Publications

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Book Chapter
Prieto, GA, Parker RL, Vernon FL, Shearer PM, Thomson DJ.  2006.  Uncertainties in earthquake source spectrum estimation using empirical Green functions. Earthquakes; radiated energy and the physics of faulting. 170( Abercrombie RE, McGarr A, Kanamori H, Di Toro G, Eds.).:69-74., Washington: American Geophysical Union   10.1029/170gm08   Abstract

We analyze the problem of reliably estimating uncertainties of the earthquake source spectrum and related source parameters using Empirical Green Functions (EGF). We take advantage of the large dataset available from 10 seismic stations at hypocentral distances (10 km < d <50 km) to average spectral ratios of the 2001 M5.1 Anza earthquake and 160 nearby aftershocks. We estimate the uncertainty of the average source spectrum of the M5.1 target earthquake by performing propagation of errors, which, due to the large number of EGFs used, is significantly smaller than that obtained using a single EGF. Our approach provides estimates of both the earthquake source spectrum and its uncertainties, plus confidence intervals on related source parameters such as radiated seismic energy or apparent stress, allowing the assessment of statistical significance. This is of paramount importance when comparing different sized earthquakes and analyzing source scaling of the earthquake rupture process. Our best estimate of radiated energy for the target earthquake is 1.24×1011 Joules with 95% confidence intervals (0.73×1011, 2.28×1011). The estimated apparent stress of 0.33 (0.19, 0.59) MPa is relatively low compared to previous estimates from smaller earthquakes (1MPa) in the same region.

Journal Article
Koper, KD, Pankow KL, Pechmann JC, Hale JM, Burlacu R, Yeck WL, Benz HM, Herrmann RB, Trugman DT, Shearer PM.  2018.  Afterslip enhanced aftershock activity during the 2017 earthquake sequence near Sulphur Peak, Idaho. Geophysical Research Letters. 45:5352-5361.   10.1029/2018gl078196   AbstractWebsite

An energetic earthquake sequence occurred during September to October 2017 near Sulphur Peak, Idaho. The normal-faulting M-w 5.3 mainshock of 2 September 2017 was widely felt in Idaho, Utah, and Wyoming. Over 1,000 aftershocks were located within the first 2 months, 29 of which had magnitudes >= 4.0 M-L. High-accuracy locations derived with data from a temporary seismic array show that the sequence occurred in the upper (<10km) crust of the Aspen Range, east of the northern section of the range-bounding, west-dipping East Bear Lake Fault. Moment tensors for 77 of the largest events show normal and strike-slip faulting with a summed aftershock moment that is 1.8-2.4 times larger than the mainshock moment. We propose that the unusually high productivity of the 2017 Sulphur Peak sequence can be explained by aseismic afterslip, which triggered a secondary swarm south of the coseismic rupture zone beginning similar to 1 day after the mainshock. Plain Language Summary During the fall of 2017, an energetic sequence of earthquakes was recorded in southeastern Idaho. The mainshock had a moment magnitude of M-w 5.3, yet thousands of aftershocks were detected. We found that the unusually high productivity of this earthquake sequence can be explained by extra sliding that occurred just after the mainshock. This extra sliding happened too slowly to generate seismic waves, but it was large enough to alter the stress in the crust such that the extra aftershocks were created. Our finding suggests that in this region of Idaho, some of the strain that is built up by tectonic forces is released in slow-slip or creep events. This discovery will ultimately lead to more accurate forecasts of seismic hazard in the region.

Wolfe, CJ, Okubo PG, Ekstrom G, Nettles M, Shearer PM.  2004.  Characteristics of deep (<= 13 km) Hawaiian earthquakes and Hawaiian earthquakes west of 155.55 degrees W. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000618   AbstractWebsite

