Re-evaluation of focal depths and source mechanisms of selected earthquakes in the Afar depression

We present a stepwise inversion procedure to assess the focal depth and model earthquake source complexity of seven moderate-sized earthquakes  (6.2 > M _w > 5.1)  that occurred in the Afar depression and the surrounding region. The Afar depression is a region of highly extended and intruded lithosphere, and zones of incipient seafloor spreading. A time-domain inversion of full moment tensor was performed to model direct P and SH waves of teleseismic data. Waveform inversion of the selected events estimated focal depths in the range of 17–22 km, deeper than previously published results. This suggests that the brittle–ductile transition zone beneath parts of the Afar depression extends more than 22 km. The effect of near-source velocity structure on the moment tensor elements was also investigated and was found to respond little to the models considered. Synthetic tests indicate that the size of the estimated, non-physical, non-isotropic source component is rather sensitive to incorrect depth estimation. The dominant double couple part of the moment tensor solutions for most of the events indicates that their occurrence is mainly due to shearing. Parameters associated with source directivity (rupture velocity and azimuth) were also investigated. Re-evaluation of the analysed events shows predominantly normal faulting consistent with the relative plate motions in the region.     

Relocation of M ≥ 2 events of the 1989 Dôbi seismic sequence in Afar: evidence for earthquake migration

The Ethiopian side of central Afar was struck in August 1989 by the largest seismic sequence (three 6.1 ≤ M _s ≤ 6.3 events, 15 with M _s or m _b ≥ 5.0) since that of Serdo in 1969. Using the Djibouti seismological network, we relocated 297 of the events of that sequence. As most of the large events took place outside the network, we assessed the accuracy and stability of earthquake relocations by using three different velocity models and two relocation codes to try to relate individual shocks to distinct faults and surface breaks. A majority of the events apparently occurred underneath the floor of the Dôbi graben, an area about 45  km long and 15  km wide, rupturing boundary and inner floor faults, in agreement with the surface cracks and scarps that we mapped in the area. The relocation shows that the principal events propagated about 50  km northwestwards along the graben in the first 40  hr. A day and a half after the beginning of the sequence, smaller events ( M ≤ 4) started to propagate more than 55  km eastwards, towards Asal Lake. Using two three-component stations installed near the Ethiopian border, we could determine reliable depths for 21 events. The depths are compatible with a seismogenic crust about 14  km thick in the Dôbi and Hanle graben area. Although the Dôbi sequence ruptured about 50  km of the fault array extending from Serdo to Asal, where the regional stress was released by earthquakes in 1969 and 1978, respectively, a seismic gap about 50  km long still subsists along the northern part of the Gaggade region (Der'êla half-graben).     

Estimation of seasonal changes in the flow of Shirase Glacier using JERS-1/SAR image correlation

This paper presents estimates of detailed seasonal variations in ice-flow velocity for Shirase Glacier calculated using data obtained by Japanese Earth Resources Satellite-1 (JERS-1) synthetic aperture radar (SAR). We used 12 pairs of images (44-day repeat cycle) over the interval from 30 April 1996 to 1 July 1998 to estimate ice-flow fields using an image correlation method. Geometric registration was performed with reference to the RADARSAT Antarctic Mapping Project (RAMP) image dataset. Error analysis based on feature mismatch indicated an absolute error of ±0.30 km/a and relative error of ±0.04 km/a in the estimated flow velocity. The obtained ice-flow velocity increases rapidly from the upstream region (1.18 km/a) to the grounding line, where it becomes stagnant (2.32 km/a), before accelerating gradually to 2.62–2.82 km/a in the downstream region and then increasing to 3.05–3.50 km/a at the terminus of the floating ice tongue. The ice-flow velocities in the downstream region are highly variable, depending on both the distance from the grounding line and the observed epoch (season). Most of the obtained seasonal variations in ice-flow velocity at the floating ice tongue are within the range of the associated error estimate, but the annual difference between 1997 (3.11 km/a) and 1998 (3.50 km/a) is significant, reflecting a possible acceleration in the ice-flow velocity in association with the disappearance of the floating ice tongue between April and May of 1998. In terms of the summer–winter difference in averaged air temperature, the large difference recorded in 1997 (17.0 °C) relative to 1996 (13.9 °C) corresponds to a reduced ice-flow velocity in 1997 (approximately 0.20 km/a) relative to that in 1996 (approximately 0.30 km/a), indicating interactions between air, sea ice, and glacier flow in Lützow-Holm Bay.     

