Surface Wave Magnitude Research Articles

Seismicity of California's North Coast

At least three moment magnitude (M) 7 earthquakes occurred along California9s north coast in the second half of the nineteenth century. The M 7.3 earthquake on 23 November 1873 occurred near the California-Oregon coast and likely was located on the Cascadia subduction zone or within the North American plate. The M 7.0+ earthquake on 9 May 1878 was located about 75 km offshore near the Mendocino fault. The surface-wave magnitude ( M S ) 7.0 earthquake on 16 April 1899 was located about 150 km offshore within the Gorda plate. There were at least three M 7 north-coast earthquakes in the 35 years before 1906, two M 7 earthquakes in the 20 years after 1906, no M 7 earthquakes from 1923 until 1980, and four M 7 earthquakes since 1980. The relative seismic quiescence after 1906 for M 7 earthquakes along California9s north coast mimics the post-1906 seismic quiescence in the San Francisco Bay area for M 6 earthquakes. The post-1906 relative quiescence did not extend to lower magnitudes in either area. The 18 April 1906 earthquake apparently influenced the rate of occurrence of M 7 north-coast earthquakes as it apparently influenced the rate of M 6 earthquakes in the San Francisco Bay area. The relative seismic quiescence along the California north-coast region after 1906 should be taken into account when evaluating seismic hazards in northwest California.

3D Analysis of S-Wave Propagation in Soft Deposits

During the 1985 Michoacan earthquake (surface-wave magnitude = 8.1), large-amplitude seismograms of extremely long duration were recorded in the lake-bed zone of Mexico City. Large amplification in soft surficial deposits has not been well understood. Ground subsidence of the soft clay deposits might heavily influence S-wave propagation. In the present study, we attempt to explain both S-wave amplification and propagation time between surface and borehole recordings under the action of gravity on the basis of a 3D nonlinear finite-element (FE) technique where a bilinear model with a Mohr-Coulomb criterion is used. We identify body-wave (S-wave) time sections in the recordings with a cross-correlation analysis. The FE simulation successfully predicts the S-wave amplification and propagation time, which cannot be explained by a purely 1D linear model. Although subsidence changes S-wave linear amplification, S-wave propagation time decreases little.

3D Analysis of S-Wave Propagation in Soft Deposits

During the 1985 Michoacan earthquake (surface-wave magnitude = 8.1), large-amplitude seismograms of extremely long duration were recorded in the lake-bed zone of Mexico City. Large amplification in soft surficial deposits has not been well understood. Ground subsidence of the soft clay deposits might heavily influence S-wave propagation. In the present study, we attempt to explain both S-wave amplification and propagation time between surface and borehole recordings under the action of gravity on the basis of a 3D nonlinear finite-element (FE) technique where a bilinear model with a Mohr-Coulomb criterion is used. We identify body-wave (S-wave) time sections in the recordings with a cross-correlation analysis. The FE simulation successfully predicts the S-wave amplification and propagation time, which cannot be explained by a purely 1D linear model. Although subsidence changes S-wave linear amplification, S-wave propagation time decreases little.

Revised world seismicity catalog (1950-1997) for strong (Ms≧ 6) shallow (h≦ 70 km) earthquakes

