Mainshock Aftershock Research Articles

Liquefaction-induced deformation of earthen embankments on non-homogeneous soil deposits under sequential ground motions

Damage of embankments during earthquakes is widely attributed to the liquefaction of foundation soil. Previous studies have investigated the dynamic response of embankments by mainly considering uniform sand foundation and a single earthquake event. However, the foundation of an embankment consists of many sublayers of soil from liquefiable sand to relatively impermeable layer, and during earthquakes a mainshock may trigger numerous aftershocks within a short time which may have the potential to cause additional damage to soil structures. Accordingly, the investigation of liquefaction-induced deformation of earthen embankments on various liquefiable foundation conditions under mainshock–aftershock sequential ground motions is carried out by a series of dynamic centrifuge tests in this study. The liquefiable foundation includes uniform sand profile, continuous layered soil profile, and non-homogeneous soil profiles. Effects of various foundation conditions on embankment deformations are compared and analyzed. From the test results, it is found that the embankment resting on non-homogeneous soil deposits suffer more damage compared to the uniform sand foundation of same relative density. The test results also suggest that the sequential ground motions have a significant effect on the accumulated deformation of embankment.

Discrete characteristics of the aftershock sequence of the 2011 Van earthquake

An intraplate earthquake of magnitude Mw 7.2 occurred on a NE–SW trending blind oblique thrust fault in accretionary orogen, the Van region of Eastern Anatolia on October 23, 2011. The aftershock seismicity in the Van earthquake was not continuous but, rather, highly discrete. This shed light on the chaotic nonuniformity of the event distribution and played key roles in determining the seismic coupling between the rupturing process and seismogeneity.I analyzed the discrete statistical mechanics of the 2011 Van mainshock–aftershock sequence with an estimation of the non-dimensional tuning parameters consisting of; temporal clusters (C) and the random (RN) distribution of aftershocks, range of size scales (ROSS), strength change (εD), temperature (T), P-value of temporal decay, material parameter R-value, seismic coupling χ, and Q-value of aftershock distribution. I also investigated the frequency–size (FS), temporal (T) statistics and the sequential characteristics of aftershock dynamics using discrete approach and examined the discrete evolutionary periods of the Van earthquake Gutenberg–Richter (GR) distribution. My study revealed that the FS and T statistical properties of aftershock sequence represent the Gutenberg–Richter (GR) distribution, clustered (C) in time and random (RN) Poisson distribution, respectively.The overall statistical behavior of the aftershock sequence shows that the Van earthquake originated in a discrete structural framework with high seismic coupling under highly variable faulting conditions. My analyses relate this larger dip–slip event to a discrete seismogenesis with two main components of complex fracturing and branching framework of the ruptured fault and dynamic strengthening and hardening behavior of the earthquake. The results indicate two dynamic cases. The first is associated with aperiodic nature of aftershock distribution, indicating a time-independent Poissonian event. The second is associated with variable slip model, varying loading rates and high seismic coupling along the ruptured area. This study concludes that the aperiodic statistical behavior of the Van aftershock sequence is one of the results of the time-independent Poissonian seismic deformation in the 2011 Van earthquake.

Earthquake swarms in Greenland

Two earthquake swarms have been detected in Greenland. One occurred on the island of Disko in August 2010, the other one was active from January 2008 to June 2009 near the South-East Greenland coast c. 200 km south of Tasiilaq. An earthquake swarm is defined as a series of earthquakes of similar magnitude located within a small area. The magnitude of the largest earthquakes in a swarm is typically less than 4 (Ma & Eaton 2009). Swarm activity is distinctly different from the more common mainshock–aftershock activity, which is characterised by one large earthquake (mainshock) followed by a series of smaller aftershocks. Earthquake swarms mainly occur in areas with tectonic and/or volcanic activity (Stykes 1970), but intraplate swarms are also found in otherwise stable environments (Gregersen 1979; Atakan et al. 1994; Uski et al. 2006; Ma & Eaton 2009). Geological boundaries and old fault zones appear to be a common setting for intraplate earthquake swarms. Earthquake swarms have previously been detected in North and North-East Greenland (Gregersen 1979) at a time when the seismograph coverage was very sparse. It was concluded that the earthquake swarms were caused by tectonic stresses in and around old sedimentary basins near the continental margin.