[ 1] High precision relocation of earthquakes recorded by the Hawaiian Volcano Observatory (HVO) seismic network provides new information on the characteristics of seismic faulting at this oceanic hot spot. Using waveform cross correlation, we have measured correlation coefficients and travel time differences for a set of 14,605 deep (greater than or equal to 13 km) earthquakes recorded from 1988 to 1998. We find that about half of the analyzed earthquakes are in similar event clusters that delineate fault zones in the lower crust and upper mantle. We suggest that much of this deep seismicity reflects rupture in the brittle lithosphere away from the magma pathways, although at Kilauea the stresses from magma movement may additionally help trigger mantle earthquakes on preexisting faults in regions with high differential ambient stresses. Focal mechanisms of similar event clusters throughout Hawaii display characteristic patterns and appear consistent with the hypothesis that deep earthquakes on preexisting faults reflect the stresses due to volcano loading and flexure. We also present the results of applying cross correlation analyses and relocation to -7000 earthquakes at all depths located west of 155.55degreesW and recorded from 1988 to 1998. The pattern of relocated earthquakes at the Kealakekua fault zone is consistent with the presence of a lowangle detachment on the west flank of Mauna Loa.

Richards-Dinger, KB, Shearer PM.  2000.  Earthquake locations in southern California obtained using source-specific station terms. Journal of Geophysical Research-Solid Earth. 105:10939-10960.   10.1029/2000jb900014   AbstractWebsite

We relocate 297,400 events recorded by the Southern California Seismic Network (SCSN) between 1975 and 1998 using spatially varying station terms to improve relative location accuracy. Our method uses existing SCSN P and S picks, a smooth one-dimensional velocity model, and an iterative grid search approach based on the L1 norm. We apply empirical corrections for three-dimensional structure by computing station timing corrections that continuously vary as a function of source position. Station terms for each event are obtained by smoothing the residuals from nearby events using a natural neighbor (Delaunay) tessellation of the seismicity and then iterating until a stable set of locations and station terms is achieved. Our approach achieves relative location accuracy comparable locally to master event methods but can be applied uniformly over large regions. Median estimated standard errors for our final locations are 328 m in horizontal position and 741 m in depth. Our locations exhibit much less scatter, particularly in depth, than those of the SCSN catalog and a greater tendency to align into linear and planar features suggestive of fault structures. Our results appear comparable to, and in some cases better than, previous SCSN relocation studies using joint-hypocenter-velocity inversion techniques. Plots of daytime versus nighttime events permit discrimination between clusters of natural and artificial seismicity We observe no simple relationship between the maximum depth of seismicity and surface geology.

Astiz, L, Shearer PM.  2000.  Earthquake locations in the inner Continental Borderland, offshore southern California. Bulletin of the Seismological Society of America. 90:425-449.   10.1785/0119990022   AbstractWebsite

The inner Continental Borderland region, offshore southern California, is tectonically active and contains several faults that are potential seismic hazards to nearby cities. However, fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms. To improve the accuracy of event locations in this area, we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the Borderland. The regions are defined by either temporal or spatial clustering of seismic activity in the Southern California Seismic Network (SCSN) catalog. Obtaining accurate locations for these events is difficult, due to the lack of nearby stations, the limited azimuthal coverage, and uncertainties in the velocity structure for this area. Our location procedure is based on the L-l norm, grid search, waveform cross-correlation method of Shearer (1997), except that we use a nearest neighbor approach (Astiz et al., 2000) to identify suitable event pairs for waveform cross-correlation and we explore the effect of different velocity models on the locations and associated station terms. In general, our relocated events have small estimated relative location errors and the events are more clustered than the SCSN catalog locations. A quarry on the south tip of Catalina Island provides a test of our location accuracy and suggests that, under ideal conditions, offshore events can be located to within 1 to 2 km of their true locations. Our final locations for most clusters are well correlated with known local tectonic features. We relate the 1981 Santa Barbara Island (M-L = 5.3) earthquake with the Santa Cruz fault, the 13 July 1986 Oceanside (M-L = 5.3) sequence with the San Diego Trough fault zone, and events near San Clemente Island with the known trace of the San Clemente fault zone. Over 3000 of the offshore events during this time period are associated with the 1986 Oceanside earthquake and its extended aftershock sequence. Our locations define a northeast-dipping fault plane for the Oceanside sequence, but in cross-section the events are scattered over a broad zone (about 4-km thick). This could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure. Events that occur near Coronado Bank in the SCSN catalog are relocated closer to the San Diego coast and suggest a shallow-angle, northeast-dipping fault plane at 10 to 15 km depth.

