Interplate Events Research Articles

1911年に喜界島近海で発生した巨大地震の震源位置の再評価

The hypocenter of the 1911 great earthquake occurred around Kikai-jima, Japan is thought to be 28.0°N, 130.0°E (about 30km southward from Kikai-jima) and 100km depth in general. This implies that the 1911 event occurred within the subducting slab at the Ryukyu trench. However, it is not clearly known what kind of seismic data were used to determine the epicenter and how the depth of hypocenter was estimated quantitatively. In the present study, we determine hypocenter of this event by using selected S-P time data not only reported ones but also re-measured ones from smoked-paper seismograms. The hypocenter is revealed to be 28.7°N, 130.6°E and 60km in depth, about 60km northeastward from Kikai-jima. The focal depth is, however, considered to be about 10km instead of 60km by referring to the present hypocenter distribution because of the low reliability of focal depth in the relocation. Recent epicenter distribution shows the low seismic activity area on the plate boundary northeast off Kikai-jima which extends about 70km length parallel to the Ryukyu trench axis. The 1911 great earthquake seems to be occurred at the shallower rim of this low seismic activity area. If the low seismic activity is the result of strong coupling on plate boundary, this area might be the asperity of the 1911 great earthquake. Tsunami heights accompanied with this event are reported recently to be more than 5m at Kikai-jima and Amami-oshima, which supports the present study that the 1911 great earthquake is the interplate event rather than the intraslab one.

Simulation of Ground Motion in the Zagros Region of Iran Using the Specific Barrier Model and the Stochastic Method

Abstract Physically based ground‐motion prediction equations for soil and rock sites in the Zagros region have been developed based on the specific barrier model (SBM) used within the context of the stochastic model. Instead of direct time‐domain simulation, random vibration theory was used to estimate measures of peak motion in terms of the pseudospectral velocity of anelastic harmonic oscillator with 5% viscous damping. To avoid the uncertainties, calibration of the source model uses a database of carefully selected strong motion data without ambiguity about the site condition. Therefore, only rock sites are selected for determining source parameters. Also, to avoid any inconsistencies caused by magnitude conversion formulas, we restricted the dataset only to events with available moment magnitudes. Regression analysis is performed using the random effects model that considers both interevent and intraevent variabilities to effectively deal with the problem of an unequal number of records from different earthquakes. No sign of self‐similarity breakdown is observed between the source radius and its seismic moment. The local and global stress drops derived for the Zagros region (39 and 116 bars, respectively) are more consistent with the values obtained by other authors for an interplate regime than the values for an intraplate region. However, from the viewpoint of source heterogeneity (as the ratio of the stress drops is an indicator of the complexity of the source and heterogeneity of slip on the fault) the Zagros events, which have a stress‐drop ratio of about three are more homogeneous than other interplate events. Stochastic simulations are then implemented to predict peak ground motion and response spectra parameters for rock and soil site conditions. Online Material: Catalog tables of earthquakes and attenuation tables.

Sources of shaking and flooding during the Tohoku-Oki earthquake: A mixture of rupture styles

Modeling strong ground motions from great subduction zone earthquakes is one of the great challenges of computational seismology. To separate the rupture characteristics from complexities caused by 3D sub-surface geology requires an extraordinary data set such as provided by the recent Mw9.0 Tohoku-Oki earthquake. Here we combine deterministic inversion and dynamically guided forward simulation methods to model over one thousand high-rate GPS and strong motion observations from 0 to 0.25Hz across the entire Honshu Island. Our results display distinct styles of rupture with a deeper generic interplate event (∼Mw8.5) transitioning to a shallow tsunamigenic earthquake (∼Mw9.0) at about 25km depth in a process driven by a strong dynamic weakening mechanism, possibly thermal pressurization. This source model predicts many important features of the broad set of seismic, geodetic and seafloor observations providing a major advance in our understanding of such great natural hazards.

El terremoto del 4 de marzo de 1924 (Ms 7,0): ¿un gran temblor interplaca relacionado al límite incipiente entre la placa Caribe y la microplaca de Panamá?

