Showing papers in "Pure and Applied Geophysics in 1995"

Laboratory Experiments of Tsunami Runup on a Circular Island

Abstract: Laboratory experiments of a 7.2-m-diameter conical island were conducted to study three-dimensional tsunami runup. The 62.5-cm tall island had 1 on 4 side slopes and was positioned in the center of a 30-m-wide by 25-m-long flat-bottom basin. Solitary waves with height-to-depth ratios ranging from 0.05 to 0.20 and “source” lengths ranging from 0.30 to 7.14 island diameters were tested in water depths of 32 and 42 cm. Twenty-seven capacitance wave gages were used to measure surface wave elevations at incident and four radial transects on the island slope. Maximum vertical runup measurements were made at 20 locations around the perimeter of the island using rod and transit. A new runup gage was located on the back or lee side of the island to record runup time series.

Linear and Nonlinear Computations of the 1992 Nicaragua Earthquake Tsunami

Abstract: Numerical computations of tsunamis are made for the 1992 Nicaragua earthquake using different governing equations, bottom frictional values and bathymetry data. The results are compared with each other as well as with the observations, both tide gauge records and runup heights. Comparison of the observed and computed tsunami waveforms indicates that the use of detailed bathymetry data with a smalI grid size is more effective than to include nonlinear terms in tsunami computation. Linear computation overestimates the amplitude for the later phase than the first arrival, particularly when the amplitude becomes large. The computed amplitudes along the coast from nonlinear computation are much smaller than the observed tsunami runup heights; the average ratio, or the amplification factor, is estimated to be 3 in the present case when the grid size of 1 minute is used. The factor however may depend on the grid size for the computation.

Coastal Sedimentation Associated with the December 12th, 1992 Tsunami in Flores, Indonesia

Abstract: This paper presents the result of a detailed granulometric investigation of sediments deposited by a modern tsunami, the 1992 tsunami in Flores, Indonesia. Eyewitness accounts indicate that sediments were deposited upon coastal lowlands over wide areas as a result of the tsunami inundation. Distinctive vertical and lateral variations in particle size composition are characteristic features of the tsunami deposits and these are intimately related to sedimentary processes associated with flood inundation. The geomorphological and sedimentary evidence is used here to establish a preliminary model of tsunami sedimentation. This information is believed to be of great value in understanding sedimentary processes associated with tsunami flooding and in the interpretation of palaeo-tsunami deposits.

Tsunami deposits from the 1993 Southwest Hokkaido earthquake and the 1640 Hokkaido Komagatake eruption, northern Japan

Abstract: The southwest Hokkaido tsunami of July 12th, 1993, left continuous onshore sand deposits along the west coast of Oshima Peninsula, Hokkaido, northern Japan. We investigated spatial distribution and lithofacies of the new tsunami deposits for its identification of ancient tsunami deposits. An eyewitness account and bent plants helped our interpretation of the onshore tsunami behavior. We regard the following properties as typical of the coastal tsunami sand deposits: (1) The deposits cover the surface almost continuously on gentle topography. (2).Deposit thicknesses and mean grain sizes decrease with distance from the sea. (3) Deposit thicknesses and lithofacies vary greatly across local surface undulation. (4) Graded bedding reflecting tsunami runup and backwash is present in thick deposits. (5) The deposits are widely distributed along the coast and extend inland several tens of meters to 100 m. We examined a candidate for the paleo-tsunami deposits associated with the 1640 Komagatake eruption, and confirmed that the similar patterns are typical of ancient tsunami deposits.

Onshore tsunami deposits caused by the 1993 Southwest Hokkaido and 1983 Japan Sea earthquakes

Abstract: Onshore tsunami deposits resulting from the 1993 Southwest Hokkaido and 1983 Japan Sea earthquakes were described to evaluate the feasibility of tsunami deposits for inferring paleoseismic events along submarine faults. Tsunami deposits were divided into three types, based on their composition and aerial distribution: (A) deposits consisting only of floating materials, (B) locally distributed siliclastic deposits, and (C) widespread siliclastic deposits. The most widely distributed tsunami deposits consist of the first two types. Type C deposits are mostly limited to areas where the higher tsunami runup was observed. The scale of tsunami represented by vertical tsunami runup is an important factor controlling the volume of tsunami deposits. The thickest deposits, about 10 cm, occur behind coastal dunes. To produce thick siliclastic tsunami deposits, a suitable source area, such as sand bar or dune, must be available in addition to sufficient vertical tsunami runup. Estimation of the amounts of erosion and deposition indicates that tsunami deposits were derived from both onshore and shoreface regions. The composition and grain size of the tsunami deposits strongly reflect the nature of the sedimentary materials of their source area. Sedimentary structures of the tsunami deposits suggest both low and high flow regimes. Consequently, it seems very difficult to identify tsunami deposits based only on grain size distribution or sedimentary structure of a single site in ancient successions.