Seismic quiescence before the M 7, 1988, Spitak earthquake, Armenia

A detailed analysis of the 35  yr of seismicity between 1962 and 1997 using a gridding technique shows that the M 7, Spitak earthquake of 1988 December 7 was preceded by a quiescence anomaly that started at approximately 1984±0.5, and lasted about 5±0.5  yr, up to the main shock. This quiescence anomaly had a radius of about 20±3  km, estimated from circular areas with 75 per cent rate decrease, centred at the point of maximum significance of the anomaly. The quiescence was clearly present in the aftershock volume during the 5  yr before the 1988 main shock, but its statistically strongest expression was located 30  km NW of the epicentre. This anomaly fulfills the association rules between precursory quiescence anomalies and main shocks, even for a tight definition, and is therefore proposed as a case of precursory quiescence. The largest value of the standard deviate Z , found by random selection of samples by gridding, was Z =14 for a time window of T _w=3  yr, using a sample size of N =300 events. This makes this anomaly the strongest observed so far, and it is the first documented in an environment of continental collision. There are no false alarms exceeding in significance the precursor. The Armenian earthquake catalogue used for this study had 4600 earthquakes with M ≥ M _min=2.2 in the area bounded by 39.5° to 42°N/42.5° to 47°E. From the point of view of homogeneous reporting this is the best catalogue we have analysed so far. The limits of the data used and the density of the grid are dictated by the data, and have no influence on the results. The choice of free parameters does not influence the results significantly within the following limits: 100≤ N ≤500, 2≤ T _w≤7, 2.2≤ M _min≤2.8.     

Seismic vertical array analysis for phase decomposition

We propose a vertical array analysis method that decomposes complex seismograms into body and surface wave time histories by using a velocity structure at the vertical array site. We assume that the vertical array records are the sum of vertically incident plane P and S waves, and laterally incident Love and Rayleigh waves. Each phase at the surface is related to that at a certain depth by the transfer function in the frequency domain; the transfer function is obtained by Haskell's matrix method, assuming a 1-D velocity structure. Decomposed P , S and surface waves at the surface are estimated from the vertical array records and the transfer functions by using a least-squares method in the frequency domain; their time histories are obtained by the inverse Fourier transform. We carried out numerical tests of this method based on synthetic vertical array records consisting of vertically incident plane P and S waves and laterally incident plane Love and Rayleigh waves. Perfect results of the decomposed P , S , Love and Rayleigh waves were obtained for synthetic records without noise. A test of the synthetic records in which a small amount of white noise was added yielded a reasonable result for the decomposed P , S and surface waves. We applied this method to real vertical array records from the Ashigara valley, a moderate-sized sedimentary valley. The array records from two earthquakes occurring at depths of 123 and 148 km near the array (epicentral distance of about 31 km) exhibited long-duration later phases. The analysis showed that duration of the decomposed S waves was a few seconds and that the decomposed surface waves appeared a few seconds after the direct S -wave arrival and had very long duration. This result indicated that the long-duration later phases were generated not by multireflected S waves, but by basin-induced surface waves.     

Kinematic multistation discriminator between local quarry blasts and earthquakes

It has long been known that S waves on seismograms of local explosions are often accompanied by strong low-frequency, low-velocity, Rg surface wave trains, often significantly diminished for earthquakes. We utilize this fact to construct a new formal discriminator between earthquakes and explosions by measuring the S -surface-wave group velocity. The method is based on analysing the velogram; that is, the display of the envelope of ground motion versus group velocity V = R/T , where R is the epicentral distance and T  the traveltime. We examine the distribution of seismic energy in time and space using envelopes of records from the Israel Seismic Network (ISN), from which we compute the velograms and observe differences in the velograms of quarry blasts and earthquakes. The data include 143 seismic events occurring in three areas (Galilee, Dead Sea, and Gilad) monitored by the ISN; the magnitude range is M _L = 1.0–2.8 at distances of 15–310 km. From the velograms we measure the group velocity, V _{m s} , within the 1–4 km s⁻¹ range at which the velogram reaches its maximum for each available station. The resulting V _{m s} (R) function is closely fitted by the empirical relationship a + b  ln  R , with a and b coefficients varying from event to event. A simple linear function c = b + 0.33a at a threshold C = 0.69 completely separates ( a,b ) pairs for the 67 Galilee events, and, for the 76 remaining events, one earthquake and four explosions are wrongly classified. After data validation and application of the Fisher linear discriminator, adapted to the events from Galilee, only two misidentified events remain for the whole data set.     