Stress Drops of the 1997–1998 Colfiorito, Central Italy Earthquakes: Hints for a Common Behaviour of Normal Faults in the Apennines

Stress drop estimates of moderate-magnitude earthquakes in the Umbria–Marche region, in the northern Apennines, exhibit a large scatter. For the two Mw 5.7 and 6.0 main shocks of 26 September 1997 near Colfiorito, several papers resulted in stress drop estimates of 20 MPa, but values as low as 2–3 MPa were proposed as well. Also for the largest aftershocks (Mw > 4), estimates spread from < 1 MPa up to values ten times larger. We have critically revisited methods and data used in the literature. We have specifically faced the trade-off between source and propagation effects, as we believe that it is responsible for a part of the large scatter. To keep this trade-off under control, we have applied a methodology that combines the best fit of both source spectra after Empirical Green’s Function (EGF) deconvolution and observed ground motion spectra, finding that the results of the two different data sets converge independently at the same solution. We have used ground motions observed in the Colfiorito basin, where an accelerograph and a co-located seismological broad-band station recorded three clusters of earthquakes in a broad magnitude interval (1.7 ≤ Mw ≤ 6.0). We have found that the mainshock–aftershock sequences result in stress drops of 2–5 MPa at Mw ≥ 5.6, with an average tendency to decrease at smaller magnitudes where stress drop variability increases. These findings confirm the source scaling recently assessed through Empirical Green’s Function deconvolution for another well-monitored seismic sequence of normal-faulting earthquakes, which struck the city of L’Aquila in the central Apennines in April 2009. The similar scaling law of the two areas suggests common mechanisms of stress release for the shallow normal faults in the Apennines. The propensity of smaller earthquakes to increase in variability, with a tendency toward smaller stress drops, may reflect an effect of fault strength heterogeneities for smaller size ruptures.

Seismic risk of base isolated non-ductile reinforced concrete buildings considering uncertainties and mainshock–aftershock sequences

Base isolation is a promising approach to retrofit seismically vulnerable buildings, but the supporting body of research on seismic risk mitigation through base isolation, particularly considering the associated uncertainties and mainshock–aftershock sequences, is deficient. Therefore, in this study seismic risk analysis was performed for an old non-ductile RC frame building before and after retrofit with base isolation. Various sources of uncertainty such as structural uncertainties, ground motions uncertainties and modeling uncertainties are discussed and propagated in the analysis procedure. A sensitivity study was also conducted to determine which structural parameters have the most significant impact on both the seismic demands of the un-retrofitted and base isolated building. A suite of recorded mainshock and aftershock ground motions was utilized to investigate the influence of considering aftershocks on the performance of these types of buildings. The study revealed that base isolation can greatly reduce the seismic risk for higher damage levels, as one would expect. More importantly, the results also indicated that neglecting aftershocks can cause considerable underestimation of the seismic risk.

Sequence-Based Probabilistic Seismic Hazard Analysis

Abstract Earthquakes are typically clustered both in space and time. Only mainshocks, the largest magnitude events within each cluster, are considered by classical seismic hazard, which is expressed in terms of rate of exceedance of a ground‐motion intensity measure at a site of interest (Cornell, 1968). This kind of probabilistic seismic hazard analysis (PSHA) is used for structural design or assessment in the long term. Recently, for short‐term risk management purposes, a similar approach has been adopted to perform aftershock probabilistic seismic hazard analysis (APSHA), conditional to mainshock occurrence (Yeo and Cornell, 2009). PSHA often refers to a homogeneous Poisson process to describe event occurrence, whereas APSHA models aftershock occurrence via a conditional nonhomogeneous Poisson process, the rate of which depends on the magnitude of the mainshock that has triggered the sequence. On the other hand, the clusters, each of which is composed of the mainshock and the following aftershocks, may be seen as single events occurring at the same rate of the mainshocks. This may allow accounting for aftershocks in hazard analysis in a relatively simple manner, as first argued by Toro and Silva (2001) and further investigated by Boyd (2012). In fact, this short note, focusing on the probabilistic aspects, shows the feasibility of analytically combining results of PSHA and APSHA to get a seismic hazard integral accounting for mainshock–aftershocks seismic sequences, which was still missing from the mentioned studies. The results of the application presented help to preliminarily assess the increase in seismic hazard in terms of rate of occurrence of events causing the exceedance of an acceleration threshold (e.g., that considered for structural design) also considering aftershocks. That is a relevant aspect from the earthquake engineering perspective.