AbstractUsing the standard earthquake catalogs provided by the International Seismological Centre (ISC), its predecessor the International Seismological Summary (ISS), and the National Earthquake Information Center (NEIC), I analyze the worldwide consistency of teleseismic reporting, completeness of the seismicity record, and homogeneity of magnitude determination, for strong shallow earthquakes (surface-wave magnitude, Ms ≥ 6; depth, h ≤ 70 km) for the period 1950 to 1997. Under the postulates that the rate of earthquake occurrence for the entire world is constant on a time scale of decades and that well after the installation of the World-Wide Standardized Seismograph Network in the middle 1960s the earthquake catalog for strong (Ms ≥ 6) shocks is complete, and the seismicity rates are typical of all periods in the century, we find that, due to the use of different formulations and criteria to calculate the parameter magnitude, the Ms of moderate (6 ≤ Ms < 7) events during the period 1950 to 1963 was systematically overestimated by as much as 0.5 magnitude unit, relative to the Ms assigned to shocks occurring after 1963. When this correction is taken into account, the new catalog of events with Ms (corrected) ≥ 6 in the period 1950 to 1997 becomes largely homogeneous in Ms. Under the foregoing postulates, this new catalog is shown to list nearly all and only the strong shocks [Ms (corrected) ≥ 6; h ≤ 70 km] that occurred in the Earth during the period, a notable exception being the time span from 1964 to 1968. The revised catalog, including the scalar moment and moment magnitude for each event, is listed on the worldwide web page http://www.ldc.usb.ve/~ojperez/catalog.

Station correction analysis for surface-wave magnitudes of New Zealand earthquakes

Station terms and standard errors are presented for 345 world-wide stations used in the determination of surface-wave magnitudes of 190 selected New Zealand earthquakes over the period 1901-1993 [1]. These will facilitate the estimation of surface-wave magnitudes of other earthquakes in the New Zealand region. The station terms and the residuals from the linear model used to estimate them are both found to be weakly related to the mean distance from the earthquakes recorded by each station. The horizontal and vertical components at a given site are treated as separate stations. The station term for the horizontal component tends to exceed that for the vertical component at mean distances in the 20°-40° range.

次世代の史料地震学 史料にみる地震津波発光

One of the intriguing events accompanying large tsunamis, especially those that strike the Pacific coasts of Japan, is the luminous phenomenon; many historical Japanese documents have described how fire balls or pillars of fire seemed to come out from the sea when tsunami approached. Among 17 events of large tsunamis from 684 to 1946, where the surface wave magnitude was greater than 8 or the tsunami magnitude was greater than 3, nine tsunami events accompanied luminous phenomena. In spite the event's high probability, few explanations have been offered as to the source mechanism except luminescent planktonic organisms, which is hardly plausible because luminous tsunamis have been eye-witnessed even in the winter season when such planktonic organisms are less active, and even in the daytime when the intensity of light emitted from planktonic organisms is unlikely to exceed the day-time brightness.Most tsunami earthquakes are thought to be associated with sediments in the accretionary prism. One recent important finding is that large volumes of stable methane hydrate are present within ocean-floor sediments at water depths exceeding about 500m at lower temperatures. When the equilibrium conditions of coupled low temperature and moderate hydrostatic pressure are disturbed by an earthquake, the hydrate abruptly decomposes. Conversely, a breakdown of hydrate may cause a further mass movement, and a cascading chain of events may occur. Some eye-witness reports in historical documents strongly suggest that luminous phenomena associated with tsunami are attributable to methane hydrate disruption, not others causes such as luminescent planktonic organisms.

Magnitudes of New Zealand earthquakes, 1901-1993

Determinations of surface-wave magnitude (Ms) are made on a consistent basis for 202 selected New Zealand earthquakes over the period 1901-1993, including most post-1942 events with local magnitude not less than 6.0 and centroid depth less than 45 km. These determinations have led to a reassessment of magnitudes and locations of some earlier events in the New Zealand Seismological Observatory Catalogue of local magnitudes (ML), in some cases with substantial revisions. The surface-wave magnitudes are compared with local magnitudes and moment magnitudes (Mw), where available, and the relations between these three variables and centroid depth are examined through regression models. The absence of surface-wave observations for some earthquakes allows an upper limit to be placed on their likely moment magnitudes. The analysis shows that estimates of Mw derived from Ms will have a standard error of about 0.15 and Mw derived from ML a standard error of about 0.3.