Aseismic fault movement before the 1995 Kobe earthquake detected by a GPS survey: implication for preseismic stress localization?

By inversion analysis of the baseline changes and horizontal displacements observed with GPS (Global Positioning System) during 1990–1994, a high-angle reverse fault was detected in the Shikoku-Kinki region, southwest Japan. The active blind fault is characterized by reverse dip-slip (0.7±0.2  m yr⁻¹ within a layer 17–26  km deep) with a length of 208±5  km, a (down-dip) width of 9±2  km, a dip-angle of 51°±2° and a strike direction of 40°±2° (NE). Evidence from the geological investigation of subfaults close to the southwestern portion of the fault, two historical earthquakes ( M _L=7.0, 1789 and 6.4, 1955) near the centre of the fault, and an additional inversion analysis of the baseline changes recorded by the nationwide permanent GPS array from 18 January to 31 December 1995 partially demonstrates the existence of the fault, and suggests that it might be a reactivation of a pre-existing fault in this region. The fact that hardly any earthquakes ( M _L>2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in this study, we evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar yr⁻¹ at a depth of about 20  km around the epicentral area of the 1995 January 17  Kobe earthquake ( M _L=7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar yr⁻¹ during 1990–1994. The results suggest that the 1995  Kobe earthquake could have been induced or triggered by aseismic fault movement.     

Post-seismic relaxation following the great 2004 Sumatra-Andaman earthquake on a compressible self-gravitating Earth

The   M _w γ 9.0  2004 December 26 Sumatra-Andaman and   M _w = 8.7  2005 March 28 Nias earthquakes, which collectively ruptured approximately 1800 km of the Andaman and Sunda subduction zones, are expected to be followed by vigorous viscoelastic relaxation involving both the upper and lower mantle. Because of these large spatial dimensions it is desirable to fully account for gravitational coupling effects in the relaxation process. We present a stable method of computing relaxation of a spherically-stratified, compressible and self-gravitating viscoelastic Earth following an impulsive moment release event. The solution is cast in terms of a spherical harmonic expansion of viscoelastic normal modes. For simple layered viscoelastic models, which include a low-viscosity oceanic asthenosphere, we predict substantial post-seismic effects over a region several 100s of km wide surrounding the eastern Indian Ocean. We compare observed GPS time-series from ten regional sites (mostly in Thailand and Indonesia), beginning in 2004 December, with synthetic time-series that include the coseismic and post-seismic effects of the 2004 December 26 and 2005 March 28 earthquakes. A viscosity structure involving a biviscous (Burgers body) rheology in the asthenosphere explains the pattern and amplitude of post-seismic offsets remarkably well.     

Source history of the 1905 great Mongolian earthquakes (Tsetserleg, Bolnay)

Two great Mongolian earthquakes, Tsetserleg and Bolnay, occurred on 1905 July 9 and 23. We determined the source history of these events using body waveform inversion. The Tsetserleg rupture (azimuth N60°) correspond to a N60° oriented branch of the long EW oriented Bolnay fault.
Historical seismograms recorded by Wiechert instruments are digitized and corrected for the geometrical deformation due to the recording system. We use predictive filters to recover the signals lost at the minute marks.
The total rupture length for the Tsetserleg earthquake may reach up to 190 km, in order to explain the width of the recorded body waves. This implies adding 60 km to the previously mapped fault. The rupture propagation is mainly eastward. It starts at the southwest of the central subsegment, showing a left lateral strike-slip with a reverse component. The total duration of the modelled source function is 65 s. The seismic moment deduced from the inversion is 10²¹ N m, giving a magnitude   M _w = 8  .
The nucleation of the Bolnay earthquake was at the intersection between the main fault (375 km left lateral strike-slip) and the Teregtiin fault (N160°, 80 km long right lateral strike-slip with a vertical component near the main fault). The rupture was bilateral along the main fault: 100 km to the west and 275 km to east. It also propagated 80 km to the southeast along the Teregtiin fault. The source duration was 115 s. The moment magnitude Mw varies between 8.3 and 8.5.
The nucleation and rupture depths remain uncertain. We tested three cases: (1) nucleation and rupture depth limited to the seismogenic zone; (2) nucleation in the seismogenic zone and rupture propagation going to the base of the crust and (3) nucleation within the crust–upper mantle interface and rupture propagation within the upper mantle.     