Shearer, PM.  1998.  Evidence from a cluster of small earthquakes for a fault at 18 km depth beneath Oak Ridge, southern California. Bulletin of the Seismological Society of America. 88:1327-1336. AbstractWebsite

A swarm of about 50 small earthquakes (M similar to 1.5) occurred for a month during 1989 beneath Oak Ridge, southern California. Location accuracy using conventional analysis of arrival-time picks is limited for these events by the weak, emergent nature of arrivals on the available seismograms. However, waveform crosscorrelation techniques are found to provide precise relative event locations due to the similarity of the waveforms recorded at individual stations. The relocated events form a small cluster about 1 km across at a depth of similar to 18 km and are aligned along a plane that dips 35 degrees to the northwest. Estimated standard errors for the locations are generally less than 50 m. The time evolution of the sequence shows a gradual migration of activity away from its initiation point. Three additional events occurred several months later; these align along the same plane but are displaced about 500 m to the southeast from the main swarm. Reliable fault-plane solutions are difficult to obtain for these events due to the small number of station records available, the limited range of takeoff angles, and the weak initial arrivals on many of the seismograms, Stacking the records at each station over the different events greatly reduces prearrival noise levels and assists in resolving the average P first motions. Analysis of these first-motion data indicates that the slip planes of probable focal mechanisms are not in agreement with the plane defined by the seismicity. The seismicity alignment may represent the extension of the Simi fault, in which case some shallowing of the fault dip would be required to match the observed 35 degrees dip at 18 km.

Matoza, RS, Shearer PM, Okubo PG.  2014.  High-precision relocation of long-period events beneath the summit region of Kilauea Volcano, Hawai'i, from 1986 to 2009. Geophysical Research Letters. 41:3413-3421.   10.1002/2014gl059819   AbstractWebsite

Long-period (0.5-5 Hz, LP) seismicity has been recorded for decades in the summit region of Klauea Volcano, Hawaii, and is postulated as linked with the magma transport and shallow hydrothermal systems. To better characterize its spatiotemporal occurrence, we perform a systematic analysis of 49,030 seismic events occurring in the Klauea summit region from January 1986 to March 2009 recorded by the approximate to 50-station Hawaiian Volcano Observatory permanent network. We estimate 215,437 P wave spectra, considering all events on all stations, and use a station-averaged spectral metric to consistently classify LP and non-LP seismicity. We compute high-precision relative relocations for 5327 LP events (43% of all classified LP events) using waveform cross correlation and cluster analysis with 6.4million event pairs, combined with the source-specific station term method. The majority of intermediate-depth (5-15km) LPs collapse to a compact volume, with remarkable source location stability over 23 years indicating a source process controlled by geological or conduit structure.

Shearer, PM.  1997.  Improving local earthquake locations using the L1 norm and waveform cross correlation: Application to the Whittier Narrows, California, aftershock sequence. Journal of Geophysical Research-Solid Earth. 102:8269-8283.   10.1029/96jb03228   AbstractWebsite

Experiments with different earthquake location methods applied to aftershocks of the October 1, 1987, Whittler Narrows earthquake in California (M-L=5.9) suggest that local event locations can be greatly improved through the use of the L1 norm, station corrections and waveform cross correlation. The Whittler Narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the Southern California Seismic Network (SCSN), a telemetered network of several hundred short-period seismographs. SCSN travel time picks and waveforms obtained through the Southern California Earthquake Center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock. Using a smoothed version of the standard southern California velocity model and the existing travel time picks, improved location accuracy is obtained through use of the L1 norm rather than the conventional least squares (L2 norm) approach, presumably due to the more robust response of the former to outliers in the data. A large additional improvement results from the use of station terms to account for three-dimensional velocity structure outside of the event cluster. To achieve greater location accuracy, waveforms for these events are resampled and low-pass filtered, and the P and S wave cross-correlation functions are computed at each station for every event pair. For those events with similar waveforms, differential times may be obtained from the cross-correlation functions. These times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked. Locations obtained from the adjusted picks show a further improvement in accuracy. Location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick. Improvements in location accuracy are indicated by the reduced scatter in the residuals, smaller estimated location errors, and the increased tendency of the locations to cluster along well-defined fault planes. Median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location, although the relative locations within localized clusters of similar events are better constrained. Seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with a M-L=5.3 aftershock. These fault planes appear to cross, and activity began on the secondary fault plane prior to the large aftershock.

