Coda Magnitude Research Articles

APPRAISAL OF THE ABAJI-ABUJA (NIGERIA) ML 2.25 EARTH-TREMOR OF 10th JANUARY 2020

This research aims at determining the epicentre, magnitude and energy dissipated during the earth-tremor which took place at Abaji-Abuja (Nigeria) on 10th January 2020 at 13:46:20 UTC. The event was recorded by Volksmeter II VMII-2RU broadband Seismographs located at the Earthquake and Space Weather laboratory, Obafemi Awolowo University, Ife, Osun state, Nigeria. The computed P-wave and S-wave arrival times and lag times were 35.8seconds, 36.1seconds and 33.3seconds recorded by three Seismographs Ife (1), Ife (2) and Ife (3) respectively. The WinQuake software was used for the analysis. Epicentre distances computed were 330.3 km, 333.5 km and 303.2 km from the three seismograms, using trilateration method. The results gave the epicentre location at 8049’N and 6047’E (Gurdi town, Yaba District, Abaji LGA, Abuja, Nigeria),focal depth of 5 km, and average epicentral distance of 322.3km from Ile-Ife. The tremor has a Local Magnitude of 2.25 MLand Coda Magnitude of 3.7Md, and the dissipated energy of 149.6 × 10-12 KJ. The implication of these results is that a local magnitude of 2.25 ML of 149.6 × 10-12KJ of energy serves as an indicator to the future occurrence of another earthquake within the said region. Necessary precautionary measures should be taken when carrying out geological and construction works within the Abaji-Abuja environs. The massive rock blasting and quarrying must have reactivated the faults within the area.

Local magnitude, coda magnitude, and radiated energy of volcanic tectonic earthquakes from October 2010 to December 2011 at Sinabung volcano, Indonesia

In August 2010, Sinabung volcano began erupting after more than a thousand years of dormancy. Following several weeks of phreatic eruptions, the eruptions ceased and Sinabung entered what became an inter-eruptive period of dominantly seismic unrest. While standard equations for understanding the size of an earthquake (local magnitude (ML), coda magnitude (MC), and seismic energy release (ER)) have long been developed, it is best practice to fine tune these relations for a given region and period of study to more accurately describe seismicity and to directly compare it with other volcanic systems. More accurate descriptions of magnitudes and energy release are vital to accurate volcanic eruption forecasting and evaluation of seismic and volcanic risk. In this study, we use high-frequency volcano-tectonic (VT) earthquakes recorded on a temporary three-component network installed between October 2010 and December 2011 in the region around Sinabung volcano to better constrain the seismic parameters of and better understand this previously unstudied volcano. We determine region-specific formulas for ML, MC, and ER as follows: $$ {\mathrm{M}}_{\mathrm{L}}={\log}_{10}A+1.1252{\log}_{10}r+0.0280\ r-2.5427,\kern0.5em {\mathrm{M}}_{\mathrm{C}}=0.7764\ {\log}_{10}{t}_{coda}+0.0676\ r-0.7185,\kern0.5em \mathrm{and}\kern0.5em {\log}_{10}\left({\mathrm{E}}_{\mathrm{R}}\right)=1.5720{\mathrm{M}}_{\mathrm{L}}+11.5258, $$ where A, r, and tcoda are maximum amplitude on a Wood-Anderson seismogram, hypocentral distance (km), and the coda duration (s), respectively. Constants in the ML equation have physically interpretable meanings. The constant for the geometrical spreading term (log10r term) equals one for perfect spherical spreading of the waveform. Our value is greater than one and thus suggests that wavefronts spread at a slightly different rate than for simple spherical spreading. The constant for the attenuation term (r term) is consistent with locally mapped attenuative deposits (limestones and tuffs) and previous 3D tomographic results. Our MC equation differs from a previous study, likely because different data in a different time period were used. Earthquake hypocenters are consistent with those located in previous tomographic studies, and we interpret the earthquakes in this study as distal VT earthquakes induced by continued magmatic intrusion at Sinabung over the period of October 2010–December 2011.