The seismogenic source, the intensity distribution and the coseismic effects are investigated for theMarch 4, 1924 (Ms = 7.0) earthquake, which occurred in the central-pacific forearc of Costa Rica. A neotectonicstudy allows to define the characteristics of the cortical faulting related to this destructive earthquake. From acareful study of macroseismic data obtained from different bibliographic sources, the intensity distribution of thisearthquake is defined. Maximum intensities between VIII and IX in the Mercalli Modified scale were defined inthe mesoseismal area, which includes the towns of Orotina, San Mateo, San Ramon and Esparza. The earthquakeproduce the collapse and damage of a great number of houses and buildings, strong damage to the railroad, greatnumber of landslides, rock falls, earth cracks and liquefaction. The earthquake aftershock sequence includesvarious events with magnitudes Ms between 5.0 and 6.4. The main shock triggered seismic activity in differentfaults along the interior region of the country, and eruptions at the Irazu and Rincon de la Vieja volcanoes.With criteria such as an epicentral location using regional seismographic stations, the trend of the S wave andthe S-P from aftershocks, the mesoseismal area, and a possible earthquake rupture, it is suggested that theseismogenic source of the earthquake was the Tarcoles fault, although it is believed that the Bijagual faultsystem also contribute to the rupture process. These are transcurrent faults trending NE to N, that belong to adeformation zone located between the Caribbean plate and the Panama microplate. For this reason, the March4, 1924 earthquake is considered an interplate event related to the incipient limit between those tectonic plates.

Intraplate Response to the Great 2004 Sumatra-Andaman Earthquake: A Study from the Andaman Segment

Abstract The 1300-km rupture of the 2004 interplate earthquake terminated at around 15° N, in the northernmost segment of the Andaman–Nicobar subduction zone. This part of the plate boundary is noted for its generally lower level seismicity, compared with the southern segments. Based on the Global Centroid Moment Tensor (CMT) and National Earthquake Information Center (NEIC) data, most of the earthquakes of M w ≥4.5 prior to 2004 were associated with the Andaman Spreading Ridge (ASR), and a few events were located within the forearc basin. The 2004 event was followed by an upward migration of hypocenters along the subducting plate, and the Andaman segment experienced a surge of aftershock activity. The continuing extensional faulting events, including the most recent earthquake (10 August 2009; M w 7.5) in the northern end of the 2004 rupture, suggest the reduction of compressional strain associated with the interplate event. The style of faulting of the intraplate events before and after a great plate boundary earthquake reflects the relative influences of the plate-driving forces. Here we discuss the pattern of earthquakes in the Andaman segment before and after the 2004 event to appraise the spatial and temporal relation between large interplate thrust events and intraplate deformation. This study suggests that faulting mechanisms in the outer-ridge and outer-rise regions could be indicative of the maturity of interplate seismic cycles.

Seismic swarm associated with the 2008 eruption of Kasatochi Volcano, Alaska: Earthquake locations and source parameters

[1] An energetic seismic swarm accompanied an eruption of Kasatochi Volcano in the central Aleutian volcanic arc in August of 2008. In retrospect, the first earthquakes in the swarm were detected about 1 month prior to the eruption onset. Activity in the swarm quickly intensified less than 48 h prior to the first large explosion and subsequently subsided with decline of eruptive activity. The largest earthquake measured as moment magnitude 5.8, and a dozen additional earthquakes were larger than magnitude 4. The swarm exhibited both tectonic and volcanic characteristics. Its shear failure earthquake features were b value = 0.9, most earthquakes with impulsive P and S arrivals and higher-frequency content, and earthquake faulting parameters consistent with regional tectonic stresses. Its volcanic or fluid-influenced seismicity features were volcanic tremor, large CLVD components in moment tensor solutions, and increasing magnitudes with time. Earthquake location tests suggest that the earthquakes occurred in a distributed volume elongated in the NS direction either directly under the volcano or within 5–10 km south of it. Following the MW 5.8 event, earthquakes occurred in a new crustal volume slightly east and north of the previous earthquakes. The central Aleutian Arc is a tectonically active region with seismicity occurring in the crusts of the Pacific and North American plates in addition to interplate events. We postulate that the Kasatochi seismic swarm was a manifestation of the complex interaction of tectonic and magmatic processes in the Earth's crust. Although magmatic intrusion triggered the earthquakes in the swarm, the earthquakes failed in context of the regional stress field.