Fan, WY, Shearer PM.  2016.  Local near instantaneously dynamically triggered aftershocks of large earthquakes. Science. 353:1133-1136.   10.1126/science.aag0013   AbstractWebsite

Aftershocks are often triggered by static- and/or dynamic-stress changes caused by mainshocks. The relative importance of the two triggering mechanisms is controversial at near-to-intermediate distances. We detected and located 48 previously unidentified large early aftershocks triggered by earthquakes with magnitudes between >= 7 and 8 within a few fault lengths (approximately 300 kilometers), during times that high-amplitude surface waves arrive from the mainshock (less than 200 seconds). The observations indicate that near-to-intermediate-field dynamic triggering commonly exists and fundamentally promotes aftershock occurrence. The mainshocks and their nearby early aftershocks are located at major subduction zones and continental boundaries, and mainshocks with all types of faulting-mechanisms (normal, reverse, and strike-slip) can trigger early aftershocks.

Castro, RR, Shearer PM, Astiz L, Suter M, Jacques-Ayala C, Vernon F.  2010.  The Long-Lasting Aftershock Series of the 3 May 1887 M-w 7.5 Sonora Earthquake in the Mexican Basin and Range Province. Bulletin of the Seismological Society of America. 100:1153-1164.   10.1785/0120090180   AbstractWebsite

We study local and regional body-wave arrival times from several seismic networks to better define the active regional fault pattern in the epicentral region of the 3 May 1887 M-w 7.5 Sonora, Mexico (southern Basin and Range Province) earthquake. We determine hypocenter coordinates of earthquakes that originated between 2003 and 2007 from arrival times recorded by the local network RESNES (Red Sismica del Noreste de Sonora) and stations of the Network of Autonomously Recording Seismographs (NARS)-Baja array. For events between April and December 2007, we also incorporated arrival times from USArray stations located within 150 km of the United States-Mexico border. We first obtained preliminary earthquake locations with the Hypoinverse program (Klein, 2002) and then relocated these initial hypocenter coordinates with the source-specific station term (SSST) method (Lin and Shearer, 2005). Most relocated epicenters cluster in the upper crust near the faults that ruptured during the 1887 earthquake and can be interpreted to be part of its long-lasting series of aftershocks. The region of aftershock activity extends, along the same fault zone, 40-50 km south of the documented southern tip of the 1887 rupture and includes faults in the epicentral region of the 17 May 1913 (I-max VIII, M-I 5.0-0.4) and 18 December 1923 (I-max IX, M-I 5.7-0.4) Granados-Huasabas, Sonora, earthquakes, which themselves are likely to be aftershocks of the 1887 event. The long aftershock duration can be explained by the unusually large magnitude of the mainshock and by the low slip rates and long mainshock recurrence times of the faults that ruptured in 1887.

Wolfe, CJ, Okubo PG, Shearer PM.  2003.  Mantle fault zone beneath Kilauea volcano, Hawaii. Science. 300:478-480.   10.1126/science.1082205   AbstractWebsite

Relocations and focal mechanism analyses of deep earthquakes (greater than or equal to13 kilometers) at Kilauea volcano demonstrate that seismicity is focused on an active fault zone at 30-kilometer depth, with seaward slip on a low-angle plane, and other smaller, distinct fault zones. The earthquakes we have analyzed predominantly reflect tectonic faulting in the brittle lithosphere rather than magma movement associated with volcanic activity. The tectonic earthquakes may be induced on preexisting faults by stresses of magmatic origin, although background stresses from volcano loading and lithospheric flexure may also contribute.

Shearer, PM.  2002.  Parallel fault strands at 9-km depth resolved on the Imperial Fault, Southern California. Geophysical Research Letters. 29   10.1029/2002gl015302   AbstractWebsite

[1] Precision relocation of hundreds of small earthquakes occurring along the Imperial Fault in Southern California during the last two decades reveals parallel steaks of seismicity at 9-km depth. These strands are spaced about 0.5 km apart within a 2 km wide zone of earthquakes near the brittle-ductile transition between the shallow locked part of the fault and a creeping zone at depth. These results suggest that the lower crustal shear zone below the Imperial Fault, site of major earthquakes in 1940 and 1979, must be at least two kilometers wide.