A new approach to earthquake prediction: The Load/Unload Response Ratio (LURR) theory

Abstract: The response to loading is different from that to unloading when the focal media is approaching instability. The ratio of the response rate during loading to that during unloading, called Load/Unload Response Ratio Y (LURR), could be a measure of the closeness degree to instability and is used in a new approach to earthquake prediction. Retrospective examination of some one hundred earthquake cases (from M 4 to M 8.6) indicates that for more than 80% of the examined ones, the value of Y is much higher than 1 for a period before the main shock, but the Y value always fluctuates slightly about 1 during two decades for seven stable regions, so that the parameter Y could indicate a dangerous degree of an impending earthquake. Several earthquakes occurring on the Chinese mainland in recent years as well as the Northridge California, U.S.A. earthquake (Jan. 17, 1994, M w 6.7) have been predicted beforehand with this method.

Edge Wave and Non-trapped Modes of the 25 April 1992 Cape Mendocino Tsunami

Abstract: The 25 April 1992 Cape Mendocino earthquake generated a tsunami characterized by both coastal trapped edge wave and non-trapped tsunami modes that propagated north and south along the U.S. West Coast. Both observed and synthetic time series at Crescent City and North Spit are consistent with the zero-order edge wave mode solution for a semi-infinite sloping beach depth profile. Wave amplitudes at Crescent City were about twice that observed at North Spit, in spite of the fact that the source region was three times farther from Crescent City than North Spit. The largest observed amplitude was due to an edge wave which arrived almost three hours after the initial onset of the tsunami; since such waves are highly localized nearshore, this suggests that the enhanced responsiveness at Crescent City is at least partly due to local dynamic processes. Furthermore, the substantially delayed arrival of this wave, which was generated at the southern end of the Cascadia Subduction Zone, has significant implications for hazard mitigation efforts along the entire U.S. West Coast. Specifically, this study demonstrates that slow-moving but very energetic edge wave modes could be generated by future large tsunamigenic earthquakes in the CSZ, and that these might arrive unexpectedly at coastal communities several hours after the initial tsunami waves have subsided.

Field survey of the East Java earthquake and tsunami of June 3, 1994

Abstract: A field survey of the June 3, 1994 East Java earthquake tsunami was conducted within three weeks, and the distributions of the seismic intensities, tsunami heights, and human and house damages were surveyed The seismic intensities on the south coasts of Java and Bali Islands were small for an earthquake with magnitude M 76 The earthquake caused no land damage About 40 minutes after the main shock, a huge tsunami attacked the coasts, several villages in East Java Province were damaged severely, and 223 persons perished At Pancer Village about 70 percent of the houses were swept away and 121 persons were killed by the tsunami The relationship between tsunami heights and distances from the source shows that the Hatori’s tsunami magnitude was m = 3, which seems to be larger for the earthquake magnitude But we should not consider this an extraordinary event because it was pointed out by Hatori (1994) that the magnitudes of tsunamis in the Indonesia-Philippine region generally exceed 1–2 grade larger than those of other regions

Analysis of Induced Seismicity for Stress Field Determination and Pore Pressure Mapping

Abstract: The focal mechanisms of some one hundred microseismic events induced by various water injections have been determined Within the same depth interval, numerous stress measurements have been conducted with the HTPF method When inverted simultaneously, the HTPF data and the focal plane solutions help determine the complete stress field in a fairly large volume of rock (about 15×106 m3) These results demonstrate that hydraulically conductive fault zones are associated with local stress heterogeneities Some of these stress heterogeneities correspond to local stress concentrations with principal stress magnitudes much larger than those of the regional stress field They preclude the determination of the regional stress field from the sole inversion of focal mechanisms In addition to determining the regional stress field, the integrated inversion of focal mechanisms and HTPF data help identify the fault plane for each for each of the focal mechanisms These slip motions have been demonstrated to be consistent with Terzaghi's effective stress principle and a Coulomb friction law with a friction coefficient ranging from 065 to 09 This has been used for mapping the pore pressure in the rock mass This mapping shows that induced seismicity does not outline zones of high flow rate but only zones of high pore pressure For one fault zone where no significant flow has been observed, the local pore pressure has been found to be larger than the regional minimum principal stress but no hydraulic fracturing has been detected there