Slip distribution of the 2003 Tokachi‐oki Mw 8.1 earthquake from joint inversion of tsunami waveforms and geodetic data

We study the 2003 Mw 8.1 Tokachi‐oki earthquake, a great interplate event that occurred along the southwestern Kuril Trench and generated a significant tsunami. To determine the earthquake slip distribution, we perform the first joint inversion of tsunami waveforms measured by tide gauges and of coseismic displacement measured both by GPS stations and three ocean bottom pressure gauges (PG) for this event. The resolution of the different data sets on the slip distribution is assessed by means of several checkerboard tests. Results show that tsunami data constrain the slip distribution offshore, whereas GPS data constrain the slip distribution in the onshore zone. The three PG data only coarsely constrain the offshore slip, indicating that denser networks should be installed close to subduction zones. Combining the three data sets significantly improves the inversion results. Joint inversion of the 2003 Tokachi‐oki earthquake data leads to maximum slip values (∼6 m) confined at depths greater than ∼25 km, between 30 and 80 km northwest of the hypocenter, with a patch of slip (3 m) in the deepest part of the source (∼50 km depth). Slip values are very low (≤1 m) updip from the hypocenter. Furthermore, the rupture does not extend on the plate interface off Akkeshi. As a significant back slip amount (∼4 m) has accumulated there since the last 1952 earthquake, this segment could rupture during the next large interplate event along the Kuril Trench.

Seismological evidence for shallow thrusting north of the Timor trough

Summary. The eastern Sunda arc in Indonesia is the site of subduction of the continental crust of Australia and its shallow seismicity is manifested by infrequent, large (Ms≥ 7) earthquakes that occur near the volcanic arc within the upper plate rather than as interplate events beneath the forearc. In the same region, volcanic activity has ceased and small shallow earthquakes are rare. We examine two of the larger earthquakes: the events of 1962 May 15 (Ms= 7.2) and 1977 August 27 (Ms= 7.0). The 1977 event occurred north of central Timor and the inversion of long-period P- and SH-waves indicates a nearly pure thrust mechanism at a depth of 10 km below sea-level and a seismic moment of 4.1 × 1026 dyne cm. The P-wave nodal planes for this solution strike roughly parallel to the mapped trace of a thrust zone (the Wetar thrust zone) at the southern margin of the backarc basin and the south nodal dipping plane is consistent with the sense of thrusting on this thrust zone. A broad zone of compressional deformation is evident in seismic reflection profiles over the epicentral region of the 1977 event and gravity data suggest that a small amount of either isostatically uncompensated crustal thickening or flexure of the underthrusting plate has occurred. The 1962 earthquake occurred approximately 300 km north-east of the 1977 earthquake and near the eastern end of the volcanically quiet section of the Banda arc. The similarity of the few available waveforms to those at the same stations for the 1977 event suggests a similar source mechanism. Modelling of Pdif waveforms for the 1962 event indicates a depth of less than 40 km and a preferred centroidal depth of 20 km, for which a seismic moment of 5 × 1026 dyne cm is estimated. These fault plane solutions indicate that most of the seismic energy released by the collision between Australia and the Sunda arc is generated by faulting within the overriding plate and we infer that a major change in the plate geometry is imminent. We suggest that most of the relative convergence between the plates is accommodated by thrusting of the marginal basin (the Banda Sea) southward beneath the arc and that the present geometry represents the initial stage of the reversal of the arc's polarity.