Zhan, ZW, Shearer PM.  2015.  Possible seasonality in large deep-focus earthquakes. Geophysical Research Letters. 42:7366-7373.   10.1002/2015gl065088   AbstractWebsite

Large deep-focus earthquakes (magnitude>7.0, depth>500km) have exhibited strong seasonality in their occurrence times since the beginning of global earthquake catalogs. Of 60 such events from 1900 to the present, 42 have occurred in the middle half of each year. The seasonality appears strongest in the northwest Pacific subduction zones and weakest in the Tonga region. Taken at face value, the surplus of northern hemisphere summer events is statistically significant, but due to the ex post facto hypothesis testing, the absence of seasonality in smaller deep earthquakes, and the lack of a known physical triggering mechanism, we cannot rule out that the observed seasonality is just random chance. However, we can make a testable prediction of seasonality in future large deep-focus earthquakes, which, given likely earthquake occurrence rates, should be verified or falsified within a few decades. If confirmed, deep earthquake seasonality would challenge our current understanding of deep earthquakes.

Ross, ZE, Trugman DT, Hauksson E, Shearer PM.  2019.  Searching for hidden earthquakes in Southern California. Science. 364:767-+.   10.1126/science.aaw6888   AbstractWebsite

Earthquakes follow a well-known power-law size relation, with smaller events occurring much more often than larger events. Earthquake catalogs are thus dominated by small earthquakes yet are still missing a much larger number of even smaller events because of signal fidelity issues. To overcome these limitations, we applied a template-matching detection technique to the entire waveform archive of the regional seismic network in Southern California. This effort resulted in a catalog with 1.81 million earthquakes, a 10-fold increase, which provides important insights into the geometry of fault zones at depth, foreshock behavior and nucleation processes, and earthquake-triggering mechanisms. The rich detail resolved in this type of catalog will facilitate the next generation of analyses of earthquakes and faults.

Shearer, PM.  2012.  Self-similar earthquake triggering, Bath's law, and foreshock/aftershock magnitudes: Simulations, theory, and results for southern California. Journal of Geophysical Research-Solid Earth. 117   10.1029/2011jb008957   AbstractWebsite

Bath's law, the observation that the largest aftershock is, on average, 1.2 magnitudes smaller than its main shock, independent of main shock size, suggests some degree of self-similarity in earthquake triggering. This behavior can largely be explained with triggering models in which the increased triggering caused by larger magnitude events is exactly compensated for by their decreased numbers, and these models can account for many features of real seismicity catalogs. The Bath's law magnitude difference of 1.2 places a useful constraint on aftershock productivity in these models. A more general test of triggering self-similarity is to plot foreshock and aftershock rates as a function of magnitude m relative to the main shock magnitude, m(max), of the largest event in the sequence. Both computer simulations and theory show that these dN/dm curves should be nearly coincident, regardless of main shock magnitude. The aftershock dN/dm curves have the same Gutenberg-Richter b-value as the underlying distribution, but the foreshock dN/dm curves have the same b-value only for foreshock magnitudes less than about m(max) - 3. For larger foreshock values, the dN/dm curve flattens and converges with the aftershock dN/dm curve at m = m(max). This effect can explain observations of anomalously low b-values in some foreshock sequences and the decrease in apparent aftershock to foreshock ratios for small magnitude main shocks. Observed apparent foreshock and aftershock dN/dm curves for events close in space and time to M 2.5 to 5.5 main shocks in southern California appear roughly self-similar, but differ from triggering simulations is several key respects: (1) the aftershock b-values are significantly lower than that of the complete catalog, (2) the number of aftershocks is too large to be consistent with Bath's law, and (3) the foreshock-to-aftershock ratio is too large to be consistent with Bath's law. These observations indicate for southern California that triggering self-similarity is not obeyed for these small main shocks or that the space/time clustering is not primarily caused by earthquake-to-earthquake triggering.