The empirical relationships between M s, m b and M L for China and vicinity

Data from 753 earthquakes are used to determine a relationship between surface-wave magnitude (Ms) and bodywave magnitude (mb), and from 541 earthquakes to determine a relationship between surface-wave magnitude (Ms) and local magnitude (ML) for China and vicinity: Ms=0.9883 mb-0.0420, Ms=0.9919 ML-0.1773. The relationship of Msversus mb is obtained for 292 events occurred in the Chinese mainland in the time period from 1964 to 1996, 291 events occurred in Taiwan in the time period from 1964 to 1995 and 170 events occurred in the surrounding area. Standard deviation of the fitting is 0.445. Relationship of Msversus ML is obtained for 36 events occurred in the Chinese mainland, 293 events occurred in Taiwan, China and 212 events occurred in the surrounding area. The total amount is 541 events. Standard deviation of the fitting is 0.4673.

Bias in surface-wave magnitudeMsdue to inadequate distance corrections

AbstractWe investigate bias in surface-wave magnitude using the complete ISC and NEIC datasets from 1978 to 1993. We conclude that although there are some small differences between the ISC and NEIC magnitudes, there is no major difference between these agencies for this presentation of the global dataset. The frequency-distance plot for reported surface-wave amplitude observations exhibits detailed structure of the body-wave amplitude-distance curve at all distances; the influence of the surface-wave amplitude decay with distance is much less apparent. This censoring via the body waves represents a large deficit in the number of potentially usable surface-wave amplitude observations, particularly in the P-wave shadow zone between Δ = 100° and 120°. We have obtained two new modified Ms formulas based upon analysis of all ISC data between 1978 and 1993. In the first, the conventional logarithmic dependence of the distance correction is retained, and we obtainM s e = log ( A / T ) max + 1.155 log ( Δ ) + 4.269 .In the second, we make allowance for the theoretically known contribution of dispersion and geometrical spreading, to obtainM s t = log ( A / T ) max + 1 3 log ( Δ ) + 1 2 log ( sin Δ ) + 0.0046 Δ + 5.370.Comparison of these formulas with other work confirms the inadequacy of the distance-dependence term in the Gutenberg and Prague formulas, and we show that our first formula, as well as that of Herak and Herak, gives less bias at all epicentral distances to within the scatter of the observed dataset. Our second formula provides an improved overall distance correction, especially beyond Δ = 145°. We show evidence that Airy-phase distance decay predominates at shorter distances (Δ≦30°), but for greater distances, we are unable to resolve whether this or non-Airy-phase decay predominates. Assuming 20-sec surface waves with U = 3.6 km/sec, we obtain a globally averaged apparent Q−1 of 0.00192 ± 0.00026 (Q ≈ 500). We argue that our second formula not only improves the distance correction for surface-wave magnitudes but also promotes the analysis of unexplained amplitude anomalies by formally allowing for those contributions that are theoretically predictable. We conclude that there remains systematic bias in station magnitudes and that this includes the effects of source depth, different path contributions, and differences in seismometer response. For intermediate magnitudes, Mts shows less scatter against log M0 than does Ms calculated using the Prague formula.

The earthquake of 16 November, 1925 (Ms=7.0) and the reported tsunami in Zihuatanejo, Mexico


 
 
 A feasibility study to develop a tsunami alert system for Mexican earthquakes, using broadband seismograms from the National Seismological Service, is currently under way. A first step in this direction is a revision of the Mexican tsunami catalogs. In these catalogs, one of the largest tsunamis of this century is reported in the Port of Zihuatanejo and has been re- lated to an earthquake which occurred on November 16, 1925. This earthquake was located at a distance of about 600 km from Zihuatanejo and had a surface-wave magnitude, Ms, of 7.0. In developing a tsunami alert system, it is important t o know if the tsunami was indeed related to the earthquake of 1925. In this note we examine available evidence and find thatthe tsunami was not related to the earthquake. There is no evidence of a local earthquake near Zihuatanejo which may have resulted in the tsunami. We conclude that the tsunami was either caused by slumping of the sea floor near Zihuatanejo or by a meteorological phenomenon in the region.
 