What controls interplate coupling?: Evidence for abrupt change in coupling across a border between two overlying plates in the NE Japan subduction zone

Interplate coupling in the southernmost extent of the northeast Japan subduction zone (east off-Kanto, Japan) is investigated in detail using a large data set of small repeating earthquakes. In this region of the trench-trench-trench type triple junction, the subducting Pacific plate is in contact with two different overlying plates (the North American and Philippine Sea plates). The border between the two overlying plates extending northwestward from the triple junction is identified along the upper surface of the Pacific plate based on the slip vectors of many interplate events including small repeating earthquakes. The interplate coupling coefficient estimated from the cumulative slip of small repeating earthquakes for the period from 1993 to 2007 reveals a distinct change from south (ca. 0.3) to north (ca. 0.7) across the border. This border also corresponds to the southern limit of the source areas of M>7 interplate earthquakes in the last 80 years along the Japan trench, again indicating the stronger coupling to the north. We also investigate hypocenter distribution and seismic velocity structure along the plate boundary from a large number of travel-time data obtained from the nationwide seismograph network. The results reveal a distinct low-velocity zone just above the Pacific plate and low seismicity along the plate boundary in the region overlain by the Philippine Sea plate, whereas there are no low-velocity zones and the seismicity is high in the region overlain by the North American plate. These observations imply that the geological difference of overlying plate controls large-scale coupling at the plate interface.

Continuous Long-Term Seafloor Pressure Observation for Detecting Slow-Slip Interplate Events in Miyagi-Oki on the Landward Japan Trench Slope

In starting continuous seafloor pressure observation in the Miyagi Oki region, where a M 7.5 interplate earthquake is expected within the next three decades, we distributed pressure stations to enable us to determine slow-slip event size and location along the top of the subducting Pacific Plate as precisely as possible. Numerical simulation indicated that ∼ 2 cm uplift is expected above a thrust faulting event of Mw=6.8 at a depth of ∼ 40 km. Combining continuous onshore global positioning system (GPS) time series and seafloor pressure observation is expected to significantly improve spatial resolution of interplate slip distribution over inversion results using only GPS data alone. Pressure change due to the predicted amount of seafloor movement is comparable to or slightly less than that caused by dynamic ocean processes. Seafloor pressure data from long-term continuous observation using the spatially dense array provided by our study is invaluable for understanding spatiotemporal patterns in seafloor pressure variations caused by two different origins – the lithosphere and the hydro-atmosphere.

Intraplate and interplate earthquakes in Chilean subduction zone: A theoretical and observational comparison

During the last decade, efforts to improve our knowledge of great Chilean earthquakes have shown that not all of the major destructive events have occurred in the contact between the Nazca and south American plates (interplate earthquakes). Waveform analysis of records from the Ms = 8.0, 1950 Antofagasta and the Ms = 7.8, 1939 Chillán earthquakes have shown that these large events are tensional, rupturing along nearly vertical, intermediate depth, fault planes within the subducting slab (intraplate events). Previous studies found that other earthquakes in Chile, like Santiago 1945 (Ms = 7.1), La Ligua 1965 (Ms = 7.1), Tal-tal 1965 (Ms = 6.9), Tocopilla 1970 (Ms = 6.5), and Tarapacá 2005 (Mw = 7.8) were also of tensional type. In the present work, we analyze theoretical and observational evidence comparing interplate and intraplate earthquakes. We found clear differences in the source characteristics between these two kinds of events, with intermediate depth, intraplate earthquakes presenting larger corner frequencies and greater seismic energy release than interplate events (for a given seismic moment). This is also reflected in the higher averaged apparent stress drop for intraplate earthquakes ( σ a ∼ 90 bar) compared to interplate events ( σ a ∼ 30 bar). From theoretical computations, we found that the rupture velocity has a minor effect on the resulting displacements; while directivity and changes in the static stress drop produced notable (and similar) results. We believe that the differences found in the data might be due to changes in the static stress drop, the effect of directivity, or both. Regardless of the cause of the observed differences in the apparent stress drop, these results should be taken into consideration into the assessment of the Seismic Hazard in Chile.