Model of the geoelectrical structure of the mid- and lower mantle in the Europe–Asia region

The conductivity structure of the Earth's mantle was estimated using the induction method down to 2100  km depth for the Europe–Asia region. For this purpose, the responses obtained at seven geomagnetic observatories (IRT, KIV, MOS, NVS, HLP, WIT and NGK) were analysed, together with reliable published results for 11  yr variations. 1-D spherical modelling has shown that, beneath the mid-mantle conductive layer (600–800  km), the conductivity increases slowly from about 1  S  m⁻¹ at 1000  km depth to 10  S  m⁻¹ at 1900  km, while further down (1900–2100  km) this increase is faster. Published models of the lower mantle conductivity obtained using the secular, 30–60  yr variations were also considered, in order to estimate the conductivity at depths down to the core. The new regional model of the lower mantle conductivity does not contradict most modern geoelectrical sounding results. This model supports the idea that the mantle base, situated below 2100  km depth, has a very high conductivity.     

Size and duration of the high-frequency radiator in the source of the 2004 December 26 Sumatra earthquake

We recover the gross space–time characteristics of high-frequency (HF) radiator of the great Sumatra-Andaman islands earthquake of 2004 December 26 ( M _w= 9.1–9.3) using the time histories of the power of radiated HF P waves. To determine these time histories we process teleseismic P waves at 36 BB stations, using, in sequence: (1) bandpass filtering (four bands: 0.4–1.2, 1.2–2, 2–3 and 3–4 Hz); (2) squaring wave amplitudes, making 'power signals' for each band and (3) stripping the propagation-related distortion ( P coda, etc.) from the power signal and thus recovering source time function for HF power. In step (3) we employ an inverse filter constructed from an empirical Green's function, which is estimated as the power signal from an aftershock. For each ray we thus obtain signals with relatively well-defined end and no coda. From these signals we extract: total duration (joint estimate for all four bands) and temporal centroid of signal power for each band. Through linear inversion, the set of duration values for a set of rays delivers estimates of the rupture stopping point and stopping time. Similarly, the set of temporal centroids can be inverted to obtain the position of the space–time centroid of HF energy radiator. The quality of inversion for centroid is acceptable for lower-frequency bands but deteriorates for higher-frequency bands where only a fraction of stations provide useful data. For the source length and duration the following joint estimates were obtained: 1241 ± 224 km, 550 ± 10 s. The estimated stopping point position corresponds to the northern extremity of the aftershock zone. Spatial HF radiation centroids are located at distances 350–700 km from the epicentre, in a systematic way: the higher is the frequency, the farther is the centroid from the epicentre. Average rupture propagation velocity is estimated as 2.25 km s^–1.     

Fractal structure and characteristic scale in the distributions of earthquake epicentres, active faults and rivers in Japan

Using a box-counting method, we examined the fractal structures of the spatial distributions of three geological systems in Japan, namely those of earthquake epicentres, active faults, and rivers. Our results show that, in the scale range from 1 to 100  km, all geological systems have a common characteristic scale of ~13  km, which divides the spatial distribution into two bands: a smaller scale r < 13  km and larger scale r > 13  km (where r is the box size). In both bands, the three systems obey a power law distribution, and therefore it is proposed that all geological systems have a band-limited fractal structure. Since the characteristic scale of ~13  km is in agreement with the depth of the brittle–ductile transition zone of the crust, we suggest that it is a common feature of the heterogeneity of the crust.     

Waveform modelling of teleseismic S, Sp, SsPmP, and shear-coupled PL waves for crust- and upper-mantle velocity structure beneath Africa