Assessment of Seismic Performance of Buildings with Incorporation of Aftershocks

Aftershock ground motions are essential for the study of aftershock synthesis into seismic risk. However, the number of available aftershock records is limited, which necessitates that the methods for synthesizing aftershocks to be developed. This paper aims to propose a methodology to synthesize aftershocks based on the information of the mainshocks, which is able to incorporate the uncertainties of ground motions and can be utilized in seismic risk analysis. The proposed method is illustrated by application to risk analysis for two nonductile RC frame buildings and is validated by comparing with the results obtained from using as-recorded mainshock-aftershock (MS-AS) sequences. The results calculated when only mainshocks are considered are also studied to investigate the MS-AS effects. The paper shows that the synthesized MS-AS sequences using the proposed method can yield results statistically close to the as-recorded sequences. The results also reveal that only considering the mainshock will underestimate the seismic risk but the underestimation is limited.

The damage investigation of inelastic SDOF structure under the mainshock–aftershock sequence-type ground motions

Structures located in seismically active regions may be subjected to mainshock–aftershock sequence-type ground motions, which are characterized by the presence of strong aftershock ground motions after the mainshock and separated by short intervals of time. This manuscript investigates the damage of inelastic SDOF structure with four hysteretic models and three response demand parameters. The relative intensity of the aftershock ground motion to the mainshock ground motion is defined, denoted by PGAas/PGAms. The aftershock ground motions are scaled to have different levels of PGAas/PGAms. The results indicate that the effect of aftershock on ductility demand is slight for PGAas/PGAms≤0.5 and can be ignored in engineering practice. The aftershock ground motion has more significant influence on the normalized hysteretic energy and damage index than on ductility demand. The effect of aftershock ground motion with larger PGAas/PGAms on the response demand is generally more obvious for non-degrading system than for degrading system.

Scaling Transition in Earthquake Sources: A Possible Link Between Seismic and Laboratory Measurements

We estimate the corner frequencies of 20 crustal seismic events from mainshock–aftershock sequences in different tectonic environments (mainshocks 5.7 < M W < 7.6) using the well-established seismic coda ratio technique (Mayeda et al. in Geophys Res Lett 34:L11303, 2007; Mayeda and Malagnini in Geophys Res Lett, 2010), which provides optimal stability and does not require path or site corrections. For each sequence, we assumed the Brune source model and estimated all the events’ corner frequencies and associated apparent stresses following the MDAC spectral formulation of Walter and Taylor (A revised magnitude and distance amplitude correction (MDAC2) procedure for regional seismic discriminants, 2001), which allows for the possibility of non-self-similar source scaling. Within each sequence, we observe a systematic deviation from the self-similar $$ M_{0} \propto \mathop f\nolimits_{\text{c}}^{ - 3} $$ line, all data being rather compatible with $$ M_{0} \propto \mathop f\nolimits_{\text{c}}^{ - (3 + \varepsilon )} $$ , where e > 0 (Kanamori and Rivera in Bull Seismol Soc Am 94:314–319, 2004). The deviation from a strict self-similar behavior within each earthquake sequence of our collection is indicated by a systematic increase in the estimated average static stress drop and apparent stress with increasing seismic moment (moment magnitude). Our favored physical interpretation for the increased apparent stress with earthquake size is a progressive frictional weakening for increasing seismic slip, in agreement with recent results obtained in laboratory experiments performed on state-of-the-art apparatuses at slip rates of the order of 1 m/s or larger. At smaller magnitudes (M W < 5.5), the overall data set is characterized by a variability in apparent stress of almost three orders of magnitude, mostly from the scatter observed in strike-slip sequences. Larger events (M W > 5.5) show much less variability: about one order of magnitude. It appears that the apparent stress (and static stress drop) does not grow indefinitely at larger magnitudes: for example, in the case of the Chi–Chi sequence (the best sampled sequence between M W 5 and 6.5), some roughly constant stress parameters characterize earthquakes larger than M W ~ 5.5. A representative fault slip for M W 5.5 is a few tens of centimeters (e.g., Ide and Takeo in J Geophys Res 102:27379–27391, 1997), which corresponds to the slip amount at which effective lubrication is observed, according to recent laboratory friction experiments performed at seismic slip velocities (V ~ 1 m/s) and normal stresses representative of crustal depths (Di Toro et al. in Nature in press, 2011, and references therein). If the observed deviation from self-similar scaling is explained in terms of an asymptotic increase in apparent stress (Malagnini et al. in Pure Appl Geophys, 2014, this volume), which is directly related to dynamic stress drop on the fault, one interpretation is that for a seismic slip of a few tens of centimeters (M W ~ 5.5) or larger, a fully lubricated frictional state may be asymptotically approached.