Anomalous seismicity in the lower crust of the Santiago Basin, Chile

In central Chile, intraplate, crustal seismicity is mostly constrained to the upper 20 km; nevertheless, sparse observations of earthquakes have been reported deeper, without a definitive explanation. In the lower crust of the Santiago Basin, located within a compressional, subduction environment, we have observed a series of events organized in two clusters. This work presents a detailed study of these earthquakes, recorded from November 2002 to April 2005, with depths from 28 to 36 km and magnitudes ranging from 2.8 to 4.3. Using a local network (composed of vertical short-periods, broadbands, and accelerometers) we performed a time and spectral waveform analysis to determine their exact location and certain source parameters: seismic moment, corner frequency, and seismic energy. We found that each cluster presents extremely well correlated waveforms, indicating that each event in the cluster ruptures the same patch in the same fashion. Even more, we determined that representative events show a large apparent stress drop, releasing six times more energy than interplate events, for a given seismic moment. The most plausible interpretation seems to be a decollement between the crust and the mantle, where few (but strong) asperities are located in a broader region that continuously creeps. We believe that this work will help improve our understanding of the processes involved in subduction zones, as well as the assessment of the Seismic Hazard for the Santiago Metropolitan Area.

Interpretation of the 1993, 2001, and 2002 Guam Earthquakes as Intraslab Events by a Simultaneous Relocation of the Mainshocks, Aftershocks, and Background Earthquakes

It has been unclear whether the 1993, 2001, and 2002 Guam earthquakes ( M w 7.8, 7.0, and 7.1, respectively) in the southern Mariana subduction zone were interplate events or intraplate ones within the Pacific slab. We relocated their mainshocks, aftershocks, and numerous long-term background earthquakes simultaneously with high relative accuracy using the modified joint hypocenter determination (MJHD) method. As a result, their mainshocks and three-day aftershocks are clearly distributed within the background slab seismicity. Therefore, we conclude that the 1993, 2001, and 2002 Guam earthquakes were not interplate events, which scarcely occur in the Mariana subduction zone, but intraslab earthquakes within the subducted Pacific plate. The 1993 fault plane is inferred to be the near-horizontal nodal plane of the mechanism solution, though the 2001 and 2002 fault planes remain unknown. The relocation of target events simultaneously with background earthquakes has turned out very effective for discriminating between large interplate and intraslab earthquakes in subduction zones.

Outer rise stress changes related to the subduction of the Juan Fernandez Ridge, central Chile

Although outer rise seismicity is less common than interplate seismicity in subduction zones, a significant level of seismicity has occurred between the Nazca trench and Juan Fernandez Ridge, in central Chile, during the past 20 years. We first study the 9 April 2001 (Mw = 7.0) event and determine its focal mechanism, depth, and source time function by body‐waveform inversion from teleseismic broadband data. The results indicate tensional faulting in the upper part of the mechanical lithosphere. Its strike (41°) is similar to those observed in events down dip of the slab at about 100 km depth, which could indicate that these earthquakes occur in preexisting structures formed at the trench. Compressive outer rise events have also occurred during the 1980s in front of the rupture zone of the 1985 Mw 7.8 Valparaíso Earthquake. To understand their relation with the state of stress of the lithosphere, we construct yield stress envelopes of the oceanic lithosphere, including static and dynamic stresses. Dynamic stresses are due either to slab pull, ridge push, resistive, and drag forces. We explain the sequence of compressive and tensional events by the accumulation of stress prior to 1985 when the subduction is assumed to be locked and after by the unlockage of the subduction by the Valparaíso interplate event. The yield stress envelope analysis enables us to quantify the accumulation of compressive forces before 1985 and the tensional force after.