Numerical Simulation of the 1992 Flores Tsunami: Interpretation of Tsunami Phenomena in Northeastern Flores Island and Damage at Babi Island

Abstract: Numerical analysis of the 1992 Flores Island, Indonesia earthquake tsunami is carried out with the composite fault model consisting of two different slip values Computed results show good agreement with the measured runup heights in the northeastern part of Flores Island, except for those in the southern shore of Hading Bay and at Riangkroko The landslides in the southern part of Hading Bay could generate local tsunamis of more than 10m The circular-arc slip model proposed in this study for wave generation due to landslides shows better results than the subsidence model, It is, however, difficult to reproduce the tsunami runup height of 262 m at Riangkroko, which was extraordinarily high compared to other places The wave propagation process on a sea bottom with a steep slope, as well as landslides, may be the cause of the amplification of tsunami at Riangkroko The simulation model demonstrates that the reflected wave along the northeastern shore of Flores Island, accompanying a high hydraulic pressure, could be the main cause of severe damage in the southern coast of Babi Island

Source characteristics of the 1992 Nicaragua tsunami earthquake inferred from teleseismic body waves

Abstract: We analyzed the broadband body waves of the 1992 Nicaragua earthquake to determine the nature of rupture. The rupture propagation was represented by the distribution of point sources with moment-rate functions at 9 grid points with uniform spacing of 20 km along the fault strike. The moment-rate functions were then parameterized, and the parameters were determined with the least squares method with some constraints. The centroid times of the individual moment-rate functions indicate slow and smooth rupture propagation at a velocity of 1.5 km/s toward NW and 1.0 km/s toward SE. Including a small initial break which precedes the main rupture by about 10 s, we obtained a total source duration of 110 s. The total seismic moment is M o = 3.4 × 1020 Nm, which is consistent with the value determined from long-period surface waves, M o = 3.7 × 1020 Nm. The average rise time of dislocation is determined to be τ ≈ 10s. The major moment release occurred along a fault length of 160 km. With the assumption of a fault width W = 50 km, we obtained the dislocation D = 1.3 m. From τ and D the dislocation velocity is D = D/τ ≈ 0.1 m/s, significantly smaller than the typical value for ordinary earthquakes. The stress drop Δσ = 1.1 MPa is also less than the typical value for subduction zone earthquakes by a factor of 2–3. On the other hand, the apparent stress defined by 2μE s /M o , where μ and E s are respectively the rigidity and the seismic wave energy, is 0.037 MPa, more than an order of magnitude smaller than Δσ. The Nicaragua tsunami earthquake is characterized by the following three properties: 1) slow rupture propagation; 2) smooth rupture; 3) slow dislocation motion.

Damage to Coastal Villages due to the 1992 Flores Island Earthquake Tsunami

Abstract: A field survey of the 1992 Flores Island earthquake tsunami was conducted during December 29, 1992 to January 5, 1993 along the north coast of the eastern part of Flores Island. We visited over 40 villages, measured tsunami heights, and interviewed the inhabitants. It was clarified that the first wave attacked the coast within five minutes at most of the surveyed villages. The crust was uplifted west of the Cape of Batumanuk, and subsided east of it. In the residential area of Wuring, which is located on a sand spit with ground height of 2 meters, most wooden houses built on stilts collapsed and 87 people were killed even though the tsunami height reached only 3.2 meters. In the two villages on Babi Island, the tsunami swept away all wooden houses and killed 263 of 1,093 inhabitants. Tsunami height at Riang-Kroko village on the northeastern end of Flores Island reached 26.2 meters and 137 of the 406 inhabitants were killed by the tsumani. Evidence of landslides was detected at a few points on the coast of Hading Bay, and the huge tsunami was probably formed by earthquake-induced landslides. The relationship between tsunami height and mortality was checked for seven villages. The efficiencies of trees arranged in front of coastal villages, and coral reefs in dissipating the tsunami energy are discussed.

