Large Regional Earthquakes Research Articles

Palaeoseismology of the North Anatolian Fault near the Marmara Sea: implications for fault segmentation and seismic hazard

Abstract We conducted palaeoseismic studies along the North Anatolian fault both east and west of the Marmara Sea to evaluate its recent surface rupture history in relation to the well-documented historical record of earthquakes in the region, and to assess the hazard of this major fault to the city of Istanbul, one of the largest cities in the Middle East. Across the 1912 rupture of the Ganos strand of the North Anatolian fault west of the Marmara Sea, we excavated 26 trenches to resolve slip and constrain the earthquake history on a channel–fan complex that crosses the fault at a high angle. A distinctive, well-sorted fine sand channel that served as a marker unit was exposed in 21 trenches totaling over 300 m in length. Isopach mapping shows that the sand is channelized north of the fault, and flowed as an overflow fan complex across a broad fault scarp to the south. Realignment of the feeder channel thalweg to the fan apex required about 9±1 m of reconstruction. Study of the rupture history in several exposures demonstrates that this displacement occurred as two large events. Analysis of radiocarbon dates places the age of the sand channel as post ad 1655, so we attribute the two surface ruptures to the large regional earthquakes of 1766 and 1912. If each was similar in size, then about 4–5 m of slip can be attributed to each event, consistent with that reported for 1912 farther east. We also found evidence for two additional surface ruptures after about ad 900, which probably correspond to the large regional earthquakes of 1063 and 1344 (or 1354). These observations suggest fairly periodic occurrence of large earthquakes (RI= c . 283±113 years) for the past millennium, and a rate of c . 16 mm/a if all events experienced similar slip. We excavated six trenches at two sites along the 1999 Izmit rupture to study the past earthquake history along that segment of the North Anatolian fault. One site, located in the township of Köseköy east of Izmit, revealed evidence for three surface ruptures (including 1999) during the past 400 years. The other trench was sited in an Ottoman canal that was excavated (but never completed) in 1591. There is evidence for three large surface rupturing events in the upper 2 m of alluvial fill within the canal at that site, located only a few kilometres from the Köseköy site. One of the past events is almost certainly the large earthquake of 1719, for which historical descriptions of damage are nearly identical to that of 1999. Other earthquakes that could plausibly be attributed to the other recognized rupture of the Izmit segment are the 1754, 1878 or 1894 events, all of which produced damage in the region and for which the source faults are poorly known. Our palaeoseismic observations suggest that the Izmit segment of the North Anatolia fault ruptures every one and a half centuries or so, consistent with the historical record for the region, although the time between ruptures may be as short as 35 years if 1754 broke the Izmit segment. Release of about 4 m of seismic slip both west and east of the Marmara Sea this past century (1912, 1999) support the contention that Istanbul is at high risk from a pending large earthquake. In that historical records suggest that the last large central Marmara Sea event occurred in 1766, there may be a similar 4 m of accumulated strain across the Marmara basin segment of the North Anatolian fault.

Precursors of large Kamchatka earthquakes based on monitoring of seismic noise

This paper presents results from a monitoring study in high frequency seismic noise in Kamchatka during 1992–2006 and reports their use for predicting large regional earthquakes (M ≥ 6.0) in an epicentral distance range Δ within 400 km. The prediction is based on an original method using earth tides as the standard external excitation to study patterns of seismic emission. The method as used at present is described in detail. Guidelines are delineated for future refinement of the prediction method.

Extending Earthquakes' Reach Through Cascading

Earthquakes, whatever their size, can trigger other earthquakes. Mainshocks cause aftershocks to occur, which in turn activate their own local aftershock sequences, resulting in a cascade of triggering that extends the reach of the initial mainshock. A long-lasting difficulty is to determine which earthquakes are connected, either directly or indirectly. Here we show that this causal structure can be found probabilistically, with no a priori model nor parameterization. Large regional earthquakes are found to have a short direct influence in comparison to the overall aftershock sequence duration. Relative to these large mainshocks, small earthquakes collectively have a greater effect on triggering. Hence, cascade triggering is a key component in earthquake interactions.

Moho topography under central Greece and its compensation by Pn time-terms for the accurate location of hypocenters: The example of the Gulf of Corinth 1995 Aigion earthquake

In this paper we expand over the whole of central Greece, the Moho map centered on the Gulf of Corinth from tomographic inversion of PmP traveltime profile data recorded by several tens of temporary stations. Our approach is based on Pn, Moho refracted waves, from a large regional earthquake recorded by both temporary stations and the permanent Hellenic network. The Moho map shows the large Moho depth under the Hellenides belt. It also highlights the shallower Moho domain towards the Aegean Sea south and east of the Corinth Gulf. The domain of shallow Moho is limited along a NE–SW prolongation ahead of the North Anatolian Fault, from the North Aegean Trough to the western tip of the Gulf of Corinth towards the Gulf of Patras. The Pn time-terms provide corrections for the permanent stations that can be used together with the 1D velocity–depth model for a first-order compensation of lateral heterogeneity and contribute to the accurate and fast location of earthquake hypocenters. As a test we relocated the 1995 Aigion earthquake in this way, using only the sparse data of the permanent stations. Hypocentral coordinates then shift close to those derived by a dedicated dense array deployed after the earthquake, implying improvement of the routine location.