High‐resolution subduction zone seismicity and velocity structure beneath Ibaraki Prefecture, Japan

We use double‐difference tomography and waveform‐derived cross‐correlation differential times to estimate earthquake locations and P and S wave velocity structure in the subduction zone under Ibaraki and neighboring prefectures of north central Honshu. We find evidence in both earthquake hypocenters and the velocity structure that the Philippine Sea plate, or perhaps a microplate fragment, may be caught between the subducting Pacific plate and overriding Okhotsk plate in this region. In the southern part of the study area, we find a zone of interplate events extending as deep as 60 km, forming a distinct lineation in cross section. Focal mechanisms support the interpretation that these are low‐angle, subduction interface events. We infer that these events probably occur on the interface between the Pacific and Philippine Sea plates rather than between the Pacific and Okhotsk plates. In the upper part of the downgoing Pacific plate, we find a zone of high Vp/Vs ratio (∼1.95), which may be explained by high pore fluid pressures within the subducting crust. At a depth of 60–80 km, this region of high Vp/Vs appears to diffuse into the overlying mantle wedge, possibly indicating the upward release of fluids from the slab.

Fault dynamics of Indian Ocean earthquakes derived from the array analysis of T-waves

The Great Sumatran earthquake of 26 December 2004 generated some of the largest and longest T-waves ever recorded, and certainly the largest T-waves recorded on modern high-dynamic range hydrophones. For example, the recordings at the Diego Garcia hydroacoustic array indicate a T-wave duration of approximately 15 min. Array processing of these signals by many investigators indicates that the T-wave generation started near the earthquake epicenter and propagated northwest along the trench for nearly 600 km. This corresponds to the faulting zone defined by the aftershock distribution. A dataset of T-wave recordings of Indian Ocean earthquakes has been examined, covering the period 2001 to the present, including events on the oceans ridges and trenches, as well as some interplate events. The recordings were made at Diego Garcia, Cape Leeuwin, and Crozet Island. Event sizes range from magnitude 4 to 8. Bearing versus time is derived via intersensor cross correlations with a time resolution of 0.010 s, corresponding to a bearing resolution of 0.4 deg. Examples of events along the Sumatran Trench are shown with extended T-wave source areas, from which estimates of fault length and rupture velocity are made. These studies further demonstrate the utility of T-phases for understanding seismic source dynamics.

Inslab Earthquakes of Central Mexico: Peak Ground-Motion Parameters and Response Spectra

We developed equations to predict pseudoacceleration response spectra (5% damping), peak ground acceleration, and peak ground velocity at free-field rock sites for intermediate-depth, normal-faulting inslab earthquakes of Central Mexico. The data set comprises 16 earthquakes (5.2 ≤ M w ≤ 7.4; 35 ≤ H ≤ 138 km) recorded at local and regional distances ( R ≤ 400 km). It represents a homogeneous catalog with respect to tectonic regime, fault mechanism, and soil class. Our results show larger amplitudes in the epicentral area from inslab events than from interplate events, a consequence of higher stress drops during the former type of earthquakes. Peak ground accelerations from moderate to large ( M w >6.0) inslab events significantly exceed those from interplate events with similar magnitude, reaching almost three times for the largest events. The ground motion due to inslab events, however, decays faster than for the thrust events. Our results are in reasonable agreement with other studies based on Japanese and the worldwide data.

3D Cohesive End-Zone Model for Source Scaling of Strike-Slip Interplate Earthquakes