Fault plane of the 1964 Niigata earthquake, Japan, derived from relocation of the mainshock and aftershocks by using the modified joint hypocenter determination and grid search methods

We relocated the mainshock and immediate aftershocks of the 1964 Niigata earthquake (Ms 7.5) using both the modified joint hypocenter determination (MJHD), and the grid search (GS), methods to obtain their hypocenters and identify the fault plane of the mainshock. We used P-wave arrival times at 13 Japanese stations reported by the Japan Meteorological Agency (JMA). Firstly, we performed the MJHD method to select readings with high accuracies and to obtain station corrections that related to the lateral heterogeneity of the Earth’s structure. Secondly, we performed the GS method by using only selected readings and assuming the station corrections. Based on the relocated hypocenters, we found that the mainshock and immediate aftershocks occurred along a plane striking N15°E and dipping 11° eastward. This result strongly suggests that one of the nodal planes of the mainshock, for which the strike and dip are N9°E and 34°, respectively, is preferable as the fault plane. When global stations are also used for relocations, the results confirm that the fault plane of the 1964 Niigata earthquake dips gently eastward. Therefore, we concluded that the fault plane of the 1964 Niigata earthquake is gently eastward-dipping.

Aftershock Statistics of Major Subduction Earthquakes

Abstract Large megathrust subduction earthquakes generate prolific aftershock sequences which last over an extended period of time and affect wide spatial areas. Among those aftershocks, the largest ones can cause additional damage and pose significant risks to population and infrastructure. Therefore, modeling aftershock sequences of large subduction earthquakes is of considerable importance for seismic‐hazard assessment and earthquake risk mitigation. It can also play a prominent role in the ground‐shaking modeling of major mainshock–aftershock sequences. In this work, we analyze statistical properties of aftershock sequences of large subduction earthquakes worldwide which occurred from 1973 to present, including recent catastrophic events in Sumatra, Chile, and Japan. We use information provided in the National Earthquake Information Center (NEIC) catalog to extract 70 aftershock sequences generated by mainshocks of M 7.0 and above. We construct their temporal decay rates and magnitude–frequency statistics. To model their temporal behavior, we estimate the parameters of the modified Omori law. In the magnitude domain, we model the frequency–magnitude statistics using the Gutenberg–Richter scaling relationship. We also analyze statistically the difference between the magnitude of the mainshock and the corresponding largest aftershock in the sequence and discuss this in terms of Bath’s law. One of the main goals of this work is to investigate the variation in parameter values of the above empirical laws with respect to the magnitude of the mainshock. Our main finding indicates that most parameters do not depend on the magnitude of the mainshock. However, they show some variation in values across different subduction settings. Online Material: The location and spatial information for elliptical regions of aftershock zones.