Field Survey of the 1993 Hokkaido Nansei-Oki Earthquake Tsunami

Abstract: Runup data in Hokkaido and in three prefectures in the Tohoku District are described with a few witnessed arrival times and with comments of tide records. The highest runup of 31.7 m was found at the bottom of a narrow valley on the west coast of Okushiri Island. In order to explain high runups of 20 m at Hamatsumae in the sheltered area, roles of edge waves, refraction of the Okushiri Spur and tsunami generation by causes other than the major fault motion should be understood. An early arrival of the tsunami on the west coast of Hokkaido suggests another tsunami generation mechanism in addition to the major fault motion.

Tsunami Generation of the 1993 Hokkaido Nansei-Oki Earthquake

Abstract: Heterogeneous fault motion of the 1993 Hokkaido Nansei-Oki earthquake is studied by using seismic, geodetic and tsunami data, and the tsunami generation from the fault model is examined. Seismological analyses indicate that the focal mechanism of the first 10 s, when about a third of the total moment was released, is different from the overall focal mechanism. A joint inversion of geodetic data on Okushiri Island and the tide gauge records in Japan and Korea indicates that the largest slip, about 6 m, occurred in a small area just south of the epicenter. This corresponds to the initial rupture on a fault plane dipping shallowly to the west. The slip on the northernmost subfault, which is dipping to the east, is about 2 m, while the slips on the southern subfaults, which are steeply dipping to the west, are more than 3 m. Tsunami heights around Okushiri Island are calculated from the heterogeneous fault model using different grid sizes. Computation on the smaller grids produces larger tsunami heights that are closer to the observed tsunami runup heights. Tsunami propagation in the nearly closed Japan Sea is examined as the free oscillation of the Japan Sea. The excitation of the free oscillation by this earthquake is smaller than that by the 1964 Niigata or 1983 Japan Sea earthquake.

Field Survey of the 1994 Mindoro Island, Philippines Tsunami

Abstract: This is a report of the field survey of the November 15, 1994 Mindoro Island, Philippines, tsunami generated by an earthquake (M = 7.0) with a strike-slip motion. We will report runup heights from 54 locations on Luzon, Mindoro and other smaller islands in the Cape Verde passage between Mindoro and Luzon. Most of the damage was concentrated along the northern coast of Mindoro. Runup height distribution ranged 3–4 m at the most severely damaged areas and 2–4 in neighboring areas. The tsunami-affected area was limited to within 10 km of the epicenter. The largest recorded runup value of 7.3 m was measured on the southwestern coast of Baco Island while a runup of 6.1 m was detected on its northern coastline. The earthquake and tsunami killed 62 people, injured 248 and destroyed 800 houses. As observed in other recent tsunami disasters, most of the casualties were children. Nearly all eyewitnesses interviewed described the first wave as a leading-depression wave. Eyewitnesses reported that the main direction of tsunami propagation was SW in Subaang Bay, SE in Wawa and Calapan, NE on Baco Island and N on Verde Island, suggesting that the tsunami source area was in the southern Pass of Verde Island and that the wave propagated rapidly in all directions. The fault plane extended offshore to the N of Mindoro Island, with its rupture originating S of Verde Island and propagating almost directly south to the inland of Mindoro, thereby accounting for the relatively limited damage area observed on the N of Mindoro.

The 1994 Shikotan Earthquake Tsunamis

Abstract: The 1994 Shikotan earthquake was one of the greatest earthquakes in recent years with a magnitude of M s 8.0. A tsunami survey was conducted by Russian and U.S. geophysicists from October 16–30, 1994, less than two weeks after the earthquake. The survey results and a numerical hindcast simulation are reported. Tsunami focusing effect at locations supposedly sheltered by the island chain is discussed. Based on the obtained data, tsunamis which attacked Shikotan Island are characterized as long waves (the order of 10–20 min wave period) with a positive leading wave. Possible consequences of the positive leading wave form are discussed in relation to the observed minimal destruction of beach vegetation and relatively small transport of marine sediment onto the shore. The high-quality tide-gage record in Malokurilskaya Bay indicates the occurrence of a 53 cm subsidence at the site.