We describe a waveform modelling technique and demonstrate its application to determine the crust- and upper-mantle velocity structure beneath Africa. Our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a Very Fast Simulated Annealing (VFSA). We match the S , Sp, SsPmP and shear-coupled PL phases in seismograms of deep (200–800 km), moderate-to-large magnitude (5.5–7.0) earthquakes recorded teleseismically at permanent broad-band seismic stations in Africa. Using our technique we produce P - and S -wave velocity models of crust and upper mantle beneath Africa. Additionally, our use of the shear-coupled PL phase, wherever observed, improves the constraints for lower crust- and upper-mantle velocity structure beneath the corresponding seismic stations. Our technique retains the advantages of receiver function methods, uses a different part of the seismogram, is sensitive to both P - and S -wave velocities directly, and obtains helpful constraints in model parameters in the vicinity of the Moho. The resulting range of crustal thicknesses beneath Africa (21–46 km) indicates that the crust is thicker in south Africa, thinner in east Africa and intermediate in north and west Africa. Crustal P - (4.7–8 km s⁻¹) and S -wave velocities (2.5–4.7  km s⁻¹) obtained in this study show that in some parts of the models, these are slower in east Africa and faster in north, west and south Africa. Anomalous crustal low-velocity zones are also observed in the models for seismic stations in the cratonic regions of north, west and south Africa. Overall, the results of our study are consistent with earlier models and regional tectonics of Africa.     

Syn‐kinematic sedimentation at a releasing splay in the northern Minwun Ranges,Sagaing Fault zone,Myanmar: significance for fault timing and displacement

The Sagaing Fault zone is the largest active fault in SE Asia, whose current displacement rate of around 1.8 cm year^?1 is well‐established from GPS data. Yet determining the timing of initiation and total displacement on the fault zone has proven controversial. The timing problem can potentially be resolved through a newly identified syn‐kinematic sedimentary section directly related to displacement on the Sagaing Fault in the northern Minwun Ranges. The northern part of the western strand of the Sagaing Fault has a releasing splay geometry that sets up a syn‐kinematic oblique‐extensional basin in its hangingwall, here called the North Minwun Basin. A series of thick ridges probably composed of alluvial fan and fluvial sandstones dipping between 20 and 70° to the north, and younging northwards comprise the basin fill over a distance of 40 km. Total stratigraphic thickness (not vertical thickness) is estimated at 25 km. The basin in terms of depositional geometries, large displacements, and large stratigraphic thickness and appearance on satellite images has parallels with the extensional Hornelen basin, Norway and the strike‐slip Ridge Basin, California. Minimum likely displacement on the fault strand is 40 km, and may possibly be in excess of 100 km. The remote and inaccessible basin has yet to be properly dated, likely ages range between Eocene and Miocene. When dated the basin will provide an important constraint on the timing of deformation. The potential for this basin to constrain the timing and displacement along the northern part of the Sagaing Fault has not been previously recognised.     

Density inhomogeneities of the upper mantle of the Central Balkans

A 3-D density model was created for the Central Balkans area down to a depth of 670  km on the basis of seismic (both artificial sources and earthquakes) and gravity data. This model is based on density columns constructed for the main geological units of the study region. The densities for these columns were obtained using the density variation method. This method makes it possible to extrapolate the density distribution from the well-studied uppermost layers to the deeper levels of the Earth. The constructed 3-D density model was interpreted in relation to the available data on the heat flow and the seismicity of the region. The subdivision of the region by the Maritza fault into two parts—the southern part including the Rhodope massif and the northern part including the structures of Alpine activation of Srednogorie and the Balkanides—was confirmed. The upraised position of the 400  km boundary within the upper mantle, which was established from the density modelling, is assumed to be a sign of development of recent geodynamical processes over the Srednogorie block. From the viewpoint of seismicity prediction, a finding of mantle inhomogeneities orthogonal to the Maritza suture is of great importance.     

Average attenuation of 0.7–5.0 Hz Lg waves and magnitude scale determination for the region bounding the western branch of the East African Rift

The investigation of L g attenuation characteristics in the region bounding the western branch of the East African rift system using digital recordings from a seismic network located along the rift between Lake Rukwa and Lake Malawi is reported. A set of 24 recordings of L g waves from 12 regional earthquakes has been used for the determination of anelastic attenuation, Q _Lg , and regional body-wave magnitude, m _{b Lg} , scale. The events used have body-wave magnitudes, m _b , between 4.6 and 5.5, which have been determined teleseismically and listed in ISC bulletins. The data were time-domain displacement amplitudes measured at 10 different frequencies (0.7–5.0  Hz). Q _Lg and its frequency dependence, η , in the region can be represented in the form Q _Lg = (186.2 ± 6.5)  f  ^(0.78±0.05). This model is in agreement with models established in other active tectonic regions. The L g -wave-based magnitude formula for the region is given by m _{b Lg} = log   A + (3.76 ± 0.38)  log   D − (5.72 ± 1.06), where A is a half-peak-to-peak maximum amplitude of the 1  s L g wave amplitude in microns and D is the epicentral distance in kilometres. Magnitude results for the 12 regional earthquakes tested are in good agreement with the ISC body-wave magnitude scale.     