An Improved Method to Determine Coda‐Q, Earthquake Magnitude, and Site Amplification: Theory and Application to Southern California

AbstractSeismic coda waves can be used to constrain attenuation, estimate earthquake magnitudes, and determine site amplification factors. We have developed a new multistation and multievent method to determine these three important seismic parameters simultaneously. We analyze 642 representative local (≤100 km) and shallow (≤20 km) earthquakes with catalog magnitudes between 1.8 and 5.4 in southern California at multiple frequency bands centered at 1.5, 3, 6, and 12 Hz. We find that the length of the moving average time window can affect the measurement of coda attenuation QC, but our tests indicate that the optimal window length is about 15 times the dominant data period. We use linear regression to fit each coda section and use only those portions that agree with the model decay rate with a correlation coefficient larger than 0.9. For a frequency‐dependent coda‐QC model (QC = Q0fn) at 1‐Hz reference frequency, our results yield estimates for Q0 and n of 107–288 and 0.42–1.14, respectively. Our coda magnitude estimates are linearly correlated with catalog magnitudes, and our observed lateral variations in coda‐QC and our site amplification factors are in general agreement with previous results, although there are notable differences at some locations. This approach provides a unified, accurate, and stable method to measure coda‐QC, earthquake magnitude, and site amplification using coda waves of locally recorded earthquakes.

Moment Magnitudes of Local/Regional Events from 1D Coda Calibrations in the Broader Middle East Region

Abstract Reliable moment magnitude estimates for seismic events in the Middle East region can be difficult to obtain due to the uneven distribution of stations, the complex tectonic structure, and regions of high attenuation. In this study, we take advantage of the many new broadband seismic stations that have become available through improved national networks and numerous temporary deployments. We make coda envelope‐amplitude measurements for 2247 events recorded by 68 stations over 13 narrow frequency bands ranging between 0.03 and 8 Hz. The absolute scaling of these spectra was calculated based on independent waveform modeling solutions of the moment magnitudes for a subset of these events to avoid circularity. Using our 1D path calibrations, we determined coda‐based magnitudes for a majority of the events. We obtain fairly good agreement with waveform‐modeled seismic moments for the larger events ( M w >4.5) at low frequencies ( 0.7 Hz) because of unaccounted 2D path effects, as well as mixing of both Sn coda and Lg coda, which have different attenuation behavior. This scatter leads to increased variance in the magnitudes estimated for smaller events in which low‐frequency amplitudes are below the noise levels and the higher frequencies are the only signals available. We quantify the expected variance in coda envelope amplitudes as a function of frequency using interstation scatter as our metric. The net results of this study provide thousands of new 1D coda magnitude estimates for events in the broad region, as well as the necessary initial starting model for use in a new related 2D coda study (Pasyanos et al. , 2016). Online Material: Table of site terms and moment magnitudes.

Attenuation of coda waves in the Garhwal Lesser Himalaya, India

Qc estimates for the Uttarkashi and the Chamoli regions of the Garhwal Lesser Himalaya have been obtained by analyzing the coda waves of 159 local earthquakes recorded during 2008 and 2009 employing a 12-station seismological network. Earthquakes around the Uttarkashi region are located in the epicentral distance range of 5.0 to 93.9 km, focal depth range of 1.63 to 42.13 km, and coda magnitude range of 0.2 to 2.9, whereas earthquakes around Chamoli region are located in the epicentral distance range of 19.8–109.2 km, focal depth range of 1.36 to 40.72 km, and coda magnitude range of 1.0 to 3.0. The coda waves of 30 s duration, recorded on 982 seismograms, have been analyzed in seven frequencies range centered at 1.5, 3.0, 6.0, 9.0, 12.0, 18.0, and 24.0 Hz for four to five lapse time windows (LTW) using the single backscattering model given by Aki and Chouet (J Geophys Res 80:3322–3342, 1975). Mean value of Qc estimates vary from 76 at 1.5 Hz to 2201 at 24.0 Hz for LTW range of 10–40 s and from 216 at 1.5 Hz to 3243 at 24.0 Hz for LTW range of 50–80 s (for the Uttarkashi region) and from 147 at 1.5 Hz to 2273 at 24.0 Hz for LTW range of 20–50 s and from 188 at 1.5 Hz to 2826 at 24.0 Hz for LTW range of 50–80 s (for Chamoli region). The Qc values thus obtained showed a clear dependence on frequency and LTW and frequency dependence Qc relationships, Qc = Q0fη, for LTWs that have been obtained as Qc = 57f1.20 (10–40 s), Qc = 97f1.07 (20–50 s), Qc = 116f1.03 (30–60 s), Qc = 130f1.03 (40–70 s), and Qc = 162f0.95 (50–80 s) for Uttarkashi region and Qc = 107f0.95 (20–50 s), Qc = 115f0.96 (30–60 s), Qc = 128f0.95 (40–70 s), and Qc = 145f0.95 (50–80 s) for Chamoli region.