 

The Rhodes earthquake of 26 June 1926

We use macroseismic and instrumental data to re-examine the large earthquake of 26 June 1926 in the Hellenic Arc and other associated events. The earthquake was felt over a large area in the Eastern Mediterranean region and caused sporadic damage, and in places destruction, over a large area in Rhodes, southwestern Anatolia, eastern Crete and in the Nile Delta. Despite its size, there has been uncertainty as to its position, depth and magnitude. The earthquake was well recorded instrumentally and we have relocated it using the readings listed in the International Seismological Summary and the present location procedure of the International Seismological Centre. We have also inspected seismograms from 14 stations. We find the intermediate depth of the event to be well established at about 115 km, with a formal error of about 10 km and supported by depth phases. There is evidence of complexity in the source, with a secondary release of energy from the same focus after about six seconds. From original records and bulletins we establish the surface-wave magnitude (Ms as 7.4 ± 0.3, corresponding to a moment release of about 2.2 × 1027 dyn.cm. The felt information is consistent with these findings, and although the earthquake was felt very widely, nowhere did it produce excessively high intensities. Our intensity assessments, usually from original sources, tend to be lower than those assigned in previous studies, the highest being IX MSK on Rhodes.

Assessment of Seismicity Model for Hong Kong Region

The seismicity model, the Gutenberg-Richter magnitude-frequency relationship. has been developed for Hong Kong region. Detailed investigation and statistical tests on the completeness of the earthquake catalogue and the regional variation of the seismicity have been carried out. It has been found that in the catalogue compiled in this study the historical records on land since 1895 are likely to be complete for surface-wave magnitude M = 4 % or greater. and that there is no record before 1920 in offshore area about 100 km from coastal lines. Almost all historical records before 1894 are on land. indicating that the historical records may be complete only on land. The earthquake spatial distribution is not uniform on land. The number of earthquakes with M = 6 or greater per year in the last 100 years is much larger than that in the previous historical time (1500–1894). It has also been found that the regional variation of the seismicity is a significant factor likely to affect the seismic hazard in Hong Ko...

Specral magnitudes and seismograms measure radiation from seismic sources

For over 100 years, efforts have been made to measure the strength of earthquakes. In 1935, Richter established the local magnitude scale, ML, for earthquakes in Southern California using Wood Anderson Seismographs. The seismic waves used had periods ranging from 0.1 s to 2 s. In 1945, Gutenberg [1945a] developed the surface wave magnitude scale (Ms) based on the amplitudes of horizontal surface waves with periods of (20±3) s, recorded at large distances. Gutenberg [1945b] also used seismic body P and S waves, with average periods of 5 s and 10 s, respectively, to introduce the bodywave magnitude scale (mb).

Seismic Evaluation of Brent-Spence Bridge

This is the second part of a two-part paper on field testing and analytical studies related to seismic behavior of the Brent-Spence bridge. This part of the paper deals with seismic evaluation of the bridge, while the first part dealt with free and ambient vibration studies of the bridge by field testing and finite-element analysis. Site-specific ground motion scenarios are developed for the bridge to represent probable earthquakes that may occur in these seismic zones. The time histories of these events are then used in the seismic analysis of the main bridge and response spectra are used for analyzing the approach spans. A three-dimensional finite-element model of the main bridge was subjected to the time histories to determine maximum responses (displacements, forces, and stresses). The seismic analysis of approach spans dealt only with the potential for loss of span due to excessive longitudinal displacement along the highway main line. For the maximum credible earthquake of magnitude 8.5 Ms (7.3 mb,Lg) on the Richter scale at New Madrid, the seismic evaluation indicates that the main bridge will survive the earthquake in the elastic range without significant damage and no loss of span. The approach spans are found to be vulnerable to loss of span failure in the 8.5 Ms event.