Shearer, P, Hauksson E, Lin GQ.  2005.  Southern California hypocenter relocation with waveform cross-correlation, part 2: Results using source-specific station terms and cluster analysis. Bulletin of the Seismological Society of America. 95:904-915.   10.1785/01200401168   AbstractWebsite

We obtain precise relative relocations for more than 340,000 southern California earthquakes between 1984 and 2002 by applying the source-specific station-term (SSST) method to existing P- and S-phase picks and a differential location method to about 208,000 events within similar-event clusters identified with waveform cross-correlation. The entire catalog is first relocated by using existing phase picks, a reference ID velocity model, and the SSST method of Richards-Dinger and Shearer (2000). We also perform separate relocations of Imperial Valley events by using a velocity model more suited to this region. Next, we apply cluster analysis to the waveform cross-correlation output to identify similar-event clusters. We relocate earthquakes within each similar-event cluster by using the differential times alone, keeping the cluster centroid fixed to its initial SSST location. We estimate standard errors for the relative locations from the internal consistency of differential locations between individual event pairs; these errors are often as small as tens of meters. In many cases the relocated events within each similar-event cluster align in planar features suggestive of faults. We observe a surprising number of such faults at small scales that strike nearly perpendicular to the main seismicity trends. In general, the fine-scale details of the seismicity reveal a great deal of structural complexity in southern California fault systems.

Allmann, BR, Shearer PA, Hauksson E.  2008.  Spectral discrimination between quarry blasts and earthquakes in southern California. Bulletin of the Seismological Society of America. 98:2073-2079.   10.1785/0120070215   AbstractWebsite

We compare P-wave spectra of quarry blasts and earthquakes recorded by the southern California seismic network (SCSN) between 2000 and 2005, with the goal of developing methods to discriminate between these events. We process the spectra using an iterative robust least-squares method to isolate source, receiver, and propagation path contributions. This corrects for first-order attenuation structure, as well as near-receiver site effects and any errors in the instrument response functions. Using the earthquake spectra and a simple omega(-2) source model, we compute an empirical Green's function (EGF) to remove the trade-off between the source terms and other parameters in our model. A constant stress-drop model gives a good fit to the observed average earthquake spectra over a wide range of moment magnitude, but provides a mediocre fit to the average quarry blast spectra, which have a generally steeper fall-off at high frequencies than omega(-2). We also compare P- and S-wave amplitudes and find modestly smaller average S amplitudes for the explosions compared to the earthquakes. For southern California, the root-mean-square (rms) misfit of P-wave spectra to an omega(-2) source model is a more reliable explosion discriminant than the S-to-P amplitude ratio and works for about 90% of the events.

Vidale, JE, Shearer PM.  2006.  A survey of 71 earthquake bursts across southern California: Exploring the role of pore fluid pressure fluctuations and aseismic slip as drivers. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb004034   AbstractWebsite

[ 1] We investigate the cause of seismicity bursts by examining a waveform-relocated catalog for southern California between 1984 and 2002 and systematically identifying 71 isolated sequences of 40 or more earthquakes occurring within a 2-km-radius volume and a 4-week interval. Fifty-seven of the 71 bursts are difficult to interpret as primarily a main shock and its Omori-law-abiding foreshocks and aftershocks because they exhibit a more complicated evolution in space, time, and magnitude; we identify 18 of these sequences as particularly swarm-like. Evidence against a simple cascade of elastic stress triggering includes the presence of an interval of steady seismicity rate, the tendency of the largest event to strike later in the sequence, the large spatial extent of some of the swarms compared to their cumulative moment, and the weak correlation between the number of events in each burst and the magnitude of the largest event in each burst. Shallow sequences and normal faulting mechanism sequences are most likely to be swarm-like. The tendencies of the hypocenters in the swarm-like sequences to occur on vertical planes and expand over time suggest pore fluid pressure fluctuations as the most likely mechanism driving the swarm-like seismicity bursts. However, episodic aseismic slip could also be at least partly responsible and might provide a more compelling explanation for the steady rate of seismicity during swarms, whereas fluid pressure perturbations might be expected to diminish more rapidly with time. Both aftershock-like and swarm-like seismicity bursts are distributed across the entire study region, indicating that they are a general feature of tectonic faulting, rather than limited to a few geological conditions such as volcanic or geothermal areas.