameters of the barrier as follows: breaking slip of about 20 cm, cohesive stress of about 100 bars, and length of end zone (nonelastic zone) of a few hundred meters. The barrier parameters for the great 1857 earthquake were also obtained from the description of surface fault breaks by Wallace (1968). The result led to the estimation of maximum acceleration of about 1.5g near the fault, under the assumption that the end zone length is proportional to the diameter of individual crack of the barrier model. Barriers for other earthquakes are discussed, and they are classified into geometrical barriers such as fault bend and corner and inhomogeneous barriers such as the high‐velocity anomaly straddling the San Andreas fault near San Juan Bautista. The barriers act not only as a stopper of rupture but also as an initiator of rupture, as well as a stress concentrator, causing twin earthquakes and migration or progression of major earthquakes along the plate boundary.
“If I were a brilliant scientist, I would be working on earthquake prediction.” This is a statement from a Los Angeles radio talk show I heard just after the Northridge earthquake of January 17, 1994. Five weeks later, at a monthly meeting of the Southern California Earthquake Center (SCEC), where more than two hundred scientists and engineers gathered to exchange notes on the earthquake, a distinguished French geologist who works on earthquake faults in China envied me for working now in southern California. This place is like northeastern China 20 years ago, when high seismicity and research activities led to the successful prediction of the Haicheng earthquake of February 4, 1975 with magnitude 7.3. A difficult question still haunting us [ Aki , 1989] is whether the Haicheng prediction was founded on the physical reality of precursory phenomena or on the wishful thinking of observers subjected to the political pressure which encouraged precursor reporting. It is, however, true that a successful life‐saving prediction like the Haicheng prediction can only be carried out by the coordinated efforts of decision makers and physical scientists.