Remote Triggering Not Evident Near Epicenters of Impending Great Earthquakes

Recently, there have been numerous great ( M w≥8), devastating earthquakes, with a rate in the last seven years that is 260% of the average rate during the 111‐year seismological history. Each great earthquake presents an opportunity to study a major fault at the very beginning and end of the inferred seismic cycle. In this work, we use these events as both targets and sources to probe susceptibility to dynamic triggering in the epicentral region before and after a large earthquake. This study also carefully addresses the possibility that large earthquakes interact in a cascade of remotely triggered sequences that culminates in further large earthquakes. We seek evidence of triggering associated with the 16 great M w≥8 events that occurred between 1998 and 2011, using regional and global earthquake catalogs to measure changes in interevent time statistics. Statistical significance is calculated with respect to a nonstationary reference model that includes mainshock–aftershock clustering. We find limited evidence that a few great earthquakes triggered an increase in seismicity at the site of the next great earthquake in the sequence. However, this evidence is not corroborated by all statistical tests nor all earthquake catalogs. Systematic triggered rate changes in the years to decades before each great earthquake are less than 19% at the 95% confidence level, too small to explain the observed rate increase. The catalogs are insufficient for the purpose of resolving more moderate triggering expected from previous studies. We calculate that an improvement in completeness magnitude from 3.7 to 3.5 could resolve the expected triggering signal in the International Seismic Center (ISC) catalog taken as a whole, but an improvement to M 2.0 would be needed to consistently resolve triggering on a regional basis.

Damage evaluation of concrete gravity dams under mainshock–aftershock seismic sequences

A large mainshock may trigger numerous aftershocks within a short period, and large aftershocks have the potential to cause additional cumulative damage to structures. This paper investigates the effects and potential of aftershocks on the accumulated damage of concrete gravity dams. For that purpose, 30 as-recorded mainshock–aftershock seismic sequences are considered in this study, and a typical two-dimensional gravity dam model subjected to the selected as-recorded seismic sequences is modeled. A Concrete Damaged Plasticity (CDP) model including the strain hardening or softening behavior is selected for the concrete material. This model is used to evaluate the nonlinear dynamic response and the seismic damage process of Koyna dam under mainshock–aftershock seismic sequences. According to the characteristics of the cracking damage development, the local and global damage indices are both established to study the influence of strong aftershocks on the cumulative damage of concrete gravity dams. From the results of this investigation, it is found that the as-recorded sequences of ground motions have a significant effect on the accumulated damage and on the design of concrete gravity dams.

Distribution of Earthquake Cluster Sizes in the Western United States and in Japan

Earthquakes occur in clusters, which classically are described as foreshock–mainshock–aftershock sequences or swarms. In this paper, every earthquake in a seismicity catalog is assigned to a cluster if it is separated from at least one other event in the cluster by less than Δ t in time and less than Δ r in space. The minimum cluster size is one earthquake. For catalogs that are complete to small magnitudes, this approach is successful in capturing the full spatial extent of an extensive cluster even for Δ r much smaller than the actual cluster dimension. The declustered catalogs are much closer to Poissonian distribution than the originals. This was applied to seismicity catalogs for Japan, Southern California, and Nevada. Cluster sizes measured by the number of earthquakes in the cluster exhibit an approximate power‐law frequency distribution. An upper bound to cluster durations is proportional to K 0.5, where K is the number of earthquakes in the cluster. This paper demonstrates an analytical approach suitable for selecting values of Δ t and Δ r that are appropriate for the earthquake catalog.