Microseismicity evidence for subduction of the Caribbean plate beneath the South American Plate in northwestern Venezuela

Over 1100 microearthquakes with body wave magnitude mb<4 have been located in western Venezuela and the southwestern Caribbean region since the installation in 1980 of the Venezuelan Seismological Array, together with 120 events of mb≥4, one of them with surface wave magnitude Ms∼6. This tectonically complex region is part of the boundary between the Caribbean and the South American plates. The main seismically active feature inland in western Venezuela is the northeast striking, 600‐km long, 100‐km wide, right‐lateral strike‐slip Boconó fault zone along the Venezuelan Andes. About 80% of the earthquakes located in the entire region in the period 1980‐mid‐1995 have occurred on this fault zone, at focal depths <20 km. Microearthquake activity at lower rates also occurs northwest of the Venezuelan Andes, both in the continental and Caribbean sea regions. Part of this activity takes place at depths down to ∼150 km. Northwest oriented seismicity depth profiles show the existence of a Benioff zone dipping to the southeast beneath northwestern Venezuela and northern Colombia. This indicates the presence of a northeast striking, southeast dipping subducted slab of the Caribbean plate beneath the South American plate. Hypocentral locations show that the northeastern end of this subduction occurs northwest of the Curaçao‐Aruba region, in the vicinity of a northwest trending, right‐lateral strike‐slip fault zone that joins up with the northeastern end of the Boconó fault zone. This latter place turns out to be the western end of the east‐west striking San Sebastián fault along the Venezuelan coast.

Estimation of seismic moment and slip distribution of the April 1, 1946, Aleutian tsunami earthquake

The 1946 Aleutian earthquake produced a tsunami of tsunami magnitude Mt=9.3, but the surface wave magnitude is only Ms=7.4, making it a tsunami earthquake. The discrepancy between the apparent size of the earthquake based on the seismic data and the size of the tsunami has been explained by several mechanisms, including a landslide and a slow earthquake, but there are few seismic data available to determine the correct mechanism. We study the generating mechanism of the tsunami using tsunami waveforms recorded on tide gauges. We have modeled the source of the 1946 Aleutian tsunami as the result of an underthrusting earthquake. We performed both forward and inverse modeling of the data using a finite difference calculation to compute synthetic tsunamis. We include both vertical and horizontal deformation of the ocean bottom due to faulting in the computation. The results of the inversion of the tsunami waveforms show that the slip on the fault is mainly concentrated in the shallow section of the fault near the epicenter. There is little slip in the area of the aftershocks, indicating that the aftershocks may represent an area of afterslip or triggered seismicity. The moment estimate of the earthquake is 23×1020 N m, or Mw=8.2. The majority of the waveforms are well‐explained by this fault model and slip distribution, including the Honolulu waveform, but the Hawaiian data also suggest that other effects, such as a landslide, may be necessary for explaining the abnormal tsunami amplitudes in Hawaii. The 1946 earthquake is similar to other tsunami earthquakes, such as the 1896 Sanriku and 1992 Nicaragua earthquakes, indicating that slow, shallow rupture may explain the disproportionally large tsunamis.

A note on the depth recurrence and strain release of large Vrancea earthquakes

The large 1940 Vrancea event with body wave magnitude m B = 7.3 was investigated in terms of the depth of rupture initiation and extent. It was found that the rupture started at 150 km depth and propagated downwards. The ruptured zones of the 1977, 1986 and 1990 Vrancea events were estimated from the distribution of aftershocks located with the joint hypocentre determination method. Their depth ranges are 90–110 km, 130–150 km and 70–90 km, respectively. Because the body wave magnitudes m b were saturated for the last three events and the surface wave magnitudes M S were underestimated (not being corrected for depth) we chose the moment magnitudes to quantify the four events: 7.7 (1940), 7.4 (1977), 7.1 (1986) and 6.9 (1990), respectively. The depth interval 110–130 km is found to be unbroken since at least 1838 or 1802 and therefore is expected to accommodate the next large moment release ( M w = 7.0 to 7.4). The strain in the vertical and in the SE-NW directions is about 0.0003, giving a moderate strain rate of about 10 −13 s −1. A correlation between the depth interval of the large events and the year of occurrence in a century is suggested, offering a physical basis to the observed quasi-cyclicity.