Anatahan, Northern Mariana Islands: Reconnaissance geological observations during and after the volcanic crisis of spring 1990, and monitoring prior to the May 2003 eruption

Anatahan island is 9.5 km east–west by 3.5 km north–south and truncated by an elongate caldera 5 km east–west by 2.5 km north–south. A steep-walled pit crater ∼1 km across and ∼200 m deep occupies the eastern part of the caldera. The island is the summit region of a mostly submarine stratovolcano. The oldest subaerial rocks (stage 1) are exposed low on the outer flanks and in the caldera walls. These include thick (∼10 m) and thin (2–3 m) lava flows, well-indurated tuffs, and scoria units that make up the bulk of the island. Rock compositions range from basaltic andesite to dacite, and most are plagioclase-phyric. On the steep north and south flanks of the volcano, these rocks are cut by numerous east–west-oriented, few-hundred-m-long lineaments of undetermined origin. Indurated breccias unconformably overlie scarps cut into stage 1 units low on the south flank. Intermediate-age eruptive units (stage 2) include caldera-filling lava flows and pyroclastic deposits and, on the outer flanks, vents and valley-filling lava flows. The youngest pre-2003 volcanic unit on Anatahan (stage 3) is a hydromagmatic surge and fall deposit rich in accretionary lapilli. Prior to 2003, this unit was found over almost the entire island, and in many places original depositional surfaces and outcrops could be found in high-energy environments along the coast, indicating a young (but undetermined) age. During reconnaissance visits in 1990, 1992, 1994, and 2001, geothermal activity (fumaroles as well as pits with boiling, sediment-laden pools) was observed in the southern part of the pit crater. In March and April 1990, increased local seismicity, a large regional earthquake, and reported increased fumarolic activity in the pit crater prompted evacuation of Anatahan village, at the west end of the island. Our first field investigation took place in late April 1990 to assess the level of volcanic unrest, conduct reconnaissance geological observations, collect rock and geothermal water samples, and set up a geophysical monitoring network. Results at this time were inconclusive with respect to determining whether the activity was anomalous. Water in some of the geothermal pits within the pit crater was boiling, and pH values as low as 0.7 were recorded in the field. An electronic distance measurement (EDM) network was installed, and over a ∼1-week period, up to 9 cm of extension occurred across some lines but not others. Seismicity was characterized by intermittent local earthquakes but no sustained swarms or tremor. A brief visit in June 1990 revealed that the shallow lake near the boiling pits was gone, but activity in the pits themselves was similar to that of April 1990. Only minor extension had occurred along a single EDM line since the previous visit, and no earthquakes >M2.5 occurred during the visit. Subsequent 1- to 2-day visits occurred in October 1990, May 1992, May 1994, and June 2001. Activity within the geothermal pits was relatively constant during every visit, although during this 11-year period the level of the water in each pit decreased. In June 2001, a ∼50-m-wide region of mud pots and steaming ground in the central part of the geothermal area had developed. No geologic evidence, however, suggested that an eruption would occur < 2 years afterward. Most of the EDM lines showed slight extension between late 1990 and 1992, followed by very gradual contraction from 1992 to 2001. A more extensive seismic-monitoring system was installed on the Northern Mariana Islands during these visits, and it recorded a small seismic swarm at Anatahan from May to July 1993. The telemetry component of the seismic equipment broke prior to 2001 and had not been repaired by the time of the May 2003 eruption, so no precursory seismic data were recorded to indicate pre-eruption unrest.

Salton Trough Regional Deformation Estimated from Combined Trilateration and Survey-Mode GPS Data

The Salton Trough in southeastern California, United States, has one of the highest seismicity and deformation rates in southern California, including 20 earthquakes M 6 or larger since 1892. From 1972 through 1987, the U.S. Geological Survey (USGS) measured a 41-station trilateration network in this region. We re-measured 37 of the USGS baselines using survey-mode Global Positioning System methods from 1995 through 1999. We estimate the Salton Trough deformation field over a nearly 30-year period through combined analysis of baseline length time series from these two datasets. Our primary result is that strain accumulation has been steady over our observation span, at a resolution of about 0.05 μstrain/yr at 95% confidence, with no evidence for significant long-term strain transients despite the occurrence of seven large regional earthquakes during our observation period. Similar to earlier studies, we find that the regional strain field is consistent with 0.5 ± 0.03 μstrain/yr total engineering shear strain along an axis oriented 311.6° ± 23° east of north, approximately parallel to the strike of the major regional faults, the San Andreas and San Jacinto (all uncertainties in the text and tables are standard deviations unless otherwise noted). We also find that (1) the shear strain rate near the San Jacinto fault is at least as high as it is near the San Andreas fault, (2) the areal dilatation near the southeastern Salton Sea is significant, and (3) one station near the southeastern Salton Sea moved anomalously during the period 1987.95-1995.11. Online material: Tables of stations used in this study and of estimated strain fields. Manuscript received 8 January 2003.