Damage spectra for the mainshock–aftershock sequence-type ground motions

Strong aftershocks have the potential to increase the damage state of the structures due to the damage accumulation. This paper investigates the damage spectra for the mainshock–aftershock sequence-type ground motions with Park–Ang damage index. A method of simulating the mainshock–aftershock sequence-type ground motions is proposed based on the modified form of Bath's law and NGA ground motion prediction equation. The damage spectra are computed using the recorded and simulated sequence-type ground motions, and the effects of period of vibration, strength reduction factor, site condition, seismic sequence, damping ratio and post-yield stiffness on damage spectra are studied statistically. The results indicate that the effect of aftershock on structural damage is significant and recorded sequence-type ground motions may underestimate the damage of long-period structures due to the incompleteness of dataset. A simplified equation is proposed to facilitate the application of damage spectra in the seismic practice for mainshock–aftershock sequence-type ground motions.

Nonlinear Response Potential of Mainshock-Aftershock Sequences from Japanese Earthquakes

Abstract A large mainshock triggers numerous aftershocks, exposing evacuees and residents to significant risk and hampering building reoccupation and restoration activities in a post‐disaster situation. It is thus important to take into account the seismic effects of mainshock–aftershock (MSAS) sequences, not just those of mainshocks. To assess the nonlinear damage potential caused by aftershocks, this study investigates the effects of aftershocks on peak ductility demand of inelastic single‐degree‐of‐freedom systems using real as well as artificial MSAS sequences. The real sequences are constructed from the K‐NET and KiK‐net databases for Japanese earthquakes. Comparison of peak ductility demand due to real mainshock events alone and real MSAS sequences renders empirical assessment of the aftershock impact on peak ductility demand. Moreover, time‐history data of artificial MSAS sequences are generated based on the generalized Omori’s law and suitable aftershock record selection procedure that takes into account key characteristics of aftershock records (magnitude, distance, and site classification). The validity of artificially generated MSAS sequences is evaluated by comparing probabilistic characteristics of peak ductility demand caused by artificial sequences with those caused by real sequences. The results indicate that peak ductility demands from real and artificial sequences are similar; thus, artificial sequences can be substituted for real sequences. Such calibration is particularly useful when an extensive data set of real MSAS sequences is not available. Online Material: Table of mainshock–aftershock sequences from Japanese earthquakes.

Underwater acoustic records from the March 2009 eruption of Hunga Ha'apai-Hunga Tonga volcano in the Kingdom of Tonga

A network of autonomous underwater hydrophones is used to monitor acoustic activity associated with Hunga Ha'apai-Hunga Tonga volcano during a period of 15months. The data provide a continuous record spanning a surtseyan eruption (VEI 2) in March of 2009, which input ~1013J of acoustic energy into the ocean soundscape. In the months before the eruption, the volcano can be identified as an intermittent source of ambient noise. The period of seismic unrest that precedes the eruption begins at 15:11 UTC on 16 March (04:11 LT on 17 March), approximately 7h before the first satellite confirmation of eruptive activity and 14h before the first eyewitness reports. The initial seismic activity, which includes a single 4.8mb event at 15:25, evolves as a typical foreshock–mainshock–aftershock sequence. By 15:38, however, the rate of small earthquakes begins to increase, marking the onset of the seismic swarm. The period of highest-amplitude acoustic energy release between 16:40 and ~17:10 is interpreted to mark the opening of the volcanic conduit. By 19:00 on 16 March, the acoustic signature of the volcano is marked by a continuous wide-band (1–20Hz) noise and a set of transient very-broadband (1–125Hz) explosion signals. This activity is characteristic of the main surtseyan phase of the eruption. Both the intensity of explosions and the amplitude of the lower frequency wide-band noise decay through time, and eruptive activity likely ends at ~09:00 on 19 March, ~2.7days after the initiation of seismic activity. At this time the continuous low frequency noise decays to near background levels and signal coherence drops suddenly. Low-level acoustic unrest persists through June of 2009, after which the volcano becomes acoustically dormant during the remaining ten months of monitoring. The analysis of volcano-acoustic signals associated with Hunga Ha'apai-Hunga Tonga volcano highlights the potential role of regional hydroacoustic monitoring in assessing volcanic hazards in arc settings.