Discrimination of small earthquakes and artificial explosions in the Korean Peninsula using Pg/Lg ratios

The purpose of this study is to develop a technique to discriminate artificial explosions from local small earthquakes ( M ≤ 4.0) in the time–frequency domain. In order to obtain spectral features of artificial explosions and earthquakes, 3-D spectrograms (frequency, time and amplitude) have been used. They represent a useful tool for studying the frequency content of entire seismic waveforms observed at local and regional distances (Kim, Simpson & Richards 1994). P and S(L g ) waves from quarry blasts show that the frequency content associated with the dominant amplitude appears above 10  Hz and Rg phases are observed at close distances. P and S(L g ) waves from the Tongosan earthquake have strong amplitudes below 10  Hz. For the Munkyong earthquake, however, a broader frequency content up to 20  Hz is found.
  For discrimination between small earthquakes and explosions, Pg/L g spectral ratios are used below 10  Hz, and through spectrogram analysis we can see different frequency contents of explosions and earthquakes. Unfortunately, because explosion data recorded at KSRS array are digitized at 20  sps, we cannot avoid analysing below 10  Hz because of the Nyquist frequency. In order to select time windows, the group velocity was computed using multiple-filter analysis (MFA), and free-surface effects have been removed from all three-component data in order to improve data quality. Using FFT, a log-average spectral amplitude is calculated over seven frequency bands: 0.5 to 3, 2 to 4, 3 to 5, 4 to 6, 5 to 7, 6 to 8 and 8 to 10  Hz. The best separation between explosions and earthquakes is observed from 6 to 8  Hz. In this frequency band we can separate explosions with log ( Pg/L g ) above −0.5, except EXP1 recorded at SIHY1-1, and earthquakes below −0.5, except the Munkyong earthquake record at station KMH.     

A test of the precursory accelerating moment release model on some recent New Zealand earthquakes

The proposal that the moment release rate increases in a systematic way in a large region around a forthcoming large earthquake is tested using three recent, large New Zealand events. The three events, 1993–1995, magnitudes 6.7–7.0, occurred in varied tectonic settings. For all three events, a circular precursory region can be found such that the moment release rate of the included seismicity is modelled significantly better by the proposed accelerating model than by a linear moment release model, although in one case the result is dubious. The 'best' such regions have radii from 122 to 167 km, roughly in accord with previous observations world-wide, but are offset by 50–60 km from the associated main shock epicentre. A grid-search procedure is used to test whether these three earthquakes could have been forecast using the accelerating moment release model. For two of the earthquakes the result is positive in terms of location, but the main shock times are only loosely constrained.     

Temporal and three-dimensional spatial analyses of the frequency–magnitude distribution near Long Valley Caldera, California

The 3-D distribution of the b value of the frequency–magnitude distribution is analysed in the seismically active parts of the crust near Long Valley Caldera, California. The seismicity is sampled in spherical volumes, containing N =150 earthquakes and centred at nodes of a grid separated by 0.3  km. Significant variations in the b value are detected, with b ranging from b ≈0.6 to b ≈2.0. High b -value volumes are located near the resurgent dome, and at depths below 5  km at Mammoth Mountain. b values are found to be much lower south of the Long Valley Caldera. We interpret this to indicate that an active magma body has advanced from depths below 8  km to depths of 4 to 5  km beneath Mammoth Mountain in 1989, and that anomalous crust, either highly fractured or containing unusually high pore pressure, such as is the case in the vicinity of active magma bodies, exists north of the seismically active area beneath the resurgent dome at all depths. We also investigate the spatial distribution of temporal variations of the frequency–magnitude distribution by introducing differential b -value maps. b values increased from b ≈0.8 to b ≈1.5 underneath Mammoth Mountain at the onset of the 1989 earthquake swarm and remained high thereafter. This suggests that an intrusion permanently altered the average distribution of cracks at 5–10  km depth, or that the pore pressure permanently increased. We propose that high b values are a necessary (but not sufficient) condition near a magmatic body, and therefore spatial b -value mapping can be used to aid in the identification of active magma bodies.