An Earthquake Instability Model Based on Faults Containing High Fluid-pressure Compartments

Abstract: It has been proposed that large strike-slip faults such as the San Andreas contain water in seal-bounded compartments. Arguments based on heat flow and stress orientation suggest that in most of the compartments, the water pressure is so high that the average shear strength of the fault is less than 20 MPa. We propose a variation of this basic model in which most of the shear stress on the fault is supported by a small number of compartments where the pore pressure is relatively low. As a result, the fault gouge in these compartments is compacted and lithified and has a high undisturbed strength. When one of these locked regions fails, the system made up of the neighboring high and low pressure compartments can become unstable. Material in the high fluid pressure compartments is initially underconsolidated since the low effective confining pressure has retarded compaction. As these compartments are deformed, fluid pressure remains nearly unchanged so that they offer little resistance to shear. The low pore pressure compartments, however, are overconsolidated and dilate as they are sheared. Decompression of the pore fluid in these compartments lowers fluid pressure, increasing effective normal stress and shear strength. While this effect tends to stabilize the fault, it can be shown that this dilatancy hardening can be more than offset by displacement weakening of the fault (i.e., the drop from peak to residual strength). If the surrounding rock mass is sufficiently compliant to produce an instability, slip will propagate along the fault until the shear fracture runs into a low-stress region. Frictional heating and the accompanying increase in fluid pressure that are suggested to occur during shearing of the fault zone will act as additional destabilizers. However, significant heating occurs only after a finite amount of slip and therefore is more likely to contribute to the energetics of rupture propagation than to the initiation of the instability.

Modeling the seismic source and tsunami generation of the December 12, 1992 Flores Island, Indonesia, earthquake

Abstract: On December 12, 1992 a large earthquake (M s 7.5) occurred just north of Flores Island, Indonesia which, along with the tsunami it generated, killed more than 2,000 people. In this study, teleseismic P and SH waves, as well as PP waves from distances up to 123°, are inverted for the orientations and time histories of multiple point sources. By repeating the inversion for reasonable values of depth, time separation and spatial separation, a 2-fault model is developed. Next, the vertical deformation of the seafloor is estimated from this fault model. Using a detailed bathymetric model, linear and nonlinear tsunami propagation models are tested. The data consist of a single tide gauge record at Palopo (650 km to the north), as well as tsunami runup height measurements from Flores Island and nearby islands. Assuming a tsunami runup amplification factor of two, the two-fault model explains the tide gauge record and the tsunami runup heights on most of Flores Island. It cannot, however, explain the large tsunami runup heights observed near Leworahang (on Hading Bay) and Riangkroko (on the northeast peninsula). Massive coastal slumping was observed at both of these locations. A final model, which in addition to the two faults, includes point sources of large vertical displacement at these two locations explains the observations quite well.

Earthquake Activity in the Aswan Region, Egypt

Abstract: The November 14, 1981 Aswan earthquake (M L = 5.7), which was related to the impoundment of Lake Aswan, was followed by an extended sequence of earthquakes, and is investigated in this study. Earthquake data from June 1982 to late 1991, collected from the Aswan network, are classified into two sets on the basis of focal depth (i.e., shallow, or deeper than 10 km). It is determined that (a) shallow seismicity is characterized by swarm activity, whereas deep seismicity is characterized by a foreshock-main shock-aftershock sequence; (b) the b value is equal to 0.77 and 0.99 for the shallow and deep sequences, respectively; and (c) observations clearly indicate that the temporal variations of shallow seismic activity were associated with a high rate of water-level fluctuation in Lake Aswan; a correlation with the deeper earthquake sequence, however, is not evident. These features, as well as the tomographic characteristics of the Aswan region (Awad and Mizoue, this issue), imply that the Aswan seismic activity must be regarded as consisting of two distinct earthquake groups.

Source Models for the 1993 Hokkaido Nansei-Oki Earthquake Tsunami

Abstract: A source model for the 1993 Hokkaido Nansei-Oki tsunami must satisfy certain conditions. Such conditions are presented in this paper, and two methods are used to determine the best source model for this event. A trial-and-error method selects DCRC-17a as the best among 24 different models. This model has three fault planes dipping westward. To reproduce well the tide gauge records at two locations, an inversion analysis is used to modify the dislocation of DCRC-17a.