Application of the telegraph model to coda Q variations in southern California

We examine waveforms and data used to construct coda magnitude in southern California to estimate the spatial variation of coda Q and its dependence on frequency. Our analysis combined with independent borehole data suggests that coda is mainly generated by multiple scattering in the upper few kilometers of the crust where large impedance contrasts occur because of surface layering or fracturing. The ubiquitous observation that coda Q increases with frequency is explained as arising from multiple reverberations in the upper crust. We suggest that the telegraph model that has been successfully used to describe reflection seismogram multiples in the exploration industry may also apply to earthquakes. Under this model the apparent increase of Q with frequency is due to trapping of high‐frequency energy in the upper crust. This behavior is expected if the associated reflector series has an exponential autocorrelation function, a feature of the telegraph model. At lower frequencies, trapping is less efficient. The combined effects give rise to an apparent absorption band that we suppose is superimposed on frequency‐independent intrinsic attenuation. Maximum apparent attenuation occurs at wavelengths equal to the dimensions of the regions of upper crust that contain the scattering layers. At lower frequencies, trapping is less effective, and attenuation decreases as the longer‐wavelength waves sample the deeper crust and upper mantle where because of overburden pressures, acoustic impedance contrasts are less extreme. By taking spectral ratios of coda waves to direct S, we estimate that intrinsic Q is high (∼3000) and that coda may be modeled as multiple scattered S waves in a region of anisotropic scattering. The exponential decay of the coda is a result of the perfectly reflecting surface of the Earth with backscattering from random near‐surface layers causing progressive leakage and loss of energy downward into the more transparent lower crust and mantle.

Local Magnitude Determinations for Intermountain Seismic Belt Earthquakes from Broadband Digital Data

The University of Utah Seismograph Stations (uuss) earthquake catalogs for the Utah and Yellowstone National Park regions contain two types of size measurements: local magnitude ( M L ) and coda magnitude ( M C ), which is calibrated against M L . From 1962 through 1993, uuss calculated M L values for southern and central Intermountain Seismic Belt earthquakes using maximum peak-to-peak (p-p) amplitudes on paper records from one to five Wood–Anderson (W-A) seismographs in Utah. For M L determinations of earthquakes since 1994, uuss has utilized synthetic W-A seismograms from U.S. National Seismic Network and uuss broadband digital telemetry stations in the region, which numbered 23 by the end of our study period on 30 June 2002. This change has greatly increased the percentage of earthquakes for which M L can be determined. It is now possible to determine M L for all M ≥3 earthquakes in the Utah and Yellowstone regions and earthquakes as small as M To maintain continuity in the magnitudes in the uuss earthquake catalogs, we determined empirical M L station corrections that minimize differences between M L s calculated from paper and synthetic W-A records. Application of these station corrections, in combination with distance corrections from Richter (1958) which have been in use at uuss since 1962, produces M L values that do not show any significant distance dependence. M L determinations for the Utah and Yellowstone regions for 1981–2002 using our station corrections and Richter’s distance corrections have provided a reliable data set for recalibrating the M C scales for these regions. Our revised M L values are consistent with available moment magnitude determinations for Intermountain Seismic Belt earthquakes. To facilitate automatic M L measurements, we analyzed the distribution of the times of maximum p-p amplitudes in synthetic W-A records. A 30-sec time window for maximum amplitudes, beginning 5 sec before the predicted Sg time, encompasses 95% of the maximum p-p amplitudes. In our judgment, this time window represents a good compromise between maximizing the chances of capturing the maximum amplitude and minimizing the risk of including other seismic events.