Paleoseismology of the Gazikoy-Saros segment of the North Anatolia fault, northwestern Turkey: Comparison of the historical and paleoseismic records, implications of regional seismic hazard, and models of earthquake recurrence

We excavated five trenches across the North Anatolia fault zone (NAFZ)along the Ganos fault (Gazikoy-Saros segment), which last produced surfacerupture in 1912, near Kavakkoy where the fault enters the Gulf of Saros. The trenches exposed faulted sediments in a flood-plain environment withabundant detrital charcoal and scattered land-snail shells. Twenty-tworadiocarbon dates place constraints on the ages of the exposed sediments,which range from less than a few hundred years to about 6000 years inage. In two closely spaced trenches, we identified five discrete earthquakeevent horizons in the upper 2.5 m of stratigraphy based on abruptupward termination of shear zones, folding, fissuring, and abruptstratigraphic thickening, four of which may corresponded to historicallyrecorded large regional earthquakes. The earliest of the identified eventsoccurs below an unconformity and dates to about 4 ka B.P. The morerecent four events all occurred within the past 1000–1200 years and maycorrespond to large earthquakes in A.D. 824, ca 1354, 1509, 1766 and1912 (Ambraseys and Finkel, 1987, 1991, 1995). In another trench,we identified at least two events that have occurred during the past 500years and probably correspond to the large events of 1766 and 1912. These observations support an average return period of about 250–300years for the Gazikoy-Saros segment of the NAFZ. They also suggest thatthis segment, which is bound both to the east and west by large releasingstepovers, behaves in a quasi-periodic fashion, at least for the past severalsurface ruptures.Most of the 23 mm/yr of dextral shear between Anatolia and Europeobserved by GPS occurs on the North Anatolian fault. We use18 mm/yr and the ∼ 250–300 year recurrence rate, as determined fromour trenching and the historical record, to suggest that each of theearthquakes observed in our trenches produced several meters of slip,consistent with their inferred sizes from the extent of historical damage. Considering that Istanbul has not suffered a large nearby event in theMarmara Sea since 1766, we suggest that about 4 m of strain hasaccumulated across faults in the Marmara during these past centuries. Thisis similar to the average slip in many of the large earthquakes on the NorthAnatolian fault this century. If released seismically, this could result in anearthquake in the M 7.2–M 7.6 range, similar to the August and November,1999 earthquakes east of the Marmara Sea.

Remotely triggered microearthquakes at the Geysers Geothermal Field, California

Microearthquake (MEQ) activity recorded on a local network at The Geysers geothermal field has been observed to increase dramatically within seconds to minutes after the arrival of surface waves from large regional earthquakes. This behavior was observed not only following the 1992 Landers earthquake, for which remotely triggered seismicity (at The Geysers and elsewhere) has been documented, but also for the following events: Gulf of Alaska (Ms 7.6, March 6, 1988); Loma Prieta (Ms 7.1, October 17, 1989); two events within the Gorda Plate (Ms 6.9 on July 13, 1991 and Ms 7.1 on August 17, 1991); Petrolia ( Ms 7.0, April 25, 1992); and Northridge (M s 6.6, January 17, 1994). The exact onset times of most swarms are difficult to pinpoint, but all start within minutes of the passage of seismic waves and in at least one case (Gulf of Alaska) the onset appears to coincide with the expected arrival of the surface waves. These swarms generally last for a few hours at most, and decay with time in a manner similar to aftershock sequences. We propose that the dynamic stresses associated with seismic waves emanating from these earthquakes may trigger the onset of these swarms. The Landers event may have been particularly efficient at generating remotely triggered swarms throughout the western U.S. because of its size and shallow focal depth, which would promote large surface waves. Our data demonstrate, however, that the phenomenon is not unique to the Landers event.

伸縮計と地震計の併用による地震観測

A seismic observation using an extensometer in combination with pendulum seismometers was carried out at the Osakayama Crustal Deformation Observatory, situated in the sowthwestern part of Shiga Prefecture. The extensometer used for the observation is equiped with a differential-transformer transducer, the output signals of which were passed through an electronic device of automatic zero adjustment in order to minimize the effect of the earth-tide and secular strain signals that contribute ‘noise’ to observation of small seismic signals.A detailed analysis of some good quality records obtained from recent earthquakes successfully demonstrated an agreement of theory and observation as to the directional response characteristics of the strain- and the pendulum seismometers, though the accuracy of the amplitude ratios of ground velocity to strain estimated from the record of the two instruments was not sufficient to provide the data for determination of phase velocity of surface waves from observation at the single station.Comparison was made between the strain and the velocity records obtained from two Chilean and 1983 Japan Sea earthquakes and it was shown that seismic waves propagating along the minor or the major arc path of the great circle, such as SS- and SSS-coupled PL waves and some G waves, can be easily discriminated by taking into account the difference of directinal responses of the two instruments to seismic waves. Two examples of ‘strain step’ recorded from large regional earthquakes were characterized by a gradual change in strain rather than step-like one.