Effects of aftershocks on peak ductility demand due to strong ground motion records from shallow crustal earthquakes

SUMMARYAftershocks induced by a large mainshock can cause additional damage to structures and infrastructure, hampering building reoccupation and restoration activities in a post‐disaster situation. To assess the nonlinear damage potential due to aftershocks, this study investigates the effects of aftershocks by using real as well as artificially generated mainshock–aftershock sequences. The real mainshock–aftershock sequences are constructed from the Pacific Earthquake Engineering Research Center—Next Generation Attenuation database for worldwide shallow crustal earthquakes; however, they are deemed to be incomplete because of missing records. To supplement incomplete real dataset, artificial sequences are generated on the basis of the generalized Omori's law, and a suitable aftershock record selection procedure is then devised to simulate time‐series data for mainshock–aftershock sequences. The results from nonlinear dynamic analysis of inelastic single‐degree‐of‐freedom systems using real and artificial sequences indicate that the incremental effects of aftershocks on peak ductility demand using the real sequences are relatively minor and that peak ductility demand estimates based on the generalized Omori's law are greater, particularly in the upper tail, than those for the real sequences. The results based on the generalized Omori's law also suggest that the aftershock effects based on the real sequences might underestimate the aftershock impact because of the incompleteness of the real dataset. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Seismicity, active faults, stress patterns, and rupture processes in the Hualien region, Taiwan, investigated using the 1990 Hualien earthquake sequence

The Hualien mainshock–aftershock sequence from December 1990 to January 1991 was used to investigate seismicity, active faults, stress patterns, and complex rupture processes in the Hualien region of eastern Taiwan. A combined data set, compiled from observations by the regional seismic network (Taiwan Telemetered Seismograph Network, TTSN) and a dense aftershock monitoring array (Hualien Temporary Seismic Network, HTSN), provided high-quality seismic data. Using these data, earthquake source parameters could be well constrained by the joint hypocenter determination (JHD) and double-difference relocation methods. These two relocation methods presented similar results and produced improved estimations of earthquake location. Station corrections determined by JHD showed significant lateral velocity variations across the Yuehmei fault. The relocated seismicity indicated that a previously known active fault, the Chimei fault, extends to offshore. Earthquake relocations also revealed a northwest dipping seismicity probably related to the Chimei fault. In addition, three sets of focal mechanism solutions are presented and their discrepancies are attributed to the complex rupture process. At first, earthquakes were initiated by thrust motions. Later, the rupture pattern changed to strike–slip motions. Variations in slip may reflect the fault geometry or rupture boundary conditions in the region. The observed stress patterns are considered to relate not only to the regional collision and mountain-building processes, but also to active faults in local structures.

Assessing historical rate changes in global tsunami occurrence

SUMMARY The global catalogue of tsunami events is examined to determine if transient variations in tsunami rates are consistent with a Poisson process commonly assumed for tsunami hazard assessments. The primary data analyzed are tsunamis with maximum sizes >1m. The record of these tsunamis appears to be complete since approximately 1890. A secondary data set of tsunamis >0.1m is also analyzed that appears to be complete since approximately 1960. Various kernel density estimates used to determine the rate distribution with time indicate a prominent rate change in global tsunamis during the mid-1990s. Less prominent rate changes occur in the early- and mid-20th century. To determine whether these rate fluctuations are anomalous, the distribution of annual event numbers for the tsunami catalogue is compared to Poisson and negative binomial distributions, the latter of which includes the effects of temporal clustering. Compared to a Poisson distribution, the negative binomial distribution model provides a consistent fit to tsunami event numbers for the >1m data set, but the Poisson null hypothesis cannot be falsified for the shorter duration >0.1m data set. Temporal clustering of tsunami sources is also indicated by the distribution of interevent times for both data sets. Tsunami event clusters consist only of two to four events, in contrast to protracted sequences of earthquakes that make up foreshock–main shock–aftershock sequences. From past studies of seismicity, it is likely that there is a physical triggering mechanism responsible for events within the tsunami source ‘mini-clusters’. In conclusion, prominent transient rate increases in the occurrence of global tsunamis appear to be caused by temporal grouping of geographically distinct mini-clusters, in addition to the random preferential location of global M >7 earthquakes along offshore fault zones.