A Study of the Local Magnitude Scale in the Basin of Mexico: Mutually Consistent Estimates of log A0 and Ground-Motion Scaling

A study of the local magnitude scale in the Basin of Mexico is presented using about 1810 synthetic Wood-Anderson peak amplitudes at 13 stations from the Valley of Mexico seismic network (rsvm). Data from all available rsvm earthquakes located in central Mexico were used, and the events within the Basin of Mexico were relocated. Parametric and nonparametric expressions for the log A ( r ) curve were determined and corrected to Richter’s reference level, which was obtained after the linear separation of source, site, and distance terms. The parametric attenuation function is best expressed as log A ( r ) = −0.48 log ( r /100) − 0.0018 ( r − 100) − 3. The results of the attenuation curve are consistent with ground-motion scaling studies (geometrical spreading, Qs , and stochastic simulations): the Basin of Mexico is characterized by a complex attenuation model at short distances, but the general trend is that of a highly attenuative region. We observe that the coda magnitude has been underestimating the excitation levels of local events in the Basin of Mexico and may require station corrections to improve its accuracy. Our study will provide local magnitudes to the seismicity catalog in the Basin of Mexico, thus contributing to more successful earthquake hazard analyses in the most important and heavily populated region of Mexico.

Probabilistic Earthquake Relocation in Three-Dimensional Velocity Models for the Yellowstone National Park Region, Wyoming

Recorded seismicity for the Yellowstone National Park region, comprising 25,267 earthquakes from November 1972 to December 2002, has been relocated using three-dimensional velocity models and probabilistic earthquake location. In addition, new coda magnitudes for earthquakes between 1984 and 2002 were computed by using an improved coda magnitude equation. Three-dimensional velocity models for earthquake location were computed by inverting subsets of high-quality data of three different periods, 1973–1981, 1984–1994, and 1995–2002, for hypocenter locations and seismic velocities. Earthquakes were relocated by using a nonlinear, probabilistic solution to the earthquake location problem. Fully nonlinear location uncertainties included in the probabilistic solution allow a better and more reliable classification of earthquake locations into four quality classes. Earthquake locations show an improvement in location accuracy with time, which we attribute to improved network geometry and more precise timing of arrival times. No large systematic shifts of the relocated earthquake locations are observed, except a systematic shift of ∼2 km to greater depth. The new relocated earthquake locations show tighter clustering of epicenters and focal depths when compared with original earthquake locations. The most intense seismicity in terms of number of earthquakes and cumulative seismic moment release in the Yellowstone National Park region occurs northwest of the Yellowstone caldera between Hebgen Lake and the northern rim of the caldera. Seismicity within the Yellowstone caldera is diffuse, and shallow individual clusters of earthquakes can be associated with major hydrothermal areas.

An application of the coda methodology for moment-rate spectra using broadband stations in Turkey

A recently developed coda magnitude methodology was applied to selected broadband stations in Turkey for the purpose of testing the coda method in a large, laterally complex region. As found in other, albeit smaller regions, coda envelope amplitude measurements are significantly less variable than distance-corrected direct wave measurements (i.e., L{sub g} and surface waves) by roughly a factor 3-to-4. Despite strong lateral crustal heterogeneity in Turkey, they found that the region could be adequately modeled assuming a simple 1-D, radially symmetric path correction. After calibrating the stations ISP, ISKB and MALT for local and regional distances, single-station moment-magnitude estimates (M{sub W}) derived from the coda spectra were in excellent agreement with those determined from multistation waveform modeling inversions, exhibiting a data standard deviation of 0.17. Though the calibration was validated using large events, the results of the calibration will extend M{sub W} estimates to significantly smaller events which could not otherwise be waveform modeled. The successful application of the method is remarkable considering the significant lateral complexity in Turkey and the simple assumptions used in the coda method.