Study of calibration function for surface wave magnitude of DK1 seismographs

The calibration function for surface wave magnitude of DK1 seismograph is deduced by using least squares method of error theory based on the records of DK1 seismograph from 1987 to 1993 and the M s magnitude defined by International Seismological Center (ISC) as basic standard, which takes the form σ DK1(Δ)=(1.66 ± 0.034)lg(Δ)+(3.2 ± 0.11) (10° ⩽ Δ ⩽ 130°, T = 8 ~ 20 s)

The October 9, 1995 Manzanillo, Mexico Earthquake

INTRODUCTION On Monday, October 9, 1995, at 9:36 am local time (3:36 pm GMT), an earthquake with an epicenter located at 19.25° N and 104.19° W, a surface-wave magnitude of Ms = 7.3 (USGS) and a moment magnitude Mw = 7.6 (USGS) struck the area near the town of Manzanillo, Colima, affecting the southwestern part of Jalisco and northern part of Colima, Mexico (Figure 1). In the epicentral region, along 80 km of coastal area, about 17,000 structures in different villages and cities either collapsed or were heavily damaged, affecting nearly 45,000 inhabitants. At least 48 people in the city of Manzanillo, Colima and 10 in the affected area of Jalisco were killed by this earthquake. Damage caused by strong ground motion and the resultant tsunami and liquefaction were reported at several communities of Jalisco and Colima. Some of the most damaged were: Cihuatlan, Jaluco, Melaque, Barra de Navidad, La...

Long‐term seismic behavior of the focal and adjacent regions of great earthquakes during the time between two successive shocks

We analyze the spatial‐temporal‐energy distribution of shallow (depth, h<70 km) seismicity (surface wave magnitude, Ms≥7) occurring in the focal and adjacent regions of very great shallow earthquakes (rupture lengths of the order of hundreds of kilometers) in the circum‐Pacific seismic belt for as many decades as possible before and after the occurrence of the main shock. For this purpose we have revised the catalog of data for the world instrumental seismicity (1900–1995) by removing a series of heterogeneities in the earthquake reporting and magnitude determination. This revised catalog is fairly complete for earthquakes with M≥∼7 since early in the century. Our findings indicate that once a main shock‐aftershock sequence occurs the focal region enters a period of relative quiescence for large (Ms≥7) earthquakes. Significant seismic activity (Ms≥7) occurs during many decades prior to the next main shock. This activity is fairly continuous or random in time and lasts until the onset of the main event but clusters in space around the ends of the forthcoming rupture and/or in the vicinity of the principal epicenter (except in the case of rare bilateral breaks the main epicenter is located at one of these ends). Thus major portions of the rupture zone experience no events with Ms≥7 during the seismic cycle. Larger aftershocks also cluster near the edges of the rupture and near the principal epicenter. Preshock activity, minutes to weeks before the main shock (if any), occurs, in general, near the main epicenter. We also find that the nucleation and ends of the rupture often coincide in space with recognizable geometrical and/or geological inhomogeneities, “barriers” existing in the lithosphere. It appears that while strain energy accumulates, high stress concentrations often occur at the ends of the future rupture zone, which results in the long‐term foreshock activity and in the subsequent initiation of the main rupture from within those regions. We suggest a simple mechanical model that explains the observed pattern of earthquake occurrence for both unilateral and bilateral ruptures. In all cases of great earthquakes that have occurred this century for which a long and complete (to a level of 2 to 3 orders of magnitude smaller than the moment of the main shock) catalog of prior seismicity exists, we can not recognize any statistically significant “quiescent” or “doughnut” patterns in the preceding seismicity at the Ms≥7 level.