Coupling of Tectonic Loading and Earthquake Fault Slips at Subduction Zones

Abstract: Because of the viscoelastic behaviour of the earth, accumulation of elastic strain energy by tectonic loading and release of such energy by earthquake fault slips at subduction zones may take place on different spatial scales. If the lithospheric plate is acted upon by distant tectonic forces, strain accumulation must occur in a broad region. However, an earthquake releases strain only in a region comparable to the size of the rupture area. A two-dimensional finite-element model of a subduction zone with viscoelastic rheology has been used to investigate the coupling of tectonic loading and earthquake fault slips. A fault lock-and-unlock technique is employed so that the amount of fault slip in an earthquake is not prescribed, but determined by the accumulated stress. The amount of earthquake fault slip as a fraction of the total relative plate motion depends on the relative sizes of the earthquake rupture area and the region of tectonic strain accumulation, as well as the rheology of the rock material. The larger the region of strain accumulation is compared to the earthquake rupture, the smaller is the earthquake fault slip. The reason for the limited earthquake fault slip is that the elastic shear stress in the asthenosphere induced by the earthquake resists the elastic rebound of the overlying plate. Since rapid permanent plate shortening is not observed at subduction zones, there must be either strain release over a large region or strain accumulation over a small region over earthquake cycles. The former can be achieved only by significant aseismic fault slip between large subduction earthquakes. The most likely mechanism for the latter is the accumulation of elastic strain around isolated locked asperities of the fault, which requires significant aseismic fault slip between asperities.

Inferred attenuation from site effect-corrected T phases recorded on the island of Hawaii

Abstract: Coda site amplification factors are used to eliminate the site effect from records of three circum-PacificT phases recorded by the Hawaiian Volcano Observatory (HVO) network on the island of Hawaii. ObservedT-phase amplitudes across the island generally decrease with increasing distance from the conversion point where acoustic waves in the SOFAR channel become seismic waves propagating through the crust. However, the decay of the observedT-phase signal across the island is not regular in regions of dense station coverage, in particular, the Kilauea caldera region. We divide the maximum observedT-phase amplitudes at a given station by the coda site amplification factor obtained for the same station and frequency band (3.0Hz); the distribution of these amplitudes reveals a smooth pattern over the entire island. The distance over which the site effect-correctedT-phase amplitude decreases by one-half, combined with the apparent velocity of propagation ofT phases across the island, allows for an approximate determination of near-surfaceQ over much of the island of Hawaii. We found a region of lowQ in the Kilauea summit area (Q≈30) and east rift zone (Q≈60) with considerably higherQ in the Kaoiki and northern portions of the island (Q≈150 to 200). The lowQ values obtained in the Kilauea summit region and east rift zone are significantly lower than estimates of codaQ in the same region, suggestingT phases may be sampling the earth's near-surface properties.

Attenuation and source parameters at Deception Island (South Shetland Islands, Antarctica)

Abstract: Deception Island is the most active volcano of the South Shetland Islands-Antarctic Peninsula group, experiencing eruptions in 1967, 1969 and 1970. Local attenuation derived from coda analysis and source parameters derived from Brune's model, for well located seismic events, have been studied in order to complement the available geophysical information. Results show abnormally lowQ0 values and an abnormally high frequency dependence, as well as large dispersion. These factors are strongly dependent on the path travelled by the seismic wave. Retrieved values of the source parameters (stress drop, seismic moment and source radius), are again abnormally low compared to world-wide average values, for example, those obtained for the Oroville, California aftershock series between June and September, 1976. These results are consistent with some aspects of the geology of Deception Island, such as the very high degree of fracturing and faulting, and the existence of a strong hydrothermal alteration affecting most of the subaerial volcanic rocks. Moreover, the pattern defined for the lateral variations ofQ0 shows minimum values in the inner bay of the island, close to the most recent eruption vents. A large reduction in spectral amplitudes over a particular frequency range occurs in several observations, corresponding to the path crossing the zone of highest attenuation. This observation suggests the existence of a hot magmatic intrusion produced during the most recent eruption, and coincides with the superficial low density mass distribution obtained from the gravimetric model and the long wave magnetic field component obtained from magnetic surveys. The width of this intrusion is estimated to be about 200 m, in agreement with the previous results obtained analyzing residuals of the location of seismic events.

Tsunami Trapping near Circular Islands

Abstract: Trapping of long water waves that are induced by submarine earthquakes and that attack circular islands is studied by applying a theoretical model (Tinti and Vannini, 1994) that is based on the linear shallow water approximation. The solution is computed as the superposition of the eigenmodes of the water basin. The tsunami trapping is seen in terms of the capability of the source to excite the “trapped” eigenmodes of the basin. The bottom depth dependence around the island is shown to be quite important in determining the trapping capability of the island: a depth profile that is downwardly concave as the distance from the island, coasts increases is substantially more efficient in amplifying the incoming waves and in trapping their energy than a profile exhibiting an upward concavity.