Assessing Hypocentral Accuracy and Lower Magnitude Completeness in the Pacific Northwest Using Seismic Refraction Detonations and Cumulative Frequency-magnitude Relationships

Absolute hypocentral location errors are traditionally estimated from the errors in the locations of quarry blasts. Catalog completeness has been evaluated by examining cumulative frequency-magnitude relationships ( e.g., Rydelek and Sacks, 1989). In the Pacific Northwest, however, the number of quarries with well defined blasting schedules is relatively sparse ( e.g., Benson et al., 1992). Seismic refraction detonations provide an independent assessment of actual location errors of surface events in the Pacific Northwest and elsewhere. Because the detonation yields are also known, their reported magnitudes can be used to investigate the detection and location thresholds for low-magnitude earthquakes. As the number and distribution of seismic stations in the Pacific Northwest has expanded (Figure 1), the location accuracy of the networks and the completeness of their catalogs has improved with time. Because the station coverage is not uniform geographically, these network properties vary with location. In this note we use 72 refraction detonations listed in the Advanced National Seismic System (ANSS) and Pacific Northwest Seismic Network (PNSN) catalogs for the Pacific Northwest to investigate the hypocentral accuracy and completeness of the earthquake catalog as a function of time and location since 1984. These reported detonations had an average charge size of 939 kg and yielded an average coda magnitude of 1.65. Applying magnitude-versus-charge-size relationships to 64 detonations not listed in the ANSS and PNSN catalogs permits us to extend our analysis back to 1965. Brocher (2003b) noted several reasons why the location errors for surface detonations may not be fully representative of location errors for tectonic earthquakes. Nonetheless, quarry blasts and refraction detonations represent seismic sources whose origin times and locations are precisely known, and they provide an independent measure of the quality of location solutions by the network. Furthermore, the regional variation in the ability of a seismic network to locate surficial …

Detonation Charge Size versus Coda Magnitude Relations in California and Nevada

Magnitude–charge size relations have important uses in forensic seismology and are used in Comprehensive Nuclear-Test-Ban Treaty monitoring. I derive empirical magnitude versus detonation-charge-size relationships for 322 detonations located by permanent seismic networks in California and Nevada. These detonations, used in 41 different seismic refraction or network calibration experiments, ranged in yield (charge size) between 25 and 10 6 kg; coda magnitudes reported for them ranged from 0.5 to 3.9. Almost all represent simultaneous (single-fired) detonations of one or more boreholes. Repeated detonations at the same shotpoint suggest that the reported coda magnitudes are repeatable, on average, to within 0.1 magnitude unit. An empirical linear regression for these 322 detonations yields M = 0.31 + 0.50 log 10 (weight [kg]). The detonations compiled here demonstrate that the Khalturin et al. (1998) relationship, developed mainly for data from large chemical explosions but which fits data from nuclear blasts, can be used to estimate the minimum charge size for coda magnitudes between 0.5 and 3.9. Drilling, loading, and shooting logs indicate that the explosive specification, loading method, and effectiveness of tamp are the primary factors determining the efficiency of a detonation. These records indicate that locating a detonation within the water table is neither a necessary nor sufficient condition for an efficient shot.

Hypocentral Accuracy of Surficial Detonations in Northern California

Terrorist attacks on the Murrah Federal Building in Oklahoma City, the World Trade Center in New York City, and the Pentagon in Washington, D.C. point to a need for the real-time monitoring and characterization of surficial detonations, particularly along lifelines in sparsely populated areas. To help evaluate the feasibility of monitoring such detonations using seismic networks, I have compiled actual hypocentral errors for 163 shallow borehole detonations used for seismic refraction experiments in northern California. The charge size used for these refraction detonations averaged about 1,000 kg, a size that has been used in terrorist attacks (Holzer et al. , 1996). The detonations compiled here were used in a series of seismic refraction experiments from 1976 to 1995. The precision and spatial coverage provided by this database of refraction detonations far exceeds those of the quarry blasts traditionally used to compute absolute hypocentral location errors ( e.g. , Olsen and Zoback, 1995). In a related paper, Brocher (2002) summarized charge size versus coda magnitude relationships for 293 seismic refraction detonations in California and Nevada ranging in size up to 106 kg (1 kiloton). Because the number and distribution of seismic stations in northern California have changed only slightly since 1976 (Figure 1), one would not expect the hypocentral location accuracy of the network to have improved dramatically since then. As the station geometry is not uniform, however, one would expect that location errors will vary with location. I thus compare location errors for all detonations in northern California to those for a subset of the data for detonations in the greater San Francisco Bay area, where the station density is higher. The accuracy of the estimates of the uncertainty of the catalog locations of the detonations is also of interest. Comparison of the actual hypocentral location errors with the …