A 3D, static fracture mechanics model of earthquake rupture that in- corporates cohesive end zones (CEZs), or zones of increased frictional strength, is tested to determine whether it helps to understand the observed scaling behavior of average slip with rupture dimensions for shallow (! 20 km), continental, interplate strike-slip earthquakes. Our new compilation of average source parameters suggests that (1) average slip increases with aspect ratio (along-strike length/down-dip width), although in decreasing proportions for progressively larger aspect ratio ruptures, and (2) a gradual scaling change exists at an aspect ratio of 6. These general trends match the functional form predicted by the CEZ model. Despite these general trends, significant scatter in average slip is apparent among similarly sized ruptures. We test the hypothesis that the CEZs represent strength heterogeneities along the rupture surface that result from velocity-strengthening frictional behavior; and that this het- erogeneity in frictional behavior along the fault is the primary reason for the failure of a universal (constant stress drop) scaling law. CEZ lengths are measured from slip and stress drop distributions determined from published inversions of geophysical data for the 1984 Morgan Hill, 1992 Landers, 1999 Hector Mine, and 1999 I zmit earthquakes, and range from 15 to 40% of the rupture segment lengths. These lengths are an order of magnitude larger than that inferred from characteristics of high-frequency seismic radiation (i.e., f max ). These data indicate that the ratio of average coseismic slip to rupture length decreases in the presence of large CEZs. Measured CEZ lengths, rupture dimensions, and average slip are used to calculate average resolved shear-driving stresses and CEZ shear-yield strengths on the order of 10-30 MPa. In our new model of earthquake rupture, stress drop is predicted to be a small fraction of fault strength and thus supports a partial stress drop model of earthquake rupture for strike-slip interplate events.

Inversion analysis of the 2004 off the Kii peninsula earthquakes using seismic intensity data

Abstract An inversion analysis has been developed to evaluate short-period radiation zones on an earthquake fault plane using seismic intensity data. It is robust and widely applicable even for historical earthquakes without ground motion, tsunami and geodetic data observed by instruments. It was applied to the 2004 off the Kii peninsula earthquakes that occurred in the Philippine Sea plate along the Nankai Trough. The short-period seismic wave radiation zone of each event during the 2004 off the Kii peninsula earthquakes from the inversion analysis was included in epicenter area of the corresponding aftershocks and slipped fault zone derived from the waveform inversion analysis as well. It may be also found that there were slip areas where little short period energy radiated. It also applied to the 1899/3/7 earthquake whose felt area was similar to that of the 2004 off the Kii peninsula earthquakes. However, it was indicated that the short-period radiation zones of the 1899 earthquake was located near the coast of the Kii peninsula and departed from that of the 2004 earthquake. The 1899 event might be an intermediate-depth earthquake in the Philippine Sea plate or a relative small interplate event at the west margin of the Tonankai earthquake source region.

Soil liquefaction during the Arequipa Mw 8.4, June 23, 2001 earthquake, southern coastal Peru

The Arequipa June 23, 2001, earthquake with a moment magnitude of Mw 8.4 struck southern Peru, northern Chile and western Bolivia. This shallow (29 km deep) interplate event, occurring in the coupled zone of the Nazca subduction next to the southeast of the subducting Nazca ridge, triggered very localized but widely outspread soil liquefaction. Although sand blows and lateral spreading of river banks and road bridge abutments were observed 390 km away from the epicenter in the southeast direction (nearing the town of Tacna, close to the Chile border), liquefaction features were only observed in major river valleys and delta and coastal plains in the meizoseismal area. This was strongly controlled by the aridity along the coastal strip of Southern Peru. From the sand blow distribution along the coastal area, a first relationship of isolated sand blow diameter versus epicentral distance for a single event is ever proposed. The most significant outcome from this liquefaction field reconnaissance is that energy propagation during the main June 23, 2001, event is further supported by the distribution and size of the isolated sand blows in the meizoseismal area. The sand blows are larger to the southeast of the epicenter than its northwestern equivalents. This can be stated in other words as well. The area affected by liquefaction to the northwest is less spread out than to the southeast. Implications of these results in future paleoliquefaction investigations for earthquake magnitude and epicentral determinations are extremely important. In cases of highly asymmetrical distribution of liquefaction features such as this one, where rupture propagation tends to be mono-directional, it can be reliably determined an epicentral distance (between earthquake and liquefaction evidence) and an earthquake magnitude only if the largest sand blow is found. Therefore, magnitude estimation using this uneven liquefaction occurrence will surely lead to underrating if only the shortest side of the meizoseismal area is unluckily studied, which can eventually be the only part exhibiting liquefaction evidence, depending on the earthquake location and the distribution of liquefaction-prone environments.