Analysis and Automatic Processing in Near-field of Eight 1992–1994 Tsunamigenic Earthquakes: Improvements Towards Real-time Tsunami Warning

Abstract: We study eight tsunamigenic earthquakes of 1992–1994 with data from single near-field 3-component long-period stations. The analysis is made from the standpoint of tsunami warning by an automatic process which estimates the epicentral location and the seismic moment through the variable-period mantle magnitude M m . Simulations of early warning based on the real-time computation of the seismic moment are also tested with this system, which would give a justified warning in each region of tsunami potentiality. By exploiting the dependence of moment rate release with frequency, the system has the capability of recognizing both “tsunami earthquakes” such as the 1992 Nicaragua and 1994 Java events, as well as instances of the opposite case of low-frequency deficiency, interpreted as indicating a deeper than normal source (1993 Guam event). We report both the results of delayed-time processing of the near-field stations, and the actual real-time warnings at PPT, which confirm the former.

Rheological Information from Geological Structures

Abstract: The contrast in rheological properties between layers of different composition or texture, and between stiff inclusions and their matrix, gives rise to perturbations in flow that result in structures. Theory and modeling allow us to understand the conditions necessary for such structures to form and, conversely, we can use the form of the structures to infer possible rheological conditions for the rocks during natural deformation. We review here several structures and their use as indicators of rheological behavior, based on theory, numerical and experimental models, and observations on natural structures.

Speculation on the Causes of Continuing Seismicity Near Koyna Reservoir, India

Abstract: The cause for continuous induced seismicity at Koyna is not well understood. A heuristic model based on various physical parameters observed at Koyna is being proposed to explain the ongoing seismicity. This model contains two essential elements: (i) intersecting faults near Koyna provide means of stress build-up in response to plate tectonic forces, (ii) The annual reservoir loading cycle and changes in the ground water table perturb this stress build-up by an influx of pore pressure in a fluid infiltrated medium. Hence, the spatial and temporal pattern of the pore pressure distribution and the seismicity will be governed by the location and hydromechanical properties of the faults and fractures. The predictions of the model can be tested by comparing the temporal and spatial pattern of seismicity with the changes in lake level and water table.

Investigations of June 7, 1988 earthquake of magnitude 4.5 near Idukki Dam in Southern India

Abstract: A mildly damaging earthquake of magnitude 4.5 and intensity VI occurred 20 km east of the Idukki reservoir, Kerala in southern India. With a network of 5 seismic stations, the aftershocks which continued for 3 1/2 months were monitored. The hypocentral parameters, b value, M 1/M 0 ratio indicate that this earthquake sequence does not qualify to be categorized as induced. The trend of the aftershocks, composite fault plane solution and local tectonics point towards reactivation of a NW-SE fault along the Kallar river. The existence of such a fault is also supported by gravity studies.

On the limits of linear moment tensor inversion of surface wave spectra

Abstract: Some theoretical and practical limits to linear moment tensor inversion of surface waves are analyzed in detail, in particular when one or few stations are used for rapid determination of source parameters. The theory is briefly outlined and steps of preprocessing, especially corrections for source history and propagation, are discussed in order to guarantee the validity of the moment tensor inversion. The inverse problem is first studied from a theoretical point of view. Then the feasibility of the moment tensor inversion is tested from synthetic computations and the formalism is improved in order to obtain a better system conditioning. Finally, some cases of practical nonuniqueness of the solution are shown and possible restrictions to the applicability of the method are discussed. All steps are illustrated with the example of theMs=6.9 Erzincan (Turkey) earthquake of March, 13, 1992.

Bayesian probabilities of earthquake occurrences in Greece and surrounding areas

Abstract: The Bayesian extreme-value distribution of earthquake occurrences has been adopted to estimate the seismic hazard in some seismogenic zones in Greece and surrounding regions. Seismic moment, slip rate, earthquake recurrence rate and magnitude were considered as basic parameters for computing the prior estimates of the seismicity. These estimates are then updated in terms of Bayes' theorem and historical estimates of seismicity associated with each zone. High probabilities for earthquakes withM≥6.0 have been obtained for the northwestern part of Greece as well as for the southwestern part of the Hellenic arc.