Baylike and continuous variations of the relative level of the late coda during 24 years of observation on Kamchatka

The relative amplitude levels of backscattered shear waves (coda) of local earthquakes as expressed in coda magnitude residuals were studied for nine Kamchatka seismograph stations. Coda magnitudes were determined from coda amplitudes measured on three‐component photograph records of 1.2‐s period displacement‐recording (between 1 and 10 Hz) instruments. A coda magnitude scale was specially designed for this study. It is rather precise, with a standard deviation of about 0.15 for log amplitudes measured at any single station. At each station, for each event we compute the station magnitude residual, i.e., station magnitude (averaged over three components) minus network average magnitude. Station corrections for each station and component were determined and included in the procedure. Time series of coda magnitude residuals including 500–1000 events for each station for the 24‐year period of observations (1967–1990) show moderate but statistically significant oscillations around a constant level. Superposed on this background, two prominent anomalies are revealed. One, at station KBG, of 3 years duration, preceded two M ≈ 8 shallow earthquakes within 100 km from the station. Another, at station APH, of 1.5 years duration, preceded a major (volume of 2.5 km3) fissure volcanic eruption within a 70‐km distance from the station. No other comparable shallow earthquakes or eruptions took place on Kamchatka during this period. The sign and the amplitude of the anomalies indicate a possible 30% increase of S ‐wave attenuation in the lithosphere under the stations.

S波コーダ部分の振幅による地震のマグニチュードの決定

A new empirical formula to determine the earthquake magnitude from the amplitude in the coda-portion of S-wave is obtained. The formula has a form, MC=alogA+b·logT+c, where A is the rms amplitude in the coda-portion of S-wave at the lapse time, T, from the origin time of an earthquake. The coefficient, a, is around 1.0 and the coefficients, b and c, depend on the observational site condition, in particular, characteristics of the seismometer. The formula is applicable to A more than twice as large as the noise level and to T more than 1.6 times as long as the S-wave travel time. The magnitude derived from the above formula, the coda magnitude, gives a close approximation to the magnitude determined by the Japan Meteorological Agency in the magnitude range from 1.3 to 4.6, when the seismometer of its natural frequency, either 1.0Hz or 4.5Hz, is used.The coda magnitude has the advantage of covering a wide range of magnitude, because the amplitude in the coda-portion of S-wave can be used even when the maximum amplitude is out of scale. It also has a high reliability, because the coda magnitude is not largely affected by the characteristics of radiation at the source. Present method is very easy to carry out, especially when the digital data are available, as the coda magnitude can be determined by an easy calculation of rms amplitude.

Coda duration magnitudes in central California: An empirical approach

AbstractA new empirical coda magnitude MD is presented for the central California seismic network (CALNET), in which station corrections are introduced to reduce the influence of hypocenter and station distributions. The new magnitude scale is obtained by relating the lapse-time τ^ to local magnitudes ML using data from 55 earthquakes with 1.1 ≦ ML ≤ 5.6 from June 1977 through 1981. Magnitude estimates are significantly improved by introducing corrections for the seismograph instrument attenuation setting αij and a site correction δj. The magnitude of the ith event 〈MD〉, is the median value of individualM D i j = − 1.03 + 2.10 log ⁡ 10 τ ^ i j + 0.00268 ⁡ τ ^ i j + α i j + δ j ,where τ^ij is the lapse-time measured at the jth station. Local site effects influence the duration of the coda and cause stations to either overestimate or underestimate 〈MD〉, typically up to 0.5 magnitude units. The distribution of the site corrections is spatially correlated, changing coherently across major tectonic and geologic features such as the San Andreas fault. It appears that the site corrections are influenced by the physical attenuation properties near the station, seemingly contradicting the concept that the coda is related to back-scattering and attenuation in a volume with dimensions on the order of the hypocenter to receiver distance.

The 1983 Borah Peak, Idaho, earthquake and its aftershocks

Abstract The 28 October 1983 Borah Peak, Idaho, earthquake ( M S = 7.3) occurred in an area of low historic seismicity within east-central Idaho along a segment of the Lost River fault active during the Holocene. A dense network of portable short-period seismographs (up to 45 stations, station spacings of 2 to 10 km) was installed beginning several hours after the main shock and operated for 22 days. In addition to records from the portable instrumentation, data from permanent seismograph stations operating in Idaho, Utah, Montana, Oregon, Washington, and Wyoming provide a good regional data base. No foreshock activity above M C (coda magnitude) 2.0 was detected for the 2-month period preceding the main shock. The epicenter of the main shock is ∼ 14 km south-southwest of the end of the surface faulting. This relationship suggests unilateral rupture propagating to the northwest. The distribution of 421 aftershocks of M C > 2 defines an epicentral pattern, 75 km × 10 km, trending north-northwest parallel to the surface rupture but displaced laterally southwest by 5 to 10 km. Aftershocks extend to depths of approximately 16 km and in the central and southeastern portion of the aftershock pattern define a zone, dipping approximately 45° southwest, that intersects the surface near the fault scarp. The entire aftershock zone as observed during the first 3 weeks was active shortly after the main shock occurred . Fault plane solutions for 47 aftershocks show predominantly normal faulting with varying amounts of strike-slip motion. Tension axis orientations indicate a dominant extension direction of NNE-SSW during the aftershock sequence. There is considerable diversity among the aftershock focal mechanisms, even along the central and southeast portions of the fault where the hypocenters appear to outline the main fault break. We therefore interpret most of the aftershocks to represent complex fracturing on subsidiary structures adjacent to the main fault .

The Lake Taupo, New Zealand, earthquake swarms of 1983

Abstract The results of microearthquake surveys made during two earthquake swarms that occurred near Lake Taupo, New Zealand, are presented. The swarm of 1983 February was concentrated in an area of 30 km2 while that in 1983 June–July covered an area of 300 km2. Composite focal mechanisms show a predominance of strike-slip faulting with a consistent, but small thrust component. This is in sharp contrast with normal surface faulting observed at the time of the second swarm. For the February swarm the strike-slip motion is right-lateral on a northeast nodal plane and for the June-July swarm it is left-lateral on a northeast plane. The observed surface deformation is an order of magnitude larger than that expected due to seismic slip alone which, together with the gradual development of the surface faulting, suggests that the surface faulting was largely aseismic and may have been in response to subsurface deformation and earthquake shaking. No evidence is found for the swarms being caused by other than ongoing regional deformation. A coda magnitude relation suitable for the Taupo Volcanic Zone is Mc = 0.03 ‡ 0.03 + (1.00 ‡ 0.03) log2 t + (0.041 + 0.007) R.

MLmeasurements for northeastern United States earthquakes

abstractLocal magnitudes (ML) are reported for 56 northeastern United States and Canadian earthquakes using records from a pair of Wood-Anderson torsion instruments which have operated at Weston Observatory since 1967. Corrections to the Richter ML formula are computed to take into account the difference in attenuation between southern California and the northeastern United States. The corrected ML values for these events were compared to the previously reported magnitudes which had been calculated either from the Nuttli (1973) mbLg formula applied to 5- to 10-Hz waves (called MN) or from a coda magnitude formula (Mc). In general, the ML determinations tend to underestimate the MN and Mc values by about 0.4 magnitude units. This confirms the fact that the MN scale is not appropriate when applied to high-frequency waves, as it is in the northeastern United States. The reason for this appears to be in the relation of the period of the wave used in the magnitude determination to the corner period of the earthquake and earth response.

The Fort Ross earthquake sequence, March and April, 1978

abstract On March 31, 1978, an earthquake of coda magnitude 3.3 (ML = 3.7 BRK) occurred 5 km off the coast of northern California near Fort Ross. A single foreshock preceded the earthquake and approximately 60 aftershocks followed. Locations based on P- and S-wave arrival times indicate that the earthquakes occurred offshore, west of the San Andreas fault in the vicinity of a fault that is visible on acoustic reflection profiles. All earthquakes had hypocentral depths less than 8 km. A fault-plane solution from P-wave first motions suggests that the focal mechanism of the main shock consisted of nearly equal components of dextral and vertical movement on a plane striking northwest. Other events in this sequence had first-motion patterns strikingly different than the main shock, indicating that more than one type of faulting occurred during the sequence. Seismic moments computed for three aftershocks ranged from 3.8 × 1019 to 1.1 × 1019 dyne-cm.